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

Abstract

Anthropogenic CO2 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 CO2. By using energy from intermittent sources (such as wind and solar) and protons from water, CO2 can be converted to a variety of useful products (i.e. CO and C2H4).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 CO2 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 m2/s) of CO2 in the liquid electrolyte.5,6 Furthermore, the H-cell configuration is restricted to laboratory scale experiments. Potential commercial processes for electrochemical CO2 reduction rely on the use of gas-diffusion layers, which greatly increases the diffusivity of CO2 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 CO2 reduction can misrepresent a catalyst’s intrinsic capability for electrochemical CO2 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 CO2. 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 CO2 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 CO2 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 CO2 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