Understanding High-Temperature Electrolysis
Authors: Foit Severin , Dittrich Lucy , Nohl Markus , Vinke I. C. , Eichel Rüdiger-A. , De Haart L. G. J. (Bert) 
Published: July 2021

Electrolysis is the technology, which provides the key for sector coupling. It enables the conversion of renewable energy to material value generation. Therefore, it is of utmost importance, that the energy is used in the most efficient way. High-temperature electrolysis on solid oxide cells provides high efficiency combined with the possibility to convert both water (H2O) and carbon dioxide (CO2) to hydrogen (H2) and carbon monoxide (CO)[1].
For a purposive improvement of the efficiency of the high-temperature electrolysis, a precise understanding of the on-going processes is needed[2]. Both the electrochemical and the procedural processes can have a significant impact on the performance, lifetime and overall effectiveness of the conversions taking place[3]. Some of the outcomes of changing operation parameters have been discussed before, although focusing on specific topics within the field. Therefore, a comprehensive sweep of parameters has to be conducted and results and findings have to be analyzed in full detail[3-6].
In this contribution, we show the summary and conclusions of multiple investigations of the high-temperature electrolysis. A series of detailed analysis has been performed using current-voltage characteristics (IV curves) and electrochemical impedance spectroscopy (EIS). The sets of experiments were conducted using commercially available cells (Elcogen) consisting of a Nickel/ 8 mol % Yttrium-Stabilized Zirconia (8YSZ) cermet fuel electrode, an 8YSZ electrolyte, a Cerium Gadolinium Oxide barrier layer and a Lanthanum Strontium Cobaltite air electrode. The variation of parameters included feed gas compositions ranging from pure water electrolysis to pure carbon dioxide electrolysis and co-electrolysis in between, temperature range of 650 °C to 900 °C, flow rates with various space-velocities, current densities from OCV up to over 2 A·cm2 and measurement times up to 500 h. In parallel a full-scale model was developed starting from fundamental thermodynamics over applied kinetics using Comsol software, in order to compare the experimental results and evaluation with the simulation results. As a first result, we are able to show a solution to the discussion of the role and impact of the direct electrochemical conversion of carbon dioxide during co-electrolysis. This is a first essential step for the understanding of high-temperature electrolysis in general.

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Figure 1