Electrolysis in a Hybrid Thermochemical Water-Splitting Cycle Based on Cobalt Oxide
Thermochemical water splitting cycles remain a promising approach to produce hydrogen from water using concentrated sunlight due to their high theoretical solar-to-hydrogen conversion efficiencies. One promising cycle is a hybrid cycle based on cobalt oxide. Hydrogen is produced in two chemical steps. In one step, concentrated sunlight is used to reduce cobalt oxide from Co3O4 to CoO in air near 1000 °C at atmospheric pressure. In the second step, the CoO is dissolved in a KOH electrolyte and electrolysis is performed. The oxidation of the CoO during electrolysis produces a Co3O4 precipitate and hydrogen near room temperature. The Co3O4 product is recycled and a fraction of the hydrogen is delivered to a fuel cell in order to provide the electrical input for electrolysis. The net effect of the cycle is the splitting of water using concentrated sunlight with an ideal solar-to-hydrogen conversion efficiency of 38% . One advantage of this approach is that the fuel production step is decoupled from the solar step and proceeds at room temperature. Hydrogen can thus be produced when and where water is readily available. For commercially viability, current densities of at least 50 mA/cm2 during electrolysis are required . When the cobalt oxide electrolyte is quiescent in our electrolysis cell, current densities are far below this value . One approach to increasing the current density is to enhance mass transfer to the electrode surface via convection of the electrolyte. To assess the impact of this technique and to better understand the kinetics involved in the process, we employed a rotating disc electrode. Our results show that the current density increases, as expected from the Levitch equation. In this presentation, we describe our current findings with convective mass transfer and relate them to our previously developed model for the electrochemical kinetics under quiescent conditions. Extending our model to convective mass transfer will allow the model to be more effectively used to develop and evaluate commercial cell designs that could be implemented for the efficient production of H2 from H2O, a sustainable solar fuel.  Palumbo, R., Diver, R. B., Larson, C., Coker, E. N., Miller, J. E., Guertin, J., Schoer, J., Meyer, M., and Siegel, N. P., 2012, “Solar thermal decoupled water electrolysis process I: Proof of concept,” Chem. Eng. Sci., 84, pp. 372-380.  Nudehi, S., Larson, C., Prusinski, W., Kotfer, D., Otto, J., Beyers, J., Schoer, J., and Palumbo, R., 2018, “Solar thermal decoupled water electrolysis process II: An extended investigation of the anodic electrochemical reaction,” Chem. Eng. Sci., 181, pp. 159-172.
Silcox, Rachel, "Electrolysis in a Hybrid Thermochemical Water-Splitting Cycle Based on Cobalt Oxide" (2019). Summer Interdisciplinary Research Symposium. 67.