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Promoting Oxygen Evolution Reactions through Introducing Oxygen Vacancies to Benchmark NiFe-OOH Catalysts Majid Asnavandi, Yichun Yin, Yibing Li, Chenghua Sun, and Chuan Zhao ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Energy Letters

Promoting Oxygen Evolution Reactions through Introducing Oxygen Vacancies to Benchmark NiFeOOH Catalysts Majid Asnavandi,a Yichun Yin,b Yibing Li,a Chenghua Sunc,† and Chuan Zhao a,† a

School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia

b

School of Chemistry, Faculty of Science, Monash University, Clayton, VIC 3800, Australia

c

Faculty of Science, Engineering & Technology, Swinburne University of Technology,

Hawthorn VIC 3122, Australia AUTHOR INFORMATION Corresponding Author Email

addresses

of

the

corresponding

authors:

[email protected]

and

[email protected]

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ABSTRACT: Advanced electrocatalysts towards oxygen evolution reaction (OER) at high current density with low overpotential remain a significant challenge for electrochemical water splitting. Herein, NiFe-based catalysts with appropriate electronic conductivity and catalytic activity have been obtained through introducing oxygen vacancies by a facile and economic NaBH4 reduction approach. The combined density functional theory calculations, physical characterization, and electrochemical studies disclose that the reductive treatment creates high amount of oxygen vacancies, high active sites, and low energy barrier for OER. The oxygen vacancy-rich catalyst yields a more than two-fold increased current density (from 100 to 240 mA cm-2) at a low overpotential of 270 mV, accompanied with good stability under OER condition. The approach is also broadly applicable for NiFe compounds synthesized via different methods or substrates.

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Electrochemical water splitting has been considered as a promising approach for producing hydrogen fuels from water in large scale. However, the kinetics of oxygen evolution reaction (OER) at the anode is slow and consequently affects hydrogen evolution reaction (HER) at the cathode.1 Iridium dioxide (IrO2) and ruthenium dioxide (RuO2) are the most active OER catalysts to increase OER kinetics, which however have high cost and their supply is not sustainable.2 Hence, alternative OER electrocatalyst on the basis of first-row transition metals and their complexes such as iron hydroxide, nickel oxide, and cobalt-oxyhydroxide-phosphate have been intensively studied.1,3,4 The introduction of Fe component in nickel-based catalysts can cause strong synergistic effects by affecting the catalyst electronic structure.5,6 Nevertheless, NiFe mixed oxides and hydroxides suffer from poor conductivity and lack of knowledge of active sites.7,8 Therefore, surface engineering is crucial to improve the NiFe-based materials electrochemical activity to catalyze OER. Here we demonstrate a straightforward approach for enhancing the electrocatalytic performance of NiFe (oxy)hydroxides for OER via engineering surface oxygen vacancies. Oxygen vacancies (OVs) can often generate interband states for metal oxides and thus may promote electron mobility and improve the conductivity.9,10,11 Another effect associated with OVs is the generation of lowly coordinated metals (LCMs), which have been identified as the active sites of metal oxides catalysts for OER8,12 while the majority surface, like the natural (0001) surface for Co-OOH, often does not perform well.13 Such LCMs are only found at the steps, edges and minority surfaces, which explains why active sites are generally not abundant. We show that the catalytic performance of NiFe-OOH on different substrates including nickel foam (NF) and glassy carbon (GC) for OER can be improved by creating OVs on the surface,


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which results in rapid charge transfer and favorable energetics for OER by more than 200% of current density of NiFe/GC electrode at overpotential of 370 mV in 1 M KOH. We firstly carried out density functional theory (DFT) computational investigation of OVs effect on the OER performance of NiFe-OOH.

Figure 1. OVs effect on OER catalysis performance predicted by DFT calculations. Intermediate states (S, S-OH, S-O, and S-OOH) and Gibbs free energy changes associated with the elementary reactions of OER on Fe-doped NiOOH (a) without OV and (b) with OV. The catalyst is modelled by a slab with (01-12) surface exposed to reactants, which has been proposed as the major active surfaces for OER in the case of NiFe-OOH.14 To evaluate the OER overpotential, Gibbs free energy changes (∆G) for four elementary reactions (Equations 1-4) are calculated using standard hydrogen electrode as the reference:15 S + OH- → S-OH + e-

(Equation 1)

S-OH + OH-→ S-O + H2O + e-

(Equation 2)

S-O + OH-→ S-OOH + e-

(Equation 3)

S-OOH + OH- → S+ O2 + H2O + e-

(Equation 4)

where S represents the catalyst active sites. OER on the surface without OVs is firstly studied, and the geometries and calculated ∆G are shown in Figure 1a with ∆Gmax= 1.78 eV, leading to


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an overpotential of 0.55 V when OER occurs at Fe-site. When an oxygen (indicated by the circle in Figure 1) is removed to generate an OV and LCMs (four-coordinated Fe and Ni), labelled as Fe4c and Ni4c, over which reactant species (O, OH, OOH) are adsorbed and fully relaxed, with the geometries and calculated ∆G shown in Figure 1b, in which ∆Gmax is 1.41 eV and overpotential η = 0.18 V. In comparison to the OV-free case, the overpotential has been reduced by 0.37 V, suggesting that LCMs associated with OVs are more reactive, which is consistent with the understanding from early works that the high catalytic performance of NiFe-OOH origins from under-coordinated metals, like defects, steps, and edges.16 To experimentally validate the hypothesis, a NiFe-OOH electrode was firstly prepared by electrodeposition of NiFe (oxy)hydroxide onto a nickel foam (NF) substrate.17 Subsequently, the as-prepared NiFe electrode was subjected to chemical reduction with NaBH4 to synthesize reduced NiFe on NF, dubbed r-NiFe/NF. As shown in Figure S1, the color and appearance of the obtained r-NiFe/NF electrode show no significant change by visual inspection after chemical reduction. The NiFe-OOH electrodeposited on the NF substrate has a nanosheet structure with open areas between the nanosheets, as displayed in Figure S2. According to Figure 2a, these rippled nanosheets form mesoporous composite film with high surface area and the NaBH4 reductive treatment does not induce any detectable change to the morphology. X-ray diffraction (XRD) patterns of NiFe/NF and r-NiFe/NF electrodes in Figure S3 show no other peaks apart those from nickel foam, suggesting that the materials deposited and after NaBH4 reduction are both amorphous (or locally ordered which cannot be detected by XRD). This was further confirmed by high-resolution transmission electron microscopy (HRTEM) where no typical lattice fringes corresponding to Ni, Fe or NiFe composites were detected in Figure S4.


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X-ray photoelectron spectroscopy (XPS) measurements were carried out to gain insight into the chemical state of Ni and Fe and the formation of OVs in the pristine and reduced NiFe samples (Figure S5a). The XPS data reveals that the Ni:Fe in both pristine and reduced material is 3:1 confirming atomic adsorption spectroscopy (AAS) results which show ca. 36% Fe content in the NiFeOOH material, before and after the NaBH4 treatment. In the Ni 2p (Figure S5b) and Ni 3p (Figure S5c) spectrums of pristine NiFe/NF, the Ni 2p1/2 at 855.8 belongs to Ni(OH)2,18 while peaks at 68.2 and 71.3 eV are attributed to Ni2+ 3p1/2 and Ni3+ 3p3/2,19,20 respectively, revealing the coexistence of Ni2+/Ni3+ and the NiOOH phase.21 The Fe 2p spectrum (Figure S5d) displays two major peaks at 724.78 and 711.58 eV with an obvious Fe 2p3/2 associated satellite peaks at around 719.0 eV, indicating that Fe-atoms in the pristine samples are presented as Fe3+ in the phase of FeOOH.22,23 Compared to the pristine NiFe sample, new peaks in Ni 2p (853.3 eV) and Fe 2p (707.6 eV) spectrums corresponding to metallic Ni and Fe, respectively,21,22 and a new broad shoulder in the Fe 3p spectrum (Figure S5c) attributed to Fe2+,24 was observed in the r-NiFe/NF sample. Moreover, there is a small peak shift for both Ni and Fe toward lower binding energies after NaBH4 treatment (See Figure S5a-d), suggesting a decrease in the oxidation state of the transition metals.25 Hence, the material comprises metallic NiFe and oxygen vacancy-rich NiFe-OOH. The decrease of Ni and Fe oxidation states is also confirmed by pre-edge peaks in the X-ray absorption near edge structure (XANES) spectra in which the peaks energy position and intensity depend strongly on the oxidation state and geometry around Ni and Fe.26 According to Figure S6, XANES spectra of Ni and Fe of the NiFe-OOH have similar absorption profiles before and after the NaBH4 treatment. It means that the treatment does not have any effect on the crystal structure of the compounds. However, the pre-edge area in Figure 2b depicts that the small peaks


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in pre-edge zone of reduced samples have shifted slightly to lower energy, suggesting lower oxidation state of Ni and Fe after the treatment.14,27 The small peaks also show higher absorption values revealing lower coordination numbers of Ni and Fe, which is caused by OV.28 All of these results suggest both nickel and iron were partially reduced to lower oxidation state by NaBH4 treatment to form new oxygen vacancies, with no effect on bulk structure of the catalyst.

Figure 2. a) SEM image of the synthesized r-NiFe/NF electrode, b) high resolution Ni and Fe pre-edge X-ray absorption near-edge structure (XANES), c) high resolution O 1s spectrum of XPS pattern of NiFe-OOH after the NaBH4 treatment, d) PL spectra, e) EPR spectra and f) bandgap energy determination via UV-vis spectroscopy. The formation of OVs in the NiFe-OOH after the reductive treatment was examined by the O 1s core level spectra. As shown in Figure 2c, there are three spin-orbit peaks including i) oxygen-


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metal bond in the lattice with lower binding energy (530.1 eV), ii) oxygen loss with low oxygen coordination in the material at medium binding energy (531.7eV) and iii) adsorbed oxygen on and within the surface with higher binding energy (533.2 eV).23,29 The OV density formed can be estimated by taking the ratio of peak area of oxygen loss to lattice oxygen.23 From Figure 2c and Figure S5e, the OV density in the r-NiFe/NF electrode is 7.0, which is more than 2 times higher than that in the pristine one which was 3.2 before the treatment. The increased density of defects is confirmed by Raman scattering spectra where weaker and broadened peaks were observed after reduction.30,31 The Raman spectra of pristine and rNiFe/NF electrode are shown in Figure S7, also confirms the presence of FeOOH at 310 and 710 cm-1,32 NiOOH at 550 cm-1,33 and nickel hydroxide at ca. 480 cm-1.34 Moreover, a shoulder is observed at ca. 3640 cm-1 due to disordered nickel hydroxide33 which is more prominent for rNiFe sample than the pristine one. This finding is further confirmed by photoluminescence (PL) spectroscopy shown in Figure 2d, as the r-NiFe/NF sample displays a strong PL emission peak at ca. 410 nm which corresponds to the recombination of holes with two-electron-trapped OV.35,36 Furthermore, electron paramagnetic resonance (EPR) spectroscopy, Figure 2e, reasserts that the r-NiFe has more defects in the structure and the signals at g = 1.99 suggest that the structural defects come from oxygen vacancies.35,37 UV-vis spectroscopy has been utilized to investigate the effect of reductive treatment on electrical properties of NiFe-OOH. Based on Figure S8, the reflectance intensity and accordingly the band gap energy (Eg) are determined to be different for the pristine and reduced NiFe samples. The Eg values calculated with the Tauc method38 give different values for NiFe and r-NiFe samples and show the treatment affects the NiFe-OOH conductivity39,40 (see Figure 2f). The difference in the catalyst conductivity is consistent with DFT calculations.


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The OER performances of the pristine NiFe/NF and reduced (r-NiFe/NF) electrodes are tested in 1 M KOH (Figure 3a). Scanning to an anodic potential above 1.45 V, a rapid increase of oxygen evolution current is detected, accompanied by vigorous gas bubble formation, at both electrodes. The oxidation peak observed between 1.3 V and 1.45 V (vs RHE) belongs to phase transformation between Ni(OH)2 to NiOOH.33 It is found that after reduction the Ni2+/Ni3+ peak shifted to a more positive potential at the r-NiFe/NF electrode, indicating changes in its electronic structure. Significant higher OER current is observed at r-NiFe/NF electrode than pristine NiFe/NF at higher potentials. At an overpotential of η = 270 mV, the r-NiFe/NF electrode can deliver 240 mA cm-2 in 1 M KOH, which is 2.4 times higher than pristine NiFe/NF (100 mA cm-2). This result confirms that NaBH4 reduction can enhance the OER activity of NiFe dramatically. Such increase in OER current without any significant change of onset potential indicates that the increase of NiFe/NF electrical conductivity and active sites induced by the large concentration of OVs and LCMs. The fabricated electrode performance is compared with some other NiFe-based OER electrodes in Table S1. The OER curves with no iR-compensation are also presented in Figure S9.


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Figure 3. Electrochemical behavior of the oxygen electrodes; a) OER polarization of the electrodes, before and after NaBH4 treatment in 1 M KOH at scan rate of 5 mV s-1 with 95% iRcompensation of NiFe/NF electrode, b) EIS response of NiFe/NF and r-NiFe/NF electrodes, c) chronopotentiometry of r-NiFe/NF electrode in 0.1 and 1 M KOH, d) OER polarization of NiFe/GC electrode. To elucidate the influence of surface area, the electrochemical surface area (ECSA) of rNiFe/NF has been determined by double layer capacitance in 1 M KOH solution1 (details in the supporting information and Figure S10), and compared with the pristine electrode. Table S2 reveals that there is difference between ECSA of NF, GC and NiFe/NF. The same ECSA values and roughness factor is determined for both pristine and reduced OER electrodes, suggesting that regardless of the electrode background, the NaBH4 reductive treatment does not induce significant change in surface area, and the enhanced OER activity is not caused by surface area increase. However, double layer capacitance is not an accurate method for quantitatively surface area calculation of nonconductive catalysts.41,42 Figure S11 displays voltammetry of NiFe/GC with different catalyst loadings and confirms that the NiFe/GC and r-NiFe/GC electrodes have similar capacitance and accordingly ECSA based on Boettcher method.43 Electrical impedance spectroscopy (EIS) is further employed to study the effect of OVs on the electrical resistance of the electrodes. Figure 3b demonstrates the Nyquist plots for pristine and reduced OER electrodes for assessing the charge transfer process. The semi-circle curve of the obtained data reveals that the charge transfer resistance (Rct) is decreased from 79 to 36 ohms after the reductive treatment, indicating higher conductivity and faster charge transport of the reduced electrode. Besides, Tafel slopes for the electrodes have been evaluated. As shown in Figure S12, the Tafel slope of pristine and reduced electrode is respectively 47 and 40 mV dec-1 in 1 M KOH


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ACS Energy Letters

which are less than IrO2, RuO2 and recent published NiFe hydroxides on nickel foam.30,44,45 This small Tafel slope of r-NiFe/NF further demonstrates more efficient kinetics of water oxidation with less polarization loss. In addition, turnover frequency (TOF) of the NiFe catalysts at overpotential of 400 mV increased from 0.075 s-1 for pristine catalyst to 0.146 s-1 for r-NiFe. It also should be mentioned that the Faradaic efficiency detected before and after reductive treatment remained almost constant (97.7%) indicating that the current density originates from evolution of oxygen on the surface of the catalysts rather than side reactions46 (Figure S13). To examine the stability of the electrode, long time bulk water electrolysis and cyclic voltammetry (CV) in 1 M KOH is performed with r-NiFe/NF. The potential changes of the electrode are negligible with very small voltage fluctuations even after 10 hours (Figure 3c), and CVs after 500 cycles are also almost identical (Figure S14), indicating the OVs are stable in the NiFe-OOH structure and the r-NiFe/NF is robust for OER. Figure S15 also displays the present of oxygen vacancy after long time OER performance. Finally, to make sure that the NaBH4 treatment strongly affects NiFe-OOH but not the Ni foam substrate, the NiFe catalyst was electrodeposited on glass carbon (GC) and then treated by NaBH4. Figure 3d shows similar enhancement effect of NaBH4 treatment is observed for NiFe/GC electrode. The NiFe/GC electrode achieves to 15 mA cm-2 at the potential of 1.6 V vs. RHE and this potential changes to ca. 1.52 V for r-NiFe/GC. Notably, similar OV-induced OER enhancement is also found for other reported NiFe based oxygen evolution electrodes such as NiFe-LDH with carbon nanotube (CNT) support47 and on NiFe/NF synthesized by hydrothermal method30 (Figure S16), suggesting OV creation can be a generic approach for enhancing the performance of NiFe based catalysts for OER.


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In summary, oxygen vacancy-rich materials possess high number of defects and edges with low coordinated metals. OVs in catalyst structures increase the number of active sites and the electrical conductivity, offering 31 mA cm-2 at overpotential of 370 mV for r-NiFe/GC rather than 15 mA cm-2 for the pristine one. It is exhibited that NaBH4 treatment is a fast and simple method to introduce OV into NiFe-OOH and enhance its OER electrocatalytic. DFT calculations also show that OER energy barrier decreased in the presence of OVs. The reductive treatment is economical and effective for OV creation in any other NiFe-based catalysts and thus can be used in water splitting applications widely. ASSOCIATED CONTENT Supporting Information contains more figures in detail and the experimental part. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Dr Majid Asnavandi: [email protected], Yichiun Yin: [email protected], Dr yibing Li: [email protected], Dr Chenghua Sun: [email protected], Prof. Chuan Zhao: [email protected]. ACKNOWLEDGMENT The authors would like to acknowledge UNSW Mark Wainwright Analytical Center for providing access to their XPS, EPR, SEM, HRTEM and XRD facilities. XAS study was undertaken at XAS beamline at the Australian Synchrotron. We thank Prof. D. B. Hibbert and Dr Bin Gong for their helpful discussion and comments. The study was financed by Australian Research Council (DP130100268, DP160103107). REFERENCES


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