Interfacing and nanostructuring nickel metal for hydrogen evolution electrocatalysis in alkaline media

Abstract

Hydrogen fuel acts as one of the most promising alternatives to petroleum fuels owning to its high gravimetric energy density and environmental friendliness. Hydrogen evolution reaction (HER), a half reaction of water splitting, plays a critical role in producing high purity hydrogen fuels. Highly active and robust electrocatalysts are required to decrease the overpotential, speed up the reaction rate, and thus to minimize the overall energy consumption. In order to couple HER with OER to realize overall water splitting in the same pH range, robust electrocatalyst is needed for high efficient alkaline electrocatalysis. However, the activity of electrocatalyst in alkaline medium is usually two to three orders of magnitude lower than that in acidic medium. Thus, it is still a challenging issue to design earth-abundant electrocatalysts with low cost, high activity and long-term stability and for HER from water splitting in alkaline solutions. Electrocatalytic property of Nickel was discovered almost a century ago, and over the years, we have witnessed a continuous and rapid development in optimizing Ni-based electrocatalysts. My thesis focuses on design of facile approaches for Ni material modification by interfacing and nanostructuring of Ni metal, aiming to increase the intrinsic activity as well as the active sites of Ni material for high-efficiency hydrogen production. This thesis contains 5 chapters. Chapter 1 mainly introduces the development of electrocatalysts for HER and the objects in my thesis. Chapter 2 introduces the experimental techniques employed in my research. Chapter 3 and 4 focus on the findings in my study. The conclusion and outlook is shown in chapter 5. In chapter 3, we report a facile synthesis route of three-dimensional porous Ni/Ni₃S₂ nano-interfaces on oxidized carbon cloth for HER in alkaline solution. This unique structure exposes a high proportion of Ni/Ni₃S₂ hetero-interface to electrolyte, creating a synergetic effect between Ni and Ni₃S₂ for enhancing HER. The synergetic effect at the interface was verified by DFT calculation, which consists in an interface-assisted heterolytic splitting of H₂O into OH^- and H⁺, and the subsequent expeditious H₂-forming reaction owing to the weakened binding between Ni and H induced by the neighboring Ni₃S₂. The resulting porous network shows a high HER activity in alkaline media, reaching 10 mA/cm² at 95 mV with a tafel slope of 66 mV/dec, which is much smaller than that of nickel metal currently being used in industry. In chapter 4, We devised a facile electrodeposition strategy for growing Ni/MoO_x heterostructure network on conductive substrate. By constructing Ni/MoOx interface, the overpotential decrease to 122 mV at current density of 10 mA/cm², 62 mV lower than that of Ni metal. We also found that Cu could greatly increase the activity of Ni/MoO_x system as substrate. Comparing catalysts grown on Ni substrate and Cu substrate, the Ni/MoO_x with Cu support manifests a factor of 3 activity increase in catalysing HER, the overpotential further dropped to 42 mV at current density of 10 mA/cm². In addition to the synergetic effect between Ni and MoOx, we propose the dramatic decrease of overpotential might be caused by Cu substrate, which might probably play a role in optimization of hydrogen adsorption and desorption energy. Chapter 5 gives the summary of my thesis and some outlook about the modification of Ni-based mateirial in the future.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology