Alcohol Oxidation Reactions on Porous PtCu/C Catalysts THESIS. the Graduate School of The Ohio State University. Heewon J. Choi, B.S.

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1 Alcohol Oxidation Reactions on Porous PtCu/C Catalysts THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Heewon J. Choi, B.S. Graduate Program in Chemistry The Ohio State University 2014 Master s Examination Committee: Dr. Anne Co, Advisor Dr. Yiying Wu

2 Copyright by Heewon J. Choi 2014

3 Abstract Pt and Pt-based metal alloys are the most effective electrocatalysts for fuel cells utilizing small organic molecules as fuels. Some of these fuels include alcohols such as in direct methanol fuel cells (DMFCs), direct ethanol fuel cells (DEFCs) and organic acids in direct formic acid fuel cells (DFAFCs). Pt-based catalysts are the most efficient in the electrocatalysis of the oxygen reduction reaction (ORR) at the cathode. A 50at% Pt 50at% Cu catalyst has been used as the catalyst for the alcohol oxidation reaction experiments. In this thesis work, I report that a PtCu/C exhibits higher activity and lower overpotential compared to a pure Pt/C. The ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib), If/Ib is utilized, to evaluate the CO poisoning resistance of Pt-based catalysts. The data shows that a PtCu/C has a higher catalytic activity, higher If/Ib ratio, and lower overpotential values for the methanol, ethanol and formic acid oxidation reactions compared to a commercial Pt/C in both acidic and basic conditions. Various AOR experiments were performed, such as the effect of Pt deposition time (0.25 min min Pt) on the composition of electrode and the effect of temperature (RT 80 C). Also, cyclic voltammograms (CV) were collected at different sweep rates (2 mv/s mv/s) and different rotation rates (0 rpm rpm). Degradation studies were also done on the Pt/C and PtCu/C catalysts deposited on the electrodes. ii

4 Dedication This document is dedicated to my family and friends. iii

5 Acknowledgments I really want to express great appreciation to my advisor Prof. Anne Co for her help, patience, support and guidance throughout this study and work. I would also like to thank the Co group members for their help throughout my research project. I would like to especially thank Eric Coleman for training me in the lab, and showing me how to perform key experiments such as the oxygen reduction reaction (ORR) experiments. I would also like to thank all my family and friends for their support, encouragement during this time. Finally, I would like to acknowledge the Ohio State University Chemistry and Biochemistry Department for funding this work. iv

6 Vita February Suji High, South Korea B.S. Chemistry, Purdue University 2012 to present...graduate Teaching Associate, Department of Chemistry, The Ohio State University Publications 1. Hurt, M. R.; Borton, D. J.; Choi, H. J.; Kenttamaa, H. I. Comparison of the Structures of Molecules in Coal and Petroleum Asphaltenes by Using Mass Spectrometry, Energy & Fuels 2013, 27, Fields of Study Major Field: Chemistry v

7 Table of Contents Abstract...ii Dedication..iii Acknowledgments..iv Vita..v Publications.v Fields of Study v Table of Contents...vi List of Tables..ix List of Figures.xi List of symbols and abbreviations...xvi Chapter 1: Introduction Background of fuel cell Fuel cell types Alcohol fuel cell..6 vi

8 1.4 Direct methanol fuel cell (DMFC) Direct ethanol fuel cell (DEFC) Direct formic acid fuel cell (DFAFC) Platinum catalyst Why PtCu/C catalyst Why nanoporous Cu (high surface area)...22 Chapter 2: Experimental Nanoporous Cu (NpCu) Synthesis Pt deposition (galvanic displacement reaction) on NpCu coated electrodes Three electrode cell Oxygen reduction reaction (ORR) activity Electrochemical characterization Cyclic voltammetry (CV) Effect of composition & temperature Chapter 3: Fuel oxidation on porous PtCu/C catalyst Background of a methanol oxidation Methanol oxidation on PtCu/C in acidic and alkaline conditions Ethanol oxidation on PtCu/C in acidic and alkaline conditions Formic acid oxidation on PtCu/C in acidic condition Comparing PtCu/C oxidation activity between methanol, ethanol and formic vii

9 acid Degradation studies (stability test of the Pt-based catalysts)...55 Chapter 4: Fuel oxidation at the anode as a function of Pt composition N2 deareated CV on PtCu/C and Pt/C in acidic and alkaline conditions Methanol oxidation on PtCu/C in acidic and alkaline conditions Ethanol oxidation on PtCu/C in acidic and alkaline conditions Formic acid oxidation on PtCu/C in acidic condition Alcohols oxidation on PtCu/C in acidic and alkaline conditions 80 Chapter 5: Effect of temperature on fuel oxidation Methanol oxidation in acidic medium Ethanol oxidation in acidic medium Poisoning as a function of temperature. 102 Chapter 6: Conclusion References viii

10 List of Tables Table 1.1 The operating temperatures, membranes, reactions and catalysts of the five different fuel cell types. Fuel cell types are as follows: proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC). 4 Table 1.2 Types of fuels at the anode, chemical reactions of fuels, standard potential value (vs. RHE) and energy density of fuel Table 1.3 The most common catalysts for anode fuels (hydrogen gas & alcohols) Table 1.4 A comparison of Pt-based catalysts for alcohol oxidation reaction (AOR), including a comparison of our PtCu/C catalyst and Pt-based bimetallic catalysts to reported values in the literatre...18 Table 1.5 The comparison of our PtCu/C catalyst and other types of PtCu catalyst (core-shell nanoparticle, bimetallic alloy nanoparticle) based on corrected electrocatalytic activity at 0.9V (specific activity of ORR) Table 4.1 The calculated electrochemical surface area (ECSA) of a platinum (cm Pt2 ) between 0.03 V and 0.4 V with standard deviation in acidic and alkaline electrolyte based on the Pt composition of a PtCu/C catalyst at varying Pt deposition times & Pt/C..58 Table 4.2 The I f/i b ratios and the corrected current densities of the forward peaks in 0.5 M MeOH and 0.1 M HClO 4 (0.1 M KOH) at 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode..71 ix

11 Table 4.3 The I f/i b ratios and the corrected current densities of the forward peaks in 0.5 M EtOH and 0.1 M HClO 4 (0.1 M KOH) at 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode...75 Table 4.4 The I f/i b ratios and the corrected current densities of the forward peaks in 0.5 M HCOOH and 0.1 M HClO 4 at 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode Table 5.1 The onset potential (V) of the forward peaks (If) and the current densities of the forward peaks (If) and backward peaks (Ib) of the MOR at low, middle and high temperatures (at RT, 40 C, 50 C, 60 C and 80 C) of a 1 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH at 10 mv/s and 1600 rpm Table 5.2 The activation energies (kj/mol) of the forward and backward peaks of the MOR at low, middle (40-50 C) and high temperatures (>80 C) of 0.1 M HClO M MeOH at various Pt deposition time (1 min and 2 min Pt desposition)...92 Table 5.3 The onset potential (V) of the forward peaks (If) and the current densities of the forward peaks (If) and backward peaks (Ib) of the EOR at low, middle and high temperatures (at RT, 40 C, 50 C, 60 C and 80 C) of a 1 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH at 10 mv/s and 1600 rpm. 97 Table 5.4 The activation energies (kj/mol) of the forward and backward peaks of the EOR at low, middle (40-50 C) and high temperatures (>80 C) of 0.1 M HClO M EtOH at various Pt deposition time (1 min and 2 min Pt desposition) Table 5.5 The I f/i b ratios and the Pt area corrected current densities of the forward peak of the MOR and EOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min Pt deposited PtCu/C in N 2 deareated 0.5 M CH 3OH (in 0.5 M C 2H 5OH) and 0.1 M HClO 4 between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm x

12 List of Figures Figure 1.1 Schematic of a fuel cell. Fuels such as hydrogen, methanol, ethanol and formic acid are oxidized at the anode and produce electrons which pass through the external circuit while positively charged protons move from the anode to the cathode through the proton exchange membrane (PEM). Protons react with oxygen from the air on the cathode and form water molecules Figure 1.2 Methanol oxidation cyclic voltammetry (CV) on PtCu/C catalyst....9 Figure 2.1 Glassy carbon electrodes (diameter 5 mm) Figure 2.2 Electrochemical cell (including gas bubbler) Figure 2.3 Galvanic displacement reaction, the electrode before depositing Pt, the electrochemical cell during depositing Pt and the electrode after depositing Pt Figure 2.4 Rotating disk electrode (RDE) apparatus Figure 2.5 PtCu/C catalyst Figure 2.6 Scanning electron microscope (SEM) image and energy-dispersive X-ray (EDX) spectrum of NpCu catalyst Figure 2.7 SEM image and EDX spectrum of PtCu/C catalyst...28 Figure 2.8 Pt mesh reference & counter electrodes Figure 3.1 The CV of a MOR; both anodic and cathodic sweeps (forward and backward peaks) xi

13 Figure 3.2 The CV of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M MeOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm Figure 3.3 Scatter plot of I f/i b Vs. ECSA (cm Pt2 ) for a commercial Pt/C (blue solid) and PtCu/C (red solid) in 0.5 M CH 3OH M HClO 4 solution Figure 3.4 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M MeOH and 0.1 M KOH and PtCu/C in N 2 deareated 0.1 M KOH between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm Figure 3.5 The CV of a PtCu/C in N 2 deareated 0.5 M MeOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.5M MeOH and 0.1 M KOH between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm..43 Figure 3.6 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M EtOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm Figure 3.7 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M EtOH and 0.1 M KOH and PtCu/C in N 2 deareated 0.1 M KOH between 0.3 V and 1.2 V at RT, 10 mv/s and 1600 rpm..47 Figure 3.8 The CV (anodic sweep only) of a PtCu/C in N 2 deareated 0.5 M EtOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.5M EtOH and 0.1 M KOH between 0.3 V and 1.2 V at RT, 10 mv/s and 1600 rpm..49 Figure 3.9 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M formic acid and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm xii

14 Figure 3.10 The CV (anodic sweep only) of a PtCu/C in N 2 deareated 0.5 M MeOH (0.5 M EtOH, 0.5 M formic acid) and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.3 V and 1.2 V at RT, 10 mv/s and 1600 rpm Figure 3.11 Degradation studies of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M CH 3OH and 0.1 M HClO 4 for 2 hour at RT Figure 4.1 The CV of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.6 V and 1.2 V at RT based on 1 min and 2 min Pt deposition times on a PtCu/C electrode, respectively Figure 4.2 The potential (V) values of the Pt oxide reduction peak of the CV (Figure 4.1) on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode Figure 4.3 The CV of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH between 0.6 V and 1.2 V at RT based on the 1 min and 2 min Pt deposition times on a PtCu/C electrode, respectively Figure 4.4 The potential (V) values of the Pt oxidation peak of the CV (Figure 4.1) on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode...64 Figure 4.5 The potential (V) values of the Pt oxide reduction peak of the CV (Figure 4.1) on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode Figure 4.6 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode...68 xiii

15 Figure 4.7 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH and 0.5 M MeOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode..70 Figure 4.8 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode..73 Figure 4.9 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH and 0.5 M EtOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode..74 Figure 4.10 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M HCOOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode Figure 4.11 The corrected current densities of the forward peaks on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH (0.5 M EtOH) and in N 2 deareated 0.1 M KOH and 0.5 M MeOH (0.5 M EtOH) at 10 mv/s and 1600 rpm based on the Pt composition of a PtCu/C electrode Figure 4.12 The I f/i b ratios on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH (0.5 M EtOH) and in N 2 deareated 0.1 M KOH and 0.5 M MeOH (0.5 M EtOH) at 10 mv/s and 1600 rpm based on the Pt composition on a PtCu/C electrode Figure 5.1 The CV (Pt area corrected current density, anodic sweep only) of the MOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO4 and 0.5 M MeOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm...86 xiv

16 Figure 5.2 The CV (Pt area corrected current density, backward, Ib peak only) of the MOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm. 88 Figure 5.3 Ln (Pt area corrected current density of forward peak) Vs. 1/T (1/K) of the MOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min and 2 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm..90 Figure 5.4 Ln (Pt area corrected current density of backward peak) Vs. 1/T (1/K) of the MOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min and 2 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm..91 Figure 5.5 The CV (Pt area corrected current density, anodic sweep only) of the EOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm Figure 5.6 The CV (Pt area corrected current density, backwards peak only) of the EOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm Figure 5.7 Ln (Pt area corrected current density of forward peak) Vs. 1/T (1/K) of the EOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min and 2 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm Figure 5.8 Ln (Pt area corrected current density of backward peak) Vs. 1/T (1/K) of the EOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min and 2 min Pt deposited PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm Figure 5.9 Figure 5.9 The I f/i b ratios of the forward peak of the MOR and EOR at RT, 40 C, 50 C, 60 C and 80 C of a 1 min Pt deposited PtCu/C in N 2 deareated 0.5 M CH 3OH (in 0.5 M C 2H 5OH) and 0.1 M HClO 4 between 0.2 V and 1.2 V at 10 mv/s and 1600 rpm xv

17 List of Symbols and Abbreviations AFC CE CV DAFC DEFC DFAFC DMFC E onset ECSA EDX EOR FAOR MCFC MOR NpCu ORR PAFC PEM PEMFC Alkaline fuel cell Counter electrode Cyclic voltammetry/cyclic voltammogram Direct alcohol fuel cell Direct ethanol fuel cell Direct formic acid fuel cell Direct methanol fuel cell Onset potential Electrochemical surface area Energy-dispersive X-ray Ethanol oxidation reaction Formic acid oxidation reaction Molten carbonate fuel cell Methanol oxidation reaction Nanoporous copper Oxygen reduction reaction Phosphoric acid fuel cell Polymer electrolyte membrane, proton exchange membrane Proton exchange membrane fuel cell xvi

18 PTFE RDE RE RHE RT SCE SEM SHE SOFC TOC WE Polyetrafluoroethylene or Teflon Rotating disk electrode Reference electrode Reversible hydrogen electrode Room temperature Saturated calomel electrode Scanning electron microscopy Standard hydrogen electrode Solid oxide fuel cell Total organic carbon Working electrode Units A C V Ampere/amp (unit of current that also equals Coulomb/second) Coulomb (unit of charge) Volt (unit of potential) xvii

19 CHAPTER 1 Introduction This chapter provides background information on fuel cells, and specifically, direct alcohol fuel cells (DAFCs). 1.1 Background of fuel cell Fuel cells were first invented in 1838 and are currently being used for many applications, including in buses, airplanes, stationary power sources, etc. A fuel cell is an energy conversion device that transforms energy stored in fuel to electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but alcohols like methanol and ethanol are also being considered due to their high energy density. Alcohols are promising fuels at the anode because most alcohols are inexpensive, abundant and relatively environmentally friendly. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen/air to maintain the chemical reaction, and therefore produce electricity continuously for as long as a constant source of fuels and oxygen/air is supplied. A fuel cell consists of an anode, which oxidizes fuel and a cathode, which reduces an oxidant, such as oxygen gas from air. The anode and the cathode are separated by a 1

20 polymer electrolyte membrane (PEM), which is also known as a proton exchange membrane. PEM is a semipermeable membrane and designed to conduct protons only. Proton exchange membrane fuel cells (PEMFCs) work with a polymer electrolyte in the form of a thin, permeable sheet. Proton exchange membrane fuel cell (PEMFC) generally consumes hydrogen at the anode, but liquid fuels such as methanol or ethanol can also be utilized. At the anode of a fuel cell, the oxidation reaction occurs. In a standard fuel cell, hydrogen is oxidized and positively charged protons are allowed to move from the anode to the cathode through the PEM. 1 Electrons travel along an external circuit from the anode to the cathode and produce an electrical current. At the cathode, the electrons reduce oxygen O2 to O 2- or OH - and positively charged hydrogen ions combine with the reduced O 2- or OH - to form water molecules that flow out of the cell. To increase the performance of fuel cells, metal catalysts are often used. The most common catalyst at the anode of a standard fuel cell is Pt or a Pt-based catalyst. Pt is the catalyst of choice for increasing catalytic activity, current density and performance of fuel cell. Pt-M where M = Sn, Ru, etc. metal alloys with other transition metals are also considered as catalyst for fuel oxidation Several approaches improve the catalytic activity, such as increasing surface area and lowering the total Pt content. 2

21 Fuel Oxidation Oxygen Reduction Hydrogen H 2(g) 2H + (aq) + 2e - Load e - e - Methanol CH 3OH(l) + H 2O(l) CO 2(g) + 4 H + + O e - 2 H 2O E = 1.23 V vs SHE 6H + (aq) + 6e - Ethanol H + Anode Cathode PEM C 2H 5OH(l) + 3H 2O(l) 2CO 2(g) + 12H + (aq) + 12e - Formic Acid HCOOH(l) CO 2(g) + 2H + (aq) + 2e - Figure 1.1 Schematic of a fuel cell. Fuels such as hydrogen, methanol, ethanol and formic acid are oxidized at the anode and produce electrons which pass through the external circuit while positively charged protons move from the anode to the cathode through the proton exchange membrane (PEM). Protons react with oxygen from the air on the cathode and form water molecules. 3

22 1.2 Fuel cell types 1 Table 1.1 Operating temperatures, membranes, and catalysts of the five different fuel cell types. Fuel cell types are as follows: proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC). Types of fuel cell Operating Temperature ( C) Membrane Catalyst Proton exchange membrane (PEM) 60 C 80 C H + Pt Alkaline 90 C 100 C OH - Phosphoric acid 100 C 250 C H + Molten carbonate 600 C 700 C CO3 2- Non-precious metal Carbonsupported Pt catalyst Ni. Non-Pt catalyst Solid oxide 800 C 1000 C O 2- Non-Pt catalyst 4

23 1.2.1 Proton exchange membrane fuel cell (PEMFC) The most common fuel cell is the proton exchange membrane fuel cell (PEMFC). The operating temperatures for PEMFC are between 60 C and 80 C. A polymer electrolyte separates O2 from H2 using Nafion that allows H + conduction Alkaline fuel cell (AFC) In alkaline fuel cells, a solution of potassium hydroxide (KOH) in water is generally used as the electrolyte. The operating temperatures are between 90 C and 100 C. Alkaline fuel cells operate on compressed hydrogen and oxygen. The efficiency of an alkaline fuel cell is about 70% Phosphoric acid fuel cell (PAFC) Phosphoric acid fuel cells (PAFCs) utilize H3PO4 (Phosphoric acid) as the electrolyte. The operating temperatures are between 100 C and 250 C. The efficiency of a phosphoric acid fuel cell (PAFC) is about 40-80% Molten carbonate fuel cell (MCFC) Molten carbonate fuel cells (MCFCs) operate at high temperatures between 600 C and 700 C. High-temperature compounds of carbonate salt are utilized as the electrolyte. The efficiency of a molten carbonate fuel cell is about 60-80%. Non-precious metals can be 5

24 used as catalysts for the anode and cathode, and the membrane conducts CO3 2- ion. The high temperature limits damage from CO poisoning of the cell Solid oxide fuel cell (SOFC) Solid oxide fuel cells (SOFCs) also operate at extremely high temperatures. The operating temperatures are between 800 C and 1000 C. Oxygen conducting ceramic oxides, for example, 8 % Y2O2 doped ZrO2, are utilized as electrolytes. The efficiency of a SOFC is about 60%. 1.3 Alcohol fuel cell Table 1.2 Types of fuels at the anode, chemical reactions of fuels, standard potential value (vs. RHE) and energy density of fuels. Anode Fuels Anode reactions E (VRHE) Energy density (kj/ml) Hydrogen (l, g) H2(g) 2H + (aq) + 2e (l) 0.01(g) Methanol CH3OH(l) + H2O(l) CO2(g) + 6H + (aq) + 6e Ethanol C2H5OH(l) + 3H2O(l) 2CO2(g) + 12H + (aq) + 12e Formic acid HCOOH(l) CO2(g) + 2H + (aq) + 2e

25 Hydrogen gas has been the most common fuel at the anode of PEM, AFC, and SOFC fuel cells. Using hydrogen gas as fuel in PEM delivers high power density and provides some advantages, due to its low weight. Also, hydrogen gas provides higher net cell voltage and lower overpotential compared with other fuels. However, a few difficulties of using hydrogen gas include safe handling, transportation, and storage. Direct alcohol fuel cells (DAFCs) such as methanol fuel cell, ethanol fuel cell, etc. are promising alternatives since liquid fuels are a lot easier to be handled, transported, and stored than gas fuels. Also, small alcohols of organic molecules such as methanol and ethanol are cheap, relatively non-toxic to the environment and have higher energy density than hydrogen gas (Table 1.2). Compared to the energy density of hydrogen gas (0.01 kj/ml), the energy densities of methanol (17.4 kj/ml), ethanol (22.7 kj/ml) and formic acid (7.2 kj/ml) are much higher. In addition, ethanol shows the highest energy density among the fuels in Table 1.2. One of the main problems for DAFCs is poisoning of the catalyst by CO, which lowers the performance of alcohol fuel cells. Liquid fuels used in DAFC are typically small organic molecules, such as methanol, ethanol and formic acid. Carbon dioxide is a final product if the alcohols are completely oxidized. The operating temperatures of direct methanol fuel cell (DMFC) are between 50 C and 120 C. The DAFC is a promising energy source as a battery replacement for portable appliances such as cellular phones and laptop computers. Larger scale DAFCs could be used as a power and energy source for larger electronic devices, possibly even for cars in the future. 7

26 1.4 Direct methanol fuel cell (DMFC) In a DMFC, methanol is utilized at the anode. Pt and Pt-based catalysts are the most promising catalytic materials used for methanol oxidation reaction (MOR) of DMFCs. Many research groups have found that Pt alloys such as PtRu are better catalysts for MOR. Alternative catalysts to replace Pt-based metal alloys are currently being developed due to the high cost of platinum. However, most non-noble catalysts to date have had problems with corrosion in acidic conditions, and degrades rapidly during intermediate steps of the MOR. Therefore, Pt-based catalytic materials are still the best catalysts for MOR. 20 8

27 Current density (A/cm Pt 2 ) I f I b E(V) Vs. RHE Figure 1.2 Methanol oxidation cyclic voltammetry (CV) on PtCu/C catalyst. The main product of the complete MOR at the anode is carbon dioxide (CO2). The halfreaction of MOR at the anode is; CH3OH(l) + H2O(l) CO2(g) + 6H + (aq) + 6e - E = VRHE The complete MOR mechanism involves several significant intermediate steps. The complete MOR on the Pt catalyst takes place over the broad potential range of 0 V ~ 1.2 V. 9

28 There are a couple of important intermediate steps (including CO poisoning) during the MOR at the anode of a fuel cell. Each of the intermediate steps takes place in specific potential regions. The detailed MOR mechanism is presented below based on the research of Goodenough, et al. 20 1) E < 0.5 V, this region shows the initial reactions of MOR on Pt catalyst deposited electrode. This is due to the dissociative chemisorption of the methanol reaction that occurs in this region. And this initial reaction gives the major adsorbed species Pt3COH. Three-step process involves: CH3OH + 2Pt PtCH2OH + PtH (1a) PtCH2OH + 2Pt Pt2CHOH + PtH (1b) Pt2CHOH + 2Pt Pt3COH + PtH (1c) Pt3COH is one of the major intermediate products in the MOR. The PtH species are lost to the solution as H +. PtH Pt + H + + e - 2) Over the potential range of 0.3 V < E < 0.9 V, Adsorbed species, which were obtained below 0.5 V react with adsorbed OH to yield several different oxidation products. This reaction is known as the equilibrium formation of OH at the surface. PtCH2OH + OHad CH2O + Pt + H2O (2a) Pt2CHOH + 2OHad HCOOH + Pt + H2O (2b) 10

29 Pt3COH + 3OHad CO2 + 3Pt + 2H2O (2c) There are small amounts of formaldehyde and formic acid formed during intermediate reactions below 0.9 V. Formaldehyde and formic acid are also products of intermediate steps during the MOR. 3) Over the potential range of 0.5 V < E < 1.1 V, The major adsorbed product Pt3COH at the low potential range produces other products at relatively high potential range during the MOR. Pt3COH Pt2CO + Pt + H + + e - (3a) Pt2CO + OH PtCO2H + Pt (3b) PtCO2H + OH CO2 + Pt + H2O (3c) Carbon dioxide (CO2) is the desired final product of MOR on a Pt-based catalyst. The unreacted Pt2CO rearranges itself to give the linearly bound carbon monoxide (CO) species, which acts as a poisoning species during the MOR by interrupting the formation of CO2. During the reverse scan, the Pt surface is uncovered by the removal of adsorbed OH which facilitates the removal of CO poisoning. Therefore, the reverse peak basically shows how much CO poisoning is on the Pt catalyst during MOR. The reaction occurring on the reverse peak could be written as, PtOH + Pt=C=O CO2 + 2Pt + H + + e - 11

30 1.5 Direct ethanol fuel cell (DEFC) A direct ethanol fuel cell (DEFC) is operated and utilized based on the same principle as the common PEMFC. Ethanol is used instead of hydrogen gas, at the anode of the fuel cell in a DEFC. Same as DMFC, Pt and Pt-based metal alloys are the most efficient catalysts in a DEFC. In addition, the normal cyclic voltammogram (CV) of the EOR looks similar to the normal CV of the MOR. The advantages of ethanol over other alcohol molecules, such as methanol and formic acid, are that ethanol has a higher energy density (22.7 kj/ml) and lower toxicity than the other alcohol molecules. Ethanol is more environmentally friendly and less dangerous than methanol. It is a liquid under atmospheric pressure and there are no problems in distributing and transporting ethanol. Therefore, ethanol is a promising liquid fuel that could be used at the anode of a fuel cell, instead of hydrogen or other natural gases and other alcohols. However, there are some disadvantages using ethanol as a fuel. Ethanol has a more complicated chemical structure compared to methanol or formic acid. This produces a lot of intermediate products during the EOR. Ethanol has a higher possibility for CO poisoning during the EOR than MOR because it is produced at multiple intermediate steps. Also, the C-C bond of ethanol is difficult to break because it has a relatively high bond energy (348 kj/mol). Methanol and formic acid do not require C-C bond cleavage and thus require less energy/lower applied potential. This increases the possibility that the Pt catalyst on the electrode is poisoned by CO during EOR. This interrupts the formation of CO2 as a final product of EOR and causes DEFCs to have a lower performance. 15,

31 The main product of the overall EOR at the anode is carbon dioxide (CO2) same as the final product of the overall MOR. And the half-reaction of the EOR at the anode is shown below. C2H5OH(l) + 3H2O(l) 6CO2(g) + 12H + (aq) + 12e - E = VRHE There are many intermediate steps during the EOR mechanism. The EOR on the Pt catalyst occurs over the broad potential range of 0 V ~ 1.2 V. Because of the complicated reaction mechanism on EOR, a specific reaction mechanism has yet to be determined, depending on catalyst of choice. However, the generally accepted mechanism is described below. The initial reaction of the EOR mechanism involves dehydrogenation and yields acetaldehyde and acetyl intermediates. CH3CHO and COCH3 are the intermediates found in EOR. 25,26 The partial EOR yields a couple of undesirable byproducts. CH3CH2OH [CH3CH2OH]ad CH3CHO CH3COOH Because of the difficulty of breaking the C-C bond in ethanol, EOR produces high concentrations of CH3CHO and CH3COOH. These undesirable byproducts attach to the surface of the Pt and Pt-based catalysts and interrupt the complete oxidation to CO2. The byproducts formed during EOR also decrease the performance and efficiency of a DEFC. 1.6 Direct formic acid fuel cell (DFAFC) Formic acid is sometimes used at the anode of a fuel cell. Pt and Pt-based metal alloys are the most efficient catalysts in DFAFC. Formic acid has a relatively lower energy density 13

32 (7.2 kj/ml) than the other alcohol molecules. And the performance of a DFAFC is comparatively lower than the other alcohol fuel cells. However, it is a liquid under atmospheric pressure, and distributing and transporting formic acid is easy. Therefore, formic acid is still in consideration for an alternative liquid fuel, instead of hydrogen or other natural gases. However, in general, DFAFC is unfavorable due to the low energy density and high possibility of getting CO poisoning on the Pt surface. The main product of the overall formic acid oxidation reaction (FAOR) at the anode is carbon dioxide (CO2), same as the final product of the overall MOR and EOR. And the half-reaction of the FAOR at the anode is shown below. CHOOH(l) CO2(g) + 2H + + 2e - E = VRHE There are two different potential mechanisms in FAOR. First, FAOR forms a reactive intermediate, this is known as the direct oxidation pathway of formic acid. Second, formic acid forms COads which is oxidized to CO2, the final product of FAOR. This mechanism is known as the indirect oxidation pathway of formic acid. Indirect oxidation causes a lot of problems for FAOR because it poisons the Pt surface and lowers the performance of the Pt catalysts

33 1.7 Platinum catalyst Table 1.3 The most common catalysts for anode fuels (hydrogen gas & alcohols). Anode fuels Hydrogen (H2) Methanol (CH3OH) Ethanol (C2H5OH) Formic acid (HCOOH) Most common catalysts Pt, Ni Pt, PtRu Pt, PtSn Pt, PtRu Pt-based catalysts are the most efficient electrocatalysts for fuel oxidation, as well as the oxygen reduction reaction (ORR). Adding secondary and ternary metal additives like ruthenium (Ru), tin (Sn), or palladium (Pd) to Pt could improve the current density and catalytic activity of the reaction compared to a pure Pt catalyst. However, Pt is an expensive material. Many scientists have tried to use alternative catalysts such as nonnoble metals to replace Pt. However, most non-noble metal catalysts suffer from corrosion in the region where the main reaction occurs. Therefore, Pt is still the best catalyst for fuel oxidation at the anode. Pt-based bimetallic alloy catalysts are more preferred than pure Pt catalyst because they reduce the amount of Pt in the catalyst and increase the stability and activity towards AOR ) PtRu: Many scientists have focused on developing Pt-based bimetallic catalysts, which could increase the oxidation current for fuel oxidation. The PtRu enhanced the speed of the fuel oxidation reaction, which means that an ethanol molecule can be 15

34 oxidized to CO2 relatively quickly compared to other Pt-based bimetallic catalysts. Therefore, the PtRu is preferred to increase CO2 selectivity at low temperatures of AOR. 13, ) PtSn: This is the most effective Pt-based bimetallic catalyst for the EOR, when compared to the other Pt-based bimetallic catalysts. The activity and efficiency of this binary catalyst for AOR depends on its composition and structure. Different composition and structure can be obtained by using various methods. In order to improve the catalytic activity of PtSn, many research groups have studied the most efficient synthesis method because catalytic activity is strongly dependent on the synthesis methods and conditions. For example, some research groups have proved that having a Pt:Sn atomic ratio of 3:1 is the optimized ratio of PtSn bimetallic catalyst because it showed high catalytic activity and current density for EOR. 4,19,39 3) PtCu: This is one of the most promising catalysts for AOR. We used a PtCu/C bimetallic alloy as the catalyst for the AOR experiment. A PtCu/C has shown high catalytic activity at both cathode and anode. It showed a much higher activity on the oxygen reduction reaction (ORR) and had better stability than a commercial Pt/C. In this work, I have shown that the PtCu/C had much higher CO poisoning resistance than the pure Pt/C during the reaction of alcohol fuels. We proved that the PtCu/C could be used as a very promising catalyst for fuel oxidation based on high current density, catalytic activity and efficiency. The recent data demonstrates that the PtCu/C bimetallic electrocatalyst is a suitable catalyst for half-cell experiments, especially for AOR at the anode. In addition to the PtCu/C, there are few promising bimetallic electrocatalysts for 16

35 AOR. The following catalysts have also been widely studied for fuel oxidation at the anode of a DAFC. 8,9, ) Other alloys: PtRh, PtPd and PtW, etc. are all promising Pt-based bimetallic catalysts which could be used at fuel oxidation. Those catalysts have relatively high current density, catalytic activity and CO poisoning tolerance on the electrode. Some research groups have studied AOR by using those Pt-based bimetallic catalysts. They are also working on finding the best synthesis method, which improves the catalytic activity and efficiency of these possible catalysts. 14,21,

36 Table 1.4 A comparison of Pt-based catalysts for alcohol oxidation reaction (AOR), including a comparison of our PtCu/C catalyst and Pt-based bimetallic catalysts to reported values in the literature. 33 Catalyst Solution (electrolyte, fuel) Reference electrode E onset Vs. SHE (V) E onset (over potential, V) If (ma/cmpt 2 ) If/Ib Sweep rate (mv/s) 0.1 M PtCu/C PtCu/C PtRu PtSn PtW PtPd HClO4, 0.5 M MeOH 0.1 M HClO4, 0.5 M EtOH 0.5 M H2SO4, 1M EtOH 0.5 M H2SO4, 1M EtOH 0.5 M H2SO4, 1M EtOH 0.5 M H2SO4, 1M EtOH RHE > 1 10 RHE < 1 10 SCE > 1 10 SCE > 1 10 SCE > 1 10 SCE >

37 1.8 Why PtCu/C catalyst We use a Pt enriched PtCu/C shell over a PtCu/C core as a catalyst for the alcohol oxidation reaction (AOR). Our goal is to galvanically deposit a very thin layer of Pt onto a porous Cu support. In this way, we could minimize the amount of Pt, maximize the active surface area and increase inherent catalytic activity. Also, we can make highly conducting PtCu/C porous supports. However, there are many different kinds of PtCu/C catalysts, which could electrochemically work at both the anode and the cathode of fuel cell. Some research groups have worked on developing PtCu/C catalyst types, such as bimetallic alloy nanoparticles, or core-shell catalysts, which improve specific activity on the ORR and give a high surface area of Pt. They have developed several PtCu/C catalysts and found ways to optimize the catalytic activity for the ORR. Several types of PtCu/C catalyst are summarized below. 1) Core-shell Pt-based catalyst: As a particular type of bimetallic catalyst, core-shell nanoparticles are a very attractive approach in making a high surface area catalyst. In general, a particle shell is enriched with the catalytic material on a core with a different composition. The electrocatalytic activity is based on the shell thickness and synthesis method. Chemical dealloying is a very important synthesis method to obtain catalytically active core-shell particles. In detail, the synthesis of Pt-based alloy nanoparticles is conducted via liquid precursor impregnation, followed by a freezing-drying method, and then annealing in a hydrogen atmosphere. 51 There are size dependent morphologies and compositions of dealloyed catalysts. First, single core-shell nanoparticles are formed by dealloying particles, making these particles less than nm in diameter. Second, 19

38 multiple cores-shell nanoparticles have more than nm of multiple cores and less than nm of pores. Lastly, porous multiple cores-shell particles are dealloyed bimetallic particles that exhibit multiple cores and a various range of porosity. 51,52 2) Bimetallic alloy nanoparticles: These have significantly different optical and catalytic properties from monometallic nanoparticles. Bimetallic nanoparticles produce enhanced catalytic activity. They also offer a wide range of compositional, and morphological configurations, and allow control of their surface properties. Electrocatalytic activity of bimetallic nanoparticles is dependent on the structure, composition, and particle size of the alloy. Thermal annealing of bimetallic precursors is the most common and important synthetic step to form bimetallic alloy nanoparticles. In detail, the preparation of PtCu precursor materials is reported as follows. A Pt:Cu ratio of 1:3 is formed using a Cu precursor impregnation on carbon-supported Pt nanoparticles followed by freeze drying. 51, ) Pt overlayers thin film: The dealloyed PtCu thin film shows a lot higher specific oxygen reduction activity than a pure Pt monolayer thin film. Alloy catalysts with Pt overlayers formed at the surface are reliable catalysts because they have high catalytic activity and significantly different chemical and electronic properties when compared to bulk Pt films. There are two ways to make Pt surface overlayers. First, they can be formed by Pt monolayers deposited on various metal substrates using vacuum techniques. Second, they can be formed by electrochemical dissolution/dealloying of non-noble elements from a bimetallic alloy (Pt-M). 44,58,59 20

39 Table 1.5 The comparison of our PtCu/C catalyst and other types of PtCu catalyst (core-shell nanoparticle, bimetallic alloy nanoparticle) based on corrected electrocatalytic activity at 0.9V (specific activity of ORR). Specific activity Catalyst comp. atom Annealing temp. of ORR at 0.9V- % ( C) IR (µa/cmpt 2 ) Reference Pt/C (pure) No annealing 476 PtCu/C (our catalyst) No annealing 1599 Pt25Cu75 (core-shell) Pt25Cu75 (core-shell) Pt25Cu75 (core-shell) Pt25Cu75 (alloy nanoparticle) Pt25Cu75 (alloy nanoparticle) Pt25Cu75 (alloy nanoparticle)

40 1.9 Why nanoporous Cu (high surface area) Alloy nanoparticles, nanostructures and core-shell catalysts are used for catalysts in electrochemical energy conversion. In this work, we used nanoporous Cu as the catalyst for fuel oxidation at the anode. We used nanoporous Cu catalysts for several reasons. The preparation of nanoporous Cu is simple and thus would be an excellent candidate for industrial scale production. We can simply fabricate Cu-Al alloys and etch the aluminum from the Cu-Al alloy with 6 M NaOH. It is also possible to modify etching conditions to tune our pore size. It does not take too much time to synthesize a nanoporous Cu catalyst. We could produce a nanoporous Cu sample that has a high surface area (BET Surface Area > 13 m 2 g -1 ) that can be tuned to have a pore size from nm within a short period of time. Having a high surface area of nanoporous Cu is very important because it can change the size of the pores and improve the catalytic activity. Cu metal with nanosized pores are connected to each other and form strong bonds. This increases conductivity and produces a highly conducting support. 60,61 22

41 CHAPTER 2 EXPERIMENTAL 2.1 Nanoporous Cu (NpCu) Synthesis A nanoporous Cu support was formed by etching Al from an in-house prepared Cu-Al alloy, with composition ranging between 20 to 50 wt% Cu. Upon etching, the nanoporous support provides a high surface area of m 2 /g. For the detailed preparation of nanoporous Cu (NpCu), Al was etched from the Cu-Al alloy with 6 M NaOH for 24 hours. CuOx was reduced to Cu in a tube furnace under H2 (Praxair, INC.) for 2 hours at 450 C. For the preparation of the NpCu/C sample, 50 % of NpCu (15 mg), 50% carbon black (15 mg) and ~10 ml deionized water (Millipore, ultrapure water 18.2 MΩcm at 25 C, total organic carbon (TOC) is less than 4 ppb) were added together and mixed in a small vial. The solution was then sonicated for 12 min in the sonicator (ultra sonicator from Fisher scientific, INC.) to make sure that the solution was mixed well. During sonication, Nafion (perfluorosulfonic acid PTFE copolymer, Alfa aesar the johnson matthey company) was added to the solution to make the catalysts adhere to the glassy carbon electrodes substrate. After sonication, the NpCu catalyst was dropped on the 23

42 glassy carbon electrodes. The electrodes were then heated at C for 1 hour and 30 min in the oven (vacuum oven from Thermo scientific, INC.). Figure 2.1 Glassy carbon electrodes (diameter 5 mm). 2.2 Pt deposition (galvanic displacement reaction) on NpCu coated electrodes Pt was deposited onto NpCu coated electrodes via a galvanic displacement reaction (Figure. 2.3) at a constant rotation rate (500 rpm). A mixture of g K2PtCl4 (>99.9% trace metal basis powder, 5 g) in 120 ml of deionized water was put in the electrochemical cell and heated to 50 C using the circular bath (Fisher scientific, INC.). The temperature was monitored using a clean thermometer. A rotating disk electrode (RDE) apparatus was used for Pt deposition onto the NpCu catalyst. RDE was rotated at 24

43 constant rotation rate (500 rpm). The galvanic displacement reaction, (the redox reaction between Pt and Cu), results in a layer of Pt on top of the Cu. Ring disk electrode (RDE) apparatus (Figure 2.4): The RDE is a hydrodynamic working electrode used in a three electrode system. The electrode rotates during experiments inducing a flux of electrolyte to the electrode (Figure. 2.4). This setup was used in all of the experiments to investigate reaction kinetics and the mechanism for fuel cells. Figure 2.2 Electrochemical cell (including gas bubbler). 25

44 Before During After Figure 2.3 Galvanic displacement reaction, the electrode before depositing Pt, the electrochemical cell during depositing Pt and the electrode after depositing Pt. 26

45 Figure 2.4 Rotating disk electrode (RDE) Apparatus. Figure 2.5 PtCu/C catalyst. 27

46 Figure 2.6 Scanning electron microscope (SEM) image and energy-dispersive X-ray (EDX) spectrum of NpCu catalyst. Figure 2.7 SEM image and EDX spectrum of PtCu/C catalyst. 28

47 2.3 Three electrode cell A ring disk electrode (RDE) was used as the working electrode, Pt mesh was used as the counter electrode and a reversible hydrogen electrode (RHE) was used as the reference electrode (Figure. 2.8). Working electrode: The working electrode in an electrochemical system is the electrode on which the reaction of interest is occurring. Common working electrodes can consist of inert metals such as gold, silver or platinum, or inert carbon such as glassy carbon. Reference electrode: An electrode, which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant concentrations of each of the participants of the redox reaction. A reversible hydrogen electrode (RHE) is a reference electrode, more specifically, a subtype of the standard hydrogen electrode, where the redox potential is adjusted to the ph of the solution which can then be directly used in the electrolyte. Counter electrode: A high surface area of Pt mesh electrode, which allows current flow from the counter electrode (CE) to the working electrode (WE). A Pt mesh electrode can be used as a convenient counter electrode for the AOR. 29

48 Figure 2.8 Pt mesh reference & counter electrodes. 2.4 Oxygen reduction reaction (ORR) activity ORR specific activity was measured at 0.9 V vs. RHE (reversible hydrogen electrode). After performing the blank measurements using CV, a saturated solution of electrolyte (0.1 M HClO4 and 0.1 M KOH) with oxygen (compressed, Prax air, INC.) bubbling for 40 minutes was created. The ORR experiment was performed and the specific activity value (I kinetic value) was obtained, especially in 0.9 V region. The ORR specific activity was used to evaluate and determine the quality of the PtCu/C. 30

49 2.5 Electrochemical characterization Methanol (CH3OH, E : VRHE, ACS certified from Fisher scientific, INC.), ethanol (C2H5OH, E : VRHE, 500 ml, 200 proof absolute from Sigma-aldrich, INC.) and formic acid (HCOOH, E : VRHE, 100 ml, reagent grade >95% from Sigmaaldrich, INC.) were used as fuels. For supporting electrolytes, 0.1 M HClO4 (perchloric acid, trace metal grade from Fisher scientific, INC.) was used as the acid electrolyte and 0.1 M KOH (potassium hydroxide pellets from Sigma-aldrich, INC.) was used as the base electrolyte. The alcohol fuel was added in the electrolyte and a mixture of 0.5 M alcohol in a 0.1 M supporting electrolytes was used for all experimentations. In detail, all experiments were performed in 0.1 M HClO M MeOH (0.5 M EtOH, 0.5 M HCOOH) and 0.1 M KOH M MeOH (0.5 M EtOH). 2.6 Cyclic voltammetry (CV) CV is an electrochemical technique, which measures the current that develops in an electrochemical cell under conditions where voltage is in excess of that predicted by the Nernst equation. CV is performed by cycling the potential of a working electrode and by measuring the resulting current. CVs for all experiments were scanned over a potential range of 0.03 V to 1.20 V. The scan rate was varied from 2 mv/s to 500 mv/s. The CVs were collected at certain rotation rates between 0 rpm and 1600 rpm. 31

50 2.7 Effect of composition & temperature Pt was deposited onto NpCu coated electrodes via a galvanic displacement reaction at a constant rotation rate at 500 rpm. Pt was deposited at 6 different deposition times on the coated NpCu electrodes. RDE was also rotated at 6 different times at a constant rotation rate. The 6 different deposition times are as follows: 0.25 min, 0.5 min, 1 min, 1.5 min, 2 min and 2.5 min. Data was collected for 5 different temperatures of the mixture (0.5 M alcohol M electrolyte) in the electrochemical cell. The 5 different temperatures are as follows: RT, 40 C, 50 C, 60 C and 80 C. The solution of 0.5 M alcohol M electrolyte was heated by the circular bath. The actual temperature of the electrochemical cell was measured by a clean thermometer. CVs were scanned over a potential range of 0.03 V to 1.20 V. Scan rates were 10 mv/s and 100 mv/s. Rotation rates were between 0 rpm and 1600 rpm. 32

51 CHAPTER 3 Fuel oxidation on porous PtCu/C catalyst 3.1 Background of methanol oxidation As an alternative fuel cell of PEMFC, DMFC is one of the most promising fuel cells in the future. Pt and Pt-based bimetallic catalysts are the most promising catalytic materials used for MOR. The main product of the overall MOR at the anode is carbon dioxide (CO2). And the halfreaction of MOR at the anode is below. 20 CH3OH(l) + H2O(l) CO2(g) + 6H + (aq) + 6e - E = VRHE The complete MOR mechanism involves several significant intermediate steps. 1) At E < 0.5 V (Figure 3.1), this region shows the initial reactions of MOR on Pt catalyst deposited electrode. The dissociative chemisorption of methanol reaction occurs in this region, and the major adsorbed species is Pt3COH. Pt2CHOH + 2Pt Pt3COH + PtH 33

52 2) Over the potential range 0.3 V < E < 0.9 V (Figure 3.1), adsorbed species react with OH to yield several different oxidation products. This reaction is called as the equilibrium reaction formation of OH at the surface. 3) Over the potential range 0.5 V < E < 1.1 V (Figure 3.1), PtCO2H, the major adsorbed product at low potential ranges, reacts further to create other compounds. PtCO2H + OH CO2 + Pt + H2O Carbon dioxide (CO2) is the desired product of MOR on Pt-based catalyst. 4) Over the backward process of the MOR in figure 3.1, the removal of OH on the Pt surface removes poisonous CO. This process basically shows CO removal step on Ptbased catalyst during MOR. 5) The total reaction of the MOR is, PtOH + Pt=C=O CO2 + 2Pt + H + + e - 34

53 Current density (A/cm Pt 2 ) I f I b E(V) Vs. RHE Figure 3.1 The CV of a MOR; both anodic and cathodic sweeps (forward and backward peaks). For the data analysis of CV of an AOR, there are three important metrics that determine the quality of PtCu/C bimetallic catalyst compared with pure Pt/C monometallic catalyst. First, we utilize the current density values in A/cmPt 2 (ma/ cmpt 2 ) at every point on the whole AOR CV. A higher current density value indicates higher quality of the PtCu/C catalyst. We also utilize the ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib), If/Ib, to evaluate the CO poisoning resistance of the catalysts. If the If/Ib ratio is higher (> 1), the PtCu/C catalyst has much higher CO tolerance during the 35

54 AOR. Lastly, we use onset potential value to evaluate the efficiency and power of the PtCu/C catalyst. If the onset potential value is lower, it means that the AOR is started earlier. Therefore, there is less overpotential during the reaction and has higher net cell voltage in DAFC. Higher net cell voltage gives higher performance and power on the DAFCs. The following data were analyzed based on these three important metrics. 36

55 Current density (ma/cm Pt 2 ) 3.2 Methanol oxidation on PtCu/C in acidic and alkaline conditions Electrolyte: 0.1 M HClO 4 Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 0.5 min I f I b 1.0 I b E(V) Vs. RHE I f PtCu/C Pt/C No Methanol Figure 3.2 The CV of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M MeOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm. Figure 3.2 shows the CV of a commercial Pt/C (blue line) and PtCu/C (red line) in 0.5 M methanol M perchloric acid (0.5 M CH3OH M HClO4). A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The Pt deposition time was 0.5 min. In 37

56 the figure, there is the blank CV, which was obtained with no methanol (black line). Based on the three metrics that determine the quality of Pt-based catalyst, there is a distinct difference between the CV of the commercial Pt/C and the PtCu/C. First, the current density at any point of the CV including the forward peak of a PtCu/C shows much higher value than the current density at any point of a commercial Pt/C. The higher current means that the PtCu/C has higher power density than the commercial Pt/C. Based on the current density values, we could prove that the rate of the MOR on the PtCu/C is much faster than the commercial Pt/C. Second, If/Ib values of the CV were used to evaluate the quality of the Pt-based catalysts and to compare the amount of CO poisoning on the PtCu/C and pure Pt/C during the intermediate steps of MOR. As shown in the CV, the If/Ib value of the PtCu/C is much higher than the If/Ib value of the commercial Pt/C. The value of If/Ib is higher than 1 on the PtCu/C (~1.4) and the value of If/Ib is lower than 1 on the commercial Pt/C (~0.7). Because If/Ib is higher than 1, we report that the PtCu/C has reasonably high CO poisoning tolerance. According to the current density values, If/Ib values of two different catalysts, we showed that the quality of the PtCu/C is better than the commercial Pt/C. Also, PtCu/C had less CO poisoning during the intermediate steps and this means that most methanol molecules were converted to CO2 molecules. 38

57 1.5 PtCu/C Pt/C 1.3 I f / I b ECSA (cm Pt2 ) Figure 3.3 Scatter plot of I f/i b Vs. ECSA (cm Pt2 ) for a commercial Pt/C (blue solid) and PtCu/C (red solid) in 0.5 M CH 3OH M HClO 4 solution. Figure 3.3 compares the If/Ib ratios (the current density of the forward peak / the current density of the backward peak) of a commercial Pt/C and PtCu/C and the ECSA (cmpt 2 ) of the Pt-based catalysts. The If/Ib ratio is used to determine the quality of Pt-based catalysts and to estimate how much CO adhered to the Pt catalyst during the MOR. As shown in figure 3.3, the ECSA values of the Pt on PtCu/C are relatively smaller than the ECSA values of the Pt on a commercial Pt/C. However, the If/Ib ratios of the PtCu/C are much higher than the If/Ib ratios of the commercial Pt/C. The If/Ib values of the PtCu/C are 39

58 between 0.9 and 1.5; the If/Ib values of the commercial Pt/C are between 0.5 and 0.8. Once the ECSA of the PtCu/C become closer to the ECSA of the Pt/C, the If/Ib values are decreased. When the ECSA gets larger, more CO poisoning occurs during the reaction, interrupting the formation of CO2. Based on the If/Ib values of the two different catalysts, the PtCu/C shows less CO poisoning and higher CO2 production than the pure Pt/C during the MOR. The high current density of the forward peak shows that most methanol molecules are completely converted to CO2 during the oxidation reaction and the fast rate of the MOR. 40

59 Current density (ma/cm Pt 2 ) Electrolyte: 0.1 M KOH Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 2 min I f I f PtCu/C E(V) Vs. RHE Figure 3.4 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M MeOH and 0.1 M KOH and PtCu/C in N 2 deareated 0.1 M KOH between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm. Figure 3.4 shows the anodic sweeps of the CV of a commercial Pt/C and PtCu/C in 0.5 M methanol M potassium hydroxide (0.5 M CH3OH M KOH). A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The Pt deposition time was 0.5 min. Figure 3.4 also shows a blank CV, which obtained in 0.1 M potassium hydroxide with no methanol (black line). 41

60 The alkaline electrolyte was used instead of the acidic electrolyte in Figure 3.4, there are a few reasons why alkaline was utilized as the electrolyte for the MOR. First, the alkaline electrolyte is less corrosive to the catalyst than the acidic electrolyte. Second, it s relatively cheaper than the acidic electrolyte. Finally, the oxidation of alcohol proceeds faster and has a lower onset potential in alkaline electrolyte than in acidic electrolyte. 61,62 The performance of the catalysts was evaluated based on the current density of the CV in Figure 3.4. There is a clear difference between the anodic sweep of CV of the commercial Pt/C and the PtCu/C. First, the current density at any point of CV including the forward peak of the PtCu/C shows a higher value than the current density at any point of the commercial Pt/C. Specifically, the current density of the forward peak (~2.7 ma/cmpt 2 ) on the PtCu/C is about four times higher than the current density of the forward peak (~0.6 ma/cmpt 2 ) on the pure Pt/C. The high current density values show that the rate of the MOR on the PtCu/C is much faster than the commercial Pt/C. According to the current density values, we proved that the quality of the PtCu/C is higher than the commercial Pt/C in the alkaline electrolyte. 42

61 Current density (ma/cm Pt 2 ) MeOH acid MeOH base 1.4 Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 1 min E(V) Vs. RHE Figure 3.5 The CV of a PtCu/C in N 2 deareated 0.5 M MeOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.5M MeOH and 0.1 M KOH between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm. Figure 3.5 shows the CV of a PtCu/C in 0.5 M methanol M perchloric acid (blue) and 0.5 M methanol M potassium hydroxide (red). A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The Pt deposition time was 1 min in both acidic and alkaline electrolyte. As mentioned before, the alkaline electrolyte has some advantages over acidic electrolyte for the AOR. The CV in acidic electrolyte and basic electrolyte could be compared 43

62 according to the three metrics that were used to compare the performance of the Pt-based metallic catalysts in the previous figures. First, the current density of the anodic sweep of the CV including the forward peak in alkaline electrolyte shows a higher value than the current density of the anodic sweep in acidic electrolyte. Specifically, the current density of the forward peak (~1.4 ma/cmpt 2 ) in basic electrolyte is about two times higher than the current density of the forward peak (~0.8 ma/cmpt 2 ) in acidic electrolyte. This shows that the rate of the MOR in alkaline electrolyte is much faster than in acidic electrolyte on the PtCu/C. In alkaline electrolyte, more charges are flowing during the reaction within the same time than in acidic electrolyte on the PtCu/C. Second, the If/Ib ratio was used to evaluate the MOR in both acidic and alkaline electrolyte on a PtCu/C. As shown in Figure 3.5, the calculated value of If/Ib is about 2.0 in alkaline electrolyte, while 1.1 in acidic electrolyte. According to the If/Ib ratios, there is less CO poisoning adhered to the Pt catalyst surface in alkaline electrolyte. Also, more CO2 and less of the intermediate products were formed during MOR in alkaline than in acidic electrolyte within the same time. As a result, we know that the PtCu/C has the higher performance and CO poisoning tolerance in alkaline electrolyte than in acidic electrolyte. Finally, we can clearly compare the onset potential values in two different electrolytes. The onset potential value is lower in alkaline electrolyte (0.5 V) than in acidic electrolyte (0.6 V). This means that there is less overpotential during the oxidation reaction in alkaline electrolyte compared to acidic electrolyte. Lower overpotential gives higher net cell voltage and MOR efficiency on the PtCu/C. Overall, based on the CV 44

63 Current density (ma/cm Pt 2 ) analysis, the alkaline electrolyte has relatively better results than the acidic electrolyte during the MOR on the porous PtCu/C. 3.3 Ethanol oxidation on PtCu/C in acidic and alkaline conditions 14.0 Electrolyte: 0.1 M HClO 4 Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 0.5 min I f 10.0 I f 6.0 PtCu/C Pt/C 2.0 No Ethanol -2.0 E(V) Vs. RHE Figure 3.6 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M EtOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm. 45

64 Figure 3.6 shows the CV of a commercial Pt/C and PtCu/C in 0.5 M ethanol M perchloric acid (0.5 M C2H5OH M HClO4). A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The Pt deposition time was 0.5 min. The blank CV, which was obtained with no ethanol is also shown in the figure (black line). The current density and the onset potential values were utilized to analyze the data and to decide the performance of the Pt-based bimetallic catalyst. Based on the current density of the anodic sweep, there is a clear difference between the CV of the commercial Pt/C and the PtCu/C. The current density at any point of the CV including the forward peak on the PtCu/C shows a higher value than the current density at any point of the commercial Pt/C. Specifically, the current density of the forward peak on the PtCu/C is about 14 ma/cmpt 2, while pure Pt/C is about 7 ma/cmpt 2. Based on the current density values, the PtCu/C is a better catalyst than the pure Pt/C for the EOR. The onset potential values also support proving the high performance of PtCu/C. As shown in Figure 3.6, the onset potential of the PtCu/C (~0.45 V) is lower than the onset potential of pure Pt/C (~0.5 V). This represents that the PtCu/C provides lower overpotential and higher net cell voltage than the pure Pt/C during the EOR. 46

65 Current density (ma/cm Pt 2 ) Electrolyte: 0.1 M KOH Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 2 min I f E(V) Vs. RHE I f PtCu/C Pt/C No ethanol Figure 3.7 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M EtOH and 0.1 M KOH and PtCu/C in N 2 deareated 0.1 M KOH between 0.3 V and 1.2 V at RT, 10 mv/s and 1600 rpm. Figure 3.7 shows the anodic sweeps of the CV of a commercial Pt/C and PtCu/C in 0.5 M ethanol M potassium hydroxide (0.5 M C2H5OH M KOH). A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The Pt deposition time was 2 min. The blank CV with no ethanol is also shown in the figure. The alkaline electrolyte was used instead of the acidic electrolyte for the EOR. The performance of the catalysts was evaluated based on the current density of CV and the 47

66 onset potential values. First, the current density at any point of CV including the forward peak of the PtCu/C shows a significantly higher value than the current density at any point of the commercial Pt/C. Specifically, the current density of the forward peak (~5.2 ma/cmpt 2 ) on the PtCu/C is about five times higher than the current density of the forward peak (~1 ma/cmpt 2 ) on the pure Pt/C. The onset potential of the PtCu/C (~0.3 V) is lower than the onset potential of the pure Pt/C (~0.45 V). The result shows that the PtCu/C has higher performance and better CO poisoning tolerance than the pure Pt/C in both MOR and EOR. 48

67 Current density (ma/cm Pt 2 ) EtOH acid EtOH base 1.8 Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 1 min I f 1.3 I f E(V) Vs. RHE Figure 3.8 The CV (anodic sweep only) of a PtCu/C in N 2 deareated 0.5 M EtOH and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.5M EtOH and 0.1 M KOH between 0.3 V and 1.2 V at RT, 10 mv/s and 1600 rpm. Figure 3.8 shows the CV of a PtCu/C in 0.5 M ethanol M perchloric acid and 0.5 M ethanol M potassium hydroxide. A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The Pt deposition time was 1 min in both acidic and alkaline electrolyte. The CV in acidic electrolyte and alkaline electrolyte could be compared according to the three metrics that were used to compare the performance of the Pt-based metallic catalyst before. In this work, the current density and the onset potential values were utilized to 49

68 compare the CV in acidic and alkaline electrolyte. First, the current density of the anodic sweep of the CV including the forward peak in alkaline electrolyte shows a higher value than the current density of the anodic sweep in acidic electrolyte. Specifically, the current density of the forward peak (~2.0 ma/cmpt 2 ) in alkaline electrolyte is about two times higher than the current density of the forward peak (~1.0 ma/cmpt 2 ) in acidic electrolyte. The onset potential value of the alkaline electrolyte is about 0.3 V; acidic electrolyte is about 0.4 V. The onset potential value is lower in alkaline electrolyte than in acidic electrolyte. Based on the result, the PtCu/C show better catalytic activity and less CO poisoning in alkaline electrolyte than in acidic electrolyte for the EOR. 50

69 Current density (ma/cm Pt 2 ) 3.4 Formic acid oxidation on PtCu/C in acidic condition Electrolyte: 0.1 M HClO 4 Rotation rate: 1600 rpm Sweep rate : 10 mv/s Pt deposition: 0.25 min I f I f PtCu/C Pt/C 0.1 No formic acid E(V) Vs. RHE Figure 3.9 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M formic acid and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.5 V and 1.2 V at RT, 10 mv/s and 1600 rpm. Figure 3.9 shows the anodic sweeps of the CV of a commercial Pt/C and PtCu/C in 0.5 M formic acid M perchloric acid (0.5 M C2H5OH M HClO4). A scan rate of 10 51

70 mv/s and the rotation rate of 1600 rpm were used. The Pt deposition time was 0.25 min. The blank CV was collected with no formic acid added in the figure. The CV of the FAOR was only collected in acidic electrolyte because there is no CO2 formation during the FAOR in alkaline electrolyte. Formate (CHOO - ), one of the intermediate products for the FAOR, forbids CO2 formation entirely in alkaline electrolyte on a PtCu/C. In the FAOR, there is an extra peak, which is shown on the anodic sweep compared to the MOR and EOR. Although the CV of the FAOR looked slightly different from the CV of the MOR and EOR, the CV was still analyzed in the same way as methanol and ethanol CV. The current density and the onset potential values were used to analyze the FAOR data and to determine the performance of the PtCu/C during the FAOR. Based on the current density of the anodic sweep, the current density of the forward peak of the PtCu/C is slightly higher than the current density of the commercial Pt/C. The current density of the forward peak on the PtCu/C is about 1.0 ma/cmpt 2 and the current density of the forward peak on the pure Pt/C is about 0.8 ma/cmpt 2. Even if there is only a small difference between the current density values of the two different Pt-based catalysts, it is clearly shown that the PtCu/C is better catalyst than the pure Pt/C for the FAOR. It is more difficult to compare the onset potential values of the PtCu/C and the Pt/C in FAOR than in MOR or EOR. However, we could estimate that the onset potential value on the PtCu/C is smaller than the one on the pure Pt/C. Even if we show that the PtCu/C is a better catalyst than the pure Pt/C, the FAOR shows really low If/Ib ratios, higher CO poisoning and lower performance on the Pt-based catalysts than the MOR and EOR. 52

71 Current density (ma/cm 2 ) Interestingly, the onset potential values of the FAOR are lower than the MOR and EOR. We can hypothesize that a different reaction occurs on the Pt-based catalysts for the FAOR compared to the MOR and EOR. 3.5 Comparing PtCu/C oxidation activity between methanol, ethanol and formic acid Electrolyte: 0.1 M HClO 4 Rotation rate: 1600 rpm Sweep rate: 10 mv/s Pt deposition: 1 min I f Methanol Ethanol Formic acid E(V) Vs. RHE Figure 3.10 The CV (anodic sweep only) of a PtCu/C in N 2 deareated 0.5 M MeOH (0.5 M EtOH, 0.5 M formic acid) and 0.1 M HClO 4 and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.3 V and 1.2 V at RT, 10 mv/s and 1600 rpm. 53

72 Figure 3.10 shows the forward peaks of the AOR on the PtCu/C including methanol, ethanol and formic acid. Each forward peak of the CV was collected in 0.5 M methanol, 0.5 M ethanol and 0.5 M formic acid at 1600 rpm and a sweep rate of 10 mv/s. The Pt deposition time was 0.5 min. All the CVs were collected in acidic electrolyte except for the blank CV, which was collected 0.1 M perchloric acid without any alcohol. All the CV of the alcohol oxidation shown in Figure 3.10 could be compared based on the current density values and the onset potential values. Three CV of the AOR show different current density values of the forward peak even though all experimental conditions, Pt deposition time, rotation rate, and sweep rate, were the same. The current density of the forward peak of ethanol (10.0 ma/cmpt 2 ) is the highest compared to other alcohols. This shows that the EOR reaction has a high power density and rate of the reaction. There is a high CO2 formation in the EOR on a PtCu/C. The high current density of the forward peak of the ethanol also means that the PtCu/C is very efficient and powerful for the EOR. The second highest current density of the forward peak (9.0 ma/cmpt 2 ) is for the MOR. The MOR also forms sufficient CO2 molecules and the rate of the reaction is relatively fast on a PtCu/C compared to formic acid. The FAR shows low current density of the forward peak (4.5 ma/cmpt 2 ). There is no sufficient CO2 formation during the FAOR and the rate of the reaction is relatively slow compared to other alcohol molecules. As shown in Figure 3.10, the onset potential value of ethanol is much closer to the equilibrium potential than the onset potential value of methanol. This shows that the EOR is started earlier than the MOR on a PtCu/C. The onset potential of the FAOR is closest 54

73 Current (ma) to the equilibrium potential even if the current density of the forward peak is lower than the other alcohols. 3.6 Degradation studies (stability test of the Pt-based catalysts) Electrolyte: 0.1 M HClO 4 Rotation rate: 1600 rpm Sweep rate: 10 mv/s PtCu/C Pt/C Time (hour) Figure 3.11 Degradation studies of a commercial Pt/C and PtCu/C in N 2 deareated 0.5 M CH 3OH and 0.1 M HClO 4 for 2 hour at RT. 55

74 With the AOR experiments, a stability test was performed on the Pt-based catalyst deposited on the electrodes. Figure 3.11 shows the chronoamperometry of the degradation on a commercial Pt/C and PtCu/C in 0.5 M methanol M perchloric acid. The chronoamperometry is an electrochemical technique in which the potential of the working electrode is held constant and the resulting current from faradic processes occurring at the electrode is monitored as a function of time. The potential of the forward peaks of the CV for the MOR on both pure Pt/C and PtCu/C were hold for 2 hours and the current drop was monitored by the chronoamperometry CV. The potential hold value for PtCu/C was V and for pure Pt/C V. These numbers were the potential values of the forward peaks chosen from the CV of the MOR on the PtCu/C and pure Pt/C. As shown in Figure 3.11, the current drop of the pure Pt/C is much faster than the PtCu/C. At the point of 1 hour of the degradation, the current of the pure Pt/C is very close to 0 ma, however, the current of the PtCu/C is about 0.5 ma. After 1.5 hour, there was no current on the pure Pt/C. Therefore, the stability of the PtCu/C is definitely higher than the stability of the pure Pt/C for the AOR. In the degradation studies of the Pt-based catalysts, we found the advantage of the bimetallic PtCu/C over pure monometallic Pt/C for the AOR in terms of the stability. 56

75 CHAPTER 4 Fuel oxidation at the anode as a function of Pt composition Pt was deposited onto NpCu coated electrodes via a galvanic displacement reaction at a constant rotation rate (500 rpm) to investigate the effect of Pt composition at 6 different Pt deposition times. The 6 different deposition times are as follows: 0.25 min, 0.5 min, 1 min, 1.5 min, 2 min and 2.5 min. 4.1 N2 deareated CV on PtCu/C and Pt/C in acidic and alkaline conditions Table 4.1 shows the calculated electrochemical surface area (ECSA) of the Pt-based catalysts (cmpt 2 ) between 0.03 V and 0.4 V with standard deviation in acidic and alkaline electrolyte based on the Pt composition at 6 different Pt deposition times. The ECSA of Pt was calculated from the charges associated with HUPD between 0.03 V and 0.4 V. The surface area of PtCu/C and Pt/C were calculated from HUPD assuming 210 μc cm 2 as the polycrystalline constant and a constant double layer current in the whole potential region of hydrogen desorption. 63 The amount of Pt (ECSA) is reported as cmpt 2. In general, Table 4.1 shows that as the Pt deposition time increases, the ECSA also increases on the PtCu/C. The ECSA of pure Pt/C is much higher than the ECSA of PtCu/C at any Pt 57

76 deposition time. When the Pt deposition time is 0.25 min, the ECSA of Pt is 0.74 ± 0.21 cmpt 2 in the acidic electrolyte, versus 0.90 ± 0.16 cmpt 2 in the alkaline electrolyte. At 2.5 min Pt deposition, the ECSA of Pt is 1.86 ± 0.17 cmpt 2 in the acidic electrolyte, while it is 1.39 ± 0.05 cmpt 2 in the alkaline electrolyte. The ECSA of Pt on a pure Pt/C is 6.57 ± 2.16 cmpt 2 in the acidic electrolyte, while it is 3.96 ± 0.09 cmpt 2 in the alkaline electrolyte. Table 4.1 The calculated electrochemical surface area (ECSA) of a platinum (cm Pt2 ) between 0.03 V and 0.4 V with standard deviation in acidic and alkaline electrolyte based on the Pt composition of a PtCu/C catalyst at varying Pt deposition times & Pt/C. Pt deposition time & Pt/C ECSA (cmpt 2 ) in acidic electrolyte ECSA (cmpt 2 ) in alkaline electrolyte 0.25 min Pt 0.74 ± ± min Pt 0.50 ± ± min Pt 0.89 ± ± min Pt 2.52 ± ± min Pt 1.41 ± ± min Pt 1.86 ± ± 0.05 Pt/C 6.57 ± ±

77 Current (ma) 0.15 Pt PtOx + xe Pt PtOx E (V) Vs. RHE 1 min Pt 2 min Pt Pt/C Figure 4.1 The CV of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 between 0.6 V and 1.2 V at RT based on 1 min and 2 min Pt deposition times on a PtCu/C electrode, respectively. Figure 4.1 shows the CV of the commercial Pt/C and bimetallic PtCu/C in 0.1 M perchloric acid under N2 at RT without alcohol. A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The CV of the 1 min and 2 min Pt deposited PtCu/C in the acidic electrolyte are shown in Figure 4.1. Previously, our ORR work has shown the potential values of the Pt oxidation peak lies at values greater than 0.8 V and the Pt oxide reduction peak is observed at values between 0.6 V and 0.8V. This indicates the oxophilicity of the PtCu/C. This, in turn correlates to the ORR activity of the catalyst. 11,13 We also have shown the corrected ORR activities (Ikinetic values) at 0.9 V are 59

78 relatively high on the Pt-based catalysts. We have reported higher ORR activities on the PtCu/C compared to the pure Pt/C. Figure 4.1 shows the potential values of the Pt oxidation peaks and Pt oxide reduction peaks on the PtCu/C in the acidic electrolyte. Figure 4.2 shows the potential values of the Pt oxide reduction peak at 6 different Pt deposition times on the PtCu/C and pure Pt/C. 60

79 Potential (V) min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Pt deposition time (min) Figure 4.2 The potential (V) values of the Pt oxide reduction peak of the CV (Figure 4.1) on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. In Figure 4.2, the peaks of the Pt oxide reduction peak are plotted as a function of the Pt composition on the PtCu/C in the acidic electrolyte. Figure 4.2 shows the potential values of the Pt oxide reduction peaks (Figure 4.1) at 6 different Pt deposition times on the PtCu/C and pure Pt/C. There is a trend of the potential values of the Pt oxide reduction peaks at 6 different Pt deposition times. The potential values of the Pt oxide reduction 61

80 peaks are constant: they are close to 0.82 V from 0.25 min Pt deposition to 1.5 min Pt deposition and decrease gradually from 1.5 min Pt deposition to 2.5 min Pt deposition in the acidic electrolyte. When the Pt deposition times are 2 min and 2.5 min, the potential values of the Pt oxide reduction peaks are about 0.80 V. The potential value of the Pt oxide reduction peak on a pure Pt/C is the lowest (about 0.77 V) indicating the most oxophilic surface is pure Pt/C. 62

81 Current (ma) 0.3 Pt PtOx + xe min Pt Pt PtOx 2 min Pt E (V) Vs. RHE Pt/C Figure 4.3 The CV of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH between 0.6 V and 1.2 V at RT based on the 1 min and 2 min Pt deposition times on a PtCu/C electrode, respectively. Figure 4.3 shows the CV of the commercial Pt/C and bimetallic PtCu/C in 0.1 M potassium hydroxide under N2 at RT without alcohols present. A scan rate of 10 mv/s and a rotation rate of 1600 rpm were used. The CV of the 1 min and 2 min Pt deposited PtCu/C in the alkaline electrolyte are shown in Figure 4.3. Figure 4.3 shows the potential values of the Pt oxidation peak (E > 0.8 V) and Pt oxide reduction peak (0.6 V < E < 0.8 V) on the PtCu/C in the alkaline electrolyte. Figure

82 Potential (V) and Figure 4.5 show the potential values of the Pt oxidation peaks and Pt oxide reduction peaks at 6 different Pt deposition times on the PtCu/C and pure Pt/C in the alkaline electrolyte min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Pt deposition time (min) Figure 4.4 The potential (V) values of the Pt oxidation peak of the CV (Figure 4.1) on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. 64

83 In Figure 4.4, the peaks of the Pt oxidation peak are plotted as a function of the Pt composition on the PtCu/C in the alkaline electrolyte. Figure 4.4 also shows the potential values of the Pt oxidation peaks (Figure 4.3) at 6 different Pt deposition times on the PtCu/C and pure Pt/C. The potential values of the Pt oxidation peak increases between 0.25 min and 0.5 min of Pt deposition and decreases between 0.5 min and 2 min in the alkaline electrolyte. The potential values of the Pt oxidation peaks at 0.25 min and 0.5 min are V and 0.88 V, respectively. The potential values of the Pt oxidation peak at 1 min, 1.5 min and 2 min decreases as follows: V, V and 0.85 V, respectively. The potential values of the Pt oxidation peaks at 2 min and 2.5 min on the PtCu/C and pure Pt/C are both roughly 0.85 V. 65

84 Potential (V) min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Pt deposition time (min) Figure 4.5 The potential (V) values of the Pt oxide reduction peak of the CV (Figure 4.1) on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. In Figure 4.5, the peaks of the Pt oxide reduction peak are plotted as a function of the Pt composition on the PtCu/C in the alkaline electrolyte. Figure 4.5 shows the potential values of the Pt oxide reduction peaks (Figure 4.3) at 6 different Pt deposition times on the PtCu/C and a pure Pt/C. There is a distinct trend in these values. The potential values of the Pt oxide reduction peaks are between 0.75 V and 0.80 V from 0.25 min Pt 66

85 deposition to 2.5 min Pt deposition and decrease on a pure Pt/C in the alkaline electrolyte. The potential values of the Pt oxide reduction peaks at 0.25 min, 0.5 min, 1 min, 1.5 min, 2 min and 2.5 min are 0.76 V, 0.80 V, 0.80 V, 0.77 V, 0.79 V and 0.77 V, respectively. The potential value of the Pt oxide reduction peak on a pure Pt/C is the lowest (about 0.70 V) again indicating the most oxophilic surface is pure Pt/C. 67

86 Current density (ma/cm Pt 2 ) 4.2 Methanol oxidation on PtCu/C in acidic and alkaline conditions min Pt 0.5 min Pt 1 min Pt 1.5 min Pt E (V) Vs. RHE 2 min Pt 2.5 min Pt Pt/C Figure 4.6 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. In this work, the experiment was performed to observe the effect of the Pt composition in the MOR on the PtCu/C in the acidic electrolyte. The figure shows the forward peaks of 68

87 the MOR on the PtCu/C. Each forward peak was collected in 0.5 M methanol, at 1600 rpm, and with a sweep rate of 10 mv/s. All the CVs were collected in 0.1 M perchloric acid. The Pt composition was changed from 0.25 min Pt deposition to 2.5 min Pt deposition. The current density of the forward peak at 2.5 min deposited Pt is the highest (~2.0 ma/cmpt 2 ); however, the current density of the forward peak at 0.25 min is less than 0.8 ma/cmpt 2. The current density of the forward peak on a pure Pt/C is the lowest (less than 0.7 ma/cmpt 2 ). The current densities of the forward peaks on a PtCu/C at any of the Pt deposition times are higher than the current density of the forward peak on a pure Pt/C in the MOR. 69

88 Current density (ma/cm Pt 2 ) E (V) Vs. RHE 0.25 min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Figure 4.7 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH and 0.5 M MeOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. Figure 4.7 shows the forward peaks of the MOR on the PtCu/C and Pt/C in the alkaline electrolyte. Each forward peak of the CV was collected in 0.5 M methanol, at 10 mv/s and 1600 rpm. All the CVs were collected in 0.1 M potassium hydroxide and the Pt deposition time was changed from 0.25 min to 2.5 min. The current density of the forward peak at 2 min deposited Pt is the highest (~3.3 ma/cmpt 2 ); however, the current 70

89 density of the forward peak at 0.25 min is about 0.8 ma/cmpt 2. The current density of the forward peak on a pure Pt/C is relatively high, which is about 3.0 ma/cmpt 2. Table 4.2 The I f/i b ratios and the corrected current densities of the forward peaks in 0.5 M MeOH and 0.1 M HClO 4 (0.1 M KOH) at 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. Pt deposition time & Pt/C If/Ib 1600 rpm, 10 mv/s Acidic medium (If-Ibase)/ECSA ( ma/cmpt 2 ) 1600 rpm, 10 mv/s Acidic medium If/Ib 1600 rpm, 10 mv/s Basic medium (If-Ibase)/ECSA (ma/cmpt 2 ) 1600 rpm, 10 mv/s Basic medium 0.25 min Pt min Pt min Pt min Pt min Pt min Pt Pt/C Table 4.2 shows the If/Ib ratios and the corrected current densities of the forward peaks on the PtCu/C and pure Pt/C catalysts at 6 different Pt deposition times during the MOR in the acidic and basic electrolyte. The If/Ib ratios and the current densities of the forward peaks were compared based on the 6 different Pt deposition times and a pure Pt in the acidic and basic electrolyte. During the MOR, the current density is relatively low at 71

90 short Pt deposition times, and the If/Ib ratios are higher in the acidic electrolyte. In general, as the Pt deposition time increases, the If/Ib ratio decreases and the current density increases in the acidic electrolyte. It is observed that there is less CO poisoning at relatively short Pt deposition times in the acidic electrolyte. When the Pt deposition time is 0.25 min, the If/Ib ratio in the acidic electrolyte is the highest, with a value of 1.56, and the current density is 0.75 ma/cmpt 2. At 2.5 min Pt deposition, the If/Ib ratio in the acidic electrolyte is 1.12, and the current density is 2.00 ma/cmpt 2. On a pure Pt/C, the If/Ib ratio in the acidic electrolyte is 0.69 and the current density is 0.56 ma/cmpt 2. This shows that there is a much lower current density in the Pt/C compared to the PtCu/C. In table 4.2, there is different trend between the basic electrolyte and the acidic electrolyte. As compared to the acidic electrolyte, it is observed that there is less CO poisoning at relatively long Pt deposition times in the basic electrolyte. When the Pt deposition time is 2 min, the If/Ib ratio in the basic electrolyte is the highest, where it is 1.96 and the current density is the highest, where it is 2.65 ma/cmpt 2. At 0.5 min Pt deposition, the If/Ib ratio in the basic electrolyte is the lowest, with a value of 0.96 and the current density is 0.39 ma/cmpt 2. On a pure Pt/C, the If/Ib ratio in the basic electrolyte is 2.22, which is even larger than the If/Ib ratios on a PtCu/C and the current density is 0.71 ma/cmpt 2. In the basic electrolyte, the pure Pt/C showed the relatively high If/Ib ratio compared to the PtCu/C. 72

91 Current density (ma/cm Pt 2 ) 4.3. Ethanol oxidation on PtCu/C in acidic and alkaline conditions E (V) Vs. RHE 0.25 min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Figure 4.8 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M EtOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. Figure 4.8 shows the forward peaks of the EOR on the PtCu/C and Pt/C in the acidic electrolyte. Each forward peak of the CV was collected in 0.5 M ethanol at 10 mv/s and 73

92 Current density (ma/cm Pt 2 ) 1600 rpm. All the CVs were collected in 0.1 M perchloric acid and the Pt deposition time was changed from 0.25 min to 2.5 min. The current density of the forward peak at 2.5 min deposited Pt is the highest (~1.4 ma/cmpt 2 ); however, the current density of the forward peak at 1.5 min is about 0.7 ma/cmpt 2. The current density of the forward peak on a pure Pt/C is about 0.6 ma/cmpt E (V) Vs. RHE 0.25 min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Figure 4.9 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M KOH and 0.5 M EtOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. 74

93 Figure 4.9 shows the forward peaks of the EOR on the PtCu/C and Pt/C in the basic electrolyte. Each forward peak of the CV was collected in 0.5 M ethanol at 10 mv/s and 1600 rpm. All the CVs were collected in 0.1 M potassium hydroxide and the Pt deposition time was changed from 0.25 min to 2.5 min. The current density of the forward peak at 2 min deposited Pt is the highest (~3.3 ma/cmpt 2 ); however, the current density of the forward peak at 0.25 min is about 0.7 ma/cmpt 2. The current density of the forward peak on a pure Pt/C is relatively high which is about 2.8 ma/cmpt 2. Table 4.3 The I f/i b ratios and the corrected current densities of the forward peaks in 0.5 M EtOH and 0.1 M HClO 4 (0.1 M KOH) at 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. Pt deposition time & Pt/C If/Ib 1600 rpm, 10 mv/s Acidic medium (If-Ibase)/ECSA (ma/cm 2 ) 1600 rpm, 10 mv/s Acidic medium If/Ib 1600 rpm, 10 mv/s Basic medium (If-Ibase)/ECSA (ma/cm 2 ) 1600 rpm, 10 mv/s Basic medium 0.25 min Pt min Pt min Pt min Pt min Pt min Pt Pt/C

94 Table 4.3 shows the If/Ib ratios and the corrected current densities of the forward peaks on the PtCu/C and pure Pt/C at 6 different Pt deposition times during the EOR in the acidic and basic electrolyte. When the Pt deposition time is 0.25 min, the If/Ib ratio in the acidic electrolyte is the highest except a pure Pt/C, where it is 0.86 and the current density is 1.38 ma/cmpt 2. At 2 min Pt deposition, the If/Ib ratio in the acidic electrolyte is the lowest, where it is 0.47 and has a current density of 1.43 ma/cmpt 2. On a pure Pt/C, the If/Ib ratio in the acidic electrolyte is 0.99 and the current density is 0.68 ma/cmpt 2, which shows little higher If/Ib ratio compared to a PtCu/C. In table 4.2, when the Pt deposition time is 2 min, the If/Ib ratio in the basic electrolyte is the highest, which is 1.01 and the current density is the highest, where it is 5.22 ma/cmpt 2. At 0.25 min Pt deposition, the If/Ib ratio in the basic electrolyte is the lowest, where it is 0.48 and has a current density of 0.19 ma/cmpt 2. On a pure Pt/C, the If/Ib ratio in the basic electrolyte is 1.23, which is even larger than the If/Ib ratios on a PtCu/C and the current density is 1.21 ma/cmpt 2. In the basic electrolyte, the pure Pt/C showed a higher If/Ib ratio compared to the PtCu/C. 76

95 Current density (ma/cm Pt 2 ) 4.4 Formic acid oxidation on PtCu/C in acidic condition E (V) Vs. RHE 0.25 min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Figure 4.10 The CV (anodic sweep only) of a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M HCOOH between 0.2 V and 1.2 V at RT, 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. Figure 4.10 shows the forward peaks of the FAOR on the PtCu/C and Pt/C in the acidic electrolyte. Each forward peak of the CV was collected in 0.5 M formic acid at 10 mv/s 77

96 and 1600 rpm. All the CVs were collected in 0.1 M perchloric acid and the Pt deposition time was changed from 0.25 min to 2.5 min. The current density of the forward peak at 2 min deposited Pt is the highest (~1.3 ma/cmpt 2 ); however, the current density of the forward peak at 1 min is less than 0.5 ma/cmpt 2. The current density of the forward peak on a pure Pt/C is about 0.75 ma/cmpt 2. The current densities of the forward peaks in the FAOR are much lower than the current densities of the forward peaks in the MOR and EOR on both pure Pt/C and PtCu/C. Table 4.4 The I f/i b ratios and the corrected current densities of the forward peaks in 0.5 M HCOOH and 0.1 M HClO 4 at 10 mv/s and 1600 rpm based on varying Pt deposition times on a PtCu/C electrode. Pt deposition time & Pt/C If/Ib 1600 rpm, 10 mv/s (If-Ibase)/ECSA (ma/cmpt 2 ) 1600 rpm, 10 mv/s 0.25 min Pt min Pt min Pt min Pt min Pt min Pt Pt/C

97 Table 4.4 shows the If/Ib ratios and the corrected current densities of the forward peaks on the PtCu/C and pure Pt/C catalysts at 6 different Pt deposition times during the FAOR in the acidic electrolyte. The If/Ib ratios and the current densities of the forward peaks were compared based on the 6 different Pt deposition times and a pure Pt in the acidic electrolyte. When the Pt deposition time is 2 min, the If/Ib ratio in the acidic electrolyte is the highest, with a value of 0.95 and the current density is 1.22 ma/cmpt 2. At 0.5 min Pt deposition, the If/Ib ratio in the acidic electrolyte is the lowest, with a value of 0.59 and the current density is 0.74 ma/cmpt 2. On a pure Pt/C, the If/Ib ratio in the acidic electrolyte is 0.24 and the current density is 0.79 ma/cmpt 2. The If/Ib ratios and current densities at 6 different Pt deposition times and a pure Pt/C in the FAOR are relatively lower than the MOR and EOR. 79

98 Current density of forward peak (ma/cm Pt 2 ) 4.5 Alcohols oxidation on PtCu/C in acidic and alkaline conditions M MeOH M HClO4 0.5 M EtOH M HClO4 0.5 M MeOH M KOH 0.5 M EtOH M KOH min Pt 0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Pt/C Figure 4.11 The corrected current densities of the forward peaks on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH (0.5 M EtOH) and in N 2 deareated 0.1 M KOH and 0.5 M MeOH (0.5 M EtOH) at 10 mv/s and 1600 rpm based on the Pt composition of a PtCu/C electrode. Figure 4.11 shows the current densities of the forward peaks on the PtCu/C at 6 different Pt deposition times and pure Pt/C during the MOR and EOR in both acidic and basic 80

99 electrolyte. In the figure, the current densities of the MOR and EOR in acidic and basic electrolyte at any Pt deposition time on a PtCu/C are much higher than the current densities on a pure Pt/C. Based on the current densities of the forward peaks, the figure clearly indicates that the overall performance of a PtCu/C is better than a Pt/C for the MOR and EOR in both acidic and basic electrolytes. During the MOR and EOR in the acidic electrolyte, there is no distinguishable trend between the current densities of the forward peaks at 6 different Pt deposition times. However, the current densities of the MOR and EOR in acidic electrolyte are high at 0.5 min Pt and 2.5 min Pt. When the Pt deposition time is 0.5 min in the acidic electrolyte (0.1 M HClO4), the current densities of the forward peaks for the MOR and EOR are about 1.8 ma/cm 2 and 1.6 ma/cmpt 2, respectively. When the Pt deposition time is 2.5 min in the acidic electrolyte (0.1 M HClO4), the current densities of the forward peaks for the MOR and EOR are about 2.2 ma/cmpt 2 and 1.8 ma/cmpt 2, respectively. However, on a pure Pt/C in the acidic electrolyte (0.1 M HClO4), the current densities of the forward peaks for the MOR and EOR are about 0.5 ma/cmpt 2 and 0.4 ma/cmpt 2, respectively. Overall, the performance of the PtCu/C and the rate of the AOR in the acidic electrolyte are optimized at 0.5 min Pt and 2.5 min Pt deposition based on the reported current densities of the forward peaks. During the MOR and EOR in the basic electrolyte, there is a clear trend between the current densities of the forward peaks at 6 different Pt deposition times. The current densities of the forward peaks increase gradually from 0.25 min Pt deposition to 2 min Pt deposition and decrease from 2 min Pt deposition to 2.5 min Pt deposition for the MOR 81

100 and EOR in the basic electrolyte. In the basic electrolyte, 2 min Pt deposition is the optimized Pt composition on a PtCu/C for the MOR and EOR. When the Pt deposition time is 2 min in the basic electrolyte (0.1 M KOH), the current densities of the forward peaks for the MOR and EOR are about 2.7 ma/cmpt 2 and 5.3 ma/cmpt 2, respectively. However, on a pure Pt/C in the basic electrolyte (0.1 M KOH), the current densities of the forward peaks for the MOR and EOR are about 0.5 ma/cmpt 2 and 1.0 ma/cmpt 2, respectively. The activity of the PtCu/C and the rate of the AOR in the basic electrolyte are overall optimized at 2 min Pt deposition. The current densities of the forward peaks on a 2 min deposited PtCu/C are about 5 times higher than the current densities of the forward peaks on a pure Pt/C for the MOR and EOR in the basic electrolyte. Overall, from Figure 4.11, the bimetallic PtCu/C is a more efficient catalyst than the pure Pt/C for the electro-oxidation of alcohols in both acidic and basic conditions. 82

101 If/Ib M MeOH M HClO4 0.5 M EtOH M HClO4 0.5 M MeOH M KOH 0.5 M EtOH M KOH min Pt0.5 min Pt 1 min Pt 1.5 min Pt 2 min Pt 2.5 min Pt Figure 4.12 The I f/i b ratios on a commercial Pt/C and PtCu/C in N 2 deareated 0.1 M HClO 4 and 0.5 M MeOH (0.5 M EtOH) and in N 2 deareated 0.1 M KOH and 0.5 M MeOH (0.5 M EtOH) at 10 mv/s and 1600 rpm based on the Pt composition on a PtCu/C electrode. Figure 4.12 shows the If/Ib ratios on the PtCu/C at 6 different Pt deposition times during the MOR and EOR in both acidic and basic electrolyte. During the MOR and EOR in the acidic electrolyte, there is no distinguishable trend of the If/Ib ratios at 6 different Pt deposition times. However, the If/Ib ratios of the MOR and EOR in the acidic electrolyte are high at 0.25 min Pt and 0.5 min Pt deposition. When the Pt deposition time is