Investigation of carbon dioxide electrolysis reaction kinetics in a solid oxide electrolyzer

Abstract

The atmosphere of Mars is a potential source of the gases essential for human exploration missions. Many international space agencies and scientists have shown great interest in developing chemical plants to make propellants and life-support consumables utilizing the red planet's atmosphere and Earth-sourced H₂. Electrolyzing carbon dioxide to produce oxygen by a solid oxide electrolysis cell has been proven to be a potential candidate. A solid oxide electrolysis cell, which consists of 8mol% yttria-stabilized zirconia sandwiched between two electrodes, is designed, manufactured and tested. The electrode/electrolyte interfacial polarization characteristics are investigated with the aid of the current interruption method using a four-electrode set-up. Activation overpotentials, which are free of ohmic losses, are measured in the temperature range from 1023 to 1123K for the carbon dioxide electrode and the oxygen electrode. Both the electrode activation overpotentials show the Tafel behavior. In order to increase the active electrochemical reaction sites, platinum yttria-stabilized zirconia cermet electrode is investigated. A solid oxide electrolysis cell with cermet electrodes shows high performance and significantly reduces anode activation overpotentials, and ohmic resistance as well. A 1-D steady state solid oxide electrolysis cell model is set up to take into account different kinetics: (1) surface exchange kinetics at the gas/electrode interface involving adsorption/desorption; (2) mass transfer of the reactants and products involving the bulk diffusion and surface diffusion; and (3) electrochemical kinetics involving charge transfer at the triple phase boundary. The solid oxide electrolysis cell's performance and voltage are predicted at any given current based on this model. The effects of surface diffusion coefficients, adsorption/desorption rate constants, and anodic/cathodic reaction rate constants on carbon dioxide electrolysis are studied. A comparison of solid oxide electrolysis cells between the numerical results and the experimental results is made.