Current is not product
A potentiostat measures electrons, not molecules. In CO2, N2 or nitrate reduction, part of the current always goes to hydrogen, and part may go to several carbon or nitrogen products at once. The only way to know what a catalyst makes is to measure every product, then compare the charge they account for with the charge that was passed.
FEi = zi F ni / Q ji = FEi × jtotalzi electrons per molecule, ni moles made, Q total charge. The partial current density ji combines activity and selectivity into one number.
The Faradaic efficiencies of all products should add up to close to 100%. A total well below that suggests a missed product, a leak or a calibration problem. A total above 100% almost always means an error.
Gas products
- Online GC. A steady flow of CO2 (or Ar, N2) carries the gas from the headspace of the cathode compartment to a gas chromatograph that samples it at set times. A thermal conductivity detector (TCD) is used for H2, and a flame ionisation detector (FID), often with a methanizer, for CO and hydrocarbons such as CH4 and C2H4.
- Calibrate each gas with certified standard mixtures over the concentration range you expect.
- Measure the flow at the outlet, with a flow meter or a bubble flow meter. CO2 is consumed and turned into carbonate in some cells, so the outlet flow can be lower than the inlet flow, which changes the calculated FE.
- Wait for a steady state before the first injection, and give the average and spread of several injections.
The calculator for gas products turns GC concentrations, flow and current into FE and partial current densities.
Products in the electrolyte
- ¹H NMR with an internal standard (for example DMSO or phenol) and water suppression measures formate, acetate, alcohols and many other organics in one spectrum.
- HPLC is common for organic acids and for the products of glycerol, glucose and HMF oxidation.
- Ammonia can be measured by the indophenol blue or salicylate methods, by ion chromatography, or by ¹H NMR of ammonium, which also tells 14NH4⁺ (a triplet) from 15NH4⁺ (a doublet). Nitrite, a common intermediate of nitrate reduction, is measured with the Griess method.
- Take samples from both compartments, since products such as formate can cross the membrane, and for ammonia include an acid trap on the gas outlet.
Control experiments
Small amounts of product are easy to make by accident. The controls below rule out the most common false positives. The more surprising the claim, the more of them are needed.
| Control | What it rules out |
|---|---|
| Same experiment under Ar instead of CO2 or N2 | Products that come from the catalyst, binder or electrolyte |
| Open circuit with the feed gas | Products that are present without any current, for example contamination in the gas |
| Bare substrate or support without catalyst | Activity of the carbon paper, Ni foam or glassy carbon |
| Blank electrolyte, gloves, membrane and tubing | Ammonia and nitrate from the lab environment, which are common at the ppb to ppm level |
| Product amount against charge or time | A constant amount that does not grow with charge, which points to contamination |
| Isotope labelling, 13CO2 or 15N2 | Products that do not come from the feed |
Nitrogen reduction is a special case. Andersen and co-workers (2019) showed that many reported N2-to-ammonia results could be explained by contamination. Their protocol cleans the feed gas, measures ammonia quantitatively, and confirms the result with quantitative 15N2 experiments, with the amount of 15NH3 matching the amount expected from the charge. Follow such a protocol before claiming N2 reduction.
Other numbers to report
- Yield rate, for example µmol h⁻¹ cm⁻² or µg h⁻¹ mgcat⁻¹, together with the potential and the electrolysis time.
- Energy efficiency of a full cell, EE = Eeq × FE / Ecell, where Eeq is the thermodynamic cell voltage for that product.
- Carbon balance and single-pass conversion in flow cells, since a large part of the CO2 can be lost as carbonate.
- Stability of the selectivity, not only of the current. Many catalysts lose selectivity before they lose activity.
Check your understanding
Quiz for lesson 2.3
Pick one answer per question. You see the explanation straight away.
References and further reading
- 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
- S. Z. Andersen et al., A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements, Nature 570 (2019) 504–508. doi:10.1038/s41586-019-1260-x
- K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces, Energy Environ. Sci. 5 (2012) 7050–7059. doi:10.1039/c2ee21234j
- S. Nitopi et al., Progress and perspectives of electrochemical CO2 reduction on copper in aqueous electrolyte, Chem. Rev. 119 (2019) 7610–7672. doi:10.1021/acs.chemrev.8b00705
- P. H. van Langevelde, I. Katsounaros, M. T. M. Koper, Electrocatalytic nitrate reduction for sustainable ammonia production, Joule 5 (2021) 290–294. doi:10.1016/j.joule.2020.12.025