Application of Fe-Ni-W Plated Film Electrode to Zinc-Air Battery

This study was aimed at the preparation of an electrode for Zinc–air battery, which had excellent catalytic activity by use of electroplating of alloys made of abundant metal, such as Fe and Ni. The oxygen overvoltage of the Fe-Ni-W alloy plated electrode was the smallest through the measurement. The elemental composition and the enlargement of the surface area were confirmed by SEM and EDX analysis. Involvement of Fe and W of Fe-Ni-W alloy plated electrode will be one factor for its high catalytic activity. Thus plated Fe-Ni-W alloy electrodes were compared with other Fe alloy plated electrodes considering to their cathode performance as Zinc-air battery. The catalytic activity of Fe-Ni-W plated electrode showed the best performance comparing to Fe-Ni alloy plated electrodes as cathode for Zinc-air battery. Also comparing to the platinum electrode which had been widely used as cathode in the field of Zinc-air battery, the Fe-based alloy plated electrode showed better performance as the electrodes considering to its oxygen evolution reaction.


Introduction
World energy consumption continues to increase along with economic growth, continuing to increase at an annual average of 2.6% from 1600 GJ in 1965, reaching 5300 GJ in 2013. The main energy source responsible for this huge energy consumption will be fossil fuel. More than 80% of the current world energy demand is covered by natural gas, coal and oil. Disadvantages of the society consuming such a large amount of fossil fuel have emerged as global warming and environmental pollution problems. In order to solve this problem, How to cite this paper: Yoshihara, S., Ohtake, H. and Sasaki, J. (2019) Application of Fe-Ni-W Plated Film Electrode to investigated. Recently, metal-air batteries have become a promising power source, based on the nature of high theoretical energy density and free-oxygen fuel from the atmosphere. Among various metal-air batteries, alkaline metal-air batteries, especially lithium-oxygen, have been intensively investigated due to their high specific energy density up to 5200 W•h•kg −1 . However, the recharge ability, safety and cost issues make them hard to be commercialized. Because of these currently unconquerable limitations, rechargeable zinc-air batteries have attracted significant attention as an alternative in the past few years. As one of the most abundant elements in the earth's crust, zinc has been the chosen electrode material for several types of primary batteries ever since the past battery, e.g. Zinc-carbon battery. Primary zinc-air batteries were introduced to the science community in the late 19 th century, and their commercial products started to enter the market in the 1930s. Among all metal-air systems, zinc-air batteries are the most practically variable choice. They possess some desirable features such as a high theoretical energy density, low cost, great safety and environmental friendliness. The theoretical energy density of zinc-air is 1086 W•h•kg −1 , about 5 times higher than that of Li-ion; its operational cost is estimated to be less than that of Li-ion. The primary zinc-air battery is a mature technology that has already been successfully applied to the fields of hearing, etc.
Their recharge ability is realized by developing new bifunctional air cathodes and bifunctional oxygen electrocatalysts that can facilitate both the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charge. Especially on bifunctional electrocatalysts for the air cathode, a comprehensive account of the current development status, challenges and possible solutions is highly necessary. Here in this paper, we focus on the fabrications of the air cathode of the electrically rechargeable and/or zinc-air batteries in alkaline electrolytes. We will propose newly developed air cathode of the rechargeable zinc-air battery, with higher bifunctional performance.

Method for Manufacturing Alloy Plated Electrode
Stainless steel (SUS 430) which is relatively inexpensive and has good corrosion resistance was selected as a substrate to be subjected to electroplating treatment. It is general to apply nickel strike plating as a pretreatment to a plating material which is prone to generate an oxide film on the surface like stainless steel. Table   1 shows the plating conditions for nickel strike. The stainless steel substrate was degreased with acetone, followed with and distilled water, and then immersed in concentrated hydrochloric acid for 2 minutes to remove the oxide film. After washing above specimen with distilled water, Nickel strike plating was performed under constant current electrolytic method using an electrochemical measurement system (HA 305 Hokuto Denko Co., Ltd.) [4]. After that, masking was done so that the surface active area was 4 cm 2 by use of masking tape (N-300 Nitto Denko Co., Ltd.). Table 2, Table 3 show the bath composition and plating conditions for Fe-Ni alloy plating, Fe-Ni-W alloy plating, respectively. For the counter electrode, Ni electrode having sufficiently large surface area with respect to the cathode was used.

Elemental, Crystal Structure Analysis and Surface Observation Method of Coated Surface
The surface of the electrode was observed with a field emission scanning electron

Method and Conditions of Electrolysis Test
LSV (linear sweep voltammetry) was carried out to evaluate the activity of the oxygen-evolution reaction, or the oxygen reduction reaction by use of electro- were used for the analysis of oxygen evolution reaction mechanism. The exchange current density and Tafel slope could be obtained from the Tafel plots.
In the case of evaluation of oxygen reduction reaction, in advance O 2 gas was bubbled in the solution for 30 minutes before LSV measurement, blank LSV were carried out also in the solution after bubbling N 2 for 30 minutes.
Real oxygen reduction current were estimated by taking the difference between the current measured after O 2 bubbling and that measured after N 2 bubbling.

Surface Analysis of Fe-Ni and Fe-Ni-W Alloy Plated Film
The surface composition analysis of alloy plating films using EDX results were shown in Table 4. It was confirmed that each films were made of metals contained in each plating bath.

Electrolytic Test
LSV measurements of oxygen evolution reaction were shown in Figure 3. LSV Table 4. Elemental analysis of alloy plating films (wt%).   Fe-Ni-W showed the highest activity for oxygen evolution reaction [5].
Tafel plot analysis results were shown in Figure 4. Tafel slope and exchange current density calculated from these Tafel plots were shown in Table 5. It was suggested that Fe-Ni-W showed relatively high activity for oxygen evolution reaction.
The Tafel slope suggested the rate determining step (rds) for oxygen evolution    LSV for the oxygen reduction reaction were shown in Figure 5. LSV for Pt plate was also shown for comparison. Comparing the current density at the same potential, Pt showed the largest reduction current. Also, the onset potential for reduction current on the iron-plated electrode were less noble to that of Pt plate electrode. The interface between vapor and liquid and solid-phase is important for oxygen reduction reaction. This suggests that the reason of decreased activity of the iron-plated electrode will be due to poor affinity between Fe-alloy and O 2 .
As mentioned above it was confirmed that more oxygen bubbles on Pt electrode remained on the surface under oxygen evolution reaction. This suggests that the affinity between Pt and O 2 will be stronger comparing to that between Fe-alloy

Conclusions
This study was aimed at the preparation of the air cathode for Zinc-air battery, which had excellent catalytic activity by use of electroplating of alloys made of abundant metal, such as Fe and Ni. Finally iron based alloy plated electrodes, especially Fe-Ni-W plated electrode were highly active for oxygen evolution reaction in compared to Pt from the results of the exchange current density obtained from LSV measurement, Tafel plots. The iron-based alloy coated electrode tends to be adsorbed by OH − species, successively release the evolved oxygen away from the surface, thus improves the oxygen evolution reaction activity.
SEM, XRD observations revealed that Fe-Ni-W plated electrode, having increased surface area caused by the roughness of it, had a fine crystalline structure than that of the Fe-Ni. In the present study, considering to the oxygen reduction activity, comparing to Pt, iron-plated electrode showed a smaller current density and high overvoltage for oxygen reduction reaction. It will be attributed to poor forming of good critical gas/liquid/solid interface between oxygen reduction in aqueous solution. In addition, oxygen-metal coupling strength will be due to the strength of covalent bonds of the 2 p orbital of oxygen and metal d-orbital. W (tungsten) having 5d orbital electron as well as Pt leads to dissociative adsorption of oxygen molecules so the activity of Fe-Ni-W was superior to that of Fe-Ni.
Further study of the iron-plated electrode for oxygen reduction reaction will be needed for the practice use of such an electrode as the air cathode for metal-air batteries.