稀有金属(英文版) 2015,34(11),802-807

收稿日期：2 June 2015

基金：financially supported by the National Natural Science Foundation of China (No. 51204022);the Special Foundation for Institute of Technology Research and Development of China (No. 2014EG115002);the Program of International S&T Cooperation of China (No. 2011DFA51840);

Optimization of electrocatalytic properties of NiMoCo foam electrode for water electrolysis by post-treatment processing

Jian-Wei Wang Yue-Fa Wang Jing-Guo Zhang Yan-Lin Yu Ge-Ge Zhou Lei Cheng Lin-Shan Wang Z.Zak Fang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

GRIPM Advanced Materials Co., Ltd, General Research Institute for Nonferrous Metals

Department of Metallurgical Engineering, University of Utah,Salt Lake City

Abstract：

Hydrogen is a potential alternative to fossil fuels in coping with the increased global energy demand,and water electrolysis is an attractive approach for H₂ production. Nickel–molybdenum–cobalt(Ni Mo Co) foam electrodes used for water electrolysis were prepared by the electrodeposition method, and the influence of heat treatments on the surface structure of Ni Mo Co foam electrodes,mechanical properties, and electrochemical performance of the synthesized electrodes was investigated in order to optimize the post-treatment processes. The residual carbon in the surface of the electrode was removed by decarbonization in the atmospheric condition. The carbon content decreases to lower than 200 ×10^-6when the temperature exceeds 500 °C. Next, the material is reduced in hydrogen atmosphere from 500 to 1100 °C to remove the surface oxides. As the temperature increases, the surface molybdenum content increases significantly between 500 and 800 °C, the surface grains become coarser, and the tensile strength and elongation increase as well. The lowest polarization overpotential is obtained at 800 °C. Below 800 °C, the electrode is only partially reduced and some black oxide zones are observed on the electrode surface,which leads to the higher polarization overpotential. Thesamples heat-treated at the temperatures of higher than 800 °C show better strength and toughness as well as brighter appearance. However, the surface particle coarsening leads to a decrease in the specific surface area and a higher overpotential.

Keyword：

Water electrolysis; Foam electrode; Ni Mo Co; Post-treatment; Overpotential;

Author： Jian-Wei Wang e-mail: jswjw@sina.com;

Received： 2 June 2015

1 Introduction

Today’s energy consumption primarily relies on fossil fuels. It is projected that the global energy consumption will increase at least twofold by midcentury relative to the level at the beginning of this century [1]. Dependence on fossil fuels causes not only potentially environmental issues, but also the issues of the depletion of conventional energy sources and the sustainable development of the economy. So in the twenty-first century, the most serious challenge is to secure the supply of clean and sustainable energy.

Hydrogen is a potential alternative to fossil fuels in coping with the increased global energy demand. Hydrogen is the most abundant element on the earth and exists in many different sources, such as water, biomass, and fossil fuels. The molecular form H₂is an environmentally attractive fuel. When it is burned or used in fuel cells, it produces only water. However, hydrogen is not a direct energy source, and it requires energy to unbound hydrogen from the other substances or compounds.

There are a variety of processes that can be employed on the hydrogen production. Currently, most hydrogen is produced from reforming natural gas [2]. The disadvan- tages of this process include that natural gas is a fossil fuel and it generates CO₂during the reforming. Among dif- ferent H₂production processes, electrolysis of water is very attractive because it is technologically very simple and does not create any harmful by-products, including no carbon dioxide emission [2, 3]. This process of hydrogen production is also sustainable since approximately 70 % of the earth’s surface is covered with water. The major drawback to this technology is the cost of electricity [4]. However, this technology can be used in conjunction with renewable energy sources [1].

The actual applied voltage to conduct water electrolysis consists of mainly three components: reversible potential, overpotential on electrodes, and Ohmic loss [5, 6]. The reversible potential is determined by the reaction condi- tions, and 1.23 e V is the minimum voltage necessary for water electrolysis under standard conditions (298 K and 0.101325 MPa). When the electrode is polarizable, extra energy is needed to drive the chemical reaction of elec- trolysis, and this extra part is the overpotential of the electrode. Ohmic loss is caused by the electric resistances of electrolyte, electrode, and circuitry. Both the overpo- tential and the Ohmic loss are a function of current density. Electrodes with highly active electrocatalysis can reduce the overpotential and increase the efficiency of water electrolysis. Therefore, developing cheap and effective electrode materials for hydrogen evolution has been the focus of many reported studies [7–11]. Among various materials, Ni and its alloys show a high electrocatalytic activity toward the reaction of hydrogen evolution [7–9, 11]. The performance can also be improved by increasing the electrode surface area [9, 10]. Therefore, various methods [7, 9–11] were employed to fabricate porous electrodes for water electrolysis. There are extensively investigations on electrodeposition and electrocatalytic properties of metal species toward hydrogen evolution reaction (HER) [12, 13], such as Ni–Mo [14], Ni–Co [15], and Ni–Mo–Co. The ternary Ni Mo Co electrode with spe- cial foam structure shows better performances for water electrolysis and is accepted widely. However, the effects on the electrolytic efficiency of Ni Mo Co foam electrode is very complex and still not fully explained, not only in the electrodeposition preparation process, but also during the post-treatment.

In the present work, the Ni Mo Co foam electrodes pre- pared by the two-step electrodeposition method were experimentally investigated. The purpose is to optimize the properties of the synthesized electrodes by examining the influence of heat treatments on the surface structure and composition, mechanical properties, and electrochemical performance. The electrode samples were first decarburized in atmospheric conditions to remove the residual carbon and then reduced in hydrogen atmosphere to remove the surface oxides. Based on the examination of polarization overpo- tential, mechanical properties, and surface morphology, the post-treatment process was optimized.

2 Experimental

The Ni Mo Co alloy foam electrodes with the designed composition of Ni-20.8 wt%Mo-1.6 wt%Co were prepared by a two-step electrodeposition method. In the first step, pure Ni (99.99 %) was deposited on the prepared poly- urethane foams by electrodeposition approach with a low current density of 2 A?dm^-2. This step was to coat a thin layer of Ni on the polyurethane foam surfaces and to make polyurethane foam substrate conductive. In the second step, Ni, Mo, and Co were electrodeposited on the poly- urethane foam prepared in the first step in a mixed solution of Ni, Mo, and Co salt to form the targeted alloy. The current density for this electrodeposition was 8 A?dm^-2. The thickness of Ni Mo Co layer was controlled by both the current density and the deposition time. The electrodes prepared were washed and dried for the post-treatment processes.

The electrodes were first treated in the muffle furnace at atmospheric conditions to remove the polyurethane foam substrate. This decarbonization treatment was processed at the temperature range of 200–1000 °C for 10 h to inves- tigate the best condition in which no residual carbon was left on the electrode. The content of carbon on the electrode was measured by high-frequency burning infrared absorp- tion spectrum method. Next, the electrodes were treated in hydrogen atmosphere from 500 to 1100 °C to reduce and remove the surface oxides produced during the decarbur- ization processing.

The electrode surface at different temperatures was observed by Carl Zeiss Axiocert 200MAT optical micro- scope (OM) for large-scale analysis. The surface mor- phology of the foam alloy electrodes was analyzed using JSM7100F scanning electron microscope (SEM). The ele- mental compositions of the electrodes before and after the post-treatment were examined by Genesis 6.0 energy-dis- persive spectroscopy (EDS). The electrochemical mea- surements were conducted in the H-type conventional three-electrode glass cell of CHI760b electrochemical working shop. The Ni Mo Co foam electrode was used as the working electrode (WE). All potentials are referred to the Hg/Hg O electrode. Furthermore, a large platinum plate was used as the counter electrode (CE). The electrode was measured in alkaline solution with 1 mol?L^-1KOH (ana- lytical grade) in triply distilled water.

3 Results and discussion

Figure 1 shows the carbon content in the electrode as a function of decarbonized treatment temperature. The car- bon content quickly decreases as the treatment temperature increases. When the decarbonized temperature reaches 500 °C, the carbon remained in the electrode is as low as 200 9 10^-6and maintains at a level below 200 9 10^-6at the decarbonized temperatures beyond 500 °C. It is con- sidered that the polyurethane foam substrate is totally removed when the carbon content is lower than 200 9 10^-6. In this work, the decarbonized treatment temperature of 550–600 °C was chosen to ensure that the polyurethane foam substrate is completely removed.

Since the decarburization process is done in atmospheric conditions, it is inevitable to produce some oxides on the surface of the electrode. So the electrode samples are reduced in hydrogen at various temperatures. The element contents on the electrode surface are shown in Fig. 2. It can be clearly seen that Ni and Mo contents on the surface change significantly with the temperature, but Co content only changes a little amount. Mo content on the surface gradually increases at the reduction temperatures of below 600 °C and quickly increases at the temperatures of above 600 °C. Mo content eventually reaches a maximum of *40 wt% at the temperature of 800–900 °C and then starts to decrease slightly above 1000 °C due to the atom exchanging between the surface layer Mo atoms with the sub-layer Ni atom, which also explains the opposite changing trend of Ni and Mo curves below 800 °C. Some molecular simulations were performed to further investi- gate the Mo surface segregation behaviors. The surface segregation is generally controlled by some factors such as the surface energies of the solute and matrix metals, the elastic energy caused by the solute atoms in the matrix, and surface chemisorptions [16–19]. Since Mo atom has a larger size than Ni atom, the segregation of Mo atoms tothe surface can release the elastic energy, which is ener- getically favorable. During the decarburization process, the adsorbed oxygen atoms also drive Mo atoms to the surface because the Mo–O bonds are stronger than the Ni–O bonds. In Fig. 2, the initial total content of Ni, Mo, and Co is about 82 wt% from the start point of each curve and the final total content of the elements is close to 100 wt% at 1000 °C, which indicates that O element is almost clear up on the surface of the electrode by the reduction process.

Fig. 1 Carbon content as a function of heat treatment temperature

Fig. 2 Composition of electrode surface as a function of reduction temperature

In order to clearly see how the reduction temperature changes the electrode surface morphology, the analyses were performed within two different scales. For the large- scale analysis, Fig. 3 shows the OM images of the elec- trode surface at the temperatures from 500 to 800 °C. The sample reduced at 500 °C in H₂atmosphere shows a deepcolor, and there exist some black oxide zones on the sample surface.

Fig. 3 OM images of electrode samples reduced at various temper- atures: a 500 °C, b 600 °C, c 700 °C, and d 800 °C

Fig. 4 Typical surface SEM image of Ni Mo Co foam electrode

The color of electrode becomes brighter with the reduction temperature increasing, indicating that the oxides on the sample surface are being removed. The SEM anal- ysis was adopted to examine the surface morphologies. As seen in Fig. 4, the electrodes have a foam-like porous structure with the pore size of about 200–400 lm. The porous structure significantly increases the specific surface area compared to ordinary alloy electrode. At relatively low temperatures of 500–700 °C (Fig. 5a–c), there are many tiny white particles distributed on the electrode sur- face. These white particles are recognized as oxides. As the temperature increases, the density of the white particles reduces and the surface morphology looks quite different from those at low reduction temperatures. At 800 °C, the white particles on the electrode surface are hardly observed, indicating that the oxides are completely removed (Fig. 5d). Figure 5e, f shows that the surface oxides do not exist and also that the surface grains become coarser at 900 and 1000 °C.

The results of the tensile strength and elongation are shown in Fig. 6. As seen in Fig. 6, the tensile strength linearly increases with the reduction temperature increas- ing although the measured data points are quite scattering. Interestingly, the elongation in Fig. 6 does not show a decreasing trend opposite to the observed trend for the tensile strength, which is true for many bulk materials [20]. This may be because the reduction process not only removes the surface oxides, but also enhances the metal- lurgical bonding in the electrode. This behavior is very similar to the sintering process in powder metallurgy parts [21, 22]. The elongation seems to have a maximum at 800 °C. As seen below, it is coincident with the tempera- ture at which the lowest overpotential is obtained. It requires a further investigation to understand this phenomenon.

Fig. 5 SEM images of Ni Mo Co foam electrodes reduced at different temperatures: a 500 °C, b 600 °C, c 700 °C, d 800 °C, e 900 °C, and f 1000 °C

Fig. 6 Tensile strength and elongation rate of electrode as a function of reduction temperature

The electrode with a low overpotential indicates its high electrochemical activity toward the hydrogen evo- lution reaction. The polarization overpotential results of a typical Ni Mo Co foam electrode are shown in Fig. 7. The absolute overpotential depends on some factors, such as the electrode composition, the solution, and the current density. However, we care mainly about the relation between the overpotential and the reduction temperature at present, so the minimum overpotential was shifted to zero in Fig. 7 for better understanding. The results for thecurrent densities of 100 and 200 m A?cm^-2are present. It is seen that the overpotential g₁₀₀corresponding to the current density of 100 m A?cm^-2is 50–150 m V lower than g₂₀₀for the current density of 200 m A?cm^-2. At 800 °C, the lowest overpotential values are obtained for both g₁₀₀and g₂₀₀. This indicates that the Ni Mo Co foam electrode reduced at 800 °C has the best electrocatalytic performance. The higher overpotentials for the electrodes reduced at the temperatures of below 800 °C are attrib- uted to the incomplete reduction of the surface oxides. The electrodes treated at temperatures of higher than 800 °C own a higher overpotential, which may be due to the composition change and the grain coarsening as seen in Figs. 2 and 5. The reduction at a high temperature also leads to a decrease in the specific surface area and thus to a higher overpotential.

Fig. 7 Relative overpotentials (g100and g200) of electrode at 100 and 200 m A?cm-2

4 Conclusion

Ni Mo Co foam electrodes were prepared by electrodepo- sition method. The best electrocatalytic activity of the synthesized electrode toward the hydrogen evolution reaction is achieved by optimizing the post-treatment processes. First, the electrode was decarbonized at various temperatures, and it is found that the decarbonized tem- perature should be about 550–600 °C. Next, the electrode was reduced in hydrogen with the temperature varying from 500 to 1100 °C. The electrode has the highest electrochemical activity when reduced at 800 °C. Below 800 °C, the oxides on the electrode surface are not completely reduced, while the reduction temperature of higher than 800 °C causes the growth of surface grain and losing of the specific surface area. The elongation rate of the electrode does not show a decreasing trend with temperature opposite to the observed trend for the tensilestrength and has a maximum value at 800 °C, which is coincident with the temperature at which the lowest overpotential is obtained.