Cobalt-boride nanostructured thin films with high performance and stability for alkaline water oxidation

Suraj Gupta*†, Harshada Jadhav‡, Sucharita Sinha‡, Antonio Miotello⁋, Maulik K. Patel†, Arindam SarkarΓ, Nainesh Patel*ζ

†School of Engineering, University of Liverpool, Harrison Hughes Building, Brownlow Hill, Liverpool, L69 3GH, United Kingdom.

‡Laser & Plasma Surface Processing Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.

⁋Department of Physics, Università degli Studi di Trento, Via Sommarive, 14-38123, Povo, Trento, Italy.

ΓDepartment of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai- 400076, India.

ζDepartment of Physics, University of Mumbai, Tilak Bhavan, Vidyanagari, Santacruz (East), Mumbai 400098, India.

Corresponding authors:

Dr. Suraj Gupta, Dr. Nainesh Patel

Email: Suraj.Gupta@liverpool.ac.uk

nainesh11@gmail.com

Abstract:

Nanocrystalline cobalt boride (Co-B) thin films prepared by pulsed laser deposition were used as an anode catalyst to study the water oxidation reaction in alkaline medium. Elemental depth profiling revealed the bulk of the film to be metallic, which helps in improving conduction of charges, while the surface of the film was rich in CoOOH type species to facilitate oxygen evolution reaction (OER). Comparison of OER performance with boron-free samples suggest that inclusion of B helps in improving the OER rate by preventing the conversion of surface Co to stable oxides. Co-B film achieved a current density of 10 mA/cm2 at merely 280 mV, with potentiostatic stability for 45 hours in alkaline medium, highlighting its superior performance than the powder catalyst. This work not only establishes the advantage of developing thin film catalysts but also presents a new approach to understand the OER mechanism in metal borides.

Keywords: cobalt boride, oxygen evolution reaction, thin film, alkaline water-splitting, XPS depth profile.

Introduction:

In the present era of renewable energy technologies, electrocatalytic water-splitting is the most viable scheme for producing clean H21,2. However, wide-scale implementation of this technology is inhibited by the use of expensive and earth-scarce metals like Pt, Ru, Ir, etc2. Amongst the numerous low-cost alternatives, metal borides have earned reputation as robust materials in alkaline medium3,4. Especially, for the energy intensive oxygen evolution reaction (OER)5,6, metal borides have shown extremely high performances in alkaline medium, surpassing that of standard RuO2 and IrO2 catalysts3,7. Majority of the inexpensive catalysts, including metal borides, are often synthesized in powder form, consisting of different nanostructures3,4,7,8. However, industrial application demands feasibility of coating these materials on suitable substrates9,10. Chemical routes11,12 to prepare catalyst coatings have been employed by many researchers but the desired precision in controlling the film adhesion and morphology is difficult to achieve with these methods. The most commonly used method to prepare catalyst coatings is by dispersing the powder catalyst in suitable solvents, along with conducting binders (usually Nafion) to prepare ink, which is then coated on test surfaces. However, in this method, the catalyst particles agglomerate, thereby losing their nano-size properties and also the inclusion of conducting binders add to the overall resistance of the coating13. In wake of these issues, development of catalyst coatings by physical deposition techniques like pulsed laser deposition (PLD) remains unexplored. PLD is a versatile technique to fabricate catalyst thin films with wide range of morphologies obtained by varying the various deposition parameters, which also offers a higher degree of freedom to control the film properties (thickness, nanostructuring, adhesion, etc.)14,15. In case of metal borides, Co-B is one of the most popular catalyst that has been explored in the form of powders and chemical coatings for water-splitting applications3,16–19. Our past work showed that nanostructured Co-B thin films prepared by PLD can produce H2 with similar rates as that of noble Pt/C catalyst by dissociation of chemical hydrides20. However, surprisingly there has been no such report for electrocatalytic water-splitting. Thus, this article explores the development of Co-B thin films by PLD and their use as anode coatings for alkaline water electrolysis.

Another interesting concept that this article tends to explore is the variation in surface chemical composition of metal borides after OER. For non-oxide catalysts (phosphides, sulphides, carbides, borides, etc.), it is well-known that OER proceeds via formation of metal oxide/hydroxide layer on the surface of the catalyst5,6. In case of metal borides, the formation of these surface species was studied by Masa et al3 for crystalline Co2B using in-situ X-ray absorption spectroscopy (XAS). Prior to this, various groups used in-situ XAS to report an increase in the oxidation state of Ni, with increase in the applied oxidation potential, in Ni borate (Ni-Bi) films21,22. Although XAS provided changes in bulk chemistry of the catalyst films during the reaction, precise modifications at the catalyst surface is hard to detect by this technique. On the other hand, X-ray photoelectron spectroscopy (XPS) would be a more suitable technique to probe changes in the near surface chemistry of a catalyst. XPS data can be taken from different depths of a material to obtain complete information about its elemental profile, without resorting to in-situ techniques. This is a useful feature to analyse the surface of an electrocatalyst where not only the reaction on the surface but also the electron mobility across the bulk of the catalyst plays a crucial role in defining the activity. Hence, this article presents direct evidence of the formation of oxide/hydroxide layer on the surface of Co-B film along with the thickness of this layer and composition below it, by using elemental depth profiling data obtained from XPS. As Co-B is developed in the form of thin film, it presents a good opportunity to differentiate between the chemical composition of the surface layer and bulk, which becomes difficult in the case of nano-powders. In the end, the surface chemical states were also confirmed by electrochemical methods and an effort was made to highlight the role of boron in facilitating OER for Co-B system.

Experimental:

Synthesis of Co-B thin films: Thin films of Co-B were deposited using PLD technique. Initially, Co-B powder was synthesized by chemical reduction of cobalt chloride using sodium borohydride, as described in our previous report16. This dried powder was cold pressed at a pressure of ~1500 kg/cm2 to form circular disc (diameter: 25 mm, thickness ~ 3-4 mm), which was used as a target material for laser ablation. Fluorine doped Tin oxide (FTO) glass (Sigma-Aldrich, resistivity = ~7 Ω/sq) was used as the conducting substrate to deposit Co-B coatings. The substrates were masked in order to expose an area of 2 cm x 1.5 cm for deposition. In order to obtain uniform ablation of the target with minimum change in the surface geometry, Co-B target was mounted on a rotating target holder, while the substrates were kept fixed on a substrate holder. Deposition of Co-B films was carried out using Nd-YAG laser operating at a wavelength of 532 nm, pulse duration of 5 ns and a repetition rate of 10 Hz. The PLD chamber was evacuated up to a base pressure of 10-5 mbar. All PLD runs were carried out at room temperature, using 30,000 pulses with laser fluence of 30 J/cm2 and keeping a fixed distance of ~ 4 cm between Co-B target and substrates.

Materials characterization: Scanning electron microscope (SEM, JEOL JSM 7001F FEG-SEM) was used to image the surface morphology of the films as well as obtain elemental maps (using Oxford Instruments INCA X-act EDX detector). High resolution transmission electron microscope (HR-TEM) and scanning transmission electron microscope (STEM) images were recorded on JEOL 2100F Cs-corrected analytical FEG S/TEM at an accelerating voltage of 200 kV. XPS measurements were performed using an AXIS Supra (Kratos Analytical) instrument equipped with a monochromatic Al Kα (1486.6 eV) X-ray source for chemical analysis of the samples. C 1s peak at 284.8 eV was used as a reference to adjust the binding energy positions of all the elements. XPS peak fitting was performed using a combination of Gaussian and Lorentzian functions within the data analysis software (ESCApe) from Kratos. XPS depth profiling was carried out by sputtering the surface of the film using Ar+ ions with a beam energy of 0.5 keV. X-ray diffraction (XRD, Rigaku Ultima IV) was performed in grazing incidence mode (GIXRD) for PLD deposited CoB films at an incident angle of 0.8˚, over the 2θ range of 30˚-70˚ (scan rate = 2/min).

Electrochemical testing: Co-B thin films were tested for OER in a typical 3-electrode assembly, using Pt wire as counter and Hg/HgO as the reference electrode. 1 M KOH (pH 14) was used as the electrolyte for all practical measurements. Cu tape (3M) was used to make contact with FTO from the uncoated area of the thin film. A potentiostat system from Gamry (Interface 1010) was used for electrochemical measurements and analysis. Prior to actual measurements, the films were subjected to 15 cycles of cyclic voltammetry (CV) scans in order to stabilize the current readings. CV and linear sweep voltammetry (LSV) scans were performed in the range of 0.0 to 1.0 V (vs Hg/HgO) at a scan rate of 10 mV/s. CV scans were also performed at a slow scan rate of 2 mV/s to record the occurrence of pre-catalytic redox peaks. Electrochemical impedance spectroscopy (EIS) was performed in the frequency domain of 10 Hz to 2 MHz, with a sinusoidal input wave of amplitude 25 mV, at the open circuit potential (OCP). The value of solution resistance (Rs) obtained from EIS data fitting was used to obtain iR-corrected curves. Long term stability test was performed by chronoamperometry measurement at a fixed potential of 1.49 V (vs RHE) for 45 hours. All the potential values were converted to reversible hydrogen electrode (RHE) using the formula: E (RHE) = E (REF) + (0.059 pH). The potentials that will be used in this article from here onwards will be referenced to RHE, unless stated otherwise.

Results and Discussion:

Fig. 1. (a) SEM, (b-e) EDAX maps, (f) HR-TEM and (g) bright field STEM images obtained from as deposited Co-B films on FTO substrate, inset in (g) shows FFT of the image.

The surface morphology of the deposited Co-B thin films was examined using electron microscopy. SEM images in Fig. 1a reveals the surface of the film is composed of spherical particulates, ranging from 0.2 microns to 2 microns, embedded in a continuous matrix. The elemental contrast seen in Fig. 1a and Fig. S1 indicates that most of these particulates have a heavy metal core (presumably Co) and are encapsulated by low atomic number elements such as B and O, along with Co, as confirmed later. This morphology is attributed to the laser-initiated phase explosion process23 in which Co liquid droplets are preferentially ablated to form particulates while boron-based film is deposited over these particulates to form core-shell type particles. EDAX elemental maps in Fig. 1b-e also confirm that the core of spherical particulates mainly consists of Co while O and B are uniformly distributed along the surface of the film. Higher magnification SEM image in Fig. S2 shows that the film also consists of smaller-sized nanoparticles, in addition to the bigger particulates. HRTEM image in Fig. 1f quantifies the size of these nanoparticles to be between 5-10 nm, with lattice spacing of 0.21 nm, across the entire matrix of the film. Bright field STEM image (Fig. 1g) confirms the measured lattice spacing and the corresponding fast Fourier transform (FFT) of the image shown in the inset of Fig. 1g is consistent with that of metallic hcp Co24,25. Thus, it becomes clear that the PLD deposited film is composed of metallic Co in the form of nanoparticles and larger particulates, which are embedded into and encapsulated by B layer, respectively, forming a nanocrystalline Co-B assembly. This morphology is consistent with our past Co-B film deposited using excimer laser20. Presence of surface oxygen is attributed to the exposure of the film to ambient atmosphere. GIXRD pattern (Fig. S3) of Co-B film shows no diffraction peaks but only a broad hump centred at 45⁰, due to the short-range ordering of nanocrystalline Co-B20. The second small peak seen in this figure is attributed to the Si substrate.

One of the main goals of this paper is to investigate the variation in chemical states of Co-B catalyst when subjected to oxidative potentials. To do so, it is essential to have a first-hand knowledge about the chemical nature of pristine Co-B films, prior to oxidation. Fig. 2a shows the XPS spectra for Co 2p3/2 states recorded on the surface of pristine Co-B film. The spectrum can be deconvoluted into three peaks, where the peaks at binding energies of 781.0, 782.5 and 786.4 eV correspond to divalent Co, indicating presence of Co(OH)2 or CoO phase on the surface26. The surface of the film is highly prone to get oxidised in ambient atmosphere and hence XPS data doesn’t reflect the presence of metallic Co. These results co-relate well with SEM EDAX maps (Fig. 1b-e) which showed presence of both Co and O together on the surface of the film.

To obtain chemical composition of the bulk (corresponding to layers below the surface), the film was etched using Ar ions upto a depth of 50 nm and XPS spectra were recorded again. Fig. 2b shows the XPS spectra for Co 2p3/2 states of the same Co-B film at a depth of 50 nm from the surface. After deconvolution, a peak at binding energy (BE) of 778.4 eV corresponding to metallic Co is now clearly visible27. This peak was not observed for the XPS spectra obtained from the surface of the film. The spectrum also shows other peaks corresponding to oxidised Co, but their intensities are very low compared to that of metallic Co. This indicates that the bulk of Co-B film is majorly composed of metallic Co, with some degree of oxidation on the surface. O 1s states represented in Fig. 2c and 2d for surface and bulk of the film respectively, are attributed to Co(OH)2 and CoO phases26. XPS spectra in Fig. 2e and 2f represent the B 1s states on the surface of the film and at a depth of 50 nm, respectively. In both cases, the deconvoluted peak positions are very similar at 191.9-192.0 eV and 192.7-192.8 eV, attributed to boron oxide species3,16,28. This implies that the oxidation state of boron remains the same on the surface and in the bulk of the film.

Fig. 2. XPS spectra obtained from as deposited Co-B films on the surface and from a depth of 50 nm for (a, b) Co 2p3/2 states; (c, d) O 1s states and (e, f) B 1s states.

After knowing the morphological and chemical nature, the as-deposited Co-B films were tested for water oxidation reaction in 1 M KOH. The films were subjected to 25 cycles of CV scans in the potential range of 0 V to 1 V (vs Hg/HgO), following which, XPS measurements were carried out again. Fig. 3a shows the XPS Co 2p3/2 spectra recorded at the surface of the film while Fig. 3b shows the same at a depth of 50 nm. After OER test, the native Co(II) species on the surface of the film are converted to Co(III), as a signature peak of CoOOH can be seen at a binding energy of 780.0 eV7,26. The other peaks at 781.0 eV and 782.8 eV, corresponding to Co(II) states are still present on the surface, as also visible in pristine films.

When etched to a depth of 50 nm, the peaks corresponding to metallic Co (778.5 eV) and three peaks of Co(II) state (779.9 eV, 781.6 eV, 784.0 eV) were evident. However, the intensity of peak corresponding to metallic Co in the bulk of the film (at 50 nm depth) decreases drastically when compared with the same peak in pristine samples, before OER test. This implies that OER test leads to a substantial growth of the top oxide layer of the film, but the metallic nature of the bottom layer in the film is still maintained. Formation of surface oxide/hydroxide layer was confirmed by the XPS O 1s spectra, which shows two peaks in the range of 529.4 – 529.6 eV and 530.8 – 531.0 eV, collected from the surface (Fig. 3c) as well as at a depth of 50 nm (Fig. 3d). The peaks in the range of 529.4 – 529.6 eV correspond to Co(OH)2 species while those in the range of 530.8 – 531.0 eV correspond to CoOOH species26, which were not observed in pristine samples. This a direct evidence that post OER, any native Co oxide species is completely transformed into oxy-hydroxy species that are responsible for initiating the OER.

Fig. 3. XPS spectra of post-OER Co-B films for Co 2p3/2 states (a, b); O 1s states (c, d) and B 1s states (e, f) on the surface and at an etched depth of 50 nm; inset in 3a shows the XPS depth profile of the post-OER film indicating the variation in concentration of Co and O up to a depth of 25 nm from the surface.

To determine the degree of surface oxidation, inset in Fig. 3a shows the variation in atomic concentration of Co and O up to a depth of 25 nm from the surface of the post-OER film. Here, it becomes clear that the concentration of Co increases rapidly while the concentration of O decreases, as the surface of the film is etched. After a depth of 15 nm, the concentration of Co and O attains a constant value, suggesting that the newly formed oxide layer has a thickness of about 15 nm. Thus, post-OER, Co-B film attains a bilayer structure with a top oxide/hydroxide layer of few tens of nm and a bottom metallic bulk layer. The formation of this bilayer structure is highly advantageous for water oxidation, as the top oxide surface facilitates OER while the bottom metallic layer provides higher conductivity for transfer of electrons29,30. In case of B 1s spectra, both surface and bulk of the film shows evidence of boron oxide, as seen in Fig. 3e and 3f, respectively. However, when compared to the untested samples, the intensities of these peaks are diminished, which ascertains the dominance of metal oxide/hydroxide species after OER tests.

To demonstrate the electrochemical prowess of film over powder, oxidative polarization tests were carried out on Co-B film as well as amorphous powder of Co-B nanoparticles, at a scan rate of 10 mV/s, in 1 M KOH. The initiation of OER takes place on the surface of metal oxides/hydroxides and hence the presence of boron in Co-B film is not justified. To illustrate the role of B, boron-free samples, in the form of Co oxide powder and thin film were also tested alongside Co-B. SEM images and GIXRD spectra confirming the Co oxide phase of these B-free samples are shown in Fig. S4 and Fig. S5, respectively. From the LSV scans in Fig. 4a, it can be seen that both the films (Co-B and Co oxide) exhibit the lowest onset potential for oxygen evolution. At the standard current density of 10 mA/cm2, overpotentials of 322 mV, 338 mV and 390 mV were observed for Co oxide film, Co-B powder and Co oxide powder, respectively. Co-B film showed the lowest overpotential of 280 mV to attain the same current density, which is also lower than that of RuO2 nanoparticles (370 mV) and maintains it for higher current densities. Thin film catalysts always have the advantage that the surface particles remain immobilized and hence do not aggregate during measurement. In spite of this advantage, Co-B powder outclasses the performance of Co oxide film at current densities over 20 mA/cm2.

(f)

(e)

(d)

(c)

(a)

(b)

Fig. 4. (a) iR-corrected linear sweep voltammograms recorded at a scan rate of 10 mV/s in 1 M KOH for Co-B and Co oxide samples along with RuO2 nanoparticles; (b) Nyquist plots obtained from EIS for Co-B film and powder, inset shows corresponding equivalent circuit; Cyclic voltammetry curves recorded at 2 mV/s in 1 M KOH for (c) Co-B and Co oxide film; (d) Co-B and Co oxide powder; (e) Tafel plots for Co-B and Co oxide samples along with RuO2 nanoparticles; (f) Potentiostatic stability test for Co-B film at an applied potential of 1.49 V maintained for 45 hours, inset shows the CV curves for Co-B film before and after recycling for 2000 cycles in 1 M KOH, recorded at 100 mV/s.

This highlights the fact that inclusion of B does play a significant role in improving the OER rate. Amongst Co-B samples, Co-B films portray higher activity which can be ascribed to two reasons: (I) immobilization of particles over the film surface and (II) formation of a distinct bilayer assembly where the bottom conducting layer facilitates fast transfer of electrons to reach the conducting substrate, while top oxide layer participates in water oxidation. Uniform dispersion of particles over the whole surface of the film is already observed with electron microscopy (Fig. 1). To validate the second speculation, the system resistance in both cases was determined by carrying out EIS tests for Co-B film and Co-B powder (Fig. 4b). The data obtained from the Nyquist plots were fit using an equivalent circuit model comprising of two resistances (Rs and Rct) and a capacitance (inset of Fig. 4b). The corresponding data obtained using this fit is represented in Table S1. One can see that the charge-transfer resistance (Rct) in Co-B films (0.7 Ω) is much lower than that in Co-B powder (8.6 Ω), confirming the higher conductivity of films.

In addition to the advantages presented by morphological distinctions of Co-B film, it is also important to explore the intrinsic activities of powder and film. This was done by determining the electroactive surface area (ESA) of Co-B film and powder. An indirect measure of the ESA can be obtained by calculating the double-layer capacitance (CDL) formed at the interface of electrode and electrolyte. Fig. S6a and S6b show the CV scans for Co-B powder and film, respectively, at increasing scan rates. From these scans, the difference in cathodic and anodic current densities were plotted against the respective scan rates, to determine the value of CDL (Fig. S6c). Co-B powder exhibited a CDL of 1.96 μF/cm2, which is much higher compared to that of Co-B film (0.40 μF/cm2). This implies that Co-B powder possess almost 5 times more number of active surface sites when compared to Co-B film. Despite possessing higher number of active sites, the observed activity is lower for Co-B powder. To remove the contribution of ESA from the electrocatalytic activity, the observed current for Co-B film and powder was normalized by the obtained CDL values, to give ICDL, which is a true measure of the intrinsic performance of any catalyst. From Fig. S7, it can be seen that the intrinsic activity of Co-B film is much higher than that of powder, even at higher potentials. This suggests that the adsorption sites in Co-B film are more active than that in Co-B powders, resulting in higher intrinsic activity of Co-B film.

From the results so far, the variation in chemical structure of Co-B film before and after OER tests is well established. The discussion has also confirmed that the highest OER performance and intrinsic activity is observed when Co-B is made in the form of film rather than powder, and the fact that inclusion of B improves the OER rate. To understand the exact difference in samples containing boron (Co-B) and boron-free samples (Co oxide), it becomes necessary to probe the samples during oxidative tests. Cyclic voltammetry is one of the most convenient and yet powerful techniques to observe the in-situ variation in chemical states of catalysts. CV scans were recorded at a rate of 2 mV/s, in the pre-OER region for Co-B and Co oxide films (Fig. 4c) and powders (Fig. 4d). In case of Co oxide, an oxidation peak at 1.33 V (for film) and 1.44 V (for powder) originating due to the conversion of surface Co(III) oxides into CoO2 phase (Co(IV)) is seen, which then drives the OER31,32. In the case of Co-B, both film and powder portray an oxidation peak at 0.99 V and 1.06 V, respectively, ascribed to the conversion of native surface Co oxide to Co oxy-hydroxy species (CoOOH)31,33. It is well reported in literature that the formation of these species is one of the crucial steps in initiating OER and the materials that readily form these species are often found to be more active for OER5,6. The formation of these surface species could be one of the contributing factors to the higher activity of Co-B samples. In addition to this peak, both Co-B samples show a second peak, which was also observed in case of Co oxide samples, corresponding to the formation of CoO2 species31,33. In case of Co-B powder, this peak is not very prominent and only a small hump (at 1.43 V) is observed, whereas Co-B film shows a broad shoulder of this peak centred at 1.3 V. It must be noted that this oxidation peak in Co-B film occurs at a lower potential, as compared to all other three samples, which indicates a lower energy barrier for formation of CoO2 species. These results imply that transforming the oxidation states of stable oxides (here Co3O4) to form CoO2 is thermodynamically more difficult than forming new hydroxy species over the surface of borides and their conversion to CoO2. This is an important result that explains the reason for lower performance of stable metal oxides for OER. In case of amorphous/nanocrystalline Co-B compound, reverse electron transfer (with respect to atomic electronegativity) from B to d-band of Co is commonly observed, as validated from past experimental as well as theoretical results4,16. Owing to this phenomenon, Co is not able to form a stable oxide in presence of B which protects Co from oxidation. Hence, Co active sites are left available for initiation of OER via formation of oxy-hydroxy species.

In case of HER, the value of Tafel slope gives an indication of the reaction pathways and the corresponding rate-limiting steps for a particular reaction. However, though the reaction pathways for OER are known, they cannot be precisely predicted based on their Tafel slope values. In the present case, it is observed that the Tafel slopes (Fig. 4e) of Co-B and Co oxide films are 60 mV/dec and 74 mV/dec while that for Co-B and Co oxide powders are 39 mV/dec and 44 mV/dec, respectively. In both cases, Co-B shows lower Tafel slope than Co oxide, which again highlights the significance of B in the samples. The differences in the range of Tafel slope values for powder and film indicates that the rate-limiting step is different when the same reaction takes place on powder as compared to their film counterparts. However, the CV curves in Fig. 4c-d indicate that the reaction mechanism of OER proceeds in similar fashion over each catalyst whether it is in the form of film or powder. From these two observations, it is expected the rate-limiting step in both cases are not related to their chemical transformation but is more or less related to the variation in morphology of the samples in the form of film and powder. However, at this point, it is difficult to isolate the exact reason leading to different Tafel slope ranges for film and powder samples.

The argument that Co-B is more efficient in the form of thin film holds value only if these films remain stable under practical operations. To demonstrate the robustness of Co-B films, they were subjected to a constant potential of 1.49 V and electrolysis was carried out continuously for about 45 hours (Fig. 4f). The catalyst film maintained a fairly stable current density over this test, indicating its stability and robust nature, in alkaline conditions. After 45 hours of test, the surface morphology of the film was observed under SEM, as shown in Fig. S8. It was found that the globular particles on the film surface were converted to hexagonal disc-shaped particles. The formation of such hexagonal discs is usually observed for Co hydroxides34, which as seen from XPS, is a dominant phase on the surface of the post-OER film. Thus, OER not only modifies the chemical nature of the film, but it also converts the morphology of the particles, which may also assist in facilitating OER. Recycling stability was also tested for as deposited Co-B films by performing 2000 CV scans at a scan rate of 100 mV/s. Inset in Fig. 4f shows a negligible increase in overpotential (~20 mV) even after 2000 cycles of operation, asserting that these films are suitable for commercial applications.

Conclusion: In summary, the OER performance of Co-B thin films prepared by PLD technique was explored in this work. Owing to the phase explosion process in PLD, the deposited film showed Co particles embedded in boron matrix to yield a nanocrystalline Co-B network. As deposited film showed highly metallic core with minor oxidation (Co(II)) on the surface, as observed from XPS depth profiling. Post OER tests, the surface oxidation layer thickens showing transformation to CoOOH type species, while the metallicity of the core remains intact. Owing to this bilayer formation where the bottom layer is metallic, Co-B film shows high charge conductivity as compared to Co-B powder catalyst, evidenced from EIS data. The role of B was also illustrated by comparing the OER performance of Co-B film and powder with that of boron-free Co oxide samples, in pH 14. The formation of CoOOH type species during OER was also confirmed by CV measurements. Finally, stability and recycling tests were performed to establish the robustness of Co-B film in alkaline conditions for water oxidation. The results presented in this work offer new insight into understanding the OER mechanism on metal borides and development of catalyst in the form of film provides a strong case for their incorporation into industrial setup.

Supporting information:

Figures: Additional SEM images and XRD spectrum of Co-B film, SEM images and XRD spectra of Co-B powder, Co oxide powder and Co oxide film, CV scans to determine ESA, CDL normalized LSV curves, SEM image of Co-B film post stability test. Table: Resistance values from EIS.

Acknowledgement:

S. Gupta acknowledges the research grant from UK Commonwealth Scholarship Commission through Commonwealth Rutherford Fellowship (INRF-2017-139). N. Patel acknowledges UGC and SERB-DST for providing financial support through Faculty recharge program and Extra Mural project (FILE NO. EMR/2016/003028). M.K. Patel acknowledges the seed funding obtained from EPSRC-GCRF No. 141131/155337. We also acknowledge the use of electron microscopy facilities at Imaging Centre at Liverpool (ICaL). The authors acknowledge the Central Surface Analytical Facility of IIT Bombay for XPS measurements.

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Table of Contents:

Synopsis:

Developed Co-B thin films exhibit excellent performance and stability, offering a practical solution for implementation in industrial setup.

2

792790788786784782780778

786.4

782.5

Co 2p

Binding Energy (eV)

Intensity (arb. units)

781.0

On surface

792790788786784782780778776

At 50 nm depth

785.3

781.6

779.3

Binding Energy (eV)

Intensity (arb. units)

Co 2p

778.4

536534532530528

530.3

Intensity (arb. units)

At 50 nm depth

532.0

O 1s

Binding Energy (eV)

535534533532531530529

532.5

On surface

Intensity (arb. units)

Binding Energy (eV)

O 1s

531.5

(a)

(b)

(c)

(d)

(e)

(f)

196195194193192191190189188

192.7

Intensity (arb. units)

Binding Energy (eV)

On surface

B 1s

191.9

196195194193192191190189188

192.8

192.0

Intensity (arb. units)

At 50 nm depth

B 1s

Binding Energy (eV)

196195194193192191190189188

B 1s

At 50 nm depth

191.4

Intensity (arb. units)

Binding Energy (eV)

196195194193192191190189188

B 1s

On surface

189.8

191.8

Intensity (arb. units)

Binding Energy (eV)

534533532531530529528

O 1s

On surface

Intensity (arb. units)

Binding Energy (eV)

529.6

531.0

535534533532531530529528527

O 1s

At 50 nm depth

Intensity (arb. units)

Binding Energy (eV)

529.4

530.8

7927907887867847827807780510152025

30

40

50

60

70

Concentration (%)

Sputter depth (nm)

Co O

On surface

788.7

782.8

781.0

Co 2p

Binding Energy (eV)

Intensity (arb. units)

780.0

(a)

(c)

(b)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(g)

(f)