Copper-based homogeneous and heterogeneous catalysts for electrochemical water oxidation

Husileng Lee a, Xiujuan Wu *a and Licheng Sun abc
aState Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China. E-mail:
bDepartment of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden
cInstitute for Energy Science and Technology, Dalian University of Technology (DUT), Dalian 116024, China

Received 10th December 2019 , Accepted 13th January 2020

First published on 15th January 2020

Water oxidation is currently believed to be the bottleneck in the field of electrochemical water splitting and artificial photosynthesis. Enormous efforts have been devoted toward the exploration of water oxidation catalysts (WOCs), including homogeneous and heterogeneous catalysts. Recently, Cu-based WOCs have been widely developed because of their high abundance, low cost, and biological relevance. However, to the best of our knowledge, no review has been made so far on such types of catalysts. Thus, we have summarized the recent progress made in the development of homogeneous and heterogeneous Cu-based WOCs for electrochemical catalysis. Furthermore, the evaluations of catalytic activity, stability, and mechanism of these catalysts are carefully concluded and highlighted. We believe that this review can summarize the current progress in the field of Cu-based electrochemical WOCs and help in the design of more efficient and stable WOCs.

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Husileng Lee

Husileng Lee received his BS from Tianjin University of Science and Technology in 2010, majoring in Applied Chemistry. He is currently pursuing his PhD under the supervision of Prof. Licheng Sun at Dalian University of Technology. His research interests are focused on transition-metal-based molecular and material electrochemical water oxidation catalysts.

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Xiujuan Wu

Xiujuan Wu is an Associate Professor at Dalian University of Technology, China. After she received her PhD from Ewha Womans University with Prof. Wonwoo Nam in 2013, she went to Dalian University of Technology for her postdoc in Prof. Licheng Sun's group and became an Associate Professor in 2018. She works on the development of efficient water oxidation catalysts and catalytic mechanism in water oxidation.

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Licheng Sun

Licheng Sun received his PhD in 1990 from the Dalian University of Technology. He went to Germany as a postdoc at the Max-Planck-Institut für Strahlenchemie with Dr Helmut Görner (1992–1993) and then as an Alexander von Humboldt fellow at Freie Universität Berlin (1993–1995) with Prof. Harry Kurreck. He moved to the KTH Royal Institute of Technology, Stockholm, in 1995 and became an Assistant Professor in 1997, an Associate Professor in 1999 (at Stockholm University), and a Full Professor in 2004 (KTH). His research interests include artificial photosynthesis, molecular catalysts for water oxidation and hydrogen generation, functional devices for total water splitting, and solar cells.

1. Introduction

The demand for energy has been consistently increasing as a consequence of fast-growing economies as well as population expansion. Since the industrial revolution, energy supply has largely depended on traditional fossil fuels (more than 70%) according to the World Energy Council.1 It is of the great concern that the proven reserves of fossil fuels will be depleted within 100–150 years at the current rate of consumption.2 Moreover, the environmental problems induced by the combustion of fossil fuels, such as the greenhouse effect, climate change, and acid rain, are also regarded as some of the greatest challenges for the sustainable development of human society.3,4 The depletion of fossil fuels and environmental deterioration have forced us to shift the center of gravity of energy exploration from traditional fuels to clean and renewable energy sources.5 However, the large-scale utilization of clean and renewable energy is restricted by natural conditions, such as daylight time (solar energy), geographical conditions (wind, wave power, etc.), and land deficiency (biomass).6,7 Furthermore, electricity generation from renewable energy sources is restricted by reliable storage devices and faces the problem of oversupply.8

The solution to this surplus problem is to convert and store the energy from such sources into stable chemical energy, such as hydrogen (H2).9,10 H2 is widely considered to be one of the most attractive candidates for both utilization and storage due to its carbon-free nature, high energy density (3.54 kW h N−1 m−3),11 and low mass density (0.0899 kg N−1 m−3). At present, H2 is mainly produced by traditional steam methane (or methanol) reforming method at high temperatures (700–1000 °C) and high pressures (3–25 bar).12 This method gives high yield but low purity (∼60–70%) and has high energy consumption.11,13 In contrast, water splitting (eqn (1)) is a mature technology that can produce H2 under ambient conditions with high purity (>99.9%) by directly applying an electric voltage during water electrolysis.13 Moreover, it is also an attractive reaction for the conversion of solar energy into chemical fuels during artificial photosynthesis.14 This process comprises two half-reactions: (1) oxygen evolution reaction (OER, eqn (2)) producing molecular oxygen (O2) and protons (H+); (2) hydrogen evolution reaction (HER, eqn (3)) reducing H+ to H2.15

2H2O → O2 + 2H2(1)
2H2O → O2 + 4H+ + 4e(2)
4H+ + 4e → 2H2(3)

At first sight, OER seems to be simple and easily accomplished because of the simplicity of the equation. Unfortunately, the goal is definitely difficult to achieve and it is currently believed to be the bottleneck in the field of electrochemical water splitting and artificial photosynthesis.14–16 It takes ΔG = 273 kJ (corresponding to potential as large as 1.23 V at pH 0) to form 1 mole of O2, which is a thermodynamically uphill reaction. In addition, it is a kinetically sluggish reaction accompanying the transformations of 4 electrons and 4 protons.14–16 Therefore, in the past few decades, considerable efforts have been devoted toward lowering the overpotential (η) by exploring various kinds of water oxidation catalysts (WOCs), including homogeneous and heterogeneous catalysts.16,17 Each of them has its own merits and drawbacks. For homogenous catalysts, the redox and catalytic properties of the metal center can be easily tuned by structural modifications of the ligand (different functional groups, aliphatic chains or backbones, etc.).16 Moreover, a homogeneous system is also amendable to the in-depth mechanistic study by means of in situ spectroscopic technologies during the reaction process.18 However, complicated synthesis and limited stability prevent the industrial application of homogenous systems. As compared to homogeneous catalysts, heterogeneous catalysts are known for their facile synthesis and favorable durability under industrial conditions.17,19 Nevertheless, due to the complexity of the catalysts’ component and configuration, it is difficult to conduct in situ mechanistic studies.19

During the past few decades, important progress has been made in the exploration of WOCs based on noble-metal complexes or oxides, such as RuII(bda)(pic)2,20,21 Cp*Ir (κ2,C2,C2-NHC)(Cl),22 RuO2,23,24 and IrO2.25,26 However, the scarcity (content in the earth crust—Ru: 0.001 ppm; Ir: 0.000003 ppm)27 and high cost (Ru: 11.45 USD per g; Ir: 55.97 USD per g, Jan 2018) hinder their large-scale applications.28–30 Therefore, abundant and cheap 3d-transition-metal-based catalysts should be developed for meeting the urgent need of highly efficient and widely applicable WOCs.31–33 Indeed, various kinds of catalysts based on first-row transition metals (Ni,34–36 Co,4,37 Fe,38–40 and Cu41–43), including homogeneous and heterogeneous catalysts, have been reported in the past few years with high efficiencies.

Among them, copper-based catalysts have attracted increasing attention for the following reasons: (1) copper is an abundant element, which is present in the earth's crust at a proportion of 50 ppm;27 (2) copper is much cheaper than noble metals (6982 USD per ton, Jan 2018); (3) copper has extensive redox properties with a variety of valence states (Cu0, Cu+, Cu2+, and Cu3+), which can be potentially applied in the development of electrocatalysts,44,45 anode materials for lithium-ion batteries,46 zinc–air materials,47 photocatalysts,48 and redox mediators in dye-sensitized solar cells;49 (4) copper is also an important metal center in enzymes (cytochrome c oxidase, hemocyanin, etc.), where the dioxygen is activated as Cu–O2 and finally integrated into organic synthesis.49 Therefore, it is an interesting target to synthesize or construct such suitably coordinated copper species that are capable of forming Cu(III)-OOH intermediates during the water oxidation process.50,51 In this review, we have prepared a comprehensive summary of the recent achievements on molecular as well as material Cu-based electrochemical WOCs (Fig. 1) and highlight their prospects in future research.

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Fig. 1 Schematic diagram of homogeneous and heterogeneous catalysts for water oxidation.

2. Parameters for evaluating catalyst performance

2.1. η

Here, η is defined as the additional potential to the thermodynamically required potential applied to a system for overcoming the kinetic barrier.10 It is always considered to be the most critical parameter for evaluating the catalytic activity of any given catalyst.

In a homogeneous system, generally, the onset overpotential (ηo) and overpotential at 1 mA cm−2 (η1) are deemed to be important activity parameters,52 which can be obtained from the cyclic voltammetry (CV) curve as well as eqn (4) and (5). In these equations, Eo represents the potential where water oxidation occurs and EO2/H2O is the thermodynamic potential for water oxidation at a given pH (eqn (6)). In the recent years, two main methods have been reported for estimating Eo: (1) the potential where the catalytic peak “onset” occurs; (2) half-peak potential (Ep/2), where half the maximum catalytic current is obtained.53 Obviously, the first method is somewhat subjective (only by naked eye judgment) and the second method is relatively reliable.

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η1 = E1EO2/H2O (V vs. NHE)(5)
EO2/H2O = 1.229 − 0.059pH (V vs. NHE)(6)

For heterogeneous catalysts, the iR drop caused by the electrolyte should be considered. Therefore, η should be described as that in eqn (7), where Eapp represents the applied potential and iR is the potential loss caused by electrolyte resistance.54

η = EappiREO2/H2O(7)

Similar to the homogeneous system, ηo and η at the defined catalytic current density (ηj) are also considered to be important activity parameters in heterogeneous systems.55 Notably, the current density is usually corrected by using the geometric area of the electrodes (expressed as mA cm−2), which exactly reflects the catalytic performance only when the experiments are conducted on smooth 2D electrodes, such as glassy carbon (GC), fluorine-doped tin oxide (FTO), and indium-doped tin oxide (ITO) electrodes. Moreover, the area of the electrode is 2 cm−2 when the catalysts are pasted or grown in situ on 3D porous electrodes, such as copper foam (CF), carbon cloth (CC), and carbon fiber paper (CFP). Interestingly, the geometric area of 3D porous electrodes can be conventionally considered as planar geometric areas and have been widely used by researchers, including our group.56,57 To determine the intrinsic activity of the catalysts, particularly those grown in situ on a 3D substrate, the term “real surface area” has been proposed by researchers. Generally, the actual surface area of materials can be determined from the Brunauer–Emmett–Teller (BET) adsorption isotherm, and the obtained BET surface area is usually used to normalize the catalytic activity, namely, jBET. However, this method cannot reveal the electrochemically active sites evolved in the reaction.10 Recently, jECSA, the catalytic activity normalized by the electrochemically active surface area (ECSA), has been considered as a promising parameter to evaluate the catalytic activity. However, there are several methods that can be used to measure ECSA; further, the ECSA values of the same material can be different when measured using different methods.56 Therefore, researchers should evaluate the catalytic activity with a combination of η and the other parameters discussed below.

2.2 Turnover frequency (TOF)

TOF is an intrinsic parameter of catalysts. In particular, TOF is defined as the amount of product converted from the reactant per mol of the effective catalyst per unit time.56,58 The TOF values of homogeneous catalysts can be commonly derived from eqn (8).59
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where ic is the catalytic current, ip is the peak current, nc is the number of electrons transferred during the catalytic process (nc = 4 for water oxidation), np is the number of transferred electrons during the redox process of the catalyst, kcat is the pseudo-first-order rate constant (TOF), and ν is the scan rate (V s−1). The kcat value can be calculated from the slope of the plot of ic/ipversus ν−1/2.

For heterogeneous catalysts, the TOF values can be derived from eqn (9) according to the method reported in the literature.60


Here, J is the current density at a given η value (A cm−2), A is the surface area of the electrode (cm2), NA is Avogadro number (6.022 × 1023), n is the number of electrons transferred when evolving one mole of the product, F is the Faraday constant (96[thin space (1/6-em)]485 C mol−1), and Γ is the surface concentration of the catalytically active site. Γ can be calculated from the slope of the plot of peak current ip and scan rate ν according to eqn (10).59

ip = n2F2AνΓ/4RT(10)
where ip is the peak current, n is number of electrons transferred during the reduction/oxidation of the catalyst, F is the Faraday constant (96[thin space (1/6-em)]485 C mol−1), A is the surface area of the electrode (cm2), ν is the scan rate (V s−1), Γ is the surface concentration of catalytically active site, R is the ideal gas constant (8.314 J mol−1 K−1), and T is the temperature (K).

2.3 Faraday efficiency (FE)

FE indicates the selectivity of catalysts under the reaction conditions, which can be defined as the efficiency of the electrons used for the desired reduction/oxidation process rather than side reactions.61 The FE of OER can be obtained by comparing the practically evolved O2versus the theoretical value during electrolysis. The former can be estimated by means of gas chromatography or oxygen sensor, and the latter can be calculated by using the following equation.
nO2 = Q/4F(11)
where Q is the passed charges during electrolysis and F is the Faraday constant (96[thin space (1/6-em)]485 C mol−1).

2.4 Tafel slope

The Tafel slope provides the preliminary kinetic information of the catalysts toward OER, which indicates the relationship between the current density and η. The Tafel equation can be expressed as follows.59
η = a + b[thin space (1/6-em)]log[thin space (1/6-em)]i(12)
η = 2.3RT[thin space (1/6-em)]log[thin space (1/6-em)]i0/nαF − 2.3RT[thin space (1/6-em)]log[thin space (1/6-em)]i/nαF(13)

Eqn (12) shows the classic equation given by Tafel in 1905 and eqn (13) is the derivate from the Butler–Volmer equation. In the latter equation, a = 2.3RT[thin space (1/6-em)]log[thin space (1/6-em)]i0/nαF is the Tafel constant and b = −2.3RT/nαF is the so-called Tafel slope; α is the charge transfer coefficient. Therefore, the lower the Tafel slope, the faster is the electron transfer during the reaction.

2.5 Stability

The stability test of homogeneous catalysts is usually used to identify the “real” active species.62 In many reported literatures, complexes are converted into their corresponding oxides/hydroxides, which are more active than the molecule itself (section 4.1).63–65 Therefore, the post-reaction electrolytes and electrodes of homogeneous systems should be carefully examined. There are certain generally used methods for determining the homogeneity of a catalytic system:66,67

(1) Spectroscopic methods: UV–vis and FT-IR spectroscopy have been used to analyze the spectral absorbance changes of complexes before and after electrolysis;68,69 mass spectroscopy (MS) and nuclear magnetic resonance (NMR) are used to monitor the complexes or free ligands in solution after electrolysis.70,71

(2) Physical characterization: Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) are usually applied to determine the morphology and composition of post-reaction electrodes, which can determine whether a heterogeneous thin film is formed on the electrode after testing. Dynamic light scattering (DLS) can determine the formation of nanoparticles in the electrolyte after electrolysis.72–74

(3) Electrochemical method: The electrochemical behaviors of the electrolyte and electrode should be investigated after performing the electrochemical test. The electrolyte is measured again with a fresh electrode to determine whether the electrochemical behavior of the complex changed after electrolysis or not; the used electrode is rinsed without polishing and reused in a fresh electrolyte to detect the formation of metal oxides or metal hydroxides on the electrode surface.75–77

(4) Metal ion scavenger: During the water oxidation process, other ligands (bipyridine,78 EDTA,70,79etc.) are put into the system to capture the metal ions released from the complexes, which can change the electrochemical behavior of the system if metal ions have been dissociated. Notably, the scavenger can only weakly coordinate with the central metal ion, but it cannot influence the chemical properties of the complex. Therefore, it is also an efficient way to estimate whether the catalysts decomposed during the electrolysis or not.

The stability of heterogeneous catalysts should be considered in the following two aspects:61

(1) Nonoxide catalysts (phosphides, selenides, etc.) are generally believed to turn into the corresponding metal oxides and/or hydroxides at higher voltages during the water oxidation process under alkaline conditions.80 However, some of the reported catalysts can maintain their original states.81 To determine the true catalysts, certain physical and spectroscopic characterizations should be carried out. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) patterns have been used to examine the crystallization of catalysts after electrolysis;82 SEM and TEM have been used to detect changes in the morphology of catalysts after electrolysis;82 EDS, XPS, and Raman spectroscopic analyses have been used to investigate the composition and chemical states of the catalysts after electrolysis.82,83

(2) For practical applications as an anode for water splitting, catalysts should yield remarkable durability to conform to harsh industrial conditions (30% KOH, 70–80 °C). In the lab, several electrochemical methods have been used to evaluate the durability of catalysts: successive and rapid 1000 cycles of cyclic voltammetry (CV) testing;84 galvanostatic testing at a defined current density (10 mA cm−2 in 1.0 M KOH;85 1000 mA cm−2 in 10 M (or 30%) KOH86), and potentiostatic testing at given potential.85

3. Homogeneous Cu-based WOCs

In the last decade, the exploration of Cu-based WOCs has dramatically increased owing to its biologically relevant and oxyphilic nature. The history of Cu-based molecular catalysts can be traced back to 2012 when Mayer and co-workers reported the first example of homogeneous molecular catalysts.41 Since then, various kinds of molecular catalysts have been reported and even developed as multinuclear catalysts to mimic the oxygen-evolving Mn4Ca core in PS II. Meanwhile, interestingly, simple CuII salts have also been reported as efficient WOCs under certain conditions, which is also an attractive field because of its low cost and simplicity. In the subsequent sections, we summarized homogeneous Cu-based WOCs from simple to complicated.

3.1. CuII salts as homogeneous WOCs

In 2012, Chen and Meyer found that simple CuII salts can efficiently catalyze water oxidation in neutral to weakly basic aqueous CO2/HCO3−/CO32−, acetate, or HPO42−/PO43− buffer solutions.87 The addition of CuII salts into these buffer solutions can dramatically increase the catalytic activity of the system. In NaHCO3 solution at pH 6.7, the CuII salts worked as homogeneous WOCs. Kinetic studies revealed that the catalytic current densities linearly varied with [CuII] in a buffered solution at pH 6.7 (Fig. 2a). Controlled potential electrolysis (CPE) was conducted at a bias voltage of 1.65 V vs. NHE with [CuII] of 1.2 mM, and the result suggested that a steady current density of 0.35 mA cm−2 could be maintained for at least 3 h at FE of 96% (Fig. 2b). Interestingly, it turned out to be a heterogeneous catalyst in a buffered solution at pH 10.8. Bulk electrolysis was conducted in 1.0 M Na2CO3 at a bias voltage of 1.30 V vs. NHE at a high concentration of CuSO4 (3 mM), and the result suggested that a steady current density of 2.5 mA cm−2 maintained at least for 6 h with FE of 97%. The deposition of a copper-carbonate thin film was observed after electrolysis, which could dissolve back in HCO3/CO32− but not in the nonbuffered solution. Kinetic studies showed that the catalytic current linearly varied with [CuII],2 contrary to the behavior under neutral conditions, suggesting that a pH change in the electrolyte altered the catalytic mechanism of the catalyst toward water oxidation.
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Fig. 2 (a) Plot of catalytic current density at 1.65 V vs. NHE against [CuII] (Copyright 2012, Wiley-VCH. Reproduced from ref. 87 with permission). (b) Controlled potential electrolysis at 1.55 V vs. NHE without (black) and with (blue) 1.2 mM CuSO4 in 0.1 M NaHCO3 solution (pH 6.7). (Copyright 2012, Wiley-VCH. Reproduced from ref. 87 with permission). (c) CVs of 0.4 mM CuSO4 in solutions of 0.8 M NaF, 1 M KF, and 1 M CsF at pH 10.4 for a boron-doped diamond (BDD) disc electrode. Copyright 2016, American Chemical Society. (d) Controlled potential electrolysis for a 0.45 M borate buffer background at pH 7 (red), 0.2 mM CuSO4 (black), and borate buffer using the same ITO working electrode used after the CPE (blue) at 1.55 V vs. NHE (Copyright 2017, the Royal Society of Chemistry. Reprinted from ref. 89 with permission).

Interestingly, in a concentrated fluoride solution, the CuII salts also acted as active and robust WOCs at neutral to weakly basic pH.88 In the system, F acted as a proton acceptor and oxidatively robust ligand. Notably, the catalytic performance was considerably enhanced in the CsF solution as compared to those in NaF and KF solutions (Fig. 2c). A less intense interaction between F and Cs+ led to more intense coordination between F and CuII, which accounted for the improved catalytic performance.

In the subsequent work, the buffer was changed to a neutral borate solution to measure the electrocatalytic capability of CuII salts for water oxidation by Lu's group.89 The CPE experiment yielded a current density of 0.20 mA cm−2 at 1.55 V vs. NHE corresponding to a TOF value of 1 h−1 and FE of 100% (Fig. 2d). Studies on the mechanism revealed that the CuII salts acted as a first-order homogeneous catalyst under the test conditions and the borate anion served as an oxygen donor in the rate-limiting O–O bond formation step, decreasing the energy barrier and enhancing the catalytic activity of CuII salts.

3.2. Mononuclear molecular Cu-based catalysts

3.2.1. Cu-Based catalysts with bipyridine-type ligands. Bipyridine complexes have long been known due to the strong electron-donating property of the ligand and simplicity of the systems. Mayer's group reported the first molecular electrochemical WOC, a copper-bipyridine complex, namely, Cu(bpy)(OH)2 (1 in Fig. 3), which can rapidly self-assemble from a solution of bipyridine and copper salt in the pH range of 11.8–13.3.41 In this pH range, the CV curves of the complex showed pH-dependent irreversible water catalytic peaks at about 1.3–1.5 V vs. NHE at a scan rate of 100 mV s−1 (Fig. 4a). In the reverse scan, a large and irreversible peak appeared at −0.3 V vs. NHE corresponding to the reduction of O2, suggesting oxygen evolution on the electrode surface. At pH 12.8, the ηo value of the catalyst was estimated to be 750 mV and the TOF value was evaluated to be 100 s−1. The CPE was conducted to calculate the FE and prove the homogeneity of the system. The FE value was up to 90% and the catalyst was proven to be homogeneous by using several methods.
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Fig. 3 Structures of Cu-based catalysts bearing bipyridine-type ligands.

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Fig. 4 (a) CVs of solutions containing 1.0 mM total CuII ion and bpy in 0.1 M aqueous electrolyte (NaOAc/NaOH) at pH 12.5 (ref. 41) (Copyright 2012, with permission from Nature Publishing Group). (b) Proposed ligand-assisted PCET in the Cu–L system90 (Copyright 2014, with permission from American Chemical Society). (c) MetalIII/II reduction potential shift with different protonation states of ligated imidazoles (Copyright 2017, the Royal Society of Chemistry, Reproduced with permission from ref. 91). (d) CV of 4 (5 mM) in 0.1 M KNO3/0.1 M KOH at pH 8 (blue) and pH 13.3 (red). Scan rate: 100 mV s−1 (Copyright 2017, American Chemical Society. Reprinted with permission from ref. 68).

Thereafter, Lin and co-workers reported a similar complex, namely, 2 (Fig. 3), bearing a hydroxy-substituted bipyridine.90 The two pendant hydroxyl groups were designed to be a ligand-assisted proton transfer site (Fig. 4b). Successfully, the resulting catalyst could drive water oxidation at ηo value of 510–560 mV in the pH range of 12.0–14.0, which is much lower than that for complex 1. Further experimental and computational studies revealed that the ligand is redox non-innocent and facilitates the proton-coupled electron transfer (PCET) process.

In the follow-up study, an imidazole ring was introduced into the bipyridine ligand by Stott et al. to further improve the catalytic activity of the system.91 This design was inspired by the deprotonation of the Mn-ligating His332 of PS II, which was expected to decrease the redox potential of CuII/I (Fig. 4c). The electrochemical studies of the resulting copper complex, Cu(pimH) (3 in Fig. 3, pimH = 2-(2′-pyridyl)-imidazole), showed that the onset E°′ cathodically shifted by 150 mV when compared with that for complex 1. Moreover, the TOF value of 35 s−1 was evaluated at a low catalyst concentration of 150 μM. The experimental results suggested that the anodic pim ligand of 3 after deprotonation had a stronger σ-donor effect than the neutral bpy ligand of 1, further resulting in an electron-rich Cu site, which could lower the potential to CuIII in the catalytic cycle. These results suggested that the deprotonation of the imidazole moiety of the ligand could lower the catalytic η value. This design may provide newer insights into molecular WOC designs.

This system is also active when one of the pyridines is replaced by O-containing moieties. Very recently, Brudvig's group designed a new copper complex, 4, namely, Cu(pyalk)2 (Fig. 3, pyalk = 2-pyridyl-2-propanoate).68 Similar to the imidazole ring in 3, the propanol moiety of the ligand also served as the deprotonation site. The CV measurements of the catalyst demonstrated that it had no significant redox features when swept under the neutral condition. However, under basic conditions (pH 13.3), it showed a large, irreversible catalytic peak when compared with the background (Fig. 4d). The ηo value for catalysis was within 520–580 mV according to the Ep/2 value between pH 10.4–13.3. The subsequent studies revealed the catalytic mechanism of complex 4 by combining the DFT calculations, UV–vis spectra, KIE, and electrochemical analyses. The results suggested that only the cis form of CuII(pyalk)2 could convert H2O to O2.

3.2.2. Cu-Based catalysts with alkylamine-type ligands. Alkylamine-type ligands are advantageous as the resulting complexes can turn to the catalytically active conformation during OER.34 In 2016, Chen reported a Cu complex bearing the 1,2-ethylenediamine (en) ligand (5 in Fig. 5), self-assembled from the solution of Cu salt and en.92 The obtained complex acted both as a homogeneous WOC in the neutral-to-weak basic conditions and the precursor for heterogeneous WOC in the basic conditions (discussed below). At pH 8.0, the complex was proven to be homogeneous and the water oxidation occurred at ηo = ∼790 mV according to the Ep/2 value with a TOF value of 0.4 s−1.
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Fig. 5 Structures of Cu-based catalysts bearing alkylamine-type ligands.

Two Cu-based WOCs, namely, 6a and 6b (Fig. 5), bearing 1,4,8,11-tetraazacyclotetradecane (cyclam) and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane (Me4cyclam, TMC), were reported by Prevedello and co-workers.93 Similar to 5, the complexes served both as a homogeneous WOC in 0.2 M neutral phosphate-buffered solution (PBS) and a precursor to heterogeneous WOC in basic PBS (pH 9–12, discussed below). In a buffer solution at pH 7.0, water oxidation occurred at ηo = ∼880 mV for both the complexes. The CPE results showed that the FEs were 75% and 80% for 6a and 6b, respectively, and no heterogeneous film was deposited on the electrode surface, which proved the homogeneity of the complexes. Subsequently, Sun's group reported the Cu[(TMC)(H2O)](NO3)2 complex (7 in Fig. 5), with a structure similar to that of 6b, except for the fact that the starting salt was Cu(NO3)2 instead of Cu(ClO4)2.94 Interestingly, the catalytic activity of obtained complex 7 dramatically increased as compared to that of 6b. The ηo value for catalysis was 610 mV according to the Ep/2 value in PBS (0.1 M, pH 7.0). The TOF value was estimated to be 30 s−1. These results suggest that the counter ion of salts also plays an important role in the catalytic performance of WOCs.

Recently, novel Cu-based complex 8 (Fig. 5), bearing a rigid macrocycle ligand, was synthesized by Wang et al., which was proven to be an efficient WOC in PBS (0.15 M, pH 12.0).95 For 8, the ηo value was estimated to be 550 mV and the TOF value was calculated to be 4 s−1. It could sustain a relatively high current density of 1.3–1.4 mA cm−2 at η = 750 mV for 3 h with FE of 50%. Studies of their mechanisms have shown that such a rigid structure inhibited transformation into the catalytically favorable cis-conformation, which is spatially beneficial for the water nucleophilic attack or HO-OH coupling. This study implied that flexible ligands should be designed for developing highly efficient WOCs.

3.2.3. Cu-Based catalysts with alkylamine–pyridine-type ligands. By combining the flexibility of alkylamine and the strong electron-donating property of an aromatic ring, many kinds of alkylamine (imine)–pyridine-type Cu-based catalysts have been synthesized in the recent years.

(1) Complexes with anionic ligands

Meyer's group developed a monomeric Cu-based WOC, namely, Cu(Py3P) (9 in Fig. 6, Py3P = N,N-bis(2-(2-pyridyl)ethyl)pyridine-2,6-dicarboxamidate), which is a stable WOC in 0.1 M H2PO4/HPO42− (pH 8.0).96 The electrochemical kinetic studies revealed that the catalyst underwent a single-site mechanism for the water oxidation reaction. In addition, phosphate-buffered anions were involved in the catalytic process through atom–proton transfer in the rate-limiting O–O bond formation step with HPO42− as the proton acceptor, thereby facilitating proton transfer.

image file: c9nr10437b-f6.tif
Fig. 6 Structures of Cu-based catalysts 9–12.

Moreover, several copper complexes with planar structures were developed as WOCs to facilitate the nucleophilic attack of H2O. Zhan and co-workers reported a soluble copper complex Na2[Cu(opba)] (10 in Fig. 6, opba = o-phenylenebis(oxamato)) for water oxidation and reduction.97 Water oxidation occurred at ηo = 626 mV, yielding a TOF value of 1.13 s−1 at pH 10.8. Subsequently, similar structural analogs of 10, a new family of copper complexes [CuLY]2−(11 in Fig. 6), were developed by Llobet's group.98 At pH 11.5, the ηo value during water oxidation was dramatically reduced to as low as 170 mV owing to the electron-donating capability of the aromatic ring (Fig. 7a). The foot-of-the-wave analysis gave a catalytic rate constant of 3.6 s−1 at pH 11.5. Experimental results and DFT calculations suggested an unusual single-electron transfer water nucleophilic attack (SET-WNA) mechanism (Fig. 7b), where complex 11 firstly oxidized to [(L)CuIII], followed by a ligand-centered oxidation to [(L˙)CuIII(OH)], which transferred to [(L)CuIII(HO⋯OH)˙] and then [(L)CuIII(HO-OH)2−] formed by two successive intramolecular single-electron transfer (Iset) processes, ultimately releasing O2. This result suggested that the redox-innocent ligand and the SET-WNA mechanism could efficiently lower the η value for water oxidation.

image file: c9nr10437b-f7.tif
Fig. 7 (a) CV and differential pulse voltammetry (DPV) for complexes 11a–d in a PBS solution at pH 11.5. (b) Calculated catalytic cycle for 11a (Copyright 2015, with permission from American Chemical Society).

To further increase the electron-donating effect of the ligand, Cao designed water-soluble Cu-based complex 12 (Fig. 6) with a dianionic tridentate ligand, namely, N,N′-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate.99 The introduction of two dimethyl–phenyl groups could considerably enhance the electron density around the metal center. Electrochemical tests showed that water oxidation happened at ηo = 650 mV and yielded a TOF value of 20.1 s−1 in a carbonate buffer at pH 10. Combined with computational evidence, the carbonate group bonded to the Cu center can serve as the proton shuttle to remove the protons for water activation, a key role for resembling the intramolecular base.

(2) Complexes with neutral ligands

Tris-(pyridylmethyl)amine (TPA) and its derivates are appealing candidates for developing Cu-based water oxidation electrocatalysts. Copper complex 13a (Fig. 8), [CuII(TPA)(OH2)]2+, was synthesized and its electrocatalytic activity and mechanism were investigated in neutral PBS by Zhang and Su.100 As compared to binuclear catalyst 25 (Fig. 15), complex 13a had higher η and lower catalytic activity toward water oxidation under the same conditions according to the electrochemical investigation. The catalyst underwent the WNA mechanism in which formal CuIV(O) was proposed as a key intermediate. In addition, Cao designed a water-soluble copper complex [Cu(F3TPA)(ClO4)] (13b in Fig. 8, F3TPA = tris(2-fluoro-6-pyridylmethyl)amine), which could catalyze water oxidation in a borate buffer solution (BBS) at pH 8.5 with a relatively low ηo value of 610 mV.101 It could also serve as a photocatalyst toward water oxidation with the assistance of electron acceptor and photosensitizer with a maximum TOF value of (1.58 ± 0.03) × 10−1 s−1 and maximum turnover number (TON) of 11.61 ± 0.23. Binuclear derivates 25 (“bis-13a”) and 28 [(13c)2-(μ-OH)2], Fig. 15] and precursor 13d for heterogeneous catalysts are discussed below.

image file: c9nr10437b-f8.tif
Fig. 8 Structures of Cu-based catalysts 13.

Four different copper complexes were developed as electrochemical WOCs by Wang and co-workers. Among them, [(bztpen)Cu](BF4)2 (14 in Fig. 9, bztpen = N-benzyl-N,N′,N′-tris(pyridin-2-yl methyl)ethylenediamine) and [(dbzbpen)Cu(OH2)](BF4)2 (15 in Fig. 9, dbzbpen = N,N′-dibenzyl-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine) were synthesized to compare the catalytic performances between the complexes bearing N4-tetradentate and N5-pentadentate ligands.102 The calculated apparent rate constants (kcat) of 14 and 15 were 13.1 and 18.7 s−1 with ηo values of 440 and 570 mV in PBS at pH 11.5, respectively (Fig. 10a). These results suggested that a small change in the ligand structure could notably affect the catalytic performance of the catalyst.

image file: c9nr10437b-f9.tif
Fig. 9 Structures of Cu-based catalysts 14–17.

image file: c9nr10437b-f10.tif
Fig. 10 (a) CVs of 14 and 15 (both in 1.0 mM) as well as the blank CV of a GC electrode in 0.1 M PBS (pH 11.5)102 (Copyright 2017, with permission from the Royal Society of Chemistry). (b) CVs of 16 and 17 (both 1.0 mM), as well as the blank CV of a GC electrode in 0.1 M PBS (pH 9.0)103 (Copyright 2017, with permission from Wiley-VCH).

In their subsequent work, they reported another two copper complexes, namely, [(L1)Cu(OH2)](BF4)2 (16 in Fig. 9, L1 = N,N′-di-methyl-N,N′-bis(pyridin-2-yl methyl)-1,2-diaminoethane) and [(L2)Cu(OH2)](BF4)2 (17 in Fig. 9, L2 = 2,7-bis(2-pyridyl)-3,6-diaza-2,6-octadiene) as electrochemical WOCs.103 Electrochemical tests revealed that 17 exhibited η1 that was 370 mV lower and TOF that was 3.7 times higher than those for 16, suggesting that the diimine moiety of 17 played an important role in enhancing the catalytic performance toward water oxidation owing to its conjugate effect (Fig. 10b). These results suggested that the fine-tuning of the ligand structure could efficiently enhance the catalytic activity of the catalyst.

Other molecular WOCs have also been developed by taking advantage of the conjugate effect of azo (N[double bond, length as m-dash]N) or diimine (C[double bond, length as m-dash]N) moieties. Wei and co-workers designed a new azo-complex, namely, [(L)CuII(NO3)] [18 in Fig. 11, L = (E)-3-(pyridin-2-yldiazenyl)naphthalen-2-ol (HL)].104 The obtained results suggested that 18 displayed high stability and activity toward the water oxidation reaction in the presence of CeIV salt at pH 11 in a PBS solution for an initial TOF value of 4.0 kPa h−1 (implying that 4.0 kPa of oxygen evolved per hour). Very recently, the copper complex [Cu(L1H)(L1)(OH2)](ClO4) (19 in Fig. 11) with a redox-active aryl oxime ligand, namely, L1H [L1H = 1-(pyridin-2-yl)ethanone oxime], was developed by Kuilya et al., and it proved to be a highly efficient electrochemical catalyst toward water oxidation.105 The catalyst yielded a remarkably high TOF value of ∼100 s−1 in neutral PBS at ηo = ∼675 mV with FE of ∼94%. The electrochemical analysis suggested that the ligand moiety was involved in the PCET step during the catalytic cycle.

image file: c9nr10437b-f11.tif
Fig. 11 Structures of Cu-based catalysts 18–19.
3.2.4. Cu-Based catalysts with peptide-type ligands. It is well known that peptides are biorelevant molecules composed of several covalently linked amino acids, whose pKa values and spatial structure can be easily adjusted by altering the type of amino acids.106 Therefore, peptides are the ideal ligands for molecular WOCs. In 2013, Meyer's group developed a self-assembled copper polypeptide complex, namely, [(TGG4−)CuII-OH2]2− (20 in Fig. 12), with a triglycylglycine macrocyclic ligand, which could efficiently catalyze water oxidation in PBS at pH 11.106 The catalyst displayed high stability and superior activity toward water oxidation under the given conditions with a high TOF value of 33 s−1. In the proposed mechanism, the peroxide intermediate of [CuIII-OOH] was generated (Fig. 13a), which was oxidized to release oxygen, thereby completing the catalytic cycle (Fig. 13b).
image file: c9nr10437b-f12.tif
Fig. 12 Structures of Cu-based catalysts bearing the peptide-type ligands.

image file: c9nr10437b-f13.tif
Fig. 13 (a) CVs of 20. Red dashed line: with scan reversal before the catalytic water oxidation wave with the CuIII/CuII couple appearing at E1/2 = 0.58 V vs. NHE. Black line: with scan reversal following a scan into the catalytic water oxidation wave with the appearance of a new wave at E1/2 = 0.38 V vs. NHE. Blue line: after the addition of a few drops of dilute H2O2. (b) Proposed mechanism for water oxidation by 20 in PBS at pH 11 (Copyright 2013, American Chemical Society. Reproduced from ref. 106 with permission). (c) Proposed mechanism for water oxidation by 23 in PBS at pH 11.5 (ref. 108) (Copyright 2018, with permission from American Chemical Society).

On the basis of the above work, Malinka's group reported two mononuclear CuII complexes with two different tetrapeptides, namely, H-Gly-Dap(H-Gly)-Gly-NH2 of 21 (Fig. 12) and H-Gly-Dap(H-Gly)-His-NH2 of 22 (Fig. 12, dap = L-2,3-diaminopropionic acid).107 The experimental results revealed that the C-terminal His extension of dap (L = 2GH) instead of Gly (L = 3GH) could efficiently lower the pKa value for intermediate CuIIIH−2L (9.36 vs. 9.98) and improve the TOF value at pH 11 (53 vs. 24 s−1).

Very recently, Maayan and co-workers designed and synthesized Cu complex 23 (Fig. 12) bearing a peptoid BPT ligand containing a bipyridine group (bound to copper), hydroxyl group (proton-acceptor site), and benzyl group (intramolecular cooperativity).108 It proved to be an active water oxidation electrocatalyst in PBS (pH 11.5) with a TOF value of 5.8 s−1 and FE of up to 91%. Remarkably, this catalyst was highly stable over at least 15 h of electrolysis and yielded record-high TON of >56 in 6 h. In a combination of experimental data and DFT computational results, the key peroxo intermediate of [BPT-CuIII(O˙)(OH)]+ was found, which yielded a unique intramolecular cooperative catalytic pathway, suggesting that the –OH group played a major role in the high stability of the catalyst (Fig. 13c).

3.2.5. Cu-Based catalysts with porphyrin-type ligands. A porphyrin ring is a macrocyclic compound generally used in the field of solar cells, electrocatalysts, and light-emitting devices.32 Very recently, a water-soluble Cu-based complex bearing tetrakis(4-N-methylpyridyl)porphyrin (24 in Fig. 14) was reported by Cao, which could catalyze water oxidation with a small ηo value under neutral conditions.42 The catalytic peak increased from ηo = 310 mV (estimated at j = 0.1 mA cm−2) and yielded a TOF value of 30 s−1 in PBS (0.1 M, pH 7). The CPE test of 24 in neutral PBS displayed a substantial and stable current density of ∼0.2 mA cm−2 at 1.30 V vs. NHE with FE in excess of 93%. Rotating ring-disc electrode (RRDE) analysis and pH-dependent electrochemical studies suggested that during the catalytic process, a H2O2-related species, instead of high-valent CuIII–O˙/CuIV[double bond, length as m-dash]O, was generated to form the O–O bond. This might be the reason why 24 could drive water oxidation in a neutral solution with low η. Moreover, this catalyst could also drive 2e water oxidation under the acidic conditions to form H2O2, which provided newer insights into the water oxidation mechanism (Table 1).
image file: c9nr10437b-f14.tif
Fig. 14 (a) Structure of Cu-porphyrin catalyst 24. (b) CVs of 24, CuSO4, and blank solution at a scan rate of 100 mV s−1. (c) Catalytic currents in CPE with or without 24 (0.75 mM) using the FTO electrode (1.0 cm2) at the applied potential of 1.30 V vs. NHE (Copyright 2019, Reprinted from ref. 32 with the permission from the Royal Society of Chemistry).
Table 1 Summary and references of mononuclear copper-based WOCs
Catalyst structure Test conditions η o (mV) η 1 (mV) TOF (s−1) CPE Test (j@η, t, FE) Ref.
a η at 1 mA cm−2. b Manually extrapolated from the CV curve. c ABS: acetate buffer solution.
1. Bipyridine type
image file: c9nr10437b-u1.tif 0.5 mM cat., pH 12.5, 0.1 M ABSc ∼750 (Ep/2) 100 0.5 mA cm−2@860 mV, 0.5 h, 90% 41
image file: c9nr10437b-u2.tif 1 mM cat., pH 12.4, 0.1 M ABS 510–560, pH = 12–14 (Ep/2) 0.4 >0.15 mA cm−2@640 mV, 150 min, 59% 90
image file: c9nr10437b-u3.tif 0.15 mM cat., pH 12, 0.1 M ABS ∼728 (Ep/2) 35 1.8 mA cm−2@580 mV, 1500 s 91
image file: c9nr10437b-u4.tif 5 mM cat., pH 12.5, 0.1 M KNO3/0.1 M KOH ∼560b (Ep/2) ∼580b 0.7 1 mA cm−2@610 mV, 10 h 68
2. Alkylamine type
image file: c9nr10437b-u5.tif 1.0 mM cat., pH 8, 0.2 M PBS ∼492b (onset) ∼592b 0.4 ∼0.4 mA cm−2@792 mV 92
image file: c9nr10437b-u6.tif 1.0 mM cat., pH 7, 0.2 M PBS ∼880b (onset) 0.22 mA cm−2@1130 mV, 1.5 h 93
image file: c9nr10437b-u7.tif ∼880b (onset) ∼730b 7 0.6 mA cm−2@1130 mV, 1.5 h
image file: c9nr10437b-u8.tif 1.0 mM cat., pH 7, 0.1 M PBS ∼610b (onset) ∼810b 30 ∼0.25 mA cm−2@820 mV, 1 h 94
image file: c9nr10437b-u9.tif 1.0 mM cat., pH 12, 0.1 M PBS ∼550 (Ep/2) ∼700b 4 1–1.5 mA cm−2@750 mV, 3 h 95
3. Pyridine-amine (amide, enamine) type
image file: c9nr10437b-u10.tif 0.7 mM cat., pH 8, 0.15 M PBS 542 (Ep/2) 3 × 10−6 0.2–0.3 mA cm−2@η ∼640 mV, 7 h, 84% 96
image file: c9nr10437b-u11.tif 0.1 mM cat., pH 10.8, 0.25 M PBS 626 (Ep/2) ∼1150b ∼1.13 5 h, 95.8% 97
image file: c9nr10437b-u12.tif 2 mM cat., pH 11.5, 0.1 M PBS a: 700; b: 400; c: 270; d: 170 (Ep/2) a: 3.56; b: 3.58; c: 0.43 d: 0.16 0.1 mA cm−2@750 mV, ∼1 h (b), 100% 98
image file: c9nr10437b-u13.tif 1 mM cat., pH 8.5, 0.1 M BBS ∼780 (onset) ∼810 33 1.6 mA cm−2@770 mV, 5 h, >90% 100
image file: c9nr10437b-u14.tif 1 mM cat., pH 8.5, 0.1 M BBS 610 (onset) 720b 0.38 2 h, 96% 101
image file: c9nr10437b-u15.tif 0.1 mM cat., pH 11, 0.1 M PBS ∼650b (Ep/2) 104
image file: c9nr10437b-u16.tif 1 mM cat., pH 10, 0.1 M CBS 650 (onset) ∼700b 20.1 0.7–0.8 mA cm−2@860 mV, 10 h, 95% 99
image file: c9nr10437b-u17.tif 1 mM cat., pH 11.5, 0.1 M PBS 440 (0.2 mA cm−2) ∼850b 13.1 0.27 mA cm−2@850 mV, 1 h, 91% 102
image file: c9nr10437b-u18.tif 1 mM cat., pH 11.5, 0.1 M PBS 570 (0.2 mA cm−2) ∼850b 18.7 0.35 mA cm−2@850 mV, 1 h, 94%
image file: c9nr10437b-u19.tif pH 9.0, 1 mM cat., 0.1 M PBS 720 (0.1 mA cm−2) ∼1050 13.5 0.20 mA cm−2@880 mV, 1 h, 69% 103
image file: c9nr10437b-u20.tif 1 mM cat., pH 9.0, 0.1 M PBS ∼450 (0.1 mA cm−2) ∼700 50.4 ∼0.3 mA cm−2@880 mV, 1 h, 94%
image file: c9nr10437b-u21.tif 0.5 mM cat., pH 7, 0.1 M PBS 675 (onset) 100 ∼1 mA cm−2@733 mV, 1 h, 96% 105
4. Peptide-type
image file: c9nr10437b-u22.tif 0.89 mM (2 mM for CPE) cat., pH 11.0, 0.25 M NaNO3 ∼520 (onset) 620b 33 0.8 mA cm−2@580 mV, ∼5 h, 99% 106
image file: c9nr10437b-u23.tif 0.5 mM cat., pH 11.0, 0.15 M PBS ∼620 (onset) ∼740b 24 1.5 h, 95% 107
image file: c9nr10437b-u24.tif 0.5 mM cat., pH 11.0, 0.15 M PBS ∼620 (onset) ∼740b 53 1.5 h, 91%
image file: c9nr10437b-u25.tif 0.5 mM cat., pH 11.5, 0.1 M PBS ∼800b (onset) ∼1050b 5.8 1 mA cm−2@η ∼ 860 mV, 15 h, 91% 108
5. Porphyrin-type
image file: c9nr10437b-u26.tif 0.5 mM (0.75 mM for CPE) cat., pH 7.0, 0.1 M PBS 310 (0.1 mA cm−2) ∼460b 30 ∼0.2 mA cm−2@η 483 mV, 10 h, >93% 42

3.3. Multinuclear Cu-based catalysts

3.3.1. Binuclear Cu-based catalysts. The first binuclear Cu-based catalyst, the oxidatively robust [Cu2(BPMAN)(μ-OH)]3+ (25 in Fig. 15, BPMAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8naphthyridine, bis-TPA), was reported by Zhang in 2015.109 This complex displayed high reactivity and stability toward water oxidation with a TOF value of ∼0.6 s−1 at ηo = ∼780 mV in neutral aqueous solutions. Computational evidence suggested that the O–O bond formation happened by an intramolecular direct coupling mechanism rather than by the WNA pathway (Fig. 16a).
image file: c9nr10437b-f15.tif
Fig. 15 Structures of binuclear Cu-based catalysts.

image file: c9nr10437b-f16.tif
Fig. 16 (a) Proposed mechanism for water oxidation by 25 in 0.1 M PBS (pH 7)109 (Copyright 2015, with permission from Wiley-VCH). (b) Proposed mechanism for water oxidation by 26 in 0.1 M neutral PBS110 (Copyright 2018, with permission from American Chemical Society). (c) Low-energy reaction pathways derived from the DFT analysis of water oxidation by 28[thin space (1/6-em)]112 (Copyright 2019, with permission from American Chemical Society).

In their subsequent work, the same group developed an unsymmetrical binuclear Cu-based complex, namely, [Cu2(TPMAN)(μ-OH)(H2O)]3+, (26 in Fig. 15, TPMAN = 2-[bis(2-pyridylmethyl)aminomethyl]-7-[N-methyl-N-(2-pyridyl-methyl)aminomethyl]-1,8-naphthyridine) for water oxidation in neutral PBS.110 This complex was a stable and efficient homogeneous catalyst with kcat value of 0.78 s−1 and ηo value of 780 mV. The mechanism of 26 toward water oxidation was fairly different from that of 25, such as the KIE effect, buffer concentration effect, and different pH-dependent effects (Fig. 16b), suggesting that the hemilabile pyridyl group in 25 acted as an intramolecular proton acceptor to facilitate the catalytic cycle.

The new complex [Cu(oxpn)Cu(OH)2] (27 in Fig. 15, oxpn = N,N′-bis(3-aminopropyl)oxamido) was developed as both water oxidation and reduction catalyst by Zhan and co-workers.111 Electrochemical tests revealed that water oxidation occurred at ηo = 289 mV with a TOF value of 6.7 s−1. Recently, Kieber-Emmons’ group reported the [(MeTPA)CuII]2-(μ-OH)2(OTf)2 complex (28 in Fig. 15), which could drive water oxidation at moderate rates (TOF = 33 s−1, η ≈ 1 V) and high FE (90%).112 Electrochemical and spectroscopic studies revealed that the binuclear complex partially dissociated under the test conditions to generate mononuclear [(L)CuII(OH)]+. DFT calculations revealed that the oxidation of either mononuclear or binuclear species resulted in a common binuclear intermediate, namely, {[L(CuIII)]2-(μ-O)2}2+, which deterred the formation of unstable Cu(IV)[double bond, length as m-dash]O/Cu(III)–O˙ intermediates (Fig. 16c) (Table 2).

Table 2 Summary and references of binuclear copper-based WOCs
Catalyst structure Test conditions η o (mV) η 1[thin space (1/6-em)]a (mV) TOF (s−1) Stability test (j@η, t, FE) Ref.
a η at 1 mA cm−2.
image file: c9nr10437b-u27.tif 1 mM cat., pH 7, 0.1 M PBS 780 830 0.6 0.5 mA cm−2@950 mV, 2 h, 98% 109
image file: c9nr10437b-u28.tif 1 mM cat., pH 7, 0.1 M PBS 780 850 0.78 0.20 mA cm−2@950 mV, 2 h, 98% 110
image file: c9nr10437b-u29.tif 1.38 mM cat., pH 10.1, 0.25 M PBS 289 6.7 5 h, 96.5% 111
image file: c9nr10437b-u30.tif 1 mM cat., pH 12.5, 0.1 M NaOTf 1000 33 1.60 mA cm−2@856 mV, ∼5 h, 90% 112

3.3.2. Trinuclear Cu-based catalysts. Polyoxometalates (POMs) have been considered to be excellent inorganic ligands for the synthesis of WOCs.113 In 2018, the first POM-based copper cluster, namely, [(α-SbW9O33)2Cu3(H2O)3]12− (29 in Fig. 17a), was developed by Ding et al., which could catalyze water oxidation at neutral pH.113 Here, 29 proved to be a homogeneous catalyst by different kinds of technologies, namely, CV, LSV, and CPE, which was completely different from the catalytic behavior of CuCl2 that was deposited on the electrode surface under the same test conditions (Fig. 17b–d). At the same time, SEM and EDX confirmed that no heterogeneous film was formed on the electrode surface after the electrolysis tests. This work may provide a new approach for developing cheap and stable Cu-based WOCs.
image file: c9nr10437b-f17.tif
Fig. 17 (a) Combined polyhedral representation of 29. The color code is as follows: copper, yellow; tungsten, blue; antimony, red; oxygen, gray. (b) LSV curves of 29 for the 1st and 100th cycle in 80 mM Tris-HCl buffer (pH 7.1). CV curves of 2.0 mM 29 (c) and CuCl2 (d) in 80 mM Tris-HCl buffer solution (pH 7.1) with cleaned GC (blue curves), as-used GC in a blank buffer solution (red), and fresh GC in a blank solution (black)113 (Copyright 2018, with permission from the Royal Society of Chemistry).
3.3.3. Tetranuclear Cu-based catalysts. Inspired by the tetranuclear manganese cluster (CaMn4O5) in PS II, several tetranuclear Cu-based complexes have been developed as WOCs. The first tetranuclear copper-based water oxidation catalyst, namely, 30 (Fig. 18), [Cu4(bpy)42-OH)2(μ3-OH)2(H2O)2]2+, was reported by Li and Zheng in 2016.114 The multinuclear Cu4O4 core was expected to be advantageous for facilitating multielectron transfer during the water oxidation process. The catalyst showed a relatively low ηo value of ∼730 mV and achieved FE of 98% at 1.80 V vs. NHE in neutral PBS. In the same year, Masaoka's group developed tetranuclear copper complex 31 (Fig. 18) by using a ligand containing two proton dissociation sites, namely, 1,3-bis(6-hydroxy-2-pyridyl)-1H-pyrazole.115 This complex exhibited a low ηo value of ∼500 mV toward water oxidation reactions under aqueous basic conditions (pH 12.5). Several techniques, such as UV–vis spectra, EDS, and electrochemical tests, proved that the catalyst worked as a homogeneous WOC under the given conditions. Subsequently, two bioinspired Cu4O4 cubanes, namely, [(Cu-LGly)4] (32a, Fig. 18) and [(Cu-LGlu)4] (32b, Fig. 18), were synthesized by Wu and co-workers.116 These cubanes were effective molecular WOCs in aqueous solutions (pH 12). With regard to the catalytic activity, a TOF value of 267 s−1 for 32a at 1.70 V vs. NHE and that of 105 s−1 for 32b at 1.56 V vs. NHE were obtained. Electrochemical and spectroscopic studies revealed that the high performance of both these catalysts could be attributed to the successive two-electron transfer process to form high-valent CuIII and CuIIIO˙ intermediates during catalysis (Table 3).
image file: c9nr10437b-f18.tif
Fig. 18 Structures of tetranuclear Cu-based catalysts.
Table 3 Summary and references of tetranuclear copper-based WOCs
Catalyst structure Test conditions η o (mV) η 1[thin space (1/6-em)]a (mV) TOF (s−1) Stability test (j@η, t, FE) Ref.
a η at 1 mA cm−2. b Manually extrapolated from the CV curve.
image file: c9nr10437b-u31.tif 1 mM cat., pH 7.0, 0.1 M PBS 733 ∼840b 0.78 mA cm−2@1000 mV, 10 h, 98% 114
image file: c9nr10437b-u32.tif 0.2 mM (0.3 mM for CPE) cat., pH 12.5, 0.1 M NaOAc/NaOH, 500 0.8 0.2 mA cm−2@800 mV, 3 h, 75% 115
image file: c9nr10437b-u33.tif 0.25 mM cat., pH 12, 0.2 M PBS ∼400b (32a) 620 (32a) 267 (32a) 1.1 mA cm−2@920 mV, 3 h, 97 ± 2% (32a) 116
∼480b (32b) 760 (32b) 105 (32b) 0.9 mA cm−2@920 mV, 2 h (32b)

Summarily, in the past decade, several kinds of homogeneous copper-based systems, including salts and complexes, have been developed for as electrocatalysts for water oxidation. For salts and mononuclear molecular catalysts, high-valent Cu intermediates were well defined by (spectro)electrochemical or spectroscopic techniques. With regard to the catalytic mechanism, the O–O bond formation step is considered to be the rate-determining step. Generally, the O–O bond formed by water nucleophilically attacked the catalytically active CuIII–O˙/CuIV[double bond, length as m-dash]O intermediate. The electrochemical characterization of the peroxide [CuIII-OOH] intermediate was firstly conducted by Meyer and co-workers. Recently, Llobet's group revealed a unique SET-WNA mechanism. Instead of conventional 2e in one step reaching to the high-valent intermediate, two successive Iset processes from [(L˙)CuIII(OH)] to [(L)CuIII(HO-OH)2−] happened, finally releasing O2. Similarly, Cu(II)-porphyrin also generated a H2O2-related species to form the O–O bond. Moreover, for Cu-based catalysts, the added base (such as HPO42−, OH, and F) and intramolecular functional group (such as –OH, carbonate, and pyridyl group) acted as a proton acceptor for accelerating the PCET process to reach the high-valent intermediate. Meanwhile, several multinuclear complexes were designed to mimic the Mn4Ca core in natural PS II. However, their activity toward water oxidation was not sufficiently high when compared with salts and mononuclear complexes. Moreover, multinuclear complexes are also difficult to synthesize and the mechanism of certain catalysts toward water oxidation is difficult to accomplish owing to structural complexations. Finally, the homogeneous catalysts mentioned above, to the best of our knowledge, have not been combined with semiconductors (or photosensitizers) to construct an artificial photosynthesis system, probably due to the relatively high η values.

4. Heterogeneous Cu-based catalysts

Heterogeneous catalysts have attracted considerable interest because of their facile synthesis and remarkable durability toward water oxidation. However, Cu-based electrocatalytic materials have seldom been reported because of their poor activity and electroconductivity. In the past decade, the field of heterogeneous Cu-based WOCs has dramatically grown as a novel research topic. In the subsequent sections, the development of Cu-based heterogeneous catalytic systems was introduced and the perspective on future development was also discussed.

4.1. Heterogenized Cu-based molecular catalysts

Immobilizing or anchoring molecular complexes on electrodes has been considered to be one of the most important goals for developing fundamental research for practical artificial photosynthesis. In 2016, Sun's group immobilized complex 7 on CC for improving catalyst durability.94 As shown in Fig. 19, blank CC yielded negligible current density at the applied potential of 1.64 V vs. NHE in PBS (0.1 M, pH 7). At the same potential and test conditions, the 7/CC electrode (0.4 mmol) exhibited a considerably enhanced current density of up to 4 mA cm−2 owing to the high catalytic capability of 7. In addition, no decay was observed in bulk electrolysis for 4 h and the FE of 7/CC was estimated to be 98%. For comparison, 25/CC, Cu(NO3)2/CC, and CuOx/CC with the same Cu loading were also prepared and their catalytic performances were investigated under the same test conditions as that in 7/CC. As shown in Fig. 19, 25/CC exhibited a lower current density than 7/CC, suggesting the superior catalytic performance of 7/CC. Moreover, Cu(NO3)2/CC and CuOx/CC electrodes showed steady current densities less than 1 mA cm−2, excluding the possibility of degradation or deposition of the molecular catalyst.
image file: c9nr10437b-f19.tif
Fig. 19 Bulk electrolysis with CC modified by complex 7, complex 25, Cu(NO3)2, and CuOx as the working electrodes in PBS (0.1 M, pH 7). Applied potential: 1.64 V vs. NHE94 (Copyright 2016, with permission from the Royal Society of Chemistry).

In the same year, Farkas et al. heterogenized Cu-branched peptide complexes 21 and 22 on an ITO substrate by combining the substances with a suitable polyelectrolyte such as poly(L-lysine) (PLL) and poly(allylamine hydrochloride) (PAH).117 Optical waveguide lightmode spectroscopy (OWLS) and its combination with electrochemistry (EC-OWLS) were used to monitor molecular adsorption in real time (Fig. 20). Optimizing the parameters, such as pH (7.5–10.5) and catalyst concentrations, was conducted to improve the bottom-up deposition of the catalyst and polyelectrolyte onto the ITO electrode. The atomic force microscopy (AFM) results revealed that the composite electrode had nanoporous topography. Electrochemical tests revealed that the catalytic performance of the fabricated electrode was improved by catalysts as compared to the blank ITO and catalytic activity of deposited 22 was higher than that of deposited 21.

image file: c9nr10437b-f20.tif
Fig. 20 (a) Parts and operating principles of OWLS flow-through cell. (b) Components for the self-assembled catalyst deposition on the ITO surface117 (Copyright 2016, the Royal Society of Chemistry. Reprinted with permission).

Very recently, Llobet's group rationally designed the copper complex [(Lpy)Cu]2− (33 in Fig. 21, 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) to support the more extended π-conjugation through a pyrene group when compared with its homologue, 11 (Fig. 21).118 The catalytic performances of both the catalysts were compared in the homogeneous and heterogeneous phases by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, electronic fine-tuning caused by pyrene functionality led to a decrease in ηo to 150 mV and a remarkable increase in kcat from 6 to 128 s−1 as compared to the corresponding values for 11. Upon anchoring, π-stacking interactions with graphene sheets provided further π-delocalization, which could improve the catalytic activities of both the catalysts. The results indicated that 33 was the most active catalyst, displaying an ηo value of 538 mV, kcat value of 540 s−1, and impressively high TON of 5300.

image file: c9nr10437b-f21.tif
Fig. 21 Structural representation and labeling code of complexes 11 (left) and 33 (right) and the hybrid materials.118

4.2. CuOx as heterogeneous WOCs

4.2.1. CuOx thin films from molecular precursors. Things are not always as they seem. In earlier studies involving homogeneous systems, certain molecular catalysts were proven to be unstable under test conditions. For example, reported molecular catalyst 18 (Fig. 11) proved to be a precursor for a heterogeneous catalyst according to the research by Najafpour et al.119 They found that the complex became CuO entirely under the same conditions as those in Wei's research,104 and the deposited CuO was the real WOC. Therefore, researchers should be careful to distinguish whether the complex is homogeneous (such as molecular catalysts) or heterogeneous (such as oxide catalysts) toward water oxidation.

Interestingly, some researchers deposited molecular complexes on the electrode surface such as metal oxides or hydroxides, which were proven to be highly efficient WOCs under certain conditions. Moreover, the metal oxide or hydroxide thin films converting from these precursors had a higher surface area and more catalytically active sites than those of catalysts formed from simple salts, which could be attributed to the unique electronic and spatial structures of molecular complexes.

Molecular WOCs, such as 5, 6, and 12a, could also function as precursors for heterogeneous catalysts under different conditions as those mentioned above. For instance, Du et al. reported nanostructured copper oxide thin films electrodeposited from 12a and 12d, which showed significant catalytic performance toward OER in alkaline aqueous solutions.120 The films afforded 1 mA cm−2 at η = 600 mV with a Tafel slope of 56 mV dec−1, which was much higher than those of commercially available CuOx and CuOx deposited from simple CuCl2 under the same conditions. Further, Chen found that complex 5 (Fig. 5) shifted from a homogeneous catalyst to a precursor for heterogeneous oxide films when the pH of the PBS solution increased to 12, where a current density of 1 mA cm−2 could be obtained by the converted oxide film at η = of 540 mV.92 The results of the electrochemical studies revealed that 5 decomposed through the oxidation of en to glyoxal and the oxidation of CuII to CuIII. In the same year, Du's group used 5, CuII 1,3-propanediamine (34), and CuII 1,4-butanediamine (35) (Fig. 22) as the precursors and compared the catalytic performances of the obtained thin-film catalysts.121 The results showed that the oxide film electrodeposited from 5 worked as a highly efficient WOC in 1.0 M KOH, which required η = ∼370 and ∼475 mV to reach the current densities of 1 and 10 mA cm−2, respectively. When 5 was replaced by 34 and 35, the catalytic performances worsened, indicating that 5 is the best candidate to be used as the precursor under the given deposition conditions. Like 5, complexes 6 also acted as the precursor in an alkaline aqueous solution (pH 9–12).93 Bulk electrolysis studies yielded a steady current density of 3.7 mA cm−2 at η value of ∼700 mV in buffer solutions at pH 12.

image file: c9nr10437b-f22.tif
Fig. 22 Structures of precursors 34–40 for preparing copper oxide thin films.

Other simple Cu-based complexes were also synthesized as precursors for the preparation of heterogeneous thin films. A copper oxide thin film, electrodeposited from the [Cu(TEOA)(H2O)2][SO4] precursor, was reported by Fu and Li (36, Fig. 22).122 The thin film had an η value of 550 mV and the long-term electrolysis showed a stable current density of 0.55 mA cm−2 at 1.3 V vs. NHE that could be sustained for at least 3 h with FE of almost 100% in ABS at pH 12.4. Another copper oxide film (Cu-Tris film), electrodeposited in situ from PBS at pH 12.0 containing 1.0 mM 37 (Fig. 22), was reported by Sun's group.123 The obtained film showed a modest η value of 390 mV at a current density of 1.0 mA cm−2 for water oxidation. Simultaneously, a significantly low Tafel slope of 41 mV dec−1 could be achieved. This film also exhibited a high and stable current density of ∼7.5 mA cm−2 at 1.15 V vs. NHE for at least 10 h. Very recently, Szunerits used CuII-microcycle complex 38 (Fig. 22) formed by the complexation between Cu(ClO4)2 and 1,4-bis(2-carboxyaldehyde phenoxy)butane as a precursor for the electrodeposition of a Cu2O thin film on ITO.124 At pH 12, the obtained film afforded a current density of 1 mA cm−2 at an η value of 400 mV with a Tafel slope of 72 mV dec−1, within the typical range of other Cu-based heterogeneous WOCs.

Some heterogeneous catalysts were prepared through the calcination of molecular complexes. For example, several copper catalysts from the pyrolysis of a mixture of graphene oxide (GO) and Cu(phen)2 (1,10-phenanthroline) (39, Fig. 22) at different temperatures were prepared by Xia, which could serve as bifunctional catalysts for effective water oxidation as well as reduction.125 Among them, CPG-900, with the lowest energetic state of ˙OH on its surface, displayed a current density of 1 mA cm−2 at an η value of 470 mV. The results showed that the electron density of Cu2+ in the catalyst was tuned by the electronic effect from both the neighboring Cu0 of Cu nanoparticles and the N ligand incorporated in rigid graphene. In addition, Du and co-workers reported a robust and bifunctional electrocatalyst for HER and OER, which was derived from copper porphyrin-based conjugated mesoporous polymers (40, CMPs, Fig. 22).126 The catalyst had a high surface area and a unique tubular porous structure. Under optimal conditions, the catalyst exhibited relatively low η for OER (350 mV for 1.0 mA cm−2 and 450 mV for 10 mA cm−2 in 1.0 M KOH solution), which was the best among Cu-based electrochemical WOCs (Table 4).

Table 4 Summary and references of heterogeneous catalysts from molecular precursors
Precursor structure Conversion conditions Test conditions η 1[thin space (1/6-em)]d (mV) Tafel slope (mV dec−1) Durability test Ref.
a Bias vs. NHE. b Manually extrapolated from the CV curve. c Best condition in the report. d η at 1 mA cm−2. e η at 10 mA cm−2.
image file: c9nr10437b-u34.tif Conc. = 0.68 mM; bias: 1.41 Va, in a 0.2 M BBS (pH = 9.2), 9 C charge passed 0.2 M BBS (pH 9.2) ∼620b 1.2 mA cm−2@1.41 Va, 2 h 120
image file: c9nr10437b-u35.tif Conc. = 0.68 mM; bias: 1.41 Va, in a 0.2 M BBS (pH 9.2), 9 C charge passed 0.2 M BBS (pH 9.2) ∼600 ∼56 1.4 mA cm−2 decrease to 1.2 mA cm−2@1.41 Va, 2 h
image file: c9nr10437b-u36.tif Conc. = 1.0 mM; bias: 1.15 Va, in 0.2 M PBS (pH 12.0), ∼1 h deposition 0.2 M PBS (pH 12.0) 540 62 ∼2.5 mA cm−2@1.15 Va, 6 h 92
Conc. = 3.0 mM; bias: 1.01 Va, in 0.1 M KOH (pH 9.2), ∼3 h deposition 1.0 M KOH 370 90 1 mA cm−2@370 mV; 10 mA cm−2@475 mV. Stable 10 h. 121
475 (η10e)
image file: c9nr10437b-u37.tif Conc. = 2 mM; bias: 1.3 Va, in a 0.1 M ABS (pH 12.4), 5 hours deposition 0.1 M ABS (pH 12.4) ∼600b 130 0.55 mA cm−2 decrease to 0.43 mA cm−2@1.30 Va, 3 h 122
image file: c9nr10437b-u38.tif Conc. = 1.0 mM; bias: 1.15 Va, in a 0.2 M PBS (pH 2.0), 6 hours deposition 0.2 M PBS (pH 12.0) ∼670 (η10e) 41 7.5 mA cm−2@1.15 Va, 10 h 123
image file: c9nr10437b-u39.tif Conc. = 0.5 mM; bias: 1.1 Va, in water, ∼5 h deposition 0.1 M KBi (pH 12) 540 72 5 mA cm−2 decrease to 4.5 mA cm−2@1.30 Va, 3 h 124
image file: c9nr10437b-u40.tif Under Ar, pyrolyze with GO at 900 °C, 2 h 0.1 M KOH ∼470 Not mentioned Not mentioned 125
image file: c9nr10437b-u41.tif Under N2, pyrolyze at different temperature (600, 700, 800, 850h, and 900 °C), 2 h 1.0 M KOH 350c 62c Not mentioned 126

4.2.2. CuOx WOCs obtained from salts and Cu substrates. Earlier, researchers usually used 2D electrodes as the working electrodes when they conducted electrodepositions and measurements. Sun's group reported a robust copper oxide thin film, namely, Cu–Bi, prepared by the facile electrodeposition of CuII from BBS at pH 9.0 on a FTO electrode.127 The obtained film exhibited high activity and stability in CuII-free BBS at pH 9. A Tafel slope of 89 mV dec−1 was achieved in BBS (0.2 M, pH 9.0), which was slightly larger than those reported for Co–Pi and Ni–Bi. Moreover, a steady current density of 1.2 mA cm−2 could be sustained for at least 10 h at 1.3 V vs. NHE corresponding to FE of 95% (Fig. 23a–c). To improve the catalytic activity, a series of unique CuOx nanostructures were obtained via hydrothermal synthesis and subsequent H2O2 treatment by Yeo and co-workers, which exhibited high and sustainable OER activity.128 In 0.1 M KOH, the catalyst reached a current density of 1 mA cm−2 at an η value of 440 mV and sustained at ∼3.4 mA cm−2 for 1 h. The kinetic studies revealed that H2O2 treatment led to the formation of transient species on the catalyst surface. Moreover, a Cu(OH)2 material was synthesized on a FTO electrode via a simple aqueous solution reaction by Chen et al., which was further calcined at 300 °C for 2 h and converted to CuO–FTO.129 The obtained electrode could catalyze water oxidation in PBS at pH 12 with an impressive η value of 500 mV to reach a current density of 1 mA cm−2 with a Tafel slope of 57 mV dec−1 (Fig. 23d–f).
image file: c9nr10437b-f23.tif
Fig. 23 (a) Profile view of a Cu–Bi film.127 (b) Tafel plot of Cu–Bi film in 0.2 M borate buffer at pH 9 with iR compensation.127 (c) CPE curves of the FTO electrodes (1 cm2) without (black line) and with Cu–Bi film (red line) in stirred 0.2 M BBS (pH 9) at 1.3 V vs. NHE127 (Copyright 2015, with permission from American Chemical Society). (d) SEM image of CuO–FTO electrodes.129 (e) Tafel plot of the as-prepared CuO–FTO electrode in 0.2 M PBS at pH 12, corrected for the iR drop of the solution.129 (f) CPEs of the Cu(OH)2-FTO and CuO–FTO electrodes at 1.2 V vs. NHE in 0.2 M PBS at pH 12 (ref. 129) (Copyright 2017, with permission from Materials Research Society).

Moreover, Du and co-workers have made a significant contribution to the field of CuOx WOCs. In 2015, they hydrothermally synthesized a series of CuO nanomaterials with different morphologies (microspheres, nanosheets, and nanowires) from a simple Cu salt.130 Among them, CuO microspheres had the best catalytic current densities from 1.10 to 1.40 V vs. NHE. In the same year, a catalytically active Cu(OH)2 material was synthesized from a simple salt, which could robustly and efficiently catalyze water oxidation.131 Under optimized conditions (pH 9.2, KBi), the catalyst reached a current density of 0.1 mA cm−2 at an η value of 550 mV and a Tafel slope of 78 mV dec−1. Subsequently, they reported an annealing strategy to synthesize a binder-free and self-supported CuO on FTO as a heterogeneous WOC.132 The obtained electrode required 360 and 430 mV to reach 0.1 and 1 mA cm−2 in 1.0 M KOH, respectively. The structure of the CuO motif in the catalyst was carefully identified by its high-energy X-ray diffraction (HEXRD) and Cu K-edge X-ray absorption (XAS) spectra. Very recently, copper nanosheets were prepared by the molten salt method at different temperatures to control the morphology of the resulting materials.133 Under the given conditions, the CuO material prepared at 900 °C demonstrated the highest catalytic performance for OER with low η values of only 420 and 520 mV to achieve current densities of 10 and 40 mA cm−2, respectively. Furthermore, after mixing the material with multiwalled carbon nanotubes (MWCNTs), catalyst loading could be reduced from 0.7 to 0.14 mg cm−2 and the η value reduced to 470 mV for affording a current density of 40 mA cm−2.

After more comprehensive research, researchers realized that 3D electrodes are much better than 2D electrodes, because 3D electrodes have a larger surface area and a porous structure, allowing much faster electron transfer and more contact with the electrolyte. Subsequently, several kinds of Cu-based catalysts grown on 3D CF were reported in the recent years. In 2016, Hou et al. presented a facile synthesis process for fabricating Cu-based nanowire arrays (Cu NWAs), such as Cu(OH)2, CuO, Cu2O, and CuOx, grown on Cu foil (Fig. 24a).134 These catalysts were found to be highly efficient and robust electrocatalysts for use in OER. Remarkably, among them, Cu(OH)2-NWs/CF showed a low η value of 530 mV at a current density of 10 mA cm−2 and almost 100% FE for water oxidation (Fig. 24b). Around the same time, a robust and active nanostructured copper oxide (CuOx-NLs) electrocatalyst was developed by Groot and Joya.135 The electrode was prepared by simple constant potential anodization in CBS at pH 11.0, exhibiting leaf-type morphology (Fig. 24c and d). The oxygen evolution occurred at an η value of 450 mV for 10 mA cm−2 and low Tafel slope of 44 mV dec−1. Moreover, bulk electrolysis showed a stable current density of >17 mA cm−2 at least for 20 h (Fig. 24e).

image file: c9nr10437b-f24.tif
Fig. 24 (a) Illustration of the in situ fabrication of Cu-based nanowire arrays on the CF surface for water oxidation.134 (b) Current density trace obtained by the controlled potential electrolysis of Cu(OH)2 without iR correction134 (Copyright 2016, reprinted with the permission from Wiley-VCH). (c and d) SEM images of CuOx-NLs.135 (e) CPE using CuOx-NLs and as-prepared CuOx-NPs at 1.81 V vs. RHE135 (Copyright 2016, reprinted with the permission from American Chemical Society).

The dendritic structure was appealing because of its high surface area and fast charge transfer ability. Dendritic Cu2O–Cu hybrid foam was synthesized by hydrogen-bubble soft-template-assisted electrodeposition for use as a high-performance electrocatalyst (Fig. 25a).43 The generated electrode exhibited a low η value of 350 mV for 10 mA cm−2, a small Tafel slope of 67 mV dec−1, and high durability of over 50 h at a current density of 10 mA cm−2 for OER in 1.0 M KOH. This unique 3D porous foam could provide fast transportation between the electrolyte and electrode and easy release of the evolved O2 bubbles. In addition, the core–shell dendrites could provide synergistic effects for electrocatalytic reaction. Meanwhile, Fontecave's group presented a dendritic nanostructured Cu/Cu2O/CuO electrode similar to that of the electrode reported by Li's group (Fig. 25b).43 The obtained electrode was highly efficient for catalyzing water oxidation with an η value of 290 mV at 10 mA cm−2 in 1 M NaOH solution. However, the rationally designed surface structure of the electrode was largely damaged after a 10 h CPE test at 1.53 V vs. RHE (Fig. 25c). Very recently, Huang and Hao reported a nanostructured CuO/C hollow shell transferred from a metal–organic framework thin layer on nano-dendritic CF (Fig. 25d).137 This electrode reached current densities of 10 and 100 mA cm−2 at η values of 286 and 482 mV and sustained a steady current density of 10 mA cm−2 for at least 50 h with no damage to the dendritic structure. This superior catalytic performance could contribute to fast electronic transmission networks and rich redox sites.

image file: c9nr10437b-f25.tif
Fig. 25 (a) Schematic illustration for the fabrication of hybrid Cu2O–CFs43 (Copyright 2016, with permission from American Chemical Society). (b) Typical dendritic structure of the material.136 (c) Plot of catalytic potential at a fixed current density during the long-term electrolysis of Cu/Cu2O/CuO electrode136 (Copyright 2017, with permission from Wiley-VCH). (d) Strategy for the fabrication of hollow-shell-coated copper oxide NDs137 (Copyright 2018, with permission from American Chemical Society).
4.2.3 Heterogeneous nonoxide copper-based WOCs. As reported in the literature, the introduction of heteroatoms (P, S, Se, etc.) into electrocatalytic materials can considerably influence the surface morphology and electronic structure around the electrochemically active sites and then boost the catalytic performance.61 Based on this consideration, researchers have directed considerable effort toward preparing nonoxide copper-based materials for catalyzing water oxidation.

Firstly, Zheng et al. developed a ternary nanoporous sulfur-doped copper oxide (Cu2OxS1−x) on CF through a three-step facile synthesis method at room temperature (Fig. 26a).138 The obtained electrode showed excellent catalytic activity toward both OER and HER. For OER, η values of 361 and 449 mV could yield current densities of 50 and 100 mA cm−2, respectively. It could be maintained stable at least for 8 h at the potential of 1.63 V vs. RHE. Subsequently, 2.1 nm-thick Cu7Te4 nanosheets were synthesized by Qin and co-workers through an ionic-liquid-assisted ionothermal route (Fig. 26b).139 Owing to the enhanced electronic conductivity and higher number of exposed active sites, the obtained nanosheets exhibited higher activity and durability than their bulk counterparts toward water oxidation, affording a current density of 10 mA cm−2 at an η value of 460 mV with a Tafel slope of 103 mV dec−1 in 0.1 M KOH. In addition, a long-time durability test revealed that the catalyst sustained for 6 h at a current density of 10 mA cm−2 in 0.1 M KOH solution.

image file: c9nr10437b-f26.tif
Fig. 26 (a) Schematic diagram of the fabrication process for Cu2OxS1−x/Cu138 (Copyright 2018, reprinted with permission from American Chemical Society). (b) Schematic illustration for the transition of Te to Cu7Te4 by a dissolution–recrystallization mechanism139 (Copyright 2017, reproduced with permission from Elsevier Ltd).

Very recently, Li group reported the in situ fabrication of Cu3P/CuO core–shell nanorods array (Fig. 27a) with the merits of high conductivity, large active area, and short diffusion paths for the electrolyte and O2.140 It exhibited an η value of 315 mV sustained for at least 50 h at a current density of 10 mA cm−2 toward water oxidation in 1.0 M KOH. After 50 h of electrolysis, the catalyst remained as that of the original, proving its stability under harsh alkaline conditions. Recently, they reported a hybrid catalyst film composed of Cu2Se and Cu2O grown on Ti foil, denoted as Cu2Se-Cu2O/TF, which was prepared through the electrodeposition method.141 The Cu2Se-Cu2O/TF required a low η value of 465 mV to achieve a catalytic current density of 10 mA cm−2 for OER in a 0.2 M carbonate buffer solution (CBS, pH 11.0, Fig. 27b). Moreover, the CPE test revealed the high stability of the catalyst, affording a current density of 10 mA cm−2 at least for 20 h (Fig. 27c).

image file: c9nr10437b-f27.tif
Fig. 27 (a) Fabrication process of Cu3P/CuO140 (Copyright 2018, adapted with permission from Wiley-VCH). (b) LSV curves of bare TF, Cu/TF, CuO/TF, and Cu2Se-Cu2O/TF in 0.2 M CBS (pH 11.0).141 (c) Chronopotentiometric curve of Cu2Se-Cu2O/TF at a current density of 10.0 mA cm−2 in 0.2 M CBS (pH 11.0) under gentle stirring141 (Copyright 2018, reproduced with permission from the Royal Society of Chemistry).

In 2017, Ren et al. introduced the in situ transfer of a nanoarray of Cu(TCNQ) (TCNQ = tetracyanoquinodimethane) on CF to form CuO nanocrystals (Fig. 28a).142 This electrode reached a current density of 25 mA cm−2 at an η value of 317 mV and sustained a steady current density of ∼30 mA cm−2 for at least 24 h. Meanwhile, the rapid and low-temperature synthesis of hollow CuS0.55 nanoparticles on CF was demonstrated by Li and co-workers, which acted as a highly active OER electrocatalyst.143 The generated electrode exhibited a low η value of 386 mV for 100 mA cm−2 and small Tafel slope of 33 mV dec−1, which is among the best performing Cu-based WOCs. Electrochemical and spectroscopic studies revealed that the large electrochemically active surface area and strong adsorption ability of Cu sites toward the targeted OOH* intermediates were beneficial for enhancing the catalytic properties. Very recently, a new convenient synthesis process for copper nitride (Cu3N) was formulated, which served as a bifunctional electrocatalyst for both OER and HER in alkaline media (Fig. 28b).144 For OER, the electrophoretically deposited Cu3N on nickel foam (NF) displayed significantly low η values, which reached a current density of 10 mA cm−2 at η = 286 ± 4 mV and sustained for 14 h in 1.0 M KOH (Table 5).

image file: c9nr10437b-f28.tif
Fig. 28 (a) Schematic diagram to illustrate the fabrication process of CuO-TCNQ/CF142 (Copyright 2018, reproduced with permission from the Royal Society of Chemistry). (b) Schematic illustration of the transition of cubic Cu3N144 (Copyright 2019, with permission from American Chemical Society).
Table 5 Summary and references of heterogeneous catalysts from salts and foam substrates
Catalyst Substrate Electrolyte η j (mV) Tafel slope (mV dec−1) Stability test Ref.
a Bias vs. NHE. b Manually extrapolated from the Tafel curve. c Bias vs. RHE. d Nickel foam. e Bias vs. NHE. f η at the defined current density.
CuO x from salts and substrates
Cu–Bi 1 cm2 FTO pH 9.0, 0.2 M BBS ∼530b (η1) 59 1.2 mA cm−2@1.30 Va, 10 h 127
H2O2-CuOx 0.071 cm2 GC pH 13.0, 0.1 M KOH 440 (η1) ∼3.4 mA cm−2@1.75 Ve, 1 h 128
CuO–FTO 1 cm2 FTO pH 12, 0.2 M PBS 500 (η1) 57 4 mA cm−2@1.20 Va, 10 h 129
CuO nanowire 1 cm2 FTO pH 9.2, 0.2 M KPi 430 (η0.1) 55 1.2 mA cm−2@1.10 Va, 10 h 130
Cu(OH)2 1 cm2 FTO pH 9.2, 0.1 M KPi 450 (η0.1) 78 ∼0.8 mA cm−2@1.20 Va, 10 h 131
CuO 1 cm2 FTO pH 13.6, 1 M KOH 430 (η1) 61 3 mA cm−2@ 1.73 Ve, 24 h 132
CuO from molten salt 0.071 cm2 GC pH 13.6, 1 M KOH 420 (η10) 53 10 mA cm−2@1.65 Vc, 8 h 133
Cu(OH)2-NWs/CF 1 cm2 CF pH 13.0, 0.1 M NaOH 530 (η10) 86 10 mA cm−2@1.76 Vc, 7 h 134
CuOx-NLs 1 cm2 CF pH 11.0, 0.2 M CBS 450 (η10) 44 >17 mA cm−2@1.81 Vc, 20 h 135
Cu2O−Cu hybrid foams 1 cm2 CF pH 13.6, 1 M KOH 350 (η10) 67 10 mA cm−2@1.58 Vc, 50 h 42
Dendritic Cu/Cu2O/CuO 1 cm2 CF pH 13.6, 1 M NaOH 290 (η10) 64 10 mA cm−2 @1.81 Vc, 20 h 136
CuO/C hollow-shell 1 cm2 CF pH 13.6, 1 M KOH 286 (η10) 66 10 mA cm−2@1.52 Vc, 50 h 137
Non-oxide copper-based materials
Cu2OxS1−x 1 cm2 CF pH 13.6, 1 M KOH 361 (η50) ∼43 mA cm−2@1.61 Vc, 8 h 138
Cu7Te4 nanosheets 0.071 cm2 GC pH 13.0, 0.1 M KOH 460 (η10) 103 10 mA cm−2@1.69 Vc, 6 h 139
Cu3P/CuO 1 cm2 CF pH 13.6, 1 M KOH 315 (η10) 74 10 mA cm−2@1.58 Vc, 50 h 140
Cu2Se-Cu2O 1 cm2 TF pH 11.0, 0.2 M CBS 465 (η10) 140 10 mA cm−2@1.70 Vc, 20 h 141
Cu(TCNQ)/CuO 1 cm2 CF pH 13.6, 1 M KOH 317 (η25) 85 ∼30 mA cm−2@1.57 Vc, 24 h 142
CuS0.55 1 cm2 CF pH 13.6, 1 M KOH 386 (η100) 33 10 mA cm−2@1.56 Vc, 5 h 143
Cu3N 1 cm2 NFd pH 13.6, 1 M KOH 286 ± 4 (η10) 118 10 mA cm−2@1.52 Vc, 14 h 144

In summary, heterogeneous Cu-based WOCs have attracted considerable attention and much progress has been made in the last decade. Firstly, the immobilization of molecular catalysts could largely improve the catalytic performance of catalysts. Secondly, electrodeposition from molecular complexes was also an effective strategy to control the morphology of CuOx catalysts, thereby improving the catalytic performance of the obtained catalysts. Moreover, in situ generation of the catalyst on porous substrates could be applied since a 3D electrode has a larger surface area and a porous structure, which allow much faster electron transfer, lower resistance between the catalyst and electrode, and more contact with the substrate. Furthermore, the introduction of heteroatoms (P, S, Se, etc.) could influence the morphology and electron structure around the catalytically active sites, thereby leading to an improvement in the catalytic performance. However, as compared to catalysts based on other transition metals, such as Ni, Fe, and Co, the catalytic performance of heterogeneous Cu-based catalysts is comparatively inferior and still has a long way to go.

5. Conclusions and further outlook

It is urgently needed to explore clean and renewable energy sources for substituting traditional fossil fuels. Water splitting via electrolysis is a commercialized and mature technology that can be used to produce highly pure H2 at a large scale. Moreover, it is a critical reaction for realizing artificial photosynthesis. However, OER is a bottleneck reaction due to its thermodynamically uphill and kinetically sluggish nature. Therefore, in the recent years, several efforts have been devoted toward exploring highly efficient and low-cost WOCs and various kinds of catalysts have been developed to this end (including homogeneous and heterogeneous catalysts) that can lower the energy barrier and therefore enhance the catalytic performance toward water oxidation. Among the catalysts reported in the literature, Cu-based catalysts have been appealing because they are cheap, abundant, and biologically relevant. Here, we have reviewed the recent advances and impressive progress made in research toward homogeneous and heterogeneous Cu-based electrochemical WOCs. Meanwhile, the catalytic performances of each type of catalyst has been discussed.

Regarding homogeneous Cu-based water oxidation catalysts, including salts and molecular complexes, salts seem to be promising candidates for achieving the goal of water oxidation because of system simplicity. Although the mechanisms led by mononuclear molecular complexes toward water oxidation have been explained and effectively proposed, all the evidence is indirect, and high-valent intermediates cannot be precisely characterized. Therefore, it is urgently needed to develop techniques in order to truly understand the catalytic process. Moreover, current studies on multinuclear Cu-based molecular catalysts and mimics of Mn4Ca in PS II are only at the beginning stage. We believe that this field can boom with future technological developments. Most importantly, it is still critical to carefully characterize the homogeneity of a system before mechanism proposition because a slight difference in the test conditions can completely alter the catalytic mechanism. Finally, the ligands for molecular complexes should be cheap and accessible since certain synthesized ligands are much more expensive than Cu itself; otherwise, the whole exercise becomes futile.

With regard to heterogeneous Cu-based catalysts, catalytic performance is still unsatisfactory when compared with those based on other nonnoble metals, such as Ni, Co, Fe, etc. Therefore, additional surface, component, and interfacial modification methods are needed to enhance the activity and durability of Cu-based catalysts. Moreover, the detection of intermediates is insufficient. Only a few reports have focused on the mechanism studies based on advanced characterization techniques, such as in situ spectroscopic experiments, X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) methods.145 Lastly, transparent catalytic films on conductive substrates (FTO, ITO, etc.) are also a good choice as they can be developed as photoelectrochemical (PEC) devices with an appropriate semiconductor in artificial photosynthesis.

In view of the above, developing rationally designed, highly active, and stable Cu-based catalysts for in-depth mechanism studies, electrochemical water splitting, and even the construction of artificial photosynthetic systems (PEC cells) is urgently needed and believed to have a bright future for resolving the problems associated with sustainable energy conversion.

Conflicts of interest

The authors declare no competing financial interest.


This work was financially supported by the Fundamental Research Funds for the Central Universities (DUT19LK44), the Natural Science Foundation of Liaoning Province (2019-MS-051), the National Natural Science Foundation of China (21403028), the National Basic Research Program of China (973 program, 2014CB239402), the Swedish Research Council (2017-00935), the Swedish Energy Agency, and the K & A Wallenberg Foundation.


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