samples/texts_merged/1117773.md

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Resolving electron transfer kinetics in porous electrodes via diffusion-less cyclic voltammetry

Shida Yang,^ac Yang Li,^b Qing Chen.^ab*

^aDepartment of Chemistry, ^bDepartment of Mechanical and Aerospace Engineering, and ^cThe Energy Institute, HKUST, Hong Kong.

*Corresponding Author E-mail: chenqing@ust.hk (Qing Chen) ---PAGE_BREAK---

Figure S1. Background current on Ti foil as assembled in the cell with the active electrolyte but without the carbon felt. (a) $K_3Fe(CN)_6$, (b) $FeCl_3$, and (c) $VOSO_4$. The currents are at least two orders of magnitude lower than those measured with the carbon felt for all three cases, so no background subtraction is necessary for the analysis. ---PAGE_BREAK---

Figure S2. Electrochemical surface area measurements of the carbon felt electrode in the electrolytes of (a) $K_3Fe(CN)_6$, (b) $FeCl_3$, and (c) $VOSO_4$. We scan CV in ranges of potential with no visible Faradaic current and plot the average currents against the scan rates. The slopes are divided with a specific capacitance of 20 µF/cm² to derive the areas. ---PAGE_BREAK---

Figure S3. X-ray photoelectron spectra of different carbon felts.

Table S1. O/C ratio of different carbon felts and the corresponding standard rate constants $k^0$ of VO$^{2+}$/VO$_2^+$ on these electrodes.

Carbon Felt	C ratio/%	O ratio/%	O/C	k0 (cm/s)
CeTech CF020, 400 °C	92.51	7.49	0.081	1.56±0.15 × 10-6
SGL GFA6EA, 400 °C	90.14	9.86	0.109	1.642±0.072 × 10-7
SGL GFA6EA, 450 °C	89.34	10.66	0.119	2.095±0.518 × 10-7
SGL GFA6EA, 500 °C	88.93	11.07	0.124	2.455±0.216 × 10-8

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Figure S4. Additional results of the RFB tests. (a) Electrochemical impedance spectroscopy (EIS) and (b) IR-corrected polarization curves of VRFB with CF baked at different temperatures.

Table S2. Polarization resistance of VRFB with different CF.

SGL CF	Ru/Ω cm²	polarization resistance/Ω cm²	corrected polarization resistance/Ω cm²
400°C	0.395	0.487	0.092
450°C	0.421	0.540	0.119
500°C	0.450	0.664	0.214

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Table S3. Summary of standard rate constants k of VO₂⁺/VO₂⁺ reported in literature.

Electrodes	Treatment	Method	Area	k (cm/s)	Ref
SGL Carbon GFD4.6	Baked at 400 °C for 12 hrs	Symmetrical RFB	Electro-chemical	2.38×10-6	[1]
Disk made from carbon felt (SigraCELL GFA6, SGL carbon)	Baked at 400 °C for 30 hrs	Linear sweep voltammetry (LSV)	Geometric	1.6-8.8×10-8	[2]
Ultra-microelectrode made from carbon felts (GrafTech)	Electrochemical oxidation and reduction	LSV and EIS	Electro-chemical	1.7-17×10-5	[3]
Carbon felt (Sigratherm GFA5)	Not mentioned	Galvanic charging / discharging	Calculated	3×10-7	[4]
Carbon felt (Liao Yang Carbon Fiber Sci-tech. Co., Ltd. China)	None	CV and EIS	Geometric	1.84×10-3	[5]
Carbon paper (29, SGL group)	Baked at 450 °C for 30 hrs	Polarization curve and EIS in a RFB	Electro-chemical	0.2-1.8×10-7	[6]
Carbon paper (10AA, SGL group)	None	Symmetrical RFB	Gas adsorption	2.05×10-6	[7]
Carbon paper (Shanghai Hesen, Ltd. HCP030 N)	Electrochemical oxidation and reduction	CV	Gas adsorption	1.04×10-3	[8]

SI references:

[1] M. V. Holland-Cunz, J. Friedl, U. Stimming, J. Electroanal. Chem. 2018, 819, 306-311. ---PAGE_BREAK---

[2] Y. Li, J. Parrondo, S. Sankarasubramanian, V. Ramani, J. Phys. Chem. C 2019, 123, 6370-6378.

[3] M. A. Miller, A. Bourke, N. Quill, J. S. Wainright, R. P. Lynch, D. N. Buckley, R. F. Savinell, J. Electrochem. Soc. 2016, 163, A2095.

[4] A. A. Shah, M. J. Watt-Smith, F. C. Walsh, Electrochim. Acta 2008, 53, 8087-8100.

[5] W. Li, Z. Zhang, Y. Tang, H. Bian, T.-W. Ng, W. Zhang, C.-S. Lee, Adv. Sci. 2016, 3, 1500276.

[6] K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, F. R. Brushett, ACS Appl. Mater. Interfaces 2018, 10, 44430-44442.

[7] D. Aaron, C.-N. Sun, M. Bright, A. B. Papandrew, M. M. Mench, T. A. Zawodzinski, ECS Electrochemistry Letters 2013, 2, A29.

[8] X. W. Wu, T. Yamamura, S. Ohta, Q. X. Zhang, F. C. Lv, C. M. Liu, K. Shirasaki, I. Satoh, T. Shikama, D. Lu, S. Q. Liu, J Appl Electrochem 2011, 8.