Highly efficient and durable water oxidation in a near-neutral carbonate electrolyte electrocatalyzed by a core–shell structured NiO@Ni–Ci nanosheet array

Abstract

The development of cost-effective and high-performance water oxidation electrocatalysts operating in carbonate (Ci) electrolytes is highly desired but still remains a great challenge. In this communication, we report the in situ electrochemical development of an amorphous Ni–Ci layer on a NiO nanosheet array supported on carbon cloth (NiO@Ni–Ci/CC) as a 3D catalyst electrode for water oxidation in 1.0 M KHCO₃ (pH: 8.3). To drive a geometrical catalytic current density of 15 mA cm⁻², such core–shell structured NiO@Ni–Ci/CC only demands an overpotential of 387 mV, with its catalytic activity being maintained for at least 25 h. And at an overpotential of 500 mV, this catalyst electrode achieves a high turnover frequency of 0.18 mol O₂ per s.

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The exploitation of clean and sustainable energy systems is in great need to solve the issues of the increasing global energy demand due to the depletion of fossil fuels and their consequent environmental pollution.^1,2 As a renewable and carbon-neutral energy carrier, hydrogen is regarded as an ideal alternative to fossil fuels in the future. Electrocatalytic water splitting offers a simple technique for large-scale production of pure hydrogen.^3–5 The anodic oxygen evolution reaction (OER) is identified as the critical bottleneck of water splitting because of the involvement of the multi-electron transfer process, imposing serious overpotential requirement and limiting the efficiency of hydrogen production.^6–8 Ru and Ir oxides, the most active water oxidation catalysts (WOCs), however, suffer from low abundance and high cost, hindering their large-scale commercial uses.⁹ As such, it is extremely attractive to design and develop earth-abundant alternatives.

Proton exchange membrane-based water electrolysis units¹⁰ and commercial alkaline water electrolysis¹¹ operate at extreme pH, causing serious corrosion issues and limiting the types of electrodes and cell components that can be used.¹² Microbial electrolysis cells¹³ and electrochemical conversion of CO₂ into fuels or chemical products^14–16 demand WOCs under benign conditions and earth-abundant such WOCs are thus also highly desirable. Transition metal phosphates^17–19 and borates^20–22 have been widely studied as efficient catalysts for water oxidation in neutral phosphate and near-neutral borate electrolytes, respectively. Carbonate is another environmentally friendly electrolyte which is potentially advantageous in combining water oxidation with CO₂ reduction.²³ Given that nanoarrays are favorable to enhanced catalytic activity due to more exposed active sites and facilitated diffusion of the electrolyte and gas evolved,^24,25 they are highly attractive but it still remains challenging to develop transition metal carbonate-based nanoarrays for water oxidation electrocatalysis under mild conditions.

In this communication, we report that an amorphous Ni–Ci layer (6–8 nm in thickness) can be in situ developed on a crystalline NiO nanosheet array supported on carbon cloth via oxidative polarization in KHCO₃ (K–Ci). As a 3D durable catalyst electrode for water oxidation in 1.0 M K–Ci, such core–shell structured NiO@Ni–Ci/CC shows high activity with the need of an overpotential of only 387 mV to drive a geometrical catalytic current density of 15 mA cm⁻², 58 mV less than that of NiO/CC, and its catalytic activity can be maintained for at least 25 h. Besides, this catalyst achieves a high turnover frequency of 0.18 mol O₂ per s at an overpotential of 500 mV.

NiO@Ni–Ci/CC was electrochemically derived from NiO/CC in 1.0 M K–Ci^26–30 (see ESI† for more details). Fig. 1a shows the X-ray diffraction (XRD) patterns of NiO/CC and NiO@Ni–Ci/CC. As observed, NiO/CC presents three diffraction peaks at 37.2°, 43.3° and 62.9° indexed to the (111), (200) and (220) planes of NiO (JCPDS no. 42-1467), respectively, and other peaks arise from the CC substrate. NiO@Ni–Ci/CC also shows characteristic peaks of NiO but with slightly decreased peak intensities, implying the formation of amorphous species on the NiO surface. Fig. 1b presents the scanning electron microscopy (SEM) images of NiO/CC, indicating that the entire CC is fully covered by the NiO nanosheet array. Surprisingly, the as-prepared NiO@Ni–Ci/CC still retains its initial nanoarray feature but with thicker nanosheets (Fig. 1c). The energy-dispersive X-ray (EDX) spectrum (Fig. S1†) confirms the existence of Ni, C and O elements. The high-resolution transmission electron microscopy (HRTEM) image of the NiO nanosheet (Fig. 1d) reveals well-resolved lattice fringes with an interplanar distance of 0.21 nm corresponding to the (200) plane of the NiO phase.³¹ The HRTEM image of NiO@Ni–Ci confirms the generation of an amorphous shell about 6–8 nm in thickness (Fig. 1e). The EDX elemental mapping images (Fig. 1f) further verify the uniform distribution of Ni, C and O elements in the resulting NiO@Ni–Ci product.

Fig. 1 (a) XRD patterns of NiO/CC and NiO@Ni–Ci/CC. SEM images of (b) NiO/CC and (c) NiO@Ni–Ci/CC. HRTEM images taken from (d) NiO and (e) NiO@Ni–Ci. (f) EDX elemental mapping images of Ni, C and O elements for NiO@Ni–Ci.

Fig. 2a shows the X-ray photoelectron spectroscopy (XPS) survey spectrum of NiO@Ni–Ci, also confirming the presence of Ni, C and O elements. As presented in Fig. 2b, the binding energies (BEs) at 855.6 and 873.1 eV in the Ni 2p region are assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively, which verifies the existence of Ni²⁺ or Ni³⁺ bound to oxygen.^32,33 The peaks at 861.4 and 879.4 eV with two shakeup satellites (identified as “Sat.”) arise from Ni²⁺ or Ni³⁺. We also collected the XPS spectra of NiO (Fig. S2†). The Ni 2p BE of NiO@Ni–Ci has a positive shift compared with that of NiO, suggesting that Ni from Ni–Ci possesses a higher valence state. In the C 1s region (Fig. 2c), the peak at 284.7 eV is observed and another one at 288.4 eV arises from the –C(O)O– group in our product.²³ The weak peak at 529.2 eV in the O 1s region is assigned to O in NiO and carbonate, and the BE at 530.8 eV is attributed to the –OH in carbonate (Fig. 2d).³⁴ These analyses confirm that the amorphous layer is Ni–Ci in nature. The in situ formation of the core–shell structured NiO@Ni–Ci nanosheet array from the NiO nanoarray could be rationally explained as follows. Ni at the NiO surface is electrochemically oxidized into higher oxidation valence states in the process of oxidative polarization, and Ci anions in K–Ci solution interact with such Ni cations for charge compensation, leading to the in situ precipitation of Ni–Ci onto the NiO surface. Such a process can proceed repeatedly until the complete coverage of the NiO surface by the amorphous Ni–Ci layer, leading to the formation of the NiO@Ni–Ci core–shell structure, finally.

Fig. 2 (a) XPS survey spectrum of NiO@Ni–Ci. XPS spectra of NiO@Ni–Ci in the (b) Ni 2p, (c) C 1s and (d) O 1s regions.

We examined the electrocatalytic water oxidation activity of NiO@Ni–Ci/CC (Ni–Ci loading: 1.8 mg cm⁻²) using a typical three-electrode setup in 1.0 M K–Ci (pH: 8.3). Because the first sweep of linear sweep voltammetry (LSV) measurements is always unstable owing to the effect of residual currents and capacitance currents, we adopted the second one for all the polarization curves.³⁵ In order to eliminate the effect of ohmic resistance and thus reflect the intrinsic behavior of catalysts, we applied iR correction to all initial experimental data except specifically explained,³⁶ and all potentials were reported on a reversible hydrogen electrode (RHE) scale unless particularly stated. For comparison study, blank CC, NiO/CC and RuO₂ on CC (RuO₂/CC) with approximately identical loading were also measured. Fig. 3a shows the polarization curves. As expected, RuO₂/CC is highly active for water oxidation with the need of an overpotential of 265 mV to achieve 15 mA cm⁻², whereas blank CC has almost no OER activity. It is to be noted that NiO@Ni–Ci/CC shows superior OER performance at the overpotential of 387 mV to drive a catalytic current density of 15 mA cm⁻², 58 mV less than that of NiO/CC (445 mV). It is to be noted that this NiO@Ni–Ci/CC catalyst electrode also compares favorably to the behaviors of reported WOCs operating in neutral or near-neutral electrolytes, such as Ni–Bi/ITO (η_{1.0 mA cm⁻²} = 425 mV),²⁰ Fe–Ci/FTO (η_{10 mA cm⁻²} = 560 mV),²³ NiO_x-Cat/GC (η_{1.15 mA cm⁻²} = 604 mV),³⁴ Co–Ci/GC (η_{9.1 mA cm⁻²} = 771 mV),³⁷ and Cu–Bi/FTO (η_{1.0 mA cm⁻²} = 525 mV).³⁸ And a more detailed comparison is listed in Table S1.† We also measured the water oxidation performances of NiO@Ni–Ci/CC and NiO/CC in 1.0 M KOH (Fig. S3†) and found that NiO@Ni–Ci/CC is also superior in activity to NiO/CC. The Tafel plots deriving from the polarization curve were employed to evaluate the OER catalytic kinetics of the as-prepared electrodes. Fig. 3b presents the Tafel plots fitted to the equation (η = b log j + a, where j is the current density and b is the Tafel slope). NiO@Ni–Ci/CC has a Tafel slope of 102 mV dec⁻¹, smaller than those of RuO₂/CC (122 mV dec⁻¹) and NiO/CC (206 mV dec⁻¹), implying more rapid catalytic kinetics for the OER on the NiO@Ni–Ci/CC electrode.³⁶ We also examined the influence of the K–Ci concentration on the water oxidation process. As presented in Fig. S4,† the increased K–Ci concentration from 0.2 to 1.0 M decreases the OER overpotentials because of the increased conductivity of the K–Ci electrolyte.³⁹ To drive a catalytic current density of 15 mA cm⁻², overpotentials of 560, 442 and 387 mV are demanded in 0.2, 0.5 and 1.0 M K–Ci, respectively.

Fig. 3 (a) LSV curves of RuO₂/CC, NiO@Ni–Ci/CC, NiO/CC and bare CC with a scan rate of 5 mV s⁻¹. (b) Corresponding Tafel plots of RuO₂/CC, NiO@Ni–Ci/CC and NiO/CC. (c) Multi-current process of NiO@Ni–Ci/CC. The current density started at 4 mA cm⁻² and ended at 40 mA cm⁻², with an increment of 4 mA cm⁻² per 500 s without iR correction. (d) Time-dependent current density curve at a fixed overpotential of 420 mV. All experiments were tested in 1.0 M K–Ci.

Fig. 3c shows the multi-step chronopotentiometric curve of NiO@Ni–Ci/CC with increasing the tested current density from 4 to 40 mA cm⁻² (4 mA cm⁻² per 500 s). The potential instantly levels off at 0.87 V vs. Ag/AgCl for the initial current value and remains constant for the rest 500 s. All other steps also present analogous results, reflecting the excellent mass transportation properties, conductivity and mechanical robustness of the NiO@Ni–Ci/CC electrode.^36,39 We probed the stability of NiO@Ni–Ci/CC by conducting continuous cyclic voltammetry scanning in 1.0 M K–Ci at a scan rate of 100 mV s⁻¹. As observed, the LSV curve after 500 cycles shows negligible loss in current density compared with the initial one (Fig. S5†), and the corresponding XRD and SEM analyses demonstrate that NiO@Ni–Ci still retains its initial phase with the preservation of its morphology feature after the test (Fig. S6†). The time-dependent current density curve at a fixed overpotential of 420 mV (Fig. 3d) suggests that NiO@Ni–Ci/CC can maintain its catalytic activity for at least 25 h, promising its practical applications.

To estimate the electrochemically active surface area (ECSA), we measured the double layer capacitance (C_dl) at the solid/liquid interface of both NiO@Ni–Ci/CC and NiO/CC electrodes using the cyclic voltammetry technique.⁴⁰ The cyclic voltammograms (CVs) were collected in the region of 1.13–1.23 V vs. Ag/AgCl, where the current response should only be ascribed to the charging and recharging from the double layer (Fig. S7†). C_dl is determined by this following equation: i_c = v × C_dl, yielding a C_dl of 13.2 mF cm⁻² for NiO@Ni–Ci/CC, much higher than that of NiO/CC (1.6 mF cm⁻²). This result suggests that NiO@Ni–Ci/CC has a larger surface area and thus more exposed active sites for OER catalysis. As shown in Fig. S8,† we also performed electrochemical impedance spectroscopies for NiO/CC and NiO@Ni–Ci/CC. It suggests that NiO@Ni–Ci/CC possesses a smaller radius of semicircle than that for NiO/CC, implying lower R_ct and thus a higher charge-transfer rate and more fast catalytic kinetics on NiO@Ni–Ci/CC.⁴¹

We also employed turnover frequency (TOF), associated with generating oxygen molecules per second, to further reflect the intrinsic activity of the as-prepared electrocatalysts at a fixed overpotential.⁴² To calculate TOF, the concentration of active sites needs to be quantified through electrochemistry according to reported work.⁴³ The CVs of NiO@Ni–Ci/CC (Fig. 4a) and NiO/CC (Fig. 4b) under different scan rates suggest a linear relationship between the oxidation peak currents for redox species and scan rates. The slope is given by the formula: slope = n²F²m/4RT, where n represents the number of electrons transferred which is equal to 1 assuming a one-electron process for the oxidation of metal centers in Ni–Ci,²²F is the faradaic constant, m is the number of active species, and R and T are the ideal gas constant and the absolute temperature, respectively (Fig. 4c). Following the equation: TOF = JA/4Fm (J is the current density, A is the geometrical electrode area, and 4 expresses the moles of electron consumption for one mole oxygen evolution), we determined the TOF of NiO@Ni–Ci/CC as 0.18 mol O₂ per s at an overpotential of 500 mV, which is much larger than that of NiO/CC (0.05 mol O₂ per s) at the same overpotential. More detailed comparison for TOF values between NiO@Ni–Ci/CC and NiO/CC is shown in Fig. 4d. Also it is to be noted that the TOF of NiO@Ni–Ci/CC is higher than those of other reported non-noble-metal WOCs like Ni–Bi/FTO (0.01 mol O₂ per s, η = 600 mV),²⁰ NiO_x-en/FTO (0.02 mol O₂ per s, η = 610 mV),⁴⁴ and Fe–Bi/ITO (0.17 mol O₂ per s, η = 600 mV).⁴⁵ We also determined the faradaic efficiency (FE) by comparing the quantity of practically evolved oxygen with theoretically calculated oxygen (assuming 100% FE). As observed in Fig. S9,† the approximate agreement of the measured value and theoretical one demonstrates nearly 100% FE for evolution oxygen on the NiO@Ni–Ci/CC electrode.

Fig. 4 CVs of (a) NiO@Ni–Ci/CC and (b) NiO/CC under different scan rates increasing from 10 to 50 mV s⁻¹ in 1.0 M K–Ci. (c) Linear relationship of the oxidation peak currents vs. scan rates for NiO@Ni–Ci/CC and NiO/CC. (d) TOF comparison between NiO@Ni–Ci/CC and NiO/CC at different fixed overpotentials.

In summary, a Ni–Ci layer has been successfully in situ developed on a NiO nanosheet array supported on carbon cloth (NiO@Ni–Ci/CC) via oxidative polarization in K–Ci. The core–shell NiO@Ni–Ci/CC behaves as a 3D catalyst electrode for water oxidation with the need of an overpotential of 387 mV to drive a geometrical catalytic current density of 15 mA cm⁻² in 1.0 M K–Ci (pH: 8.3). It also demonstrates strong long-term electrochemical durability with a high TOF of 0.18 mol O₂ per s at an overpotential of 500 mV. This study is important because it not only provides us an attractive catalyst material for efficient and durable electrochemical water oxidation in carbonate media, but would open up an exciting new avenue to the rational design and fabrication of transition metal carbonate-based nanoarrays⁴⁶ for electrolysis^{13–16,47,48} and sensing applications.^49–51

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21575137 and 21375076).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c7se00218a

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