Strategies to Promote Accurate Gaseous Product Quantification for CO₂RR at Industrial-Relevant Current Densities

Anthropogenic CO₂ emissions are the primary drivers of global warming and climate change.¹ The electrochemical reduction of CO₂ (CO₂RR) offers a green alternative for mitigating CO₂ emissions while producing valuable fuels, chemicals, and feedstocks for the chemical industry.^2,3 The simultaneous electroreduction of CO₂ and H₂O to form syngas (H₂/CO) is particularly promising, with tunable syngas ratios (H₂/CO) between 0.3 and 3 meeting the requirements of various downstream processes.^2,4 Using a NiO_x/Ni-N-C catalyst, syngas formation can be achieved at industrially relevant current densities (&gt; 200 mA cm^-2).^1,2 Accurate quantification of CO₂RR products at high current densities is essential for evaluating reaction selectivity and efficiency. In this study, we emphasize the importance of optimizing gas product quantification via online gas chromatography to ensure accurate Faradaic efficiencies (FEs) for H₂/CO syngas. We explored the effects of gas quantification on FE using a gas diffusion electrode comprising NiO_x/Ni-N-C on H23C8 carbon paper (Freudenberg, from Quintech) in a flow cell electrolyzer (1 M KOH). Our initial results at a current density of -300 mA cm^-2 showed a suboptimal syngas ratio of 0.24 and a total FE of 64.5% (<i>j_CO</i> = -156 mA cm^-2), with a noticeable gap attributed to product quantification challenges at this current density. To address this, we optimized experimental conditions, including calibration for linearity in H₂/CO formation and product dilution with N₂. These adjustments successfully brought H₂/CO products into the linear range of the gas chromatograph calibration, with total FEs approaching 100% up to a remarkable current density of -900 mA cm^-2. At -300 mA cm^-2, the total FE reached 94.9%, with a superior <i>j_CO</i> of -251 mA cm^-2. The highest H₂/CO ratio of 1.77—suitable for hydrocarbon synthesis via Fischer-Tropsch or alcohol synthesis—was achieved at -500 mA cm^-2. These compelling results underscore the importance of reporting reliable and accurate CO₂RR data, enabling more consistent comparisons with previous and future studies.<br/><br/><br/><b>References</b><br/>(1) Segets, D.; Andronescu, C.; Apfel, U.-P. Accelerating CO₂ electrochemical conversion towards industrial implementation. <i>Nat. Commun.</i> <b>2023</b>, <i>14 </i>(1), 7950. DOI: 10.1038/s41467-023-43762-6.<br/>(2) Sanjuán, I.; Kumbhar, V.; Chanda, V.; Machado, R. R. L.; Jaato, B. N.; Braun, M.; Mahbub, M. A. A.; Bendt, G.; Hagemann, U.; Heidelmann, M.; Schuhmann, W.; Andronescu, C. Tunable Syngas Formation at Industrially Relevant Current Densities via CO₂ Electroreduction and Hydrogen Evolution over Ni and Fe-derived Catalysts obtained via One-Step Pyrolysis of Polybenzoxazine Based Composites. <i>Small</i> <b>2024</b>, <i>20 </i>(23), e2305958. DOI: 10.1002/smll.202305958.<br/>(3) Liu, K.; Smith, W. A.; Burdyny, T. Introductory Guide to Assembling and Operating Gas Diffusion Electrodes for Electrochemical CO₂ Reduction. <i>ACS Energy Lett.</i> <b>2019</b>, <i>4 </i>(3), 639–643. DOI: 10.1021/acsenergylett.9b00137.<br/>(4) Cui, S.; Yu, C.; Tan, X.; Huang, H.; Yao, X.; Qiu, J. Achieving Multiple and Tunable Ratios of Syngas to Meet Various Downstream Industrial Processes. <i>ACS Sustainable Chem. Eng.</i> <b>2020</b>, <i>8 </i>(8), 3328–3335. DOI: 10.1021/acssuschemeng.9b07255.