Enhanced electrocatalytic water splitting by Sm and Gd-doped ceria electrocatalysts on Ni foam substrate

CeO2 is attractive for water splitting due to its various valence oxidation states, however, its catalytic perfor-mance in alkaline medium is challenging. In this study, we synthesized novel ceria-based electrocatalysts: Gd-doped CeO2 nanocrystals (GDC) and Sm-doped CeO2 nanospheres (SDC) on Ni foam using sol-gel and one-step hydrothermal methods, respectively. These ceria-based electrocatalysts, GDC and SDC, depict excellent activity in the OER and HER, respectively. The GDC/NF showed promising OER performance, achieving an exceptional overpotentials of 300 and 420 mV to keep current densities of 10 and 100 mA cm-2, respectively. In addition, SDC/NF exhibited excellent HER activity with overpotentials of 117 and 325 mV to reach current densities of 10 and 100 mA cm-2, respectively. Furthermore, both GDC and SDC were ultra-stable at a fixed current density of 50 mA cm-2 for 25 h. Moreover, the complete anion-exchange membrane (GDC/NF||SDC/NF) demonstrated 1.6 Vcell to accomplish 10 mA cm-2 current density, with ultra-durability for 40 h at 50 mA cm-2. The versatile electrochemical performance of ceria-based catalysts is attributed to diverse-valence Ce3+/Ce4+ redox states and positive entropy contribution of the f0-f1 transition. Notably, doping with Gd and Sm led to an easy and strong reduction owing to the oxygen voids created and hydroxyls. Moreover, dopants stabilized ceria and assisted the generation of metal-H groups, which improved the kinetic activity of ceria for electrochemical water splitting.