High-Temperature Co-Electrolysis

Abstract

High-temperature Solid Oxide Electrolysis Cells (SOECs) possess the unique property to convert carbon dioxide and/or steam with a faradaic efficiency of 1 into carbon monoxide and/or hydrogen, respectively. High-temperature co-electrolysis of steam and carbon dioxide offers a suitable technology to provide 'white' syngas. Syngas, a mixture of hydrogen and carbon monoxide, as well as pure carbon monoxide are used in chemical industry for the synthesis of many basic chemicals as well as synfuels. Using renewable energy sources and the climate gas CO2 the high-temperature CO2 and co-electrolysis technology presents a sustainable alternative to conventional processes. The technology of co-electrolysis allows the adjustment of the H2 to CO ratio required for various down-stream catalytic processes in one step.Investigations of the high-temperature electrolysis of steam or CO2 and the co-electrolysis of a mixture of both in the feed gas were performed on so-called fuel electrode supported cells consisting of a 8mol% yttria stabilized zirconia (8YSZ) electrolyte, a Ni/8YSZ support and fuel electrode and a (La,Sr)(Co,Fe)O3 air electrode. The experiments showed that the performance of the cell during CO2 electrolysis is lower than for steam and co-electrolysis, as is shown in the Figure. The performance can be characterized by the slope of the current-voltage curve, i.e. the area specific resistance (ASR). For the CO2 electrolysis the ASR at 900 °C and 1.5 A·cm-2 amounts to 410 mΩ·cm-2. For steam and co-electrolysis, the ASR values are nearly the same: 245 mΩ·cm-2. The lower performance for CO2 electrolysis is attributed to the slower kinetics of the CO2 reduction reaction compared to the H2O reduction reaction. Similar observations were made in fuel cell operation, in which the CO oxidation reaction is around a factor 5 slower than the H2 oxidation reaction. The similar ASR values for steam and co-electrolysis are attributed to the occurrence of the reverse water gas shift (RWGS) reaction, in which CO2 and H2 react to CO and H2O during co-electrolysis. The slower kinetics of the CO2 reduction reaction are by-passed in the RWGS reaction using the hydrogen generated through the steam reduction.Further experiments were conducted to investigate the role and importance of the RWGS reaction during co-electrolysis. At the boundary to pure CO2 electrolysis, the performance of feed gas mixtures with low steam contents were investigated. First results show a significant increase of the area specific resistance at a reduced (RWGS) equilibrium steam concentration of about 5 %. Above 15 % steam in the feed gas (in the RWGS equilibrium) the ASR is no longer influenced by the feed gas composition and similar to that of pure steam electrolysis. Experimental results are furthermore compared to theoretical calculations of syngas production on a thermodynamic and partially kinetic level. Figure 1