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Physical Electrochemistry & Electrocatalysis Physical Electrochemistry & Electrocatalysis Viktoriia Saveleva Electrochemical Kinetics . Methods to Measure Electrode Reactions Cyclic Voltammetry 4/26/2019 Physical Electrochemistry & Electrocatalysis 2 Cyclic Voltammetry Recording of current density when the electrode potential is linearly changed between two potential limits with defined sweep rate Linear Sweep Voltammetry (single sweep from E1 to E2) Cyclic Voltammetry (cyclic sweep between E1 to E2 and back to E1) E1: negative potential limit E2: positive potential limit dE n: sweep rate (in V/s) n dt During potential scan diffusion processes (=transport of electroactive species to electrode surface) and faradaic charge transfer processes at the electrode surface are occuring, resulting in a defined current peak 4/26/2019 Physical Electrochemistry & Electrocatalysis 3 Cyclic Voltammetry 0.6 E 0.4 pos 0.2 0.0 E [V] -0.2 -0.4 Eneg -0.6 0 20 40 60 80 time [s] 4/26/2019 Physical Electrochemistry & Electrocatalysis 4 Cyclic Voltammetry A A e Diffusion effect starts to dominate 0.08 Start of noticeable 0.06 effect of diffusion as jd falls with time 0.04 Rate controlled by jd j [mA/cm²] 0.02Exponential increase with potential 0.00 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 E [V] 4/26/2019 Physical Electrochemistry & Electrocatalysis 5 Cyclic Voltammetry A A e 0.08 0.06 0.04 j [mA/cm²] 0.02 A A A cx0 0 cx0 cbulk 0.00 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 E [V] 4/26/2019 Physical Electrochemistry & Electrocatalysis 6 Diffusion Equation A A e Basis: Linear Diffusion, no migration, no convection Single potential sweep c A 2c A D t A x 2 c A 2c A D t A x 2 c A c A Boundary conditions j nFDA nFDA t x0 t x0 4/26/2019 Physical Electrochemistry & Electrocatalysis 7 Diffusion Equation A A e Basis: Linear Diffusion, no migration, no convection Single potential sweep Boundary conditions If no A+ in solution present A A t 0 x 0 c cbulk A A t 0 x c cbulk t 0 x 0 c A 0 t 0 x c A 0 4/26/2019 Physical Electrochemistry & Electrocatalysis 8 Diffusion Equation, CV A A e Case 1: reversible reaction, very fast electron transfer single potential sweep LSV 2.3RT c A Nernst Behavior: E E 0 log x0 nF A cx0 E E1 nt dE n 2.3RT c A dt E E 0 log x0 E nt nF A 1 cx0 c A nFE nt E 0 t 0 x 0 x0 exp 1 A RT cx0 4/26/2019 Physical Electrochemistry & Electrocatalysis 9 Reversible vs Irreversible A A e Reversible voltammetry was seen for “fast” electrode kinetics, irreversible for “slow”. What are they fast or slow related to? to rate of mass transport to the electrode 풌ퟎ measures the rate of electron transfer kinetics 푫 풎 = measures the rate of mass transport 푻 휹 푹푻 푫 휹~ 푫풕 풕~ 풎 ~ 푭υ 푻 푹푻/푭υ Reversible Quasi-reversible Irreversible ퟎ ퟎ 풌 ≫ 풎푻 풌 ≪ 풎푻 풌ퟎ Λ = Λ ≥ ퟏퟓ 15 > Λ > ퟏퟎ−ퟑ Λ ≤ ퟏퟎ−ퟑ 푭푫υ ퟏ/ퟐ 푹푻 @ RT 4/26/2019 Physical Electrochemistry & Electrocatalysis 10 Diffusion Equation, CV A A e Case 1: reversible reaction, very fast electron transfer Λ ≥ ퟏퟓ 풌ퟎ ≥ ퟎ. ퟑυퟏ/ퟐ single potential sweep LSV 1/ 2 nF 1/ 2 A 1/ 2 Function j nF DA cbulk PnE E1/ 2 RT 3/ 2 1/ 2 A 1/ 2 j peak 0.4463n DA cbulk Randles-Sevcik Equation 4/26/2019 Physical Electrochemistry & Electrocatalysis 11 Diffusion Equation, CV A A e Case 1: reversible reaction, very fast electron transfer ퟎ ퟏ/ퟐ single potential sweep LSV Λ ≥ ퟏퟓ 풌 ≥ ퟎ. ퟑυ =P At peak maximum 4/26/2019 Physical Electrochemistry & Electrocatalysis 12 Diffusion Equation, CV A A e Case 1: reversible reaction, very fast electron transfer Λ ≥ ퟏퟓ 풌ퟎ ≥ ퟎ. ퟑυퟏ/ퟐ single potential sweep LSV 1/ 2 0 2.3RT DA Half-wave Potential E1/ 2 E log nF DA RT E E 1.1 Peak Potential peak 1/ 2 nF 4/26/2019 Physical Electrochemistry & Electrocatalysis 13 Cyclic Voltammetry A A e Λ ≥ ퟏퟓ 풌ퟎ ≥ ퟎ. ퟑυퟏ/ퟐ 0.08 E E peak 1/2 0.08 0.06 0.07 0.06 0.05 0.04 0.04 [mA/cm²] P 0.03 100 mV/s j j [mA/cm²] 0.02 3/ 2 1/ 2 A 1/ 2 j 0.4463n0.02 D c 0.01 peak A bulk 0.00 10 mV/s 0 5 10 n0.5 0.00 1mV/s -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 E [V] 4/26/2019 Physical Electrochemistry & Electrocatalysis 14 Diffusion Equation, CV A A e Case 2: reversible reaction, Cyclic Potential Sweep Λ ≥ ퟏퟓ 풌ퟎ ≥ ퟎ. ퟑυퟏ/ퟐ 0.09 0.08 Epeak 0.07 E1/2 0.06 0.05 0.04 0.03 100 mV/s 0.02 0.01 10 mV/s 0.00 1mV/s -0.01 j [mA/cm²] -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 -0.5 0.0 0.5 E [V] 4/26/2019 Physical Electrochemistry & Electrocatalysis 15 Diffusion Equation, CV A A e Case 2: reversible reaction, Cyclic Potential Sweep Λ ≥ ퟏퟓ 풌ퟎ ≥ ퟎ. ퟑυퟏ/ퟐ 0.09 0.08 Epeak 0.07 E1/2 0.06 0.05 0.04 0.03 100 mV/s 0.02 0.01 0.00 -0.01 j [mA/cm²] -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 57mV -0.5 0.0 0.5 E [V] 4/26/2019 Physical Electrochemistry & Electrocatalysis 16 Diffusion Equation, CV A A e Case 3: irreversible reaction, single potential sweep LSV Λ ≤ ퟏퟎ−ퟑ 풌ퟎ ≤ ퟐ ∙ ퟏퟎ−ퟓυퟏ/ퟐ 1/ 2 1/ 2 0 RT D nF E peak E 0.780 ln ln Peak Potential nF k0 RT nF 0 j peak 0.227nFc Ak0 exp EPeak E Peak current RT F 3 / 2 1/ 2 A 1/ 2 j peak 0.496 n DA cbulk RT 4/26/2019 Physical Electrochemistry & Electrocatalysis 17 Cyclic Voltammetry A A e 0.06 E peak E1/2 0.36 0.35 0.05 0.34 Slope = 1.15RT/nF 0.33 0.04 0.32 0.31 100 mV/s 0.30 0.03 0.29 [V] P 0.28 E 0.27 j [mA/cm²] 0.02 0.26 10 mV/s 0.25 0.01 0.24 0.23 1mV/s 0.22 0.00 1 10 F 3 / 2100 1/ 2 A 1/ 2 j 0.496 n n D c peak RT A bulk -0.5 0.0 0.5 E [V] 4/26/2019 Physical Electrochemistry & Electrocatalysis 18 face-centered cubic structure (fcc) of Pt: 1 Pt atom at each cube corner & face center (100) surface (“number 5”): 1.28·1015 atoms/cm2 = 2.13·10-9 mol/cm2 = 205mC/cm2 (110) surface (“number 6”): 0.92·1015 atoms/cm2 = 1.53·10-9 mol/cm2 = 147mC/cm2 (111) surface (“hexagonal”): 1.5·1015 atoms/cm2 = 2.49·10-9 mol/cm2 = 240mC/cm2 average: 197mC/cm2 commonly used for polycrystalline Pt: 210mC/cm2 2 2 2 (note: 210mC/cm Pt = 2.04 nmolPt/cm Pt 235m Pt/gPt using MPt =195.7 gPt/molPt) (100)-face (110)-face (111)-face 2 2 Charge (Coulombs) from cyclic voltammetry divided by 210mC/cm : cm Pt surface 2 2 2 specific surface area: cm Pt/gPt m /gPt - from 30-120m /gPt for Pt/C (catalyst property) 2 2 2 2 roughness factor: 0.4mgPt/cm MEA · 60m Pt/gPt = 240cm Pt/cm MEA 4/26/2019 Physical Electrochemistry & Electrocatalysis 19 Peaks in the HUPD Region: Pt Single Crystals 2 (0.283cm Pt electrodes at 50mV/s) shape in the HUPD-region depends on surface orientation Pt/C HUPD-signature is less specific Markovic et al., J. Chem. Soc. Farad. Transactions 92, 3719 (1996) 4/26/2019 Physical Electrochemistry & Electrocatalysis 20 + - + - H2 2H + 2e Pt-Had Pt + H + e RDE Nafionfilm Catalysts In-situ cathode CV's at 20mV/s and 25°C: MEA: H2(500sccm) / N2 (62sccm); both overhumidified RDE: N2 (1000sccm) in 0.1M HClO4 20 ~1µm Glassy-Carbon (RDE) 10 + - Pt + H2O PtOHad + H + e ~6mm ] 0 Pt MEA A/g PtOH + H+ + e- Pt + H O i [ -10 ad 2 N H Electrode 2 Reference Electrodes , Working , 2 , Counter/, -20 thin-film GC in 0.1M HClO4 50cm2 MEA w. 63 sccm N2 -30 + - Pt + H + e Pt-Had -40 0.0 0.2 0.4 0.6 0.8 1.0 1.2 E/V [RHE] + - 2H + 2e H2 2 2 . H-adsorption/desorption on RDE (13mgPt/cm ) or in MEA (0.4mgPt/cm ): “H-titration” of Pt 2 2 . State-of-the-art 47%wt Pt/HSC (TKK): 92/79m /gPt (RDE/MEA) vs. 235 m /gPt theoretical limit . H-adsorption/desorption is independent of H2 partial pressure, H2-evolution is not 4/26/2019 Physical Electrochemistry & Electrocatalysis 21 Electrochemical Kinetics . Methods to Measure Electrode Reactions Stripping Voltammetry 4/26/2019 Physical Electrochemistry & Electrocatalysis 22 Anodic Stripping Voltammetry (ASV) ASV is an extremely sensitive electro-analytical technique that can determine trace quantities of analyte at the parts-per-billion level. I step: pre-concentration phase, where analyte is reduced/deposited at the working electrode surface in a stirred solution at a suitable reduction potential 푴풏+ 풂풒 + 풏풆− → 푴(풆풍풆풄풕풓풐풅풆) II step: working electrode potential is scanned so that the reduced/deposited analytes are oxidizes back to their ionic form, i.e.
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