Performance and Processes of Pure CO2 Electrolysis in Solid Oxide Cells

Abstract

In solid oxide electrolysis cells (SOECs) carbon dioxide (CO2) is converted to carbon monoxide (CO). In the chemical industry CO is an important reactant to produce base chemicals such as acetic or formic acid or fine chemicals produced by carbonylation processes. CO is also used in the reduction of oxides to metals. Conventional processes for CO production are based on fossil resources such as coal or natural gas. CO2 electrolysis presents a sustainable method to utilize renewable energy for CO production and additionally transforms the greenhouse gas CO2 into a resource.Carbon dioxide is already used and investigated in the co-electrolysis process where it is converted alongside steam to produce syngas, a mixture of hydrogen and carbon monoxide. However, the analysis of pure CO2 electrolysis for CO production hasn’t been investigated in detail. Some studies report alternative materials or first degradation results. In this contribution the CO2 electrolysis is discussed in-depth from an electrochemical point of view. A detailed analysis by current-voltage characteristics (IV curves) and electrochemical impedance spectroscopy (EIS) was performed on commercially available cells by Elcogen consisting of a Nickel/ 8 mol % Yttrium-Stabilized Zirconia (8YSZ) cermet fuel electrode, a 8YSZ electrolyte, a Cerium Gadolinium Oxide barrier layer and a Lanthanum Strontium Cobaltite air electrode. IV curves and EIS spectra were measured at varied CO2/CO ratios, temperatures, flow rates and current densities.The results show that with increasing CO2/CO ratio the total area specific resistance (ASR) increases at open circuit voltage and decreases under load (Figure 1). A similar decrease of resistance is seen for increasing flow rates. The main resistance contribution determined from impedance analysis comes from diffusion/concentration losses[1]. The analysis by impedance spectroscopy provides information on the underlying processes and is not only of relevance for understanding pure CO2 electrolysis but also for understanding the role of CO2 reduction during co-electrolysis. Figure caption: Figure 1: ASR, |i|1.4 V and OCV for varied CO2/CO ratios at 800 °C[1]. 1Two values are given due to a hysteresis. The first value represents the forward scan and the second value represents the backward scan of the IV curve. Literature [1] S. Foit, L. Dittrich, T. Duyster, I. Vinke, R.-A. Eichel, Haart, L. G. J. de, Processes 2020, 8, 1390. Figure 1