A team at the University of Tokyo has demonstrated steam electrolysis using a solid acid electrolysis cell (SAEC) for the production of hydrogen. The SAEC used a CsH₂PO₄/SiP₂O₇ composite electrolyte and Pt/C electrodes; hydrogen production was successfully demonstrated with Faraday efficiencies around 80%. Their paper appears in the journal ChemSusChem.

According to thermodynamics, enthalpy change of the water electrolysis reaction (H₂O → H₂ + ½O₂) can be written as ΔH = ΔG + TΔS, where H, G, S, T are enthalpy, Gibbs free energy, entropy, and temperature, respectively. This tells us that the total energy demand (ΔH) can be satisfied by electrical energy (ΔG) and heat (TΔS). ΔG decreases with the temperature while ΔH slightly increases. This means that the electric power required for the electrolysis becomes smaller at higher temperatures.

Solid Oxide electrolysis is conducted at > 600°C using oxide ion-conducting solid oxide electrolytes. Based on the thermodynamic consideration, solid oxide electrolysis can provide the highest conversion efficiency from electricity to hydrogen if appropriate heat sources are available. However, the high temperature leads to fast degradation of the cells and is not suitable for quick start-up and shutdown. Thus, solid oxide electrolysis at relatively low temperatures are also examined. For example, utilization of proton-conducting solid oxide electrolytes may lower the operation temperature to 400-600 °C.

The abovementioned status of the electrolysis research indicates the need for electrolysis technologies feasible in the intermediate temperature range (100-600 °C). The intermediate temperature range has potential for both the small cell overpotentials and the flexible operability suitable for the utilization of renewable energy resources. Here, we focus on an emerging intermediate-temperature electrolysis method, which is called solid acid electrolysis. Solid acid electrolysis is performed at around 200 °C by employing proton-conducting solid acids as electrolytes.

—Fujiwara et al.

In their study, the researchers found that the cell voltage under a constant current load increased with time. The performance degradation was more severe at higher temperatures.

SEM-EDX measurements showed that a certain part of the electrolyte migrated into the porous anode layer during the operation, filling the anode pores and preventing gas diffusion. It was suggested that the surface of the Pt/C catalyst in the anode was partially covered by the migrated electrolyte and became electrochemically inactive.

The team also found that the carbonaceous materials in the anode, primarily the carbon support of the Pt/C catalyst, was oxidized to carbon dioxide. The oxidation of carbon can decrease the number of electrochemically active sites in the anode. The researchers said that these phenomena were the possible causes of the performance degradation of the SAEC.

To mitigate the problems relating to the Pt/C anode, the team used a Pt mesh as an alternative anode. With a constant current load of 10 mA cm^-2, the cell voltage at 220°C was almost unchanged at around -2 V for 48 h.

The superior stability of the Pt mesh anode demonstrated the importance of the anode design. Future investigations for durable and practical SAECs should focus on the control of the electrolyte migration and the development of cost-effective anodes with high oxygen evolution activity.

—Fujiwara et al.

Resources

• Fujiwara, N., Nagase, H., Tada, S. and Kikuchi, R. (2020), “Hydrogen Production by Steam Electrolysis in Solid Acid Electrolysis Cells.” ChemSusChem. Accepted Author Manuscript. doi: 10.1002/cssc.202002281