Generation of Hydrogen Peroxide In ORR Over Low Loadings of Pt/C Catalysts

Transcription

1 Generation of Hydrogen Peroxide In ORR Over Low Loadings of Pt/C Catalysts Raja Swaidan The Cooper Union Advisor: Dr. Branko N. Popov Electrochemical Engineering 26 July 2007

2 Overview of Research Studied the phenomena that lower loadings of Pt/C electrocatalysts demonstrate preference for 2e - reduction routes to H 2 O 2 Encounter a struggle between physical and electrochemical factors Fundamental question: Which plays the dominant role? 2 Studied the selectivity of Pt catalysts supported on different types of porous carbons as a function of Pt loading on the Rotating Ring Disk Electrode (RRDE) Carbon supports included: Vulcan Carbon, Etched Vulcan Carbon, Ketjen Black, USC MFC, USC CC XRD, TEM for carbon surface and Pt dispersion analysis as well as Pt particle sizes B.E.T. Surface Area and distribution of mesoporous and microporous areas Comparative study of all Pt/C catalysts at one loading of Pt (µg Pt cm -2 ) to determine the effects of support properties like surface area and porosity on selectivity

3 Oxygen Reduction Reaction (ORR) 3 Oxidation of H 2 gas produces protons and electrons that transfer to cathode for ORR Cathode ORR O 2 4H + + 4e - 2H 2 O 2e - H 2 O 2 2e - H 2 O H 2 O 2 Int. Flow into Electrolyte Adsorb on catalyst Primary PEMFC Failure Mechanisms Aggravated by H 2 O 2 Membrane thinning Reactant crossover Internal shorting

4 RRDE Technique 4 Pt Ring, 0.46V vs MMS [Detection of H 2 O 2 intermediates that escape a series 2e - reduction to H 2 O] Teflon [Electrical separation between Disk and Ring allowing separate voltages on each] CATALYST COAT Glassy Carbon Disk [ORR] Electrolyte flow over the catalyst layer is increased with increasing RPM of electrode. Convection transports H 2 O 2 intermediates to the ring for detection and regulates catalyst s O 2 supply.

5 Low Pt/C Loading Phenomena 5 E-TEK 19.6% Pt/C Selectivity 900 RPM, 0.5M H 2 SO 4, 25 o C, 5 mv s -1 Selectivity, Vulcan Carbon 900 RPM ug/cm^2 Generated % H 2O Disk Potential [V] Generated % H 2O ug/cm^2 4 ugcm-2 8 ugcm-2 16 ugcm-2 20 ugcm-2 60ugcm Disk Potential [V] Further testing reveals that E-TEK 60 µg Pt cm -2 (240 µg VC cm -2 ) loading yields the lowest peroxide. The impression is that Pt is causing this phenomena. On the contrary, as shown here, it is the support which is the guiding factor.

6 Low Pt/C Loading Phenomena 6 20 wt.% Pt/KB Selectivity 900 RPM, 0.5 M H 2 SO 4, 25 o C, 5 mv s -1 Selectivity, KB 900 RPM 7 50 Generated %H 2O Generated % H 2O ugcm ugcm Disk Potential [V] ugcm-2 8 ugcm-2 12 ugcm-2 20 ugcm-2 Disk Potential [V] Further testing reveals that the Pt/KB 20 µg Pt cm -2 (80 µg KB cm -2 ) loading yields the lowest peroxide. Moreover, at any given loading, the catalyst with the support having larger B.E.T. surface area (KB) produces lower H 2 O 2. This catalyst also reaches a minimum in H 2 O 2 production at a lower Pt loading.

7 Ignore absolute values of ECSA (different Pt deposition methods and support modifications) Platinum Utilization Concept of A Saturation Point 7 ECSA [m 2 /g Pt ] Electrochemically Active Pt Surface Area [ECSA] Note: Electrode Geometrical Surface Area is 0.5 cm µg 50 VC cm µg 20 KB cm Pt Loading onto Working Electrode [µg Pt cm -2 ] E-TEK 20 wt.% Pt 20 wt.% Pt/KB

8 A Model Effect of Agglomeration of Pt/C Catalyst on Hydrogen Peroxide Formation M. Inaba et al. 8 Legend: = C = Pt = H + = e - = O 2 = H 2 O = H 2 O 2 O 2 Gas Feed H + from Anode 2 e - released 2 more e - released O 2 reduces to H 2 O 2 intermediate upon contact with active Pt/C particle H 2 O 2 is likely to encounter nearby Pt/C particles in the agglomerates and reduce to H 2 O before being swept to the ring for oxidation and detection

9 S.E.M. Images of Catalyst Layer E-TEK (Low B.E.T. Surface Area Support) 4 ugptcm-2 20 ugptcm-2 Agg. ~ 20 µm Agg. ~ 60 µm 60 ugptcm-2 Agg. ~ 130 µm 100 ugptcm-2 Agg. ~ 130 µm Pt/KB (High B.E.T. Surface Area Support) 4 ugptcm-2 Agg. ~ 40 µm 12 ugptcm-2 Agg. ~ 100 µm 20 ugptcm-2 Agg. ~ 130 µm 40 ugptcm-2 Agg. ~ 130 µm 9

10 10 Carbon Support VS Physical Concept A link between the two is clear, but which is the dominant player?

11 The Carbon Support 11 Agglomerate Size, Peroxide, and Real Pt Surface Area Generated % H 2O Agglomerate Size 60 µm µm µm 0.0 Note: Electrode geometrical area is 0.5 cm 2 Note: Numbers in legend have units µg Pt cm -2 ETEK Pt/C, 20 Pt/KB, 20 ETEK Pt/C, Disk Potential [V] The link pivots upon properties of the carbon support

12 ECSA Saturation Point Max Conclusions Low Loading Phenomena The Carbon Support, Not Pt Pt C Interactions Pt Particle Dispersion on Carbon B.E.T. Surface Area Mesoporous, Microporous Area Distributions Pt/C Agglomerate Size Max H 2 O 2 Formation Min Carbon Support is steering, or causing, the physical phenomena Increasing particle agglomeration parallels decreasing H 2 O 2 levels At the S.A. saturation point, particle agglomeration reaches maximum and H 2 O 2 level reaches a minimum Future Work 12 Compare selectivity of each of the aforementioned carbon supports at a given loading, holding Pt and carbon amounts on electrode constant. Highlight which support properties are impacting the selectivity of the catalyst

13 Acknowledgments 13 Many thanks to Dr. Gang Wu Dr. Prabhu Ganesan Dr. Branko N. Popov Members of the Fuel Cell Division My Family for the intellectual enlightenment and encouragement that allowed me to make the best of my first research experience. The NSF-REU fund is greatly appreciated.