2.2.1. Layered Hydroxides and Related Ni–Fe-Based Materials
Layer double hydroxides (LDHs) are a class of layered materials, which are also known as anionic clays. Their characteristic structural feature is the generic layer sequence [AcB Z AcB]
n, where
c represents layers of metal cations,
A and
B are layers of hydroxide (HO
−) anions, and
Z are layers of other anions and neutral molecules [
27]. Nowadays, LDHs find widespread application in dye-sensitized solar cells, photocatalytic water oxidation, chemical sensors, and supercapacitors, thanks to their open structures and chemical versatility [
28]. Recently, several layer double hydroxide catalysts have been developed for OER catalysis [
29,
30,
31,
32,
33].
Presently, NiFe-LDH catalysts are among the leading LDH-based WOC materials. Even though water oxidation on nickel hydroxide has been studied since the 1960s, the clear-cut assignment of Ni as the active site remains under debate for mixed metal LDHs. Some recent studies proposed that the incorporation of Fe into Ni-LDH structures provides more active sites as a potential for the enhanced OER activity [
34,
35]. This behavior has been further supported by Nocera et al., who argued that the enhanced OER activity is promoted by the Lewis acidity of Fe(III) [
36]. The group of Duan et al. demonstrated that a charge imbalance in the M(OH)
6 layers is caused by the redox-active Fe(III) ions, which is then compensated by interlayer anions [
37]. All of the representative layered hydroxide OER catalysts discussed below and summarized in
Table 2 covered research of the last three years. The maximum performing NiFe-LDH-based catalyst to date is a NiFe-LDH array, which is also specified in
Table 2 [
37]. The following selected studies contributed to new catalyst architectures and designs with improved stability. Furthermore, considerable efforts were made over the past few years to replace Fe with other transition metals.
Wu et al. reported on ultrathin CoNi double hydroxide/CoO nanosheets, via an in situ reduction and interface-directed assembly method in air, as shown in the schematic illustration of
Figure 6 [
29]. Interfacial tension resulted in the strong extrusion of hydrated metal-oxide clusters, which led to the formation of the CoNi LDH/CoO nanosheets. The current density for OER was determined as 150 mA cm
−2 at 0.3 V
RHE, i.e., 1.5 times greater than that for RuO
2. The durability of the catalyst was also tested and the current density remained stable for 10 h. The obtained CoNi LDH/CoO nanosheets exhibited high turnover frequencies (TOFs) of 1.4 s
−1 at the overpotential of 0.4 V
RHE. The authors argued that the performance of their catalysts was mainly related to the higher +3 valence of the Co and Ni centers, as well as to the low crystallinity, which gave rise to a number of exposed defects and active sites. Furthermore, the functionalization of the surface with OH
− groups and the OH
−/O
2 adsorption and transport in the porous structure contributed to the high OER activity.
Ping et al. introduced a CoAl-LDH nanosheets catalyst on a 3D graphene network [
33]. In their study, a porous catalyst was prepared by self-assembly of the exfoliated single-layer CoAl nanosheets onto 3D graphene, via electrostatic interaction as illustrated in
Figure 7. Prior to the exfoliation, the CoAl-LDH (CO
32−) was fabricated through a hydrothermal process, based on urea hydrolysis, and subjected to anion exchange. The resulting catalyst outperformed an IrO
2 reference, by exhibiting a very low overpotential of 0.28 V
RHE and a Tafel slope value of 36 mV/dec. The TOF values of the 3DGN/CoAl-NS were found to be ~0.6 s
−1 and ~1.2 s
−1 at overpotentials of 0.3 and 0.35 V
RHE, respectively. Furthermore, about 22.5 μmol of O
2 was produced at a constant current of 1 mA cm
−2. Chronoamperometry and chronopotentiometry experiments were carried out and indicated excellent electrochemical stability for 30 h (
Figure 8). The electrocatalytic efficiency of the catalyst was ascribed to the fast electron/charge transfer, during the OER, and acceleration of the reaction kinetics thanks to the uniform coating of the single layer CoAl nanosheets onto 3D graphene.
Yang et al. demonstrated the effect of Fe and Al contents in Co–Fe LDH catalysts on the OER activity [
30]. A co-precipitation method with Al
3+ or Fe
3+ ions in the range of 15 to 45 at % was applied. The optimal Fe content was reported at 35 at %, which resulted in a synergistic effect on the Co–Fe LDH catalyst. The role of both ions as trivalent species was essential for the stabilization of the LDH structure. The OER activity was enhanced when the optimal Fe content was incorporated, whereas the oxygen evolution was suppressed in the presence of Al
3+ species. The onset potential and overpotential of the optimized Co
0.65Fe
0.35(OH)
2 catalyst were observed at 1.58 V
RHE and 0.36 V
RHE, respectively. The catalyst showed a very good stability in aqueous alkaline electrolyte, for 48 h, with a small Tafel slope of 49 mV dec
−1.
An alternative LDH catalyst was introduced by You et al. [
31]. An electrodeposition method was employed to fabricate a CoFe-LDH nanosheet array coated with an ultrathin CoFe-borate layer supported on Ti mesh. The electrocatalytic performance of the 3D catalyst electrode was evaluated in near neutral pH in 0.1 M K
2B
4O
7 solution. The CoFe-B
i@CoFe-LDH NA/TM displayed a high catalytic OER activity, with a small overpotential of 0.42 V
RHE. HRTEM images depicted the formation of an ultrathin amorphous layer (CoFe-B
i), with a thickness between 5 and 8 nm and a nanoarray feature of the catalyst was, furthermore, observed during morphological characterizations (
Figure 9) and retained after electrocatalytic activity for 50 h. The TOF was calculated at 0.482 mol O
2 s
−1 at the overpotential of 0.6 V
RHE.
Xie et al. reduced MoO
42− intercalated nickel-iron LDHs and obtained a NiFe-MoO
x nanosheets catalyst [
38]. In terms of water oxidation, the authors obtained comparable results to RuO
2. These results were ascribed to the high surface area, porosity, and surface electronic structure of the new LDH material. Their strategy led to an increase of active sites and an electronic modification of the NiFe alloy surface. The TOF for this catalyst was estimated at 0.19 s
−1 at the overpotential of 0.3 V
RHE and a faradaic efficiency around 95% for OER was determined.
Furthermore, a Ni–V monolayer double hydroxide was introduced by Fan and co-workers [
32], applying a hydrothermal method to synthesize the bulk LDHs. The performance of this material is comparable to the best-performing NiFe-LDHs to date in the literature. In particular, the Ni
0.75–V
0.25 LDH gave a current density of 57 mA cm
−2, at an overpotential of 0.35 V
RHE, for the OER. Experimental O
2 evolution was evaluated by gas chromatography, where the Ni
0.75–V
0.25 LDH exhibited a TOF of 0.05 s
−1 at 0.35 V
RHE overpotential. The abundance of the active sites in the LDH structure was identified as the main reason for the high performance of Ni
0.75–V
0.25-LDH. Typical TEM images of α-Ni(OH)
2 and Ni
0.75–V
0.25 LDH show the 2D nanosheet nature of the former (
Figure 10a,b), whereas a three-dimensional morphology assembly of ultrathin nanosheets was observed upon incorporation of vanadium (
Figure 10c,d).
Li et al. reported a new view of the significance of iron doping in a Ni hydroxide catalyst, which promoted the formation of Ni
4+ and, therefore, played an important role in the water oxidation [
36]. The strong Lewis acidity of Fe
3+ was identified as a crucial factor to increase the Ni valence and, thereby, to promote the oxyl character of the intermediates during the O–O bond formation.
Görlin et al. discussed the importance of high-surface area conductive supports and of the electrolyte pH [
39]. A microwave-assisted autoclave synthesis under solvothermal conditions was applied for the synthesis of a NiFeO(OH) catalyst. Catalyst support and pH > 13 resulted in enhanced catalytic activity, and higher particle dispersion allowed more metal centers to be accessible and to be electrochemically active. The highest TOFs were determined at 0.1 and 0.2 s
−1, respectively, in 0.1 M KOH. The O
2 levels were found to be close to 100% of the faradaic efficiency after online gas chromatography-mass spectrometry (GC-MS) measurements. The role of Fe centers in the metal redox activity was, furthermore, discussed in new detail, based on the UV-vis monitoring data.
A NiFe
2O
4–NiOOH catalyst was prepared through a simple anodization method by Zhang et al. [
40]. The catalyst afforded currents up to 30 mA cm
−2 at an overpotential of 0.24 V
RHE and only required 0.41 V
RHE to achieve a very high current density of 3000 mA cm
−2. The faradaic efficiency of O
2 evolution was close to 100%. In this study, it was stated that the excellent OER performance arose from the integration of amorphous NiFe
2O
4 and NiOOH. Finally, an ultrafast synthetic approach of only 5 s was reported by Zou and co-workers for the fabrication of a Ni–Fe-OH@Ni
3S
2/NF electrode material [
41]. A schematic illustration of this ultrafast method is displayed in
Figure 11. The catalyst gave currents up to 1000 mA cm
−2 at 0.469 V
RHE and displayed excellent catalytic stability for 50 h at the applied potential of 1.6 V
RHE, at the current density of 1000 mA cm
−2 in alkaline media. Furthermore, the faradaic yield of O
2 was found to be up to 95%.
Wu et al. highlighted the importance of designing well-controlled and aligned structures to enhance the catalytic capabilities of electrocatalysts for high-performance electrodes. Starting from this main incentive, for the first time, they fabricated unique hierarchical hollow Ni/NiFe (oxy)hydroxide heterostructured nanotubes radially aligned on a Ni foam, as seen in
Figure 12 [
42].
The material was used in a home-made electrochemical cell driven by a single battery of 1.5 V and showed excellent stability and overpotential at around 0.2 V
RHE, outperforming the state-of-the-art noble metals Ru and Pt. Hunter et al. employed a pulsed laser ablation technique in liquid media and prepared NiFe-LDH nanosheets with different interlayer anions (i.e., CO
32−, SO
42−, OH
−, F
−, Cl
− and I
−) to probe their role in water oxidation catalysis [
43]. The anion binding was evaluated by XPS and DFT calculations. Outcomes of their study revealed a relationship of water oxidation activity with the pK
a value of the interlayer anions and suggested that Bronsted or Lewis basicity of the anions was a key parameter for the water oxidation mechanism.
Furthermore, the authors found that nitrite species bound to edge-site Fe centers were correlated with high water oxidation activity, thus, pointing out the active role of the latter in the water oxidation process. The NiFe-LDH nanosheets showed remarkable stability for 3.5 h in 1 M KOH. The overpotential values varied between 0.27 V
RHE and 0.45 V
RHE, depending on the various interlayer anions. Jia et al. synthesized a highly active heterostructured NiFe-LDH nanosheet on defective graphene, via a co-precipitation method [
44]. DFT calculations suggested that the high electrocatalytic activity of the NiFe LDH-NS@DG catalyst towards water oxidation arose from the interaction of exposed 3d-transition metal atoms with carbon defects. This synergism was also found to cause the robust stability of the catalyst for 10 h, in 1 M KOH. The catalyst showed current densities above 50 mA cm
−2 at the applied potential of 1.5 V
RHE. The overpotential at the current density of 10 mA cm
−2 was found to be 0.21 V
RHE. A small Tafel slope value of 52 mV dec
−1 was also found between the range of 0.1 and 0.15 V
RHE of applied potential. The onset potential of the NiFe LDH-NS@DG catalyst was observed at 1.41 V
RHE and the faradaic efficiency of O
2 levels was 97.5%, after 5 h of chronoamperometric tests, at the current density of 100 mA cm
−2.
2.2.2. Spinels
Complex spinel-type oxides have attracted a lot of research interest as water oxidation catalysts, over the last years, due to their favorable structural properties, giving rise to tunability, high conductivity, and robust operational stability. Spinel oxides adopt the general formula AB2X4, where A and B are cations occupying the octahedral and tetrahedral sites in the anionic sublattice (with X representing anions, typically oxygen or less often chalcogens/halogenides). Co3O4 is a well-known example of a binary, spinel-type WOC. Ternary spinel-type cobaltites exhibit similar structural and electronic properties to Co3O4 with the general composition MCo2O4 (M = Zn, Ni, Cu, Mn, etc.).
The gold standard of Co-oxide WOCs, over the past years, has been the amorphous CoP
i catalyst introduced by Nocera et al. [
5,
45]. The selection of articles below was taken from the most cited and hot papers in the field, reflecting the progress in WOC understanding and performance, over the last three years. A comparison of the electrocatalytic performance of the selected WOC materials is given in
Table 3.
Recently, various synthetic strategies were employed to access Co
3O
4 spinels [
46,
47,
48,
49]. Wang and co-workers used a sol–gel method to synthesize regular cobalt spinels, comprised of Co
2+ on the tetrahedral site and two Co
3+ ions on the octahedral site [
47]. Their work was focused on the investigation of site-dependent OER activity, by substituting Co
2+ and Co
3+ with inactive Zn
2+ (ZnCo
2O
4, Co
3+ octahedral) and Al
3+ (CoAl
2O
4, Co
2+ tetrahedral), respectively. The electrochemical performance was tested by cyclic voltammetry, and interestingly the redox reaction was found to occur only at the Co
2+ tetrahedral sites, as shown in the voltammogram in
Figure 13. The Tafel slope value was found to be smaller in the case of Co
3O
4, which suggested a different rate-determining step. This study also pointed out the importance of operando investigations in such systems. From operando extended X-ray absorption fine structure (EXAFS) and electrochemical impedance spectroscopy (EIS) studies, the conclusion was drawn that cobalt oxyhydroxide (CoOOH) was formed and acted as a reactive center for the oxygen evolution reaction. In contrast, the ZnCo
2O
4 catalyst was negatively affected by the strongly bonded –OH groups on the surface, and so, its electrocatalytic activity was limited compared to its counterparts. The larger Tafel slope supported the latter observation, where the OER process is rate limited due to these strong –OH bonds.
Table 3.
Comparison of the electrocatalytic activity of spinel catalysts (cf.
Table 1 for definitions of “durability” and “stability”).
Table 3.
Comparison of the electrocatalytic activity of spinel catalysts (cf.
Table 1 for definitions of “durability” and “stability”).
Catalyst | Preparation Method | Onset Potential (V) | η (at 10 mV/cm2) (V) | Tafel Slope (mV dec−1) | Durability (h) | Stability | Electrolyte | Ref. |
---|
Co3O4/N-graphene | Hydrothermal | 1.49 | 0.96 | 121.8 | 4 | Good * | 0.1 M KOH | [50] |
Co3O4 assembled hollow spheres | Solvothermal & calcination | 1.43 | 0.29 | 86 | 10 | Prominent | 1 M KOH | [46] |
Co3O4, ZnCo2O4 & CoAl2O4 | Sol-gel | - | - | 69, 113 & 56 | - | - | 0.1 M KOH | [47] |
Co@Co3O4/N-CNT | Pyrolysis in H2 atm. & oxidative calcination | 1.62 | 0.39 | 54.3 | 45 | Good * | 0.1 M KOH | [51] |
Hollow Co3O4 microtube arrays | Hydrothermal & electrochemical treatment | 1.52 | 0.151 | 84 | 12 | High | 1 M KOH | [33] |
ZnCo2O4 QDs/N-CNT | Hydrothermal | 1.56 | 0.43 | 70.6 | 10 | High | 0.1 M KOH | [52] |
NiFe/Ni Co2O4 | Hydrothermal & electrodeposition | 1.47 | 0.34 | 38.8 | 10 | Excellent | 1 M KOH | [53] |
NiCo2O4 NPs | Two-step solution method | 1.49 | 0.157 | 75 | 20 | Excellent | 1 M KOH | [54] |
CoFeOx, CoFeNiOx & FeNiOx | Electrodeposition | 1.43 | 0.24 | 37 | - | - | 1 M KOH | [55] |
CoFe2O4@Co-Fe-Bi | Hydrothermal | - | 0.46 | 127 | 20 | Good | 0.1 M K2B4O8 | [56] |
Co3O4 | Hydrothermal | 143 | 0.35 | 69 | - | - | 1 M KOH | [57] |
An electrochemical sacrificial strategy was applied by Zhu et al. for the fabrication of hollow Co
3O
4 microtube arrays [
48], which is schematically represented in
Figure 14. More precisely, the CoHPO
4 microrods were immersed in 1 M NaOH and a constant potential was applied. Upon the anodic bias application, the microrods’ surface became rough and nanoplates were formed. The protonated CoHPO
4 was dissolved in the NaOH solution and Co
2+ species acted as source of cobalt oxides to be generated. The nanoporous nature of the cobalt oxide shells provided transport pathways for the OH
− and Co
2+ diffusion. The fully converted CoHPO
4 resulted in Co
3O
4 spinel. These micro/nanostructures were found to be competitive with IrO
2 and Pt references as OER catalysts. The authors proposed that such design and engineering of micro/nanostructures (
Figure 15) can be an efficient pathway for the enhancement of catalytic activity through the facilitation of mass transport by exposing more electroactive sites. During the water oxidation reaction, the catalyst showed impressive performance with current densities at around 300 mA cm
−2, almost two times greater, compared to IrO
2. The Tafel slope value was found to be 84 mV dec
−1, and the catalyst showed high stability during an operational time of 12 h. The corresponding voltammograms and Tafel slopes are displayed in
Figure 16. The catalyst, furthermore, exhibited a high faradaic efficiency of 96.3% for OER.
Hollow spheres assembled by α-Co(OH)
2 nanosheets were developed by Xie and co-workers via a solvothermal method followed by further calcinations, which led to mesoporous Co
3O
4 nanostructures [
46]. By tuning the amount of polyvinylpyrrolidone (PVP) different samples were synthesized as shown in
Figure 17. The obtained monodisperse hierarchical structure exhibited a low onset potential at around 1.44 V
RHE, with low overpotential (0.29 V
RHE) and relatively small Tafel slopes, in the range of 86 and 97 mV dec
−1. The samples indicated an excellent stability in alkaline media which renders them promising for further energy conversion as well as storage applications.
A subcategory of spinels are the above-mentioned cobaltites, which have been used as OER catalysts due to their increased conductivity and versatile electrochemically active sites compared to monometallic oxide catalysts. A two-step solution method was reported by Guan et al. as a way of utilizing metal–organic frameworks (MOFs), for the development of hollow NiCo
2O
4 nanowall arrays on carbon cloth, as shown in
Figure 18 [
54]. A uniform coverage of the Co-MOF on carbon cloth and the Co-based nanowalls was first obtained, and after annealing in air in the final step, the nanoarray structure of NiCo
2O
4 was maintained. The catalyst displayed high stability for 20 h and low overpotential of around 0.16 V
RHE in alkaline media, which was ascribed to its hollow nature. The material was proposed for other applications besides water oxidation, such as in supercapacitors.
Xiao et al. proposed a 3D hierarchical porous catalyst architecture consisting of NiFe/NiCo
2O
4/Ni foam [
53]. The synthesis procedure of this material is displayed in
Figure 19. The catalyst reached very high current densities in the range of 1200 mA cm
−2 at an overpotential of 0.34 V
RHE and onset potential at 1.47 V
RHE in 1 M KOH at pH 14. The stability and durability of this material was evaluated through cyclic voltammetry and the current density remained stable, over an operational time of 10 h.
Morales-Guio et al. classified a variety of oxidatively electrodeposited thin films, according to their electrocatalytic activity into the following three categories [
55]. Single-metal oxides falling into the first category, such as NiO
x, MnO
x, and FeO
x, showed the lowest performance. The second category included CoO
x and CoNiO
x, which showed a medium activity. Finally, the third category consisted of the most active catalysts, namely FeNiO
x, CoFeO
x, and CoFeNiO
x, which revealed high electrocatalytic performance, due to their synergistic effects. Their efficiency towards water oxidation reached currents up to 100 mA cm
−2, at overpotentials of around 0.24 V
RHE in 1 M KOH, at pH 14. The TOF values as a function of catalyst loading were also evaluated at the overpotential of 0.35 V
RHE. In particular, at the relatively low loading of 5 μg cm
−2, the TOF values of the more active FeNiO
x, CoFeO
x, and CoFeNiO
x catalysts were found to be around 0.5, 5, and 4 s
−1, respectively. The calculated faradaic efficiency of O
2 was also close to 100%.
Ji et al., on the other hand, used a complex core-shell CoFe
2O
4@Co-Fe-B
i nanoarray architecture in a potassium borate (pH 9.2) electrolyte and the catalyst showed a superior long-term stability, for at least 20 h, along with a high faradaic efficiency of 96% of O
2 evolution [
56].
Regarding the operational behavior of cobalt oxides, Zhang et al. monitored the chemical transformation of multiwall carbon nanotubes supported with hydrothermally prepared Co
3O
4 nanoparticles (Co
3O
4-MWCNTs) to CoO(OH), via ambient pressure X-ray photoelectron spectroscopy (APXPS) [
57]. The aforementioned transformation was only observed upon a potential application, during operando conditions. When the potential was removed, the catalyst was converted to its as-synthesized form, namely Co
3O
4. It was suggested that this rapid operando conversion was caused by proton concentration gradients in the presence of water vapor in the local environment, close to the solid/liquid interface. The electrocatalyst afforded current densities up to 100 mA cm
−2, at an applied potential of 1.65 V
RHE. The overpotential at the current density of 10 mA cm
−2 was observed at 0.35 V
RHE in 1 M KOH. The onset potential was observed at 1.43 V
RHE and the Tafel slope value was calculated at 69 mV dec
−1.
Recently, in one of our studies, we have synthesized cobalt oxide spinel (Co
3O
4) nanocrystals via a hydrothermal method. Our work provided deeper insight into the control parameters of hydrothermal WOC formation processes, through a combination of mechanistic in situ and ex situ analytical methods. This comprehensive study provided guidelines about how the WOC performance of binary oxides can be improved through an empirical understanding of their growth mechanisms [
58].
2.2.3. Perovskite-Based WOCs
Perovskite oxides display the general formula ABO
3, where the A-site ion on the corners of the elemental cell is usually an alkaline earth or rare-earth element, with a coordination number of 12. On the other hand, the octahedrally coordinated B-site ions in the center of the elemental cell are mainly 3d, 4d, and 5d transition metals. Ever since pioneering works of the 1970s [
59], numerous studies on the structure and properties of perovskites have been reported in the literature. More recently, perovskite oxides were proposed as efficient electrocatalysts for the oxygen evolution reaction [
59,
60,
61,
62,
63,
64,
65]. The following studies contributed to the development of new perovskite representatives, which significantly improved their activity and stability as water oxidation catalysts. A comparison of the electrocatalytic performance of the materials of this category is summarized in
Table 4 and covers research of the last three years.
An electrospinning method was employed by Zhao et al. for the development of PrBa
0.5Sr
0.5Co
1.5Fe
0.5O
5+δ nanofibers [
61]. The intrinsic activity and mass activity of this material were increased 4.7 and 20 times, respectively, compared to IrO
2, while the diameter of the particles was reduced to 20 nm. The aforementioned characteristics led to a high stability for 12 h, at a lower overpotential of 0.37 V
RHE, compared to IrO
2. The importance of the morphology and electronic properties of the structure resulted in an enhanced water oxidation, showing the optimization potential of these parameters. Furthermore, Zhu et al. used an electrospinning method to fabricate SrNb
0.1Co
0.7Fe
0.2O
3−δ perovskite nanorods, which displayed an excellent stability and activity in alkaline media [
64]. A representative scheme of the synthesis protocol is shown in
Figure 20. The results were compared with the-state-of the-art IrO
2 and showed seven times higher current densities at 0.4 V
RHE, combined with a lower Tafel slope. The material displayed an excellent stability for 30 h. The structural properties led to an increase of both charge transfer as well as of the electrochemically active surface, which boosted the water oxidation performance.
A ball-milling-assisted solid-state reaction was used by Xu and co-workers for the development of BaCo
0.7Sn
0.3O
3−δ and BaCo
0.7Fe
0.1Sn
0.2O
3−δ perovskite oxides, respectively [
63]. They demonstrated that the electrocatalytic activity of the catalysts can be easily tuned by modifying the dopant concentration. This example highlights the importance of doping strategies to enhance the conductivity in a wide range of materials, which in turn, increases their electrocatalytic activity towards OER. In parallel, the work of Zhu et al. focused on a sol–gel preparation method and by tuning the cation deficiency enhanced OER activity was accomplished [
60]. In particular, a La
0.95FeO
3−δ perovskite was developed;
Figure 21 shows how the significant change in OER activity is related to the formation of oxygen vacancies and Fe
4+, in the A-site of the perovskite.
Mefford et al. proposed a reverse-phase hydrolysis method for the synthesis of La
1−xSr
xCoO
3−δ [
65]. Their mechanistic study provided further evidence of how oxygen vacancies become a crucial parameter for improving the electrocatalytic activity of metal oxides. Briefly, it was stated that the controlled substitution of Sr
2+ for La
3+, across the full-phase width, while maintaining the perovskite structure, allowed the effects of covalence, vacancy defects, and oxygen exchange to be probed. Lee et al. reported for the first time a hexagonal perovskite, BaNiO
3, as water oxidation catalyst in alkaline media and demonstrated that this new family of perovskites outperformed IrO
2, by at least 19 mA cm
−2 during the first cycle of OER [
66]. The study of Black et al., first investigated quaternary hafnium oxynitrides with a reduced band gap and showed the potential of these compounds for photoelectrochemical water splitting devices [
67]. The new RHfO
2N perovskites, with R = La, Nd, and Sm, display structures resembling the GdFeO
3-type. The LaHfO
2N and NdHfO
2N compounds, in particular, showed increased resistance towards photocorrosion, compared to the SmHfO
2N photocatalyst.
2.2.4. Mixed Oxides
Mixed oxides frequently excel through their increased stability and synergistic effects, in a wide range of properties and applications [
55,
56,
68,
69,
70,
71,
72,
73].
Table 5 summarizes progress in the performance of mixed oxide water oxidation catalysts, over the last three years.
Gholamrezaei and co-workers proposed a new SrMnO
3 nanostructure for chemical water oxidation [
74]. Various preparation methods were chosen and compared by the authors, and sonochemical synthesis was found to afford the best results, with respect to morphology and nanoparticle size. Electrocatalytic water oxidation was carried out in a (NH
4)
2Ce(NO
3)
6 solution, where the effect of the Ce(IV) concentration was studied. O
2 evolution was increased with higher amounts of cerium. The results suggested that changes in the synthesis method and reaction conditions altered the morphology of the catalyst, as well as the size and the uniformity of the particles. The overall efficiency of the catalyst was enhanced along with the high uniformity of the particles.
Table 5.
Comparison of the electrocatalytic activity of mixed oxide catalysts (cf.
Table 1 for definitions of “durability” and “stability”).
Table 5.
Comparison of the electrocatalytic activity of mixed oxide catalysts (cf.
Table 1 for definitions of “durability” and “stability”).
Catalyst | Preparation Method | Onset Potential (V) | η (at 10 mV/cm2) (V) | Tafel Slope (mV dec−1) | Durability (h) | Stability | Electrolyte | Ref. |
---|
NiCeOx | Electrodeposition | - | 0.27 | - | >24 | Excellent | 1 M KOH | [68] |
Sn-Fe2O3 NWs | Hydrothermal | 0.1 | - | - | 10 | Excellent | 1 M KOH | [69] |
Ni-FeOx & Ni-Fe-CoOx NPs | Impregnation in the presence of aniline | - | 0.2 | - | 1 | Good * | 0.1 M KOH | [70] |
CoNiOx/rGO | Precipitation | - | 0.28 | 45 | 20 | Advanced | 0.1 M & 1 M KOH | [71] |
FexCoyO | Precipitation | - | 0.35 | 36.8 | 2.5 | Good | 0.1 M KOH | [72] |
Fe3O4-Co9S8 NPs/rGO | Two solvothermal steps | 1.48 | 0.34 | 82.8 | 6 | High | 0.1 M KOH | [75] |
Na1−xNiyFe1−yO2 | Hydrothermal | 1.35 | 0.26 | 44 | 12 | Excellent | 1 M KOH | [76] |
CoNiO, MnNiO & FeNiO | Precipitation reaction with urea | CoNiO: 1.60, MnNiO: 1.61 & FeNiO: 1.54 | - | CoNiO: 39, MnNiO: 43 & FeNiO: 18 | - | - | 0.1 M KOH | [77] |
Fe2−xCrxO3 | Coprecipitation | <1.3 | 0.45 | - | 1.5 | Good * | 100 mM PBS | [78] |
MnVOx@N-rGO | Hydrothermal | 1.32 | 0.39 | 271 | 4 | Excellent | 0.1 M KOH | [79] |
MnOx/N-CNT | Hydrothermal | ca. 1.4 | 0.36 | 75 | 2 | Catalyst detachment | 1 M KOH | [80] |
MnxSb1−xOx | Physical Vapor Deposition (PVD) | 1.5 | 0.3 | 75 | 25 | High * | 1 M H2SO4 | [81] |
Ho2O3/MnOx | Hydrothermal | Varied according to calcination | - | - | 0.5 | High | 0.25 M LiClO4 | [82] |
The study of Desmond-Ng et al. was focused on a theoretical model and experimental considerations aiming for the engineering of catalytic systems that are competitive with the best water oxidation electrocatalysts to date [
68]. To this end, the authors synthesized a NiCeO
x-Au catalyst, with an outstanding OER activity in alkaline media. A highlight of this approach was the new combination of electronic, geometric, and catalyst support effects. The TOF of NiCeO
x-Au catalyst at 0.28 V
RHE overpotential was determined as 0.08 s
−1.
Li et al. proposed Sn as a dopant in hematite nanowire photoanodes [
69]. The morphology was considered the key factor leading to the high performance of the Sn-doped hematite, which was furthermore coated with a cobalt oxide catalyst for enhanced photocurrent densities. A schematic illustration of the catalyst preparation is shown in
Figure 22. In this paper, the importance of retaining the concentration and uniformity of dopants along the nanowire growth was highlighted as another crucial parameter that is essential for high catalyst activity. The authors applied a silica encapsulation method, which retained the morphology of the hematite nanowire and further provided tuned nanowire lengths for maximum light absorption.
Bates et al. proposed a different way of increasing the electrocatalytic efficiency of a Ni–Fe–Co mixed metal oxide (MMO) catalyst. The morphology was optimized by addition of aniline as a capping agent, in order to generate a high surface area [
70]. Charge-transfer effects contributed to a higher conductivity among the Ni–Co sites and, therefore, increased their catalytic activity in water oxidation.
A promising strategy towards the development of OER electrocatalysts was proposed by Li et al., where a CoNiO
x nanocomposite was deposited onto a reduced graphene oxide [
71]. The exceptional performance of this nanocomposite in water oxidation was ascribed to the hierarchical sheet-on-sheet structure, affording a high surface area and high porosity. The catalyst showed a low overpotential at 0.28 V
RHE and a Tafel slope of 45 mV dec
−1. Proposing that all metal sites are active during the electrochemical reaction, the TOF value was estimated at 0.03 s
−1, at the overpotential of 0.35 V
RHE.
Zhuang et al. fabricated a high surface area catalyst consisting of Fe
xCo
y-O nanosheets (261.1 m
2 g
−1), with an ultrathin thickness of 1.2 nm [
72]. These features led to a superior activity, compared to commercial RuO
2, with an overpotential at 0.35 V
RHE and a Tafel slope at 36.8 mV dec
−1. The TOF value at the overpotential of 0.35 V
RHE was calculated at 0.02 s
−1. This outstanding performance of the catalyst was ascribed to the large number of oxygen vacancies in the sheets, which further acted as active centers. The proposed OER mechanism of a Co
3+ active site, near an oxygen vacancy, is illustrated in
Figure 23.
Applying a two-step solvothermal method, Yang et al. fabricated Fe
3O
4-decorated Co
9S
8 nanoparticles on reduced graphene oxide [
75]. The composite catalyst gave an excellent stability and catalytic activity. This was ascribed to electron transfer from the Fe species to Co
9S
8, which promoted the cleavage of the Co–O bond in the stable configuration of the Co–O–O superoxo group.
A layered Na
1−xNi
yFe
1−yO
2 double oxide was reported by Weng and co-workers with excellent stability and electrocatalytic activity, compared to the state-of-the-art RuO
2 and IrO
2 [76]. As a result of Na extraction, the valence states of Ni and Fe were raised and the catalyst exhibited excellent OER activity. This catalyst was also integrated in a perovskite solar cell, which delivered a solar-to-hydrogen efficiency of around 11.2%.
A new catalytic material comprising Fe
2−xCr
xO
3 particles is under investigation by Kanazawa et al. and is proposed for both photoelectrochemical and electrochemical water oxidation [
78]. Findings of their study revealed that Cr-substitution resulted in a five-fold enhancement of the electrochemical water oxidation performance, compared to α-Fe
2O
3, as well as lowered charge transfer resistance, giving rise to an improved OER activity, with nearly a 100% faradaic efficiency.
Mn oxides are cost-effective, environment-friendly, and redox active. The {CaMn
4O
5} oxygen evolving center of photosystem II is a constant source for bio-inspired approaches, in many fields of catalysis. Due to their versatility, Mn oxides have even been referred to as “Swiss army knife” [
83]. In the following, selected recent studies on mixed oxide catalysts, based on manganese are summarized.
Xing et al. proposed a new class of composite catalysts comprising MnVO
x on N-doped reduced graphene oxide [
79]. A hydrothermal method was applied, starting from a [Mn
4V
4O
17(OAc)
3]
3− polyoxometalate precursor. The authors showed that the precursor was successfully deposited on carbon and then converted to WOCs, with recorded OER current densities of 80 mA cm
−2, at the applied potential of 1.67 V
RHE. The overpotential of the MnVO
x/N-rGO electrocatalyst was determined as 0.39 V
RHE, at 10 mA cm
−2 current density, with a Tafel slope of 271 mV dec
−1. The synthesis method provided nanoparticulate Mn–V oxides with excellent stability, which can be further used for a variety of technological applications.
Antoni and co-workers employed high surface area N-functionalized CNTs for the oxidative deposition of MnO
x under reflux and mild hydrothermal conditions [
80]. The synthesis was carried out in the presence of CsMnO
4 and KMnO
4 as manganese sources, which gave mainly rise to the formation of crystalline birnessite manganese oxides. The MnO
x-modified N-CNTs catalysts afforded higher current densities, at around 20 mA cm
−2, compared to the unmodified N-CNTs in alkaline electrolyte. The overpotential of the catalyst was recorded as 0.36 V
RHE at 10 mA cm
−2 current density and the reported Tafel slope was 75 mV dec
−1. The faradaic efficiency was also increased from 75% to 90% towards the OER, after 10 min of electrocatalytic tests, and this was attributed to the oxidation of manganese.
A physical vapor deposition approach via reactive radio frequency magnetron co-sputtering of Mn and Sb was employed by Zhou et al., for the synthesis of rutile-type solid solutions, Mn
xSb
1−xO
x, with OER activity in strong acidic conditions [
81]. The mixed oxides showed current densities above 50 mA cm
−2, at the applied potential of 1.79 V
RHE. The overpotential at the current density of 10 mA cm
−2 was found to be 0.58 V
RHE, with a Tafel slope value of 75 mV dec
−1. Notably, the fraction of Mn
3+ increased with the overall amount of Mn in the mixed oxides. This was corroborated by XAS, AP-XPS, and computational investigations and linked to the overall increase in OER activity. Likewise, Morgan Chan et al. showed via spectroscopic and computational studies that the enhancement of MnO
2 oxygen evolution catalyst is related to the introduction of Mn
3+ species, and the suppression of Mn
3+ oxidation to Mn
4+, due to structural restrictions, was considered crucial for higher activity [
84].
Najafpour and co-workers proposed and hydrothermally fabricated a new manganese oxide supported on holmium oxide for OER [
82]. Their catalyst was very stable, recoverable, and it could be used for long operational OER. The observed self-healing properties were explained with the presence of Ce(IV) species. O
2 was produced from water in the presence of Mn oxides and Ce(IV) and water, and MnO
4− was further formed from the oxidation of intermediate Mn(II) ions by Ce(IV). These MnO
4− ions were eventually converted back to Mn oxide, resulting in self repair of the catalyst.
2.2.5. Other Hydroxides and (Oxy)hydroxides
Commercial water electrolyzers require efficient electrocatalysts with high current densities, usually above 500 mA cm
−2, with long-term stability, at overpotentials below 0.3 V
RHE. To this end, metal hydroxides and (oxy)hydroxides have attracted a lot of research attention as target structures, to fabricate catalysts with the aforementioned characteristics [
18,
36,
39,
40,
41,
85,
86,
87,
88,
89,
90,
91,
92].
Table 6 summarizes progress in the electrocatalytic activity of hydroxide and (oxy)hydroxide catalysts, over the last three years. In addition to noble metal oxides, such as RuO
2 and IrO
2, which have been established as the best OER catalysts to date, Fe-doped NiOOH ranks among the best, noble metal-free catalysts.
Recently, Shin et al. investigated new dopants using a density functional theory approach, in order to determine the atomistic mechanism for the OER of Ni
1−xFe
xOOH [
93]. In their study, 17 transition metals were considered to replace Fe. They found out that the most promising materials were Co, Rh, and Ir, and the resulting overpotentials were estimated to be 0.27, 0.15, and 0.02 V
RHE, respectively. The authors concluded that Fe
4+ and Ni
4+ species played an essential role in the OER. The promotion of a radical environment on the metal-oxo bond was also found to be a key prerequisite for high OER activity.
Table 6.
Comparison of the electrocatalytic activity of hydroxide and (oxy)hydroxide catalysts (cf.
Table 1 for definitions of “durability” and “stability”).
Table 6.
Comparison of the electrocatalytic activity of hydroxide and (oxy)hydroxide catalysts (cf.
Table 1 for definitions of “durability” and “stability”).
Catalyst | Preparation Method | Onset Potential (V) | η (at 10 mV/cm2) (V) | Tafel Slope (mV dec−1) | Durability (h) | Stability | Electrolyte | Ref. |
---|
NiFeOH/NiFeP | Hydrothermal & PH3 plasma treatment & electrodeposition | - | 0.258 | 39 | 24 | High * | 1 M KOH | [85] |
NiFe-OH | Stepwise electrodeposition | - | 0.24 | 38.9 | 5 | Good * | 1 M KOH | [86] |
W0.5Co0.4Fe0.1 oxyhydroxide coralloids | Hydrothermal | 1.53 | 0.25 | 32 | >500 | Superior | 1 M KOH | [87] |
CoFe-H hydroxide & CoFe-H/BiVO4 | Electrodeposition | 0.23 | 0.28 | 28 | 45 | High * | 1 M KOH | [88] |
CeO2/FeOOH | Electrodeposition | 0.21 | 0.25 | 32 | 50 | High | 1 M NaOH | [89] |
FeOOH/Co/FeOOH | Electrodeposition | 0.22 | 0.25 | 32 | 50 | High | 1 M NaOH | [90] |
FeCoW oxyhydroxides | Modified sol-gel | - | 0.191 | 37 | 550 | High | 1 M KOH | [91] |
NiFe(OH)/NiFe:Pi | Electrodeposition & phosphorylation | 1.43 | 0.29 | 38 | 10 | Excellent | 1 M KOH | [92] |
Ag-decorated Co(OH)2 | Selective reduction-oxidation | - | 0.27 | 67–109 (depending on the amount of Ag) | 10 | Good | 1 M KOH | [94] |
NiFeOOH/TiO2 core-shell | Hydrothermal | | 0.273 | 86.9 | 24 | High * | 0.01 M Na2SO4 | [95] |
α-NiCo(OH)2 | Electrodeposition | 1.43 | 0.26 | 25-30 | 6 | High * | 1 M KOH | [96] |
NiPS3@NiOOH | Chemical vapor transport & liquid exfoliation | 1.48 | 0.35 | 80 | >160 | Excellent | 0.1 M KOH | [97] |
Liang et al. used a PH
3 plasma-assisted method developed by their group, to fully convert a NiFe hydroxide into porous NiFeP, as shown in
Figure 24 [
85]. This method facilitated the rapid synthesis of phosphides, at around 15 min, at low temperatures of approximately 200 °C. The catalyst showed high geometric current densities of 300 mA cm
−2 at the corresponding overpotential of 0.2 V
RHE, a low overpotential of 0.258 V
RHE at 10 mA cm
−2, and a small Tafel slope of 39 mV dec
−1. The low overpotential of 0.25 V
RHE corresponded to 0.036 s
−1 TOF, as calculated by the authors. The high activity was ascribed to the morphological architecture and the electronic interaction between the α-NiFe-OH and NiFeP, respectively.
Electrodeposition was employed by many groups, in order to synthesize catalysts using FeOH or FeOOH-coupled with other co-catalyst metals or metal oxides, such as Ni, CeO
2, and BiVO
4 [
86,
87,
89,
90,
92]. Of note is the work by Li et al., which dealt with the surface wettability [
92]; the authors employed electrodeposition and phosphorylation methods to deposit an α-NiFe-OH nanosheet layer onto a NiFe:P
i 3D hierarchical nanostructure. From the results, the authors concluded that surface wettability is a key parameter for controlling the morphology, while it is also capable of modifying the chemical properties of a catalyst. The catalyst showed a low overpotential of 0.29 V
RHE at 10 mA cm
−2 and an excellent stability in the aqueous media. The faradaic efficiency was found to be 98.2%, which suggested that the main product evolved on the catalyst was O
2. Furthermore, the phosphorylation assisted the enhancement of the surface area, which provided more catalytic active sites for water adsorption and oxidation.
A trimetallic W
0.5Co
0.4Fe
0.1 oxyhydroxide corraloid was introduced by Pi et al.; this exhibited a superior stability and durability in alkaline media, for 21 days [
87]. Their catalyst outperformed most of the reported Co-based nanomaterials and showed continuous electrolysis currents with imperceptible decay after more than 500 h. The authors followed a wet-chemical method to synthesize corraloid-like trimetallic oxyhydroxides on nickel foam and carbon nanotubes. The preparative route and the electron microscopy characterization of the catalyst are displayed in
Figure 25. The images showed disintegrated and twisted nanosheets and the selected area electron diffraction data revealed the high crystallinity of the oxyhydroxides. The special structure of this catalyst went hand-in-hand with a low overpotential of 0.25 V
RHE, at the current density of 10 mA cm
−2 and a small Tafel slope of 32 mV dec
−1. XPS data showed that an optimized amount of Fe went hand in hand with the optimization of the binding energy of oxygen intermediates, which led to a high OER activity.
A similar trimetallic catalyst consisting of FeCoW oxyhydroxides was synthesized via a modified sol–gel method by Zhang and co-workers [
91]. These gelled FeCoW oxyhydroxides exhibited superior durability for 550 h, i.e., approximately 23 days, with a very low overpotential of 0.191 V
RHE at 10 mA cm
−2, in alkaline media. The intrinsic activity of gelled-FeCoW oxyhydroxides was quantified by determining the TOF value, which was found to be 0.46 s
−1, at an overpotential of 0.3 V
RHE. The faradaic efficiency of O
2 evolution recorded on the catalyst was calculated to be close to 100%. The synergistic effects between the three metals produced a favorable local coordination environment, which, along with their electronic interaction, significantly enhanced the OER activity.
Electrodeposited amorphous α-phase Ni–Co(OH)
2 nanodendrite forests were synthesized on a stainless steel foil by Balram and co-workers [
96]. The key parameter for the formation of nanodendritic structures was the addition of small amounts of water to the deposition bath. A limited water electrolysis and hydrogen bubble production occurred near the electrode interface with the electrolyte, which caused highly localized pH gradients, resulting in the aforementioned three-dimensional structure. The electrocatalyst afforded current densities higher than 50 mA cm
−2, at the applied potential of 1.6 V
RHE. The overpotential at the current density of 10 mA cm
−2 was found at 0.25 V
RHE and the Ni–Co(OH)
2 material showed an excellent stability during the operational time of 6 h.
Konkena and co-workers employed a chemical vapor transport and liquid exfoliation method, in order to fabricate an NiPS
3@NiOOH core-shell structure [
97]. Ni(III)-based (NiOOH) species of the shell structure enhanced the overall OER, as shown by a combination of spectroscopic and computational analyses. Additionally, the high OER activity was attributed to the highly active metal-edge sites. The catalyst excelled through a low-onset potential at 1.48 V
RHE, as well as an overpotential value of 0.35 V
RHE at the current density of 10 mA cm
−2. The Tafel slope was recorded as 80 mV dec
−1 and the catalyst showed a high stability after 6 h of electrocatalytic tests.