No abstract

OER) are two of the most important electrochemical reactions that limit the efficiencies of fuel cells, metal-air batteries, and electrolytic water-splitting. [4][5][6][7] Although some noble metals and their associated compounds, such as Pt, RuO 2 , and IrO 2 , exhibit high ORR or OER catalytic activity, [8][9][10][11][12] the high cost and scarcity of such precious metals prevent their large-scale use. [13,14] Perovskite-structured (ABO 3 ) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare-earth nickelates (RNiO 3 ) thin films is investigated with controlled A-site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three-electrode system in O 2 -saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A-site element ionic radius which lowers the conductivity of RNiO 3 (R = La, La 0.5 Nd 0.5 , La 0.2 Nd 0.8 , Nd, Nd 0.5 Sm 0.5 , Sm, and Gd) films, with LaNiO 3 being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La 0.2 Nd 0.8 NiO 3 . Moreover, the OER activity remains comparable within error through Sm-doped NdNiO 3 . Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni 3+ to Ni 2+ as a result of oxygen vacancies, which increases the average occupancy of the e g antibonding orbital to more than one. The work highlights the importance of tuning A-site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts.

This review outlines the semiconducting-(quasi)metallic phase transition of 2D-TMDs and discusses current developments in the application of this unique phase. Challenges and future developmental trends of TMD-based systems are also highlighted.