Recent advances in earth-abundant first-row transition metal (Fe, Co and Ni)-based electrocatalysts for the oxygen evolution reaction

The oxygen evolution reaction (OER) is of fundamental importance as a half reaction and rate-controlling step that plays a predominant function in improving the energy storage and conversion efficiency during the electrochemical water-splitting process. In this review, after briefly introducing the fundamental mechanism of the OER, we systematically summarize the recent research progress for nonprecious-metal-based OER electrocatalysts of representative first-row transition metal (Fe, Co and Ni)-based composite materials. We analyze the effects of the physicochemical properties, including morphologies, structures and compositions, on the integrated performance of these OER electrocatalysts, with the aim of determining the structure-function correlation of the electrocatalysts in the electrochemical reaction process. Furthermore, the prospective development directions of OER electrocatalysts are also illustrated and emphasized. Finally, this mini-review highlights how systematic introductions will accelerate the exploitation of high-efficiency OER electrocatalysts.


INTRODUCTION
The unrestricted consumption of traditional fossil fuels not only aggravates the global energy crisis but also causes significant damage to the natural ecological environment [1][2][3][4] . Therefore, it is extremely urgent to push forward extensive research into renewable clean energy sources, such as solar energy, wind energy, H 2 energy, and so on [5][6][7][8][9] , as alternatives for non-renewable fossil fuels. Among these, H 2 can be considered as the most promising renewable clean energy source due to its inherent advantages of all-weather utilization, extensive sources, zero-pollutant emission and high combustion value [10][11][12] . Realistically, the vigorous development of H 2 production technologies is a prerequisite for its commercial utilization [12,13] . In this regard, the H 2 production derived from electrochemical water-splitting can be served as a preferential selection, owing to its approvable characteristics of the earth abundance and accessibility of water recourses and particularly the clean, large-scale and sustainable hydrogen production, which makes it more feasible for commercialization [14] .
The water electro-dissociation procedure consists of two essential half reactions, namely, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which simultaneously occur on the cathode and anode of the water-splitting alkaline electrolyzer respectively [15] . The water-splitting process exhibits a thermodynamically endothermic reaction and requires a thermodynamics Gibbs free energy (ΔG) of 237.2 kJ mol -1 , corresponding to a standard potential (ΔE) of 1.23 V vs. a reversible hydrogen electrode (RHE) [16] . Nevertheless, compared with the two-electron transfer HER, the OER is the rate-determining step due to its sluggish kinetics as a consequence of the four-electron transfer in the formation of the O=O bond, which eventually controls the integrated energy transformation efficiency of the electrochemical watersplitting process [17,18] . Thus, the fabrication of high-efficiency OER electrocatalysts is essential for reducing the kinetic energy barriers, which is the key to realizing their industrial application.
To date, noble metal-based materials, including RuO 2 and IrO 2 , are recognized as the benchmark OER electrocatalysts because they are well known for their high-performance OER activities. Unfortunately, their intrinsic disadvantages of high expenditure and low abundance have restricted their commercial large-scale application [19,20] . Thus, multiple efforts have been executed to develop earth-abundant first-row transition metal-based electrocatalysts, such as typical oxides [21] , carbides [22,23] , sulfides [24,25] , selenides [26,27] , nitrides [28] and phosphides [29,30] , which have made the greatest contribution to OER activities [31] . Accordingly, it is essential to directly carry out investigations and summaries of these materials as efficient OER electrocatalysts [32] .
In this review, we concentrate on the catalytic performance of earth-abundant first-row transition metal (Fe, Co and Ni)-based composite materials as electrocatalysts, which are demonstrated to be a subject of increasing interest. More specifically, the physicochemical properties of these electrocatalysts, such as the morphologies, compositions, promoting abilities and presence of metal-support interactions, which are mainly responsible for the OER electrocatalytic activities, are systematically introduced to elucidate their structure-activity relationships. Furthermore, the prospective exploitation direction of OER electrocatalysts is also illustrated and emphasized. This mini-review provides general guidelines for the state-of-the-art architecture of OER electrocatalysts and a discussion of their challenges and further prospects.

OER ELECTROCATALYTIC SYSTEMS
Noble metal-based materials were the first electrocatalysts used for hydrogen production by electrocatalytic water splitting [33] . As far as elemental noble metal electrocatalysts are concerned, the corresponding order of OER catalytic activity is as follows: Pt < Rh < Pd < Ir < Ru. Kim et al. [34] found that in the OER process, although the high operating potential causes the oxidation of Ru and Ir surfaces to produce RuO 2 and IrO 2 , respectively, these oxides have the characteristics of high conductivity and excellent electronic structure, so they still maintained excellent OER catalytic activities. Hu et al. [35] summarized the OER catalytic properties of Ru, Ir and their corresponding oxides. It was demonstrated that the order of catalytic activities is Ru > Ir ≈ RuO 2 > IrO 2 . Although RuO 2 and IrO 2 exhibited excellent catalytic activity under acidic and alkaline conditions, their corrosion resistance was undesirable and thus their stability needed to be further improved by compounding them with other metals or metal oxides. At present, the reported Ir-based composite electrocatalysts mainly included IrM (M = Cu, Fe, Co or Ni), IrCoNi, IrCuNi, IrO 2 and MO x (M = Nb, Ti, Ta or Zr). These composite Ir-based electrocatalysts delivered excellent integrated OER performance [36] . For Ru-based composite electrocatalysts, the only Ru x Ir 1-x O 2 bimetallic oxide system formed by successful doping Ir into RuO 2 has been reported, which not only effectively inhibited the decomposition of the electrocatalyst but also significantly improved the OER catalytic stability [37] .
In short, noble metal-based materials present excellent OER electrocatalytic activities but their commercialscale applications are restricted by the inherent drawbacks of easy corrosion, scarcity and high cost [38] . Therefore, the key to the development of OER electrocatalysts is seeking transition metal materials with abundant reserves, low cost and high activity potential. In this regard, considering that transition metal materials are easily susceptible to oxidation and the corresponding high-valence metal ions have more oxidation properties during the OER electrochemical process, the representative earth-abundant first-row transition metal (Fe, Co and Ni)-based composites as OER electrocatalysts have attracted widespread attention and have been demonstrated to possess excellent performance for the OER.

Iron-based electrocatalysts
Fe has become the most attractive metal for OER electrocatalysts because of its high intrinsic conductivity, low toxicity and negligible and inexhaustible impact on the environment [39] . It has been concluded that although a single iron electrocatalyst can give poor OER activity, it can also serve as a promoter of polymetallic OER electrocatalysts [40,41] . For example, Dutta et al. [42] developed a Fe 3 O 4 /Ni x P electrocatalyst with the architecture of an amorphous Ni x P shell and a crystalline Fe 3 O 4 core. It is noteworthy that the Fe 3 O 4 nucleus in the electrocatalytic structure could activate the electrochemical process of Ni and therefore significantly improve the OER electrochemical activity. Zhang et al. [43] synthesized a NiFe layer double hydroxide-supported Au electrocatalyst (Au/NiFe-LDH), which delivered a low overpotential for the OER (ƞ 10 = 237 mV) and high durability. Density functional theory (DFT) calculations illustrated that Fe in the Au/NiFe-LDH should be the active center of the OER and Au could simultaneously help to control the charge distribution of the hybrid by transferring electrons to the LDH, so as to synergistically contribute to the overall performance of the OER catalytic activity. Sun et al. [44] found that when compared with the activities of Co(OH) 2 and Fe 3 O 4 (ƞ 10 = 480 and 540 mV, respectively), Co(OH) 2 nanosheets (NSs) modified with Fe 3 O 4 as OER electrocatalysts presented a higher OER activity (ƞ 10 = 390 mV). This was mainly attributed to the fact that the addition of Fe could optimize the electronic structure and surface properties of the electrocatalyst, which effectively promoted the transformation of intermediates during the OER catalytic process.
Feng et al. [45] synthesized FeOOH/Co/FeOOH-HNTA/NF hybrid electrocatalysts with anisotropic and hollow nanostructures. Due to the eminent synergistic effect between the FeOOH and Co layers in the material, not only was the conductivity of the Co metal central layer improved but a convenient channel for electron transmission during the OER process was also provided to improve the OER electrocatalytic activity. Zhang et al. [46] fabricated a gelation FeCoW hydroxyl oxide electrocatalyst. On account of the excellent synergistic effect of the Fe, Co and W elements in the structure, it provided favorable electronic structure and coordination environment for the effective water oxidation reaction, thus enhancing the catalytic performance of the OER. Zhang et al. [47] prepared polyaniline-coated Prussian blue analogs as multifunctional catalytic materials for total electrolytic water (PBAs@PANI). By accurately controlling the reaction parameters, PANI was evenly wrapped on the surfaces of the PBA nanocubes and the thickness of its shell could be easily adjusted. This advanced nanostructure could significantly enhance the charge transfer, elastic buffer and corrosion protection, thus accelerating its OER catalytic performance. Zhang et al. [48] took advantage of a pyrolysis method to bind the active sites of Ni(Fe)OOH and Ni/Fe-N-C to three-dimensional (3D)-interconnected PANI nanochains (Ni 2 Fe 1 @PANI-KOH900) [ Figure 1]. In this regard, the PANI matrix not only improved the conductivity of the electrocatalyst but also maintained the stability of its chain structure, so as to obtain more electron channels for improving its bifunctional catalytic performance for the OER and ORR.
Dong et al. [49] confirmed that transition metal ion (TMI)-doped conductive polymers [such as Fe (III)doped PANI] with foamed nickel could effectively accelerate the catalytic performance of the HER or OER. This was because the TM-N bond in the TMI-doped conductive polymers could produce active center sites with high activities, which significantly reduced the energy barrier of the HER or OER intermediates and relevant products. Liu et al. [50] synthesized amorphous cobalt iron hydroxide (CoFe-H) using a facile electrodeposition strategy as an efficient OER electrocatalyst to achieve photoelectrochemical water splitting. The CoFe-H/BiVO 4 photoanode was constructed with CoFe-H NSs and a BiVO 4 semiconductor, with a good photocurrent density was obtained. It was demonstrated that the improved OER kinetics and high-quality interface of CoFe-H/BiVO 4 , as well as the excellent optical transparency of the CoFe-H NSs, contributed to the enhancement of the photoelectrocatalytic OER performance. Yang et al. [51] developed a ferroelectric-enhanced photoelectrocatalytic system using BiVO 4 and Co 3 O 4 as photocatalysts and cocatalysts to couple BiFeO 3 to form BiVO 4 -BiFeO 3 heterojunctions. The cocatalysts could provide the additional electrocatalytically active sites for OER and the BiVO 4 -BiFeO 3 heterojunction promoted carrier separation and BiFeO 3 could form a local internal electric field through ferroelectric polarization at low voltage, which further promoted carrier separation and increased the photocurrent. Zhu et al. [52] used a facile electrodeposition synthesis method to fabricate NiFe-LDH-modified α-Fe 2 O 3 photoanodes with enhanced photoelectrocatalytic OER performance. Compared with reversible hydrogen electrodes, the photocurrent density of α-Fe 2 O 3 /Ni 0.5 Fe 0.5 -LDH electrodes was much higher than that of bare α-Fe 2 O 3 and it also exhibited good durability. Mott-Schottky and electrochemical impedance spectroscopy measurements indicated that the significant enhancement of the photoelectrocatalytic performance of α-Fe 2 O 3 /NiFe-LDH derived from the decrease in charge transfer resistance and the increase in carrier density.
Recently, Xu et al. [53] synthesized a nickel-iron diselenide ether derived oxide (Ni x Fe 1-x Se 2 -DO) electrocatalyst and compared its OER electrocatalytic activity with a NiSe-DO electrocatalyst. The results illustrated that the OER overpotential of the Ni x Fe 1-x Se 2 -DO electrocatalyst (ƞ 10 = 195 mV) was lower than that of the NiSe-DO electrocatalyst (ƞ 10 = 253 mV), so the addition of Fe substantially improved the OER electrocatalytic activity of the Ni x Fe 1-x Se 2 -DO electrocatalyst. Hung et al. [54] carried out an in-depth study of the effect of the geometric position of Fe and Co ions in an iron-doped cobalt oxide electrocatalyst (CoFe y ) with different mass ratios on the OER activity. It was found that the OER activity increased with increasing Fe content in CoFe y (CoFe 0 < CoFe 0.16 < CoFe 0.28 < CoFe 0. 38   [Co 2+ (Td)], which dramatically promoted the corresponding OER activities (ƞ 10 = 229 mV and ƞ 100 = 281 mV). Zhuang et al. [55] fabricated Fe x Co y -O NS electrocatalysts, which had the advantages of rich oxygen vacancies, high specific surface area and ultrathin thickness. Therefore, the NSs showed excellent OER performance in a 0.1 M KOH solution (ƞ 10 = 308 mV and ζ = 36.8 mV dec -1 ). Jin et al. [56] investigated the effect of Fe and Mo doping on the OER activity of a Ni(OH) 2 /NiOOH-NS electrocatalyst. As shown in Figure 2, when compared with the Ni(OH) 2 /NiOOH-NSs (ƞ 100 = 400 mV), MoFe/Ni(OH) 2 /NiOOH delivered an excellent OER activity (ƞ 10 = 280 mV) and high stability. This was mainly because the addition of Fe and Mo in the Ni(OH) 2 /NiOOH-NSs could enable the occurrence of the synergistic effect of Fe and Mo, which strengthened the interaction ability between Ni and OER intermediate products, thus improving its electrocatalytic activity. Based on the above studies, it is well known that although Fe is not the active center, it plays a prominent role as the OER promoter in such electrocatalysts.
Furthermore, it has been confirmed that iron nitride or iron phosphide embedded in a 3D porous structure or grown on a conductive substrate can act as the OER active center. The porous structure and conductivity of such materials can facilitate the contact area of the electrolyte solution and the charge transfer capacity between the catalytic active sites and substrate [28] . For example, Yu et al. [57] utilized the thermal nitriding method to in-situ realize the fabrication of nanoporous membrane iron nitride (Fe 3 N/Fe 4 N) into a highconductivity 3D graphene/nickel foam. This advanced architecture had the advantages of high specific surface area, abundant active sites, a porous structure and electrical conductivity, which were conducive to enhancing the mass transfer capacity of charge between the electrolyte and the electrode during the OER process, thus improving the corresponding electrocatalytic OER performance.
Realistically, the doping of Fe into electrocatalysts not only significantly accelerates the OER electrocatalytic activities but also plays an important role in improving the HER activities [58] . For example, Fan et al. [59] [56] .
doped Fe into Ni 3 C-based NSs, indicating that suitable Fe doping could optimize the electronic properties and surface composition of Ni 3 C and further improve the HER and OER catalytic performance. In particular, Ni 3 C-based NSs (Fe-Ni 3 C-2%) doped with 2 at.% Fe exhibited a low overpotential (292 mV) and small Tafel slope (41.3 mV dec -1 ) for the HER in an alkaline solution, which presented the best performance. Liu et al. [60] prepared NiFe precursors using an electrodeposition strategy and then fabricated Ni 3 FeN with not only a 3D porous structure but also interlinked nanoparticles in a NH 3 atmosphere at 400 °C. The results showed that Ni 3 FeN has higher HER catalytic activity than Ni 3 N and Fe 2 N (Ni 3 FeN: ƞ = 105 mV at 10 mA·cm -2 in 1.0 M KOH). Deng et al. [61] successfully synthesized FeCo nanoparticles in N-doped carbon nanotubes, which offered an excellent performance of ƞ 10 = 110 mV with ζ = 74 mV dec -1 . Its electrocatalytic activity mainly originated from Fe and Co alloying and their encapsulation in the conducting carbon nanotubes reduced the charge transfer resistance during the HER process.
Furthermore, considerable studies also disclosed that iron selenide, phosphide and sulfide exhibited excellent HER electrocatalytic performance. For example, Theerthagiri et al. [62] immobilized iron diselenide (FeSe 2 ) nanorods on graphene oxide NSs, which showed a better performance of ƞ 9.68 = 250 mV and ζ = 64 mV dec -1 . Zhang et al. [63] synthesized Ni(OH) 2 -Fe 2 P by electrodeposition using Fe 2 P as a precursor, where the amorphous Ni(OH) 2 was deposited on crystalline Fe 2 P and had a nanoarray structure. The electrocatalyst had good catalytic activity and stability, as well as excellent HER activity (ƞ of 76 mV at 10 mA·cm -2 ). Furthermore, Guo et al. [64] synthesized graded hollow microspheres of FeP as HER electrocatalysts, which showed reasonable stability of ƞ 10 = 144 mV, ζ = 58 mV dec -1 and 1000 CV cycles. Similarly, Zhu et al. [65] reported hollow mesoporous FeP-coated with a carbon layer (HMFeP@C). The results illustrated that the electrocatalysts exhibited a lower ƞ 10 value (115 mV), higher stability (constant j for 20 h) and similar ζ (56 mV dec -1 ). Due to the synergistic interaction between the cubic and porous structures, it displayed good HER catalytic activity and stability.
Recently, Chung et al. [66] synthesized FeP NPs coated with carbon shells (FeP/C), exhibiting the high electrocatalytic activity and stability of ƞ 10 = 71 mV, ζ = 52 mV dec -1 and 5000 CV cycles, with no significant change in performance [ Figure 3]. The high stability was attributed to the coated carbon shell, which effectively protected the FeP NPs from oxidation under the HER. Konkena et al. [67] reported the study of nickel-chromite-iron ore with a composite of Fe 4.5 Ni 4.5 S 8 as a HER electrocatalyst, which delivered excellent activity and stability in a 0.5 M H 2 SO 4 solution (ƞ 10 = 280 mV and ζ = 72 mV dec -1 ). Yu et al. [68] synthesized hierarchical porous microflowers of 3D-ferric nickel sulfide on foamed nickel (Ni 0.7 Fe 0.3 S 2 /NF) by a hydrothermal sulfidation method. Due to the synergistic effect of the Fe-Ni alloy and the doping of S, the charge resistance in the reaction process was reduced, so the electrocatalyst had good HER electrocatalytic activity in a 1 M KOH solution (ƞ 10 = 155 mV). In addition, Li et al. [69] prepared ultrathin CuFeS 2 NSs, delivering better electrocatalytic performance of ƞ 10 = 88.7 mV, ζ = 47 mV dec -1 , JE = 0.35 mA·cm -2 and 15 000 CV cycles in a 0.5 M H 2 SO 4 solution. DFT calculations showed that the HER activity of CuFeS 2 NSs was attributed to the high-density exposure of active S 2species.

Cobalt oxide
It has been demonstrated that among non-noble metal oxides, the representative Co 3 O 4 has the lowest theoretical overpotential for the OER. Therefore, efficient Co 3 O 4 -based electrocatalysts for the OER could be designed by changing their morphology and structural composition [70] . There are two different oxidation and coordination environments for Co atoms in Co 3 O 4 , namely, tetrahedral coordination Co (II) (Co 2+ Td) with an intermetallic distance of 3.36 Å and octahedral coordination Co (III) (Co 3+ Oh) with intermetallic distances of 2.85 and 3.36 Å. In the anodic potential region before the initial potential, the cobalt oxyhydroxide (β-CoOOH) intermediates transformed from Co 2+ Td possess a stronger water oxidation ability than pure Co 3 O 4 . Therefore, the catalytic activity of Co 2+ Td could be further ameliorated by adjusting and increasing the ratio of Co 2+ Td to Co 3+ Oh [71] . Wang  Menezes et al. [73] prepared spinel Co 3 O 4 nanoparticles partially substituted with Mn 3+ (MCO) and introduced metal defect sites to the surface of MCO by removing Mn 3+ ions in post-treatment [ Figure 4]. The removal of Mn 3+ ions from the octahedral position made the MCO surface with highly exposed Co 2+ Td centers, which promoted the formation of active intermediates over the β-CoOOH during the OER process. Compared with the original electrocatalyst, the OER electrocatalyst that was rich in defect sites derived from MCO has a lower overpotential (ƞ 10 = 320 mV). Although the removal of the Co 3+ Oh site makes the OER more feasible, it may have a negative impact on the electronic structure of the electrocatalyst and may significantly reduce its conductivity. Therefore, while maintaining the total amount of metal ions in the lattice, it was more practical to transform the octahedral Co 3+ Oh center sites with lower activity into tetrahedral Co 2+ Td center sites with higher activity. For example, Xu et al. [74] etched Co 3 O 4 with oxygen plasma to remove the oxygen atoms on its surface and convert the high-valence Co 3+ ions into low-valence Co 2+ ions, so as to improve the abundance of Co 2+ Td active center sites in Co 3 O 4 and promote the efficient oxidative decomposition of water. In order to further improve the conductivity and activity of Co 3 O 4 , the anoxic Co 3 O 4 could be hybridized with a high-conductivity carbon matrix, which provided a convenient channel for electron transfer during the OER process. Copyright 2017, American Chemical Society [66] .  [73] .
Tong et al. [75] synthesized ultrathin NSs (CoO x NPs/BNG) of graphene that were rich in boron and nitrogen and coupled with anoxic CoO x NPs. The reason for the enhanced activity of the OER electrocatalyst was intimately related to the oxygen defect structure of CoO x , which not only regulated the electronic structure of the electrocatalyst, but also encouraged its own oxygen affinity. Furthermore, the formation of Co-N-C and Co-B-N bonds also increased the conductivity of the electrocatalyst and promoted the OER electron transfer of the Co metal center. Although adjusting the composition of Co 3+ and Co 2+ in Co 3 O 4 provides a simple and feasible method to obtain a high-activity electrocatalyst, its poor conductivity still needs to be resolved. Therefore, the first row of transition metals, including Fe, Ni, Cu, Cr and V, could be doped into Co 3 O 4 to alter its electronic band structure and increase its affinity for OER active species (O*, HO* and HOO*) [76] . For instance, Lin et al. [77] confirmed that embedding electrophilic Cr 3+ ions (Cr 0.75 Co 2.25 O 4 ) in spinel cobalt oxide enhances the oxygen affinity of the central Co 2+ sites. In addition, Cr 3+ ions replaced Co 3+ ions inert to the OER at the octahedral center [ Figure 5A], which also enhanced the electron transport capacity of the whole electrocatalyst surface. Tahir et al. [78] loaded NiO/Co 3 O 4 hybrid nanoparticles on Ndoped carbon nanotubes as OER electrocatalysts. The XPS analysis showed that compared with NiO and Co 3 O 4 NPs loaded alone, the Ni and Co atoms in the hybrid material had a high oxidation state and there was a strong coupling ability between the metal center and the N atoms in the carbon matrix. This enhanced the oxygen affinity and electronic conductivity and improved the OER electrochemical performance [ Figure 5B]. Patel et al. [79] synthesized a semitransparent and porous p-type Co 3 O 4 film and proved that it could be used for photoelectrocatalytic hydrogen production. Simultaneously, they found that the study of Co 3 O 4 splitting seawater not only exhibited desirable photoelectrocatalytic HER activity but also contributed to the efficient formation of sea salt.
PANI has recently become the preferred choice for the preparation of electrocatalytic materials because of its simple monomer, convenient synthesis, strong plasticity, high conductivity and mechanical strength and stable chemical properties [80,81] . For example, Sun et al. [82] synthesized PANI@Co-Fe LDH-layered nanomaterials by a hydrothermal method, benefiting from the excellent conductivity of PANI and the twodimensional layered nanosheet structure of Co-Fe LDHs, it delivered superior OER electrocatalytic activity (ƞ 10 = 261 mV and ζ = 67.85 mV dec -1 ). Dang et al. [83] fabricated CoNiNDC/PANI-NF composites by reacting cobalt acetate and nickel acetate with 2,6-naphthalenediformate dipotassium and in-situ deposition of their products on a PANI-NF substrate. The PANI-NF could narrow the size of the CoNiNDC NSs and distribute them evenly on the surface, so that more active sites could be exposed to the outer surface, which would further contribute to the OER electrocatalytic activity.
In our research, we developed a facile and feasible strategy to realize the in-situ assembly of CoOOH NSs into a PANI network (Co/PANI NSs) for OER performance. The nitrogen species derived from PANI building blocks could function as bridging sites to preferentially coordinate with Co metal ions, which imparted coupling effects between CoOOH NSs and PANI, as well as structural stability. In addition to the Co-N coordination, the occurred electron delocalization between Co d-orbitals and PANI π-conjugated ligands could also modulate the electronic structural states of the Co/PANI HNSs, enabling the efficient interfacial electron transfer from CoOOH to PANI. Furthermore, the Co/PANI HNSs possessed a hierarchical porous with both mesopores and macropores that allowed electrolytes to be more efficiently transported to the highly oxidative active sites, resulting in fast reaction kinetics for the OER [84] . PANI had large amounts of amino and imino functional groups, which could provide lone-pair electrons and were easy to coordinate with transition metals, such as Co, so that the interaction between post-consumed PANI and hydroxides and metal oxides derived from these metals was enhanced, giving its composite surface excellent electronic structure and high structural stability [85,86] .
In our group, novel defect-induced nitrogen-doped carbon-supported Co 3 O 4 NPs were also successfully fabricated as OER electrocatalysts (denoted as Co 3 O 4 /CN HNPs) through a wetness-impregnation treatment of Co/PANI, followed by thermal annealing. This favorable architecture of the Co 3 O 4 /CN HNPs could not only improve their conductivity and electrocatalytically active sites but also generate a large number of oxygen vacancies and crystal defects. This effectively exerted the preponderance in facilitating the interfacial electronic transfer and optimizing the adsorption energy for intermediates, thus imparting the  [77] . (B) (1) Synthesis process and schematic structure of NiO/Co 3 O 4 @NC and (2) XPS spectra of Co 2p and Ni 2p. Reproduced with permission. Copyright 2017, American Chemical Society [78] . extraordinary activities in catalyzing the OER. In addition, there was evidence demonstrating the formation of C-N coordination bonds through the strong interaction of the interconnected interface and the generation of pyridinic-N species after the annealing treatment. These factors enabled the structural stability to obtain further strength and accelerated the oxygen release for the reduction of the OER overpotential, respectively [87] .

Cobalt phosphide
Although Co 3 O 4 has the highest stability and corrosion resistance in the OER process, its intrinsic catalytic performance is significantly lower than that of cobalt phosphide (CoP x ). This is because the electronegativity difference between the Co and P atoms in cobalt phosphide (Co 2 P, CoP and CoP 2 ) is very small, so it exhibits high inherent conductivity, resulting in stronger metal properties [88] . In addition, under the condition of anodic polarization, the surface of CoP x is oxidized to produce a CoO x active layer. These CoO x layers are mainly composed of cobalt (oxygen) hydroxyl species, similar to the β-CoOOH active intermediate in Co 3 O 4 , which is the main reason why water is easy to oxidize [89] . For example, Chang et al. [90] prepared a carbon-supported CoP nanorod electrocatalyst (CoP-NR/C). After anodic polarization, the surface of the CoP electrocatalyst was oxidized to nanosized CoO x , thus improving the OER electrocatalytic activity. Moreover, the catalytic performance of CoP x could be further enhanced by doping it with transition metals. The introduction of transition metals into the lattice could change the electronic structure of CoP x , so as to adjust the binding energy of OER intermediate species at the Co center site, which considerably reduced the free energy of HOO* and O* intermediate species in the OER rate-controlling step. Xiao et al. [91] identified that metal-doped CoP x electrocatalysts could produce excellent OER performance. Preeminent OER electrocatalysts could be obtained by doping with transition metal cobalt phosphide NSs (CoM-P-NS, M = Cu, Zn or Mn) [ Figure 6]. DFT calculations demonstrated that the incorporation of Ni into the CoP lattice could decrease the ΔGo* of the OER in the rate-controlling step from 1.62 to 1.57 eV, sequentially promoting the release of O 2 . Similarly, Mendoza-Garcia et al. [92] prepared (Co 0.54 Fe 0.46 ) 2 P nanoparticles doped with Fe and Co as electrocatalysts, revealing preferable OER electrocatalytic performance. Generally, the metal properties of CoP x make it a promising OER electrocatalyst and its activity can be adjusted by heteroatom doping strategies.

Cobalt sulfide
For OER cobalt-based electrocatalysts, although the water oxidation performance of cobalt sulfide is inferior to cobalt oxide or cobalt phosphide, it still remains the focus of cobalt-based electrocatalytic materials. At present, cobalt sulfide, such as Co 3 S 4 , Co 1-x S, CoS 2 , CoS and Co 9 S 8 , has been frequently utilized as OER electrocatalysts but their catalytic performance is severely restricted by low conductivity and reduced exposure of active sites. Therefore, increasing their specific surface area and complex synthesis with high conductivity heteroatom doped carbon matrix is the most effective strategy to address the above-mentioned challenges [72,73] . For example, Ganesan et al. [93] in situ grew cobalt sulfides of CoS 2 and Co 9 S 8 phases on N, S-Co-doped graphite oxide plates, showing good OER electrocatalytic performance. Similarly, Qiao et al. [94] confirmed that Co 1-x S hollow nanospheres (Co 1-x S/N, S-G) hybridized with graphene NSs doping with N and S heteroatoms had excellent electrocatalytic activity as OER electrocatalysts. This was mainly due to the hollow structure of cobalt sulfide nanoparticles in the electrocatalyst, with most of their active centers being completely exposed. Otherwise, O was directly doped in CoS x lattice, resulting in the formation of a Co active center ΔGo*, which obviously contributes to the further release of O 2 .
Cai et al. [95] synthesized an oxygen-doped cobalt sulfide porous nanocube (CoS 4.6 O 0.6 ) electrocatalyst [ Figure 7] and its OER catalytic activity was equivalent to the RuO 2 benchmark electrocatalyst. In addition to non-metallic doping, transition metal doping can also improve the OER properties of cobalt sulfide. For example, Cu doping could provide more O-binding sites for a Co 3 S 4 electrocatalyst and optimize its electronic structure, hence improving the catalytic performance of the OER. Chauhan et al. [96] prepared layered nanosheet aggregates of CuCo 2 S 4 as OER electrocatalysts, which exhibited a high specific surface area and interconnected nanosheet structure, thus ensuring the high conductivity and availability of active sites for excellent OER electrocatalytic performance. Sun et al. [97] first obtained self-supporting layered electrode materials by plating a PANI network layer on foam nickel and determined the growth position and orientation of CuCo 2 S 4 nanoarrays. The nitrogen species from PANI could be combined with metal ions as the bridge sites, which provided a strong coupling effect for the in-situ growth of CuCo 2 S 4 nanoarrays. Furthermore, the grid structure of PANI could divide the growth region into many smaller nanoblocks. Compared with the nitrogen sites evenly distributed on the mesh plane, only a small part of the nitrogen sites were located on the narrow fence structure, which made it difficult for CuCo 2 S 4 to grow to the fence area, thus limiting its self-growth space. These nitrogen sites evenly distributed on the reticular plane could make CuCo 2 S 4 grow perpendicular to its surface and limited their growth size simultaneously and this excellent structural feature could further enhance its OER electrocatalytic activity. Shit et al. [98] developed a noble metal-free cobalt sulfide nanoparticle-grafted porous organic polymer nanohybrid (CoS x @POP) as a photoelectrocatalyst. Its good photoelectrocatalytic activity for the HER could be attributed to the intrinsic synergistic effect of CoS x and POP, which formed a unique high-porosity CoS x @POP nanostructure. This structure permitted the easy diffusion of electrolytes and efficient electron transfer from POP to CoS x during hydrogen generation with a tunable bandgap, which straddled between the reduction and oxidation potential of H 2 O. Zhou et al. [99] prepared a CoS/BiVO 4 photoanode through surface modification by electrochemically modifying CoS onto the BiVO 4 surface. The resulting CoS/BiVO 4 photoanode exhibited a significantly enhanced photocurrent of 3.2 mA·cm -2 compared to a RHE under 1.23 V illumination, which was ~2.5 times higher than that of pristine BiVO 4 . This surficial modification strategy was proven to be a favorable method to effectively enhance the photoelectrocatalytic OER activity. .

Nickel-based electrocatalysts
Nickel and nickel alloys Due to Ni being a 3D transition metal with earth-abundant sources, Ni-based electrocatalysts have the advantages of low expenditure, excellent electrocatalytic activity, outstanding stability and a high alloying degree with other metals. Ni electrocatalysts have important applications in the catalytic reaction of many industrial processes, especially in the fields of hydrogenation, secondary alkaline battery and water electrolysis [100] . It has been demonstrated that although Ni electrocatalysts exhibit superior OER electrocatalytic activity, they are vulnerable to corrosion, thus leading to final degradation in the consecutive reaction process [101] . As a consequence, the OER electrocatalytic performance of Ni electrocatalysts could be further enhanced by covering a conductive carbon layer on the electrocatalyst or providing support to prevent its agglomeration. For example, Ramakrishnan et al. [102] encapsulated Ni nanoparticles derived from Ni organic complexes in nitrogen-doped mesoporous carbon nanostructures (NCNPs) [ Figure 8A], which displayed remarkable OER electrocatalytic performance in alkaline solutions. Li et al. [103] assembled FeNi bimetallic nanoparticles on a MOF-derived carbon matrix as an OER electrocatalyst (FexNiy-BDC, BDC = benzenedicarboxylate) [ Figure 8B]. In this regard, the porous structure of the MOF-derived carbon matrix provided a broad specific surface area and the electronic synergy between Fe and Ni stabilized the OER active components of this electrocatalyst. Therefore, the FexNiy-BDC electrocatalyst presented better OER electrocatalytic performance. Wang et al. [104] utilized a MOF-74 derived matrix to anchor NiCo/Fe 3 O 4 hybrid nanoparticles and used them as OER electrocatalysts [ Figure 8C]. DFT calculation showed that NiCo species could promote the stability of the OER active species in the electrocatalytic process. In addition, the synergistic effect between Fe 3 O 4 and NiCo in the NiCo/Fe 3 O 4 hybrid materials also helps to boost the OER electrochemical activity.

Nickel selenide
Matching metal Ni, nickel selenide (NiSe 2 ) has been examined for a wider range of applications and can easily be synthesized or grown with various porous nanostructures with high conductivity and specific surface area. NiSe 2 can be converted into nickel oxide and hydroxide at oxidation potentials as an active species of OER electrocatalysts. In addition, NiSe 2 deposited on a conductive carbon matrix or alloyed with other transition metals could significantly improve its OER electrocatalytic activity and stability [105] . Swesi et al. [106] fabricated metal-rich NiSe 2 on a gold-plated silicon substrate using an electrodeposition strategy. The electrodeposited NiSe 2 film containing Ni-Ni bonds had a phase structure similar to that of the sulfur mineral heap iron ore. Due to annealing at 300 °C, more Se defect sites were introduced, so the catalytic efficiency and overall crystallinity of the electrocatalyst were accordingly improved. Gu et al. [107] prepared iron-doped nickel selenide electrocatalysts (Fe-NiSe 2 -UNWs) and investigated their effect on the OER performance [ Figure 9]. The results demonstrated that in a 0.1 M KOH solution, the 8.4% Fe/NiSe 2 electrocatalyst had the best OER electrocatalytic performance (ƞ 10 = 268 mV and ζ = 41 mV dec -1 ). According to DFT calculations, Fe doping could adjust the electronic structure of NiSe 2 , effectively reduce the energy barrier of OER intermediate species and improve the corresponding catalytic performance.
Liu et al. [108] developed a nickel selenide electrode with PANI surface functionalization (NiSe-PANI) [ Figure 10]. The modified PANI layer finely modulated the surface electronic structure of NiSe, optimized the surface Se-rich structure of NiSe and improved the formation of Ni 3+ active species. When NiSe-PANI was used as a bifunctional electrocatalyst for total electrolytic water reaction, the NiSe-PANI electrode exhibited significant electrocatalytic activity (ƞ 10 = 300 mV), equivalent to the performance of Pt and IrO 2 combined electrodes. Similarly, other PANI-functionalized nickel sulfur electrodes also exhibited good total  [107] .
electrolytic water reaction performance, which proved that the electronic modulation strategy of PANI surface functionalization had universal applicability for improving the intrinsic OER electrochemical activity. Hou et al. [109] reported a 3D hybrid electrocatalyst that was constructed through the in situ anchoring of Co 9 S 8 NSs onto the surface of Ni 3 Se 2 NSs vertically aligned on an electrochemically exfoliated graphene foil. Benefiting from the synergistic effect between Ni 3 Se 2 and Co 9 S 8 , it could be easily integrated with a macroporous silicon photocathode for highly active solar-driven photoelectrocatalytic water-splitting for the OER. Lee et al. [110] developed three possible polymorphic forms of nickel selenide (orthorhombic NiSe 2 , cubic NiSe and hexagonal NiSe) as bifunctional electrocatalysts for photoelectrocatalytic systems. Photocathodes or photoanodes were fabricated by depositing the nickel selenide NCs onto p-or n-type Si nanowire arrays. Experiments revealed that compared to the other two types, the orthorhombic NiSe 2 NCs were more metallic and formed fewer surface oxides, which increased the photocurrent and transfer onset potential, resulting in better photoelectrocatalytic performance and exerted efficient water-splitting ability.

Nickel hydroxide and hydroxyl oxides
Among the nickel-based materials that catalyze OER, nickel hydroxide and its hydroxyl oxides (collectively  [108] . referred to as NiOx) are a class of electrocatalytic materials with excellent application prospects [111] . Since the sluggish kinetics of the OER are the main limitations of the performance of hydrogen production from electrolytic water, improving the catalytic performance of NiOx on the OER has been the focus of major research [112] . In order to better understand the effect of NiOx species on the catalytic performance of the OER, it is very important to analyze the structural and phase changes in the water oxidation reaction. Luan et al. [113] synthesized a 2D-Ni(OH) 2 catalytic material by a layered reverse micelle method because the 2D-Ni(OH) 2 phase could be formed at an anodic potential γ-NiOOH active phase, thus showing better OER catalytic activity and stability. Gao et al. [114] controllably prepared α-Ni(OH) 2 and β-Ni(OH) 2 by a solvothermal system and compared their OER catalytic activities. The results indicated that α-Ni(OH) 2 than β-Ni(OH) 2 presented better OER activity and higher stability [ Figure 11], which was mainly due to the formation of γ-NiOOH through α-Ni(OH) 2 , thus promoting the diffusion of OER intermediates and the corresponding conversion to O 2 . After characterizing and analyzing the surface morphology of α-Ni(OH) 2  [113] . and β-Ni(OH) 2 for 500 CV cycles by TEM, it was demonstrated that the α-Ni(OH) 2 phase could be easily transformed into γ-NiOOH without material expansion or mechanical deformation, whereas for β-Ni(OH) 2 , a large number of nanoparticles were observed to accumulate and the surface was severely corroded after CV cycling, manifesting that β-Ni(OH) 2 had weak structural stability during the OER process due to Jahn-Teller distortion (spontaneous symmetry break in solid-state system) at the low spin d 7 Ni 3+ center. Similarly, Dou et al. [111] compared the OER activities of only γ-NiOOH with both containing γ-NiOOH and β-NiOOH. It was found that the electrocatalytic activity of γ-NiOOH for OER was much higher than that of NiOOH mixed with γ-and β-phases. The high OER electrochemical activity of γ-NiOOH phase was attributed to its large layer spacing, which facilitated the diffusion of electrolytes and the desorption of O 2 molecules on the electrocatalyst surface.
Combined with the above discussion on the OER activities of β-NiOOH and γ-NiOOH, Li et al. [115] directly confirmed that the OER catalytic activity of γ-NiOOH was better than β-NiOOH. Nevertheless, γ-NiOOH or β-NiOOH with high activity on OER was still controversial. Some research has elucidated that γ-NiOOH is the active center of the OER, while other researchers have claimed that β-NiOOH is the active center of the OER. Therefore, all these contradictory studies on NiOx active centers illustrate that, in addition to the active phase, the OER activity of NiOx electrocatalyst also depends on its preparation method, initial precursor (mainly containing trace transition metal impurities, such as Fe, Ce, Cd, Pb and Zn), electrode cycle, morphology, active surface area and electrolyte [116,117] . More specifically, Corrigan et al. [116] found that the inclusion of Fe into the Ni(OH) 2 could significantly accelerate OER kinetics, while doping other transition metal elements, such as Cd, Pb and Zn, hindered the catalytic performance of the OER. According to the experimental observation, the significant effect of Fe on the OER activity of Ni(OH) 2 was mainly associated with the change of electrocatalyst conductivity and the formation of the active center, which was more favorable to catalyzing OER intermediates. Correspondingly, Trotochaud et al. [117] detected Fe impurities in NiOOH from an analytical grade KOH solution. They found that the presence of a small amount of Fe impurities in NiOOH could significantly promote the OER activity, while NiOOH exhibited inferior OER activity in a Fe-free KOH electrolyte (raw KOH was completely purified from Fe impurities).
Many theoretical and experimental groups have researched the role of Fe into Ni(OH) 2 and NiOOH for the OER activity to judge whether Fe itself acts as the active center or just activated Ni(OH) 2 and NiOOH to obtain better OER performance [118] . Friebel et al. [119] carried out DFT calculations of the OER on γ-NiOOH and Fe-doped γ-NiOOH. The results demonstrated that the overpotential of Fe-doped γ-NiOOH to the OER (0.43 V) was much lower than that of γ-NiOOH (0.56V) and Fe in Ni 1-x Fe x OOH was the active center of the OER. In addition, the effect of Fe on the structure of Ni(OH) 2 /NiOOH electrocatalyst was investigated by EXAFS characterization. The results concluded that the addition of Fe did not change the Ni-O bond length but the Fe-O bond length exhibited an obvious shrinkage in the oxidation of Ni 1-x Fe x OOH, which optimized the binding adsorption energy of OER intermediates and reduced the OER overpotential. Wu et al. [120] compared the OER catalytic activities of NiO/NF and Fe-doped NiO/NF (Fe 11% -NiO/NF). The results presented that Fe 11% -NiO/NF had better OER electrocatalytic performance and higher stability than NiO/NF. The Fe doping of Fe 11% -NiO/NF electrocatalyst and its mesoporous structure was mainly responsible for the excellent OER catalytic performance [ Figure 12]. Among them, the mesoporous NS structure of NF provided rich open space for facilitating the diffusion of electrolytes and the close contact between electrolytes and electrocatalysts. More importantly, Fe doping reduced the energy barrier of molecular oxygen generated by OER intermediates.
Li et al. [121] compared the OER activities of NiFe-LDH and Ni (OH) 2 NS electrocatalysts. The results showed that the OER activity of NiFe-LDH was smaller than that of Ni(OH) 2 NSs. Furthermore, they also researched the effect of vanadium doping into the NiFe LDH (NiFeV-LDH) electrocatalyst on OER activity. The results disclosed that NiFeV-LDHs delivered high OER catalytic activity and stability. The result was attributed to the reason that the electronic structure of the whole electrocatalyst was changed by vanadium doping, which improved its conductivity and the number of catalytic active sites. Li et al. [122] deposited double oxygen evolution catalyst (OEC) layers (FeOOH and NiOOH) over the nanotube array-like WO 3 (WA) surface to form a WA-OEC photoanode. Therein, FeOOH greatly reduced the WA/OEC interface electron-hole pairs recombination rate, while NiOOH restricted the recombination of electron-hole pairs at the OEC/electrolyte interface and significantly improved the corresponding OER activity. The WA/OEC photoanode had a photocurrent density of 120 μA·cm -2 under simulated sunlight illumination, showing a good photoelectrocatalytic water-splitting efficiency. Wei et al. [123] composited NiOOH with CdS as a photoelectrocatalyst (CdS/NiOOH). After a 3600 s photoelectrocatalytic stability test, the stability of CdS/NiOOH was significantly improved by 44.50% compared with pure CdS due to NiOOH preventing the oxidation of CdS by trapping photogenerated holes. Furthermore, the deposition of NiOOH was more beneficial in accelerating the separation of photogenerated carriers, thereby enhancing the photoelectrocatalytic activity for the water-splitting process. Pirkarami et al. [124] synthesized a novel 3D CdS@NiCo-LDH material as a cost-effective, bifunctional and efficient photoelectrocatalyst for water splitting, which had the advantages of high specific surface area, fast electron transfer and multiple channels  [120] .
to release gaseous products, resulting in better electrocatalytic OER activity in alkaline environments. When using this electrocatalyst for the HER, it could achieve current density values of 10 and 100 mA·cm -2 at voltages of 379 and 202 mV, respectively.
Nickel is the fourth largest metal element in terms of reserves and has the same main group as platinum with high electrocatalytic activity. Therefore, research into Ni-based materials as HER electrocatalysts has proved both popular and fruitful [125] . For example, Zhang et al. [126] successfully prepared NiCo 2 P x nanowires (NWs) and after conducting characterization and performing experiments, concluded that for the HER in 1.0 M KOH, the NiCo 2 P x NWs delivered negligible attenuation at 250 mA·cm -2 after 5000 CV cycles, indicating reasonably high durability. They also showed relatively high HER activity (ƞ = 58 mV at 10 mA·cm -2 ) and negligible attenuation at 30 h under an overpotential of ƞ = 100 mV, indicating its ultralong stability [ Figure 13]. Li et al. [127] obtained porous sea urchin-like Ni 0.5 Co 0.5 P by the combination of calcinating and phosphating treatment of NiCo(CO 3 )(OH) 2 in an Ar atmosphere. For the HER in 1.0 M KOH, the electrocatalyst presented negligible attenuation after 1000 CV cycles, indicating its high durability and activity for the HER (ƞ of 87 mV at 10 mA·cm -2 ). Wang et al. [128] synthesized a Ni-reduced GO (rGO) nanostructure using an electrodeposition strategy in a supergravity field. Benefiting from its high surface area, good electrical conductivity and synergistic effect between Ni nanoparticles and rGO sheets, the Ni-rGO exhibited reasonable stability at 250/100 mA·cm -2 for the HER during a durability test in 1.0 M NaOH. It also showed excellent HER activity (ƞ of 36 mV at 10 mA·cm -2 ). Similarly, Ni/C 3 N 4 nanostructured composites prepared by electrodeposition under a supergravity field could also improve the stability and activity for the HER. Wang et al. [129] reported that nanostructured Ni/C 3 N 4 exhibited high stability and activity for the HER in 1.0 M NaOH. More specifically, the Ni/C 3 N 4 presented high HER activity (ƞ of 222 mV at 10 mA·cm -2 ) and negligible attenuation at 100 mA·cm -2 within 12 h.
In addition, like iron, nickel phosphide and sulfide also have good HER electrocatalytic activity. For example, Wang et al. [130] prepared Ni(OH) 2 •0.75H 2 O by a hydrothermal method and then phosphated the nanostructured Ni 5 P 4 in a N 2 atmosphere at 370 °C. The electrocatalyst exhibited high conductivity, which significantly enhanced its high activity and stability for the HER. For the HER in 1.0 M KOH, Ni 5 P 4 showed negligible decay after 3600 CV cycles, indicating its very high durability and HER activity (ƞ of 47 mV at 10 mA·cm -2 ). Wang et al. [131] reported that the high HER activity of Ni x P y could be achieved by simply growing in 3D-NF. 3D-NF had a porous layered structure and large specific surface area, which significantly reduced the diffusion path length of ions and improved the electron and ion conductivity in the HER process. In addition, Lado et al. [132] found that Al-doped Ni-P (AlNiP) provided higher HER catalytic activity (ƞ 10 = 111 mV) than pure NiP (ƞ 10 = 175 mV). This indicated that the doping of Al changed the state density of NiP to reduce the energy barrier of the HER dynamics.
Tong et al. [133] prepared Ni 3 S 2 by a hydrothermal method and showed that Ni 3 S 2 nanorods@Ni 3 S 2 NSs exhibited higher HER activity than Ni 3 S 2 nanorods. Ni 3 S 2 formed by a nanorod@nanosheet homojunction can provide abundant active sites to reduce electron transport and promote gas release, which might enhance its high durability and activity for the HER [ Figure 14]. For the HER in 1.0 M KOH, Ni 3 S 2 showed negligible attenuation after 10,000 CV cycles, indicating very high durability, it also delivered very high HER activity (ƞ of 48.1 mV at 10 mA·cm -2 ) and reasonable stability at 48 mV for 24 h. Yang et al. [134] first fabricated nanoporous Cu by electrodeposition and etching and finally prepared nanoporous Ni 3 S 2 @Cu by an electrodisplacement strategy. The porous structure of the electrocatalyst provided a large number of active sites, which enhanced its catalytic activity and durability for the HER. For the HER in 1.0 M KOH, the Ni 3 S 2 @Cu activated catalyst exhibited negligible decay at 200 mA·cm -2 after 2000 CV cycles, which indicated its much high durability, and much higher HER activity (ƞ of 60.8 mV at 10 mA·cm -2 ). In addition, Long et al. [135] synthesized Fe-Ni ultrathin NSs as an acidic HER electrocatalyst, which exhibited excellent HER activity (ƞ 10 = 105 mV and ζ = 40 mV dec -1 ) and high stability. Qu et al. [136] developed vanadium-doped Ni 3 S 2 NW on NF, which presented good stability of ƞ 10 = 68 mV and 8000 CV cycles in alkaline solution. In addition, Wang et al. [137] synthesized a platinum nickel/nickel sulfide nanowire (Pt 3 Ni/NiS nanowire) as a HER electrocatalyst, which displayed j = 37.2 mA·cm -2 in ƞ = 70 mV in a 1 M KOH solution, 9.7 times better than 20% Pt/C.
It is precisely because these transition metals (Fe, Co and Ni) have the advantages of low expenditure, chemical stability, unique and adjustable electronic structure, rich redox states and high intrinsic activity that they are not only regarded as the best potential alternatives to replace traditional noble-metal-contained Copyright 2017, Wiley [126] .  [133] .
OER/HER electrocatalysts but can also be used as energy storage materials in the field of clean energy, such as lithium-ion batteries, fuel cells and supercapacitors [138] . Although some important research progress has been made in the preparation and theory of these metals and some matrix materials as functional materials, there still exist several problems to be resolved. On the one hand, due to the facile aggregation characteristics of these nanomaterials, the exposed quantity and quality of functional active sites are undesirable. On the other hand, due to the characteristics of the metastable structure of quantum-level nanomaterials, their structures were vulnerable to structural collapse and agglomeration. These phenomena could easily cause the functional exposed active sites to be covered and inactivated. Therefore, it still remained a big challenge to resolve the high-quality and high-density active site exposure and structural stabilities of these materials employed in these fields.

CONCLUSION AND OUTLOOK
The water-splitting electrolyzer is regarded as a promising technology for dissociating water into highpurity hydrogen in a low-expenditure, zero-pollution emission and high-efficiency manner. Potential strategies for designing the earth-abundant first-row transition metal (Fe, Co and Ni)-based composite materials as electrocatalysts to substitute noble-metal-based electrocatalysts have been profoundly discussed in this review. Noticeably, the recently developed catalysts of these categories have exhibited excellent OER electrocatalytic activities, which are comparable or superior to precious-metal-based electrocatalysts (Ru, Ir, RuO 2 and IrO 2 ). Moreover, the influence of morphologies, compositions and heteroatom doping on the OER kinetics have been described. Highly efficient electrocatalysts made of Fe and Ni-based compositions are also extensively applicable to the field of the HER and have been elaborated in a short separate section, which differentiates the strategy in making good Fe and Ni HER electrocatalysts from the OER, demonstrating the wide applicability of these Fe and Ni-based nanocomposites. Nevertheless, the fabrication of high-performance water-splitting electrocatalysts remains at a preliminary stage and future works are needed to continue to develop novel high-performance OER electrocatalysts.
Realistically, there are some investigations illustrating that transition metal nitride nanomaterials are regarded as the best potential materials to replace traditional noble metal-based electrocatalysts because of their low-expenditure, chemical stability, unique and adjustable electronic structure, rich redox states and high electrocatalytic intrinsic activity. In this regard, the selection of appropriate precursors is an important prerequisite for the design and optimization of transition metal nitride electrocatalyst synthesis routes. Nitrogen-containing polymers (NCPs) are considered as appropriate precursors for the preparation of transition metal nitride. This is because NCPs can effectively coordinate with transition metal ions (TMI) to form a strong TM-N bond, which is more conducive to strengthening the anchoring and dispersion of transition metal ions. Therefore, M/NCP precursors prepared by impregnation methods (M = Fe, Co or Ni) can convert TM-N bonds into M-N-C bonds in MN x /C under high-temperature carbonization treatment. The active site of this nanostructure not only has high stability and corrosion resistance under alkaline conditions, but also has high OER intrinsic catalytic activity. MN x /CN electrocatalyst carbonized from M/NCP precursor exhibits high conductivity, so it strengthens the electrical transfer capability of OER process. Therefore, rendering NCPs as C and N sources to prepare MN x /CN electrocatalysts with different metal coordination environments by controlling carbonization temperature and metal loading has become an important means to obtain efficient OER electrocatalysts. Furthermore, NCPs can be used as precursors to prepare high-density M-N-C single-atom electrocatalyst. The electrocatalyst with this structure can not only fully improve the atomic utilization efficiency of metal but also exposes the active sites of metal to the greatest extent and promote the occurrence of catalyzing the OER. In addition, a single regular M-N-C single-atom electrocatalyst is helpful to reveal the "structure-activity" relationship between its active site structure and OER electrocatalytic activity. Furthermore, NCPs can also serve as precursors to preparing high-dispersed supported bimetallic electrocatalysts. The synergistic effect between metals can adjust the electronic state of the electrocatalyst surface, as well as simultaneously helping to strengthen the adsorption energy of OER intermediate species, so as to improve the integrated electrochemical performance of the OER.

Authors' contributions
Conceived the manuscript: Chen X, Liu J