SILAR Synthesized Binder-Free, Hydrous Cobalt Phosphate Thin Film Electrocatalysts for OER Application: Annealing Effect on the Electrocatalytic Activity

Highly e ﬃ cient and robust electrocatalysts intended for the oxygen evolution reaction (OER) are essential for energy conversion devices; the structural and morphological fractions of the electrocatalyst can also greatly in ﬂ uence the OER performance. Therefore, developing a high-performing electrocatalyst with desired properties is crucial through a simple and cost-e ﬀ ective chemical process. So, the binder-free, hydrous cobalt phosphate (Co 3 (PO 4 ) 2 .nH 2 O) thin ﬁ lm electrocatalysts are prepared via the successive ionic layer adsorption and reaction (SILAR) method onto stainless steel (SS) substrates at ambient temperature. Additionally, the impact of annealing on the OER e ﬃ ciency of thin ﬁ lm electrodes made of hydrous cobalt phosphate was observed by subjecting the electrocatalysts to di ﬀ erent temperatures (200 ° C and 400 ° C). The SILAR synthesized hydrous Co 3 (PO 4 ) 2 .nH 2 O with the short-range ordered agglomerated particles transformed into discrete nanoparticles with an annealing temperature. The as-prepared hydrous cobalt phosphate (CP) demonstrated outstanding OER performance with the least overpotential ( η ) of 265mV at 10 mAcm -2 current density and the lowest Tafel slope of 37 mV dec -1 , and the overpotential ( η 10 ) increased upon the annealing of catalysts (CP 200 and CP 400). Moreover, the as-prepared electrocatalyst demonstrated overall water splitting at the lowest potential of 1.56V (@10mAcm -2 ) in the alkaline electrolysis system (CP//Pt). The present study reveals that the electrocatalytic performance of the as-prepared cobalt phosphate thin ﬁ lm catalyst is signi ﬁ cantly associated with the hydrous content present in catalysts and demonstrates the practical applicability of SILAR-synthesized binder-free cobalt phosphate thin ﬁ lm electrocatalysts.


Introduction
The potential of hydrogen gas (H 2 ) as a green and sustainable energy source is widely recognized within the industry, with an anticipated significant contribution to the expanding renewable energy sector [1,2].Among various hydrogen production processes, electrochemical water splitting (2H 2 O⟶2H 2 +O 2 ) is one of the attractive paths to produce hydrogen (H 2 ); however, electrochemical water splitting at a large scale is significantly hindered by the sluggish oxygen evolution reaction (OER) at the anode [3,4].Within the OER, multiple surfaceadsorbed intermediates result from four proton-coupled electron transfer processes [5,6].Hence, the focus of most of the research community pivoted towards OER kinetics and the continuous search for synthetic catalysts to overcome this problem, which can play an essential role in minimizing overpotential towards OER.To attain maximum effectiveness, it is essential for the electrocatalyst substance to possess various catalytic active sites, excellent conductivity, uncomplicated transport of mass, unhindered detachment of gas molecules, and enduring stability [7].
In general, noble metal-containing catalyst materials like platinum (Pt), ruthenium (Ru), and iridium oxide (IrO 2 ) are classed as state-of-the-art OER catalysts because of their excellent electrical conductivity, low overpotential, and Tafel slope [8,9].Despite their potential benefits, commercializing them is prohibitively expensive due to their scarcity.Moreover, their instability in highly acidic or alkaline media severely restricts their practical applications [10,11].Producing electrocatalysts of superior caliber is a prerequisite to ensuring desirable functionality and endurance without relying on precious metals.Therefore, various transition metal-based compounds such as transition metal oxides (TMOs) [12], transition metal hydroxides (TMOHs) [13], transition metal sulfides (TMSs) [14,15], transition metal carbides (TMCs) [16], transition metal chalcogenides (TMChs) [17], and transition metal phosphides/phosphates (TMP/Pi) [18] have been studied as promising electrocatalysts.Regarding this, the TMPi are fascinating electrocatalyst materials for water splitting owing to the large number of active sites provided by polyhedral metal centers and tetrahedral phosphate ligands, which facilitate the adsorption and stabilizing of active centers [19].Additionally, regional atomic geometry and phosphate ligands facilitate a conducive environment for the adsorption and oxidation of electrolyte ions [20].Cobalt phosphate (CP) is one of the finest catalyst materials among several TMPi for electrochemical water splitting; however, there is still a significant need for cost-effective advancement efforts in enhancing the activity and durability of CP electrodes towards optimal OER performance.
The physicochemical properties of a catalyst material greatly determine its electrocatalytic performance, and various morphologies and microstructures of cobalt phosphate electrocatalysts have been explored for OER application.However, innovative attention has recently been paid to amorphous/nanocrystalline electrocatalysts since they offer structural flexibility and rich defects, are fully active for electrolytic ion adsorption, formation of intermediate states, and electron transfer, and can provide enormous opportunities for electrochemical water splitting [21,22].Furthermore, compared to their ordered crystalline counterparts, amorphous phases exhibit a greater abundance of bonds arranged randomly while possessing unsaturated electronic configurations [23].Recently, it has been reported that the nanoconfined fluids, i.e., structural water or the hydrous content present in the material, play a vital role in expanding electrochemical double-layer capacitance (EDLC) by means of providing a facile interface to electrolytic ions [24].It can also benefit the ion transfer and facile interface during the electrochemical catalytic process.Therefore, synthesizing an amorphous/nanocrystalline hydrous electrocatalyst is crucial to achieving high electrocatalytic performance.

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International Journal of Energy Research the surface morphology and particle size of preparing thinfilm electrocatalysts can be altered by varying growth kinetics through adsorption, reaction, and rinsing time.Furthermore, unlike hydrothermal and other wet chemical methods, the SILAR method avoids the wastage of materials due to the precipitation by homogeneous growth while preparing the binder-free electrodes.So, precision, uniformity, versatility, and affordability make the facile SILAR method viable to produce binder-free, amorphous, CP thin film electrocatalysts on a conductive substrate.Therefore, the influence of annealing at 200 and 400 °C on the as-prepared cobalt phosphate electrocatalyst by the SILAR method and its consequent effect on OER performance have been investigated for the first time.
In the present work, binder-free, hydrous cobalt phosphate thin film electrocatalysts are successfully synthesized via the facile SILAR method herein.The role of annealing of CP thin film catalysts on their physicochemical properties and, consequently, electrochemical catalysis is examined.Additionally, a comparative performance of as-prepared hydrous CP (Co 3 (PO 4 ) 2 .nH 2 O) with annealed samples (CP 200 and CP 400) experimentally demonstrates that asprepared hydrous CP is a more efficient OER catalyst in 1 M KOH electrolyte.The as-prepared CP thin film electrocatalyst attains a current density of 10 mA cm -2 at a low overpotential of 265 mV with a small 37 mV dec -1 Tafel slope.Also, the overall water-splitting performance is evaluated for 30 minutes using the best-performing CP and Pt electrocatalysts for OER and HER.Accordingly, the present work not only offers an analytical approach to studying the impact of annealing on electrocatalysts but also brings new perceptions into the OER performance of amorphous, hydrous CP-based electrocatalysts.

Experimental Detail
2.1.Synthesis of CP Thin Film Electrocatalyst.To prepare hydrous CP thin film electrocatalysts, a wet chemical SILAR method is adopted since it includes a successive accumulation of cations and reaction with the anions over the surface of the substrate from distinctly placed aqueous precursor solutions to form a thin film, as illustrated in schematic Figure 1(a).The CoCl 2 •6H 2 O (with a purity of >98%) and K 2 HPO 4 (with a purity of >98%) chemicals were procured from Sigma-Aldrich.In one beaker, 0.05 M CoCl 2 •6H 2 O (0.594 gm dissolved in 50 mL of double distilled water, abbreviated as DDW) was prepared as the cationic bath.Simultaneously, in another beaker, 0.05 M K 2 HPO 4 (0.435 grams dissolved in 50 mL DDW) was prepared as the anionic bath.Meanwhile, two DDW beakers were arranged for rinsing after cationic and anionic baths.The well-polished stainless steel (SS) substrates (1 cm × 5 cm pieces) were washed with the laboratory liquid soap and DDW many times.Furthermore, SS substrates are ultrasonicated in the ethanol solution and DDW and further used for the deposition of an electrocatalyst.
In the first step, well-cleaned SS substrates were plunged into the first beaker for 10 seconds containing a cationic pre-cursor to allow metal ion (Co) adsorption on the substrate surface.In the next step, the cation's (Co 2+ ) adsorbed substrates were rinsed for 5 seconds in the first DDW (second beaker) rinsing bath to dismiss the improperly adsorbed and surplus cations from the substrate surface.In the next step, cations adsorbed substrates submerged in the third beaker for 10 seconds, containing a precursor of anionic, where anions reacted with preadsorbed cations to make a monolayer of CP on the substrate.Lastly, the deposited substrates were dipped into the second rinsing bath for 5 seconds (fourth beaker) to confirm the removal of unabsorbed and nonadherent ions.The procedure completes one cycle for the deposition of a monolayer of CP.After similar 80 cycles, the uniform and well-adherent CP thin films are deposited with the desired thickness of 0.59 mg cm -2 .Moreover, other optimized preparative parameters, such as pH of the solution, concentrations of precursors, dipping time, deposition temperature, and number of cycles, are provided in Table S1 (see ESI).After drying the CP films at room temperature, the deposited films were annealed at 200 and 400 °C using a tube furnace with a 2 °C min -1 heating rate, maintained for 2 h in a vacuum, and designated as CP 200 and CP 400, as shown in the photographs of the electrocatalyst shown in Figure 1(b).

Characterizations.
The temperature profile of 30 to 1000 °C is examined by means of the HITACHI STA thermal analyzer, where thermogravimetric analysis and differential thermal analysis curves are acquired.The rate at which heating occurs during this process is set at a steady pace of 5 °C/minute.The Rigaku Miniflex X-ray diffractometer was employed to record the X-ray diffraction patterns, covering an angular range of 5 to 80 °for 2θ, where Cu-Kα target radiation is λ = 1 5406.Fourier transform infrared spectrometry (FT-IR) (Alpha (II) Bruker) was employed to probe the existing functional groups in the as-prepared and annealed catalyst thin films.Raman spectroscopy was performed for molecular vibrational analysis in the 100 to 4000 cm -1 wavenumber range using the Renishaw Raman microscope with an excitation wavelength laser of 532 nm.For the X-ray photoelectron spectroscopy (XPS) study, the Thermo Scientific ESCALAB 250 instrument was used.Moreover, to investigate the surface morphology and composition properties of the sample, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses were performed utilizing JEOL JSM-7001F FE-SEM equipment.
2.3.Electrocatalytic Activity Measurement.The electrodes were subjected to an electrocatalytic examination through a VERSASTAT 4-500 electrochemical workstation, employing the customary three-electrode arrangement in an electrolytic cell.The prepared thin-film electrocatalyst and platinum metal plate (Pt) are used as working and counter electrodes.The mercury/mercury oxide (Hg/HgO) containing 1 M KOH was used as a reference electrode.To study OER, 1 M KOH electrolyte (pH = 14) was used at room temperature.The measured voltages were converted from Hg/HgO 3 International Journal of Energy Research to a reversible hydrogen electrode (RHE) scale by using the Nernst equation [7].
where E RHE is the converted voltage vs. RHE, E Hg/HgO is the experimentally measured potential against the Hg/HgO reference electrode, and E °Hg/HgO is the standard potential (0.098) of the Hg/HgO reference electrode.The LSV curves (not fully or partially iR compensated) were recorded at a scan rate of 1 mV s -1 in the 1.0 to 1.7 V vs. RHE potential window to calculate the overpotentials at the current density of 10 mA cm -2 .The following equation calculates the Tafel plots [7].
where a is the intercept, the slope is denoted by b, and j is the current density.The electrochemical active surface area (ECSA) was assessed by acquiring cyclic voltammetry plots while altering scan rates within the non-Faradaic potential range from 0.7 to 1.05 V vs. RHE.At 10 mV AC amplitude, the electrochemical impedance spectroscopy (EIS) was recorded in the 0.1 MHz to 10 MHz frequency range at 10 mV amplitude.To check the durability, a chronoamperometry (CA) study for 24 h at overpotential was conducted for the best-performing thin-film electrocatalyst.Moreover, full-cell electrolysis was performed using a two-electrode arrangement, where a high-performing thin film electrocatalyst and a platinum plate are utilized as an anode and cathode, respectively, and electrochemically characterized in the same alkaline electrolyte for overall water splitting and durability tests.

Formation of Hydrous CP Thin Film and Reaction
Mechanism.In the present study, the synthesis of a binderfree and hydrous CP thin film electrocatalyst is achieved by the facile SILAR method.The production of CP thin films involves the immersion of the substrate into two distinct precursor solutions, one cationic and the other anionic.These solutions are separated by washing vessels during the process of immersing, resulting in the acquisition of CP thin films, which restrict the highly crystalline growth of material and enable the trapping of water (hydrous content) during deposition.Hence, the SILAR method allows the preparation of hydrous, amorphous materials in thin film form over any substrate.Thus, in the current study, an approach was taken where the SS substrate underwent immersion in a Co 2+ cationic precursor solution followed by exposure to HPO 4 2-anionic solution.The interaction between preadsorbed Co 2+ and HPO 4 2-ions resulted in the creation of a firmly attached hydrous CP layer on top of the substrate, as described by an equation.
Thus, the ion-by-ion growth process forms a hydrous CP thin film electrocatalyst, where the film deposition ascends by nucleation, followed by the coalescence and stacking of particles [42].The synthesis of hydrous CP thin films using the SILAR method has several attractive features: it leads to amorphous/nanocrystalline material formation in the thin film owing to abruption in growth due to rinsing baths, and it can also introduce hydrous content into the structure of the material.Significantly, a low deposition temperature prevents adverse impacts such as dopant redistribution, interdiffusion, and contamination.Hence, the SILAR method favors producing a thin film of hydrous CP (Co 3 (PO 4 ) 2 .nH 2 O).
Furthermore, CP thin film electrocatalysts were annealed at 200 and 400 °C for 2 hours to dehydrate hydrous CP films.In order to determine the effect of annealing temperature on hydrous content in CP material, TGA-DTA analysis was conducted.The analysis was conducted over a temperature range of 30 °C to 1000 °C with a heating rate of 5 °C per minute, as depicted in Figure S1(a).There are three significant stages of weight loss: between room temperature (30 °C) and 100 °C, 200 °C, and 400 °C.From the TGA plot, an initial ~3% weight loss with an exothermic process up to 100 °C is observed due to the loss of surface adsorbed moisture.The weight loss of ~11% in the temperature region up to 200 °C is caused by the partial removal of structural water from the bulk of the cobalt phosphate electrocatalyst, and the exothermic peak in the DTA curve confirms the desorption of water and phase transformation.The percentage weight decrease of partial dehydration towards Co 3 (PO 4 ) 2 .4H 2 O is almost the same as the theoretical value.Finally, it has been observed that the cobalt phosphate electrocatalyst experiences a weight loss of up to approximately 24%, which demonstrates that crystal water of hydrous CP (Co 3 (PO 4 ) 2 .8H 2 O) dehydrated through the evaporation of remaining nanoconfined water at temperatures up to 400 °C and is almost the same as the theoretical values.The TGA graph does not show significant visual further weight loss after 400 °C and turns out to be a thermodynamically stable phase, which reveals the formation of a dehydrated phase of cobalt phosphate thin film electrocatalysts.
The XRD patterns of the CP series as prepared and annealed are demonstrated in Figure 2(a), representing CP, CP 200, and CP 400 samples.It is observed from these patterns that instead of clear, strong peaks, a broad hump indicates an absence of crystalline structure, confirming that prepared cobalt phosphate materials are amorphous.Additionally, the XRD patterns of CP, CP 200, and CP 400 thin films deposited on the SS substrate are shown in Figure S1(b).Initially, the XRD of the CP, CP 200, and CP 400 electrocatalyst powder samples scratched from the substrate was measured to avoid the suppression of peaks in the CP sample due to the dominance of crystalline SS peaks.The amorphous structures in both unannealed and even after being heated at two different temperatures offer versatility in terms of structural adaptability for electrocatalysts, while the defects associated with such materials have the potential to enhance electrolytic water separation activity via increasing their efficiency due to their hydrous amorphous nature [43].Additionally, the FT-IR spectra of annealed and as-prepared CP samples were analyzed to investigate how annealing affects their structural characteristics.The spectrum presented in Figure 2(b) displays a band at 1637 cm -1 (α4) and a broader band at 3400 cm -1 (α5), both related to the O-H bond bending and stretching of physically adsorbed/ nanoconfined water present in the CP catalyst.Interestingly, the intensity of the α4 and α5 peaks decreases gradually with temperature rise, which implies the progressive elimination of physically adsorbed water and nanoconfined hydrous content.However, moisture is still present even after being annealed at 400 °C, as the (α4, α5) peaks do not completely vanish.The alteration of the aforementioned distinct peaks is accompanied by an elevation in temperature, leading to a surge in the peak intensity at 474 cm -1 (α1) and 1054 cm -1 (α3), which are attributed to P-O bending modes and asymmetric stretching modes of PO 4 3-ions.Also, a peak that appears at 570 cm -1 (α2) is associated with the bending vibrations of Co-O in CoO 6 octahedra.Moreover, Raman spectroscopy is employed for as-deposited and annealed CP samples between 100 and 4000 cm -1 wavenumber range to examine dehydration levels, as illustrated in Figure S2(a) (see ESI).The observed spectral features in the analyzed samples can be attributed to specific vibrational modes of molecular groups and ions.Two distinct peaks were observed at 370.6 cm -2 (α1) and 578 cm -2 (α2), related to bending vibrations of O-Co-O bonds in cobalt phosphate species and H 2 PO 4 fragments, respectively.Moreover, a major peak with high intensity was detected at 971 cm -1 (α3) for both samples, corresponding to the stretching mode of a cobalt phosphate moiety.In addition, another feature showing a shoulder peak is assigned due to the symmetric stretching vibration mode [PO 4 ] within the tetrahedral geometry configuration formed by the phosphate ion coordinated with the metal center atom Co.Additionally, minor signals appearing at frequencies around 1600 cm -2 (α4) and 3316 cm -2 (α5) signify different oscillatory behaviors such as bending vibrations involving H-O-H molecules or OH-stretching motions happening International Journal of Energy Research mainly along hydrogen bonding positions located on a surface area near our sample interfaces [44,45].Based on the FT-IR and Raman analyses, it has been confirmed that the CP material prepared is indeed hydrous.Further, the complete disappearance of peaks owing to water molecules in Raman analysis confirms the dehydration of nanoconfined water molecules from the surface of CP films.However, only a decrease in the intensity of FT-IR peaks corresponding to water molecules upon annealing CP samples confirms the incomplete dehydration and the presence of minimal nanoconfined water in the CP series electrocatalyst.The XRD, FT-IR, and Raman analyses demonstrate the preparation of hydrous CP (Co 3 (PO 4 ) 2 •nH 2 O) and the gradual dehydration of the CP electrode with annealing temperature.
Additionally, XPS analysis was conducted to investigate the electronic structures of the constituent elements in the produced material.The comprehensive XPS examination of all three samples verifies through Figure S2(b), available in ESI, that cobalt (Co), phosphorus (P), and oxygen (O) are indeed present within this composite.The Co 2p highresolution spectrum depicted in Figure 3(a) presents two distinct sharp peaks with binding energies centered at 781.63 and 797.63 eV for Co 2p 3/2 and Co 2p 1/2 , respectively, along with additional oscillatory satellite peaks positioned at around 785.73 and 803.13 eV.In the Co 2p XPS spectra of the CP sample, the broad peak due to the 2p 3/2 level can be deconvoluted into two peaks situated at 781.17 and 785.73 eV, with the sharp peak assigned to Co 3+ , while the broad peak is the satellite peak of Co 2+ species.Based on the peak positions and energy separation of the Co 2p 3/2 and Co 2p 1/2 peaks, it can be confirmed that the cobalt significantly remains in a divalent state across all samples.However, upon annealing at 200 °C and 400 °C, a slight shift in sharp peaks can be observed towards lower binding energy, and the intensity of sharp peaks belonging to Co 3+ slightly increases, indicating a mild signature of a higher oxidation state owing to the surface atmospheric oxidation [7,20].Moreover, Figure 3(b) shows the deconvoluted XPS spectrum of P 2p, in which two peaks correspond to the P 2p 3/2 at 133.20 eV and P 2p 1/2 at 134.12 eV core-level characteristic peaks of the pentavalent phosphate [(PO 4 ) 3-] and metaphosphate [(PO 3 ) -] groups.It is observed that the area under the broad peak P 2p 3/2 belongs to phosphate [(PO 4 ) 3-], which increases with annealing temperature.However, the intensity and area under the curve of P 2p 1/2 belonging to decreased metaphosphate, suggesting subtle changes in the chemical environment and the exclusion of impurities upon annealing (CP 200, CP 400) from the cobalt phosphate electrocatalyst [46,47].Figure 3(c) reveals the existence of two separate peaks with binding energies measured at 531.3 and 532.82 eV in the oxygen O 1s peak spectrum.These individual peaks correspond to P-O bonding, and -OH species confirm the presence of a phosphate group and nanoconfined structural water in prepared CP series electrodes [48].Fascinatingly, as the annealing temperature increases, the O 1s peak shoulder diminishes with decreased intensity and slight shifting (532.82eV to 533.28 eV) but does not disappear entirely.This confirms that some moisture residues remain present in the CP 400 sample.Thus, XPS analysis confirmed that the as-prepared electrocatalyst is hydrous cobalt phosphate, where Co is present in 2+ and P is in a 5+ electronic oxidation state and dehydrating with increased annealing temperature.
Moreover, FE-SEM images were captured at different magnifications (×10,000 and ×50,000) to record the surface morphological transformation of CP catalysts after annealing, as illustrated in Figure 4.At lower magnification (×10,000), the FE-SEM image shows that the films are deposited with compact nanoparticle-like morphology, and particles are well covered over the substrate (Figure 4(a)).Upon closer observation at higher magnification, it has been noted that the as-prepared CP samples contain some Thus, it is observed that morphology changes from shortrange agglomerated particles like with an average size of ~130 nm to spherical nanoparticles of ~70 ± 10 nm due to material dehydration.Figure S3   7 International Journal of Energy Research present in atomic percentages of 29.31%, 25.79%, 44.90%, 26.10%, 23.42%, and 50.48%, respectively, and reveal a consistent distribution of Co, P, and O elements throughout the CP series electrocatalyst samples.The formation of hydrous nanoparticle CP on the SS substrate has been confirmed through the FE-SEM images and EDS analysis.Amorphous, hydrous CP catalysts with particle-like morphology offer structural flexibility that is expected to result in a larger electrochemical active area and greater exposure of active sites compared to crystalline counterparts, which may lead to superior electrocatalytic performance.
Furthermore, to understand the structure of the asprepared Co 3 (PO 4 ) 2 .nH 2 O, an HRTEM analysis is conducted.The HRTEM image in Figure S4(a) illustrates that the amorphous CP exhibits an agglomerated particle morphology with an average diameter of approximately 117.75 nm.Furthermore, the HRTEM images also indicate an amorphous nature, as discernible lattice fringes are absent, showing no crystallinity evidence, suggesting that the particles are amorphous (Figure S4(b-d)).Likewise, the selected area electron diffraction (SAED) pattern displayed solely diffuse rings, solidifying the characterization of the synthesized CP particles as amorphous in nature (Figure S4 (inset of d)) [49].In summary, the amorphous nature, as revealed by the XRD data, is firmly substantiated by the HRTEM analysis.

Electrocatalytic Performance Analysis.
The present study investigates the influence of annealing of catalysts on their electrocatalytic OER performance, and, therefore, three typical samples of hydrous cobalt phosphate (CP) and partially dehydrated cobalt phosphate (CP 200/400) are prepared as OER catalysts and tested.The SS is used as a substrate due to its easy availability, low cost, nonreactivity in a wide pH range of electrolytes compared to Ni-foam/mesh, and good electrical conductivity compared to other conducting backbones such as carbon cloth and carbon paper.The LSV curves for CP, CP 200, and CP 400 catalyst electrodes were recorded at a 1 mV s -1 sweep rate in 1 M KOH electrolyte and are displayed in Figure 5(a).The curves show similar progressions, with higher overpotential increasing current density.The overpotentials of CP series thin-film catalysts are measured at a current density of 10 mA cm -2 , and the as-prepared CP catalyst requires the lowest overpotential   International Journal of Energy Research (η 10 ) of 265 mV (@10 mA cm -2 ) towards OER.The overpotentials of the annealed CP 200 and CP 400 thin-film catalyst electrodes were found to be 279 and 297 mV, respectively, which is explicitly higher than the overpotential of the asprepared CP electrode.Moreover, a series of LSV experiments were measured at higher scan rates, such as 5 mV s -1 and 10 mV s -1 , in addition to the standard at 1 mV s -1 for the as-prepared CP catalyst.The overpotential slightly increases from 265 mV to 278 mV (@10 mA cm -2 ) with the increase in scan rate (1 to 10 mV s -1 ), as shown in Figure S5(a).However, a negligible change in overpotential can be observed at higher current densities (<100 mA cm -2 ).Thus, the LSV at different scan rate results anticipated that the electrocatalyst's overpotential exhibits increasing trends due to mass transport limitations and reaction kinetics varying at higher scan rates.To evaluate reaction rate kinetics for the OER, the Tafel slope was obtained by plotting applied potential (vs.RHE) as a function of logarithmic current density at the current shoot point, as shown in Figure 5(b).The corresponding Tafel slope is smaller for the as-prepared CP electrode (37 mV dec -1 ) than that of the CP 200 (42 mV dec -1 ) and CP 400 (55 mV dec -1 ) electrodes, respectively, suggesting a more rapid electrochemical kinetics of the as-prepared CP electrode for OER.The lower Tafel slope recommends low activation energy and fast reaction kinetics of adsorption, formation of intermediate states, and removal of oxygen gas molecules.The lower overpotential and Tafel slope suggest tremendous electrocatalytic activity of the as-prepared amorphous, hydrous CP electrode for OER.In contrast, there was a significant increase in the overpotential observed in samples CP 200 and CP 400 due to the release of hydrous content from the material during annealing, leading to its dehydration form.
Moreover, ECSA is calculated for all three samples since several studies have proposed that the number of active sites of the electrocatalyst is directly associated with the ECSA, and it can be evaluated by using double-layer capacitance (C dl ).To calculate the ECSA using C dl , CV curves were recorded within a non-Faradaic region where no significant redox reaction occurs, and so, CV curves were measured at different rates (25 to 200 mV s -1 ) in a potential window of 0.7 to 1.05 V vs. RHE, as illustrated in Figures 6(a)-6(c).The C dl is determined by plotting the double-layer charging currents (i c ) versus the scan rate and using the following [7]: where the scan rate is denoted by "v."The graph shown in Figure 6(d) yields a straight line with a slope equal to C dl , and it discloses C dl values of 0.238, 0.170, and 0.148 mF cm -2 for CP, CP 200, and CP 400 thin-film electrodes, respectively.The ECSA and roughness factor (RF) were determined from the corresponding C dl values.The ECSA was calculated by the following equation [7]: where C s is the specific capacitance of a 1.0 M KOH electrolyte solution with a value of 0.04 mF cm -2 for the atomically smooth planar surface per unit area.The as-prepared hydrous CP catalyst provides an ECSA of 5.95 cm -2 , which is higher than the ECSAs of 4.25 cm -2 and 3.70 cm -2 for CP 200 and CP 400 electrodes, respectively.Correspondingly, the  9 International Journal of Energy Research electrocatalytic interface texture of electrodes, i.e., roughness factor, is calculated by the following equation [7]: where A Geometric is the geometric area of the electrode in contact with the electrolyte.The present study uses a unit area of thin film electrodes for the analysis.The RF values are 5.95, 4.25, and 3.70 for CP, CP 200, and CP 400 electrodes, respectively.Accordingly, the maximum ECSA and RF factors of the as-prepared CP catalyst confirm the manifestation of more active sites, contributing to its exceptional OER activity.Therefore, it can be concluded that the observed maximum C dl originated from the amorphous structure, and the lower overpotential can be attributed to nanoconfined structural water (hydrous content), which facilitates the facile interaction between the electrolytic ions and the host catalyst [24].
In addition, a stability study has been performed over 24 h at the obtained overpotential (η 10 ) to deliver a 10 mA cm -2 current density to ensure the durability and robustness of the CP catalyst for continuous oxygen evolution, as shown in Figure 7(a).After the initial surge in current density, the current density is stabilized near 10 mA cm -2 for 24 h, which suggests continuous oxygen  International Journal of Energy Research evolution at an overpotential of 265 mV with more than 99% operational stability.It is crucial to note that the annealing temperature of the catalyst has a significant impact on the initial surge of current density in stability graphs, which declines with increased annealing temperature (CP 200 and 400), as evident in the stability graph shown in Figure S5(b).Furthermore, the stability of the catalysis experienced a 3% reduction for the CP 200 and CP 400 electrodes after 24 hours.These findings emphasize the importance of closely monitoring the annealing temperature to maintain optimal OER performance.Therefore, an initial surge in current density in the stability curve of as-prepared CP and CP 200 samples indicates practical interaction between electrolyte and catalyst material owing to the nanoconfined structural water (hydrous content).Thus, the results specify that the OER activity of as-prepared CP is highly stable, suggesting a promising catalytic OER activity in alkaline electrolytes.Furthermore, LSV measurements are performed for CP, CP 200, and CP 400 electrocatalysts after 24 h stability, and the overpotentials (η 10 ) are found to be 269, 287, and 306 mV, respectively, as shown in Figure 7(b) (BS: before stability; AS: after stability).The overpotentials (η 10 ) are slightly increased after stability, and the Tafel slope is relatively increased to 40, 43, and 56 mV dec -1 for all three catalysts, which reveals the sluggish reaction kinetics after long-term stability, as shown in Figure 7(c).Moreover, the electrocatalytic OER performance of all three catalysts before and after 24 h stability tests is tabulated in Table S2 (see ESI).The electrochemical impedance spectroscopy technique is used to assess the as-prepared CP series catalysts for their electrochemical conductivity and charge transfer kinetics.The Nyquist plots of the as-prepared CP, CP 200, and CP 400 catalysts taken at open circuit potential depicted welldefined semicircles in the high-frequency region and a straight line at low frequency, as shown in Figure 7(d).Subsequently, we utilize Z-View software to fit EIS data from all samples of the CP series using Randle circuit-generated quantitative measures listed in Table S3.It was noted that annealing temperature caused an increase in solution resistance (R s ) values, slightly implying pathway hindrance between electrolyte and catalyst due to dehydration effects induced by the annealing process.Additionally, the chargetransfer resistance (R ct ) in the electrochemical reaction is attributed to the diameter of the semicircle, and a smaller diameter indicates a lower R ct and faster charge transfer rate.The as-prepared CP electrode has a distinctly smaller R ct value of 0.34 Ω compared to other catalysts, suggesting the existence of more facile charge transfer kinetics.After 24 h of stability, the R ct quite increases to 0.54 Ω due to the decrement in active sites by the continuous oxygen evolution process.The higher values of R ct for CP 200 and CP 400 electrodes, respectively, confirm low OER catalytic performance due to dehydration of the catalyst after annealing.The smaller R s and R ct of the asprepared CP electrode suggest an excellent interface between electrolyte and catalyst owing to the nanoconfined water (hydrous content), which further agrees with the less Tafel slope, indicating rapid electrode reaction dynamics.
Furthermore, to get detailed insights into the catalytic performance of as-prepared and postmortem CP electrocatalysts, electrodes were additionally characterized (ex situ) for morphological (FE-SEM) and electronic variation (XPS) after long-term stability.The FE-SEM image of the as-synthesized CP thin film electrocatalyst after continuous 24 h electrocatalytic performance at a magnification of ×5,000 and ×30,000 is shown in Figure S6(a, b).It is clearly seen that the particlelike morphology is not disturbed much and looks similar to before electrochemical measurements (Figure 4(b)), except for some increased roughness and agglomeration, possibly due to the mild corrosion with a strong base.However, the negligible change in surface morphology observed at higher magnification (×30,000), with a reduction in particle size of ~90 nm (Figure S6(b)), indicates less damage in the particlelike microstructure of amorphous CP material, which confirms good electrochemical stability.Moreover, elemental mapping of the electrocatalyst CP sample depicts the presence of Co, P, and O elements on the surface of the electrocatalyst, with slight degradation of the phosphorus element, as shown in Figure S6(c-e).Furthermore, the XPS analysis is performed before and after the chronoamperometry test to investigate the electronic variation after the durability test of CP catalysts (Figure S7).In the XPS survey shown in Figure S7(a), the spectra of CP before and after the stability test display almost similar characteristic peaks of Co, P, and O elements, with a slight reduction in the intensity of the P 2p element.Moreover, the deconvoluted Co 2p 3/2 and Co 2p 1/2 spectra into four primary peaks featured two distinct suppressed satellite peaks after the stability test, corresponding to Co 3+ and Co 2+ for Co 2p 3/2 and Co 2p 1/2 .As presumed, the Co 2+ state is scarcely suppressed and exhibits a reasonable increase in the peak intensity of the Co 3+ state at 781.17 eV in the Co 2p spectra after the stability test, which displays the transition of Co states from Co 2+ to Co 3+ after the stability test (Figure S7(b)) corresponds to in situ formation of CoOOH [35,[50][51][52][53][54].Also, the minor shift and drop in intensity of the P 2p (phosphorus) spectra seen after the stability test confirm the degradation of the phosphorus pentavalent states near the electrode's surface (Figure S7(c)).Furthermore, as shown in Figure S7(d), a modest change of P-O (531.3 eV) and an increase in OH -(532.82eV) bonds in the O 1s spectra confirm the insertion of hydroxyl ions (OH) throughout the chronoamperometric test process and further confirm the in situ formation of CoOOH near the surface.According to the overall XPS examination, the hydroxyl ions intercalate by using the vacancy sites of (PO 4 ) 3 and form CoOOH, which interestingly could be the active sites along with CP in the electrocatalyst, further enhancing the OER electrocatalytic activities [20,35,50].
Also, overpotentials at different current densities (10, 25, 50, and 100 mA cm -2 ) for the as-prepared CP electrodes before and after 24 h stability tests are measured, as presented in Figure 8(a).The OER-performances of asprepared hydrous cobalt phosphate catalysts are rigorously compared with existing literature on cobalt phosphatebased compounds, as shown in Figure 8(b), and other details of catalysts in terms of methodology and morphology are tabulated in Table S4.Among them, the Co-Pi nanoarray prepared by Xie et al. [25]   13 International Journal of Energy Research density with a 70 mV dec −1 Tafel slope.Also, Pramanik et al. [29] achieved a low 380 mV overpotential at 10 mA cm -2 current density with a Tafel slope of 58.7 mV dec -1 for the Co-Pi electrode prepared by a sol-gel method.Cai et al. [30] synthesized Co phosphate nanoparticles by electrodeposition and achieved 360 mV overpotential at 20 mA cm -2 current density with a Tafel slope of 80 mV dec −1 .In comparison to prior research, the hydrous cobalt phosphate synthesized in the present work displays exceptional OER performance, exhibiting an overpotential of less than 265 mV at a current density of 10 mA cm -2 with a Tafel slope of 37 mV dec -1 .Nevertheless, Qi et al. [21] reported better catalytic activity for anhydrous, crystalline Co 3 (PO 4 ) 2 plates owing to Co (tetrahedral) sites than Co 3 (PO 4 ) 2. 8H 2 O with pure Co (octahedral) sites.However, the present work demonstrates that amorphous and hydrous CP (Co 3 (PO 4 ) 2. nH 2 O) thin film electrocatalysts exhibit excellent activity compared to annealed CP 400 electrocatalysts.These inconsistent results may be due to incomplete dehydration and the amorphous nature of CP electrodes, where nanoconfined water plays a critical role in connecting and agglomerating nanoparticles.
Besides, several overall advantages of SILAR-prepared hydrous CP (Co 3 (PO 4 ) 2 .8H 2 O) catalysts are accountable for the outstanding OER performance, which are as follows: (1) The nanoconfined water creates molecular cavities, acts as an electrochemically active center, allows OH -ions to travel inside the structure, and favors the adsorption to form intermediate states (OH * , O * , and OOH * ) [55,56].(2) A spherical particle-like morphology is one of the advantages of getting a high ECSA for electrode material and the large number of nanograin boundaries acting as centers for water molecule adsorption and oxidation.(3) The defect-rich amorphous and hydrous electrocatalyst provides long-term durability since an electrode does not suffer from structural damage.(4) Moreover, binder-free synthesis of thin-film electrocatalysts by the simple SILAR route provides facile and quick charge transfer in an electrocatalytic process.Thus, the amorphous, short-range nanoparticles and hydrous nature of the CP electrocatalyst show exceptional catalytic performance towards OER with excellent durability.
Furthermore, overall water electrolysis is evaluated by preparing a full cell using a best-performing hydrous CP The O 2 generation rate of 0.033 mL min -1 is precisely half of the H 2 generation rate of 0.066 mL min -1 , and it is close to the theoretical volume of OER at 10 mA cm -2 current density, which further affirms the efficient water splitting by hydrous CP//Pt catalyst.Furthermore, the polarization curve was recorded after 24 h of continuous catalysis and compared with the polarization curves before stability for the full test cell, as shown in Figure 9(b).The comparative LSV plots confirmed a slight increment in overpotential from 1.56 to 1.57 V for 10 mA cm -2 , and such an increase in voltage can be due to slight material degradation in the electrolyte during the catalysis process.Also, the Tafel plots shown in Figure 9(c) demonstrate an increment from 138 mV dec -1 to 141 mV dec -1 after 24 h of catalysis.The stable catalytic performance for 24 h of an as-prepared hydrous CP electrode in a full electrolysis test cell proves its strongest candidature for upcoming alkaline electrolysis devices.The overall water splitting suggests the practical applicability of hydrous, amorphous cobalt phosphate thin film catalyst towards water splitting in alkaline electrolytes.

Conclusions
In conclusion, the hydrous, amorphous cobalt phosphate (thin film) electrocatalysts are prepared using the binder-free SILAR method at room temperature.The dehydration of hydrous, amorphous cobalt phosphate thin films with a decrease in average particle size upon annealing up to 400 °C is confirmed by XRD, FT-IR, Raman spectroscopy, XPS, and FE-SEM analysis.Furthermore, the present study demonstrates that the hydrous content of the CP thin film electrocatalysts strongly affects the electrochemical OER performance.An electrocatalytic OER analysis shows that the hydrous content associated with cobalt phosphate in the as-prepared CP sample acts as an electrocatalytic site and provides facile interaction between catalyst and electrolyte ions at a reduced overpotential (η 10 ) of 265mV.Moreover, overall water splitting at 1.56 V (@10 mA) in a full test cell (CP//Pt) demonstrates that the synthesized, amorphous, hydrous cobalt phosphate thin films can be reliable electrocatalysts in electrochemical OER.These results clearly demonstrate that binder-free, hydrous cobalt phosphate thin film electrocatalyst presents a superior alternative to costly commercial catalysts in terms of both cost-effectiveness and ease of preparation.Thus, it is imperative to consider leveraging this material for the development of efficient and robust OER electrocatalysts.

Figure 1 :
Figure 1: (a) Schematic representation of SILAR method for the deposition of cobalt phosphate thin film electrocatalyst.(b) Photographs of prepared CP, CP 200, and CP 400 catalysts.

Figure 5 :
Figure 5: Electrochemical OER analysis of the cobalt phosphate (CP, CP 200, and CP 400).(a) LSV curves at a scan rate of 1 mV s -1 , inset shows magnified view at current shoot point, (b) Tafel plots.

Figure 6 :
Figure 6: Electrochemical active surface area (ECSA) analysis of the cobalt phosphate (CP, CP 200, and CP 400).The CV curves of (a) CP, (b) CP 200, (c) CP 400, and (d) graph of anodic current density Vs scan rate with a linear fit for C dl values.

Figure 8 :
Figure 8: (a) Overpotentials of CP catalyst before and after stability at different current densities.(b) Comparative electrochemical catalytic performance of cobalt phosphate-based catalyst with as-prepared CP catalyst.