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Nanostructured Nickel-Cobalt-Titanium Alloy Grown on Titanium Substrate as Efficient Electrocatalysts for Alkaline Water Electrolysis Pandian Ganesan, Sivanantham Arumugam, and Sangaraju Shanmugam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00353 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

Nanostructured Nickel-Cobalt-Titanium Alloy Grown on Titanium Substrate as Efficient Electrocatalysts for Alkaline Water Electrolysis Pandian Ganesan, Sivanantham Arumugam, and Sangaraju Shanmugam*

Department of Energy Systems and Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1 Sang-Ri, Hyeongpung-Myeon, Dalseong-gun, Daegu, 42988, Republic of Korea

Keywords: Nickel-cobalt-titanium, titanium substrates, alkaline water splitting, electrocatalysts, electrodeposition, OER, HER, alloy.

ABSTRACT: One of the important challenges in alkaline water electrolysis are to utilize a bifunctional catalyst for both hydrogen evolution (HER) and oxygen evolution (OER) reactions to increase the efficiency of water splitting devices for the long durable operations. Herein, nickel-cobalt-titanium alloy directly grown on a high corrosion resistance titanium foil (NCT) by a simple, single and rapid electrochemical deposition at room temperature. The electrocatalytic activity of NCT alloy electrodes is evaluated for both HER and OER in aqueous electrolyte. Our nickel-cobalt-titanium electrocatalyst exhibits low overpotentials around 125 and 331 mV for HER and OER, respectively in 1 M KOH. In addition to this outstanding activity, the bifunctional catalyst also exhibits excellent OER and HER electrode stability up to 150 h of

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continuous operation with a minimal loss in activity. Further, the nickel-cobalt-titanium alloy directly grown on titanium foil is used to directly construct membrane electrode assembly (MEA) for alkaline electrolyte membrane (AEM) water electrolyzer which makes the practical applicability. This single-step electrodeposition of nickel-cobalt-titanium on titanium foil with high activity and excellent electrode stability suitable for replacing alternative commercial viable catalyst for the alkaline water splitting.

1. Introduction The electrochemical hydrogen production using renewable energy received tremendous importance due to its high energy density, and not releasing pollutant gas. 1-5 However, the use of noble metal based catalysts limited its production in the commercial scale.4,

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The use of

perfluorinated membranes in polymer electrolyte membrane (PEM) water electrolyzer and noble catalysts are key limitations for successful commercialization of electrocatalytic hydrogen generation.9-14 An alternative to the acid membrane water electrolyzers, alkaline electrolyte membrane (AEM) water electrolyzers can reduce the cost of catalyst and component.13,14 The electrode materials used in the practical alkaline membrane water electrolyzer are not only costly but also less efficient, which is due to slow kinetics involved in the anodic OER reaction and the hydrogen evolution reaction (HER) at the cathode in basic environments.9-11 To further minimize the cost of alkaline electrolyte membrane water electrolyzer, the synthesis of catalyst fabrication in direct conjunction with the desired current collector is highly preferred.13, 14 If the catalysts were developed on the substrate such as nickel foam, titanium, platinum coated stainless steel, etc. it will further reduce the cost and increase the electrode efficiency due to their stability in the alkaline medium. Titanium and its alloys are well-known materials widely used in aerospace, automobile, and power generation sectors, due to their light weight, high mechanical strength,


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excellent corrosion resistance, and thermal stability.15 Based on the Pourbaix diagram, titanium is highly stable in the highly oxidative and reductive environment in the electrochemical water splitting potential region in the high alkaline medium.16 Previously, the porous titanium was reported as a suitable current collector for the both alkaline and acid water electrolysis.17 The nickel and cobalt coating on the titanium was widely used to solve the wear resistance and galling issues.18 Also among the transition metals, nickel and cobalt are the best elements for the electrochemical water oxidation.19 Various cobalt and nickel based catalysts such as cobalt particles embedded on the nitrogen-rich carbon, cobalt corrole catalyst, cobalt molybdenum nanoparticles, metallic nickel, β-Ni(OH) on carbon and Ni-Co composite with graphene were reported as hydrogen evolution catalysts (HER).20-25 The monolayer cobalt nanoparticles, nickel oxide particle, cobalt and nickel on glassy carbon, nickel cobalt on a copper substrate, nickel titanium alloy on the stainless steel substrate, Ni-Co binary oxide, nickel incorporated copper on titanium were reported for the oxygen evolution reactions in alkaline medium.26-31 Moreover, Ni/N/C for bi-functional water splitting catalyst, Co-Ti layered double oxides for lithium-air batteries and NiCo alloy oxides for the zinc air batteries.32-34 Also, composites such as Ni-Mo, Ni-Co-Mo-Fe, NiCo-O, Ni-Co-P used for OER

35-38

and Ni-Co, Ni-Co-Y alloy; nickel titanium

composites 26, 39, 40 were studied for the HER. However, one notable exception in the previously reported nickel and cobalt alloy based catalysts was that the most corrosion resistive titanium support was not used for the full water splitting catalysts in the alkaline medium. In the present study, we introduce an easy electrochemical in-situ synthesis to grow nickelcobalt titanium alloy on titanium foil. Subsequently, nickel-cobalt-titanium alloy was used to construct the MEA for commercial alkaline water electrolysis. The prepared nickel-cobalttitanium alloy on the titanium foil electrocatalyst is highly active for HER with low


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overpotentials of 125 mV at the current density of -10 mA cm-2 and 331 mV at the current density of 100 mA cm-2 for OER and affords very high current densities for long-term OER and HER activity. Its performance is much better than that of the metal oxides, and metal sulfides reported previously

36, 41, 42

.

Further, the nickel-cobalt-titanium alloy catalyst also exhibits

excellent HER performance close to the state-of-the-art noble catalysts (e.g., Pt/C) and outstanding OER performance compared to the state-of-the-noble art catalysts (e.g., RuO2). 2. Experimental Section 2.1. Materials and Methods. Titanium foil (Ti, thickness: 0.1 mm) and nickel foam (Ni, thickness; 1.6 mm) were purchased from MTI Korea Ltd. Nickel sulphate and cobalt sulphate were purchased from Alfa Aesar. The hydrogen fluoride and nitric acid were purchased from Alfa Aesar with the spectroscopic grade. The as-deposited nickel-cobalt-titanium was characterized using X-ray diffraction (XRD) for phase confirmation. X-ray diffractometer (XRD) studies were measured using the instrument from Rigaku and MiniFlex 600 model with CuKα radiation (1.5418 Å). The structural morphology of the electrodes was investigated by the field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800II, 3 kV). For the SEM characterization, the samples were coated using osmium sputtering. The transmission electron microscope (TEM) studies were analyzed with Hitachi HF-3300, 300 kV equipment. The nickelcobalt-titanium was removed from nickel-cobalt-titanium electrode using sonication in isopropanol with the help of ultrasonic agitator; a few drops of the suspension was drop cast onto a copper grid, dried and utilized for TEM measurements. X-ray photoelectron spectroscopy (XPS) from Thermo-scientific, ESCALAB 250Xi and studies were performed in 10-9 mbar vacuum. UV-visible spectroscopy (CARY5000, Aligant technology solutions) and FT-IR


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(Continuum, Thermo-scientific) were used to characterize the electronic energy level and functional group analysis of the nickel-cobalt-titanium electrodes, respectively. 2.2. Synthesis. The titanium foil was pre-treated with HF solution contains 1:2:7 ratio of hydrogen fluoride, HNO3, and DI water. Electrochemical deposition was carried out using 7 g of nickel sulphate, and 1 g of cobalt sulphate were dissolved in 100 mL de-ionised water as an analyte. Nickel foam and titanium foil with 4 cm2 area each act as anode and cathode, respectively. The electrodeposition was carried out by applying a constant current at 100 mA cm2

using the DC power source to the nickel anode and titanium cathode. The nickel and cobalt

alloy were deposited on titanium on various time intervals such as 2.5, 5 and 10 min which is labeled as NCT-1, NCT-2, and NCT-3 respectively18. The deposition was also carried out using the individual precursors of nickel and cobalt alone and labeled as CT and NT. 2.3. Electrochemical Measurements. The electrochemical activities of all electrodes were evaluated using linear sweep voltammetry (LSV) in an Ar-saturated atmosphere for hydrogen evolution reaction (HER) and without saturation for the oxygen evolution reaction (OER) using potentiostat (Bio-Logic) instrument. The typical three-electrode system with working, reference and counter electrodes were immersed in the 1M potassium hydroxide (KOH) aqueous electrolyte. The mercury/mercurous oxide (Hg/HgO) was used as a reference electrode, and a platinum coil was used as counter electrode. All HER and OER measurements were carried out at a scan rate of 10 mVs-1. To compare OER and HER activity of nickel-cobalt alloy electrode, the commercial Pt/C (40%) and RuO2 catalysts were used. The catalyst ink was prepared using 5 mg of the catalyst dispersed in a 200 micro liter solution mixture consists of isopropanol, DI water, and Nafion(5%) in the ratio 16:3:1 and dispersed using ultrasonication for 30 min to get a homogeneous ink. The 1.5 µL was dispersed uniformly on glassy carbon disk with 0.07 cm2 area


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and dried. The potentials studied for all the electrodes were measured from the Hg/HgO which was converted to the reversible hydrogen electrode (RHE) scale using the relation, RHE = E (Hg/HgO) + 0.942 V in

1 M aqueous KOH electrolyte. The LSV of all the electrodes were

characterized by the electrochemical impedance spectroscopy (EIS) with a frequency range of 200 to 100 mHz at 0 V to calculate iR correction, charge transfer resistance, and double layer capacitance. The iR correction was carried out based on the solution resistance about 2.0 ohm obtained from the EIS. 2.4. Preparation of MEA. The electrochemically prepared NCT-2 is used as anode and cathode with the active area of 2 cm2. The titanium electrodes were used as electrodes and anionic membrane from Tokuyama (A201) which was pre-treated with potassium hydroxide solution for one day and then soaked in DI water for 1 h and then used for MEA assembly. The fuel cell equipment from the Heliocentric device (Germany) was used as the PEM water electrolyzer. Also, the benchmark catalysts such as RuO2 and Pt/C (40%) about 4.5 mg cm-2 coated on each side of the membrane were prepared for comparison. 3. Result and discussion 3.1 Structure and Morphology Analysis. XRD patterns of NCT-1, NCT-2, and NCT-3 were depicted in Figure 1. The NCT-1 exhibits the nickel, nickel-titanium (NiTi) alloy and titanium phases with ICDD nos. 00-003-1051, 01-088-2325, 03-065-0145 and 00-044-1288, respectively. The existence of NiTi phase in the NCT-1 is due to the less deposition time about 2.5 min, and non-existence of the cobalt may be due to low availability of cobalt precursor during a deposition in the electrolyte. Also, NCT-2 and NCT-3 samples exhibit individual metallic nickel (ICDD nos. 00-003-1051) and cobalt (ICDD nos. 01-088-2325) phases with deviation in the peaks (Figure 1a). Moreover, in Figure 1b, the NCT-2 exhibits diffraction patterns of (111), (200) and


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(220) planes for nickel and cobalt with ~ 0.30 negative shift and ~ 0.30 positive shift, respectively. The shift in the nickel and cobalt peaks for the NCT-2 sample confirms that the nickel and cobalt exist as an alloy which was also supported by the previous literature.43, 44 However, in the case of NCT-3 sample exhibits nickel peak without any shift due to the high amount of nickel compare to cobalt in the alloy due to the longer deposition time (Figure 1b). To understand the effect of electrolyte concentration, the cobalt sulphate concentrations was varied as 0.8, 1.0 and 1.2 g and nickel suphate concentration and time of deposition (5 min) were kept constant (Figure 2). When sample prepared with the CoSO4 concentration of 0.8 g exhibits nickel and cobalt phases along with the titanium phase, but when concentrations of cobalt suphate increased to 1.0 and 1.2 g in the electrolyte and obtained samples exhibit only cobalt and nickel metallic phases. This observation shows that as the concentration of cobalt precursor increases in the bath, the deposition of the nickel and cobalt content in the NCT-2 and NCT-3 samples increased and also the existence of titanium in the NCT-1 shows that the initial formation of nickel-cobalt alloy with titanium surface and then a huge deposition of nickel-cobalt alloy was observed which confirms the involvement of titanium foil surface in the alloy formation. Figure 3 displays the SEM morphology of NCT-1, NCT-2, and NCT-3 samples. Figure 3a,b show the SEM images of the NCT-1 electrode which exhibits less distributed nickel-cobalt-titanium alloy coating on titanium and Figure 3b&c display SEM morphology of NCT-2 electrode which shows the agglomerated nano-sized nickel-cobalt alloy on the titanium foil. Similarly, NCT-3 electrode also exhibits the thicker agglomerated nickel-cobalt-titanium alloy on titanium (Figure 3e,f). Further, to understand the composition of nickel and cobalt in the alloy with respect to the deposition time, the cross-sectional SEM elemental mapping and energy dispersive spectroscopy (EDS) analysis for NCT-1, NCT-2, NCT-3, NT, and CT were carried out and corresponding


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results are given in the supporting information Figure S1-S10 and Table S1-S5. From the SEM elemental mapping and EDS analysis discussions (supporting information), it is observed that the bottom layers of the nickel-cobalt alloy samples NCT-1, NCT-2, and NCT-3 contain a high amount of titanium compared to the middle and upper layers (Table S1-S3). Also, it was inferred that as the deposition time increases from 2.5 min to 10 min, the nickel and cobalt contents were gradually increased. However, the upper surface of NCT-2 (5 min deposition), the nickel and cobalt contents were found to be 33.06 and 67.11%, respectively which is approximately similar to the nickel (41.22%) and cobalt (57.67%) contents in the middle region of NCT-3. This observation reveals that initially, nickel was the most preferred deposit on the titanium surface and then cobalt also compete with the nickel deposit until 5 min deposition (NCT-2). However, after 5 min deposition, nickel content was predominant compared to cobalt amount in the coating. It was further confirmed by the XRD results of the NCT-2 (5 min deposition) and NCT3 (10 min deposition, Figure 1b) in which NCT-2 exhibits a shift of 0.30 for both cobalt and nickel XRD patterns. This behavior reveals that the NCT-2 sample exhibits maximum cobalt content and further increasing the deposition to 10 min, NCT-3 sample exhibits more nickel than cobalt contents. Hence, based on the EDS analysis of NCT-1, NCT-2, and NCT-3 samples, the percentage of nickel and cobalt were plotted in Figure 4. Even though NCT-2 and NCT-3 sample exhibits only cobalt and nickel, the EDS mapping shows there was an appreciable amount of titanium which demonstrates the involvement of titanium in the alloying formation. Hence, the shift in the nickel and cobalt XRD peaks were evident, which could be due to the effect of titanium in Ni-Co-Ti formation (Figure 1b). The deposition of nickel and cobalt electrodes alone NT and CT, show 100% nickel and cobalt respectively. The NCT-1 exhibits 81% nickel and 19% cobalt which reveals that 2.5 min deposited sample, contains a high amount


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of nickel compared to cobalt. However, in the case of NCT-2 sample (5 min) exhibits 68% nickel and 32% cobalt indicates the increase of cobalt content compared to cobalt content of NCT-1 sample. Moreover, the NCT-3 sample exhibits 75% nickel and 25% cobalt, which shows a low amount of cobalt compared to cobalt in nickel-cobalt-titanium. The TEM morphology of NCT-2 (Figure 5) shows particles are agglomerated together (Figure 5a). The high-resolution TEM (HR-TEM) image presented in Figure 5b confirms the formation of the nickel with (111) and (220) planes with which corresponds to the lattice fringe widths of 2.02 and 1.24 Å, respectively and cobalt with (111) plane with lattice fringe width 2.04 Å. Also, the TEM energy dispersive spectroscopy (EDS) point analysis was made in three different areas of NCT-2 sample as shown in Figure S11, and results confirmed the presence of nickel, cobalt, and oxygen with the average atomic weight percentage of 56.6, 36.27 and 7.17%, respectively (Table S6). 3.2. XPS Analysis. We examined the XPS to understand the chemical environment of the NCT-1, NCT-2, and NCT-3 samples (Figure 5c&d). The XPS interpretation of NCT-1, NCT-2, and NCT-3 in the Co2P3/2 region featured a cobalt in zero oxidation state at 777.13, 777.02 and 777.76 eV, respectively (Figure 5c).41 The binding energy (BE) of Co 2P3/2 in the NCT-1, NCT2, and NCT-3 exhibit slight deviations from the characteristic metallic cobalt 777.3 eV peak (Figure 5c). Also, the cobalt contents based on the XPS quantification of the peaks of the NCT1, NCT-2, and NCT-3 were 30.59, 18.13 and 29.20%, respectively (Table S7). Meanwhile, the NCT-1, NCT-2, and NCT-3 samples exhibit Co(OH)2 peak at 782.4 eV each with satellite peaks 786.8, 786.8 and 786.9 eV, respectively (Figure 5c).45-48 Further, the XPS results of NCT-1, NCT-2, and NCT-3 in the Ni2P3/2 region featured a nickel in zero oxidation state at 851.6, 851.7 and 852.0 eV, respectively (Figure 5d).45 The peak shift confirms the alloy formation in all the samples. Also, the nickel contents based on the XPS quantification of peaks for the NCT-1,


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NCT-2, and NCT-3 were 67.24, 59.96 and 69.55%, respectively (Table S7). Moreover, the NCT-1, NCT-2, and NCT-3 samples exhibit Ni(OH)2 peak at 857.6, 857.6 and 857.7 eV, respectively (Figure 5d).45-48 Also, the nickel hydroxide contents based on the XPS quantification of peaks for the NCT-1, NCT-2, and NCT-3 were 32.79, 40.02 and 30.55%, respectively (Table S7). The shifts in the Ni0 and Co0 oxidation states in all the samples reveal the effect of deposition time during the electrodeposition. Also, the cobalt hydroxides contents based on the XPS quantification of the peaks of the NCT-1, NCT-2, and NCT-3 were 69.41, 81.87 and 70.80%, respectively (Table S7). We have also calculated the total metal content and metal hydroxide content from XPS peak area. The NCT-1, NCT-2, and NCT-3 exhibits total cobalt and nickel metal contents is about 57.36, 52.40 and 56.14%, respectively and the total cobalt and nickel hydroxide contents of NCT-1, NCT-2, and NCT-3 possess the metal hydroxide contents about 42.64, 51.60 and 23.96%, respectively (Table S7). Based on the XPS analysis, the amount of metal content is higher than the metal hydroxide contents due to the atmosphere. To further understand the effect of cobalt content which influences the cobalt and nickel peak shifts in binding energy (∆E), the peak changes in the Co2p3/2 region and Ni2p3/2 region were plotted against the cobalt content. The metallic cobalt (Co2p3/2) and nickel (Ni2p3/2) peaks for NCT-1 exhibit at 777.13 and 851.6 eV when the cobalt content is 19% (Figure 5c,d,e&f), respectively. The Co2p3/2 and Ni2p3/2 binding energy values of NCT-1 display the negative shifts from the characteristic metallic cobalt (777.3 eV) and nickel (851.9 eV) (Figure 5e&f). A binding energy shift of 0.17 eV (Co) and 0.3 eV (Ni) was observed, and this shift may be beneficial in the movement of electronic cloud in the bonding which may induce the surface charges in the alloy that allows the adsorption of the hydroxides and hydrides for the improvement of the OER and HER. Moreover, this is the reason for the existence of OER and


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HER activity when compared to the titanium foil alone (Figure 7 & 8). For NCT-2 sample, a large negative shift (0.28 eV) was observed for the cobalt Co2p3/2 metallic peak and less noticeable negative shift (0.02 eV) for the nickel Ni2p3/2 metallic peak as shown in Figure 5e&f. A large cobalt peak shift of NCT-2 than NCT-1 sample was attributed from the increased cobalt content of NCT-2 compared to the NCT-1. The increased negative shift about 0.28 eV in the metallic cobalt of NCT-2 may be due to the higher amount of cobalt involving weakening the bond strength of the cobalt and nickel alloy and favors the uneven distribution of electronic cloud in the bonding energy levels which may provide induced surface charges (Figure 5e&f). This surface charge is responsible for effective hydroxide and hydride adsorption during OER and HER reactions. Hence, the NCT-2 exhibits 125 and 231 mV overpotentials for HER and OER, respectively which is much lower than the overpotentials observed for NCT-1 sample. However, exceptionally, the NCT-3 sample exhibits a positive shift of 2P3/2 peak for both metallic cobalt (0.46 eV) and nickel (0.1 eV) from the characteristic individual metal peaks (Figure 5e&f)). In NCT-3 sample, the alloy particles were highly agglomerated which is responsible for the positive shift of binding energy that reflects in the increased bond strength of Ni and Co compared to the NCT-1 and NCT-2 samples. The positive shift observed for NCT-3 sample is responsible for higher overpotential of 30 mV (OER) and 135 mV (HER) than the NCT-2 electrode. 3.3. UV-visible and FT-IR spectra. To understand the surface nickel cobalt hydroxide formation due to the adsorbed oxygen, the FT-IR was carried out for NCT-1, NCT-2 and NCT-3 samples (Figure 6a). The presence of the broad peak around 3100-3400 cm-1 in all the electrodes confirms the existence of the adsorbed hydroxyl on the metal surface. However, the existence of strong peak at 1630, 1636, and 1634 cm-1 for NCT-1, NCT-2, and NCT-3, respectively shows


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that negligible amount of the metal hydroxide.48-50 Meanwhile, the NCT-1, NCT-2, and NCT-3 exhibit adsorption peaks at 1381, 1340, and 1345 cm-1, respectively due to the carbonates.49 The FT-IR analysis explains that the formation of the hydroxide on nickel and cobalt is due to the adsorbed oxygen or water from the atmosphere.48 - 50 The UV-visible spectroscopy diffuse reflectance was carried out to understand the nature of nickel and cobalt on the titanium substrate. Based on Mie’s theory, the surface plasmon for the nickel and cobalt appear for NCT-2 and NCT-3 at 352 and 357 nm (Figure 6b), respectively.44, 50 However, the NCT-1 sample shows nickel peak with a blue shift at 260 nm from characteristic metallic nickel peak at 303 nm due to the less available nickel particles involved in SPR.44, 52 Even though NCT-2 and NCT-3 samples show SPR behavior, the NCT-3 exhibits red shifts in wavelength due to the increased particles size due to the agglomeration.44,

51-53

Also, NCT-3

exhibits the peak at 270 nm relates to the nickel metal which deviates from the characteristic metallic nickel peak at 303 nm. This may be due to the less available nickel particles involved in SPR. Meanwhile, individual metal deposits NT and CT were also characterized by UV-Vis spectroscopy, the NT shows only nickel peak at 338 nm, whereas CT shows very low reflectance value to qualify the peak. (Figure S12).44, 51- 52 This indicates that during the electrochemical synthesis initially most preferred deposition is nickel and then subsequently cobalt also actively contribute to the alloy formation. Hence, unlike the NCT-1, NCT-2 and NCT-3 electrodes show the SPR due to the bi-metallic alloy behavior which confirms the interaction of nickel and cobalt. 3.4. Electrochemical Activity. The HER activities of NCT-1, NCT-2, NCT-3, and Pt/C supported on Ti electrodes were evaluated using linear sweep voltammetry in the potential range from 0.2 to -0.40 V vs. RHE in 1 M KOH (Figure 7). The NCT-1, NCT-2, NCT-3 and Pt/C supported on Ti electrodes exhibit HER overpotentials of 177, 125, 260, and 106 mV,


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respectively at a current density of -10 mA cm-2 (Figure 5a). The HER Tafel slopes for the NCT1, NCT-2, and NCT-3, were 57, 90, 47 and 41 mV dec-1, respectively (Figure 7b). The NCT-2 exhibits lower Tafel slope value compared to the NCT-1 and NCT-3 due to the fast electrode kinetics attributed by the strong Tafel-Volmer steps ((1) & (2)) in the Tafel-Volmer-Heyrovsky mechanism ((1), (2) & (3)). 41 H2O + M + e-

MHads

+ OH-

(1)

H2O + MH ads + e-

H2 + M

+ OH-

(2)

MH + MHads

H2 + 2M

(3)

The NCT-3 electrode exhibits a 30 mV less overpotential than NCT-2 due to the low available content of cobalt with nickel in the NCT-3 alloy. Further, the controlled (CT and NT) samples show the HER overpotentials of 380 mV and 263 mV, respectively indicating that the alloying effect of nickel and cobalt is responsible for the better performance for the NCT-2 (Figure S13a). The NCT-2 electrode exhibits 125 mV HER overpotential to offer -10 mA cm-2. The overpotential observed for NCT-2 electrode is much lower when compared with the recently reported non-precious catalysts such as NiαCo3-αO4 nanowires (138 mV @ 10 mA cm-2), Co-NiG (330 mV@-10 mA cm-2), NiCo alloy (439 mV@ -10 mA cm-2), CoOx@CN (232 mV @ -10 mA cm-2), cobalt embedded nickel on carbon (249 mV@-10 mA cm-2) as listed in Table S8.24, 27, 54 – 56

Based on the robust HER activity, we also evaluated the OER activity for the NCT-1, NCT-2, NCT-3, and RuO2 supported on Ti electrodes in 1 M KOH (Figure 8). The NCT-1 electrode requires an overpotential of 291 mV to afford 100 mA cm-2. However, NCT-2 and NCT-3 electrodes require the overpotentials of 331 and 361 mV, respectively at 100 mA cm-2 (Figure 8a). The NCT-2 electrode shows much lower OER overpotential value of 331 mV compared


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with the benchmark catalysts, RuO2 (351 mV). The OER Tafel slope values for the NCT-1, NCT-2, NCT-3, and RuO2 supported on Ti were observed to be 90, 36, 60 and 96 mV dec-1, respectively (Figure 8b). Based on the literature, the low Tafel slopes of NCT-2 may follow the mechanism through forming the active β-oxy-hydroxides during the electrochemical performance which is given in the below equation.41 MO(OH) + OH-

[MO(OH)2]+ + e-

(4)

[MO(OH)2]+ + 2OH-

[MO]+ + O2 +2H2O+ 2e-

(5)

[MO]+ + OH-

MOOH

(6)

The least performance of NCT-1 electrode might be due to the less active Ni-Ti phase for OER and the best activity of NCT-2 due to the optimum ratio of nickel (68%) and cobalt (32%) deposited on the titanium. Further, controlled samples cobalt (CT) and nickel (NT) show OER overpotentials of 491 and 551 mV, respectively indicating that the alloy nature of nickel and cobalt responsible for the better performance for the NCT-2 (Figure S13 b). To further understand the enhanced activity of NCT-2 sample, we analyzed the effect of alloying on the metal oxidation potential by the cyclic voltammetry in the region of 0.6 to 1.8 V vs. RHE (Figure S13c). The anodic peak potential of 1.18, 1.36, 1.40, 1.26 and 1.27 V was observed for NCT-1, NCT-2, NCT-3, NT, and CT, respectively. The NCT-1 sample shows a peak potential of 1.18 V, whereas the NT (Ni-Ti) exhibits a peak at 1.26 V (Table R1 and Figure S13c). The Co incorporation in Ni shifts the oxidation potential to more cathodic direction. However, when increasing the cobalt content from NCT-1 to NCT-2 displays the peak shift from 1.18 to 1.36 V (Figure S13c and Table R1) which is due to the increased amount of nickel in the NCT-2 alloy. This is also further understood from the peak potential differences in the NT and CT about 1.26 and 1.28 V, respectively. Even though the metal content in the alloy plays an important role in


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the activity, the important reason behind the activity is due to the in-situ formation of active hydroxides (M(II) to M(III) (M= Ni, Co)) of nickel and cobalt during the oxygen evolution reaction which is responsible for improved activity in the OER.57 The better activity of the NCT-1 compared to the NCT-2 may be due to the existence of the NiTi phase which is different from other control samples. Moreover, the NCT-2 catalyst exhibits the better activity about 221 mV overpotential to achieve 10 mA cm-2 current density when compared to the recently reported non-precious catalysts such as NiαCo3-αO4 nanowires (337 mV @ 10 mA cm-2), β-Ni(OH)2 (340 mV@10 mA cm-2), CoOx@CN (260 mV@ 10 mA cm-2), cobalt embedded nickel on carbon (328 mV@10 mA cm-2 ) and NiCo nanowires (302 mV@10 mA cm-2 ) (Table S8).24, 36, 55-55,58 To understand the effect of nickel and cobalt compositions on the HER and OER activity, a histogram was plotted with the HER and OER overpotentials at -10 mA cm-2 and 100 mA cm-2, respectively for the NT, CT, NCT-1, NCT-2 and NCT-3 (Figure 9a) and the results were also listed in Table 1. Control samples (NT and CT) exhibit very high overpotentials for the both HER and OER compared to the nickel-cobalt alloys. This behavior reveals that presence of cobalt with the nickel increases the HER and OER activity. Among the alloy electrodes, NCT-2 with nickel (68%) and cobalt (32%) shows least HER and OER overpotential compared to the NCT-1 and NCT-3 electrodes. The NCT-3 is less HER and OER active compared to the NCT-2 due to reduced synergistic effect between nickel and cobalt due to low cobalt concentration compared to the nickel. The high activity of NCT-2 due to the fast electrode kinetics which can be understood from the Nyquist plot of NCT-1, NCT-2, and NCT-3 as shown in Figure 9b. The impedance spectrum reveals much smaller charge transfer resistance (Rct) about 0.587 Ω for NCT-2 compared to that of NCT-1 and NCT-3 about 2.48 and 1.065 Ω, respectively. Even though in the case of HER, NCT-1 exhibits higher resistance, it outperforms


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compared with NCT-3. This could be due to the existence of the NiTi alloy which may possess high activity. However, in the case of the OER, as the deposition time increases from 2.5 min to 5 min in the NCT-1, and NCT-2, the OER overpotential is decreased to offer 100 mA cm-2 current density. Even though in the NCT-3 the deposition time is 10 min, it exhibits slightly increases overpotential than NCT-2. This could be due to the lack of cobalt content in the alloy between the surface of nickel-cobalt bi-metallic alloy and titanium substrate.58, 59 3.5. Durability Tests. Further to understand the stability of the electrodes, we specifically chosen the best NCT-2 electrode to perform chronopotentiometry of OER and HER for approximately 150 h

of continuous operation.

Figure 9c represents the HER

chronopotentiometry behavior of NCT-2 in 1 M KOH at a current density of 10 mA cm-2. The NCT-2 electrode exhibits increment in potential from 1.087 to 1.213 V with 12% loss. Moreover, the NCT-2 shows an OER performance for continuous 150 h operation with 18% loss with increment in potential from 0.657 to 0.821 V (Figure 9d). The long-term stability of the NCT-2 may be due to the stability of nickel and cobalt in alkaline medium and strong corrosion resistance in alkaline medium. 3.6. Post analysis. After durability test of NCT-2, the electrode was characterized using XRD analysis. The XRD pattern of the electrode after HER durability test showed the presence of Ni2O3, Co2TiO4, and Ti6O (Figure S14). The formation of several compounds is due to the reduction in the metal oxidation state thereby a loss in the activity. After 150 h of OER operation, the electrode exhibits the Ti, TiO2, NiTi and Ni phases (Figure S15). The formation of metal oxides may be due to the oxidation of electrode surface under constant anodic potential perturbation. We also examined the SEM morphology of the NCT-2 electrode after the 150 h HER and OER durability studies (Figure S16). The XRD pattern of the electrode after HER


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durability test showed titanium di-oxide (TiO2) (Figure S15). Based on XRD patterns of after OER tests of NCT/Ti electrodes, there were phase changes during oxidation of metal (Figure S15) might be the reason for the potential loss during long-term operation. The high and low magnification images of after HER durability test, the electrode shows the change of morphology from agglomerated particle size to porous structure (Figure S16a& b). Meanwhile, unlike the after HER electrodes, OER electrode exhibits also porous morphology (Figure S16c&d) with wrecks which are different from the initial NCT-2 electrode (Figure 3). 3.7. MEA Performance. The MEA performance using NCT-2//AEM// NCT-2 electrodes as both anode and cathode were studied. For comparison, the MEA was also constructed using Pt/C//AEM//RuO2. Figure 10 displays the alkaline electrolyte membrane water splitting using NCT-2 as both anode and cathode which represents the full water splitting properties in the plot of the voltage versus current density operation of the electrolyzers constructed using NCT-2 (both anode and cathode). The cell voltage plotted against the current density was displayed in the Figure 10a which exhibits 170 mA cm–2 at 2.0 V which is better than the MEA assembled with benchmark catalysts Pt/C and RuO2 exhibits 142 mA cm–2 at 2.0 V. The amount of hydrogen generated was calculated using water displacement and also used to calculate the columbic efficiency (Figure 10b). Both the calculated and experimental amount of hydrogen generated were well correlation each other, except slight disagreement at 1.8 V. This may be due to the lack of porous network in titanium which facilitates the proper water management. Moreover, NCT-2 exhibits outstanding performance about 170 mA cm–2 at 2.0 V where the cell constructed with reported iron as anode and Ni-Fe as cathode exhibits around 140 mA cm–2 at 2.0 V

60

and the electrodes with platinum exhibits 71 mA cm–2 at 2.0 V (Figure 10c).61,

62

Moreover, to understand the electrode stability during the AEM water electrolysis, the


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chronopotentiometric durability test at 100 mA cm-2 was carried out for NCT-2 (both anode and cathode) using the peristaltic pump. The constant current stability test of NCT-2 exhibits the performs loss about 22% after 13 h operation which is much better overpotential loss compared to the recent literature reported for the IrO2 and Pt/C as electrocatalysts for AEM.63 Thus, this prototype alkaline water electrolyzer constructed with the NCT-2 can be able to produce hydrogen if the electricity is driven by the solar energy as reported 64, 65 and also this catalyst can also be applicable to the KOH feed alkaline water electrolyzers.66, 67 4. Conclusions We have developed the single-step electrodeposition of the nickel-cobalt alloy on the titanium cost effectively. We found that the content of the nickel, cobalt, and titanium on the titanium foil various with the time of deposition. The electrochemical activity of the nickel cobalt-titanium alloy (NCT-2) shows the better HER and OER performance with overpotentials about 125 mV at -10 mA cm-2 and 331 mV at 100 mA cm-2, respectively. The NCT-2 electrode also exhibits excellent durability with 150 h continuous operation for HER at -10 mA cm-2 and OER at 100 mA cm-2. Based on the above observation, the earth abundant nickel-cobalt-titanium alloy with titanium foil electrode is a promising candidate for alkaline water electrolysis. Also, the MEA assembled with NCT-2 as anode and cathode showed 170 mA cm–2 at 2.0 V which confirms it is competitive towards its commercialization. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org/page/4authors/index.html. SEM elemental mapping and energy dispersive spectroscopy (EDS) analysis for NCT-1, NCT-2, NCT-3, NT, and CT in Figure S1-S10 and Table S1-S5. TEM energy dispersive spectroscopy analysis of NCT-2 in Figure S11. Diffuse reflectance using UV-Visible spectroscopy for the CT, NCT-2, and NT in Figure S12. The HER and OER polarization curves of CT, NCT-2, and NT in


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Figure S13. XRD pattern of before and after 150 h HER and OER durability tests of NCT-2 electrodes in Figure S14 & S15. SEM morphology after durability tests if NCT-2 in Figure S17. Literature comparison in the Table S8. AUTHOR INFORMATION Corresponding Author * Email: [email protected] OrchidID orcid.org/0000-0001-6295-2718 Funding Sources The authors acknowledge the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (17-IT-02) for financially supported. ACKNOWLEDGMENTS We thank DGIST-Center for Core Research Facilities (CCRF) for providing various facilities for sample analysis. REFERENCES (1) Giovanni, C. D.; Reyes-Carmona, A.; Coursier, A.; Nowak, S.; Greneche, J. M.; Lecoq, H.; Mouton, L.; Roziere, J.; Jones, D.; Peron, J.; Giraud, M.; Tard, C. Low-Cost Nanostructured Iron Sulfide Electrocatalysts for PEM water electrolysis. ACS Catal. 2016, 6, 2626-2631. (2) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy) Hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549-7558. (3) Pinaud, B. A.; Benck, J. D.; Seitz, L.C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983-2002. (4) Selamet, O. F.; M. S. Ergoktas, M. S. Effects of Bolt Torque and Contact Resistance on The Performance of The Polymer Electrolyte Membrane Electrolyzers. J. Power Sources 2015, 281, 103-113.


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(57) Godwin, I. J.; Lyons, M. E. G. Enhanced Oxygen Evolution at Hydrous Nickel Oxide Electrodes via Electrochemical Ageing in Alkaline Solution. Electrochem. Commun. 2013, 32, 39-42. (58) Bae, S. H.; Kim, J. E.; Randriamahazaka, H.; Moon, S. Y.; Park, J. Y.; Oh, I. K. Seamlessly Conductive 3D Nanoarchitecture of Core-Shell Ni-Co Nanowire Network for Highly Efficient Oxygen Evolution. Adv. Energy Mater. 2016, 7, 1601492. (59) Wang, N.; Hang, T.; S. Shanmugam, Li, M. Preparation and Characterization of NickelCobalt Alloy Nanostructures Array Fabricated by Electrodeposition. CrystEngComm 2014, 16, 6937-6943. (60) Rashid, M. M.; Mesfer, M. K. A.; Naseem, H.; Danish, M. Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis, and High Temperature Water Electrolysis. Int. J. Adv. Technol. Eng. Sci. 2015, 4, 80-93. (61) Opu, M. S. Effect of Operating Parameters on Performance of Alkaline Water Electrolysis. Int. J. Therm. Environ. Eng. 2015, 9, 53-60. (62) Lu, A. Y.; Yang, X.; Tseng, C. C.; Min, S.; Lin, S. H.; Hsu, C. L.; Li, H.; Idriss, H.; Kuo, J. L.; Huang, K. W.; Li, L. J. High-Sulfur-Vacancy Amorphous Molybdenum Sulfide as High Current Electrocatalyst in Hydrogen Evolution. Small 2016, 12, 5530-5537. (63) Leng, Y.; Chen, G.; Mendoza, A. J.; Tighe, B. T.; Hickner, M. A.; Wang, Y. C. SolidState Water Electrolysis with an Alkaline Membrane. J. Am. Chem. Soc. 2012. 134, 9054-9057. (64) Rozain, C.; Mayousse, E.; Guillet, N.; Millet, P. Influence of Iridium Oxide Loadings on the Performance of PEM Water Electrolysis Cells: Part II-Advanced Oxygen Electrodes. Appl. Catal., B 2016, 182, 123-131.


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(65) Chi, J.; Yu, H.; Li, G.; Fu, L.; Jia, J.; Gao, X.; Yi, Baolian, Y.; Shao, Z. Nickel/Cobalt Oxide as a Highly Efficient OER Electrocatalyst in an Alkaline Polymer Electrolyte Water Electrolyzer. RSC Adv., 2016, 6, 90397-90400. (66) Ahn, S. H.; Yoo, S. J.; Kim, H. J.; Henkensmeier, D.; Nam, S. W.; Kim, S. K.; Jang, J. H. Anion Exchange Membrane Water Electrolyzer with an Ultra-Low Loading of Pt-Decorated Electrocatalyst. Appl. Catal., B 2016, 180, 674-679. (67) Liu, J.; Yang, Y.; Ni, B.; Li, H.; Wang, X. Fullerene-Like Nickel Oxysulfide Hollow Nanospheres as Bifunctional Electrocatalysts for Water Splitting. Small 2016, Doi: 10.1002/smll.201602637.


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Figures and caption

Figure 1. (a) XRD pattern of NCT-1, NCT-2, and NCT-3. (b) Ni and Co peak shifts in NCT-2 and NCT-3.


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Figure 2. XRD pattern of NCT-2 (0.8g), NCT-2 (1.0g) and NCT-2 (1.2g).


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Figure 3. SEM images: (a) low and (b) high magnification images of NCT-1. (c) low and (d) high magnification images of NCT-2 and (e) low and (f) high magnification images of NCT-3.


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Figure 4. Nickel and cobalt content plots for the NT, CT, NCT-1, NCT-2 and NCT-3.


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Figure 5. (a)TEM image and (b) HR-TEM images of NCT-2 with indices of nickel and cobalt planes. XPS spectra of (c) Cobalt (Co2p) and (d) Nickel (Ni2p) of NCT-1, NCT-2 and NCT-3. The binding energy shifts in NCT-1, NCT-2, and NCT-3 in the (e) Co2p3/2 and (f) Ni2p3/2 region were plotted against the cobalt content.


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Figure 6. (a) Fourier Transformed Infrared spectroscopy (FT-IR) of NCT-1, NCT-2, and NCT3. (b) Diffuse reflectance spectra using UV-Visible spectroscopy for the NCT-1, NCT-2, and NCT-3 electrodes.


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Figure 7. (a) The HER polarization curves of NCT-1, NCT-2, NCT-3, RuO2 on Ti and Titanium (Ti) and (b) its corresponding Tafel slopes of NCT-1, NCT-2, NCT-3 and RuO2 on Ti.


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Figure 8. (a) The OER polarization curves of NCT-1, NCT-2, NCT-3, RuO2 on Ti and Titanium (Ti) and (b) its corresponding Tafel slopes of NCT-1, NCT-2, NCT-3 and RuO2 on Ti.


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Figure 9. (a) Comparison of the HER and OER overpotentials for the NT, CT, NCT-1, NCT-2, and NCT-3. (b) Electrochemical Z-fitted impedance spectra of NCT-1, NCT-2, and NCT-3 (c) HER chronopotentiometric durability of NCT-2 and (d) OER chronopotentiometric durability of NCT-2.


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Figure 10. (a) Cell voltage versus current density plot of AEM using NCT-2 as both anode and cathode, (b) hydrogen generation rate in mmol h-1cm-2 for the alkaline electrolyzer membrane (AEM) constructed with NCT-2 as both anode and cathode (solid blue line for H2 generation rate from displacement and red dotted line for the H2 generation rate from the coulombic charge obtained) and (c) Comparison of the AEM electrolyzer performance with relevant literature. (d) Chronopotentiometric response of MEA constructed with NCT-2 as anode and cathode.


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Table 1. Comparison of nickel and cobalt composition for all the nickel and cobalt alloys with HER and OER activity.

Samples

Ni (%)

Co (%)

HER overpotential at -2 -10 mA cm

OER overpotential -2 at 100 mA cm

(mV)

(mV)

NT

100

-

263

551

CT

-

100

380

491

NCT-1

81

70

177

-

NCT-2

68

32

125

331

NCT-3

75

25

260

361


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Table of Content


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Nanostructured NickelâCobaltâTitanium Alloy ... - ACS Publications

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