Reaction Mechanism and Rate-Determining Step Speculation of Reversible CO/CO2 Electrochemical Conversion on the Nickel Patterned Electrodes

The nickel(Ni)-patterned electrode enables researchers to quantify the triple-phase boundary (TPB) length and Ni surface area, exclude the influence of bulk gas diffusion and clearly separate the active regions of the chemical/electrochemical reactions. This tool has been studied for identifying the reaction mechanism in SOFC but rarely in SOEC. In this study, the Ni-patterned electrodes with the stripe width of 100 μm were tested in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H 2 O/H 2 and further H 2 O/CO 2 /H 2 /CO. The experimental test shows the stability of the Ni-patterned electrode in the H 2 O/H 2 atmosphere was much poorer than that in the CO/CO 2 atmosphere due to the nickel reacting with steam. Thus, the Ni-patterned electrode needed to operate at a temperature of ≤700 o C with the inlet H 2 /H 2 O molar ratio of >7 and the current-applied time shouldn’t be over 5 h to keep the accuracy of the TPB length and Ni surface area. The effects of the temperature, partial pressure of H 2 O and H 2 are investigated. The activation energy of the Ni-patterned electrodes was 0.386 eV. The electrochemical performance had a positive correlation with both the partial pressure of H 2 and H 2 O, especially showed higher sensitivity to the partial pressure of H 2 O. Further, a possible mechanism of the H 2 O/H 2 electrochemical conversion was proposed according to the experimental data and existing literature, which contains two-step charge-transfer reaction: H(Ni)+O 2- (YSZ)OH - (YSZ)+(Ni)+e - and H(Ni)+OH - (YSZ)H 2 O(YSZ)+(Ni)+e - . Based on the experiments, an analytical calculation was performed to investigate the rate-determining steps in the Ni-patterned electrodes by bridging the connection between the kinetic parameters and operating conditions. The results indicate the rate-limiting steps may be different for the SOFC and SOEC modes. In SOFC mode, the H 2 electrochemical oxidation could depend more on the charge-transfer reaction H(Ni)+O 2- (YSZ)→OH - (YSZ)+(Ni)+e - , however in SOEC mode, the rate-determining step of the H 2 O electrochemical reduction should be H 2 O(YSZ)+(Ni)+e - →OH - (YSZ)+H(Ni). In this way, a reversible mechanism could be applied for reversible SOFC (RSOFC) to describe the conversion between hydrogen and steam by different charge-transfer-reaction domination in the SOFC and SOEC mode. Figure 1