Effect of the Electrode Shape on CO2 Electroreduction Product Selectivity over Ag and Cu Catalysts
Authors: Sharifi Golru Samaneh , Karaiskakis Alexandros N , Biddinger Elizabeth J 
Published: February 2020

CO2 emissions from fossil fuels and industrial processes are impacting climate change. Electroreduction of CO2 into valuable chemicals and fuels could be a promising technology to offset CO2 emissions 1, 2. Product selectivity in CO2 electroreduction is impacted by parameters such as catalyst roughness, crystal orientation, and pH2, 3. Altering the current distribution and consequently the electric field in the CO2 electroreduction cell can also alter the product distribution. Enhancing the electric field close to the electrode surface can increase the CO2 molecules adsorbed on the surface due to the high concentration of cations on the surface (FIRC effect) 4. Moreover, it can more stabilize intermediates on the surface and enhance the formation of C+2 products 5, 6. The shape of electrode is an important parameter that can alter the current distribution in the electrochemical cell and thus have an impact on product selectivity.
We have studied the impact of electrode shape on the product selectivity in CO2 electroreduction. Ag and Cu electrodes with two different shapes - wire and flag were used. The results showed an enhancement in C2 products formation for both Cu wire-shaped and Ag wire-shaped electrodes compared to the flag electrodes. The Ag wire-shaped electrodes had the lower onset potentials for hydrocarbon products and had 40% increase in CO and 75% increase in CH4partial current density at -1.3V. Ethylene was not observed on Ag flag even at high potentials (up to -1.3 V), while ethylene was observed on the Ag wire starting at -1.2 V. By changing the electrode from flag to wire, the total current density increased up to 60% on Ag, while current densities for Cu flag and Cu wire were comparable. Moreover, the results indicate that the partial current density and %FEs for ethylene and ethanol on Cu wire were higher than those on Cu flag at all studied potentials. The Cu wire-shaped electrodes had lower onset potentials for ethanol formation (-0.9 V). The main hydrocarbon on Cu flag was methane, while the main hydrocarbon on Cu wire was ethylene. This superior catalytic performance can be attributed to the higher surface current intensities on the wire-shaped electrodes due to their geometry. These results indicated that the shape of electrode is an important parameter that affects the product selectivity and partial current density during CO2 electroreduction.
References

Cave, E. R.; Shi, C.; Kuhl, K. P.; Hatsukade, T.; Abram, D. N.; Hahn, C.; Chan, K.; Jaramillo, T. F., Trends in the catalytic activity of hydrogen evolution during CO2 electroreduction on transition metals. ACS Catalysis
2018,
8(4), 3035-3040.

Karaiskakis, A. N.; Biddinger, E. J., Evaluation of the impact of surface reconstruction on rough electrodeposited copper-based catalysts for carbon dioxide electroreduction. Energy Technology
2017,
5(6), 901-910.

Gattrell, M.; Gupta, N.; Co, A., A review of the aqueous electrochemical reduction of CO2to hydrocarbons at copper. J. Electroanal. Chem.
2006,
594(1), 1-19.

Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P. G.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H., Enhanced electrocatalytic CO2reduction via field-induced reagent concentration. Nature
2016,
537(7620), 382-386.

Montoya, J. H.; Shi, C.; Chan, K.; Nørskov, J. K., Theoretical insights into a CO dimerization mechanism in CO2electroreduction. J. Phys. Chem. Lett.
2015,
6(11), 2032-2037.

Chen, L. D.; Urushihara, M.; Chan, K.; Nørskov, J. K., Electric field effects in electrochemical CO2reduction. ACS Catal.
2016,
6(10), 7133-7139.