Fuel cells are increasingly regarded as an effective tool to address global environmental and energy problems [1, 2]. The first fuel cell that transformed chemical energy into electrical energy was developed in 1839 [3]. Since then, an abundance of studies have concentrated on improving the energy conversion efficiency, leading to the development of various fuel cells types (Fig. 1) [4]. Although the materials used for anodes and electrolytes vary between fuel cells, the reaction at the cathode is the same, i.e., the oxygen reduction reaction (ORR). In practice, both the ORR (at the cathode) and the fuel-oxidation reaction (at the anode) need catalysts to yield acceptable reaction efficiencies, but the obstacle for large-scale uptake of fuel cells is the sluggish rate of the ORR. Among the many catalysts developed over the past decades,Pt has shown the best catalytic performance, but its high cost has limited its large-scale application. Pt-alloys have been widely investigated to address the cost issue while making maximum use of Pt, with some alloys exhibiting much better catalytic performance than commercial Pt/C [9]. A number of substitutes for Pt have also been considered. These include Pd-based catalysts [11] and non-noble metals and functionalized carbon materials [13]. Among the non-noble metal catalysts,Fe [14],Co [15], and Mn [16] are the most studied materials, though the body of work on using Cu-based materials to catalyze the ORR has also been growing.
As an earth-abundant non-noble metal,Cu has been widely investigated in a number of fields; notably including transparency electrodes [17] and organocatalysis [17]. Indeed,Cu-based nanomaterials have been shown to exhibit excellent electrocatalytic activities in glucose oxidization [18], hydrogen peroxide reduction [19, 20] and methanol oxidization [21]. Early studies reported the ORR kinetic parameters for a bulk Cu electrode [22]. Lately, via a one-pot method, our research group synthesized a type of subnanometer-sized Cu clusters with high ORR activity [4]. The catalytic activity of Cu clusters has also been found to depend on their core size [23].
The use of Cu-based catalysts for the ORR has clearly progressed and this review highlights the recent relevant advances. We first detail a general introduction to the ORR and measuring and evaluating the impact of electrocatalysts on it. The main section of this article then summarizes the recent development of Cu-based catalysts for ORR, disaggregating the catalysts according to whether they are Cu complexes,Cu compounds or other Cu-based nanostructures. Finally, we discuss the potential directions for future development of Cu-based ORR catalysts.
Although enormous effort has been devoted globally to the study of ORR, the complexity of dynamic processes mean the reaction is not yet fully understood at the atomic level [25, 26]. Nonetheless, it is widely accepted that the ORR involves the net transfer of four electrons. This transfer results in zero-valent O2 being reduced to various negatively charged, bivalent oxygen species, depending on the reaction conditions. Under acidic conditions,O2 can be converted to H2O through a direct 4e− process, which is the most favorable path for the creation of useful electrical power. However,O2 may also be converted to H2O2, a common intermediate product during ORR, and then reduced to H2O through a 2e− + 2e− pathway, which is less desirable. Similar observations can be made for the ORR under basic conditions: O2 can be totally reduced through a direct 4e− pathway or via a 2e− + 2e− pathway but with an intermediate product of HO2−instead. The theoretical reaction paths are as follows:
In acid:
In base:
For the most common Pt-based catalysts,O2 is usually reduced through the most efficient 4e− pathway under normal conditions. Nevertheless,O2 can be reduced via the 2e− pathway in many cases, especially when less understood non-Pt catalysts are used. Although theoretical calculations can be used to predict the various reaction intermediates [27], their direct detection is still limited and this is the main obstacle to fully understanding the ORR mechanism and effectively designing ORR catalysts. Recently, however, a number of ORR intermediates have been directly identified experimentally by in situ spectroscopic techniques [28].
The activity of ORR catalysts is usually evaluated by fabricating a membrane electrode assembly (MEA) or by depositing the catalyst on a rotating disk electrode (RDE). A MEA includes electrodes (anode and cathode), a diffusion media (also called substrate), and a proton-exchange membrane [29]. Although a MEA is more suitable for testing the performance of a catalyst for practical applications, it is complicated by the fabrication of apparatus required for carrying out the measurements. Conversely,RDE activity tests are very easily conducted making them a popular option for most ORR research groups. Testing ORR activity by the RDE method requires the prepared catalysts to first be dispersed in a mixture of water, isopropanol/ethanol and Nafion (5 wt%) according to a volume ratio of 4:1:0.025 [30]. However, the proportion and components of the catalyst ink need not be fixed and mixtures with other ratios are also acceptable if the catalyst ink can be uniformly deposited on a glassy carbon (GC) electrode by forming a thin film. Controlling the mass-loading of catalyst in the thin film formed on GC is very important for obtaining accurate activity results (see further experimental details in Ref. [31]). Normally, two electrolytes—an acidic solution and a basic solution—are used, but buffersolutions with differen t pH values can also be used for some complex catalysts [31]. In general, the electrolyte is chosen according to the properties of the catalyst being tested.
An electrocatalyst’s ORR activity can be evaluated qualitatively by different electrochemical techniques. The kinetics of the reaction are usually measured by linear sweep voltammetry (LSV). Here, a RDE is used in an O2-saturated electrolyte with a low sweep-rate to avoid any interruption from capacitive currents [27]. Fig. 2 shows the typical LSV curve for the ORR [31]. The kinetic data can be obtained using the following K-L (Koutecky-Levich) equation [32]:
where i refers to the experimental current,id is the diffusion-limited current,ik represents the mass-transport-free kinetic current. The value of id can be obtained from the Levich equation:
where n is the number of electrons transferred per oxygen molecule,F is the Faraday Constant (96485 C/mol),A is the geometric area of the working electrode,D is the diffusion coefficient of O2 in the electrolyte,v is the kinematic viscosity of the electrolyte,w is the RDE rotating rate in rad/s (if rotating rate is expressed in rpm, the constant 0.62 should be replaced by 0.2), and CO2 is the saturated oxygen concentration in the electrolyte. The values of ik and n can, respectively, be derived from the intercept and slope of K-L plots (1/i − w1/2 plots).
The H2O2 yield from a catalyst under test can be obtained using the rotating ring disk electrode (RRDE) test and Eq. (9).
where iR is the current from the ring,iD is the current from the disk, and N is the current-collection efficiency. In addition to using Eq. (8), the number of electrons (n) can also be determined by the RRDE according to following equation:
In general,n is between 2 and 4 with lower yields of H2O2 resulting in higher n values and better catalytic performance.
Overpotential and specific activity are two other key parameters that are used to evaluate a catalyst’s ORR performance [34]. Overpotential is the energy gap between the standard equilibrium potential and the activation energy and originates in ORR from the strong O2 double bond (498 kJ) [35]. The onset potential (Eonset) and half-wave potential (E1/2) can be read directly from the LSV curve with more positive values of Eonset and E1/2 signifying a lower overpotential. Meanwhile, specific activity can be defined as the value of ik normalized to either the catalyst mass (mass activity) or its electrochemical surface area (area activity). In short, a lower overpotential and a higher specific activity signals a more active ORR catalyst.
Another critical issue for ORR catalysts is stability [36]. This is usually evaluated by comparing the LSV curves before and after electrochemical tests but may also be investigated using chronoamperometry at E1/2 [40, 41]. Recently, the development of practical catalysts has aroused interested in using various other electrochemical techniques to investigate resistance to methanol crossover and CO poisoning [42].
Interest in using laccases (Cu-containing multi-Cu oxidases) for the ORR has stemmed from their ability to catalyze O2 reduction [43, 44] and their low overpotential for the ORR of 20 mV, which is negligible compared with that of Pt-based ORR catalysts (approximately 300 mV) [48]. Blanford et al. [49] used fungal laccases to catalyze the ORR by attaching them to a substrate-like anthracene-modified pyrolytic graphite. They found that the laccase on the modified graphite was more stable than that on the untreated graphite and ascribed this to the strong adsorption of laccase on anthracene-based units on the modified graphite. A number of other materials have been investigated as laccase supports to study their effect on the activity of the ORR [50]. Similarly, researchers investigated the kinetics and mechanism of ORR with laccase by supporting it on carbon nanotubes using 1-pyrenebutanoic acid, succinimidyl es ter or steroid biosurfactant sodium cholate as a linking agent [30]. These work directly monitored the enzyme electron transfer during ORR, paving the way for the direct use of enzymes in electrochemical applications. Notably,Holmberg et al. [51] recently reported two laccase isoforms with high ORR activity and stability at high temperature, potentially offering a new way to overcome problems of instability with laccase catalysts.
Although laccase can efficiently catalyze the ORR, it cannot achieve high current densities because of its large size (approximately 160 nm3) [52]. Further, as a biological protein, laccase is very vulnerable to changes in the local environment and only functions in a very narrow pH range [53, 54]. To overcome the drawbacks of natural laccase, a number of Cu complexes that mimic laccase have been developed to catalyze ORR, as described in Gewirth et al.’s excellent and comprehensive review [55]. The review summarized Cu complexes with different ligands, including porphyrins, phthalocyanines, amino-alkyl ligands, substituted 1, 10-phenanthrolines, aromatic N-donor ligands, tris (2-pyridylmethyl) amine and substituted triazoles. The ORR activities of different Cu complexes were compared according to the working electrode, pH range, and values of E1/2 and Eonset. Rather than duplicate the previous review, the present work focuses on selective updates that have been reported since its publication.
Subsequent to their review,Gewirth et al. [52] designed, synthesized, and tested a Cu-based molecular ORR catalyst with multi-functional ligands. Through a series of comparative experiments, they found that the proton-transfer step was hugely influential for the ORR catalyst’s overall activity. In a similar vein,Wang et al. [55] took on the challenge of mimicking the Cu2+ active sites in laccase directly and designed a graphene- supported Cu-complex-hybrid that had a similar d-electron density of Cu at the active site. The hybrid was prepared through the high-temperature pyrolysis of Cu2+-1, 10- phenanthroline. The hybrid showed excellent ORR activity in both acidic and basic conditions. Indeed, under basic conditions the ORR activity was better than that of a commercial Pt/C catalyst, with a value of Eonset of 0.978 V (versus RHE) and a low Tafel slope of 49 mV per decade. The authors proposed that the hybrid’s performance benefited from a Cu2+ coordination environment that allowed effective electron-transfer from Cu2+ to N (Fig. 3). The same hybrid also exhibited great activity in catalyzing the oxygen evolution reaction (OER) at a suitable potential.
Thermal decomposition of Cu complex with carbon support is a decent solution to solve the instability problem of Cu complex catalyst in ORR [59]. However, the addition of heat impacts the structure of most complexes, potentially altering the active site. Therefore, theory suggests that it is quite difficult to accurately tune the activity through structural design. In a practical example,Xi et al. [63] adopted a dinucleating ligand (triazole-dipyridine,TADPy) to covalently immobilize a multicopper complex on reduced graphene oxide (RGO) without high temperature treatment (Fig. 4). The covalently linked hybrid (RGO-TADPyCu) exhibited Eonset and E1/2 values of 0.951 and 0.795 V (vs. RHE), respectively. After 10, 000 continuous cycles, the E1/2 fell by approximately 4 mV, indicating an outstanding stability. The ORR activity of the covalently linked hybrid was better than that of a physisorbed mononuclear RGO-TAPyCu hybrid, a physisorbed dinuclear RGO-TADPyCu hybrid, and pure RGO, suggesting that the superior performance comes from the strong covalent connection between the Cu complex and the support. In follow-on work,Cu complexes were also covalently bonded to other supports establishing this as an effective method in the search for active and stable ORR catalysts [64].
Cytochrome c oxidase (CcO) has been extensively investigated since its discovery in 1927 [67]. As a terminal oxidase used during respiration, it can reduce O2 quite effectively and has therefore been studied as a model catalyst for, and to explore the reaction mechanism of, the ORR [68]. A density functional theory (DFT) study showed that CcO has better ORR catalytic activity and a much lower overpotential than Pt-based catalysts [69]. However,CcO is too large (molecular weight,MW, of approximately 200 kDa) to obtain a high current density [70]. Moreover, as a membrane protein,CcO is not stable in a fuel cell’s working environment. It has therefore been important to investigate the catalytic mechanism of CcO to guide the design and synthesis of CcO analogues with high ORR activity and stability [71]. Many studies have proved that the catalytic site of CcO is a binuclear heme-Cu center, but, because native CcO contains multiple metal ions, it is challenging to study the heme-Cu center directly [72]. Given these difficulties, the synthesis of a Cu-containing catalyst may be an effective method of mimicking CcO’s ORR benefits.
Good design and preparation of active and stable ORR catalysts requires an understanding of the reaction process, the intermediates and the electron transfer numbers involved. Wikstrm et al. [73] reported that the reduction potential (E°) of an active site could significantly influence a catalysts’ ORR activity. To investigate the influence of E° in detail,Bhagi-Damodaran et al. [69] synthesized a small binuclear heme-copper protein and varied E°. They established a relationship that showed that the ORR rate increased as E° was increased.
To investigate the reaction mechanism of ORR,Kakuda et al. [74] recently prepared two Cu (Ⅱ) complexes: (1) [(PV-tmpa) CuⅡ](ClO4)2 where PV-tmpa = bis(pyrid-2-ylmethyl) [6-(pivalamido) pyrid-2-yl]-methyl-amine; and (2) [(tmpa) CuⅡ] (ClO4)2 where tmpa = tris(2-pyridylmethyl) amine). The only difference was that Complex 1 had an extra pivalamido group compared with Complex 2 (Fig. 5(a)). Complexes 1 and 2 were used to catalyze the ORR by decamethylferrocene (Fc*) in trifluoroacetic acid (CF3COOH) acetone solution. Under catalysis,Fc*,CF3COOH, and acetone acted as reductant, proton source, and solvent, respectively. A number of methods were used to study the process and mechanism, including a photodiode-array spectrophotometer, the stopped-flow technique,EPR spectra,ESI mass spectrometry and DFT analysis. The authors used the experimental results to propose the 4e− reaction mechanism illustrated in Fig. 5(b). In the first, rate-determining step,Fc* donates an electron to Complex 1, producing Fc*+ and a CuⅠ complex. The CuⅠ complex then rapidly couples with O2 to produce a superoxo-copper(Ⅱ) species, which in turn rapidly reacts with another CuⅠ complex to form a peroxo-dicopper(Ⅱ) complex. The peroxo-dicopper(Ⅱ) complex is protonated by coupling with a hydrio n before being reduced by Fc* and trifluoroacetic acid to water, regenerating Complex1. Alongside the importance of the 4e− reduction of O2 in fuel cells, understanding the 2e− reduction of H2O2 is also vital. This is because H2O2 is an undesired intermediate in many ORR reactions, and so efficient reduction of H2O2 can also improve the catalytic efficiency. Besides, research on the reduction of H2O2 is also critical for H2O2 fuel cells [74]. Compared with Complex 2,Complex 1 was reported to be approximately four times more active in ORR and approximately three times more active in the 2e− reduction of H2O2, with both findings linked to the extra pivalamido group.
In 2010,Fukuzumi et al. [79] reported a mononuclear Cu complex [(tmpa) CuⅡ](ClO4)2, that effectively catalyzed the 4e− reduction of O2 in HClO4. The same research group later synthesized a new dinuclear Cu complex [CuⅡ2(XYLO)(OH)]2+ that efficiently catalyzed the ORR at a significantly lower overpotential and found that using HClO4 as the proton source changed the electron transfer number from two to four [80]. Even more recently, the group reported that when a Lewis acid was used to in place of a Brnsted acid as the proton source, the number of transferred electrons changed back to two [85].
Chatterjee et al. [92] synthesized three Cu-bearing complexes (Fig. 6) to mimic CcO and to detect possible intermediates in the ORR. To study the function of Cu in the complexes, an Fe-only complex (i,6L-Fe) was prepared alongside a Cu-Fe complex (ii,6L-FeCu) and its imidazole adduct (iii, 6L-Fe(Im) Cu). The complexes were first adsorbed onto a graphite (working) electrode to study their electrochemical properties by cyclic voltammetry (CV) tests in a phosphate buffer solution (pH = 7). The complexes were then immobilized on a roughened Ag surface, which was modified by a C8SH self-assembled monolayer. Surface-enhanced resonance Raman spectroscopy (SERRS) was then performed to investigate the intermediates formed during the in situ reaction while RDE and RRDE techniques were used to evaluate the complexes’ electrocatalytic activity and selectivity during the ORR. Finally, a coupled SERRS-RDE technique was employed to observe the intermediates. This series of measurements allowed the identification of a bridging peroxo intermediate on the surface of the as-synthesized Cu-bearing complexes in aqueous media.
In addition to pure metals, their unique structures and properties have led to investigations into the potential use of metal compounds (including, among others, oxides, sulfides, and nitrides) in the ORR [95]. This is particularly true of Cu-based compounds, which have been widely studied.
Cu3N nanocubes with a mean diameter of approximately 11 nm, but which could be tuned by changing the capping agent, were first prepared through a simple one-phase method [105]. Electrochemical studies showed that the as-synthesized Cu3N nanocubes possessed excellent catalytic activity for the ORR under basic conditions. Separately,Han et al. [93, 94] investigated the ORR activity of Cu2Se nanowires with tetragonal and cubic phase structures, finding the former to be much more catalytically active. In addition, the tetragonal versions were only associated with the direct 4e− mechanism, while the cubic versions involved a mixed electron transfer mechanism that included both the 2e− and 4e− pathways.
Many research groups have developed methods for the preparation of nitrides and selenides, though few works that addressed electrochemical studies have been reported [95]. A much wider body of work has focused on the use of Cu’s oxides and sulfides as catalysts for the ORR. Recent developments in these fields are summarized below.
Although pure Cu oxides can catalyze the ORR, poor electrical conductivity limits their electrochemical applications [99]. A great deal of research has shown using graphene as a support enhances the electrochemical performance of Cu oxides [100, 101]. By dispersing Cu2O nanoparticles on RGO,Yan et al. [102] found that the composite showed excellent resistance to CO-poisoning and methanol-crossover during the ORR. Zhang et al. [103] studied the electrochemical properties of graphene-supported CuO with NH3·H2O used to remove the partial Cu2O at the interface of CuO and graphene to fulfill the interfacial adjustment. The interfacial interaction between CuO and graphene was found to greatly affect the electrochemical properties, namely via the “butterfly effect”. Zhang et al. [104] developed an electro-deposition method for depositing n-type Cu2O on activated carbon. A high power density of approximately 1.4 W/m2 was produced by using the Cu2O/activated carbon composite as a catalyst at the cathode during the ORR in a single-chamber microbial fuel cell.
The coordination effect has also been used to improve the ORR activity of metal catalysts [108]. Zhou et al. [109] supported CuO on N-doped RGO (CuO/N-RGO) (Fig. 7) and found the process to be a 3.7 e− process. This indicated that the hybrid was an efficient ORR catalyst while control experiments revealed that the current density and value of Eonset of the hybrid-catalyzed reactions were much better than those for CuO/GO and RGO. The enhancement was later attributed to the hybrid’s coordination of Cu and N.
The structure of nanoparticles has widely been accepted to significantly influence their activity [110]. To explore the relationship between the crystal face and ORR activity of Cu2O,Li et al. [112] synthesized a series of Cu2O nanocrystals with different morphologies and crystal planes. Among the three different Cu2O nanocrystals,Cu2O nanocubes with a dominant {100} face exhibited the best ORR performance, with a large electron transfer number (n = 3.7) and a low H2O2 yield (about 15%). The nanocubes were also less sensitive to poisoning by CO or methanol-crossover than a commercial Pt/C catalyst. By comparing the kinetic characteristics of the ORR for three different Cu2O nanocrystals, the authors found that the intermediate migration and charge transfer on {100} planes were much faster than those on {111} planes, indicating that the {100} plane of Cu2O is more active than the {111} plane.
Metal sulfides have been considered excellent electrocatalysts for many electrochemical reactions [111]. Cu sulfides, in particular, have also been studied as ORR catalysts and, as with Cu oxides, a proper support is usually needed to overcome the low electrical conductivity. Shih et al. [112] synthesized Cu(2-x)S nanoparticles on the support of C nanodots (Cu(2-x)S/C dots) to catalyze the ORR in an acidic media. The Cu(2-x)S/C dots exhibited a positive Eonset value (0.92 V vs. Ag/AgCl) and high tolerance to methanol. Later, the same group also prepared Cu9S8 nanoparticles on the support of C nanotubes (Cu9S8/CNTs) and examined their ORR activity under basic conditions [113]. Although the current density and working potential of the hybrid were worse than those of commercial Pt/C, similar to the Cu(2-x)S/C dots,Cu9S8/CNTs possessed good resistance to CO and methanol. Recently, a new type of CuxSy/nanoporous C composite (CPS-CuS) was fabricated by carbonizing a mixture of Cu on a metal-organic framework (MOF) and graphite oxide (GO) [114]. The ORR activity of CPS-CuS was indicated by an obvious reductionpeak in its CV results in O2-saturated 0.1 mol/L KOH compared with the CV results obtained with a N2-saturated electrolyte. The high activity was further confirmed by a high ORR current density observed during LSV testing. A comparison with the catalytic activity of CPS alone made it clear that CuS was the dominant catalytic agent.
Recently,Wang et al. [115] synthesized a series of Cu(2-x)S nanoplates, varying the amount of initial Cu, and discussed the effect of Cu-deficiency on a catalyst’s ORR activity (Fig. 8). Elemental analysis suggested that a higher amount of Cu in the precursors resulted in a lower Cu content in the Cu(2-x)S product. This counterintuitive phenomenon was proposed to be caused by the simultaneous occurrence of precursor dissolution and complexation, and nanoparticle nucleation and growth. ORR tests revealed that the Cu(2-x)S nanoplates with the highest Cu precursors exhibited the best catalytic performance with the most positive Eonset and E1/2 values (0.9 and 0.7 V vs. RHE, respectively). Supporting the Cu(2-x)S nanoplates on carbon black and RGO enhanced the ORR performance by varying degrees. Although the Cu(2-x)S nanoplates did not perform as well as a commercial Pt/C catalyst, this work opened a new pathway for the exploration of novel Cu-based, non-precious metal catalysts.
Because of their low cost, relative abundance and environmental benignity, spinel materials have attracted much attention for their potential as bi-functional catalysts in the ORR and the OER [116]. A previous study showed that Cu ions doped in Co3O4 improved the OER activity compared with that of the Co3O4 structure alone [117]. To determine whether the phenomenon could be replicated for the ORR, the same group investigated the ORR activities of CuCo2O4 and Co3O4 [118, 119]. Cu was again found to be active as increasing the amount of CuCo2O4 enhanced catalytic activity and increased the electron transfer number from 2.2 to 3.4. Serov et al. [120] then systematically investigated the ORR performance of various CuCo2O4 structures that were synthesized by different methods (sacrificial support methods [SSM], spray pyrolysis, sol-gel, and pore forming). The CuCo2O4 prepared by SSM exhibited the most positive E1/2 value with DFT analysis pinpointing the lack of CuO as the key to the increased activity. However, the electrical conductivity and ORR activity decreased as the amount of CuO was increased because of CuO’s intrinsic inertness.
To improve the support effect,De Koninck et al. [122] used Nb-doped TiO2 nanoparticles to substitute conventional C supports (poly[vinylidene fluorideco-hexafluoropropylene],Ebonex powder and Vulcan XC72R). The doped Nb was believed to have hindered the phase transformation and the grain growth of TiO2, therefore improving the electrical conductivity of the support material. Ning et al. [126] studied the ORR activity of spinel CuCo2O4 nanoparticles loaded on N-RGO (Fig. 9(a),(b)). As shown in Fig. 9(c), the ORR peak obtained for CuCo2O4/ N-RGO in O2-saturated 1 mol/L KOH solution was only 50 mV less than that of commercial Pt/C. Moreover, the current-time (i-t) curve (Fig. 9(d)) shows that the durability of CuCo2O4/ N-RGO was remarkably better than that of the commercial Pt/C catalyst.
Recently,Wu et al. [123, 124] developed a new strategy to improve the activity of spinel ORR catalysts based on structural reversal. Taking FeCo2O4 as example to study the structure-activity relationship in detail, they reversed the normal structure by adjusting the spinel’s Fe content. The inverse structure showed better catalytic performance than commercial Pt/C, with Eonset and E1/2 values more positive than those of Pt/C in 0.1 mol/L KOH solution (65 and 42 mV, respectively). A DFT study then revealed that the increased activity may have derived from a “dissimilarity effect” between the Co and Fe atoms that modulated the adsorption energy and elongated the O-O bonds.
Mn oxides have recently been the focus of considerable attention as potential ORR catalysts [124]. Mao et al. [125] studied several different oxides and found their order of ORR activity to be: γ-MnOOH > Mn2O3 > Mn3O4 > Mn5O8. Cao et al. [127] performed a similar study that focused on different phases of MnO2 and found that activity varied as follows: α-MnO2 ≈ δ-MnO2 > γ-MnO2 > λ-MnO2 > β-MnO2. Meanwhile,Cheng et al. [128] reported that β-MnO2 was more active than λ-MnO2 because of the latter’s higher electrical conductivity. Other work has focused on the effects of oxygen vacancies, catalyst shape, and heteroatom-doping [129].
Lambert et al. [130] introduced Cu to this area by studying the ORR activity of Cu-doped α-MnO2 and found that compared with pristine α-MnO2 the Cu-doped α-MnO2 exhibited a more positive Eonset value (−75 vs. −112 mV), a higher current density (2.41 vs. 1.42 mA/cm2) and a higher electron transfer number (3.5 vs. 3.1). The support effect was also investigated with available C,RGO, and graphene-like C, the last of which improved ORR performance the most. The same group then varied the amount of Cu involved to better understand Cu’s role [131]. All Cu-doped α-MnO2 samples showed considerably better catalytic activities than pristine α-MnO2. The researchers concluded that enhanced activity was derived from the increase of Mn3+ on the surface of Cu-doped α-MnO2 because the Mn3+/Mn4+ couple is directly related to the rate-limiting step of O2/OH− exchange [132].
First reported in 2005, covalent organic frameworks (COFs) are a type of strongly bound porous organic structure that have been investigated for their use in a diverse range of applications [134]. A type of COF, covalent triazine frameworks (CTFs), consist of 1, 3, 5-triazine units [135]. Although their use as electrocatalysts has been limited by their established poor electrical conductivity, the use of CTFs in electrocatalysis could offer several potential advantages. These advantages include: (1) CTFs contain many nitrogen atoms that have unpaired electrons, which can be used to coordinate metals; (2) it is easy to modify the outer electron of coordinated metals; and (3) covalent bonding means that CTFs structures are relatively stable. In light of this,Kamiya et al. [136] prepared ORR catalysts by incorporating C particles into CTFs to improve their electrical conductivity and by modifying Pt on CTFs to increase their catalytic activity. Although the modified-Pt CTFs exhibited much higher ORR activity than that of commercial Pt/C, practical considerations mean that further exploration of the use of non-precious metal catalysts remains important. Recently, the same group used the same synthesis method but with Cu [126]. The modified-Cu CTFs showed outstanding ORR activity in neutral pH conditions, with the most positive value of Eonset (−0.81V vs. RHE) of all Cu-based ORR catalysts reported to date. Extended X-ray absorption fine structure (EXAFS) spectra and DFT calculations revealed that the high activity originated from CTF’s relatively stable, unsaturated Cu atoms (Fig. 10).
Analogous to COFs,MOFs are also porous organic structures that have been extensively investigated in the field of catalysis [136]. Cu-BTC (BTC = 1, 3, 5-tricarboxylate) has been the most commonly studied. Although MOFs tend to possess an outstanding gas adsorption capacity, which is very beneficial for ORR, they have rarely been applied in electrocatalysis because of their instability in electrolyte solutions. Mao et al. [1]overcame this by synthesizing a new, relatively stable,Cu-MOF (Cu-bipy-BTC, bipy = 2, 2′-bipyridine). This first attempt to catalyze the ORR with a Cu-based MOF was successful with the Cu-bipy-BTC MOF showing good ORR activity and stability in neutral conditions (pH = 6). Jahan et al. [2] prepared a composite by assembling Cu-MOF on GO via a hydrothermal route. Interestingly, the morphology of the composite changed depending on the amount of GO in the composite. Moreover, as a multifunctional electrocatalyst, the composite exhibited distinctive catalytic ability for the ORR,OER and hydrogen evolution reactions. In particular, the composite delivered 76% of the power density of the commercial Pt/C when tested in a MEA at 80 ℃. Recently,Jiang et al. [3] also investigated the ORR activity of a new type of Cu-MOF,NPC-4 (NPC: Nanoporous cage),Cu2(TMBDI)(H2O)2,(TMBDI = 2, 3, 5, 6-tetramethyl- benzene-1, 4-di-isophthalate). They found that the NPC-4 was unable to catalyze the ORR until it was activated by solvent-exchange, but that the activated NPC-4 was not stable enough to undergo electrochemical measurements. Incorporating RGO onto a GC electrode that was modified with activated NPC-4 catalyst considerably improved the ORR activity and the catalyst stability compared with those of NPC-4. In this case,RGO acted both as a mediator between the catalyst and the electrode (improving the speed of electron transport) and prevented the detachment of the catalyst from the electrode (enhancing the catalyst’s stability).
Only one reported work was found that focused on the ORR activity of isolated Cu nanoparticles. Liu et al. [4]prepared Cu nanoparticles with 1-decyne as a ligand and found that in basic conditions the alkyne-capped Cu nanoparticles exhibited obvious ORR activity when the results were compared with those of bulk copper. FTIR and photoluminescence spectroscopic studies indicated the formation of Cu−C≡ bonds. For supported Cu nanoparticles,Yang et al. [5] found that nanoparticles supported on Co3O4 microspheres had improved catalytic stability and better ORR activity. The same research group also studied the ORR activity of various Cu nanoparticles supported on defective TiO2 with Cu nanoparticles modified with 1-decyne showing the best performance [6]. Interestingly, the authors found that Ti3+ was present at the interface between TiO2 and Cu for the nanoparticles modified with 1-decyne but was absent for those without 1-decyne and suggested that Ti3+ accounted in part for the improved ORR activity. Liu et al. [7] also reported recently that graphene quantum dots had been used to support Cu nanoparticles for ORR.
Incorporating MOFs,Ania et al. [8] used the pyrolysis method with a Cu-based MOF and GO as precursors to prepare a Cu/RGO hybrid that showed high ORR activity (because of direct contact between Cu and the conductive graphene phase) and strong methanol tolerance. An interesting piece by Noh et al. [9] reported that Cu nanoparticles had been coated with N-doped C (Cu@N-C) and then activated by CO2 at high temperature (1000 ℃). The CO2 treatment decreased the thickness of the Cu’s C coating and improved the ORR activity of the Cu@N-C hybrid such that it was comparable to that of commercial Pt/C (Fig. 11,Eonset and E1/2 = 0.87 and 0.71 V vs. RHE in 0.1 mol/L KOH solution, respectively). The Cu@N-C also showed good stability (Fig. 11(d)). DFT studies were used to calculate oxygen-adsorption energies. These results showed that the hybrid was more amenable to oxygen adsorption than N-C without Cu (oxygen was too weakly adsorbed) and pure Cu (oxygen was too strongly adsorbed).
Throughout its development, the design of Cu-based ORR catalysts has mainly focused on mimicking the Cu-containing enzymes that transform O2 to H2O, especially laccase and cytochrome c oxidase. Early studies attempted to graft enzymes onto electrodes and directly catalyze the ORR, but these grafted catalysts were hindered by poor stability in practical applications. Successful attempts to improve stability then saw enzymes attached to support materials, but the large size of the enzymes limited current density, preventing their use in fuel cells. Studies of the active site and reaction mechanism then allowed the synthesis of various enzyme-mimicking Cu complexes and identified the coordination of Cu and N as key to a catalyst’s ORR activity.
Although the catalytic activity of Cu-based catalysts for ORR remain lower than those of commercial Pt/C catalysts, a growing number of studies have experimentally and theoretically shown the possibility of using Cu-based catalysts for ORR. Building on these studies, more work should be conducted to better understand precisely how enzymes catalyze the ORR as this knowledge will help to inform the design of more advanced Cu-based electrocatalysts.
Another branch of research has focused on Cu’s ability to improve other non-noble metal ORR catalysts, the most developed of which are Fe- and Co-based. Indeed, a number of Fe-N-C and Co-N-C catalysts already exhibit outstanding ORR electrocatalytic performance. Work in this field has shown the need for an appropriate metal salt, a nitrogen source and a carbon support to improve catalytic performance. Although currently relatively unexplored, incorporating knowledge obtained from Fe- or Co-based ORR catalysts could see Cu-based catalysts develop rapidly. In particular, further development of Cu-based ORR catalysts may focus on the following: (1) the design and synthesis of Cu-complexes that mimic enzymes to investigate the active site and ORR reaction mechanism; (2) developing Cu-N-C systems coupled with various Cu compounds, similar to previous work with Fe and Co; and (3) manipulating the structure of existing Cu catalysts. As an important non-noble metal,Cu-based nanomaterials will hopefully offer a class of highly active, stable, and poison-resistant ORR catalysts.
2016,
Vol. 37 Issue (7): 1049-1061 
PDF (1242 KB)
2016,
Vol. 37 Issue (7): 1049-1061 PDF (1242 KB)