Electrode reaction properties using a reactant gas addition method in a commercial 100 cm2 class solid oxide fuel cell

The world is gradually shifting away from the consumption of fossil fuels in favor of alternative green energy sources due to the depletion of fossil fuels and their corresponding negative impacts on the environment. To date, several fuel cells have been commercialized for green electricity production. Among them, solid oxide fuel cell (SOFC) is a promising next-generation stationary power source owing to its low carbon emissions, fuel flexibility and high efficiency. Since SOFCs require high operating temperatures, achieving better cell performance requires a thorough understanding of the reaction mechanisms of the electrolyte/electrode materials used in SOFC.

The gas-phase mass transfer and the charge transfer are some of the main factors controlling the reaction characteristics of the electrodes. The mass transfer effects in SOFC depend on a number of factors, including temperature, interconnects on the SOFC stacks, porosity, particle size and gas composition. However, there is no consensus on the oxygen reduction mechanism, especially at the cathode. Despite the extensive research on SOFCs, most have focused on pore diffusion of the gas-phase mass transfer with limited studies on the diffusion outside the pores and the effects of gas transport through the channels despite its significant contribution to the mass transfer process.

Herein, Dr. Samuel Koomson and Professor Choong-Gon Lee from Hanbat National University investigated the reaction characteristics of an electrode in a commercial 100 cm² anode-supported SOFCs using the reactant gas addition method. Since adding the reactant gas to the electrode affected its partial pressure and flow rate, this technique was applied to analyze the changes in the overpotential to establish the mass-transfer effect of specific reactants on the electrode reaction. Their work is currently published in the peer-review International Journal of Hydrogen Energy.

The research team revealed that the anode had considerable mass transfer-induced overpotential mostly consisting of the H₂ and H₂O species. At the measured current in the range 0 – 150 mA cm^-2, the H₂O species provided most of the anodic overpotential. This indicated that the anodic reaction was under more intense H₂O-induced mass transfer resistance than H₂-induced mass transfer resistance. On the other hand, the cathodic overpotential was found to be caused by the deficiency of O₂ species in the gas phase mass transfer rather than in the solid phases.

The effects of reactant mass transfer on the electrodes were satisfactorily illustrated by studying the relationships between the partial pressure, flow rate and overpotential of the reactant gases. The low overpotential reported in this study was mainly attributed to the utilization of thin electrodes. The overpotential was highly dependent on the gas flow rate and reactant utilization, indicating that the anodic reaction is mainly a mass-transfer controlled process. Consequently, the gas-phase mass transfer-controlled process was the dominant one over the solid-phase mass transfer process at the cathode.

In summary, the authors demonstrated effectiveness and practicability of the reaction gas addition method in studying the reaction properties of 100 cm² class planar anode-supported SOFCs. The obtained overpotential values agreed well with those obtained via inert-gas step addition (ISA), implying that the reactant gas addition method is a feasible and valuable tool for characterizing electrode reactions. In a statement to Advances in Engineering, Professor Choong-Gon Lee stated that the presented reactant gas addition method is a promising tool and would contribute to an in-depth understanding of the reaction mechanisms and processes in fuel cells.