NixCo3−xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution

Oxygen evolution reaction (OER) is coupled with a number of important cathodic processes, for instance water splitting for hydrogen production. An effective electrocatalyst can reduce the overpotential and thus enhance the energy efficiency. Therefore extensive research efforts have been invested in developing inexpensive and efficient OER electrocatalysts that have sufficient stability in prolonged exposure to oxidizing conditions. Co3O4 has been demonstrated to have good efficiency and corrosion stability for the OER. It also has much lower cost than RuO2or IrO2-based catalysts. In prior reports of Co3O4, the electrodes were thin films or particle agglomerates bound together by polymers. Comparatively speaking, selfstanding nanowire (NW) arrays should have the advantages of efficient mass transfer and large surface area. To the best of our knowledge, there are still no OER studies of Co3O4 NW arrays. Moreover, despite the fact that Ni doping has been demonstrated to enhance the electrocatalytic efficiency of Co3O4, [5] there is no report on NixCo3 xO4 NWs yet. In this Communication, we report the first synthesis of mesoporous NixCo3 xO4 NW arrays and their electrocatalytic performance in OER. NW arrays grown directly on conductive substrates have several structural advantages: the open space between NWs can facilitate the diffusion of active species; the large surface areas associated with NWs and their mesoporous structures accelerate the surface reaction; and the direct contact of NWs to the underneath conductive substrate ensures each NW to participate in the reaction and also allows their direct use in the electrochemical cells. The NW arrays were grown on Ti foils in an aqueous solution containing Co(NO3)2, via the ammonia-evaporation-induced growth. Different amounts of Ni(NO3)2 were added to the solution in our efforts to tune the Ni-doping level. It is interesting to notice that different Ni(NO3)2 concentrations result in different NW surface roughness as shown in the scanning electron microscopy (SEM) images in Figure 1. Pure Co3O4 NWs are about 400 nm in diameter and 15–20mm in length (Fig. 1A and 1B). They have relatively smooth surfaces. As we increased the ratio of starting Ni(NO3)2 to Co(NO3)2 precursors to 0.5:1.0 and 1.0:1.0, while keeping the total concentration of metal salts constant at 0.2 M, the corresponding NW products (denoted as NCO-1 and NCO-2 respectively) became thicker and rougher (Fig. 1C–F). The spinel crystal structure was maintained after doping, and no