The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis

May 31, 2016 - This catalyst was found to produce current densities of 10 mA/cm2 at the relatively low overpotential of 320 mV in alkaline electrolyte...
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The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-catalysts” Oluwaniyi Mabayoje,† Ahmed Shoola,† Bryan R. Wygant,† and C. Buddie Mullins*,†,‡,§ †

Department of Chemistry, ‡Department of Chemical Engineering, and §Texas Materials Institute, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Oxygen evolution catalysts composed of a metal (Ni, Co, or Fe) and a pnictide or chalcogenide (P, S, or Se) counterion are a promising class of electrocatalysts for the oxygen evolution reaction (OER), an important reaction for the photoelectrochemical splitting of water. We synthesized a nickel-based oxygen evolution catalyst derived from pulse-electrodeposited nickel sulfide. This catalyst was found to produce current densities of 10 mA/cm2 at the relatively low overpotential of 320 mV in alkaline electrolyte (1 M KOH). Importantly, we found that the sulfur anion in the nickel sulfide is depleted in the active form of the electrocatalyst and that the NiS is converted into an amorphous nickel oxide in the potential range where water is oxidized to oxygen. The superior catalytic activity of this nickel sulfide is thus unrelated to the sulfur anions in the active catalyst but is instead related to the metal sulfide’s ability to act as a precursor to a highly active nickel oxide OER electrocatalyst. The nickel oxide derived from nickel sulfide was found to be amorphous with a relatively high surface area, two factors that have been previously shown to be important in oxygen evolution electrocatalysis.

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A promising class of catalysts reported for the OER are metal chalcogenides (CoSe27 and Co3S48) and metal pnictides (NiN,9,10 CoP,11 and Ni2P12). Following this trend, we investigated the activity of electrodeposited nickel sulfide (NiS) as a catalyst for the OER. Beyond simply discovering a new catalyst, we also wanted to study the role of anions, in this case the sulfide, in OER catalysis. Here, we were driven by our suspicion that the surfaces of most catalyst materials are typically partially or fully oxidized to the corresponding metal oxide at potentials necessary for water oxidation. In notable works on the surface of metal-based catalysts for the OER, researchers attributed the higher activity of a bifunctional HER/ OER nickel phosphide catalyst (relative to the conventional metal oxide) to substrate effects brought about by the presence of metal phosphide below the metal oxide layer in contact with the electrolyte.13 For a CoS2-derived OER catalyst, Cui and coworkers ascribed the activity of the “electrochemically tuned metal oxide” to a greater number of active sites in cobalt oxide catalysts derived from cobalt disulfide.13

riven by a desire to store the energy of the sun using chemical bonds, various forms of solar energy-based alternative sources are being explored as possible replacements for fossil fuels.1 One possible solar fuel is hydrogen produced in a photoelectrochemical (PEC) cell by splitting water.2 Water splitting involves two electrochemical reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Water oxidation, involving four electrons, is the more difficult of the two reactions that make up water splitting. The search for OER catalysts that can be used as overlayers for light-absorbing semiconductors has dominated PEC water splitting research in recent times. These catalysts are needed to carry out kinetically difficult electrochemical reactions, while also protecting the light-absorbing semiconductor from degradation in aqueous solutions. This protection is necessary because most semiconductors are unstable in the potential range in which the OER takes place. Metal oxide semiconductors used for the OER typically lose up to ∼66% of their initial photocurrent within a 24 h period.3 This deterioration is worse for classic nonoxide semiconductor electrodes like Si and GaAs.4,5 Outside of water splitting, the oxygen evolution reaction is also essential to the development of other technologies such as metal air batteries.6 © XXXX American Chemical Society

Received: April 22, 2016 Accepted: May 31, 2016

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phase, and a minor sulfur-deficient Ni9S8 (Godlevskite) phase. Integrating the peaks of the Ni 2p and S 2p regions of the X-ray photoelectron spectroscopy (XPS) spectra also gave a nickel− sulfur ratio close to one, consistent with EDS results (Figure S2). The electrocatalytic activity of NiS and NiO films were probed after a preanodization step in a 1 M KOH electrolyte (Figure S3). A constant current density of 2 mA/cm2 was applied to the NiS and control NiO electrodes, and the potential needed to achieve the specified current density gradually decreased during this anodization step. This process was completed in 30−35 min. The NiS samples were transformed from light black or gray films into visually transparent films after the anodization process. Anodic conditioning has been shown to improve the activity of many nickel-based oxygen-evolving catalysts.16,17 The potential needed to achieve current densities of 10 mA/cm2 decreased by 40−50 mV for both the NiO and NiS films during the anodization step. Anodic conditioning of films likely involved a change in phase of nickel oxide, a change in the oxidation state of nickel16 or the incorporation of iron impurities from the electrolytes.18 Similar levels of improvement after anodization for both the control NiO samples and the NiS-derived samples indicate that both catalysts absorb similar amounts of iron impurities from the electrolytes. Furthermore, ICP-MS results show that both samples incorporate similar levels of Fe. Linear sweep voltammetry plots recorded after anodic conditioning for both the nickel sulfide sample and the nickel oxide are shown in Figure 2a. The NiS-derived catalyst required a potential only 320 mV more positive than the reversible potential for water oxidation (1.23 V vs RHE) to achieve a current density of 10 mA/cm2. The annealed nickel oxide control sample film, in contrast, required a higher overpotential of 370 mV to reach

Here, we report a nickel-based oxygen evolution catalyst derived from pulse electrodeposited nickel sulfide. We study its activity as a catalyst for the oxygen evolution reaction, and importantly, we discuss the role of the sulfur counterion in the active form of the electrocatalyst. We find that the active catalyst is an amorphous nickel oxide and that the superior activity of the nickel sulfide-derived material has little to do with sulfur counterions. Instead, the metal sulfide acts as a precursor to a highly active nickel oxide OER electrocatalyst. We hypothesize that metal oxides are ultimately the active components of oxygen-evolving catalysts, and metal chalcogenides act as precursors to these highly active metal oxides. The multipotential technique (Figure S1) used in preparing these catalysts14 led to a ∼1:1 Ni:S ratio as determined by energy-dispersive X-ray spectroscopy (EDS) (Figure S2). The first potential step at a reducing potential of −0.9 V vs Ag/AgCl led to the deposition of nickel-rich Ni−S, while the oxidative polarization step removed some of the excess nickel formed in the first step. A typical current−time plot obtained during the deposition process is shown in Figure S1. A 900 s (30 cycle) process led to the deposition of NiS on the fluorine-doped tin oxide (FTO) substrate. Nickel hydroxide thin films, as a precursor to NiO control samples, were also prepared by a cathodic electrodeposition procedure in a 0.1 M Ni(NO3)2 bath. At suitable cathodic potentials, hydroxyl ions are generated from the reduction of nitrate ions, which causes the precipitation and deposition of Ni(OH)2 on the substrate.15 Films with nickel mass loadings similar to the NiS are obtained after eight current pulses (1 mA/cm2 for 5 s followed by 10 s at zero current). Crystalline NiS and NiO control films are obtained after annealing in a tube furnace at 350 °C in argon for 1 h. Inductively coupled plasma-mass spectrometry (ICPMS) analysis showed that the NiS and NiO films had similar Ni loadings of ∼5 μg/cm2 before and after anodization. Initial attempts at identifying the as-deposited films using Xray diffraction (XRD) on FTO showed only peaks attributable to SnO2 from the substrate. To minimize the influence of the substrate on these measurements, XRD patterns were collected from powder material scraped from relatively thick films (120 cycles) prepared in the same manner as the films used for electrochemical testing and used to probe the crystallinity of the electrodeposited material. Powder XRD patterns (Figure 1) showed peaks attributable to both a primary NiS (Millerite)

Figure 1. XRD patterns for electrodeposited NiS thin films annealed in Ar at 350 °C. Peaks marked by a black star (*) represent a Millerite NiS phase (PDF 01-074-7329) and peaks marked by a red star (*) represent a Godlevskite Ni9S8 phase (PDF 01-078-3209).

Figure 2. (a) LSV curves on NiS and NiO thin films. (b) Amperometry curves of a NiS film held at a potential of 1.55 V vs RHE for 6 h. Both experiments are carried out in 1 M KOH. 196

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Figure 3. (a) XRD patterns for NiS and NiO samples after cyclic voltammetry in the potential region where the OER takes place. (b) Ni 2p XPS region and (c) S 2p XPS region for NiS samples after using these materials as OER catalysts. A sample in a near-neutral pH 9.2 KBi buffer was used to investigate if the transformation from NiS to NiOx depends on the pH of the electrolyte.

reported by Cui and co-workers for CoS2-derived materials.13 These authors also reported the superiority of the CoS2-derived sample compared to crystalline cobalt oxide prepared by thermal annealing. Methods aimed at creating amorphous phases of metal oxides/hydroxides as catalysts for the OER have previously yielded promising results. For example, electrodeposited amorphous cobalt phosphate (Co−Pi)20 and nickel borate (Ni−Bi)16 have shown excellent activity for the OER in neutral and near neutral (pH 9.2) electrolytes. Previously, our group has also shown that thin films of amorphous FeOOH catalysts achieve higher currents at lower overpotentials than crystalline FeOOH catalysts reported in the literature.21,22 Similar results were reported by Berlinguette and co-workers for amorphous metal oxides derived from the photochemical decomposition of metal−organic precursors.23,24 XPS (Figure 3b,c) was used to confirm the transformation of NiS into nickel oxide. A comparison of the XPS spectra for the as-deposited and electrochemically oxidized samples shows that peaks in the Ni 2p region of the XPS spectra were shifted to higher binding energies, with the major peak located at 852.8 eV before testing and shifting by 2.5 to 855.3 eV after electrochemical cycling. More dramatically, the samples polarized in both basic potassium hydroxide and near-neutral (pH 9.2) potassium borate (KBi) buffered electrolytes show no peaks in the S 2p region of the XPS spectra (Figure 3c). We Ar sputtered thin films of electrochemically oxidized NiS samples and repeated XPS measurements (Figure S8) in the S 2p region to investigate the concentration of sulfur in the bulk of films as well as at the surface and found no peak, indicating that sulfur was completely depleted from the material. On the basis of these XPS results and the XRD results discussed earlier, we conclude that the NiS sample, after being used as an OER catalyst, is transformed into an amorphous nickel oxide (NiOx). The mechanism for the oxidation of nickel sulfide is beyond the scope of the present study. Metal sulfides like pyrite (FeS2) are known to be converted to metal oxides in the presence of oxygen gas in aqueous solutions.25 We speculate that the oxygen produced during water oxidation and the oxidizing potentials needed for the OER may be responsible for the conversion of the nickel sulfides to the corresponding metal oxides. Likely many additional studies are needed and will be required by the electrocatalysis community to uncover the very complicated mechanism for the creation of the amorphous oxide beginning with the sulfide. Because pyrite-type (MS2) metal disulfides are also converted to metal oxides when used as OER catalysts,13 we further hypothesize that this trans-

the same current density. The overpotential needed for the NiS-derived film was within 30 mV of the best nickel-based catalysts reported for the OER; nickel phosphides at mass loadings of 140 μg/cm2 required 290 mV of overpotential to achieve current densities of 10 mA/cm2.11 The OER overpotentials required for several reported nickel-based catalysts to achieve current densities of 10 mA/cm2 on nonporous substrates (FTO or glassy carbon) are shown in Table S1. Amperometric experiments conducted over a 6 h period were used to investigate the electrochemical stability of NiS-derived catalysts for the OER. Results presented in Figure 2b show that the current produced at 1.55 V vs RHE decreased by less than 0.2 mA/cm2 during this period, indicating that these films are stable for the OER. Shorter (2 h) stability tests showed the films retained similar nickel loadings after amperometric measurements. To probe the effects of annealing temperature on OER performance, we also tested nonannealed NiS samples. We found that nonannealed films performed better as catalysts for the OER than the annealed samples shown in Figure 2a, reaching current densities of 10 mA/cm2 at an overpotential of 310 mV, but they were not as stable as annealed samples, as shown by a larger decrease in current density during a 6 h stability test (Figures S3 and S4). Thinner NiS films prepared using only 10 deposition cycles were also compared to thicker samples. Expectedly, these thinner films performed worse than the films deposited using 30 cycles.19 As shown in Figure S5, the thinner sample required 335 mV to achieve current densities of 10 mA/cm2. Increasing the number of deposition cycles beyond 30 did not improve the activity of these catalysts. A YSI 5100 dissolved oxygen measurement probe was used to compare the amount of oxygen evolved during 30 min to the theoretical amount of oxygen calculated from the charge passed when passing a current of 10 mA over the same period (Figure S7). The expected amount of oxygen evolved was compared to the actual amount of oxygen evolved and found to be between 90 and 95%, indicating that the catalyst selectively oxidized water to oxygen. XRD patterns of the NiS sample after electrochemical cycling (Figure 3a) were also obtained from powder samples scraped from FTO substrates. These postcycled films, referred to as NiS → NiOx from here on, display none of the NiS or Ni9S8 peaks seen in the noncycled samples, indicating that the samples became X-ray amorphous after being used as OER catalysts. The control NiO, on the other hand, still shows the characteristic peaks expected of nickel oxide after electrochemical testing. The transformation of metal sulfides into amorphous metal oxides during oxidative cycling has been 197

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ACS Energy Letters formation happens for all the phases of nickel sulfides in neutral and basic electrolytes. To test this hypothesis, we prepared a Ni3S2 sample on Ni foil by hydrothermal sulfurization using thioacetamide26 and found these samples were also transformed into an amorphous sample after electrochemical anodization in base (Figure S10). These results raise questions about the role of chalcogenide and even pnictide counterions in OER catalysis. The higher activity for the OER observed for these films, in the literature and in this study, is consistent compared to analogous crystalline metal oxides even in the case of the sulfides where the counterion is depleted after electrochemical oxidation. Many enlightening studies have previously reported these materials as OER catalysts, attributing the activity of these materials to the presence of chalcogenide and pnictide counterions without characterization of the surface or bulk composition of samples during and after OER catalysis.8,27,28 In the case of the pnictides12 which are more resistant to bulk oxidation, reports speculate on the role of metal pnictides in the bulk of the films without studying the role of the metal oxides formed by electrochemical transformation on the catalyst’s surface. Nath and co-workers have studied the role of heavier chalcogenides like selenides in water oxidation,29 and while they show that the selenium content was not fully depleted after the OER, the role of bulk selenides in OER electrocatalysis is not yet fully understood. Here, we focus attention on the oxide formed during the OER. We show that the reported chalcogenide and pnictide materials may be better described as “pre-catalysts”, or precursors to the true metal oxide OER catalysts. We also used XPS to compare the oxidation state of nickel in the amorphous NiS → NiOx catalyst and the crystalline nickel oxide control catalyst. A report comparing amorphous and crystalline cobalt−iron oxides by X-ray absorption spectroscopy (XAS) showed that the oxidation state of cobalt in amorphous samples was closer to Co(III), while that in the crystalline samples was more similar to Co(II).30 The possibility of different oxidation states and different metal coordination environments was probed by comparing the Ni 2p and Ni 3p regions of both the NiS-derived sample and crystalline NiO. XPS measurements (Figure 4) in the Ni 2p region (Figure 4a) showed no difference in peak shapes and peak positions for the NiS → NiOx and NiO samples. The insensitivity of XPS peak positions to oxidation states in this region has previously been attributed to the multiplet effect in XPS measurements. Other researchers have reported better sensitivity to oxidation states in the 3p region of 3d transition metal oxides.31 Looking at the Ni 3p region (Figure 4b) of the NiS → NiOx and NiO control samples, we observe a difference of 0.3 eV in the peak position of these two samples, with the NiS-derived sample shifted to more positive binding energies. We consider this indicative of the presence of higher oxidation states for Ni or of a difference in the coordination of Ni in the NiS-derived samples. Nickel vacancies, known to exist in NiO, have been well studied by researchers examining this material as a p-type light absorber for dye-sensitized solar cells (DSSC) where the vacancies are known to lead to the creation of Ni(III) surface states which serve as recombination centers.32,33 These are generally passivated by atomic layer deposition of Al2O3, and cyclic voltammetry plots of these passivated films show smaller “preoxidation” peaks.33 The presence of nickel vacancies may lead to slightly higher oxidation states, and we postulate the presence of Ni vacancies as a possible explanation

Figure 4. (a) Ni 2p and (b) Ni 3p XPS regions comparing NiSderived NiOx samples with NiO samples grown by thermal annealing.

for the shifts in the Ni 3p peak positions of NiS → NiOx relative to NiO. This shift is analogous to the slight shifts observed for oxygen vacancies induced by annealing metal oxides like TiO2 in a reducing atmosphere.34,35 Nickel vacancies likely exist to a larger extent in the typically amorphous or slightly crystalline NiOx grown by electrochemical oxidation when compared to NiO grown by thermal annealing. The imperfect growth of an amorphous oxide by electrochemical oxidation of the NiS is what likely creates these Ni(III) sites. These defects may lead to a preponderance of active sites for the oxygen evolution reaction when nickel sulfide is converted to nickel oxide.9 To gain a better understanding of the superior activity of the amorphous NiS-derived (NiS → NiOx) samples when compared to NiO, we examined the possible influence of morphology on the activity of these thin films. The performance of nickel-based oxygen evolution catalysts have shown high dependence on factors like film morphology and nanostructure.36 Simple changes like adding additives in electrodeposition baths37 and using pulsed deposition19 (as opposed to continuous deposition) have recently been shown to affect the morphology and activity of nickel oxide/hydroxide water oxidation catalysts. We consider the possibility of forming high surface area films using metal sulfide materials electrochemically converted into oxides. Liu and Xue have previously reported on the generation of high surface area materials when metal sulfides are converted into metal oxides.38 Pertinently, a higher electrochemical surface area (ECSA) was reported for CoS2-derived amorphous cobalt oxide samples compared to deposited and annealed CoO thin films.13 For the low mass loadings used in these experiments (∼10 μg/cm2), the apparent 198

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arise from the insulating nature of the films in potential regions negative of the “peroxidation” peak near 1.4 V vs RHE, representing the oxidation of Ni(II) to Ni(III).19 The ECSA obtained using capacitance measurements from AC impedance spectroscopy, taken over a frequency range of 0.1 to 100 kHz at several potentials positive of the Ni2+/Ni3+ peroxidation peak, yields more reasonable measurements of the electrochemical surface area. Capacitance values were measured by fitting the AC impedance measurements to a modified Randle’s circuit with a resistor (Ru) connected in series to a resistor (Rct) and a constant phase element which represents the double-layer capacitance. The capacitance values in Figure 6 show that the NiS-derived samples had a value for capacitance almost double those measured from crystalline NiO samples. An increase in the capacitance, and therefore the electrochemical surface area, reasonably correlates with an increase in the activity as reported for many metal oxides.39 In conclusion, we have synthesized a nickel-based oxygen evolution catalyst by cycling pulse-electrodeposited nickel sulfide at potentials necessary for water oxidation. This catalyst was found to produce benchmark current densities of 10 mA/ cm2 at a relatively low overpotential of 320 mV in 1 M KOH electrolytes. Using XPS, we observed that the sulfur anion in the nickel sulfide is totally depleted in the active form of the electrocatalyst. X-ray diffractograms and XPS spectra also show the NiS is converted into an amorphous nickel oxide in the potential range where water is oxidized to oxygen. Therefore, the superior catalytic activity of this metal chalcogenide is not related to the sulfur in the active catalyst; instead, the metal sulfide serves as a precursor to a more active nickel oxide OER electrocatalyst. The nickel oxide derived from nickel sulfide had properties similar to active nickel oxide catalysts reported in the literature in that they were all amorphous and had relatively high surface areas. Using XPS measurements in the Ni 3p region, we speculate that these NiS-derived materials also had higher oxidation states of nickel than crystalline nickel oxide control samples, induced by the presence of nickel vacancies. In summary, using a sulfide “pre-catalyst” it is possible to create a metal oxide OER catalyst with physical and chemical properties which result in catalytic performance superior to that of the native oxide.

surface roughness of the films was mostly determined by the roughness of the underlying FTO substrate, with feature sizes being similar for the NiS sample (Figure 5a), and the FTO

Figure 5. (a) SEM images for (a) FTO substrates, (b) NiS, and (c) electrochemically oxidized NiS samples.

substrate, (Figure 5c) as can be seen in the scanning electron microscopy (SEM) images. Interestingly, we also observe that the electrochemically oxidized NiS (NiS → NiOx) has a morphology slightly different from as-deposited NiS. While the NiS film is made up of sphere-like nanoparticles of about 100− 200 nm diameter, the oxidized form looks rougher, indicative of a structural and morphological evolution when metal sulfides are used as OER catalysts. Furthermore, and more quantitatively, we estimate the relative electrochemical surface area of the NiS → NiOx and control NiO films by measuring their capacitance (Figure 6).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00084. Detailed experimental information, EDX data, linear scanning voltammetry curves for unannealed NiS samples and thinner NiS samples, long-term tests, oxygen detection experiments, Bode plots for both NiS and NiS → NiOx samples (PDF)

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Figure 6. Capacitance values for NiS and NiO samples obtained through AC impedance measurements.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Finding the capacitance of thin films through conventional cyclic voltammetric measurements was not possible for these nickel-based thin films reported here because these values tend to be significantly underestimated when measured using cyclic voltammetry, as previously observed by researchers in the literature.19,39 As reported by Boettcher et al., the difficulties in estimating the capacitance and ECSA using cyclic voltammetry

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the U.S. Department of Energy (DOE) Grant DE-FG02-09ER16119 and Welch 199

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Foundation Grant F-1436. O.M. gratefully acknowledges National Science Foundation (NSF) Integrative Graduate Education and Research Traineeship (IGERT) Grant 0966298 for financial support. We also acknowledge the National Science Foundation (Grant 0618242) for funding the X-ray photoelectron spectrometer used in this work.

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