Host-Guest Engineering of Layered Double Hydroxides towards Efficient Oxygen Evolution Reaction : Recent Advances and Perspectives

Electrochemical water splitting has great potential in the storage of intermittent energy from the sun, wind, or other renewable sources for sustainable clean energy applications. However, the anodic oxygen evolution reaction (OER) usually determines the efficiency of practical water electrolysis due to its sluggish four-electron process. Layered double hydroxides (LDHs) have attracted increasing attention as one of the ideal and promising electrocatalysts for water oxidation due to their excellent activity, high stability in basic conditions, as well as their earth-abundant compositions. In this review, we discuss the recent progress on LDH-based OER electrocatalysts in terms of active sites, host-guest engineering, and catalytic performances. Moreover, further developments and challenges in developing promising electrocatalysts based on LDHs are discussed from the viewpoint of molecular design and engineering.


Introduction
Electrochemical water splitting holds great promise for clean energy resources and has aroused broad study interest in recent years [1][2][3][4][5].Among all the studies, the development of electrocatalysts for the anode oxygen evolution reaction (OER) is one of the key issues to decrease the overpotential of practical water splitting due to its sluggish four-electron process [6][7][8][9][10].It is well-known that ruthenium and iridium oxides demonstrate high activity for water oxidation in acid and alkaline electrolytes, respectively [11,12].However, an efficient alternative is still needed because of the high cost and scarcity of noble metal-based catalysts, which is difficult to meet the large-scale applications.Recently, various transition metal compounds (e.g., oxides, [13][14][15], hydroxides [16][17][18][19][20], and phosphides [21][22][23][24]) have emerged as a new family of OER electrocatalysts.Especially, the homogeneous mixed-transition-metal compounds without phase segregation have been reported with higher OER activity, probably owing to the effectively modulated 3d electronic structures.For instance, NiFe-based electrocatalysts have become a kind of dazzling material attributed to their high OER activity, since first investigated by Corrigan in the 1980s [25], and significantly promoted by Dai in 2013 [26].
Layered double hydroxides (LDHs) are a large class of two-dimensional (2D) intercalated materials which can be described by the general formula [M II  1−x M III x (OH) 2 ] z+ (A n− ) z/n •yH 2 O (M II and M III are divalent and trivalent metals respectively; A n− is the interlayer anion compensating for the positive charge of the brucite-like layers) [27][28][29].Recently, LDHs, especially NiFe-LDH, are believed to be one of the ideal and promising electrocatalysts for water splitting due to their excellent OER activity, high stability in basic conditions, and low cost [30][31][32][33][34].To date, various LDHs, as well as their nanocomposites, have been synthesized for better OER performances.For example, the design and synthesis of LDHs/conducting-material composites can overcome the intrinsically poor conductivity of LDHs and provide a rapid transport of electrons/ions [35][36][37][38][39][40][41][42].To improve the intrinsic activity, the role of host transition layers and guest interlayer anions in increasing the OER activity have also been considered [43][44][45][46][47][48][49].Boettcher et al. found that the incorporation of Fe shows a more than 30-fold increase in conductivity, as well as a partial-charge transfer activation effect of Fe to Ni sites [50].Jin et al. detected the presence of Fe IV in NiFe-LDH during steady-state water oxidation by using operando Mössbauer spectroscopy [51], which has important implications for stabilizing the NiOOH lattice.Although, with this progress, how to determine the real active sites, as well as how to rationally design much more efficient electrocatalysts based on LDHs, still remains highly desirable and challenging.
Along with the increasing development of LDH-based OER catalysts, some important reviews on different aspects have already been reported.For example, Dai et al. first gave a mini review about NiFe-based materials (including alloys, oxides, and hydroxides) for OER in 2015 [52], where the related mechanism and applications have been briefly discussed.Our group summarized the development of LDH materials for electrochemical energy storage and conversion [53], in which the OER applications have been discussed from the viewpoint of electrode materials design.Strasser et al. further published a review article focusing on the progresses of NiFe-based (Oxy)hydroxide catalysts [54].Other excellent reviews also mentioned LDH-based OER electrocatalysts, including the perspective on the OER activity trends and design principles based on transition metal oxides and (oxy)hydroxides by Boettcher et al., [55] and nanocarbon-based electrocatalysts summarized by Zhang et al., [56].
Nevertheless, the discussion on the LDH-based OER catalysts from the viewpoint of their supramolecular intercalated structures is seldom considered.In this review, we will focus on the roles of the host layer and guest interlayer anions in the OER performances of LDHs.The recent advances in the host layer designs will be first discussed in order to provide a systematic digestion of previous achievements in the unveiling of the active nature of metal ions in promoting water oxidation.The interlayer anions also play an indispensable synergistic effect, which will be demonstrated in the next discussion.We also hope to display future efficient OER electrocatalysts based on LDHs from the molecular design and engineering.

The Role of Host Layer Metal Ions
Electrochemical water splitting under both acidic and alkaline conditions has been studied for more than a half century as a means of storing clear energy.It is noted that OER is a four electron-proton coupled reaction, while hydrogen evolution reaction (HER) is only a two electron-transfer reaction, which implies that a higher energy is required for OER process to overcome the reaction barrier.In the past decades, nickel-based electrodes have been widely used as anode catalysts for alkaline water splitting due to the merits of earth-abundance, high activity, and good stability [57][58][59][60][61]. Corrigan found that a low concentration of iron impurity in nickel oxide increased the OER kinetics obviously [25].Moreover, the oxygen evolution overpotential for the sample with iron impurities of 10-50% in the nickel oxide is substantially lower than that of either nickel oxide or iron oxide.Although without a clear understanding about this interesting phenomenon, a synergetic effect between nickel and iron species for catalyzing the OER process has been rationally assumed.This inspired the intensive investigations to study various NiFe mixed compounds and the catalytic roles for Ni and Fe in order to obtain better OER electrocatalysts.As a family of typical two-dimensional inorganic materials, LDHs consist of brucite-like [Mg(OH) 2 ] host nanosheets with edge-sharing metal-O 6 octahedra (lateral particle size ranging from nanometer to micrometer-scale).The easy tunability of metal ions without altering the structure, as well as anion exchange properties of LDHs, make them interesting alternatives for applications in electrochemistry.Bell et al. synthesized (Ni,Fe) oxyhydroxides across the entire composition range (Figure 1a,b) [62].It was found that the addition of 25% Fe to Ni(OH) 2 results in up to a 500-fold higher OER current density compared to pure Ni and Fe oxyhydroxide films at the overpotential of 0.3 V. Zhang et al. also investigated the effect of different Ni/Fe molar ratios to their OER performances [63].It was found that moderate metal substitution into the host hydroxide framework (Fe into Ni or Ni into Fe) substantially enhanced the OER activity with a decrease of both the Tafel slope and overpotential.To further accurately control the Fe content, Boettcher et al. developed a method for purification of KOH electrolyte by using precipitated bulk Ni(OH) 2 to absorb Fe impurities [50].As a result, no significant OER current is observed until >400 mV overpotential for Ni(OH) 2 films aged in purified KOH electrolyte.The excellent conductivity of electrocatalysts is important to achieve a fast charge transfer through the catalyst film with a negligible potential drop.To study the conductivity trends of LDHs with different metal ion ratios, various NiFe-LDH have been synthesized by using the purified KOH electrolyte.The conductivity measurement showed that all NiFe-LDH films have low conductivities at low potential, while a sharp increase along with the Ni oxidation (Figure 1c).The NiFe-LDH films with Fe content range from 5% to 25% gives σ ≈ 3.5 to 6.5 mS cm −1 , significantly higher than that of Fe-free film (σ ≈ 2.5 mS cm −1 ), indicating that Fe increases the conductivity of NiOOH. Figure 1d [50].As a result, no significant OER current is observed until >400 mV overpotential for Ni(OH)2 films aged in purified KOH electrolyte.The excellent conductivity of electrocatalysts is important to achieve a fast charge transfer through the catalyst film with a negligible potential drop.
To study the conductivity trends of LDHs with different metal ion ratios, various NiFe-LDH have been synthesized by using the purified KOH electrolyte.The conductivity measurement showed that all NiFe-LDH films have low conductivities at low potential, while a sharp increase along with the Ni oxidation (Figure 1c).The NiFe-LDH films with Fe content range from 5% to 25% gives σ ≈ 3.5 to 6.5 mS cm −1 , significantly higher than that of Fe-free film (σ ≈ 2.5 mS cm −1 ), indicating that Fe increases the conductivity of NiOOH.In addition to the enhanced conductivity, the catalytically-active metal redox state of the NiFe-based catalyst has remained under debate.It has long been assumed that Ni is the reactive site for water oxidation in NiFe oxide electrocatalysts on the basis of the high activity of Ni oxide electrocatalysts.For example, Strasser et al. found that the Fe centers consistently remain in the Fe III state regardless of potential and composition [64], which was determined by operando differential electrochemical mass spectrometry (DEMS) and X-ray absorption spectroscopy (XAS) under OER conditions.On the other hand, Ni IV at Fe content below 4% has been detected under catalytic conditions, while Ni atoms stabilized in a low-valent oxidation state when further increasing the content of Fe.This difference in metal valent states mainly depends on the rate of water oxidation−metal-reduction (k OER ) and metal oxidation (k Mox ) (Figure 2a).The lower k Mox /k OER ratio reflects a dramatically increased rate constant of water oxidation (k Mox ), which may exceed the rate of the metal oxidation (k OER ).It concluded that high catalytic OER activity of the mixed Ni−Fe catalysts demonstrate a sharply-decreased k Mox /k OER ratio.They further give a catalytic OER cycle (Figure 2b), where the buildup of oxidation equivalents from Ni II to Ni IV sites is followed by the O−O bond formation with the subsequent release of molecular oxygen.However, there is substantial evidence from X-ray absorption and Mössbauer experiments that Ni IV and Fe IV are both found in NiFe-LDH at OER potentials during different studies [51,65,66].Recently, Stahl et al. detected the Fe IV species (up to 21% of the total Fe) during steady-state water oxidation on NiFe-LDH [51].The stable presence of Fe IV can be ascribed to the increased electron-donating ability of the π-symmetry lone pairs of the bridging oxygen atoms between Ni and Fe (Figure 2c,d), which makes the NiOOH lattice a more stable environment for high-valent metal ions.Based on the previous results and the above discussion, the brief conclusions can be obtained: (1) iron impurity in nickel oxide and hydroxide significantly promote the electrocatalytic water oxidation, and suitable Ni/Fe ratio (from 2:1 to 4:1) can further improve OER activity; (2) the conductivity of Ni(OH) 2 /NiOOH increases when combined with iron element at a suitable level (from 5% to 25%) due to the Fe-induced charge transfer; (3) the Ni or Fe as active sites have both been reported, which has been verified by the detected high valence states of Ni IV or Fe IV , while the presence of high valence metal ions probably depends on the metal and water oxidation rate.Therefore, different metal ions introduced into the LDH host layer induces a varied chemical and electronic environment, which thereby varied their OER performances.In addition to NiFe-LDH, other LDHs with various host metal ions (e.g., NiCo, NiMn, ZnCo, and CoAl) have been demonstrated as OER electrocatalysts (Figure 3) .However, their activity is still lower than that of NiFe-LDH.

The Exfoliation of LDHs
The electrochemical properties of an electrocatalyst are affected by its nanostructures [92][93][94][95].For instance, the atomically-thin 2D inorganic materials usually demonstrate unique properties compared with their bulk counterparts [96][97][98][99][100].LDHs are composed of atomically-thick positive brucite-like host layers and interlayer anions.In practice, LDHs are stacked with several layers, which limits their electrochemical performances due to the inaccessibility to the inner surfaces of the host layers.In the past decades, ultrathin LDH nanosheets with atomic thickness have been synthesized by both "bottom-up" and "top-down" approaches [101][102][103][104][105][106], which provide opportunities in maximizing the utility of the layers and improving their physicochemical properties (e.g., specific surface area and conductivity).Particularly, the "top-down" delamination method is the most widely developed for producing thin LDH platelets with a thickness of a few atomic layers.Hu et al. first used the exfoliation strategy to promote the OER performances of LDHs [91].The CoCo, NiCo, and NiFe-LDH with Br -anions are prepared as representative LDH materials, which are exfoliated into single-layer nanosheets in the formamide solution.The OER current densities at an overpotential of 300 mV were enhanced by 2.6-, 3.4-, and 4.5-fold upon exfoliation of CoCo, NiCo, and NiFe-LDH, respectively, compared with their bulk materials.Additionally, the water oxidation activity has the order of NiFe > NiCo > CoCo for both exfoliated nanosheets and bulk LDHs.Following this work, they further synthesized ultrathin CoMn-LDH nanoplatelets (3-5 nm) by a coprecipitation method [80], which gives a current density of 42.5 mA cm −2 at η = 350 mV.This value is about 7.6, 22.5, and 2.8 times higher than that of Co(OH)2 + Mn2O3, spinel MnCo2O4+δ, and IrO2, respectively.
Exfoliated ultrathin LDH nanosheets display enhanced active site exposure.However, the

The Exfoliation of LDHs
The electrochemical properties of an electrocatalyst are affected by its nanostructures [92][93][94][95].For instance, the atomically-thin 2D inorganic materials usually demonstrate unique properties compared with their bulk counterparts [96][97][98][99][100].LDHs are composed of atomically-thick positive brucite-like host layers and interlayer anions.In practice, LDHs are stacked with several layers, which limits their electrochemical performances due to the inaccessibility to the inner surfaces of the host layers.In the past decades, ultrathin LDH nanosheets with atomic thickness have been synthesized by both "bottom-up" and "top-down" approaches [101][102][103][104][105][106], which provide opportunities in maximizing the utility of the layers and improving their physicochemical properties (e.g., specific surface area and conductivity).Particularly, the "top-down" delamination method is the most widely developed for producing thin LDH platelets with a thickness of a few atomic layers.Hu et al. first used the exfoliation strategy to promote the OER performances of LDHs [91].The CoCo, NiCo, and NiFe-LDH with Br − anions are prepared as representative LDH materials, which are exfoliated into single-layer nanosheets in the formamide solution.The OER current densities at an overpotential of 300 mV were enhanced by 2.6-, 3.4-, and 4.5-fold upon exfoliation of CoCo, NiCo, and NiFe-LDH, respectively, compared with their bulk materials.Additionally, the water oxidation activity has the order of NiFe > NiCo > CoCo for both exfoliated nanosheets and bulk LDHs.Following this work, they further synthesized ultrathin CoMn-LDH nanoplatelets (3-5 nm) by a coprecipitation method [80], which gives a current density of 42.5 mA cm −2 at η = 350 mV.This value is about 7.6, 22.5, and 2.8 times higher than that of Co(OH) 2 + Mn 2 O 3 , spinel MnCo 2 O 4+δ , and IrO 2 , respectively.
Exfoliated ultrathin LDH nanosheets display enhanced active site exposure.However, the LDHs' exfoliation in liquid usually suffers from strong adsorption of solvent molecules, as well as the restacking when removing the surface solvent [107,108].In addition to liquid exfoliation of LDHs, Wang et al. developed an efficient strategy for the exfoliation of LDHs into stable and clean ultrathin nanosheets by plasma etching [78].The high-energy plasma destroys the ionic bonds and hydrogen bonds in the interlayers of the bulk LDHs, which interrupted the host-guest charge balance and separated the brucite-like host layers from each other.For instance, the thickness of CoFe-LDH have been successfully decreased from ~20 nm to 0.6 nm by subjecting bulk CoFe-LDH to Ar plasma etching for 60 min.Moreover, the coordination number of the Co-O OH octahedra is lower than that in the bulk CoFe-LDH, suggesting the presence of oxygen vacancies (VO).The as-prepared LDH ultrathin nanosheets demonstrate much-improved OER performance with a low overpotential of 266 mV at 10 mA cm −2 .Recently, they further reported the exfoliation of CoFe-LDH by a water plasma-assisted strategy (Figure 4a) [109], which was accompanied with the formation of multi-vacancies, including O, Co, and Fe vacancies.The as-exfoliated ultrathin LDHs nanosheets with multi-vacancies show significantly promoted electrocatalytic activity for water oxidation.As shown in Figure 4b, water-plasma exfoliated CoFe-LDH nanosheets just require a low overpotential of 290 mV to reach 10 mA cm −2 while the pristine CoFe-LDH need an overpotential of 332 mV.Therefore, the effective exfoliation, as well as the defect introduction, both promotes the OER activity of LDHs.

Construction of LDH Nanoarrays
LDH nanosheet arrays (NSAs), that have highly-dispersed nanoplatelets, well-uniformed orientation, and improved conductivity compared with LDH powdered samples, have been recently constructed as efficient OER electrocatalysts [110][111][112][113][114][115][116][117][118].Various LDH NSAs have been perpendicularly grown on the surface of conducting substrates (metals [115], conducting glasses [116], carbon fibers [117], and papers [118]) by in situ procedures.One of the most effective methods for the fabrication of LDH NSAs is the hydrothermal process [119,120].To design an highly-active OER electrocatalyst, Huang et al. reported a single-crystalline NiFe-LDH NSA array on a Ni foam with the assistance of a direction agent of NH 4 F [120].The top and cross-sectional Scanning Electron Microscope (SEM) images of NiFe-LDH NSAs reveal a highly-oriented flake array nanostructure that is in vertical contact with the substrate (Figure 5a,b), with an edge length of 1-3 µm and a uniform thickness of less than 20 nm.The high-resolution transmission electron microscopy (HRTEM) image and corresponding selected area electron diffraction (SAED) pattern illustrate a single crystal phase of LDH (Figure 5c).The NiFe-LDH NSAs exhibits superior OER activity compared with the coated NiFe-LDH film, as well as a RuO 2 film electrode, achieving the overpotentials of 210 mV, 240 mV, and 260 mV at the current densities of 10, 50, and 100 mA cm −2 , respectively.Moreover, it is found that the single-crystalline NiFe-LDH arrays display smaller overpotentials than that of the reported amorphous NiFe materials and other analogous LDH-based materials.The hexamethylenetetramine (C 6 H 12 N 4 ) also can be used as a direct agent to prepare vertically-aligned LDH NSAs [72].For example, the NiFe-LDH NSAs grown on the nickel foam were created by an in situ co-precipitation approach using a reaction solution containing Ni(NO 3 ) 2 , Fe(NO 3 ) 3 , C 6 H 12 N 4 , and CH 3 OH.SEM images of the NiFe-LDH NSAs reveal a three-dimensional (3D) porous architecture with a LDH thickness of about 15 nm.TEM elemental mapping images of as-synthesized NiFe-LDH scratched off nickel foam suggest that Ni, Fe, and O elements are uniformly distributed over the NiFe-LDH (Figure 5f).The OER performances of LDH@nickel foam (NF) NSAs, Ni(OH) 2 @NF NSAs, and NF were evaluated in a typical three-electrode electrochemical cell in 1.0 M KOH solution at room temperature.Figure 5g displays the OER polarization curves of LDH@NF, Ni(OH) 2 @NF, and NF.It is clear that NiFe-LDH@NF demonstrates the highest OER activity compared with the contrast samples, with the lowest overpotential of 210 mV at 10 mA cm −2 , which are 88, 110, and 161 mV less than those of NiCo-LDH@NF, Ni(OH) 2 @NF, and NF, respectively.In addition, the well-uniformed NiFe-LDH NSAs also displays promising HER performances in 1.0 mM KOH solution with low overpotential of 133 mV at 10 mA cm −2 .The bi-functional electrocatalysts for OER and HER were further used for the overall water splitting in a two-electrode electrolysis cell (Figure 5h), which just needs a cell voltage of 1.59 V to give a water splitting current density of 10 mA cm −2 in 1.0 M KOH solution with a scan rate of 2 mV s −1 (Figure 5i).It is found that the nanosheet array architecture has increased the electrochemical surface area, which provides more catalytic active sites and favors the efficient adsorption and transfer of reactants.In addition, the well-ordered arrays also benefit the gas evolution reaction and subsequently enhance the electrocatalytic activity [121].
In addition to direct co-precipitation, the template-directed method is another effective strategy for the synthesis of LDH arrays as efficient OER electrocatalysts [122][123][124].Sun et al. develop a two-step hydrothermal method to synthesize hierarchical NiCoFe-LDH NSAs [125].In this process, Co 2 (OH) 2 CO 3 nanowire arrays grown on the Ni foam were first achieved to provide a Co source and to support the growth of NiCoFe-LDH NSAs in the presence of Fe III and urea (Figure 6a).The introduction of Ni in the LDH can be ascribed to the dissolution of the Ni foam substrate at a low pH value (pH = 1.2) due to the hydrolysis of Fe III and precipitation of Ni II in the solution.The density of the LDH NSAs on the nanowires can be tuned by simply changing the molar amount of Fe III during the second hydrothermal step.With a low concentration of Fe III (0.5 mmol), just a few LDH nanoplatelets grew on the nanowires (denoted as H-LDH-0.5, Figure 6b).When the amount of Fe III increased to 1 mmol, the density of LDH NSAs around the nanowires clearly increased and the diameter of the individual nanowire@nanoplatelet expanded to 250 nm (H-LDH-1, Figure 6c).The hierarchical nanoarray architecture benefits the improvement of the electrochemical properties of active materials by exposing more active sites.As a result, a high OER rate of 80 mA cm −2 for the H-LDHs could be readily achieved by applying a small overpotential (257 mV for H-LDH-1), much better than that of the Co 2 (OH) 2 CO 3 nanowire arrays (420 mV) and LDH NPs (492 mV).Zhao et al. fabricated CoFe-LDH NSAs by a solution phase cation exchange method at room temperature by dipping the Cu foam loaded with Cu 2 O nanoarrays into an aqueous solution of CoCl 2 and FeCl 2 (Figure 6d) [74].In this process, OH − was generated in situ along with the etching of electrocatalysts for OER and HER were further used for the overall water splitting in a two-electrode electrolysis cell (Figure 5h), which just needs a cell voltage of 1.59 V to give a water splitting current density of 10 mA cm −2 in 1.0 M KOH solution with a scan rate of 2 mV s −1 (Figure 5i).It is found that the nanosheet array architecture has increased the electrochemical surface area, which provides more catalytic active sites and favors the efficient adsorption and transfer of reactants.In addition, the well-ordered arrays also benefit the gas evolution reaction and subsequently enhance the electrocatalytic activity [121].for overall water splitting of Ni5Fe-LDH@NF and NF in a two-electrode configuration.Reproduced from [72], Copyright Wiley, 2017.
In addition to direct co-precipitation, the template-directed method is another effective strategy for the synthesis of LDH arrays as efficient OER electrocatalysts [122][123][124].Sun et al. develop a two-step hydrothermal method to synthesize hierarchical NiCoFe-LDH NSAs [125].In this process, Co2(OH)2CO3 nanowire arrays grown on the Ni foam were first achieved to provide a Co source and to support the growth of NiCoFe-LDH NSAs in the presence of Fe III and urea (Figure 6a).The introduction of Ni in the LDH can be ascribed to the dissolution of the Ni foam substrate at a low pH value (pH = 1.2) due to the hydrolysis of Fe III and precipitation of Ni II in the solution.The density of the LDH NSAs on the nanowires can be tuned by simply changing the molar amount of Fe III during the second hydrothermal step.With a low concentration of Fe III (0.5 mmol), just a few LDH nanoplatelets grew on the nanowires (denoted as H-LDH-0.5, Figure 6b).When the amount of Fe III increased to 1 mmol, the density of LDH NSAs around the nanowires clearly increased and the diameter of the individual nanowire@nanoplatelet expanded to 250 nm (H-LDH-1, Figure 6c).The hierarchical nanoarray architecture benefits the improvement of the electrochemical properties of active materials by exposing more active sites.As a result, a high OER rate of 80 mA cm −2 for the H-LDHs could be readily achieved by applying a small overpotential (257 mV for H-LDH-1), much better than that of the Co2(OH)2CO3 nanowire arrays (420 mV) and LDH NPs (492 mV).Zhao et al. fabricated CoFe-LDH NSAs by a solution phase cation exchange method at room temperature by dipping the Cu foam loaded with Cu2O nanoarrays into an aqueous solution of CoCl2 and FeCl2 (Figure 6d) [74].In this process, OH − was generated in situ along with the etching of Cu2O nanowires by S2O3 2− , which then leads to the precipitation of metal ions.The resulting CoFe-LDH NSAs inherited the geometry of the Cu2O template (Figure 6e,f).The Cu2O nanoarrays show negligible OER activity, while Co0.70Fe0.30-LDHNSAs give an OER onset overpotential as low as 220 mV, a small Tafel slope at 62.4 mV dec −1 , as well as excellent long-term durability (>100 h).A facile method to prepare electrode materials with the merits of fast and one-pot synthesis on the conducting substrates is a critical step when considering the practical operations.The electrosynthesis approach is often used to fabricate electrochemical active films on conducting A facile method to prepare electrode materials with the merits of fast and one-pot synthesis on the conducting substrates is a critical step when considering the practical operations.The electrosynthesis approach is often used to fabricate electrochemical active films on conducting materials' surfaces [126][127][128].Numerous materials have been developed by using the electrosynthesis process for applications in energy storage and conversion, such as transition metal oxides [129] and hydroxides [130], and conducting polymers [131].The morphology and thickness of as-synthesized films can be manipulated by monitoring electrochemical variables, such as the potential and coulombic charge.Our group developed an electrochemical approach for the fast, precisely-controllable, and economic fabrication of various Fe-containing LDH hierarchical nanoarrays (Figure 7a) [68].This electrosynthesis process was achieved by the following proposed reduction reaction on the working electrode: NO 3 in which the resulting OH − leads to the precipitation of LDHs.The whole electrosynthesis process is finished successively within hundreds of seconds at room temperature and the thickness of the LDH NSAs can be controlled by the deposition time.Figure 7b shows the SEM image of the as-obtained NiFe-LDH NSAs, where ultrathin (8 nm in thickness) and uniform NiFe-LDH platelets were grown perpendicularly to the surface of nickel foam substrate.The TEM image further shows a thin sheet-like nanostructure.In addition, LDHs' NSAs with uniform and homogeneous surface morphology can be synthesized on the foam nickel substrate with different sizes, such as from 2 cm 2 to 100 cm 2 (Figure 7d).This is further adequate for the fabrication of NiFe-LDH NSAs on other conducting substrates, including conducting cloths and glasses (Figure 7e,f).The OER activity of NiFe-LDH NSAs was further studied, which displays the lowest onset potential of OER current and the highest current density at the same overpotential (η) compared with the CoFe-and LiFe-LDH NSAs.Moreover, it shows high energy conversion efficiency from electric energy to chemical energy with a Faradaic efficiency of 99.4% after a testing period of 10 min.The electrosynthesized NiFe-LDH NSAs also gives significantly long-term stability and the current density of OER remains constant at each given potential after 50 h of continued measurement.
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 19 materials' surfaces [126][127][128].Numerous materials have been developed by using the electrosynthesis process for applications in energy storage and conversion, such as transition metal oxides [129] and hydroxides [130], and conducting polymers [131].The morphology and thickness of as-synthesized films can be manipulated by monitoring electrochemical variables, such as the potential and coulombic charge.Our group developed an electrochemical approach for the fast, precisely-controllable, and economic fabrication of various Fe-containing LDH hierarchical nanoarrays (Figure 7a) [68].This electrosynthesis process was achieved by the following proposed reduction reaction on the working electrode: NO3 − + H2O + 2e −  NO2 − + 2OH − , in which the resulting OH -leads to the precipitation of LDHs.The whole electrosynthesis process is finished successively within hundreds of seconds at room temperature and the thickness of the LDH NSAs can be controlled by the deposition time.Figure 7b shows the SEM image of the as-obtained NiFe-LDH NSAs, where ultrathin (8 nm in thickness) and uniform NiFe-LDH platelets were grown perpendicularly to the surface of nickel foam substrate.The TEM image further shows a thin sheet-like nanostructure.In addition, LDHs' NSAs with uniform and homogeneous surface morphology can be synthesized on the foam nickel substrate with different sizes, such as from 2 cm 2 to 100 cm 2 (Figure 7d).This is further adequate for the fabrication of NiFe-LDH NSAs on other conducting substrates, including conducting cloths and glasses (Figure 7e,f).The OER activity of NiFe-LDH NSAs was further studied, which displays the lowest onset potential of OER current and the highest current density at the same overpotential (η) compared with the CoFe-and LiFe-LDH NSAs.Moreover, it shows high energy conversion efficiency from electric energy to chemical energy with a Faradaic efficiency of 99.4% after a testing period of 10 min.The electrosynthesized NiFe-LDH NSAs also gives significantly long-term stability and the current density of OER remains constant at each given potential after 50 h of continued measurement.In addition, the electrosynthesis method can be extended to the various micro-substrates, which gives uniform core-shell nanostructures and improved electrochemical properties [132][133][134][135][136].

The Engineering of Interlayer Guests
Various anions can be intercalated into the interlayer of LDHs, which varies their chemical environment of host layers.Layer-by-layer assembly (LBL) of exfoliated LDH nanosheets with different interlayer anions through electrostatic interaction is a good way to introduce different interlayer anions [137][138][139][140][141]. Our group has reported the assembly of well-ordered CoNi-LDH NS/iron porphyrin (Fe-PP) ultrathin film by means of the LBL strategy, giving rise to an excellent OER performance [142].Compared with other interlayer anions, like sodium polystyrenesulfonate (PSS) and sodium dodecyl sulfate (SDS), CoNi-LDH NS assembled with Fe-PP shows superior OER activity, which may benefit from the excellent conductivity of Fe-PP.Moreover, LDHs with various host composition has also been combined with Fe-PP, such as CoMn-, CoFe-, and ZnCo-LDH, to form LDH NS/Fe-PP ultrathin films (UTFs).As a result, the OER performance of all these LDH NS/Fe-PP UTFs is significantly improved (Figure 8a), which means the conductive interlayer anions, like Fe-PP can efficiently improve the OER catalytic activities of LDHs.In addition, the increased intersheet spacing would be expected to facilitate OH − /O 2 transport through the film.Xu [143].As shown in Figure 8b, overpotentials of [NiFe]-LDH materials with different interlayer anions illustrate that the measured overpotentials, which reflects the water oxidation abilities, do not match with the basal spacing.Further study shows that strong correlation can be found between the pKa values of the interlayer anoins and overpotentials.As shown in Figure 8c, there is a midpoint of 3.4 ± 0.7.Considering the existence of 1 M OH − in the electrolyte, it is clear that the di-and trivalent anions outcompeted the hydroxide that presented in the interlayer.This means that di-and trivalent anions can be seen as strongly-bound proton acceptors which can reduce the activation barrier for water oxidation.Interestingly, the DFT calculations suggest that the XPS signal at 405.1 eV correspond to the nitrite bound by its N-atom to edge-site iron, which is a symbol of high water oxidation activity (Figure 8d).In addition, the oxidation of NiFe-LDH in the alkaline media will generate the NiFeOOH along with the detachment of H from the topmost surface of the regular LDH.Zhang et al. found that the presence of interlayer CO 3 2− anions stabilized the active sites of LDHs [144].As discussed above, the distance of interlayer space, as well as the chemical environment induced by the interlayer anions, play an important role in the OER activity of LDHs.Additionally, iron sites at the edges of [NiFe]-LDH nanosheets may be active in water oxidation catalysis through the related studies [145].We expect that these findings can be potentially used in the engineering of OER catalysts.

Conclusions and Perspectives
In summary, we have systematically introduced the recent progress about LDHs, themselves, for electrochemical water oxidation from the view of host and guest engineering.The potential active sites of LDH have been scientifically discussed based on previous experiments and calculation.The metal ion ratio in the LDHs' host layer obviously affects their OER activity.The determination of the valence state of metal ions during water oxidation has been widely used to investigate the active sites of LDH, while this probably depends on the metal and water oxidation rate.As for guest anions, changing the space of the interlayer and the chemical properties (such as acidity and alkalinity) provides a chance to optimize of electrochemical performances for LDHs.Since the nanostructures have a profound impact on electrode materials, it is significant to synthesize uniform and highly-dispersed LDHs with all the active sites exposed to the electrolyte, such as ultrathin nanosheet arrays.
In the future, molecular-level control remains promising to entirely develop the potential of LDH-based OER electrocatalysts.Host-guest interaction of LDHs provides a large space for turning their electrochemical properties for molecule adsorption and catalytic transformation.Additionally, the defect chemistry of LDHs have also been paid increased attention, owing to the significantly-improved surface electronic structures.This also extends the question as to whether the surface or sites show a higher activity for the OER process because of their different defects.Despite the importance of optimizing the material properties, it is still a long way to the synthesis of low-cost and high-quality LDH-based electrocatalysts with precise control over their composition, structure, and morphology.To utilize solar energy in water splitting, how to combine the novel LDH-based OER catalysts with photocatalysts will be a promising method to achieve an efficient photoelectrochemical process.

Conclusions and Perspectives
In summary, we have systematically introduced the recent progress about LDHs, themselves, for electrochemical water oxidation from the view of host and guest engineering.The potential active sites of LDH have been scientifically discussed based on previous experiments and calculation.The metal ion ratio in the LDHs' host layer obviously affects their OER activity.The determination of the valence state of metal ions during water oxidation has been widely used to investigate the active sites of LDH, while this probably depends on the metal and water oxidation rate.As for guest anions, changing the space of the interlayer and the chemical properties (such as acidity and alkalinity) provides a chance to optimize of electrochemical performances for LDHs.Since the nanostructures have a profound impact on electrode materials, it is significant to synthesize uniform and highly-dispersed LDHs with all the active sites exposed to the electrolyte, such as ultrathin nanosheet arrays.
In the future, molecular-level control remains promising to entirely develop the potential of LDH-based OER electrocatalysts.Host-guest interaction of LDHs provides a large space for turning their electrochemical properties for molecule adsorption and catalytic transformation.Additionally, the defect chemistry of LDHs have also been paid increased attention, owing to the significantly-improved surface electronic structures.This also extends the question as to whether the surface or sites show a higher activity for the OER process because of their different defects.Despite the importance of optimizing the material properties, it is still a long way to the synthesis of low-cost and high-quality LDH-based electrocatalysts with precise control over their composition, structure, and morphology.To utilize solar energy in water splitting, how to combine the novel LDH-based OER catalysts with photocatalysts will be a promising method to achieve an efficient photoelectrochemical process.
compares the turnover frequency (TOF) as a function of the thickness for Fe-free NiOOH and Ni 0.75 Fe 0.25 OOH deposited on two kinds of substrate (Au and GC).It is concluded that Fe enhances the activity of NiOOH through a Ni−Fe partial-charge-transfer activation process.Catalysts 2018, 8, x FOR PEER REVIEW 3 of 19 exchange properties of LDHs, make them interesting alternatives for applications in electrochemistry.Bell et al. synthesized (Ni,Fe) oxyhydroxides across the entire composition range (Figure 1a,b) [62].It was found that the addition of 25% Fe to Ni(OH)2 results in up to a 500-fold higher OER current density compared to pure Ni and Fe oxyhydroxide films at the overpotential of 0.3 V. Zhang et al. also investigated the effect of different Ni/Fe molar ratios to their OER performances [63].It was found that moderate metal substitution into the host hydroxide framework (Fe into Ni or Ni into Fe) substantially enhanced the OER activity with a decrease of both the Tafel slope and overpotential.To further accurately control the Fe content, Boettcher et al. developed a method for purification of KOH electrolyte by using precipitated bulk Ni(OH)2 to absorb Fe impurities Figure 1d compares the turnover frequency (TOF) as a function of the thickness for Fe-free NiOOH and Ni0.75Fe0.25OOHdeposited on two kinds of substrate (Au and GC).It is concluded that Fe enhances the activity of NiOOH through a Ni−Fe partial-charge-transfer activation process.

Figure 1 .
Figure 1.(a) Schematic illustration of the crystal structure of NiFe-LDH.(b) Measured OER activity of mixed Ni−Fe catalysts as a function of Fe content.Reproduced from [62], Copyright American Chemical Society, 2014.(c) Conductivity data for the Ni1−xFexOOH with various Fe content.(d) TOFs as a function of film thickness for Fe-free NiOOH and Ni0.75Fe0.25OOH.Reproduced from [50], Copyright American Chemical Society, 2014.

Figure 1 .
Figure 1.(a) Schematic illustration of the crystal structure of NiFe-LDH.(b) Measured OER activity of mixed Ni−Fe catalysts as a function of Fe content.Reproduced from [62], Copyright American Chemical Society, 2014.(c) Conductivity data for the Ni 1−x Fe x OOH with various Fe content.(d) TOFs as a function of film thickness for Fe-free NiOOH and Ni 0.75 Fe 0.25 OOH.Reproduced from [50], Copyright American Chemical Society, 2014.

Catalysts 2018 ,
8, x FOR PEER REVIEW 4 of 19 state regardless of potential and composition[64], which was determined by operando differential electrochemical mass spectrometry (DEMS) and X-ray absorption spectroscopy (XAS) under OER conditions.On the other hand, Ni IV at Fe content below 4% has been detected under catalytic conditions, while Ni atoms stabilized in a low-valent oxidation state when further increasing the content of Fe.This difference in metal valent states mainly depends on the rate of water oxidation−metal-reduction (kOER) and metal oxidation (kMox) (Figure2a).The lower kMox/kOER ratio reflects a dramatically increased rate constant of water oxidation (kMox), which may exceed the rate of the metal oxidation (kOER).It concluded that high catalytic OER activity of the mixed Ni−Fe catalysts demonstrate a sharply-decreased kMox/kOER ratio.They further give a catalytic OER cycle (Figure2b), where the buildup of oxidation equivalents from Ni II to Ni IV sites is followed by the O−O bond formation with the subsequent release of molecular oxygen.However, there is substantial evidence from X-ray absorption and Mossbauer experiments that Ni IV and Fe IV are both found in NiFe-LDH at OER potentials during different studies[51,65,66].Recently, Stahl et al. detected the Fe IV species (up to 21% of the total Fe) during steady-state water oxidation on NiFe-LDH[51].The stable presence of Fe IV can be ascribed to the increased electron-donating ability of the π-symmetry lone pairs of the bridging oxygen atoms between Ni and Fe (Figure2c,d), which makes the NiOOH lattice a more stable environment for high-valent metal ions.

Figure 2 .
Figure 2. (a) XAS-derived structural motifs prevalent during OER catalysis at high and intermediate Ni-content.(b) Simplified scheme of the electrochemical water splitting cycle.Reproduced from [64], Copyright American Chemical Society, 2016.(c) Electronic effects that could rationalize the observation of Fe IV in NiFe, but not Fe oxide catalysts.(d) Schematic representation of a layered NiOOH lattice containing Fe ions in different sites (orange-brown).Reproduced from [51], Copyright American Chemical Society, 2015.

Figure 2 .
Figure 2. (a) XAS-derived structural motifs prevalent during OER catalysis at high and intermediate Ni-content.(b) Simplified scheme of the electrochemical water splitting cycle.Reproduced from [64], Copyright American Chemical Society, 2016.(c) Electronic effects that could rationalize the observation of Fe IV in NiFe, but not Fe oxide catalysts.(d) Schematic representation of a layered NiOOH lattice containing Fe ions in different sites (orange-brown).Reproduced from [51], Copyright American Chemical Society, 2015.
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 19Therefore, different metal ions introduced into the LDH host layer induces a varied chemical and electronic environment, which thereby varied their OER performances.In addition to NiFe-LDH, other LDHs with various host metal ions (e.g., NiCo, NiMn, ZnCo, and CoAl) have been demonstrated as OER electrocatalysts (Figure3).However, their activity is still lower than that of NiFe-LDH.

Figure 3 .
Figure 3. Overpotentials required at j = 10 mA cm −2 for various LDHs (the error bars indicate a range of overpotentials).

Figure 3 .
Figure 3. Overpotentials required at j = 10 mA cm −2 for various LDHs (the error bars indicate a range of overpotentials).

Catalysts 2018 ,
8, x FOR PEER REVIEW 6 of 19nanosheets by plasma etching[78].The high-energy plasma destroys the ionic bonds and hydrogen bonds in the interlayers of the bulk LDHs, which interrupted the host-guest charge balance and separated the brucite-like host layers from each other.For instance, the thickness of CoFe-LDH have been successfully decreased from ~20 nm to 0.6 nm by subjecting bulk CoFe-LDH to Ar plasma etching for 60 min.Moreover, the coordination number of the Co-OOH octahedra is lower than that in the bulk CoFe-LDH, suggesting the presence of oxygen vacancies (VO).The as-prepared LDH ultrathin nanosheets demonstrate much-improved OER performance with a low overpotential of 266 mV at 10 mA cm −2 .Recently, they further reported the exfoliation of CoFe-LDH by a water plasma-assisted strategy (Figure4a)[109], which was accompanied with the formation of multi-vacancies, including O, Co, and Fe vacancies.The as-exfoliated ultrathin LDHs nanosheets with multi-vacancies show significantly promoted electrocatalytic activity for water oxidation.As shown in Figure4b, water-plasma exfoliated CoFe-LDH nanosheets just require a low overpotential of 290 mV to reach 10 mA cm −2 while the pristine CoFe-LDH need an overpotential of 332 mV.Therefore, the effective exfoliation, as well as the defect introduction, both promotes the OER activity of LDHs.
Cu 2 O nanowires by S 2 O 3 2− , which then leads to the precipitation of metal ions.The resulting CoFe-LDH NSAs inherited the geometry of the Cu 2 O template (Figure 6e,f).The Cu 2 O nanoarrays show negligible OER activity, while Co 0.70 Fe 0.30 -LDH NSAs give an OER onset overpotential as low as 220 mV, a small Tafel slope at 62.4 mV dec −1 , as well as excellent long-term durability (>100 h).

Figure 5 .Figure 5 .
Figure 5. (a) SEM image of NiFe-LDH flakes grown on the nickel foam, and (b) is the corresponding cross-sectional image.(c) HRTEM image and SAED pattern (inset in (c)) of a single NiFe-LDH layer.Reproduced from [120], Royal of Society, 2016.(d,e) SEM images of Ni5Fe LDH@NF.Inset in (e) is a HAADF-STEM image of the Ni5Fe-LDH scratched off NF. (f) TEM elemental mapping images of Ni5Fe LDH scratched off NF with blue for Ni, purple for Fe, and red for O. (g) OER polarization curves of NF, Ni(OH)2@NF, Ni5Co-LDH@NF, and Ni5Fe-LDH@NF.(h) Optical images of the Ni5Fe-LDH@NF electrode and overall water splitting device.(i) The steady-state polarization curve Figure 5. (a) SEM image of NiFe-LDH flakes grown on the nickel foam, and (b) is the corresponding cross-sectional image.(c) HRTEM image and SAED pattern (inset in (c)) of a single NiFe-LDH layer.Reproduced from [120], Royal of Society, 2016.(d,e) SEM images of Ni 5 Fe LDH@NF.Inset in (e) is a HAADF-STEM image of the Ni 5 Fe-LDH scratched off NF. (f) TEM elemental mapping images of Ni 5 Fe LDH scratched off NF with blue for Ni, purple for Fe, and red for O. (g) OER polarization curves of NF, Ni(OH) 2 @NF, Ni 5 Co-LDH@NF, and Ni 5 Fe-LDH@NF.(h) Optical images of the Ni 5 Fe-LDH@NF electrode and overall water splitting device.(i) The steady-state polarization curve for overall water splitting of Ni 5 Fe-LDH@NF and NF in a two-electrode configuration.Reproduced from [72], Copyright Wiley, 2017.

Figure 7 .
Figure 7. (a) Scheme of the synthetic route to MFe-LDH (M=Co, Ni and Li) NSAs.(b) SEM and (c) TEM image of NiFe-LDH NSAs.(d) Photographs of NiFe-LDH NSAs synthesized on the foam nickel substrates at various scales (inset: the SEM image of NiFe-LDH on 100 cm 2 substrate).SEM images for NiFe-LDH nanoplatelet arrays on (e) the conducting cloth and (f) FTO substrate (inset: their corresponding photographs).Reproduced from [68], Copyright Royal of Society, 2015.

Figure 7 .
Figure 7. (a) Scheme of the synthetic route to MFe-LDH (M=Co, Ni and Li) NSAs.(b) SEM and (c) TEM image of NiFe-LDH NSAs.(d) Photographs of NiFe-LDH NSAs synthesized on the foam nickel substrates at various scales (inset: the SEM image of NiFe-LDH on 100 cm 2 substrate).SEM images for NiFe-LDH nanoplatelet arrays on (e) the conducting cloth and (f) FTO substrate (inset: their corresponding photographs).Reproduced from [68], Copyright Royal of Society, 2015.

Figure 8 .
Figure 8.(a) LSV curves for various LDHs assembled with Fe-PP.Reproduced from [142], Copyright Royal of Society, 2016.(b) Observed overpotentials η as a function of the basal spacing of NiFe-LDH materials with different interlayer anions.(c) Overpotentials η of NiFe-LDH materials with different interlayer anions A m− .(d) Measured (black) and calculated X-ray photoelectron spectroscopy (XPS) binding energy (BE) core level shifts (is-CLS) for differently-bound nitrates (blue) and nitrites (green) with a calculated structure that is consistent with the feature at 405.1 eV.Atom colors: Ni, green; Fe, maroon; N, blue; O, red; H, white.Reproduced from [143], Copyright Royal of Society, 2016.

Figure 8 .
Figure 8.(a) LSV curves for various LDHs assembled with Fe-PP.Reproduced from [142], Copyright Royal of Society, 2016.(b) Observed overpotentials η as a function of the basal spacing of NiFe-LDH materials with different interlayer anions.(c) Overpotentials η of NiFe-LDH materials with different interlayer anions A m− .(d) Measured (black) and calculated X-ray photoelectron spectroscopy (XPS) binding energy (BE) core level shifts (is-CLS) for differently-bound nitrates (blue) and nitrites (green) with a calculated structure that is consistent with the feature at 405.1 eV.Atom colors: Ni, green; Fe, maroon; N, blue; O, red; H, white.Reproduced from [143], Copyright Royal of Society, 2016.
et al. synthesized NiFe-LDH by introducing CO 3 2− to replace NO 3 − , leading to the reducing of interlayer spacing and poor OER catalytic activity [71].On the basis of OH − being the main reactant in alkaline solution, the NO 3 − has a better exchange ability with OH − in interlamination, which also affects the OER performance of NiFe-LDH.Hunter et al. synthesized NiFe-LDH with 12 different interlayer anions, which shows different activities during OER reactions