摘要：近幾十年來，氧析出反應因其在能量儲存和轉換技術中的關鍵作用而受到了廣泛關注。然而，它需要高效的催化劑例如IrO2和RuO2，來加速其緩慢的反應動力學。在所開發(fā)的低成本材料中，鎳鐵層狀雙氫氧化物（NiFe LDH）較為有前景，其在堿性電解質中表現(xiàn)出出色的氧析出性能，過電位很低，在10 mA·cm?2處僅需200–300 mV。雖然人們在開發(fā)基于NiFe LDH的高效電催化劑方面做出了巨大努力并取得了一些成果，但是要進一步降低其過電位具有相當?shù)奶魬?zhàn)性。為了克服這個瓶頸，就需要明確識別其活性位點和催化機理，從根本出發(fā)來探究新的解決方案，以獲得具有超低過電位的催化劑。本綜述首先回顧了NiFe LDH的結構、組成和發(fā)展歷史。雖然人們在研究催化活性位點和機制方面付出了巨大努力，但其真正的催化位點和機制仍然是模棱兩可并存在爭議的。我們對催化位點研究的代表性工作進行了全面分析，希望對催化機理和活性位點能提供一些深入認識和理解。此外，我們還就增強其催化活性的各種策略，如雜原子摻雜和引入空位等，進行了總結并基于電子和幾何結構對其活性提高原理進行了分類，為開發(fā)高性能的NiFe LDH基催化劑提供新的見解和方向。此外，催化劑的穩(wěn)定性，尤其是在高電流密度等技術條件下的穩(wěn)定性至關重要，但常常被人們忽視。最新的研究表明，NiFe LDH基催化劑在高電流密度下運行一段時間就會出現(xiàn)嚴重的活性衰減。因此，本綜述強調了穩(wěn)定性問題的重要性，以引起更多研究者對此問題的關注，并分析了NiFe LDH基催化劑的衰減機理，總結和討論了基于這些衰減機理開發(fā)的改善穩(wěn)定性問題的最新策略。最后，本綜述討論了制備兼具優(yōu)異催化活性和穩(wěn)定性的NiFe LDH基的高效催化劑的可能發(fā)展方向。

關鍵詞：鎳鐵層狀雙氫氧化物；氧析出反應；催化機制；電極設計；穩(wěn)定性

中圖分類號：O646

Use of NiFe Layered Double Hydroxide as Electrocatalyst in OxygenEvolution Reaction： Catalytic Mechanisms， Electrode Design， andDurability

Abstract： In recent decades， the oxygen evolution reaction （OER） has attractedsignificant attention because of its critical role in energy storage and conversiontechnologies. This reaction requires highly efficient catalysts such as IrO2 and RuO2 toaccelerate its slow reaction rate. Among existing developed low-cost materials， NiFelayered double hydroxides （NiFe LDH） have demonstrated great potential for use inOERs in alkaline electrolytes with low overpotential （200–300 mV at 10 mA·cm?2）.Extensive efforts have been devoted to developing efficient electrocatalysts based onNiFe LDHs; Further reducing their overpotential can be a challenging task. To overcomethis bottleneck， it is necessary to clearly identify the catalytic mechanism and active sitesand finding new solutions to obtain catalysts with ultra-low overpotential. Through thisreview， we thoroughly examined the structure， composition， and development history of NiFe LDHs. Despite the extensiveinvestigation of the catalytic active sites and mechanism， it still remains elusive and controversial. Herein， existing studiesthat have aimed to elucidate the catalytic sites are presented and comprehensively analyzed， providing an insightfulunderstanding of the catalytic mechanism and active sites of NiFe LDHs. Additionally， various strategies， such asheteroatom doping and the introduction of vacancies， have been proposed to enhance the catalytic activities of thesematerials. Considering the electronic and geometrical structures of NiFe LDHs， this review summarizes and categorizesactivity enhancement methods based on different enhancement mechanisms， offering new insights and directions fordeveloping high-performance NiFe LDH-based catalysts. Furthermore， despite being crucial to the practical use of thecatalyst， catalyst stability is often overlooked， especially under technological conditions such as high current densities.Recent works have suggested that NiFe LDH-based catalysts suffer severe activity fading under high current densitiesafter a short period of operation. It is important to update recent research on the stability of these catalysts. This reviewemphasizes the stability issues of NiFe LDH-based catalysts to draw more attention toward research and analyses relatedto the decay mechanisms of these catalysts. We have summarized and discussed the recent strategies that have beenproposed to reduce the stability problem developed based on these decay mechanisms. Finally， the review concludes witha discussion of possible directions for producing NiFe LDHs with extraordinary catalytic activities and stabilities.

Key Words： NiFe LDH; OER; Catalytic mechanism; Electrode design; Durability

1 Introduction

In recent decades， the environmental deterioration caused bythe consumption of the fossil fuels and increasing demand inenergy have speeded the transition from fossil fuels to cleanenergies （such as solar， wind and tidal energy）. However， thesetypical sources of renewable energies are uncontrollable owingto the seasonal and regional factors， which limit their widespreadapplication 1–5. Hence， it is imperative to develop efficientenergy storage or energy conversion technologies such aselectrochemical water-splitting devices and rechargeable metalairbatteries 6–14. The oxygen evolution reaction （OER） is one ofthe most important half reactions comprising these energyconversion or storage devices. However， owing to the sluggishreaction kinetics， especially the huge energy barrier resulting fromthe four-step proton-coupled electron transfer process，electrocatalysts are required to reduce the overpotential 15–21.Iridium （Ir） and ruthenium （Ru） oxides are state-of-art catalyststowards OER while the scarcity of Ir and Ru undoubtedly hindersthe widespread application in practice 22–26. Alternatively， nonpreciousfirst row （3d） transition-metal oxides or hydroxideshave aroused extensive research interest because of the low costand natural abundance of these elements 27–31. Among them， theNiFe LDH has shown outstanding performance with ultra-lowoverpotential in alkaline media and is regarded as one of themost promising electrocatalysts toward OER 32–36. As a memberof the large family of the two-dimensional anionic clay materials（Fig. 1a）， the NiFe LDH compound can be expressed as theformula

[Ni21+?xFex3+（OH）2]x+（An?）x/n·y（H2O）

where x is the molar ratio of Fe， An? represents the chargebalancingn-valent anion， and y is the amount of the water that isintercalated into the layers. This inclusion of Fe in the layers canimprove the charge transport of the material， which can beunderstood via a mixed mechanism involving an ‘electronhopping’ along the layers 37. It is generally realized that anominal value of x typically varies between 1/5 and 1/3， and alarger or smaller value of x may cause the formation ofhydroxide or hydrous oxides of a single metal 38. Theunderstanding of the structure of NiFe LDH can start from thatof the brucite-type Ni（OH）2 which is composed of edge-sharedNi2+―O octahedras. The partial substitution of Ni2+ by Fe3+ willintroduce excessive positive charges to the host layers， which iscompensated by the intercalated anions along with the watermolecules， such as CO3 2?， Cl? and OH?. The NiFe LDH has amuch larger intersheet spacing than that of β-Ni（OH）2 becauseof the intercalated anions and associated water molecules 39.

The research on NiFe LDH as electrocatalyst toward OER canbe dated back to the pioneering work by Corrigan in the 1980s （Fig. 1b）， it was pointed out that trace iron impurities can boostthe catalytic activity of nickel oxide 41，42. Since then， tremendousefforts have been devoted to investigating the catalyticmechanism and elucidating the active sites， thanks to theadvancement of in situ and ex situ techniques such as M?ssbauerspectroscopy and X-ray absorption spectroscopy （XAS），insightful and comprehensive understanding of the catalyticroles of the metals have been achieved 43. Additionally， variousstrategies have been introduced to enhance the activity， such asdoping heteroatoms， designing heterostructures， manipulatingthe intercalated anions， creating anion or cation vacancies， andexfoliating the layered structure to expose more active sites 32，44–48.Great achievements have been made in the aspects of activitywith overpotentials as low as 200 mV at current density of 10mA·cm?2 49–51， which is better than that of the benchmark IrOx.Boettcher et al. studied thin films of OER through quartz-crystalmicrobalance electrodes and found that the turnover frequency ofNi0.9Fe0.1OOH is 10-fold higher than that of IrOx 52. Apart from theactivity， extensive knowledge of the decay mechanism of the NiFeLDH after operating long period under industrial conditions in thewater splitting devices has been acquired 53，54. Previous reviewsmainly focused on the progress of the synthetic techniques，modification strategies and application of the layered doublehydroxides in energy conversion and storage technologies 55–57;nevertheless， NiFe LDH as the most active OER electrocatalysthas rarely been extensively and systematically summarized andreviewed. For the rational design and optimization of the NiFeLDH catalysts， it is highly desirable to provide timely updatesand insightful perspective for this field.

In this review， we present the recent advancements of the NiFeLDH electrocatalyst toward OER. It covers the catalyticmechanism and active sites， the various strategies to modify thecatalyst and the stability issues.

2 The catalytic sites and mechanisminvestigation

In the 1980s， Corrigan reported the high activity of NiFematerials and realized the strong catalytic activation of Ni（OH）2by iron incorporation 41. Because the activity is affected by anensemble of factors such as the geometrical and electronic structure （oxidation state） of the metals， the non-covalentinteractions and the steady state of the surface configurations， itis extremely challenging to decipher the nature of the catalystand large uncertainties exist concerning the intrinsic catalyticsites and mechanism as well as their relationship after the Feaddition to Ni（OH）2/NiOOH which results in the formation ofmultiple possible active sites such as Fe-O， Ni-O， or Ni-O-Fe.The optimum composition with the highest activity is reportedto fall in the range of 15%–50% Fe （at%） （Fig. 2a） 58， and higherFe content will lead to the unavoidable formation of neighboredFe octahedral with the evolution of the FeOxHy phases 39，59.There is much controversy and dispute in respect of the roles ofFe in the catalysis of OER. For the roles of Fe， it is believed thatthe Fe can affect the catalysis from various aspects such aschanging the electronic structure of the surrounding metals andthe physical properties. For example， Boettcher et al. found thata gt; 30-fold increase in film conductivity with Fe addition to theNi（OH）2， which is expected to enhance the catalytic activity，although this change in conductivity cannot explain the activityincrease 60. Gray et al. trapped a reactive cis-dioxo-Fe（VI）intermediate in solution depleted of water and confirmed thedetected intermediate is active in water oxidation 61. Both theresearch groups of Corrigan and Stahl directly detected thehighly oxidized iron species （Fe4+） during steady-state catalyticturnover through the in situ M?ssbauer spectroscopy， while it isnot observed in Fe oxide catalyst 42，62. Also， Sham et al. observedthe highly valent Fe（IV）―O bond under OER process togetherwith the increase of the Ni valent from Ni（+3） to Ni （+3.6） viathe in situ X-ray absorption spectroscopy， and it is put forwardthat the charge transfer between the Ni and Fe through a“Ni―O―Fe” bond leads to the high catalytic activity 63.Similarly， Boettcher et al. proposed that Fe-induced partialcharge transfer is responsible for the activity improvement 60，which is in line with the report by Bell et al. that the OER activityof NiOOH films increases on noble metal substrates and thiseffect is more pronounced on more-electronegative substratesand smaller on less-electronegative substrates 64. Further， theirresearch group evidenced that the outstanding OER activity ofNi（Fe）OxHy is not determined by Fe in the bulk or by averageelectrochemical properties of the Ni cations measured by thevoltammetry such as Ni redox potential， redox peak shape， andaverage e? per Ni in the redox wave， instead， the local structureplays a more important role （Fig. 2b）. They proposed that the Felocated at the edge or defect sites after the rapid incorporation ofFe into the NiOxHy from Fe cations in solution are the reactivesites （Fig. 2c） 65. Bell et al. identified the origin of theexceptional activity through the operando XAS and DFTcalculations， and revealed that the Fe3+ occupying the octahedralsites with unusually short Fe―O bond distances are the activesites with near optimal adsorption energies of OERintermediates. By contrast， the Ni sites are not active sites for theoxidation of water 66. Bard et al. conducted surface-selective andtime-dependent redox titrations to investigate the surface OER kinetics of Ni and Fe in NiFe LDH， the iron dispersed in theNiOOH matrix was identified as the “fast” active sites with anOER rate constant of 1.7 s?1 per atom 67. While， Nocera et al.shows that the role of Fe in promoting the OER seems to be aLewis acid effect. The incorporation of Lewis acid Fe3+ wouldengender a larger population of Ni4+ by increasing the acidity ofOHx moieties coordinated to nickel， which in turn leads togreater oxyl （Ni3+―O·） character and thus promotes the O―Ocoupling 68. Bell et al. found that the aged Ni films （the iron effectis excluded） shared similar Tafel slope and reaction order in OH?with those of the mixed Ni-Fe film， which implies a commonreaction pathway. They believe that the Ni sites neighbored bythe Fe are responsible for OER and the Fe incorporation modifiesthe local environment around Ni―O 58. However， their latter workshowed that the aging of α-Ni（OH）2 to β-Ni（OH）2 in purifiedKOH will undergo activity loss instead of activity increase， theprevious works believe that the aging of Ni（OH）2 will improveits activity because of the phase transition from kinetically moreaccessible α-Ni（OH）2 to the thermodynamically more favorableβ-Ni（OH）2 69. Actually， it was the Fe impurities in KOH ratherthan the phase transition that caused the activity improvement ofNi（OH）2 during the aging 70. Hu et al. studied the OER behaviorsof Ni（OH）2 and NiFe LDH by combining the 18O labeling experiments and the in situ Raman spectroscopy， they inferreddifferent catalytic mechanisms for the Fe-free and Fe-containingNi oxides 71. For the Fe-free Ni oxides， lattice oxygen atomsparticipate in the reactions and the oxygen evolves through aNiOO?. Upon Fe incorporation， a new and highly active reactionsite is created based on Fe and lattice oxygen no longerparticipates in the OER. Their latter work evidenced that theturnover frequency of the Fe sites is 20–200 times higher thanthat of the Ni sites. However， owing to their quantitativesuperiority， the Ni sites are catalytically relevant with an Fecontent of 2.3%， and above the Fe sites dominate the catalysis 72.Meanwhile， some works suggest that the Ni and Fe might playsynergistic roles for the water oxidation. Gong et al. used theoxygen atom transfer probes， and the operando Ramanspectroscopy detected a resting Fe＝O intermediate and a ratedeterminingchemical coupling step between the Fe＝O and thevicinal bridging O―（Fe―O―Ni） 73. Based on the operando Xrayscattering and absorption spectroscopy， and DFTcalculations， Strasser et al. suggest that the OER takes placefollowing a Mars van Krevelen-type mechanism through the Fesites and their synergy with the nearest neighbored Ni sites byforming the O-bridged Fe-Ni reaction centers which stabilize theOER intermediates that are unstable on Ni-Ni centers or pure Fesites 74. Goddard III et al. examined the reaction mechanismthrough the DFT calculations and found that the （Ni， Fe）OOHsystem features a synergy between the Fe and Ni：Fe（IV）stabilizes the O radical and Ni（IV） catalyzes the O-O coupling（Fig. 2d） 75.

These various studies point to three origins underlying thehigh water-oxidation activity catalyzed by the NiFe（oxy）hydroxides： （1） the Fe acts as the highly active catalytic center;（2） the Fe modifies the local environment of Ni and the Ni actsas the catalytic site; （3） the Fe and Ni synergistically catalyze theOER. However， it remains elusive and debatable about thecatalytic mechanism and active sites; the catalytic sites andmechanisms are believed to be subject to variation based on thelocal structure and coordination environment of metals. It needsmore advanced and powerful techniques to have deeper insightsinto the complicated mechanisms underlying the performance ofthe best OER catalyst.

3 The modification strategies to improve theactivity of NiFe LDH

To further improve the catalytic performance of the NiFeLDH， various strategies have been put forward in the past fewyears such as heteroatoms doping， construction ofheterostructure， exposing of more active sites， lattice strain， andso on. In this part， these modification techniques will besummarized and reviewed for the rational design of the catalysts.

3.1 Heteroatoms doping

Several mechanisms have been put forward to explain theactivity enhancement after the element doping， such asregulating the band structure and band gap of the catalyst， tuningthe electron densities of the Fe and Ni， exposing more active sitesby creation of porous structure or vacancies， and introduction ofnew active sites. In the following part， some representativeworks will be introduced.

3.1.1 Regulating the band structure

Sun et al. fabricated a vanadium doped NiFe LDH nanosheetsarray on the nickel foam， which manifested outstanding OERactivity with a low Tafel slope of 42 mV·dec?1 and a smalloverpotential of 195 mV at 20 mA·cm?2 in 1.0 mol?L?1 KOHsolution. The DFT calculations revealed that the V can narrowthe bandgap thereby improving the electrical conductivity of thecatalyst 76. Wang et al. reported that the doping of Yttrium caninduce the decrease of the bandgap after incorporation into theNiFe LDH 77. Shao et al. doped different metal atoms （Ti， V， Cr，Mn， Co， Cu， Zn， Mg and Al） into the NiFe LDH to tune theelectronic structure of the LDH host （Fig. 3a）. Except the Al andMg， all the others can promote the apparent activity of the NiFeLDH 78. The DFT calculated density of states （DOS） suggest thatthe incorporation of the transition metal atoms can reduce thebandgap width to guarantee faster electron transportation duringthe reactions （Fig. 3b）. The intercalated cations can alsoinfluence the electronic structures of the host， Strongin et al.used the classical molecular dynamics to investigate the impact of intercalated Co2+ on the structure of NiFe LDH and theelectron transfer process （Fig. 3c） 79， it was revealed that theintercalated cations can deform the 2D sheet to form small“pockets” which facilitate the formation of small clusters ofcations， solvent exchange rates are higher in these “pockets”than other sections of the LDH， resulting in quicker electrontransfer in the host. The reduction of the bandgap width can leadto an improvement in the electron transportation rate， therebyenhancing the catalytic activity.

3.1.2 Tuning the electron density of Fe and Ni

Du et al. doped the NiFe LDH with Mn and found thatchemical bonds in Mn doped NiFe LDH are shorter than thosein pure LDH， and both [NiO6] and [FeO6] units werecompressed. It was further revealed that the Mn facilitates thetransition of Ni2+ to Ni3+ through the increased hybridizationbetween Ni 3d and O 2p orbitals， while Fe promotes theoxidation of Ni3+ into Ni4+， and eventually benefits the wateroxidation 80. Lee et al. reported the activity enhancement of NiFeLDH by Ce doping and proposed that the Ce can gain electronsand change the electronic structure of the metal ions through theNi―O―Fe―O―Ce bridge （Fig. 4a） 81. Sun et al. introducedthe Mn2+ with weaker electronegativity to Ni and Fe to the NiFeLDH host， which boosted the OER activity. The DFTcalculations revealed that the Mn2+ can donate electrons to thesurrounding Ni2+ and Fe3+ （Fig. 4b）， which facilitates thedeprotonating step 11. Zhang et al. fabricated the NiFe（oxy）sulfide catalyst with the substitution of O by S， the polarized Sand nonpolarized O synergistically regulate the electronicstructure of the metal sites 82. Similarly， Meng et al. enhancedthe activity of NiFeOOH by substitution of O by N， which tunedthe d-band center of the surface metal atoms 50. Refining theelectron density of metals enables the modulation of the bindingenergy of oxygenates， which ultimately enhances the efficiency of water oxidation.

3.1.3 Creation of new active sites

Francisco et al. reported the integration of Rh atoms into theNiFe LDH to replace the Fe ions， which manifested much betterOER activity than NiFe LDH at an optimized Rh/（Rh + Fe） ratioof ～4% 83， the theoretical calculation shows that the Rh becomesthe new active site to catalyze the OER. Zhang et al. constructedthe ultrathin W6+-doped NiFe LDH and proposed that the W6+ withlow spin d0 can stabilize the O radical and preferably acts as thecatalytic site 84.

3.1.4 Exposing of more active sites

Jin et al. reported the introduction of the trivalent Al ions intothe NiFe LDH and largely increased the number of the activesites and low-coordinated metal atoms by partial dissolution ofthe Al in strong alkaline （Fig. 5a）， which significantly improvedthe catalytic activity 85. Hu et al. found that the F-incorporatedNiFe LDH can go through the surface reconstruction and bereconstructed into mesoporous and amorphous NiFe oxide hierarchical structure by F etching （Fig. 5b）， an outstandingactivity was obtained with a low overpotential of 176 mV at 10mA·cm?2 51. Dou et al. prepared a nanoporous NiFe LDHnanosheets with abundant defects via the cation-exchangeprocess （Fig. 5c）， it exhibited larger electrochemical surface areaand improved surface wettability as well as much higher activitycomparing to the NiFe LDH 49. By creating pores， more activesites are exposed to the electrolyte and thus improve the catalyticactivity.

3.2 Heterostructure construction

The influence of the heterostructure on the OER performanceof NiFe LDH can be classified into two categories： （1） Tuningthe electronic structure of the NiFe LDH; （2） Synergisticcatalysis at the interface. In the following section， somerepresentative works will be introduced.

3.2.1 Tuning the electronic structure

Sun et al. reported that the interfaced FeOOH nanoparticlescan adjust the local electronic structure of the NiFe LDH throughthe strong interfacial interaction （Fig. 6a）， it was proposed thatthe Fe（3+δ）+ species with high valence and shorted bond from theFeOOH can promote the oxidation of the Ni2+ species in theNiFe LDH into Ni3+/Ni4+， which creates genuine active sites forOER 86. Liu et al. fabricated a NiFe LDH/（Ni， Fe）Se2heterostructure with much improved activity， the DFTcalculations revealed that the DOS of the heterostructure aroundFermi level is continuous and higher than that of NiFeOOH，which can improve the conductivity of the catalyst 87. Huang etal prepared an interface between the NiFe LDH and polyanilineto increase the OER activity， the experimental and theoreticalresults suggest that the strong interfacial interaction betweenNiFe LDH and polyaniline can promote the formation of manyelectron-rich Ni and Fe active sites at the nanointerface andincrease the DOS near the Fermi level and narrow the band gap，thus facilitating the electron transfer 88. Hwang et al. exfoliatedthe NiFe LDH and RuO2 into positively and negatively chargednanosheets， respectively， and make them assembled into NiFeLDH-RuO2 hybrid nanosheets via the electrostatic interaction（Fig. 6b）. The obtained catalyst manifested much enhancedcatalytic activity， the DFT calculations suggest that the electrontransfers from the LDH to RuO2， which increased theelectrophilicity of the LDH and promoted the rate-determiningsteps such as the adsorption of the OH/OOH· species 89. Yao etal. fabricated a heterostructure by coupling of exfoliated NiFeLDH nanosheet and defective graphene， the DFT calculationsrevealed that the electron transfers from the NiFe LDH to thegraphene at the defective carbon sites， which improves the OERperformance 90. Yang et al. prepared an exfoliated NiFe LDH（+）intercalated birnessite MnO2（?） hybrid catalyst with muchimproved activity， where the positively charged LDHcompensates the charge between the negatively charged MnO2（Fig. 6c）. The electron transfer between the two oppositelycharged 2D layers and the interlayer electric field lead to theenhanced OER kinetics 91. Zhang et al. anchored the single-atomAu on NiFe LDH and observed a 6-fold increase of the activity，the theoretical calculations suggest that the charge flows fromthe Au to the surrounding O， Ni and Fe in the LDH （Fig. 6d），which promotes the adsorption of OH? and adjust the adsorptionenergies of O* and OOH* intermediates 92. The heterointerfacecan facilitate electron transfer at the interface 93， and tuning theelectronic structure — including electron density and DOS — ofmetals can ultimately enhance catalytic activity.

3.2.2 Synergistic catalysis

Du et al. engineered a NiO/NiFe LDH intersection with amuch-reduced overpotential of 205 mV at 30 mA·cm?2 in 1mol?L?1 KOH， the outstanding activity originates from thetridimensional adsorption which varies with the type of theintermediates （Fig. 7a）， as such the binding energy of theintermediates can be tuned independently， providing anopportunity to elude the scaling relationship. As shown in Fig.7b， the active site （L1） obeys the scaling relationship， and itsactivity is at the top of the activity volcano 94. On the contrary，the interfacial active sites （S1） deviate from the volcano becauseof its lower overpotentials at low ΔG*O–ΔG*OH. Pan et al.reported that the surface-adsorbed SO4 2? can stabilize theintermediate *OOH on the active site， thus facilitate the OERprocess （Fig. 7c，d） 95. Multiple sites at the interface canparticipate in the OER process， including spillover， which couldboth facilitate the rate-determining step and even bypass theLSR.

3.3 The interlayer anions

It is mentioned above that the excessive positive charges ofthe LDH host would be compensated by the intercalated anions;it has been proved that the interlayer anions can be modifiedthrough the anion exchange reactions using inorganic or organicmolecules. The degree of hydration， size， and orientation and thecharge of the anions can impact the interlayer distance and thusinfluence the catalytic activity. For example， Coronado et al.synthesized a surfactant-intercalated family of NiFe LDHs withvarious basal spacing ranging from 8.0 to 31.6 ? by anionexchange reactions （Fig. 8a） and found that increase of the basalspace leads to an increase of the electrochemical surface area， adecrease of the Tafel slopes and the resistance related to theoxygen chemisorption that promote the kinetic behavior （Fig.8b） 96. While， Song et al. directly synthesized the non-carbonateanion containing NiFe LDHs such as Cl? and SO4 2? （Fig. 8c）.Although the catalytic behavior of these materials varies， similarelectrochemical catalytic activity was observed after thenormalization of the electrochemical surface area 97. Müller etal. found a correlation between catalytic activity and the pKavalues of the conjugate acids of the interlayer anions （Fig. 8d），and the activity can be improved by di-valent （such as CO3 2?） andtri-valent （such as PO4 3?） anions. The multi-valent anions withhigher charge are stronger proton acceptors and electron donors，it is inferred that the strong proton acceptor can lower the energybarrier of the OER 98. The interlayer anions exert their effect onthe catalytic performance through two mechanisms： firstly， byaltering the electrochemical surface area， and secondly， byfacilitating water splitting through their role as proton acceptors.

3.4 Vacancy

Engineering NiFe LDH with abundant edge sites （coordinatively unsaturated sites such as the metal or oxygenvacancies） has been evidenced to be an effective strategy toimprove the catalytic performance 99. For example， Zhang et al.synthesized porous monolayer NiFe LDH with abundant defectsthrough a fast nucleation method， the DFT calculations suggestthat all the oxygen， Fe and Ni vacancy can enhance the ability ofNiFe LDH to bond OH and thus improve the OER 100.Waterhouse et al. proposed that Fe3+ vacancies can promote theevolution of the γ-（NiFe）OOH active site during OER and adjustthe binding of OER intermediates on the catalyst surface，enhancing the OER activity 101. Sun et al. reported a fluoride precoveredsurface strategy to introduce the oxygen vacancies ofNiFe LDH catalysts （Fig. 9a）. The fluoride is introduced duringthe crystallization process， and can be easily cleaned byelectrochemical treatment， creating a controllable amount ofunsaturated metal sites （oxygen vacancies） with high activity forOER 102. Yang et al. introduced metal and oxygenmultivacancies in the NiFe LDH via the introduction of electronwithdrawingorganic molecule methyl-isorhodanate （CH3NCS）（Fig. 9b）. The high activity of the prepared catalyst is realized bythe co-existence of the O and metal vacancies， where theelectroactivity of Ni sites and O sites is activated by the defectiveregion， promoting the electron transfer and intermediatetransformation 103. Wang et al. reported two NiFe LDHsnanosheets with Ni2+ or Fe3+ vacancies， respectively， the muchimprovedactivity is attributed to the introduction of vacancies，which efficiently changes the surface electronic structure atreactive sites and promotes the adsorption of OERintermediates 104. Zhang et al. also demonstrated that the Ovacancy can lower the charge transport resistances for favorableaccessibility of catalytic active sites 105. Vacancies， whichinclude both anion and cation vacancies， have the potential toeither modify the electronic structure or expose more activesites， thereby enhancing the catalytic activity.

3.5 Other modification strategies

There are some other widely used strategies to improve thecatalytic activity of the NiFe LDH such as the lattice strain and exfoliation of the material 106，107. For example， Liu et al.developed a facile ball-milling strategy to strengthen the bindingstrength of the oxygenates by creating the lattice tensile strain（Fig. 10a）， which increased the electron density around the Niand Fe sites and thus facilitated the OER 108. Duan et al.introduced asymmetrical gradient effect into NiFe LDH atnanoscale through a nanoarray construction method on Ni foamsubstrate， where the Ni ： Fe ratio decreases from 3.5 ： 1 to 2.4 ：1 from the bottom to the top in NiFe LDH nanoarray （Fig. 10b）.The long-range gradient effect facilitated the electron and holestransfer in the catalyst and thus boosted the OERperformance 109. Li et al. exposed a high-activity （012） edgeplane comparing to the traditional （003） dominated NiFe LDHs，which manifested much enhanced activity （Fig. 10c，d）. The Featom is 4-O coordinated in the （012） plane， it can promote theformation of O* and OOH* with low barriers （Fig. 10e） 110. Du etal. prepared the ultrathin NiFe LDH with 100% exposed metalsites via the alcohol intercalation process. The catalyst displayshigh activity toward OER with an overpotential of 210 mV @10mA·cm?2 due to improved mass transportation 48.

4 Stability

As mentioned above， great research attention has been put inimproving the catalytic activity and revealing the reactionmechanism as well as the active sites of the NiFe LDH， only fewworks studied the catalytic stability despite that stability ishighly critical for the development of technologically viablewater oxidation electrodes. Many published literature reportedsatisfactory OER stability of NiFe LDH because the assessment was conducted at low current densities （～10 mA·cm?2） over ashort period of time 111–115. However， the practical application ofthe catalyst requires large current densities under harshconditions such as concentrated electrolyte and hightemperature， which may accelerate the degradation of thecatalyst. For example， Lin et al. observed a 24% loss of the initialcurrent density （～200 mA·cm?2） after performing 15 h at 1.63 Vvs. RHE and the fading happened quickly from the verybeginning 54. Simultaneously， severe Fe dissolution has beendetected， while the dissolution rate of Ni is much lower than thatof Fe 116. They found that the long-term chronoamperometry（CA） measurement will induce obvious FeOOH phasesegregation due to the dissolution of Fe and site-selectiveredeposition， and the Fe segregation is energetically favorable atthe edge sites. Unlike the stability test by continuous CAmeasurement， no abrupt current density loss was observed in thestability test via the CV. The authors believe that the cathodicpolarization during CV treatment can repair the structural andchemical degradation due to the anodic polarization. Therefore，they conducted the CA treatment with reduction for severalminutes between every 1 h of CA measurement and muchenhanced stability was observed （Fig. 11a–d）. Similarly，Markvoic et al. revealed that the surface of the catalyst isdynamic （concomitant dissolution and redeposition of activesites） 117. They revealed that the activity of the FeNiOxHy showssevere activity loss after just 1h of potential hold at 1.7 V in pureKOH （Fig. 11e）， however， the addition of Fen+ salt to theelectrolyte can prevent this deactivation under the sameconditions. It is proposed that addition of the Fen+ can balancethe rate of Fe dissolution and redeposition （Fig. 11f），establishing dynamically stable Fe active sites. Liu et al.speculated that the slow diffusion rate of the OH? into theinterlayers of NiFe LDH during the OER will induce a localacidic environment and cause the dissolution of the catalyst，which is more severe under large current densities 53. To improvethe diffusion rate of the OH?， they delaminated bulk NiFe LDHinto atomically thin layers and alleviated the dissolution of theactive sites during the OER. However， Schmidt et al. deducedfrom the thermodynamic formulations that the metal oxides mustbecome unstable during OER irrespective of the pH value 118， itis derived that the lattice oxygen evolution reaction （M2n+On 2? →M2n+（aq） + n/2O2 + 2ne?） always feels a higher overpotentialthan the one applied for OER （2OH? → 1/2O2 + H2O + 2e?），therefore， a concomitant oxidation of the lattice oxygen and thedissolution of the metal cations will occur at the onset of OER.

Therefore， there are two possible directions to developtechnologically stable NiFe LDH： （1） Lower the mobility of theoxygen in the lattice. For example， Waterhouse et al. evidencedthat the introduction of the Fe3+ vacancies can strength the metal-O bonding and thus alleviate the metal dissolution. （2） Establisha balance between the dissolution and redeposition of the Fe. Forexample， Markvoic et al. demonstrated that addition of the Fen+can balance the rate of Fe dissolution and redeposition，establishing dynamically stable Fe active sites.

5 Conclusions and perspectives

With the rapid advancement of water electrolysis technology，there has been a significant focus on developing highly efficientOER electrocatalysts in recent decades. Among these catalysts，NiFe LDH has emerged as a particularly promising non-preciousmetal catalyst due to its ultra-low overpotentials ofapproximately 300 mV at a current density of 10 mA·cm?2. As aresult， extensive research efforts have been dedicated toinvestigating the catalytic mechanisms of NiFe LDH， improvingits activity， and addressing durability challenges. Despite theseefforts， there is still considerable debate surrounding the realcatalytic centers of NiFe LDH. Currently， there are three mainviewpoints regarding the catalytic sites， namely the Fe sites， theNi sites after Fe incorporation， and the involvement of both Feand Ni in the catalysis. While many of these conclusions havebeen drawn based on observations of structural and propertychanges such as valence and metal―O bond distance using in situand ex situ techniques in combination with theoreticalcalculations， more work is needed to fully elucidate the activesites. In particular， future research should focus on atomicvirtualization of the catalytic process and the local structurerather than bulk properties to reveal the origin of the high activityobserved after Fe incorporation.

There is still ample room for further improvement of thecatalytic activity of NiFe LDH. Reported modification strategiescan be categorized into several types：

（1） Regulation of band structure： the band gap of the NiFeLDH can be reduced by heteroatoms doping or creation of aheterostructure. This improves electronic conductivity andfacilitates the OER.

（2） Tuning the electron density of metal centers： theadsorption energy of the oxygenates on the metal surface can bemodified by tuning the electron density of the metal centers viastrategies like heteroatoms doping， creation of heterostructures，lattice strain， and vacancies.

（3） Exposing more active sites： there are various strategies toexpose more catalytic sites， such as digging holes on the NiFeLDH nanosheets， exfoliating the catalyst， preferably exposinghigh active facets such as the （012） of the NiFe LDH， andintercalating larger anions （such as dodecyl sulfate and octadecylsulfate） into the interlayer to increase the surface area of thecatalyst.

（4） Synergistic catalysis： synergistic catalysis usually takesplace at the interface where all of the components are directlyinvolved in the OER process. The kinetics of the ratedeterminingsteps can be facilitated by the involvement of othercomponents. For example， the interfaced ―SO4 can stabilizethe *OOH through the interaction between H and O in ―SO4，facilitating the formation of *OOH from *O on NiFe LDH，which is considered to be the rate-determining step on the edgedFe sites in NiFe LDH.

To further lower the overpotential of the NiFe LDH， breakingthe thermodynamic limits is necessary. It has been demonstratedthat the binding energy of *OH， *O， and *OOH is linearlyrelated and cannot be tuned independently because they interactwith the catalyst surface via the same atom， which results in aconstant binding energy difference of ～3.2 eV between *OOHand *OH. This binding energy difference is 0.74 eV away fromthe ideal value of 2.46 eV and results in the smallestoverpotential of 0.37 V. Therefore， breaking or eluding thisthermodynamic limit is necessary to further reduce theoverpotential. Various strategies such as by designing singleatom catalysts or introduction of external forces like light ormagnetic forces can be used to elude or break the abovementionedlinear scaling relationship. For example， Du et al.reported the tridimensional adsorption of the intermediates at theNiO/NiFe LDH interface （Fig. 7a）， which varies with the type ofthe intermediates， and independently tuned the binding energyof the intermediates to elude the thermodynamic restriction.In addition to its catalytic activity， the stability of NiFe LDHis another challenge that must be addressed to enable itsapplication in industrial water splitting processes. Whensubjected to harsh conditions such as high current density andtemperature， Fe dissolution becomes severe， and the catalyticactivity deteriorates over time during long-term OER.Thermodynamic formulations suggest that metal dissolution isinevitable regardless of pH， as the lattice oxygen evolutionreaction （M2n+On 2? → M2n+（aq） + n/2O2 + 2ne?） requires a higheroverpotential than that applied for OER （2OH? → 1/2O2 +H2O + 2e?）， leading to concomitant oxidation of lattice oxygenand dissolution of metal cations at the onset of OER. Severalstrategies have been proposed to mitigate this problem， such asthe addition of Fen+ to the electrolyte， which can balance the rateof Fe dissolution and redeposition to establish dynamicallystable Fe active sites. Furthermore， the stability of the catalystcan be enhanced by inhibiting the mobility of oxygen anionswithin the bulk lattice， which effectively prevents the dissolutionof the catalyst. However， acquiring more insightful knowledgeis necessary to direct the design of robust NiFe LDHelectrocatalysts.

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