Novel High-Temperature Alkaline Water Electrolysis Using Molten KOH–H₂O System

LSV and galvanostatic electrolysis were performed in molten KOH–H₂O (85 wt% KOH) at 150 °C using a three-electrode cell with an electrochemical measurement system. The cell was consisted of a perfluoroalkoxy alkanes (PFA) beaker, a polytetrafluoroethylene (PTFE) cover, and electrodes. The mass of KOH–H₂O was 500 g. A Ni plate (thickness: 0.1 mm) with a flag shape and 3 mm diameter was used as the working electrode. A Pt rod was used as the counter electrode. A palladium hydride (Pd–H) electrode was used as the reference electrode, and the potential was calibrated with respect to a reversible hydrogen electrode (RHE). The equilibrium potential of the Pd–H electrode for the RHE was determined using thermodynamic calculations based on the relationship between the equilibrium hydrogen pressure and the temperature in the two-phase coexistence region of Pd–H. ¹⁸ The potential of the Pd–H reference electrode was very stable during the electrochemical measurements (Fig. S1). For comparison, similar electrochemical measurements were conducted using a 30 wt% KOH aqueous solution at 80 °C, assuming the conditions of conventional commercial AWE. In this case, RHE was used as the reference electrode. All measured potentials were IR-compensated. All measurements were performed in stationary electrolytes under Ar gas flow at atmospheric pressure.

Polarization measurements were performed on Ni electrodes by LSV at a slow scan rate of 10 mV s⁻¹ (Fig. 2). The polarizations for both the HER and OER significantly decreased in molten KOH–H₂O at 150 °C compared with those in the conventional 30 wt% KOH aqueous solution at 80 °C.

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Figure 3 shows the Tafel plots constructed from the LSVs. The Tafel slope for the HER is –100 mV dec⁻¹ in aqueous KOH, whereas the Tafel slope in molten KOH–H₂O is much lower, at –34 mV dec⁻¹. For the OER, the Tafel slope is 87 mV dec⁻¹ in aqueous KOH, and 72 mV dec⁻¹, which is 15 mV dec⁻¹ lower, in molten KOH–H₂O. The changes are also observed in the Tafel region. For the HER, the Tafel region was 10–200 mA cm⁻² in aqueous KOH and 5–1500 mA cm⁻² in molten KOH–H₂O, a significant expansion toward the high current density range. Similarly, for the OER, the Tafel region was 3–30 mA cm⁻² in aqueous KOH and 3–70 mA cm⁻² in molten KOH–H₂O, indicating the expansion of the Tafel region to the high current density range. The decrease in the Tafel slope with the change of electrolyte from aqueous KOH to molten KOH–H₂O suggested a change in the rate-determining steps for the HER and OER. The expansion of the Tafel region to the high current density range was also considered to have promoted mass transfer. These effects were more significant for the HER than the OER. These results suggest that the change in the rate-determining step and the promotion of mass transfer associated with the change of electrolyte from aqueous KOH to molten KOH–H₂O were two of the main reasons for the decrease in polarizations for the HER and OER.

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Galvanostatic electrolysis was conducted at ±500 mA cm⁻², which corresponds to the high current density of a conventional commercial AWE, and at ±1000 mA cm⁻², which is double the current density, to measure the steady-state potential. Figure 4 shows the change in overpotentials in aqueous KOH and molten KOH–H₂O. Similar to the LSV results, the overpotentials for the HER and OER significantly decreased with the change of electrolyte from aqueous KOH to molten KOH–H₂O. Specifically, at 500 mA cm⁻², the HER overpotential decreased by 261 mV, from 467 mV (aqueous KOH) to 206 mV (molten KOH–H₂O). The OER overpotential decreased by 75 mV, from 553 mV (aqueous KOH) to 478 mV (molten KOH–H₂O). Thus, the total overpotential decreased by 336 mV, from 1020 mV (aqueous KOH) to 684 mV (molten KOH–H₂O). Here, the standard theoretical decomposition voltage decreased by 28 mV as the temperature increased from 80 °C to 150 °C. According to the aforementioned values, the total overpotential and standard theoretical decomposition voltage decreased by 364 mV as the electrolyte changed from aqueous KOH to molten KOH–H₂O. Therefore, the thermodynamic effect on the decrease in polarization was 8%, whereas the kinetic effect was 92%, indicating that the polarization decreased with the change of electrolyte mainly owing to kinetic effects. As shown in Fig. 4, even when the current density was doubled from 500 mA cm⁻² to 1000 mA cm⁻², the total overpotential decreased by 361 mV, from 1149 mV (aqueous KOH) to 788 mV (molten KOH–H₂O). These results indicate that water electrolysis using the molten KOH–H₂O system has the potential to improve energy efficiency, even when operated at higher current densities than conventional AWE.

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In this study, to develop highly efficient water electrolysis, we investigated the HER and OER behavior of a Ni electrode in a molten KOH–H₂O system (85 wt% KOH) at the high temperature of 150 °C. In LSV, the polarizations for the HER and OER at the Ni electrode were significantly reduced in the novel system compared to those in the conventional system (30 wt% KOH aqueous solution at 80 °C). The Tafel plots revealed that the change in the rate-determining step and the promotion of mass transfer associated with the change of electrolyte from aqueous KOH to molten KOH–H₂O were two of the main factors that reduced polarizations for the HER and OER. Steady-state potentials were measured during galvanostatic electrolysis at ±500 mA cm⁻², which correspond to the high current density for conventional commercial AWEs. As a result, the HER overpotential decreased by 261 mV and the OER overpotential decreased by 75 mV as the electrolyte changed with a total decrease of 336 mV. The thermodynamic effect on the decrease in polarization between aqueous KOH at 80° C and molten KOH–H₂O at 150 °C is 8%, whereas the kinetic effect is 92%, indicating that as the electrolyte changed, the polarization decreased, mainly due to kinetic effects. Even when the current density was doubled from 500 mA cm⁻² to 1000 mA cm⁻², the total overpotential decreased by 361 mV.