Recent Advances in the Understanding of Nickel-Based Catalysts for the Oxidation of Hydrogen-Containing Fuels in Alkaline Media

Nickel is a very abundant transition metal in the Earth's crust, and it finds numerous applications in electrochemical processes where metallic Ni or its oxides are thermodynamically stable, particularly in alkaline environments. This contribution addresses electrocatalytic properties of Ni-based catalysts in reactions of fuel oxidation in alkaline media. It first details the electrochemical behavior of Ni in alkaline media and approaches to determine the active surface area of Ni electrodes. Second, the electrocatalytic activities of Ni-based electrocatalysts for the alkaline hydrogen oxidation reaction are described (an endeavor for the development of anion exchange membrane fuel cells), along with a detailed analysis of the strategies put forward to improve them. It is notably shown that the state of Ni surface (oxidized or reduced) largely determines its electrocatalytic activity. This state of the surface also conveys a pivotal importance regarding the activity of Ni for the oxidation of complex fuels (borohydride, boranes, and hydrazine). Finally, emphasis is made on the durability of Ni-based catalysts in alkaline environments. It is shown that, in such media, the material durability of Ni-based electrodes can be high, but this does not necessarily warrant stable electrocatalytic activity, because of possible deactivation following surface oxide or bulk hydride formation in operation.