Defected NiFe layered double hydroxides on N-doped carbon nanotubes as efficient bifunctional electrocatalyst for rechargeable zinc-air batteries

Layered double hydroxides (LDHs) are highly active materials for the oxygen evolution reaction (OER); however, they have low activity for the oxygen reduction reaction (ORR). The surface modification by the defects engineering is a great strategy to boost their bifunctionality for rechargeable Zn-air batteries (ZABs). In this work, NiFe LDHs were synthesized varying the composition (Ni:Fe compositions of 1:1, 2:1 and 3:1), and BET surface areas of 262.3, 165.9 and 79.5 m2 g-1 were found. Despite the differences in BET surface areas, the NiFe LDH 3:1 presented higher activity for the OER, and thus, oxygen vacancies (Ov)-type surface defects were introduced to this LDH. This surface engineering decreased the oxygen atomic percentage from 75.25 to 57.4 %, and as result, the resistance to charge transfer drastically decreased due to electronic modifications, which can be rationalized in terms of the theoretical density of states obtained by density functional theory for a model system including oxygen vacancies. The combination between defected NiFe LDH with N-doped carbon nanotubes allowed to obtain the same current density for ORR than benchmarked Pt/C with a potential difference of 95 mV, and better activity for OER (1.46 V @ 10 mA cm-2). In the ZAB, a cell voltage of 1.44 V was achieved with a power density close to 36 mW cm-2, being this power similar to that of Pt/C.