Reporting electrocatalysis properly

Why reporting matters

Electrocatalysis papers compare numbers across materials, labs and years: an overpotential, a Tafel slope, a Faradaic efficiency. Those numbers only compare if they were measured and processed the same way. Several community guides, including Voiry et al. (2018), Wei et al. (2019) and, for CO₂ reduction, Clark et al. (2018), agree on a small set of habits that remove most of the confusion. This lesson goes through them, with a calculator for each step.

Potentials on the RHE scale

Report potentials vs RHE for reactions that involve protons or hydroxide, and say how you got there: the reference electrode and its filling, the value used for it, the pH and whether you calibrated against a hydrogen electrode. Lesson 1 has the details and a converter.

Overpotential is the distance from the equilibrium potential of the reaction, for example η = E_RHE − 1.23 V for the OER and η = E_RHE − 0 V for the HER. Avoid the term "onset potential" unless you define it, because papers use it for quite different current thresholds.

Correcting for iR drop

At 10 mA/cm² and a few ohms of uncompensated resistance, the ohmic drop is already tens of millivolts, about as large as the differences between good catalysts. It has to be measured and removed, and the paper has to say how.

• Measure R_u by impedance at, or close to, the working potential. The high-frequency intercept on the real axis of the Nyquist plot is R_u. Current interrupt, offered by many potentiostats, is another option.
• Compensate during the measurement or correct afterwards. Positive-feedback compensation is often set to about 85% of R_u, because full compensation can make the potentiostat oscillate. The remaining part can be corrected afterwards. Say which you did and how much.
• Report both if you can. Some reviewers ask for curves with and without correction, which is easy if you keep the raw data.

Which area to divide by

The same current can be expressed per geometric area, per electrochemically active surface area (ECSA) or per mass of catalyst. Each answers a different question, so state which one you used and, ideally, give more than one.

• Geometric current density, mA per cm² of electrode, is what matters for a device and is the usual basis for the overpotential at 10 mA/cm².
• Mass activity, A per g of catalyst or of active metal, needs the loading. It is useful for precious metals.
• Specific activity, current per ECSA, gets closest to the intrinsic activity of the surface, but only as good as the ECSA estimate.

For Pt-group metals, ECSA is usually measured from hydrogen underpotential deposition or CO stripping. For oxides and other materials, the double-layer capacitance is the common route. Record CVs in a narrow window where no Faradaic reaction happens, at several scan rates, and plot half the difference between anodic and cathodic currents against the scan rate. The slope is C_dl. Dividing by a specific capacitance C_s, often 0.040 mF/cm² in alkaline and 0.035 mF/cm² in acid following McCrory et al. (2013), gives the ECSA.

C_s varies from material to material by a factor of several, so ECSA from C_dl is best used to compare closely related catalysts measured in the same way. Treat absolute values, and specific activities based on them, with caution.

Activity and the Tafel slope

The most quoted activity number for water splitting is the overpotential needed for 10 mA/cm² of geometric current density. McCrory et al. (2013) chose it as a figure of merit because it is roughly the current density of a solar fuel device working at 10% efficiency. Report it with the loading, the iR correction and the scan rate. Values at 100 mA/cm² or more matter for electrolysers.

• Measure slowly. Use linear sweeps at 5 mV/s or slower, or steady-state steps (chronoamperometry or chronopotentiometry) to reduce the charging current and the effect of changes in the catalyst during the scan.
• Watch for redox peaks. Ni- and Co-based OER catalysts oxidise just before the OER and give a peak that overlaps the onset. Reading the activity from the reverse, cathodic-going scan avoids that oxidation current.
• The Tafel slope b, from η = a + b·log|j|, hints at the rate-determining step, but only when the fit range is truly kinetic, free of mass-transport limits and after iR correction. State the range fitted and how the data were taken.

Stability

A short polarisation curve says little about how a catalyst ages. A common minimum is a constant-current test (chronopotentiometry) at 10 mA/cm², or a constant-potential test, over many hours, together with a polarisation curve before and after. Longer tests at higher current densities are more convincing. Two checks make a stability claim stronger.

• Characterise the catalyst after the test, for example by XPS, XRD or electron microscopy, since many OER catalysts rebuild their surface into an oxyhydroxide.
• Analyse the electrolyte after the test, for example by ICP, for dissolved metals from the catalyst or the counter electrode.

Selectivity and Faradaic efficiency

Faradaic efficiency (FE) is the fraction of the charge that ends up in a given product. For gases, it is commonly measured with an online gas chromatograph (GC) fed by the gas leaving the cell. For liquid products, aliquots of the electrolyte are analysed by NMR or HPLC. The FEs of all products should add up to close to 100%. A large gap suggests a product that was missed or a calibration problem.

Hidden traps

• Impurities in the electrolyte. Trotochaud et al. (2014) showed that traces of Fe in commercial KOH are taken up by Ni oxyhydroxide and raise its OER activity a lot. Use high-purity or purified KOH when studying Ni-based catalysts, and say what you used.
• Counter electrode contamination. Dissolved Pt from a counter electrode can redeposit on the working electrode (Lesson 1).
• Substrate activity. Ni foam, carbon paper and steel can be active themselves, and carbon supports oxidise at OER potentials. Always measure the bare substrate.
• Bubbles. Gas bubbles block the surface and make curves noisy. Rotating the electrode or stirring helps, and the effect should be mentioned.
• Charging current. In a fast LSV, part of the current only charges the double layer and is not catalysis.

A reporting checklist

Check your understanding

References and further reading

1. D. Voiry, M. Chhowalla, Y. Gogotsi, N. A. Kotov, Y. Li, R. M. Penner, R. E. Schaak, P. S. Weiss, Best practices for reporting electrocatalytic performance of nanomaterials, ACS Nano 12 (2018) 9635–9638. doi:10.1021/acsnano.8b07700
2. C. Wei, R. R. Rao, J. Peng, B. Huang, I. E. L. Stephens, M. Risch, Z. J. Xu, Y. Shao-Horn, Recommended practices and benchmark activity for hydrogen and oxygen electrocatalysis in water splitting and fuel cells, Adv. Mater. 31 (2019) 1806296. doi:10.1002/adma.201806296
3. C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction, J. Am. Chem. Soc. 135 (2013) 16977–16987. doi:10.1021/ja407115p
4. E. L. Clark, J. Resasco, A. Landers, J. Lin, L.-T. Chung, A. Walton, C. Hahn, T. F. Jaramillo, A. T. Bell, Standards and protocols for data acquisition and reporting for studies of the electrochemical reduction of carbon dioxide, ACS Catal. 8 (2018) 6560–6570. doi:10.1021/acscatal.8b01340
5. L. Trotochaud, S. L. Young, J. K. Ranney, S. W. Boettcher, Nickel–iron oxyhydroxide oxygen-evolution electrocatalysts, the role of intentional and incidental iron incorporation, J. Am. Chem. Soc. 136 (2014) 6744–6753. doi:10.1021/ja502379c
6. A. Ch. Lazanas, M. I. Prodromidis, Electrochemical impedance spectroscopy, a tutorial, ACS Meas. Sci. Au 3 (2023) 162–193. doi:10.1021/acsmeasuresciau.2c00070