H-Cell Vs Gas Diffusion Electrolyzer for Evaluating Intrinsic Activity of Nanocatalysts for Electrochemical CO2 Reduction

Anthropogenic CO 2 emissions are a leading contributor toward climate change, making closing the carbon cycle of the utmost importance to the research community. 1 One potential technique to do this effectively is via the sustainable electrochemical conversion of CO 2 . By using energy from intermittent sources (such as wind and solar) and protons from water, CO 2 can be converted to a variety of useful products (i.e. CO and C 2 H 4 ). 2 However, controlling the selectivity towards which particular product is formed and preventing the parasitic hydrogen evolution reaction is a crucial challenge that must be overcome before commercial implementation can occur. 3 Typically, when exploring fundamental mechanisms and new catalyst materials to overcome this challenge, preliminary studies begin in an H-type cell (Figure 1a) where CO 2 and gas products diffuse to and from the catalyst surface through the liquid electrolyte. 4 However, this can place a considerable restriction on the electrochemical selectivity and current due to the low solubility (~34.2 mmol/L) and diffusivity (~2x10 -9 m 2 /s) of CO 2 in the liquid electrolyte. 5,6 Furthermore, the H-cell configuration is restricted to laboratory scale experiments. Potential commercial processes for electrochemical CO 2 reduction rely on the use of gas-diffusion layers, which greatly increases the diffusivity of CO 2 and gas products by enabling transport through the gas phase (Figure 1d). 7,8 When a catalyst or system of catalysts are evaluated solely in the H-cell the selectivity and activity for products of electrochemical CO 2 reduction can misrepresent a catalyst’s intrinsic capability for electrochemical CO 2 reduction. We present a comparison between a series of nanoparticle (NP) based electrodes, prepared similarly, in a liquid H-type cell vs. a gas-diffusion electrolyzer (GDE) for electrochemical reduction of CO 2 . In the case of Au NPs (Figure 1 a-b) we found ~45% faradaic efficiency and ~0.5 A/g current density towards CO at -0.5 V vs. RHE (the only electrochemical CO 2 product), which would suggest that these particular Au NPs are an ineffective catalyst. However, when reevaluated in the GDE the selectivity and activity are much higher (~80% and ~220 A/g) at the same potential and with the same electrolyte. The stark contrast in both selectivity and activity for the catalyst’s capability of converting CO 2 to CO depending on the reactor chosen demonstrates the importance of considering operational conditions when evaluating a class of materials true potential to carry out electrochemical CO 2 reduction in commercially relevant systems. (1) Olah, G. A.; Prakash, G. K. S.; Goeppert, A. Anthropogenic Chemical Carbon Cycle for a Sustainable Future. J. Am. Chem. Soc. 2011 , 133 (33), 12881–12898. https://doi.org/10.1021/ja202642y. (2) Whipple, D. T.; Kenis, P. J. A. Prospects of CO2 Utilization via Direct Heterogeneous Electrochemical Reduction. J. Phys. Chem. Lett. 2010 , 1 (24), 3451–3458. https://doi.org/10.1021/jz1012627. (3) Verma, S.; Kim, B.; Jhong, H.-R. “Molly”; Ma, S.; Kenis, P. J. A. A Gross-Margin Model for Defining Technoeconomic Benchmarks in the Electroreduction of CO2. ChemSusChem 2016 , 9 (15), 1972–1979. https://doi.org/10.1002/cssc.201600394. (4) Raciti, D.; Mao, M.; Ha Park, J.; Wang, C. Mass Transfer Effects in CO 2 Reduction on Cu Nanowire Electrocatalysts. Catalysis Science & Technology 2018 , 8 (9), 2364–2369. https://doi.org/10.1039/C8CY00372F. (5) Gupta, N.; Gattrell, M.; MacDougall, B. Calculation for the Cathode Surface Concentrations in the Electrochemical Reduction of CO2 in KHCO3 Solutions. J Appl Electrochem 2006 , 36 (2), 161–172. https://doi.org/10.1007/s10800-005-9058-y. (6) Raciti, D.; Mao, M.; Wang, C. Mass Transport Modelling for the Electroreduction of CO2 on Cu Nanowires. Nanotechnology 2018 , 29 (4), 044001. https://doi.org/10.1088/1361-6528/aa9bd7. (7) Weng, L.-C.; Bell, A. T.; Weber, A. Z. Modeling Gas-Diffusion Electrodes for CO2 Reduction. Phys. Chem. Chem. Phys. 2018 , 20 (25), 16973–16984. https://doi.org/10.1039/C8CP01319E. (8) Wu, K.; Birgersson, E.; Kim, B.; Kenis, P. J. A.; Karimi, I. A. Modeling and Experimental Validation of Electrochemical Reduction of CO2 to CO in a Microfluidic Cell. J. Electrochem. Soc. 2015 , 162 (1), F23–F32. https://doi.org/10.1149/2.1021414jes. Figure 1