Influence of Electrooxidation of Ni Wire Electrodes on the Kinetics of Oxygen Evolution Reaction Studied in 0

This work reports on oxygen evolution reaction (OER), studied at nickel wire electrode material. Electrocatalytic behavior of non-oxidized and electrooxidized Ni wire samples was evaluated in 0.1 mol dm NaOH solution for the potential range: 1600-2000 mV vs. RHE. The performance of nickel electrodes was examined by alternating current (a.c.) impedance spectroscopy, cyclic voltammetry and Tafel polarization measurements. Electrochemical oxidation of Ni wire radically increased the kinetics of the OER, exhibited through reduced charge-transfer resistance parameter and considerably modified Tafel polarization slopes.


Introduction
The growth of environmental problems in regard to increasing demand for fossil fuels and energy encourages the world community to search for new energy technologies that will provide an acceptable level of pollution and, at the same time, would not slow down economic growth.2][3] Hydrogen is an environmentally friendly fuel, which reserves are almost inexhaustible.The above makes hydrogen a great alternative for oil and natural gas. 4,51][12] In contrast to the hydrogen evolution reaction (HER), the process of OER proceeds more slowly due to the need to transport four electrons, as compared to two electrons for the HER.For this reason, in current study the authors concentrated on the selection and development of effective catalysts for the OER, which would be characterized by low overpotentials, good performance and relatively low cost of their mass production. 3,13,14ickel and its metallic compounds are widely used by different industries (e.g.Ni-Fe in metallurgy, nickel-based alloys and Ni-based superalloys are used in aviation, shipbuilding and chemical industry). 15,16][19][20][21] In this paper, the OER was conducted on nickel wire (nonoxidized and electrooxidized) electrodes in 0.1 mol dm -3 NaOH electrolyte, primarily by means of alternating current (a.c.) impedance spectroscopy examinations.

Experimental
A Direct-Q3 UV ultra-pure water purification system from Millipore (18.2 MΩ cm water resistivity) was used to prepare all solutions.Basic solution of 0.1 mol dm -3 NaOH was made up from Aesar, 99.996% NaOH pellets.Additionally, a separate solution dedicated to charging of a Pd reversible hydrogen electrode was prepared from sulfuric acid of highest purity available (SEASTAR Chemicals, BC, Canada).
An electrochemical cell, made of Pyrex glass, was used during the course of this work.The cell included three electrodes: a Ni wire working electrode (WE) in a central part, a reversible Pd hydrogen electrode (RHE) as reference and a Pt counter electrode (CE), both placed in separate compartments.Before conducting the HER experiments, each Ni electrode was additionally reduced in 0.1 mol dm -3 NaOH solution by cathodic polarization at 50 mA for 600 s in order to remove any spontaneously formed Ni oxide layer.After conducting experiments on fresh nickel electrode, it was once more subjected to cathodic polarization and then was electrochemically oxidized in 0.1 mol dm -3 NaOH solution to examine properties of NiOOH (nickel oxyhydroxide layer) formed on its surface.Oxidation of the Ni electrode surface was carried-out by cyclic voltammetry (CV).
Cyclic voltammetry, electrochemical impedance spectroscopy and quasi steady-state polarization methods were used in this work.All measurements were recorded at room temperature by means of Solatron 12,608 W Full Electrochemical System.Data analysis was carried-out with ZView 2.9 (Corrview 2.9) software package, where the impedance spectra were fitted by means of a complex, non-linear, least-squares immitance fitting program LEVM 6, written by Macdonald. 22All other experimental details, including pre-treatments applied to electrochemical cell and electrodes, and employed a.c.impedance protocol were as those given in publications by Pierozynski et al.; 23 Pierozynski and Mikolajczyk. 24

Scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS) characterization of electrooxidized Ni wire electrode
An EDS analysis was employed to determine the difference in the content of oxygen between "as received" (fresh) (Figure 1a) and oxidized (Figure 1b) Ni wire electrodes.The analysis was carried-out at an acceleration voltage of 20 kV.This experiment confirmed that the presence of oxygen in the oxidized electrode (ca. 3 wt.%)was 3× higher compared to that of the fresh electrode (ca. 1 wt.%), but due to simultaneous excitation of C element from an ultra-thin carbon tape, O assessment for both samples was rather of semi-quantitative nature.On the other hand, the SEM micrograph pictures of both electrodes did not show any visible difference between the electrodes (Figure 1c: "as received" and Figure 1d: oxidized Ni electrode).
OER on Ni wire and oxidized Ni wire electrodes in 0.1 mol dm -3 NaOH solution The a.c.impedance characterization of the OER for "as received" and electrooxidized Ni wire electrodes experimentally recorded in 0.1 mol dm -3 NaOH is shown in Figure 2 and Table 1.For all examined potentials, both electrodes exhibited single, "depressed" semicircles (a single-step charge-transfer reaction) in the explored frequency range (examples of Nyquist impedance plots for the overpotential of 370 mV are shown in Figure 2).The electrochemical parameters (Faradic reaction resistance (R ct ) and double-layer capacitance (C dl )) were obtained by means of a constant phase element (CPE)-modified Randles equivalent circuit model (Figure 3), whereas their presentation is given in Table 1.[27]  Hence, the R ct parameter for non-oxidized Ni wire electrode decreased from 6.044 Ω cm 2 at 370 mV to 0.567 Ω cm 2 at the overpotential of 770 mV.On the other hand, electrooxidation of the Ni wire electrode caused a significant fall in the R ct parameter, which came to 3.874 and 0.475 Ω cm 2 at 370 and 770 mV overpotential values, respectively.Thus, the R ct parameter was reduced by almost 1.6 times for the oxidized Ni wire electrode for an initial (370 mV) overpotential value (see Table 1 for details).In comparison with another important work by Hana et al., 28 conducted on three dimensional (3D) nickel oxide/nickel (NiOx/Ni) electrode, the recorded R ct parameter reached 4.33 and 2.46 Ω cm 2 at the overpotential of 390 mV, before and after surface oxidation, respectively.As a result, electrooxidation of nickel electrode led to the diminution of the R ct parameter by ca.1.8 times. 28oreover, double-layer capacitance for both types of electrode had a tendency to somewhat decrease with rising potential.The C dl value for pure Ni wire electrode reached 2,930 µF cm -2 s f1-1 at 370 mV and 1,768 µF cm 2 s f1-1 at 770 mV.Similarly, for electrooxidized Ni wire specimen, the C dl parameter declined from 4,578 to 4,004 µF cm -2 s f1-1 for the corresponding overpotentials.An increase of the double-layer capacitance by about 1.6 times for the oxidized Ni wire electrode was most likely caused by the formation of NiOOH (nickel oxyhydroxide) layer on the electrode surface during the process of electrooxidation.On the other hand, the reduction of the C dl parameter along with rising overpotential could be explained by partial blocking of electrochemically active electrode surface by freshly formed oxygen bubbles. 29,30he difference in the electrochemical performance for "as received" and the electrooxidized Ni electrodes could clearly be seen from CV data presented in Figure 4. Hence, surface-electrooxidized Ni wire exhibited significant increase of current-density within the corresponding CV profile in comparison with a baseline (fresh) electrode.In addition, an anodic peak A (Figure 4), observed between potentials 1.38 and 1.47 V vs. RHE for oxidized Ni wire sample, illustrated the process of formation of NiOOH from Ni(OH) 2 , while the cathodic peak B (1.18-1.36V vs. RHE) represents the reduction of nickel oxyhydroxide layer.Furthermore, a linear dependence of -log R ct vs. overpotential (η) in Figure 5 is consistent with kinetically controlled reactions (Volmer-Heyrovski route). 32The latter allowed to derive the OER's exchange current-densities (j 0 ) for these types of electrodes.The calculations were conducted by means of the Butler-Volmer equation and through utilization of the relation between the exchange current-density and the R ct parameter for overpotential approaching zero value. 33,34Hence, for the oxidized Ni wire, the recorded j 0 value for high overpotential range is almost twice as high as that for the non-oxidized Ni electrode (3.45 × 10 -4 and 6.44 × 10 -4 A cm -2 ).Thus, the obtained j 0 values are quite close to those derived in other OER works.[37] Furthermore, the potentiostatic Tafel polarization plots are presented in Figure 6.Here, the recorded anodic slopes (b a ) approached 146 (fresh Ni wire) and 119 mV dec −1 (electrooxidized Ni wire) electrodes.Also, the corresponding Tafel-based j 0 parameter values for the OER came to 1.2 × 10 −3 and 2.6 × 10 −3 A cm −2 for fresh and electrooxidized Ni wire samples, respectively.However, a radical difference between the Tafel-calculated and the impedance-derived values of the j 0 parameter resulted from the fact that the linear Tafel region for the studied Ni wire electrodes was not very well pronounced.Hence, the Tafel-based data presented in this work should only be treated qualitatively.

Conclusions
Surface electrooxidation treatment was used to enhance electrochemical performance of Ni wire electrodes.It was proven that electrooxidized Ni wire catalyst was significantly more electrochemically active for the OER in regard to greater values of exchange current density than baseline Ni wire sample.In conclusion, obtained results indicated practical opportunities for electrooxidized nickel wire materials in industrial alkaline water electrolysers.

Figure 1 .
Figure 1.EDS spectrum for (a) "as received" Ni wire electrode and (b) for oxidized electrode; (c) SEM micrograph picture of (c) "as received" Ni wire electrode and (d) for oxidized Ni electrode taken at 86× magnification with visible selected area for EDS measurement.

Figure 2 .
Figure 2. Complex-plane impedance plots for the OER on Ni wire and electrooxidized Ni wire electrodes in contact with 0.1 mol dm -3 NaOH electrolyte, recorded at room temperature for the overpotential of 370 mV.The solid lines correspond to representation of the data according to an equivalent circuit shown in Figure 3.

Figure 3 .
Figure 3. Equivalent circuit model used for fitting the impedance data for Ni wire electrodes, obtained in 0.1 mol dm -3 NaOH solution.The circuit includes a constant phase element (CPE) for distributed capacitance; R ct and C dl (CPE dl ) elements correspond to the OER charge-transfer resistance and double-layer capacitance components; R sol is solution resistance.

Figure 4 .
Figure 4. Cyclic voltammograms for Ni wire and electrooxidized Ni wire electrodes in contact with 0.1 mol dm -3 NaOH electrolyte, recorded at a sweep rate of 50 mV s -1 for the potential range 1.2-1.9V vs. RHE.

Figure 5 .
Figure 5. -log R ct vs. overpotential relationship, obtained for the OER in 0.1 mol dm -3 NaOH solution for Ni wire electrodes.Symbols represent experimental results and lines are data fits.

Figure 6 .
Figure 6.Quasi-potentiostatic anodic Tafel polarization curves (recorded at a scan-rate of 0.5 mV s −1 ) for Ni wire and electrooxidized Ni wire electrode surfaces, carried out in 0.1 mol dm -3 NaOH solution (appropriate iR correction was made based on the solution resistance derived from the impedance measurements); calculated Tafel slopes for Ni wire and electrooxidized Ni wire samples came to b a = 146 and 119 mV dec −1 , respectively.