Cactus-like NC/CoxP electrode enables efficient and stable hydrogen evolution for saline water splitting

Abstract

Designing efficient and robust catalysts for hydrogen evolution reaction (HER) is imperative for saline water electrolysis technology. A catalyst composed of CoxP nanowires array with N-doped carbon nanosheets (NC) was fabricated on Ni foam (NF) by an in-situ growth strategy. The material is designated as NC/CoxP@NF. In the preparation process, Co(OH)2 nanowires were transformed into a metal organic framework of cobalt (ZIF-67) on NF by the dissolution-coordination of endogenous Co2+ and 2-methylimidazole. The resulting cactus-like microstructure gives NC/CoxP@NF abundant exposed active sites and ion transport channels, which improve the HER catalytic reaction kinetics. Furthermore, the interconnected alternating nanowires and free-standing nanosheets in NC/CoxP@NF improve its structural stability, and the formation of surface polyanions (phosphate) and a NC nanosheet protective layer improve the anti-corrosive properties of catalysts. Thus, the NC/CoxP@NF has an excellent performance, requiring overpotentials of 107 and 133 mV for HER to achieve 10 mA cm−2 in 1.0 mol L−1 KOH and 1.0 mol L−1 KOH + 0.5 mol L−1 NaCl, respectively. This in-situ transformation strategy is a new way of constructing highly-efficient HER catalysts for saline water electrolysis.

1. Introduction

Currently, the vast and inexhaustible reserves of seawater have made it a more appealing option for large-scale hydrogen production through water splitting[1–4]. Unfortunately, the complex composition of seawater has rendered direct seawater electrolysis with high selectivity and stability a highly challenging endeavour[5–6]. Particularly, the sluggish hydrogen evolution reaction (HER) kinetics significantly compromises the efficiency of seawater electrolysis[7–9].

However, the scarcity and inferior stability severely limits the large-scale applications of Pt-based catalysts in seawater conditions[10–11]. Therefore, developing highly active and robust noble-metal-free electrocatalysts is urgently needed for sustainable hydrogen production in seawater electrolysis[12–13]. Among the various materials, transition metal phosphides (TMPs) demonstrate superior activity for HER due to the easily adjustable electronic structure and improved conductivity. Most importantly, the negatively charged P sites can capture protons and modify the adsorption energy of intermediate products on the catalyst surface to realize accelerated HER kinetics[14–17]. Particularly, the Gibbs free energy (△GH*) of hydrogen adsorption on CoP surface is close to 0, demonstrating the moderate interaction with the intermediates on its surface, making it a promising candidate for efficient HER catalysts[18–19]. However, due to the insufficiency of active sites and poor stability, its catalytic activity and stability must be optimized further to meet the demands for practical application. Some constructive strategies such as morphological engineering[15], electronic structure engineering[20] and protective layer construction[21–23], have been proposed to enhance the electrocatalytic activity and increase tolerance of CoP. Especially, a porous carbon layer with high specific surface area in principle should be able expose more active sites. More importantly, it can protect the components from dissolution by the physical barrier effect[24–26]. Therefore, the combination of phosphides and carbon materials shows advantages in enhancing activity and durability of TMPs-based electrocatalysts[24]. Metal-organic frameworks (MOFs) assembled by metal nodes and organic ligands, featuring high porosity and well-defined and tailorable structures, are considered as ideal candidates for the construction of TMPs.