Copper facilitated nickel oxy-hydroxide films as efficient synergistic oxygen evolution electrocatalyst

Highlights

• Mixed copper-nickel mixed oxides film deposited using cost effective method chemical solution deposition method.

• Present in 1:1 ratio gives the best electrocatalytic efficiency for water oxidation.

• Copper facilitates the formation of NiOOH resulting in improved efficiency.

Abstract

Efficient catalysts made of cheap and abundant metal ions are in need to overcome the sluggish kinetics of the anodic water oxidation reaction. The development of an inexpensive catalyst with improved performance is key to produce hydrogen by the electrolytic water splitting reaction. The user friendly chemical solution deposition method was applied to prepare films with predecided mole concentrations of copper and nickel. The mixed metal oxide with equal mole ratio of Cu:Ni (1:1) achieved the maximum activity with the onset of water oxidation at overpotential of 0.40 V (1.63 V vs RHE) and achieved the current density of 1 mA/cm² at an overpotential of 0.420 V at pH 13. The activity of this combination is attributed to the copper facilitated in-situ formation of a layered nickel oxy-hydroxide structure. The presence of both metal ions was found to be necessary indicating a synergy between Cu and Ni for the oxygen evolution reaction. The present work representing the simple synthesis process of the catalyst with the improved water oxidation activity is a promising step to develop electrodes for water electrolysis.

Introduction

Hydrogen produced by electrochemical water splitting provides a promising path to produce a clean energy source. Molecular hydrogen has the potential to replace the global energy dependence from the traditional fossil fuels to sustainable and clean energy source. For the large scale production of hydrogen, the key factor is the efficiency [1]. The electrochemical water splitting process involves the oxygen evolution reaction at the anode and the hydrogen production reaction at the cathode [1], [2], [3], [4], [5], [6]. Although, in this process, the hydrogen evolution reaction is of major interest, it is the anodic oxygen evolution reaction that hampers the efficiency of the overall water splitting reaction. The water oxidation reaction is a 4e⁻-4H⁺ transfer process that involves the OH bond breaking and formation of the OO bond [4], [5]. This process is kinetically hindered and energetically demanding thereby adversely affecting the efficiency of overall electrolytic process. This challenge can be overcome by developing appropriate catalysts that can drive the overall water splitting process at lower overpotentials [2], [4], [5], [6], [7], [8], [9]. Currently, precious metal ions like Ruthenium and Iridium oxides are the best known water oxidation catalysts, but they are expensive and also present in limited amount [1]. Efforts are in progress to develop catalyst from first row 3d transition series metal ions specifically Co, Ni, Mn and Fe [9], [10], [11], [12], [13], [14], [15], [16], [17]. Though significant progress has been made, still substantial work needs to be done to develop the catalyst that not only be made of earth abundant metal ions but should also have the improved activity.

Among the first row transition series metal ions copper based catalysts have been known to play an important role in various biological oxidation reactions [18], [19], [20], [21]. Compared to other 3d transition series metal ions copper based electrocatalysts are still not so well explored for the oxygen evolution reaction. Meyer et al. extensively developed copper based complexes for the oxygen evolution reaction in a homogenous medium [22], [23], [24]. Homogeneous catalysis is very useful for the fundamental studies but not favorable for the large scale production of hydrogen. Liu and group electrodeposited copper oxide films using inorganic complexes and metal salts for the electrocatalysis application [25], [26], [27], [28]. Yu et al. demonstrated copper oxide films electrochemically deposited using a borate buffer as efficient electrocatalyst achieving the typical current density of 1 mA/cm² at an over potential of 0.60 V (pH = 9) [29]. Electrocatalytic water oxidation by CuO has shown encouraging results, with pure CuO demonstrated to achieve a current density of 1 mA/cm² at over potentials from 0.43 mV to 0.78 mV under different testing conditions used. Efforts have been made to develop mixed copper oxides for efficient electrocatalytic performance. Wang and group [14], reported enhanced catalytic performance of copper in presence of nickel. In another report by Kibria et al. [15] studied the electrochemical activity of nickel copper electrode for the water oxidation reaction. Zheng et al. [16] developed nickel-copper based bimetallic organic framework nanosheets as an efficient catalyst for the oxygen evolution reaction. The MOF was synthesized using hydrothermal process. In a similar effort by Sun and group [17], trimetallic metal/metal oxide electrode containing nickel-iron-copper was developed that showed efficient electrocatalytic performance. McCrory and group [30] developed a series of metal oxide based catalyst. They reported NiCuO_x film to achieve 10 mA/cm² current density at overpotential of 0.40 V. Though, almost all of the work is performed on electrodeposited films. Electrodeposition is a user friendly and cost effective method but often suffers from films with poor reproducibility and non-uniform structure [30], [31]. Beside this, it is very difficult to produce films with different composition.

It has been reported that mixed oxides possess higher catalytic activity compared to respective single component oxides. Mixed oxidation states of the cations may increase the electrical conductivity that enhances the adsorption of the reactant and thereby facilitate the catalytic activity [32]. Recently significant progress has been made in the field of perovskites for the oxygen evolution application [33], [34]. Though perovskites are prepared in powder form and then tested for electrocatalytic application using an additive such as Nafion. The drawback of using these additives is their instability under anodic conditions. Another drawback is the difficulty in developing large surface area electrodes for the water splitting using these additives [35], [37], [38], [39]. These drawbacks can be overcome by using the chemical solution deposition (CSD) method, which is not only a cost effective and user friendly technique but it also results in the deposition of a homogenous film with accurate composition (multiple components) [40]. Also the deposition can be carried out on a range of substrates with good reproducibility [31], [41]. Recently, Bick et al developed Pr_0.5Ba_0.5Co_3-d based perovskites films for the electrocatalytic application using spin coating as deposition method [42], [43], [44]. Among the first row transition series metal ion Ni containing mixed metal oxides and perovskites have demonstrated impressive performance for the electrocatalytic oxygen evolution reaction [12], [14], [15], [30], [35]. The emerging results on the copper oxide and the known promising catalytic response from nickel containing catalysts prompted us to investigate the effect of mixing copper and nickel in different ratios and investigate the electrocatalytic response of the mixed oxides.

Herein, we report a facile CSD method for the copper and copper-nickel mixed oxides for the electrocatalytic oxygen evolution reaction. It was found that presence of both metal ions in equal proportion results in maximum activity. These results indicated that both of the metal ions show the synergistic effect during water oxidation reaction. Interestingly the pure copper oxide deposited using this simple method results in achieving the current density of 1 mA/cm² (pH = 13) at an over potential of 0.540 V which further reduces to 0.420 V in presence of nickel. This study on the copper-nickel mixed oxides will be important for further development of copper based catalysts for the oxygen evolution reaction.