Bio-inspired iron-nickel sulfides for tunable electrocatalytic CO₂RR

Mathias Smialkowski, Ruhr-Universität Bochum, Bochum, Germany
Kai junge Puring, Ruhr-Universität Bochum; Fraunhofer UMSICHT, Bochum; Oberhausen, Germany
Mathias Smialkowski, Ruhr-Universität Bochum, Bochum, Germany
Stefan Piontek, Ruhr-Universität Bochum, Bochum, Germany
Ulf-Peter Apfel, Ruhr-Universität Bochum; Fraunhofer UMSICHT, Bochum; Oberhausen, Germany

The efficient utilization of excess CO₂ was attempted within many investigations to minimize artificial CO₂ emissions. Herein, a promising approach is the electrocatalytic conversion of the greenhouse gas CO₂ (CO₂RR) into fuels or commodity chemicals. This aim can be achieved by focusing on the development of proper catalysts, able to efficiently convert the kinetically challenging CO₂ molecule into desired products (e.g. short hydrocarbons (C1, C2…) or carbon monoxide (CO)). Unfortunately, current electrocatalysts suffer from a low stability, activity and selectivity for CO₂RR and usually is outcompeted by HER that is lowering the overall yield of the formed carbon products.[1] Inspiring role models for an efficient catalyst design are naturally occurring enzymes - for example the nickel-containing carbon monoxide-dehydrogenase (CODHNi) is able to selectively and efficiently reduce CO₂ to afford CO.[2,3] Herein, we present the iron-nickel sulfides (Fe,Ni)9S8 which share structural similarities with the active site of the (CODH)Ni as promising catalysts for HER and CO₂RR. To explore the influence of proton concentration on the CO₂ conversion, different CO₂ saturated organic electrolyte solutions with varying amounts of water were subsequently used for electrocatalysis. Furthermore, we show that the CO₂RR product distribution is highly dependent on the concentration of protons in solution and the used solvent. While protic solvents tend to promote HER, utilization of aprotic solvents with low water content efficiently enables CO₂ reduction to afford CO and CH₄. Moreover, we demonstrate that changing the water concentration in acetonitrile also affects the product distribution during electrocatalysis. High water contents (>5000 ppm) favor HER with high faradaic efficiencies (> 90%), while very low water contents promote the formation of CO with faradaic efficiencies up to 90%. We thus postulate, that for an efficient CO₂RR the electrolysis reactor and the catalyst must be fine-tuned and taken into account. In fact, optimizing the chemical environment for CO₂RR, namely the electrolyte solution, is as important criteria for a rational design of the electrolyzer.

References
[1] J. Qiao, Y. Liu, F. Hong, J. Zhang, Chem. Soc. Rev. 2013, 43, 631–675.
[2] B. Konkena, K. J. Puring, I. Sinev, S. Piontek, O. Khavryuchenko, J. P. Duerholt, R. Schmid, H. Tueysuez, M. Muhler, W. Schuhmann, et al., Nat. Commun. 2016, 7, 12269–12277.
[3] S. Piontek, C. Andronescu, A. Zaichenko, B.

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