Superaerophobic P-doped Ni(OH)2/NiMoO4 hierarchical nanosheet arrays grown on Ni foam for electrocatalytic overall water splitting

Abstract

Transition metal (TM) oxides and hydroxides are one of the important candidates for the development of durable and low-cost electrocatalysts towards water splitting. The key issue is exploring effective methods to improve their electrocatalytic activity. Herein, we report a new type of P-doped Ni(OH)₂/NiMoO₄ hierarchical nanosheet array (abbr. P-Ni(OH)₂/NiMoO₄) grown on Ni foam (NF), which can act as a highly efficient electrocatalyst towards overall water splitting. Such a composite was obtained by a three-step preparation process. In the first two hydrothermal reactions, the crystalline Ni(OH)₂ hierarchical nanosheet arrays were grown on NF and then the low crystallinity NiMoO₄ was grafted on the Ni(OH)₂ nanosheets. In the third phosphorization step, P element was doped into the composite Ni(OH)₂/NiMoO₄. Electrocatalytic experiments show that P-Ni(OH)₂/NiMoO₄ possesses a smaller overpotential (60 mV) and lower Tafel slope (130 mV dec⁻¹) toward HER in 1 M KOH. When it was employed as an integrated water splitting catalyst, only a potential of 1.55 V was required to achieve a current density of 10 mA cm⁻². This catalytic activity is even better than those of electrolyzers constructed with noble metals Pt/C∥IrO₂. The superior electrocatalytic performance of P-Ni(OH)₂/NiMoO₄ can be attributed to the high quality of crystalline Ni(OH)₂ nanosheet arrays grown on NF, which dramatically improve the conductivity. Furthermore, the hierarchical structure not only increases the surface area and exposes more catalytically active sites, but also provides a superaerophobic surface, which helps to accelerate the release of generated bubbles. Moreover, the synergistic effects between P-Ni(OH)₂ and P-NiMoO₄ efficiently promote the HER and OER processes also. This work may suggest new a way to explore TM oxide/hydroxide-based durable electrocatalysts with highly efficient electrocatalytic activities towards overall water splitting.