Au-NiCo2O4 supported on three-dimensional hierarchical porous graphene-like material for highly effective oxygen evolution reaction

A three-dimensional hierarchical porous graphene-like (3D HPG) material was synthesized by a one-step ion-exchange/activation combination method using a cheap metal ion exchanged resin as carbon precursor. The 3D HPG material as support for Au-NiCo2O4 gives good activity and stability for oxygen evolution reaction (OER). The 3D HPG material is induced into NiCo2O4 as conductive support to increase the specific area and improve the poor conductivity of NiCo2O4. The activity of and stability of NiCo2O4 significantly are enhanced by a small amount of Au for OER. Au is a highly electronegative metal and acts as an electron adsorbate, which is believed to facilitate to generate and stabilize Co4+ and Ni3+ cations as the active centres for the OER.

Electrochemical hydrogen evolution from water splitting by coupling renewable energy devices such as wind energy and solar energy with water electrolysis has attracted more and more attention in alkaline media due to continuous consumption of fossil fuels and ever-increasing environmental problems 1 . Hydrogen can be used as a fuel to get a reliable power for almost every application that fossil fuels are used. The hydrogen produced by electrolysis can be used for methanation of CO 2 , combustion processes, and conversion back into electricity by fuel cells 2 . In alkaline media, electrochemical water electrolysis consists of two half-reactions: the cathodic hydrogen evolution reaction (HER, 2H 2 O + 2e = 2OH − + H 2 ) and the anodic oxygen evolution reaction (OER, 4OH − = 2H 2 O + 4e + O 2 ). Of two half-reactions, the OER requires to form two oxygen-oxygen bonds in the four-electron redox processes by transfer protons and electrons, which results in more kinetically demand for the OER 3,4 . So, the OER needs relatively high overpotential at the anode, which is a major cause of high energy consumption. Thus, a lot of efforts have been devoted to explore the electrocatalysts with low OER overpotential.
The rutiletype oxides of RuO 2 and IrO 2 show the lowest OER overpotential, however theses oxides suffer from poor chemical stability in alkaline media and the high price and limited supply of Ru and Ir 5,6 . So other metal oxides such as Cu oxide 7 , Mn oxide 8 have been developed. Among of all the oxide catalysts, particular attention has been paid to the cobalt oxide 9,10 and nickel oxide 11,12 , due to their high abundance, low cost, small overpotential and fast kinetics of the OER. Many researchers have studied the other oxides to enhance the performance of OER for Ni oxide 13,14 and Co oxide 15,16 . Trotochaud and his cooperators have reported that the conductivity of Ni oxide shows a > 30-fold increase with Fe oxide addition 13 . On the other hand, the presence of Fe alters the redox properties of Ni, causing a positive shift at the potential of Ni(OH) 2 /NiOOH redox reaction, a decrease in the average oxidation state of the Ni sites, and a concurrent increase in the activity of Ni cations for the OER 14 . The electrocatalytic synergism of mixed oxides of Co and Ni has been studied by many researchers 17,18 30 . Conductivity is an importance index for designing and developing available electrocatalysts for OER. With metallic conducting property of a conductivity of 10 −4 S cm −1 , RuO 2 and IrO 2 give the best OER activity 31 . However, many oxides such as NiCo 2 O 4 suffer from low electrical conductivity. So, how to improve their poor intrinsic conductivity is still challenging for oxides. Therefore, carbon materials such as carbon nanotubes have been induced into oxides to improve the electrical conductivity [32][33][34] . Currently, graphene-based carbon materials including monolayer and multilayers nanosheets are highly promising materials as the new-generation supporting materials for electrocatalysts, owing to their high specific surface area, high electrical conductivity, and outstanding chemical and electrochemical stability 35 19 and CuCo 2 O 4 24 has been reported for OER. Long and his cooperators reported that a synergy between the catalytic activity of the FeNi oxide and the enhanced electron transport arising from the graphene results in superior electrocatalytic properties for the OER 39 . The graphene supported NiCo 2 O 4 has been reported for OER [41][42][43] . Zhao and coworkers have prepared an active catalyst composed of porous graphene and cobalt oxide (PGE-CoO), which has demonstrated high porosity, large specific surface area and fast charge transport kinetics 37 . The catalyst also exhibits excellent electrochemical performance towards OER with a low onset potential and high catalytic current density. The enhanced catalytic activity could be ascribed to porous structure, high electroactive surface area and strong chemical coupling between graphene and CoO nanoparticles. Moreover, this OER catalyst also shows good stability in the alkaline solution. The high performance and strong durability suggest that the porous structured composite is favorable and promising for water splitting. However, the intrinsic hydrophobic properties of graphitized basal plane structures cause a great difficulty in uniformly loading metal nanoparticles on the surface of graphene. Though the hydrophilicity of reduced graphene oxide (RGO) could be improved via introducing oxygen functional groups, their electronic conductivity is still insufficient due to their partly restored graphitic structures. Based on this fact, it is fundamental interest to develop the novel graphene-based carbon materials with high specific surface area, high electronic conductivity as well as strong affinity to foreign constituents, beyond the continuous development of hybrid architectures for electronics and various electrochemical systems 35 . Shen and coworkers have developed a novel active three-dimensional hierarchical porous graphene-like (3D HPG) material with hierarchical pores synthesized through an efficient ion-exchange-assisted synthesis route 44 . The 3D HPG material shows high electronic conductivity and strong cohesive force and distribution effects toward the catalyst nanoparticles 45 . The 3D HPG material can provide a highly conductive structure in conjunction with a large surface area to contact the MnO 2 nanoparticles and effectively enhance the mechanical strength of the composite during volume changes as well as suppress the aggregation of MnO 2 nanoparticles during Li-ion insertion/extraction 46 .
In recent years, a lot of efforts have been made to enhance the electrocatalytic activity catalysts and several strategies have been proposed. Among them, bifunctional mechanism, modified with highly electronegative metals, such as Pt 47 , Pd 48 and Ru 49 , has been demonstrated as one of the most effective methods to improve the electrocatalytic efficiency. However, the high price and limited supply of Pt, Pd and Ru are major barriers to the development of OER catalysts using Pt-based, Pd-based and Ru-based catalysts. Scientists have pained more attention to Au because it is much more abundant and more available than Pt, Pd and Ru on the earth. Gold has been used to enhance the oxide activity of OER such as Co oxide [50][51][52][53] , Mn oxide 54,55 . A small amount of Au nanoparticles (<5%) in α-MnO 2 /Au catalysts significantly improves the catalytic activity up to 6 times compared with the activity of pure α-MnO 2 for OER 54 . Bell and coworkers have developed noble metal-supported cobalt oxide and found that the OER activity of cobalt oxide deposited on Au is nearly three times higher than that of bulk Ir 51 . The Au/NiCo 2 O 4 nanoarrays exhibit excellent OER activity, which is almost four times higher than that of Ir/C 56 .
In this paper, we focused on the development of high performance Au/NiCo 2 O 4 catalysts supported on the 3D HPG material for the OER. It is widely accepted that 3D HPG is an outstanding matrix as support material with high electrical conductivity, good electrochemical stability, controllable specific surface areas as well as pore structure 46,57 .

Results
The morphology of the 3D HPG was characterized by scanning electron microscopy (SEM) as shown in Fig. 1a,b. The Fig. 1a shows a 3D interconnected porous structure with well-developed open macropores. The magnified SEM (Fig. 1b) exhibits sub-micrometer-sized pores. The thickness of the carbon sheet is about 6 nm. The degree of crystallinity of the HPG was characterized by Raman spectrum as shown in Fig. 1c. The G band peaked at 1578 cm −1 is related to the in-plane bond-stretching motion of the pairs of carbon sp 2 atoms, which indicates the presence of crystalline graphene layers. The D band peak at 1329 cm −1 is assigned to disordered carbon and highly sensitive to graphitic defects within the graphite layers 58 . When the ratio of the peak intensity of D band to that of G band (I D /I G ) is smaller, the degree of crystallinity will be higher 59 Fig. 2c can be assigned to the Ni2p 3/2 and Ni2p 1/2 , which can be assigned to Ni 2+ . The binding energy values of XPS spectrum of Co 2p are 780.1 eV and 795.9 eV for Au-NiCo 2 O 4 (wt 1:5)/HPG as shown in Fig. 2d can be assigned to the Co 2p 3/2 and Co2p 1/2 , which can be assigned to Co 3+ . The relatively narrow peak width, the 2p 3/2 to 2p 1/2 separation of 15.9 eV, and the absence of any shake-up peak all reveal that no Co 2+ exists in the NiCo 2 O 4 phase 56 . It means that the Co cation is composed of lots of Co 3+ .
The typical high-resolution transmission electron microscopy (HRTEM) images of the Au-NiCo 2 O 4 (wt 1:5)/ HPG are shown in Fig. 3. It can be observed that the catalyst particles are well dispersed on the surface of HPG with a narrow size distribution of 3-8 nm (Fig. 3a) and the average particle size is around 5.9 nm (Fig. 3b). The parallel fringe with a spacing of 0.203 nm is corresponding to the (400) plane of NiCo 2 O 4 and the parallel fringe with a spacing of 0.235 nm is corresponding to the (111) plane of Au as shown in Fig. 3c,d. The nanoparticles of NiCo 2 O 4 and Au contact each other or exit in one particle. The element mapping images (Fig. 4) show homogeneous C (Fig. 4a), Au (Fig. 4b), Ni (Fig. 4c), Co (Fig. 4d) and O (Fig. 4e) distributions in the Au-NiCo 2 O 4 (wt 1:5)/HPG (Fig. 3a) . The element mapping images also shows that the catalyst particles are well dispersed on the surface of HPG.
In order to illustrate the advantages of the Au-   The stability of OER on the all electrodes is investigated by chronoamperometry. The chronoamperometry curves for OER in 0.1 mol L −1 KOH solution under a potential of 0.7 V was shown in Fig. 6. The current has a wave because the oxygen evolution on the NiCo 2 O 4 /HPG and Au-NiCo 2 O 4 (wt 1:5)/HPG electrodes, which also exhibits that NiCo 2 O 4 /HPG and Au-NiCo 2 O 4 (wt 1:5)/HPG catalysts have good activity for OER. When the OER happens, the oxygen bubbles will form on the surface of electrode and block the active sites on the surface of electrode, then the current of water oxidation will decrease. As the reaction is proceeding, oxygen bubbles grow up gradually, and the current of water oxidation also decreases. When the oxygen bubbles grow up enough to separate from the surface of electrode, the current of water oxidation will increase significantly due to the active sites on the surface of electrode are released for further electrochemical reaction. Until the end of the experiment, the oxidation current density on the Au-NiCo 2 O 4 (wt 1:5)/HPG electrode is 2.2 mA cm −2 , which is 1.7 times as bigger as that on the NiCo 2 O 4 /HPG electrode (1.3 mA cm −2 ). The result shows that OER on the Au-NiCo 2 O 4 (wt 1:5)/ HPG electrode has a higher current density than that on the NiCo 2 O 4 /HPG electrode with the same potential.  For further understanding of the intrinsic reaction of the OER performance on the Au-NiCo 2 O 4 (wt 1:5)/HPG catalyst, the XPS data of pure NiCo 2 O 4 /HPG and Au-NiCo 2 O 4 (wt 1:5)/HPG catalysts were compared as shown in Fig. 2c,d. The XPS data show that the binding energy value of Ni 2p has a 0.2~0.4 eV positively shift and that of Co 2p has a 1.1~1.6 eV positively shift after loading with Au. Au is a highly electronegative metal and acts as an electron adsorbate, which generates and stabilizes cobalt and nickel ions at higher oxidation states (e.g. Co 4+ and Ni 3+ ). Such a big shift in binding energy will promote the formation of Co 4+ and Ni 3+ cations. A general understanding is that the Co 4+ and Ni 3+ cations are the active centres for the OER. The presence of strong electrophilic Co 4+ and Ni 3+ cations will accelerate to form the OOH via nucleophilic reaction with O 62 . Depend on electrochemical oxidation, progressive oxidation from Co 3+ to Co 4+ and Ni 2+ to Ni 3+ is supposed as rate-limiting step, so the increased population of Co 4+ and Ni 3+ cations results in enhanced OER performance. Similar results were reported in OER using metal oxides (Co and Ni) with noble metals, where the noble metals generate and stabilize metal ions at higher oxidation states (e.g. Co 4+ and Ni 3+ ). Casella and his cooperators 63 and Yeo and his cooperators 64 demonstrated that the growth of Ni hydroxide on a gold electrode favors the oxide states of Ni 3+ over Ni 2+ . Yeo and his cooperators also noted that the cobalt oxide deposited on Au electrodes exhibits a high occurrence of Co 4+ species on the surface 52 . The enhanced activity was correlated to the electronegativity of noble metals.
In conclusion, a 3D HPG material was synthesized by a one-step ion-exchange/activation combination method using a cheap metal ion exchanged resin as carbon precursor. The 3D HPG material as support for Au-NiCo 2 O 4 gives good activity and stability for OER. The 3D HPG material is induced into NiCo 2 O 4 as conductive support to increase the specific area and improve the poor conductivity of NiCo 2 O 4 . The activity of and stability of NiCo 2 O 4 significantly are enhanced by a small amount of Au for OER. The Au-NiCo 2 O 4 (wt 1:5)/HPG shows the highest activity for OER. The value of E onset on the Au-NiCo 2 O 4 (wt 1:5)/HPG electrode is 8 and 77 mV lower than that on the NiCo 2 O 4 /HPG and Au/HPG electrodes. The values of j 0.7V are 1.4, 6.8 and 9.1 mA cm −2 on the Au/HPG, NiCo 2 O 4 /HPG and Au-NiCo 2 O 4 (wt 1:5)/HPG electrodes. Benefiting from the synergistic effect, the as-prepared Au-NiCo 2 O 4 /HPG catalyst shows significantly higher activity and better stability than NiCo 2 O 4 / HPG catalyst. The XPS data show that the binding energy value of Ni 2p has a 0.2~0.4 eV positively shift and that of Co 2p has a 1.1~1.6 eV positively shift after loading with Au. Au is a highly electronegative metal and acts as an electron adsorbate, which is believed to facilitate to generate and stabilize Co 4+ and Ni 3+ cations as the active centres for the OER.

Methods
Materials synthesis. The 3D HPG was synthesized by a one-step ion-exchange/activation combination method using a cheap metal ion exchanged resin as carbon precursor according to the Li method 44 . Firstly, the pretreated macroporous acrylic type cation exchange resin was impregnated with targeting ions of nickles in 0.05 mol L −1 nickel acetate solution for 6 h. Secondly, the exchange resin was washed with deionized water and dried at 333 K for 12 h. And then the exchanged resin was added into a 400 mL KOH/ethanol solution which contained 40 g KOH and stirred at 353 K for 6 h. After that, the mixture solution was dried at 343 K for 48 h and smashed by a disintegrator. Finally, the mixture was heated at 1123 K for 2 h in N 2 atmosphere. When the resulted sample cooled down to room temperature, 3 mol L −1 HCl solution was added in it with a specific volume for more than 12 h with magnetic stirring. After that, the sample was repeated washed until the pH value was 7 and dried at 343 K. NiCo 2 O 4 /HPG was prepared through a typical heterogeneous reaction method. 1 mmol of Ni(NO 3 ) 2 ·6H 2 O, 2 mmol of Co(NO 3 ) 2 ·6H 2 O and 0.4811 g HPG were added into H 2 O (40 mL), followed by the addition of 5 mmol NH 4 F and 12 mmol urea. After being stirred for 1 h, the obtained mixture was transferred to a Teflon-lined stainless steel autoclave and heated to 393 K for 6 h. The resultant precipitate was washed several times with deionized water until the pH of the filtrate became 7 before being dried in a vacuum oven for 12 h. Finally the obtained powder was then annealed at 673 K for 2 h in air. The Au-NiCo 2 O 4 /HPG electrocatalysts were prepared by reduction of HAuCl 4 solution on the NiCo 2 O 4 /HPG powders using an excess 0.01 mol L −1 NaBH 4 solution. Electrode preparation. The electrocatalyst powders were dispersed in deionized water with 5 wt% PTFE (polytetrafluoroethylene) on the surface of a graphite rod with a geometric area of 0.33 cm 2 . The loading of carbon black and PTFE on the electrodes was accurately controlled at 0.23 mg cm −2 and 0.1 mg cm −2 . The total loading of amount of Au and NiCo 2 O 4 in the catalysts on the electrodes was accurately controlled at 0.1 mg cm −2 .
Characterization. XRD was carried out using a Panalytical X'Pert powder X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). SEM images were obtained using a Quanta 400 FEG microscope (FEI Company). Transmission electron microscopy (TEM) images were carried out on a JEOL JEM-2010 (JEOL Ltd.). Raman spectroscopic measurements were carried out on a Raman spectrometer (Renishaw Corp., UK) using a He-Ne laser with a wavelength of 514.5 nm. XPS measurements were performed in an ESCALAB 250 spectrometer under vacuum (about 2 × 10 −9 mbar). All electrochemical measurements were carried out in 0.1 mol L −1 KOH solution by using the solartron 1287 electrochemical work station using a standard three-electrode cell at 298 K. Solutions were freshly prepared before each experiment. A platinum foil (3.0 cm 2 ) was used as counter electrode. All the potentials were measured versus a saturated calomel electrode (SCE, 0.241 V versus SHE) electrode. A salt bridge was used between the cell and the reference electrode.