Electrochemical Synthesis of Ni-Cu-P Alloy As an Efficient Water Splitting Electrocatalyst

Abstract

In recent years, global warming due to increased carbon dioxide emissions has emerged as the largest environmental problem in the world. Hydrogen has been recognized as an eco-friendly energy source that has no carbon dioxide emission, helps to reduce fine dust, thus attracting attention as an eco-friendly energy source which will be commercialized in the future [1,2]. Although the use of hydrogen energy does not emit air pollutants such as carbon dioxide, fossil fuels are currently used in the process of hydrogen production, so that a large amount of carbon dioxide and pollutants are emitted, making hydrogen unlikely to be a truly environmentally friendly energy source. Hydrogen production by water splitting is considered as a possible method to produce a large amount of hydrogen without the emission of pollutants. Water splitting is divided into two reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The most efficiently known catalysts in each reaction are platinum and iridium, respectively, and both are very expensive noble metal catalysts. High cost of the catalysts has been a major obstacle to the application of massive water electrolysis. In recent years, numerous attempts have been made to replace precious metal catalysts with relatively inexpensive transition metals. Nickel and its alloys are widely used as an alternative material for efficient water splitting, owing to their excellent activity and stability [3]. In this study, we developed a technology to make cheap alloy catalysts by electrodeposition and electroless plating with nickel and copper as alloying elements, and apply this catalyst to both oxygen generation and hydrogen generation reaction under alkaline solution. Compared with the use of nickel single metal catalysts, the electronic structure of nickel can be controlled when alloys are synthesized. The increase or decrease of the activity depending on the alloy ratio is observed due to the change of the electronic structure, so that the activity can be expected to be increased through the optimization. Copper has been suggested as one of alloying candidate in this work. Earlier, interaction between nickel and copper in solid solution has been reported with various XRD and XPS studies, which eventually confirmed that catalytic activity change of solid solution is related to the degree of alloying [4,5]. According to previous studies, binding energy of Ni has shifted with addition of copper. Many studies of metal-phosphorous based catalysts already found that phosphorous content causes enhancement of their activities by inducing partial positive charge on active metal centres [6]. Electroplating enables synthesis of metal and metal alloys with relatively low cost. High temperature and high pressure processes are not required for the production of alloys, and electrodeposited alloys are highly compatible for application as electrochemical catalysts. Electroless nickel is a widely used, characterized by excellent uniformity and corrosion resistance. Plating using an autocatalytic reaction enables uniform plating since the precipitated metal acts as a catalyst. If nickel plating using sodium hypophosphite (NaH2PO2) is used, phosphorus alloy is naturally deposited depending on the plating condition, and copper can be added to the solution as an additional alloying element. Most of reported water splitting catalysts have given emphasis on nanostructure holding high surface area. These catalysts usually assume the form of powders including nanostructures, which may require conductive adhesive polymers to be applied on electrode surface. Such process may reduce active surface area and electrical conductivity considerably, resulting in decreased catalytic activity, while electrochemical plating enables production of alloys with excellent adhesion. Through previous research, it has been confirmed that the electrodeposited nickel-copper-phosphorus ternary alloy exhibits excellent water oxidation activity as compared with the nickel-phosphorus alloy, which is already known as promising OER electrocatalyst [7]. This was extended to overall water splitting of the nickel-copper-phosphorus catalyst. Carbon fiber paper (CF) was used as a template of the deposition, which has high surface area compared to film-type substrate. The catalyst on CFP is also adaptable on gas diffusion layer (GDL) on fuel cell device applications. Reference S. Chu and A. Majumdar, Nature 488, 294 (2012).S. Anantharaj, S.R. Ede, K. Sakthikumar, K. Karthick, S. Mishra and S. Kundu, ACS Catal. 6, 8069 (2016).A.J. Bard and M.A. Fox, Acc. Chem. Res. 28, 141 (1995).A.R. Nagahash, T.H. Etsell and S. Xu, Chem. Mater. 18, 2480 (2006).S. Hufner, G. K. Wertheim and J. H. Wernick, Phys. Rev. B, 8, 4511 (1973).L. Stern, L. Feng, F. Song and X. Hu, Energy Environ. Sci. 8, 2347 (2015).B.K. Kim, S.-K. Kim, S. K. Cho and J. J. Kim, Appl. Catal. B, 237, 409 (2018).