Lecture 12: Electroanalytical Chemistry (I) 1
Electrochemistry Electrochemical processes are oxidation-reduction reactions in which: Chemical energy of a spontaneous reaction is converted to electricity or Electrical energy is used to drive a nonspontaneous reaction 2
Electrochemistry is everywhere Energy conversion and storage E-chem sensors Electrolysis Corrosion Materials research Paul A Garris, Nature Methods, 2010 3
An electrochemistry cell Electrodes Working, reference, counter Two electrodes? Solution Solvent supporting electrolyte Redox species Three electrode cell: Working (WE), reference (RE), counter (CE) WE: reaction of interest studied on WE RE: provides a stable reference point for the potential on WE CE: provides a path for most of the current passes through the circuit 4
Electrochemical reactions e e Reactions are on surfaces Electrons as special reagent working electrode K + Cl - Fe(CN) 6 3- reference electrode Electrostatic potential can be continuously varied Reaction kinetics can be easily monitored Electrodes may participate in the reaction 5
Electrode potential and electrochemical reactions Electrostatic potential on the electrode generates & changes the reactions at the electrode/solution interface Co 3+ + e Co 2+ +1.81 Au + + e Au +1.69 Ce 4+ + e Ce 3+ +1.61 AgCl + e Ag + Cl +0.22 Sn 4+ + 2e Sn 2+ +0.15 6
Electrode/solution interface 7
Electrochemical reaction rate Current as an expression of the reaction rate: Faraday s Law: Q = nfn i dq dt nf dn i nf dt dn i dt nf dn dt -0.5 voltage 1.2 V rate( mol s 1 cm 2 ) i nfa 8
Faradaic & nonfaradaic processes, double layer Faradaic process involves electron transfer at the electrode/solution interface ET Ox + - + - Red + - + - Metal IHP OHP f 1 f 2 + + + Double layer charging is a nonfaradaic process which only involves movement of ions in and out of the double layer ET Ox + - + - Red + - + - Metal f M f S f 2 s i s d s S = s i + s d = -s M x
The thickness of electric double layer is dependent on salt concentration Electric double layer f Metal 3k 1 k 1 Debye length metal f(x) f solution solution At room temperature, in H 2 O For a 1:1 electrolyte, e.g., KCl: 3k -1 ~ 10/C 1/2 Å. C molar concentration. C / M 1 3k 1 1 nm 10-1 3 nm 10-2 10 nm 10-4 0.1 mm 10-6 1mm Normal region for Echem investigations 10
Double layer charging current Faradaic Process E i Double layer charging t E 11
Pathway of a general electrode process Electrode surface region Bulk solution Electrode ET O surf O bulk R surf R bulk 12
Mass transport Modes of mass transfer: Diffusion, Migration, Convection Nernst-Planck equation: J Diffusion coefficient, D Stokes Einstein Relation i Ci ( x) zif f( x) ( x) Di DC i i Civ( x) x RT x D kt 6 r Ions: Cl -, Ru(NH 3 ) 6 3+, 0.5 10-5 0.5 nm molecule, M? 10 nm protein, P? 100 nm nanoparticle? 13
Standard hydrogen electrode The convention is to select a particular electrode and assign its standard reduction potential the value of 0.0000V. This electrode is the Standard Hydrogen Electrode. 2H + (aq) + 2e H 2 (g) H 2 Pt The standard aspect to this cell is that the activity of H 2 (g) and that of H + (aq) are both 1. This means that the pressure of H 2 is 1 atm and the concentration of H + is 1M, given that these are our standard reference states. H + 14
Standard potential tables All of the equilibrium electrochemical data is cast in Standard Reduction Potential tables. F 2 + 2e 2F +2.87 Co 3+ + e Co 2+ +1.81 Au + + e Au +1.69 Ce 4+ + e Ce 3+ +1.61 Br 2 + 2e 2Br +1.09 Ag + + e Ag +0.80 Cu 2+ + 2e Cu +0.34 AgCl + e Ag + Cl +0.22 Sn 4+ + 2e Sn 2+ +0.15 2H + + 2e H 2 0.0000 Pb 2+ + 2e Pb -0.13 Sn 2+ + 2e Sn -0.14 In 3+ + 3e In -0.34 Fe 2+ + 2e Fe -0.44 Zn 2+ + 2e Zn -0.76 V 2+ + 2e V -1.19 Cs + + e Cs -2.92 Li + + e Li -3.05 15
Lecture 13: Electroanalytical Chemistry II 16
Electrochemical reaction rate How can one use current as an expression of the reaction rate? Faraday s Law: Q = nfn dq dt i nf dn nf dt dn dt dn dt An electrochemical reaction is a heterogeneous process, so we should consider the area of the electrode i nf rate( mol s 1 cm 2 ) i nfa 17
i / ma Cyclic voltammetry Peak current: pa, na, µa, ma Peak potential: V or mv vs REF Peak-peak separation: (60/n) mv for a reversible reaction Scan rate: mv/s 20 E p,a 10 i p,a E E HL 0 i p,c E initial t E final -10 potential waveform -20 E p,c -0.2-0.1 0 0.1 0.2 E E o / V 18
Concentration of redox at the electrode surface and the current response The Nernst equation: C t 1 t 2 5 t 6 E Red Ox ne Re d E o Red RT nf i a ln( a Ox Red ) t 3 t 5 t6 Ox Red t 3 t 4 x t 2 t 1 E 19
Peak current is proportional to ν 1/2! E p i E 1/2 E p/2 Diffusion controlled process! i p i p 3/ 2 1/ 2 * 1/ 2 ( 2.69 10 5 ) n AD O C O E E p RT E1 / 2 1.109 1/ 2 mv nf ( E 28.5/ n ( ) i p 20
Charging current in cyclic voltammetry Charging current is proportional to the ν! ic C DL 21
Ultramicroelectrodes (UMEs) r < 25 mm, ultramicroelectrode r < 100 nm, nanoelectrode 25 mm r 50 nm 6-nm Pt disk nanoelectrode 22
UME in neurochemistry -2e - -2H + Wightman group, UNC Oxidation of dopamine Carbon-fiber microelectrodes Venton lab, U Virginia Sombers lab, NCSU 23
Preparation of UMEs a) Sealing a microwire in glass b) Pulling a Pt microwire with glass Pt microwire 24
UME and exocytosis Soma Dendrite Axon Neurotransmitters Secretory vesicles Synapse G.D. Fischbach, Sci. Amer. 1992, 267, 48-57. Neuronal Communication Ca 2+ Plasma membrane Extracellular Fluid Fusion Pore Exocytosis is a Vesicle Fusion Process 25
UME and exocytosis Carbon Microelectrode Amperometry: Constant applied potential Measures current as a function of time High temporal resolution Quantification of released molecules Cell Single-cell recording -2e - -2H + Oxidation of dopamine N Q nf Faraday s Law 26