A low temperature hydrothermal synthesis of delafossite CuCoO₂ as an efficient electrocatalyst for the oxygen evolution reaction in alkaline solutions

Abstract

Herein, we report the low temperature hydrothermal synthesis of delafossite CuCoO₂ crystals at 100 °C. The structural, morphological and compositional characterization of CuCoO₂ crystals was performed by powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the thermal stability of CuCoO₂ in air and its electro-catalytic activity toward the oxygen evolution reaction (OER) have also been investigated.

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Introduction

The discovery of new, economical and efficient catalysts for electrochemical energy conversion and storage is of prime importance to address the energy demand and the increasing concerns about environmental pollution. Among the electrochemical processes, the oxygen evolution reaction (OER) or water oxidation is the bottleneck of electrochemical water splitting (H₂O → H₂ + ½O₂). Nowadays, noble metal oxides (IrO₂ and RuO₂) have been demonstrated to be the state-of-the-art electrocatalysts for the OER, but their high cost limits their large-scale industrial application. Therefore, lots of effort has been made in the exploration for efficient and low-cost catalytic materials comprising earth-abundant elements as alternatives to noble metal electrocatalysts.^1,2

From the 1970s, transition metal oxides such as ABO₃ perovskite oxides (which are composed of rare and alkaline earth (A) and 3d transition metal cations (B)) have gained particular interest in an increasing number of scientific and industrial applications such as heterogeneous OER catalysts.^3–8 A series of perovskite structure oxides such as ternary compounds LaNiO₃,⁴ LaCoO₃ ⁵ and quaternary compounds Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ,^6,7 and (Ln_0.5Ba_0.5)CoO_3−δ (Ln = Pr, Sm, Gd and Ho)⁸ have been demonstrated to show an intrinsic activity comparable to the standard OER catalysts (IrO₂ and RuO₂). Besides, another kind of transition metal oxides such as delafossite oxides (the ternary oxides ABO₂, where ‘A’ is a monovalent (Cu⁺ and Ag⁺) and ‘B’ is a trivalent (Al³⁺, Ga³⁺, In³⁺) cation) also have great potential as electro-catalysts and photo-catalysts for applications in electrochemical water splitting into hydrogen (H₂) and oxygen (O₂). A group of delafossite structure oxides such as CuRhO₂, CuCoO₂ and CuGaO₂ have been reported to be promising candidates for OER electrocatalysis.⁹ Therefore, it is expected that these delafossite oxides could be useful for sustainable energy conversion applications.

In the past few years, photo-electrochemical properties of delafossite oxides for solar cell applications were extensively studied,^10–12 but only a few papers reported their applications in electrochemical water splitting. J. Ahmed and co-workers synthesized CuGaO₂ nanoparticles from a sol–gel technique and used them as bifunctional electro-catalysts in O₂ and H₂ generation by the splitting of water.¹³ Reiko Hinogami and co-workers reported CuRhO₂ delafossite as an active electrocatalyst for the OER in 1.0 M KOH electrolyte, and its cyclic voltammogram characteristic was comparable to that of Co₃O₄.¹⁴ J. Ahmed and co-workers investigated the electrocatalytic (OER) properties of CuGaO₂, and found that the nanocrystalline CuGaO₂ hexagons show an enhanced electrocatalytic activity compared to sub-micron-sized CuGaO₂ plates and micron-sized CuGaO₂ particles.¹⁵ Besides these studies on CuRhO₂ and CuGaO₂, there have been very few reports on the electro-catalysis of delafossite oxides, e.g. CuCrO₂,⁹ CuFeO₂,¹⁶ CuScO₂¹⁷ and CuMnO₂.¹⁸ Copper based delafossite oxides have demonstrated excellent electronic properties but their electrochemical studies are still lacking, which motivates us to study delafossite oxides regarding their electrochemical performance for the OER or HER (hydrogen evolution reaction).

Generally, copper based delafossite oxides can be obtained through solid state reactions or sol–gel techniques, but the high reaction temperature (900–1000 °C) usually results in crystals with a large micro-meter size.¹⁹ The low temperature synthesis of delafossite oxides with controlled particle shape and size has been challenging. In our earlier studies, nano-sized delafossite oxides as active photoelectrode materials for solar cell devices such as CuCrO₂,^20–22 CuAlO₂,²³ CuGaO₂,²⁴ and AgCrO₂ ²⁵ were obtained through a low temperature hydrothermal method at around 200 °C. More recently, we developed a facile hydrothermal route to synthesize CuFeO₂ and CuMnO₂ nanocrystals at 80–100 °C.^26,27 In this work, we further report the preparation and electro-catalysis application of CuCoO₂ crystals, which are also obtained through the low temperature hydrothermal method at 100 °C. To the best of our knowledge, there are only very few papers published on the preparation and properties of CuCoO₂ crystals via the solid-state ion exchange (metathesis) reaction at 590 °C for 2 days,^28,29 and the hydrothermal reaction at 210 °C for 60 hours,^9,30 and these previous studies were unable to explicitly describe the crystal size and morphology and the resulting product contains an unknown phase⁹ or impurity of Co₃O₄.³⁰ The present work displays the preparation and electrocatalytic application of CuCoO₂ crystals. The crystal phase, morphology, composition and chemical states of elements, and the thermal stability and the OER performance of CuCoO₂ crystals have been comprehensively studied. The as-fabricated GC@CuCoO₂ electrode (GC@CuCoO₂, the glassy carbon electrode (GC) supported CuCoO₂ electrode) with an optimal loading of CuCoO₂ catalysts exhibits efficient catalytic activity for the OER in alkaline solutions, requiring a low overpotential of 440 mV to attain an anodic current density of 10 mA cm⁻² and showing fast kinetics for the OER with a small Tafel slope (92.8 mV dec⁻¹) and charge transfer resistance.

Experimental details

Materials synthesis

All of the chemicals in these experiments were purchased from Sigma Aldrich, were of analytical gradeand used as received; CuCoO₂ crystals were prepared according to our previous studies.^26,27 Typically, 15 mmol Cu(NO₃)₂·3H₂O and 15 mmol Co(NO₃)₂·6H₂O were dissolved in 70 ml deionized water at room temperature; 4.40 g NaOH was added to the above solution and stirred for 10–20 minutes. And then, the solution was loaded into a 100 ml Teflon-lined autoclave. After the reaction was kept at 100 °C for 24 h, the obtained gray precipitate was washed with deionized water and absolute alcohol in sequence several times, and then stored in absolute alcohol solution.

Structural characterization

The morphology, microstructure, and chemical composition of CuCoO₂ were examined by field-emission scanning electron microscopy (FESEM, FEI Quanta 650) equipped with energy-dispersive X-ray spectroscopy (EDX). The crystalline structure of samples was studied by X-ray diffractometry (XRD, PANalytical X'Pert PRO) using Cu Kα radiation (λ = 1.540598 Å) and a PIXcel detector. The surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). The thermal stability of CuCoO₂ powders was investigated using a differential scanning calorimetry–thermogravimetric analysis (DSC-TG) device (Diamond TG/DTA, Perkin-Elmer Instruments); these samples were heated in air from room temperature to 800 °C at a heating rate of 10 °C min⁻¹.

Electrochemical measurements

The OER performance was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode configuration in 1.0 M KOH (pH = 13.5) using a Biologic VMP-3 potentiostat/galvanostat. A platinum wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The working electrodes of GC@CuCoO₂ were fabricated as follows. CuCoO₂ powders were dispersed in a mixture of 500 μL water, 480 μL isopropanol and 20 μL Nafion (5 wt%, Sigma), and cast onto a piece of the GC electrode (CuCoO₂ loading mass: 0.1, 0.2 and 0.3 mg cm⁻²). Cyclic voltammetric (CV) scans were recorded between 1.25 and 1.85 V vs. reversible hydrogen electrode (RHE) at the scan rate of 5 mV s⁻¹. The electrochemical double-layer capacitance (C_dl) of each sample was measured given that C_dl is positively proportional to the effective surface areas (ESA). C_dl can be extracted through CV scans at different rates (from 10 to 100 mV s⁻¹) in the non-faradaic potential window of −0.05 to 0.05 V vs. SCE. The EIS measurements were performed in the frequency range of 20 mHz–200 kHz under a constant potential of 1.60 V vs. RHE.

All current density values are normalized with respect to the geometrical surface area of the working electrode. All CV curves presented in this work are iR-corrected. The correction was done according to the following equation:

E_c = E_m − iR_s, (1)

where E_c is the iR-corrected potential, E_m is the experimentally measured potential, and R_s is the equivalent series resistance extracted from the Zero-Input Response (ZIR) measurements. Unless otherwise specified, all potentials are reported versus the reversible hydrogen electrode (RHE) by converting the potentials measured vs. SCE according to the following formula:

E(RHE) = E(SCE) + 0.241 + 0.059 pH. (2)

Results and discussion

Fig. 1a shows a SEM (scanning electron microscopy) image of the hydrothermal reaction products, where the delafossite feature of the hexagonal structure can be clearly seen. A close inspection reveals that the size of these crystals is around 1–2 μm, and the thickness of the lamellar crystal is around 300–400 nm (the inset in Fig. 1a). The XRD (X-ray diffraction) patterns of the reaction products can be indexed as CuCoO₂ (JCPDS card No. 21-0256) from Fig. 1b, as evidenced by the characteristic diffraction peaks located at 15.5°, 31.5°, 36.9°, 37.7°, 39.6°, 42.1°, 57.2°, 61.8°, 65.1°, 66.8° and 74.2°, respectively, and no impurity phase was detected. It is worth mentioning that we, for the first time, obtained the pure CuCoO₂ phase through the low temperature (100 °C) hydrothermal method, which is advantageous against the drawbacks of the high temperature hydrothermal reaction (e.g. 210 °C)^9,30 by which impurities are usually inevitably generated.

Fig. 1 SEM images (a) and XRD patterns (b) of hydrothermally synthesized products. The inset in (a) is a high magnification SEM image.

To analyze the chemical composition of CuCoO₂, SEM-EDX (scanning electron microscopy-energy-dispersive X-ray spectroscopy) mapping was employed to characterize the powders. Fig. 2a shows a typical selected area for EDX mapping measurement, and the EDS analysis (Fig. 2e) indicates that the sample contains the elements Co, Cu and O. Moreover, the C originates from carbon tapes, which was applied to hold CuCoO₂ powders. Furthermore, it can be observed that all of the Cu, Co and O elements are homogeneously distributed from Fig. 2b–d. In addition, the elemental chemical states of the CuCoO₂ crystals have been investigated by XPS (X-ray photoelectron spectroscopy). The survey spectrum again confirms the presence of Cu and Co in freshly prepared CuCoO₂ samples (Fig. S1, ESI†). The peaks located near 932.4 and 952.1 eV (Fig. 2f) correspond to the binding energies of Cu 2p_3/2 and Cu 2p_1/2, which were similar to those of CuCrO₂, CuFeO₂ and CuMnO₂ in the Cu-based delafossite family with monovalent Cu ions. Besides, there are two shakeup satellites (identified as “Sat.”) in the Co 2p spectrum, and the other peaks close to 779.9 and 794.9 eV (Fig. 2g) represent the binding energies of Co 2p_3/2 and Co 2p_1/2, which were in agreement with the reported values of Co³⁺ in Co₂O₃ and Co₃Se₄.^31–33 Moreover, the main peak close to 530.0 eV (Fig. 2h), resembling the binding energy of O 1s, suggests the metal–oxygen bonding of M–O.³⁴ The weak peak located around 531.5 eV could be identified as H₂O, which originated from absorbed water on the surface of the sample.³⁴ Therefore, the monovalent state of the copper cation (Cu⁺) and the trivalent state of the cobalt cation (Co³⁺) in the samples can be explicitly confirmed.

Fig. 2 SEM image (a), elemental mapping (b, Cu; c, Mn; d, O), EDS spectrum (e) and XPS spectra (f, Cu 2p; g, Co 2p; h, O 1s) of CuCoO₂ crystals.

From the TG (thermogravimetric) curve shown in Fig. 3a, the initial weight loss may be caused by the evaporation of absorbed water from the CuCoO₂ powders. The mass of CuCoO₂ increases sharply above 500 °C in air, which should be due to the oxidation of CuCoO₂; for example, monovalent copper (Cu⁺) was oxidized into divalent copper (Cu²⁺). The TG result of CuCoO₂ is analogous to other copper based delafossite oxides reported before, e.g. CuAlO₂,²³ CuCrO₂,²⁰ CuGaO₂ ³⁵ and CuMnO₂.²⁷ The XRD patterns (Fig. 3b) of CuCoO₂ sintered at different temperatures (e.g. 300 and 600 °C) support our analysis. There is no remarkable difference for diffraction patterns between the CuCoO₂ powders before and after sintering at 300 °C, and all these peaks can be identified as the pure CuCoO₂ (JCPDS card No. 21-0256) phase. However, the diffraction pattern of CuCoO₂ powders sintered at 600 °C is distinctly different from that of the as-prepared CuCoO₂, and can be indexed as a mixture of the CuCo₂O₄ phase (JCPDS card No. 01-1155) and CuO phase (JCPDS card No. 45-0937), suggesting that the oxidization reaction of CuCoO₂ has been completed. These results are consistent with the SEM images shown in Fig. 3c and d, where the morphology of these CuCoO₂ crystals has an appearance similar to nanoplates, but the surface of CuCoO₂ crystals melts after sintering at 600 °C (Fig. 3d), maybe owing to the newly generated copper cobalt composite oxide of CuCo₂O₄ and CuO. Thus, it is suggested that the following chemical reaction should be involved during the high-temperature (600 °C) sintering:

4CuCoO₂ + O₂ = 2CuCo₂O₄ + 2CuO. (3)

Fig. 3 (a) Thermogravimetric (TG) curves of CuCoO₂ at a heating rate of 10 °C min⁻¹ in air. Also shown are the corresponding XRD patterns (b) and SEM images of CuCoO₂ powders after sintering at different temperatures (c, 300 °C; d, 600 °C) in air for 1 hour.

The electrocatalytic activity of CuCoO₂ towards the OER was evaluated in O₂-saturated 1.0 M KOH solution using cyclic voltammetry (CV). The glassy carbon (GC) supported CuCoO₂ electrode (i.e., GC@CuCoO₂) was prepared using a method modified according to our previous report.³⁴Fig. 4a shows the CV curves of GC electrodes with different loading masses of CuCoO₂. For comparison, the OER activity of bare GC was also measured. The bare GC shows only a very small anodic current up to 1.70 V vs. RHE (reversible hydrogen electrode). After loading with CuCoO₂ crystals, the anodic current densities of electrodes (GC@CuCoO₂) are remarkably improved. From the reduction branch of the CV curves, GC@CuCoO₂-0.1 mg shows a low onset potential (the potential at which the anodic current density is 1 mA cm⁻²) of 1.64 V vs. RHE (i.e., η_on = 410 mV), and this can reach 10 mA cm⁻² at 1.73 V vs. RHE (i.e., η₁₀ = 500 mV, the inset in Fig. 4a). GC@CuCoO₂ with a high loading mass of CuCoO₂ (such as 0.2 or 0.3 mg CuCoO₂) exhibits better OER activity compared to that of GC@CuCoO₂-0.1 mg, which shows a lower onset potential of 1.62 V vs. RHE (i.e., η_on = 390 mV), and can reach 10 mA cm⁻² at 1.67 V (i.e., η₁₀ = 440 mV) for the GC@CuCoO₂-0.3 mg electrode. The small difference in the overpotential (η_on, η₁₀) for these three samples is around 10–30 mV, indicating that a higher loading of CuCoO₂ does not help to further boost the OER activity of the electrode. The OER activity of these GC@CuCoO₂ electrodes is comparable to or better than many other oxide based OER catalysts recently reported in the literature (see Table S1, ESI†), such as perovskite oxide catalysts (LaNiO₃ (η_0.12 ≈ 400 mV),⁴ LaCoO₃ (80 nm, η₁₀ = 490 mV),⁵ Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (η₁₀ = 370 mV),⁶ LaFeO₃ (η₁₀ = 510 mV),³⁶ SrCoO₃ (η_0.7 ≈ 350 mV),³⁷ SrCoO_3−δ (η₃ ≈ 320 mV),³⁸ Pr_0.5Ba_0.5Co₂O_3−δ (η_0.3 ≈ 320 mV)³⁸) or other delafossite oxide catalysts (micron-sized CuRhO₂ particles (η₁₀ ≈ 420 mV),⁹ sub-micron-sized CuCoO₂ plates (η₁₀ ≈ 430 mV),⁹ micron-sized CuFeO₂ plates (η₅ ≈ 670 mV),⁹ micron-sized CuCrO₂ plates (η₅ ≈ 710 mV),⁹ CuGaO₂ nanoparticles (η₁₈ ≈ 370 mV),¹³ micron-sized CuRhO₂ plates (η_0.1 ≈ 380 mV),¹⁴ micron-sized CuGaO₂ particles (η₅ ≈ 370 mV),¹⁵ and sub-micron sized CuGaO₂ plates (η₂₃ ≈ 370 mV)).¹⁵

Fig. 4 (a) CV curves and (b) Tafel plots of the bare GC and GC@CuCoO₂ electrodes, (c) Nyquist plots of the GC@CuCoO₂ electrodes, the inset of (c): equivalent circuit model. (d) Half of the difference between anodic and cathodic current densities (ΔJ = (J_a − J_c)/2) at 0 V vs. SCE plotted against the scan rate. The linear slope is equivalent to the double-layer capacitance C_dl that is proportional to the ECSA of GC@CuCoO₂ electrodes. All measurements were performed in 1.0 M KOH at room temperature (ca. 25 °C).

To gain additional understanding of the OER process, Tafel analysis was carried out in the linear potential region by fitting the reduced branches of the CV curves with the Tafel equation.³³ The Tafel slope is found to be 125.9, 96.7, 96.1 and 92.8 mV dec⁻¹ for the bare GC, GC@CuCoO₂-0.1 mg, GC@CuCoO₂-0.2 mg and GC@CuCoO₂-0.3 mg electrodes, respectively (Fig. 4b). The Tafel slope of GC@CuCoO₂ electrodes is substantially lower than that of many oxide-based OER catalysts (Table S1, ESI†), suggesting much faster kinetics towards the OER. This was also corroborated by our EIS study. Fig. 4c displays the EIS Nyquist plots of the bare GC and GC@CuCoO₂ electrodes, which are fitted with the equivalent circuit model shown in the inset of Fig. 4c. According to the fitting results (Table S2, ESI†), the charge transfer resistance (R_ct) of GC@CuCoO₂-0.3 mg electrodes is 65.4 Ω cm⁻², smaller than that of the GC@CuCoO₂-0.2 mg (85.2 Ω cm⁻²) or GC@CuCoO₂-0.1 mg (87.9 Ω cm⁻²), the R_ct values of GC@CuCoO₂ electrodes are smaller than those of other oxide catalysts, such as LaCoO₃ (at the potential of 1.67 V vs. RHE)⁵ and LaFeO₃ (at the potential of 1.65 V vs. RHE).³⁶ These results indicate that the CuCoO₂ crystals, as an efficient OER catalyst, can dramatically accelerate the OER process. The electrochemical double-layer capacitance (C_dl) of all the GC@CuCoO₂ electrodes were measured to compare the effective surface areas (ESA), given that C_dl is positively proportional to ESA; a larger C_dl represents a higher surface area of the investigated catalyst. C_dl was measured through CV scans (Fig. S2–S4, ESI†) in the non-faradaic potential region (−0.05 to 0.05 vs. SCE) at different scan rates (20, 40, 60, 80, 100 mV s⁻¹). As shown in Fig. 4d, the C_dl values of the GC@CuCoO₂-0.1 mg, GC@CuCoO₂-0.2 mg and GC@CuCoO₂-0.3 mg electrodes are 1.3, 1.5 and 2.5 mF cm⁻², respectively. The ESA was calculated from the C_dl according to the following equation:

ESA = C_dl/C_s, (4)

where C_s is the specific capacitance of the sample, around 0.040 mF cm⁻², a typical value reported for a metal electrode in alkaline solutions.³⁹ The ESA values of the GC@CuCoO₂-0.1 mg, GC@CuCoO₂-0.2 mg and GC@CuCoO₂-0.3 mg electrodes are 32.5, 18.75 and 20.83 m² g⁻¹, respectively, which is much higher than that of metal-oxide catalysts, such as LaCoO₃ and LaNiO₃.⁴⁰ Therefore, the GC@CuCoO₂ electrodes would possess a higher ESA with the increasing loading mass of CuCoO₂, allowing more active sites to be exposed and contributing to the high electrocatalytic activity.

Conclusions

In summary, we have synthesized delafossite CuCoO₂ through a hydrothermal method at a low temperature (100 °C) for the first time. CuCoO₂ supported on GC electrodes exhibits an efficient catalytic activity for the oxygen evolution reaction in alkaline solutions. Given the low-cost nature and ease of up-scale fabrication of electrodes through the low temperature hydrothermal method, delafossite CuCoO₂ holds substantial promise for use as an efficient and inexpensive OER catalyst in water electrolyzers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to express sincere thanks for financial support by the National Natural Science Foundation of China (No. 51402223, 51772224).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00621g

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