Co-Electrolysis, Quo Vadis?

Abstract

Co-electrolysis, the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to produce syngas, can be regarded as the enabling step to the sustainable production of synfuels and high-value chemicals. This advantageous method of beneficial CO2-valorization is predetermined to couple energy-intense sectors such as “traffic and transportation” and “chemical industry”. The needed products for these markets presuppose different syngas compositions through their production routes and according to that determine the applied technology of syngas generation by its achievable H2:CO ratio. The concept of high-temperature co-electrolysis offers both an efficient electrochemical conversion and accessibility to H2:CO distributions from 1:1 to 3:1 by a given set of parameters (temperature, current density, material,…). Though, this is yet to be shown as research has been focused on electrochemical analysis of materials, cells and stacks rather than on product distribution depending on operating parameters and an all-embracing quantitative product analysis[1]. Herein, we examine the possibilities and variety on syngas production with focus on (electrochemical) performance and syngas yield and composition under different conditions, e.g. temperature, current density, substrate composition. The experiments were executed on a cathode supported cell (CeramTec) composed of an LSCF anode, a CGO barrier-layer, an 8YSZ electrolyte and a Ni/8YSZ cathode. The testing took place in the temperature range of 750 to 900 °C with preset cell voltages up to 1.4 V. The ratios of water vapor to carbon dioxide have been set to 3:1, 2:1 and 1:1. Furthermore, the cells have been tested with a 50% water – 50% hydrogen mixture for comparison. I/V-measurements and electrochemical impedance spectroscopy have been conducted under the combinations of mentioned conditions. In addition, a full spectrum analysis of the product gases has been performed, i. e. water was not condensed from the product stream prior to the mass spectrometer. Current densities of up to over 1.7 A·cm-2 with polarization resistances of 24 mΩ·cm-2 have been obtained at 900 °C and a cell voltage of 1.4 V with a 9 nl·h-1 flow of 40% water, 40% CO2 and 20% H2 used. I/V-curves decrease in slope and increase in current density with increasing temperature (figure 1). Further investigation is ongoing and the electrochemical performance and efficiency of syngas production will be presented and discussed in detail. [1] S. Foit, I. C. Vinke, L. G. J. de Haart, R.-A. Eichel, Angewandte Chemie International Edition 2016, DOI: 10.1002/anie.201607552 Figure 1: I/V characteristics of a Ni/YSZ//YSZ//CGO//LSCF single cell during co-electrolysis of different H2/CO2 mixtures in dependence of temperature Figure 1