An approximate analytical model of reduction of carbon dioxide in solid oxide electrolysis cell by regular and singular perturbation methods

• An approximate analytical model of CO2 reduction in SOEC.
• A comparison between CO-CO2 of SOEC and SOFC.
• Precisely calculated boundary layer.

The objective of this study was to describe the behaviour of carbon dioxide reduction in Solid Oxide Electrolysis Cell (SOEC) in an easy way, so that it can be further applied to cell unit analysis. In this article, an approximate analytical model was established. Concentration overpotential, activation overpotential and Ohmic overpotential were carefully expressed. Regular perturbation method (RPM) was firstly used to get implicit solutions. Then singular perturbation method (SPM) was further developed. Combining RPM and SPM, explicit solutions describing gas concentration and overpotential distributions along electrode were accessible. Calculating time was largely saved, with clear and concise expressions of electrochemical process. Similar to fluid boundary, by comparing ionic current to operating current, a precise description of electrochemical reaction boundary layer (approximate 6%-11% electrode) was developed. It gave us a deeper understanding of the mechanism where electrochemical reaction happens and how it is influenced in SOEC. Using similar analytical models, comparisons of overpotential and ionic current distribution, gas concentration distribution between Solid Oxide Fuel Cell (SOFC) and SOEC in a CO2-CO binary system were given, with ratio of reactant/product varying from 0.5 to 2.0. Meanwhile, analysis on important impact factors through analytical way in SOEC system, such as current density (from 0 to 0.3 A cm−2), temperature (973 K and 1073 K), triple phase boundary (TPB), was provided.