Steam Electrolysis Using Solid Acid Electrolytes at Intermediate Temperatures
Authors: Xu Ke , Obata Keisuke , Katayama Masao , Takanabe Kazuhiro 
Published: December 2024

Solid acid electrolysis cells (SAECs), operating at intermediate temperatures (150-300 °C), has potentials to perform electrochemical conversions using renewable energy resources with a small cell voltage. One of the examples is a steam electrolysis to produce hydrogen and oxygen. The composite of CsH2PO4 solid acid and SiP2O7 matrix is a popular proton-conducting electrolyte in SAECs. The formation of CsH5(PO4)2 at the interface of CsH2PO4 and SiP2O7 provides high conductivity in a wide temperature range from 150 °C to 250 °C.[1] However, CsH5(PO4)2 becomes a liquid phase above 150 °C with a strong acidity, which leads to a low mechanical strength of the electrolyte and a fast cell degradation. In this study, SiO2 precursor was explored to obtain a SiP2O7 matrix, which reduced the generation of CsH5(PO4)2 in the composite and improved the stability of steam electrolysis. Additionally, the cell voltage was greatly reduced by modifying Aquivion proton conducting ionomer on a IrO2/Ti anode, which is likely due to the increasing OER (oxygen evolution reaction) active sites on the IrO2 surface.
Commonly used SiP2O7 was prepared from a fumed silica which has a particle size of 3 nm and a surface area of 395 m2/g.[1] Firstly, the chronopotentiometry test was conducted using the traditional CsH2PO4/SiP2O7 electrolyte (molar ratio: 1/2) at 250 °C, the cell voltage rapidly increased after operating for 3 hours (Figure 1. black curve). X-ray diffraction (XRD) patterns showed TiP2O7 generated on the IrO2/Ti anode after the test, suggesting that the corrosion of Ti anode substrate by the strong acid of CsH5(PO4)2 might be the reason for cell degradation. The liquid CsH5(PO4)2 in the electrolyte also caused a severe deformation of the electrolyte pellet, highlighting the necessity to control the amount of CsH5(PO4)2 in the electrolyte. The SiO2 precursor was replaced to the one with particle size of 200 nm and a surface area of 10 m2/g while adjusting the heating time during SiP2O7 synthesis. The resultant SiP2O7 matrix contained a high proportion of hexagonal crystal structure (SiP2O7-hex). Scanning electron microscope (SEM) and differential thermal analysis (DTA) results exhibit that the SiP2O7-hex has a larger particle size and a lower hygroscopicity compared to the traditional SiP2O7. Chronopotentiometry tests was conducted for CsH2PO4/SiP2O7-hex electrolyte with small doping (16%) of CsH5(PO4)2 on the electrolyte surface to ensure a good contact between the electrolyte and the electrode. The steam electrolysis was stably operated for more than 20 hours (red curve in Figure 1), and the electrolyte pellet maintained its original shape after the test. These results suggest that SiP2O7-hex has a low reactivity with CsH2PO4 under humidified conditions, which limits the generation of liquid phase CsH5(PO4)2, and prevents the electrolyte deformation and the cell degradation.
However, as seen from Figure 1, the cell voltage increased in the initial stage and finally stabilized at 2 V under only 10 mA cm-2. It is reported that both CsH5(PO4)2 and CsH2PO4 solid acid migrate towards the anode side during OER.[2,3] Energy dispersive X-ray spectroscopy (EDS) confirmed that the Cs salt almost fully covered the IrO2 after the test, which might lead to the high cell voltage. Initially, there were abundant triple phase interfaces (gas-electrolyte-IrO2 interface) where H2O and O2 can be effectively transported. However, as the solid acid migrated on the IrO2 surface with time, the effective gas diffusion pathway was blocked, and the cell voltage gradually increased. By modifying the IrO2/Ti with Aquivion, which is a popular ionomer used in proton exchange membrane fuel cells (PEMFCs) to provide proton pathways and improve water management on the electrode, the cell voltage at 10 mA cm-2 was decreased to 1.6 V. The achievable current density also increased from 30 mA cm-2 to 50 mA cm-2. It is hypothesized that the modification of Aquivion on IrO2 helps to maintain the active sites at the electrolyte-IrO2 interface, thus improving the efficiency of steam electrolysis.
Overall, this study highlights the significance to control the amount of conductive liquid phase in the solid acid electrolyte and to maintain the accessibility to the electrochemical active sites.

Reference:

[1] T. Matsui et al., J. Electrochem. Soc, 2006, 153, A339.
[2] M. Wagner et al., Sustainable Energy Fuels, 2020, 4, 5284.
[3] N. Fujiwara et al., ChemSusChem, 2021, 14, 417.

Figure 1