Facile Access to an Active γ‐NiOOH Electrocatalyst for Durable Water Oxidation Derived From an Intermetallic Nickel Germanide Precursor

Abstract Identifying novel classes of precatalysts for the oxygen evolution reaction (OER by water oxidation) with enhanced catalytic activity and stability is a key strategy to enable chemical energy conversion. The vast chemical space of intermetallic phases offers plenty of opportunities to discover OER electrocatalysts with improved performance. Herein we report intermetallic nickel germanide (NiGe) acting as a superior activity and durable Ni‐based electro(pre)catalyst for OER. It is produced from a molecular bis(germylene)‐Ni precursor. The ultra‐small NiGe nanocrystals deposited on both nickel foam and fluorinated tin oxide (FTO) electrodes showed lower overpotentials and a durability of over three weeks (505 h) in comparison to the state‐of‐the‐art Ni‐, Co‐, Fe‐, and benchmark NiFe‐based electrocatalysts under identical alkaline OER conditions. In contrast to other Ni‐based intermetallic precatalysts under alkaline OER conditions, an unexpected electroconversion of NiGe into γ‐NiIIIOOH with intercalated OH−/CO3 2− transpired that served as a highly active structure as shown by various ex situ methods and quasi in situ Raman spectroscopy.


Introduction
Theelectrocatalytic oxygen evolution reaction (OER) by water oxidation plays ad ecisive role in the development of clean renewable energy conversion and storage systems,asit provides electrons that can be used for critical reactions such as to reduce water to hydrogen, carbon dioxide to carbonaceous fuels,aswell as metal ions to the corresponding metals in recharging of metal-air batteries. [1] However,t he conversion efficiency of this crucial anodic half-reaction is limited by the inherently slow reaction kinetics due to the demanding four-proton coupled electron transfer pathway. [2] In order to lower the kinetic barriers of OER, numerous active electrocatalysts have been investigated for the past decades, however, with limited success.T herefore,r ecent research is mainly focused on exploring alternative electrocatalysts for water oxidation that are efficient, economic, competitive,yet durable. [3] In the last few years,n ickel-based catalysts have drawn enormous attention not only because of their promising activity and stability in alkaline electrolytes but also due to their function as anodes in large-scale commercial alkaline electrolyzers. [4] Over the last few years,t he suitability of numerous nickel-based oxidic and non-oxidic materials has been widely explored for efficient alkaline OER also in the context of structural-activity relationships. [4b, 5] Thep ronounced activity of such Ni-based electrodes has been ascribed to the presence of nickel oxyhydroxide (NiOOH) species at the surface,w hich serve as the OER active site. [6] Fort he non-oxidic Ni-based materials (chalcogenides,p nictides,o ra lloys), it has been shown that the transformation occurs prior to the OER either only at the surface of the electrocatalyst forming core-shell type structures or progressively throughout the bulk ensuring ad efect-rich and high surface area NiOOH. [7] Lately,s everal strategies including heteroatom doping, morphology and size control, compositional tuning, defect creation as well as interfacial engineering have been successfully utilized to enhance the density of active sites as well as electronic conductivity of Ni-based electrodes resulting in reasonable OER catalytic performances. [8] Thus,d iscovering as uitable and novel Ni-based (pre)catalyst to substantially improve the activity and durability of current OER catalysts remains af ormidable challenge and is one of the intensive efforts of current research. [9] Intermetallic nickel phases such as nickel germanides are an important class of metal semiconductors that are wellknown for their high thermal stability as well as their applications in the areas of microelectronics,p hotovoltaics, magnetism, and thermoelectrics. [10] Among the existing NiGe x phases,n ickel monogermanide (NiGe) has been regarded as ah ighly promising material due to its low electric resistivity (metallic property), enhanced stability in awide temperature range,low-temperature synthetic approach, and facile chemical processing. [11] Thus,itisthe most favored contact material in complementary to metal-oxide-semiconductor devices. [10a,d, 12] Until now,h owever, the synthesis of NiGe nano-materials is challenging and has been rarely reported. [10b] Further,i ts application for electrocatalysis has not been perceived so far.
Considering NiGe is astable intermetallic phase with high intrinsic conductivity with the active catalytic center,w e aimed to address in this report the following five research questions:(i) Can we adopt arational approach to synthesize well defined ultra-small NiGe nanostructures with high surface area from am olecular precursor?, (ii)w ill the structure of this intermetallic phase remain stable or is it merely ap recursor (precatalyst) in electrochemical alkaline OER?, (iii)what is the structural and functional role of Ge?, (iv) what type of transformation (active structure) can be expected and how does it influence the electrocatalytic OER activity and durability?, and (v) can we provide ag uideline for access to other transition-metal germanides towards OER based on our soft molecular approach?
In the last few years,significant progress has been made in developing various synthetic routes for the preparation of intermetallic compounds towards applications in electrocatalysis. [4a, 13] As traightforward strategy for the synthesis of nanostructured intermetallics is the low-temperature molecular precursor approach. [14] Thelatter enables agood control over the composition, structuring,morphology,and electronic properties of the materials. [13b] Taking advantage of this approach, we have now designed an ovel molecular germanium(II)-nickel(0) precursor complex for the preparation of nanostructured intermetallic NiGe that displayed remarkable catalytic activity and durability in alkaline water oxidation.
Here,w er eport ap remediated synthesis of the first xanthene-based bis(germylene)-Ni complex 2 (Scheme 1) and its utilization as al ow-temperature precursor to form monodisperse ultra-small NiGe nanocrystals.T he NiGe has been electrophoretically deposited on nickel foam (NF) and fluorine-doped tin oxide (FTO) electrodes and applied for alkaline OER for the first time,resulting in substantially low overpotentials surpassing the state-of-the-art Ni-, Fe,Co, and benchmark NiFe, and noble-metal-based catalysts.Strikingly, the NiGe also exhibits superior long-term stability over three weeks (505 h). Ac ombination of extensive ex situ methods and quasi in situ Raman spectroscopy revealed that NiGe is merely ap recatalyst for OER and, under applied potentials, transforms completely into OH À /CO 3 2À intercalated g-NiOOH that serves as an active catalyst to facilitate OÀO bond formation.

Results and Discussion
Am olecular bis(germylene)Ni precursor to access nickel germanide nanoparticles Germylenes represent heavy analogues of carbenes that possess adivalent Ge atom with alone pair of electrons;they can serve as potential ligands towards transition-metals. [15] Especially,t he N-heterocyclic germylenes (NHGes), featuring aGe II atom with p-donor interactions with the neighbor N atoms and/or from an intramolecular Lewis donor, are promising steering ligands akin to the N-heterocyclic silylene (NHSi)a nd carbene (NHC) congeners. [16] Taking advantage of the additional chelating effect of bis-NHGes and bis-NHSis,wehave successfully developed several bis-NHSis and bis-NHGes supported transition-metal complexes for homogeneous catalysis with remarkable reactivity. [17] Given their thermal lability,weenvisioned that abis(NHGe)Ni 0 complex could act as as uitable single-source molecular precursor for al ow-temperature synthesis of intermetallic nickel germanides.
Compound 2 is diamagnetic and has been unambiguously characterized by 1 Ha nd 13 CNMR spectroscopy,h igh-resolution mass spectrometry (HRMS), elemental analysis,a nd IR-spectroscopy (synthesis of complex 2 in Supporting Information). Them olecular structure of 2 established by an single-crystal X-ray diffraction analysis is depicted in Figure 1. In stark contrast to its silicon homologue, [18] in which the bis(silylene)-Ni is coordinated by an isomer of cod (1,3cyclooctadiene) resulting from 1,5-cyclooctadiene through proton migration, complex 2 features a1 ,5-cod ligand. Besides the coordination of both C=Cb onds of cod, the nickel atom is coordinated by the two germanium(II) centers of the bis(germylene) 1 with aG e-Ni-Ge angle of 101.6(2)8 8. TheNi1-Ge1 [2.315(4) ]and Ni1-Ge2 [2.311(4) ]distances are similar to the sum of theoretically predicted covalent radii for Ni-Ge single bonds [2.31 ]. [19] Thec rystal data, refinement parameters,and selected interatomic distances of 2 are given in Tables S3 and S4.

Synthesis and structural characterization of nanostructured NiGe
Them olecular complex 2 was subjected to thermolysis under hot-injection conditions at 250 8 8Ci no leylamine to Scheme 1. Synthesis of the molecularbis(germylene)-Ni complex 2 starting from 1.

Electrocatalytic OER activity
TheO ER catalytic performance of the prepared NiGe materials was evaluated in aqueous 1MKOH solutions using at hree-electrode cell. Prior to the OER measurements,t he NiGe was deposited on the high surface area, low-cost, conductive,and 3D open porous nickel foam (NF) as well as on fluorinated tin oxide (FTO) by electrophoretic deposition (EPD) that directly served as working electrodes.S ubsequently,t he as-deposited films were also characterized extensively which confirmed preservation of the chemical identity (stability) of NiGe on the electrode substrates (Figures S12-S19). To have af air comparison of its catalytic OER activity,t he literature reported highly active Ni-based catalysts (NiOOH and Ni(OH) 2 )w ere also synthesized ( Figure S20) and deposited through EPD on both NF and FTO under identical conditions with the same mass loading ( % 1a nd 0.4 mg cm À2 for NF and FTO,respectively). Figure 3a,t he cyclic voltammetry (CV) curves revealed that the NiGe/NF electrode displayed substantially enhanced OER activity compared to that of the as-deposited Ni(OH) 2 /NF and NiOOH/NF while the bare NF (subjected to the same EPD protocol) exhibited alimited activity.R emarkably,t he overpotential of the NiGe/NF electrode was only 228 AE 3mVa tac urrent density of 10 mA cm À2 ,w hich is lower than those of Ni(OH) 2 /NF   (292 AE 5mV), NiOOH/NF (308 AE 5mV) and bare NF (485 AE 9mV). Prior to the onset of OER, all measured Ni materials showed distinct redox peaks between 1.1-1.5 Vvs. reversible hydrogen electrode (RHE) associated with the oxidation of Ni II ! Ni III [Ni(OH) 2 + OH À ! NiOOH + H 2 O + e À ]t hat has widely been regarded as the catalytically active site for OER. [4a, 21] Theb road redox feature of NiGe/NF further suggested the oxidation of Ni occurs with the concomitant loss of Ge from the structure (see below). Moreover,t he OER kinetics of investigated materials was examined by Tafel analysis. [22] TheT afel plots were calculated by their corresponding LSV polarization curves and are shown in Figure S21. Impressively,N iGe/NF exhibits the lowest Tafel slope of 56 AE 2mVdec À1 compared to Ni(OH) 2 /NF (79 AE 1mVdec À1 )a nd NiOOH/NF (83 AE 1mVdec À1 )s uggesting am ore effective electron transfer, highly efficient reaction kinetics,a nd efficient catalytic OER activity of nanostructured NiGe/NF.

As shown in
Electrochemical impedance spectroscopy (EIS) was conducted to offer insights into the electrode kinetics during the OER process. [23] Thec harge-transfer resistance (R ct )a cross the electrode/electrolytei nterface was obtained from the Nyquist plot (Table S6). As shown in Figure 2b,t he R ct of NiGe/NF was lower than those of the other three Nielectrodes,d emonstrating its faster charge transport leading to an improved electrocatalytic OER activity.One of the most important criteria to evaluate the performance of the catalysts for practical applications in water splitting is their long-term durability under strongly alkaline operating conditions. [3a] Strikingly,t he OER chronoamperometric measurements (OER CA)o fN iGe/NF electrocatalyst displayed favorable operating stability,p roviding as table 10 mA cm À2 current density at ap otential of 1.48 Vv s. RHE for ap eriod of 24 h ( Figure S22). Motivated by this,w ef urther extended the durability measurements over three weeks that showed unceasing stability and sustainability of the NiGe/NF without any drop in the current density signifying its capability as an efficiently stable anode for alkaline water electrolysis (Figure 2c). Besides,aFaradaic efficiency of 95 AE 2% for OER was calculated by quantifying the evolved O 2 (gas) during electrolysis that matched well with the theoretically calculated values (Table S7). As the large-scale alkaline electrolyzers are operated at elevated temperature, [24] we conducted ac hronopotentiometric measurement at 100 mA cm À2 at 65 8 8Cfor 20 hthat exhibited astable potential curve supporting its usability at higher current densities ( Figure S23).
In order to assess the origin of the greatly enhanced water oxidation activity,t he double-layer capacitance (C dl )w as determined, which is proportional to the electrochemically active surface area (ECSA). [25] Thec haracteristic CV curves of presented materials with different scan rates in an onfaradaic region are shown in Figure S24. Thec orresponding capacitive current value as af unction of the scan rate was used to calculate the C dl .AC dl value of 1.36 AE 0.04 mF cm À2 was attained for NiGe/NF while much lower values,0 .85 AE 0.02, 0.79 AE 0.01, and 0.64 AE 0,01 mF cm À2 were obtained for Ni(OH) 2 /NF,N iOOH/NF and NF,r espectively ( Figure S25), suggesting that more catalytically active sites of NiGe/NF are available for the OER process.I ti sk nown from previous OER studies that non-oxide phases can undergo severe corrosion in strongly alkaline conditions leading to apartial or complete loss of metal/metalloid. [13c-f,26] To verify this,w e measured the C dl of NiGe/NF after 24 ho fO ER CA giving rise to av alue of 2.82 AE 0.04 mF cm À2 ,w hich is twice larger than the as-deposited NiGe/NF ( Figure S26). Theincrease in C dl (or ECSA) is an indication of the rapid loss of Ge into the electrolyte with significant structural transformation. Furthermore,t he currents of NiGe/NF,N i(OH) 2 /NF,a nd NiOOH/NF catalysts were normalized by (i)g eometric area, (ii)m ass,( iii) C dl (ECSA), and (iv) BET surface area and presented in Figure S27 that supported the derived conclusions (Table S8). Most importantly,i ti sw ell known that the comparison of overpotentials is only meaningful if the catalysts are tested in identical conditions with optimized parameters (mass loading, iR correction, geometric area, type of electrode substrate,set-up design, etc.). [27] Taking this into account, we synthesized, electrodeposited, and compared activities of several non-noble metal-based state-of-the-art electrocatalysts with Co-, Fe-and NiFeO x as well as commercial IrO 2 and RuO 2 for the alkaline OER in identical conditions ( Figures S28-S35). Remarkably,t he obtained OER overpotentials of these catalysts were still substantially higher than the one of NiGe/NF (Figures 2d and S36). In addition, the OER activity comparison of the presented catalysts with those of reported benchmark NiFe-or Ni-based catalysts are listed in Tables S9 and S11.
After achieving encouraging results on NF,w ef urther investigated the electrocatalytic OER performance of the same materials deposited on FTO,p rimarily to understand the inherent role of the electrode substrates influencing the catalytic activity.T he LSV profiles of NiGe/FTO,N i(OH) 2 / FTO,N iOOH/FTO, and FTOa re shown in Figure S37. Similar to the high activity and low overpotential demonstrated on NF,the NiGe/FTOrequired only an overpotential of 322 AE 2mVtoyield acurrent density of 10 mA cm À2 ,which is much smaller than those required for Ni(OH) 2 /FTO(380 AE 6mV) and NiOOH/FTO( 444 AE 6mV). As observed in the case of NiGe/NF,the CV of NiGe/FTOalso featured aredox pair evidencing that Ni II is reversibly converted to Ni III (NiOOH) which serves as the active site for OER (Figure S38). [21a] Theconducted EIS and ECSA measurements on NiGe/FTO, Ni(OH) 2 /FTO,a nd NiOOH/FTOe xhibited accelerated charge transfer and higher density of active sites for NiGe/FTOc ompared to the other investigated reference catalysts ( Figures S39, S40 and Table S10). TheO ER CA response of NiGe/FTO was examined at 1.58 Vv s. RHE for 24 h, which displayed excellent stability of the electrocatalyst ( Figure S41). Furthermore,t he normalized current densities followed the same trend as that of deposited catalysts on NF ( Figure S42). Finally,t he superiority of NiGe/FTO was illustrated by comparing the overpotentials of the as-deposited non-noble and noble-metal-containing catalysts on FTO as well as literature reported Ni-based catalysts ( Figures S43-S45, Table S9).

Ex situ post-catalytic characterization
To gain an in-depth insight into the bulk and surface structure and to uncover the nature of active species for determining as tructure-activity relationship,t he post-OER (CA24h)NiGe/FTO electrode was extensively characterized through diffraction, microscopic,s pectroscopic,a nd analytical methods.T he PXRD pattern of NiGe/FTOa fter OER exhibited ab road diffraction pattern similar to as-prepared material indicating the small particle size (Figure S46 and  S47). Interestingly,t he SEM images of NiGe after OER showed porous-type morphology with an agglomeration of particles ( Figure S48). TheE DX mapping of the film exhibited an almost total loss of Ge (over 95 %) and concomitant incorporation of Ointo the structure that shows homogeneous dispersion of Ni, suggesting complete corrosion of the NiGe phase to an OER active NiO x H y structure ( Figure S49). Thep ercentage of distribution of the elements derived by EDX mapping is also consistent with the result acquired from the ICP-AES and XPS analysis (Figures S50 and S51, Table S5). As observed for SEM, the TEM images showed arigorous transformation of initial NiGe into ahollow nanostructure (Figure 4aand Figure S52a-c). Acloser look at the nanostructures in the HR-TEM images exhibited lattice fringes of 0.23 nm that can be assigned to (102) plane of g-NiOOH (JCPDS 6-75) (Figure 4b,inset). More importantly, the SAED pattern produced weak diffraction rings at 0.34, 0.23, and 0.21 nm for (006), (102), and (105) crystallographic planes,respectively,corresponding to the g-NiOOH structure ( Figure S52d). With this information in hand, we evaluated the FT-IR spectra of OER CA that showed the appearance of new IR bands compared to the initial NiGe phase (Figure S53). Notably,t he band at 570 cm À1 can directly be corroborated to Ni III -O stretching vibrations of g-NiOOH and is ar epresentative band for the literature reported phases of g-NiOOH. [28] Surprisingly,anew band at 1382 cm À1 was observed too that can be attributed to carbon-oxygen stretching vibrations of CO 3 2À . [29] Thec arbonate anion originates from the KOHe lectrolyte that consumes CO 2 from ambient air. [30] Such intercalation of carbonate anions between interlayers of g-NiOOH has shown to be beneficial to enhance the OER activity. [28c, 30] Besides,IRbands responsible for surface hydroxylation were also evident. [28b,c] Thes urface valence states of fully converted NiGe after OER was obtained by XPS analysis (Figure S54a). TheNi2p 3/ 2 and Ni 2p 1/2 spectrum displayed sharp peaks at the binding energy of 856.0 and 873.7 eV (along with two typical satellite peaks) that can only be ascribed to Ni III of the structure,while the peaks responsible for Ni 0 were absent, confirming the complete electrooxidation of NiGe surface to g-NiOOH under alkaline OER (Figure 4c). [21b,28a,c, 31] Moreover,t he binding values obtained here are typical of g-NiOOH phases reported in the literature. [28a, 32] TheG e3 ds pectrum did not show any peak corresponding to Ge 0 ,indicating the oxidation of the material (Figure 4d). Thed econvoluted peaks at 32.2 and 33.3 eV corresponding to Ge 3d 5/2 and 3d 3/2 ,which could be due to the Ge IV O 2 generated from the adsorption at the film surface ( Figure S54b). [33] TheO1 ss pectrum was deconvoluted into two peaks,at% 531.0 eV and 531.7 eV consistent with the formation of g-NiOOH ( Figure S54c). [34] As it has been shown that Fe from the electrolyte can be incorporated into the Ni-based catalysts,w ee xamined the XPS survey spectrum ( Figure S55) to detect the presence of Fe on the surface of the films.The spectrum was inconclusive due to the overlapping Fe 2p and Sn 3p signals,and therefore,ICP-AES was conducted that showed am inute amount of Fe (< 0.4 AE 0.1 %) after 24 hofC A.
To address the question on the dissolution rate of Ge into the electrolyte and extent of phase transformation in OER conditions,wealso examined the NiGe/FTOfilm after initial CV (OER CV,3 cycles) measurements (Figures S56-S60,  Table S5) by SEM, EDX, elemental mapping,I CP-AES,FT-IR, and XPS analysis.S urprisingly,t he OER CV characterization results matched exactly with that of OER CA demonstrating that the electroconversion of NiGe to g-NiOOH is an instantaneous process.B esides,O ER CV and CA,t he NiGe/NF electrodes were also characterized after three weeks (505 h) of durability tests.A sa nticipated, the obtained SEM, EDX, elemental mapping, and XPS mapping results were found to be similar to that of CV and CA experiments further confirming the porous nature of electrodes,the absence of Ge in the structure as well as the formation of Ni III throughout the bulk (Figures S61-S64). Thus,from the ex situ characterizations at various time intervals of OER, it was apparent that the NiGe suffers spontaneous restructuring under an applied bias finally forming a g-Ni III OOH as the catalytically active structure.A ccording to the Pourbaix diagrams, [35] the structural conversion of NiGe to g-NiOOH under alkaline OER can be explained via oxidative leaching of Ge in the form of deprotonated germanic acid (GeO 3 2À ), and subsequent oxidation of nickel [from Ni d+ to Ni III , Eq. (1)],w hich should proceed through the following equation: Quasi in situ and ex situ Raman spectroscopy To uncover the active structure under quasi in situ conditions,w ep erformed (resonance) Raman spectroscopy of NiGe,w hich is av ery sensitive probe to the surface structure of the catalysts.T he as-prepared NiGe powder and the as-deposited NiGe film did not show any distinguishable features indicating that NiGe does not possess any Raman allowed bands.W et hen performed quasi in situ Raman measurements on the NiGe film treated for 24 hofOER CA where Raman bands at 481 and 554 cm À1 were observed. [4f, 5c, 36] By comparing with the reported g-NiOOH structure,t he first band at 481 cm À1 is clearly associated with the depolarized E g mode (bending) whereas the band at 554 cm À1 can be assigned to the polarized A 1g mode (stretching). [28c,34b,37] From Figure 5, one can note that for the E g mode the oxygen atoms vibrate along the plane while in the A 1g mode they vibrate perpendicular to the plane. [38] In addition to this,abroad band between 850 cm À1 and 1200 cm À1 also appeared that has been previously attributed to n(O-O) of an active oxygen species (NiOO À )i no xyhydroxide structure ( Figure S65). [4f, 36, 37c, 38] Very similar features were also observed for the ex situ Raman spectrum of NiGe film after OER CA with additional small bands at 630-660 cm À1 that can be attributed to the presence of oxygen vacancies. [39] Insights into the active structure Nickel-(oxy)hydroxides,i np articularly, a-Ni(OH) 2 , b-Ni(OH) 2 , b-NiOOH, and g-NiOOH have widely been utilized as cathodes in primary and secondary alkaline batteries as well as supercapacitors. [40] Thep hase transformation mechanism of these materials during the electrochemical redox process or aging was described by Bodesd iagram, [41] according to which the g-NiOOH can be achieved by the oxidation of a-Ni(OH) 2 or overcharging of b-NiOOH (Figure S66). [28b,32, 42] The a to g transformation involves more than one electron transfer,and the maximal oxidation state of Ni, in this case,i sl imited to + 3.6. [42,43] Most importantly,t he crystal structure of g-NiOOH comprises of alarge interlayer spacing of % 7 and contains intercalated species such as water or ions that are absorbed between the layers. [28b] Such large interlayer spacing in g-NiOOH has shown to favor the ionic intercalation of OH À or CO 3 2À (from the dissolved electrolyte) anions to expose alarge number of active sites for the evolution of oxygen ( Figure 6). [28c, 30] Our results are also consistent with the previous study on the role of interlayer anions in NiFe-or Ni-based layered double hydroxides (LDH) where the highest catalytic OER activity can be achieved when hydroxide/carbonates species are present in the interlayers of LDH. [28c, 30] Therefore,t he enhanced performance of g-NiOOH derived in situ from NiGe,compared to the as-prepared and other literature known NiOOH electrocatalysts, [4b] can be attributed to the structural flexibility of Ni sites through the generation of (defected) structure with ionic intercalation of OH À /CO 3 2À between the large interplanar spacing of g-NiOOH, higher ECSA values, better electronic conductivity,superior OER kinetics,and an accelerated charge transfer resistance to facilitate OER.

Conclusion
We have successfully addressed the research questions (i)-(v) as mentioned above in the introduction. With respect to question (i), ar ational protocol to synthesize well-defined ultra-small NiGe nanoparticles could be achieved, starting from an ew Ni-Ge molecular precursor.T oa nswer the questions (ii)-(v), the as-prepared NiGe was electrophoretically deposited on NF and FTO and investigated for the Figure 5. Quasi in situ and ex situ Raman spectroscopy of NiGe (OER CA, 24 h) on FTO collected at apotential of 1.58 Vversus RHE in alkaline electrolyte. The stretching A 1g and bending E g vibrational modes of g-NiOOH is represented by green octahedra.F or comparison, the Raman spectra of the as-prepared and as-deposited (inactive) NiGe are also shown. Figure 6. Crystal structure of NiGe and electrochemically generated g-NiOOH. Under applied potential( V) in an alkaline KOH electrolyte, acomplete dissolution of Ge was observed forming g-NiOOH. From the large interplanar spacing, it can be seen that the ionic intercalation of OH À /CO 3 2À is favored for the OER process that is reflected in higher ECSA values.
alkaline OER for the first time.T he catalytic activity and stability of the NiGe was found to be strikingly higher than that of benchmarked Ni-, Fe-, Co-, and NiFe-based catalysts in identical conditions.U nder electrochemical conditions, avigorous electroconversion of NiGe occurred indicating that the NiGe is indeed aprecatalyst for OER. Thedissolution of Ge further confirmed the insignificant structural or functional role of Ge for OER, however, it is essential to direct the structure to form the most active catalyst. From the combination of advanced ex situ and quasi in situ Raman spectroscopy,i ti sd emonstrated that the transformation of NiGe is instantaneous,f orming ah ighly-active g-NiOOH phase that acts as the most active structure to facilitate O 2 evolution. It could be expected that the large interplanar spacing of g-NiOOH provides the ionic intercalation of OH À / CO 3 2À ,w hich is favored for the OER process as reflected in higher ECSA values.W eb elieve that the insights offered in this study can easily be generalized to the other transition metal germanides that are also expected to follow the same transformation in alkaline media as that of NiGe.T he presented molecular approach might be referenced to investigate and explore alarge number of unexplored class of solidstate intermetallic materials for water splitting that otherwise are impossible to attain in their nano-form.