Water Splitting in Alkaline Electrolytes at Elevated Temperatures Using Nickel-, Tin-, and Iron-Coated Electrodes

The temperature dependences of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) were investigated on Ni-mesh (NM) electrodes modified with catalysts comprising any combination of Ni, Sn, and Fe, as well as on unmodified NM electrodes in aqueous alkaline solutions maintained at 25, 40, and 50 °C. The reactions required appropriate temperature compensations from the indicated potentials of the reference electrode and the thermodynamic equilibrium potentials of the corresponding reactions. The results indicated that the intrinsic overpotentials of all electrodes for the OER and HER to achieve a current density of 10 mA cm–2 decreased with the increasing electrolyte temperatures. The changes in these intrinsic overpotentials also depended on the catalyst: for example, the OER overpotentials of Ni–Sn–Fe/NM and the unmodified NM electrodes decreased by 0.014 and 0.028 V, respectively, as the temperature changed from 25 to 50 °C. Conversely, the HER overpotentials of Ni–Sn/NM and the unmodified NM electrodes decreased by 0.121 and 0.144 V, respectively, as the temperature changed similarly. The corresponding Tafel slope did not display any significant change with the temperature for any of the catalysts, although the exchange current density (j0) increased with increasing temperature. The electrochemical impedance measurements revealed that the charge-transfer resistance values of the Ni–Sn–Fe- and Ni–Sn-modified NM electrodes were significantly lower than that of the unmodified NM electrode. Furthermore, the electrolytic tests at constant current density revealed that the Ni–Sn–Fe- and Ni–Sn-modified NM electrodes exhibited high stability against temperature changes, whereas the unmodified NM electrodes exhibited significant irreversible degradation when electrolyzed at 40 or 50 °C. Next, a two-electrode cell comprising a Ni–Sn–Fe/NM anode and Ni–Sn/NM cathode was assembled. The cell voltage during electrolysis at constant current (600 mA cm–2) was 2.107 V at 25 °C, decreasing to 1.945 V at 50 °C. However, that of the symmetric bare NM//bare NM was 2.910 V at 25 °C, decreasing to 2.633 V at 50 °C.