Deciphering the work function induced local charge regulation towards activating an octamolybdate cluster-based solid for acidic water oxidation†

Harshita Bagdwal^a, Parul Sood^a, Arshminder Kaur Dhillon^a, Ashi Singh^b and Monika Singh*^a
aInstitute of Nano Science and Technology, Sector-81, Knowledge City, Sahibzada Ajit Singh Nagar, Punjab 140306, India. E-mail: monika@inst.ac.in
^bDepartment of Chemistry, Indian Institute of Technology, Delhi, India

First published on 13th August 2024

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1. Introduction

Fossil fuels are significant non-renewable energy sources in today's world. However, CO₂ released from their combustion is a major concern, contributing to the greenhouse effect, and adversely affecting our ecosystem.¹ Thus, there is a pressing need to explore alternative, renewable energy sources. Hydrogen has emerged as a promising candidate, a green source of energy that can be easily transported and produced through water electrolysis and fuel cell reactions.^2,3 Water splitting (2H₂O → O₂ + 2H₂) is considered one of the cleanest methods for hydrogen production, involving two half-cell reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) at the anode and cathode, respectively. The oxygen evolution reaction (OER) at the anode is crucial not only for hydrogen production, but also for CO₂ reduction, dinitrogen reduction, and nitrate reduction, which are essential for a sustainable environment. However, the OER (2H₂O → O₂ + 4H⁺ + 4e⁻) is a slow reaction occurring at the anode, significantly impacting the production of hydrogen due to its sluggish kinetics.^4–6 Therefore, there is a critical need to develop an electrocatalyst capable of enhancing the OER to facilitate the water-splitting process and hydrogen production.⁷ While noble metal-based electrocatalysts like RuO₂ and IrO₂ exhibit excellent water oxidation properties,⁸ their high cost and instability limit their widespread use.⁹ As a result, attention has shifted towards catalysts based on Earth-abundant transition metals, which offer cost-effectiveness along with activity in electrocatalytic water oxidation.¹⁰ Several reports highlight catalysts based on metals (Earth-abundant) that show activity towards electrochemical water oxidation, primarily in alkaline pH environments. In one such report, Te Tsai et al. synthesized a FeCoNi-based electrocatalyst for alkaline water splitting.¹¹ However, pH levels between acidic and neutral are ideal for water oxidation, as the high concentration of H⁺ ions at low pH levels thermodynamically supports hydrogen production. Cheng Long et al. synthesized an iridium-based derivative A_xIr_yO_z which was used as an electrocatalyst for acidic water oxidation requiring an overpotential of 305 mV to reach a current density of 10 mA cm⁻².¹² But the OER at lower pH faces immense difficulty in terms of activity and stability. Unfortunately, many catalysts, including those based on low-cost Earth-abundant metals, corrode in acidic environments, leading to poor stability.^13,14 Consequently, current efforts are focused on synthesizing an electrocatalyst that is both economical and stable in acidic media, thereby reducing the energy barrier for hydrogen production.¹⁵

On the basis of the pH of the electrolyte, there is a different pathway that is followed for O₂ generation. The reaction mechanisms for OER in acidic,¹⁶ neutral, and alkaline¹⁷ media are given below:

In basic/neutral medium:

M + OH− → MOH + e−

MOH + OH− → M−O + H2O

MO + OH− → MOOH + e−

MOOH + OH− → M + O2 H2O

In acidic medium:

M + H2O → MOH + H+ + e−

MOH → M−O + H+

MO + H2O → MOOH + H+

MOOH → M + O2 + H+

The alkaline OER is more favorable than the acidic OER because of its better reaction kinetics as it has an abundance of hydroxyl groups. On the other hand, the acidic OER requires very high energy to break the O–H bond of H₂O, hence making the reaction sluggish. However the acidic OER is preferred because of its application in Proton Exchange Membrane (PEM) water electrolyzers. In the context of renewable electricity, when compared to alkaline electrolyzers they offer various advantages such as lower ohmic resistance, high gas purity, and high operating pressure at low temperature to produce high-purity H₂ gas. However, their use is limited to noble metals like Ru and Ir. So there is a high demand to develop a noble metal-free, highly stable electrocatalyst that can enhance and support acidic water oxidation for their use in PEM electrolyzers.^18,19

To date, a plethora of organic, inorganic, and hybrid functionality materials have been employed for electrocatalysis. The different classes of electrocatalyst materials involved are metal–organic frameworks (MOFs), transition metal-based heterostructures, layered double hydroxides (LDHs), and metal oxides/phosphides/sulfides/nitrides.^20–22 POMs are emerging materials in the field of electrocatalysis due to their multi-electron redox behavior, dealing with the electrolyte of acidic to neutral range.^23,24 However, the less conductive nature and huge overpotential set a drawback for POMs to be employed as electrocatalysts.²⁵ Polyoxometalates (POMs) can maintain various oxidation states without undergoing significant structural alterations, demonstrating remarkable stability even under demanding operational conditions.²⁶ They are considered metal–oxygen anionic clusters wherein the metal oxide (MO_x) serves as the elemental structural motif. However, the metals in their highest oxidation state in the POM cluster wrapped with the oxygen atom make it an auspicious contender for catalyzing acidic water oxidation.^27,28 Their inherent anionic character endows them with heightened activity and robustness within an acidic environment.²⁹ Distinctive attributes such as redox dynamics and multielectron transfer proficiency make them tempting materials for catalyzing the acidic OER.⁵ However, there are a few more advantages of POMs over other classes of catalysts:³⁰

(1) POM-based compounds consist of various metal ions and polyoxoanions, which allows for extensive control over their composition and structure. This capability enables the fine-tuning of their catalytic performance by adjusting these parameters for specific reactions.

(2) POMs have numerous active sites that act as catalytic centers for electrocatalytic reactions. Their multi-metallic structure and dispersed ion coordination contribute to high activity and selectivity, leading to exceptional performance in such reactions.

(3) POMs exhibit significant stability in electrocatalytic reactions. Their multi-metallic structure and ion coordination help resist corrosion and structural degradation in various environments, including oxidative, reductive, acidic, and alkaline conditions, thereby extending their lifespan.

(4) The structural parameters of POMs can be precisely controlled by modifying their composition, topology, and pore structure. This adjustability allows for the customized design of POMs to meet the specific requirements of electrocatalytic reactions, optimizing their catalytic performance.

Nonetheless, pristine POMs contend with inherent deficiencies: their susceptibility to solvation causes instability, while substantial overpotential impedes their efficiency as OER catalysts, owing to inadequate conductivity.¹⁶ Additionally, the recent literature indicates that annealing or transforming POM cluster-based solids into inorganic hybrid materials (metal phosphides/borides) can significantly improve the catalytic activity.^31,32 Undoubtedly, the POM-derived electrocatalysts perform well, but the core POM structure gets completely destroyed.^33,34 Numerous studies detail the in situ synthesis of metal oxide catalysts through the calcination of POMs as precursors. In one such report, Han and his team immobilized and calcined [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ and synthesized a solid-state CoWO₄ catalyst showing improved electrocatalytic performance.³⁵ Thus, the need of the hour is to look for an alternative to precisely activate the POM, without affecting its mean structure. In addressing these demands, POMs may be immobilized through hybridization or composite integration with carbon-based super-active materials.³⁶ An array of investigations has probed nanocomposite formulations incorporating polyoxometalates alongside conductive agents like graphene oxide and acetylene black, culminating in enhanced electrocatalytic efficacy.³⁷ In one such report, Joshi et al. have shown an enhanced alkaline oxygen evolution activity by an Anderson-type polyoxometalate with acetylene black as a co-catalyst.³⁸ Amidst the exhibition of carbonaceous options, reduced graphene oxide (rGO) emerges as an outstanding synergistic companion to POMs, owing to its elevated conductivity, high stability, and expansive surface area.²⁵ The precise activation of an octa-molybdate POM was accomplished using rGO, which has significantly improved acidic OER electrocatalysis for green hydrogen generation at lower pH.

For efficient electrocatalysis, the regulation of the internal electric field plays a critical role in achieving an optimized electronic structure of POMs. The electronic structure and the structural defects due to the 2D-carbon-based material also affect the electrochemical behavior. To date, many research articles have been published where POM was composited with carbon-based materials.³⁹ However, the question is, what makes POMs so active after compositing? More specifically, in this work, the intrinsic structure of the POM was studied in terms of internal electric field and structural disorder. This work demonstrates a unique approach that focuses on activating the POMs at room temperature without affecting their core structure. Uniquely, for the first time we have investigated the underlying mechanisms of POM activation by optimizing the work function and defects within the carbon structure. The POM:rGO nanocomposite was optimized in various ratios to achieve a lower work function and a higher I_D/I_G ratio, enhancing its conductive properties for electrochemical reactions.^40,41 A lower work function signifies reduced energy required for an electron to transition from the Fermi level to the vacuum level.⁴² The non-equivalent electronic charge distribution between a POM and rGO, dispersed during nanocomposite formation, alters the electronic structure of the POM. These findings not only enhance the catalytic activity of POMs, but also present an exciting roadmap to tailor their electronic structure for future electrocatalysts, with our findings displaying a significant increase in electrocatalytic efficiency. More specifically, we synthesized a nanocomposite comprising an octamolybdate POM and reduced graphene oxide (rGO) for application in acidic OER electrocatalysis. Typically, an octa-molybdate POM, denoted as [{Cu(pz)₄}₂Mo₈O₂₆]·2H₂O (Cat 1) was synthesized, via a hydrothermal route.⁴³ Subsequently, this POM was functionalized with rGO through a green method involving mechanical grinding for 15 minutes, employing varying amounts of rGO. Notably, this synthesis method avoids the need for high temperatures (calcination) or pressures, aligning with environmentally benign practices. The composite material demonstrated outstanding oxygen evolution reaction (OER) performance in an acidic environment, achieving an impressively low overpotential of 185 mV while maintaining a current density of 10 mA cm⁻². Particularly noteworthy is the substantial reduction in overpotential, observed at 180 mV, upon the incorporation of rGO into the POM matrix at a ratio of 2:1 (POM:rGO), as compared to the pristine POM based solid (Cat 1) which exhibited an overpotential of 365 mV. The selection of polyoxometalates, such as {Cu(pz)₄}₂Mo₈O₂₆·2H₂O, as water oxidation catalysts is driven by several key factors. Firstly, their extensive framework facilitates and supports charge transfer.⁴⁴ Moreover, the synthesis of Cu(pz)₄₂Mo₈O₂₆·2H₂O is straightforward and cost-effective. Also, in the recent literature, Co-based POMs were reported more as compared to other transition metals (Cu, Ni, and Fe).^45,46 Herein, a Cu-based POM was designed and engineered to optimize its electronic structure for improved acidic water oxidation.

2. Results and discussion

An octamolybdate type polyoxometalate [{Cu(pz)₄}₂Mo₈O₂₆]·2H₂O (Cat 1), has been synthesized through a hydrothermal method. The crystal structure of [{Cu(pz)₄}₂Mo₈O₂₆]·2H₂O (Cat 1) is already known⁴³ and the asymmetric unit is shown in the ESI (Fig. S1†). Cat 1 was further mixed with rGO in different ratios through continuous mechanical grinding for 15 minutes. The use of rGO was preferred for the synthesis of the POM-based nanocomposite because of its relatively larger surface area and high conductivity, enhancing the OER activity and stability of Cat 1 (Scheme 1).^47,48 To characterize the synthesized nanocomposite various instrumental techniques were employed as given in ESI section 2.† The experimental PXRD of Cat 1 matches well with the simulated pattern. Also, the PXRD pattern for POM–rGO (2:1) nanocomposite shows a peak-to-peak match with the simulated pattern of Cat 1 with some humps which are due to the incorporation of rGO corresponding to the (002) and (001) planes at 25 and 43.2°, respectively (Fig. 1).⁴⁹

However, the low crystallinity of the as-prepared powdered POM-rGO nanocomposite due to the presence of carbon can be one of the reasons for low-intensity peaks. The ratio of POM to rGO in the nanocomposite was optimized (1:1, 2:1, and 4:1) to precisely activate the Cat 1 towards acidic OER. The nanocomposite with a POM to rGO ratio of 2:1 showed the best catalytic activity and was further studied. A comparative XRD plot for 1:1, 2:1, and 4:1 is given in Fig. S2,† where 4:1 shows more peaks of the octamolybdate-based solid (Cat 1) as compared to 2:1 and 1:1 due to the higher content of Cat 1 as compared to the other two (Fig. S2†).

The valence state characterization was performed via XPS analysis. The survey spectra for 1:1 (Fig. S3†) and 4:1 ratios are provided in the ESI (Fig. S4†) along with the comparative high-resolution XPS spectra of Mo 3d and Cu 2p (Fig. S5†). The wide scan spectrum of 2:1 and Cat 1 indicates the existence of Cu, Mo, C, N, and O, in Fig. 2a. To verify the valence state of each element, the binding energy (B.E) was calibrated with the C 1s peak at 284.8 eV.^50,51 In Fig. 2b, a comparative high-resolution Mo 3d XPS is shown, having two peaks at a binding energy of 232.6 and 235.7 for Mo 3d_5/2 and Mo 3d_3/2 respectively for Cat 1⁵² and at 232.8 and 235.9 for 2:1 indicating Mo in its highest oxidation state (Mo⁶⁺). In Fig. 2c the high-resolution Cu 2p XPS for Cat 1 was deconvoluted into two peaks Cu¹⁺ 2p_1/2 and Cu¹⁺ 2p_3/2 at 952.7 and 932.8 respectively;³ moreover, two tiny shoulders were observed beside those peaks, which indicates the presence of a small amount of Cu²⁺. Additionally, for 2:1 four peaks were deconvulated for Cu¹⁺ 2p_1/2, Cu²⁺ 2p_1/2, Cu¹⁺ 2p_3/2, and Cu²⁺ 2p_3/2 at a binding energy of 952.9, 955.2, 933, and 935.2 respectively. Two satellite peaks can also be observed. It can be seen that there is a positive shift in the binding energy of Mo 3d and Cu 2p from Cat 1 to the 2:1 nanocomposite which is due to the oxidation of Cat 1 when ground with rGO to form the 2:1 nanocomposite. The appearance of Cu²⁺ peaks in the 2:1 nanocomposite also confirms the oxidation of Cat 1. The high-resolution O1s XPS indicates the peaks corresponding to Cu–O⁵³ and Mo–O, as reported in the literature (Fig. 2d).^38,54 Due to the oxidation of Cu 2p in Cat 1, the intensity of the Cu–O peak is increased in the case of the 2:1 nanocomposite. Additionally, the high-resolution C1s XPS results in the ESI Fig. S6† indicate the characteristic peaks for rGO, involving the C–C, O–CO, and C–OH peaks.⁵⁵ The comprehensive XPS analysis confirms the successful creation of the nanocomposite comprising POM and rGO after the oxidation of Cat 1.

To probe into the morphological features of the catalyst, FESEM and TEM were performed. The FESEM image in Fig. 3a shows the micrograph of pristine Cat 1 with a nanosheet-like morphology. However, the FESEM images clearly show the breaking of bigger sheets into non-uniform smaller sheets on the formation of the 2:1 nanocomposites due to the mechanical grinding of Cat 1 (Fig. 3b). The TEM nanographs were also in agreement with the FESEM results, indicating the nanosheet width of around 720 nm for Cat 1 (Fig. 3c) and 740 nm for 2:1 (Fig. 3d) respectively. The HRTEM image for the 2:1 composite is shown in Fig. S10.† The crystallite size was also calculated using the Scherrer equation (D = Kλ/βcosθ) and was found to be 458 nm. In Fig. 3e, the elemental mapping nanograph is shown with an inset as the electron overlay for all the elements. To evaluate the homogeneous distribution of the elements, elemental mapping for the 2:1 nanocomposite was performed, and it was found that Cu, Mo, N, and O are uniformly spread over the catalytic surface (Fig. 3f). Similarly, for Cat 1 elemental mapping is provided in ESI (Fig. S7†).

TGA was performed to analyze the thermal stability of Cat 1 (Fig. S8†). Thermal stability was assessed at room temperature up to 900 °C under a nitrogen gas (N₂) atmosphere. The TGA curve for Cat 1 displays three clearly delineated stages of weight reduction. The initial decrease in weight, occurring at around 280 °C, correlates with the release of lattice water molecules and trapped moisture. The weight loss recognized from 310 °C to 380 °C is related to the decomposition of organic moieties, whereas a subsequent decrease in weight occurring after 380 °C is ascribed to the collapse of the remaining structure. Cat 1 is stable thermally up to 280 °C. The FTIR results are evident for the presence of all possible bond formations in the POM-based nanocomposite (Fig. S9†). The characteristic peak for Cu–O can be observed below 600 cm⁻¹ and the broad peak around 3400 cm⁻¹ can be ascribed to O–H stretching, respectively.^56,57 The evident peaks for Mo–O can be seen in the range of 800–1100 cm⁻¹ which is due to the octamolybdate core.⁵⁸ The coordination complex of Cu with pyrazole suggests the Cu–N bond which is around 430 cm⁻¹. Additionally, the sp² carbon of rGO can also be observed in the FTIR spectra.⁵⁹

To get further details about the reason for the better activity of the 2:1 nanocomposite compared to Cat 1 and other composites, ultraviolet photoelectron spectroscopy (UPS) analysis was performed to evaluate the work function of Cat 1 and all the composites. It can be seen in Fig. 4a that the 2:1 nanocomposite has the lowest work function of 1.74 followed by 1:1, Cat 1, and, 4:1 nanocomposites. The lower the work function, the more conducive nature of the catalyst toward electrocatalysis, as its value indicates the energy required for an electron to move from the Fermi level to the vacuum level.⁴² As 2:1 has the lowest work function, it has the best activity towards water oxidation. The UPS data are in accordance with the electrochemical activity of Cat 1 and all other composites (Table S2†). Raman spectroscopy was also performed for composites in all the ratios (2:1, 1:1, and 4:1) and it was found that 2:1 has the highest I_D/I_G ratio (1.0039) followed by 4:1 and 1:1 as shown in Fig. 4b. As previous reports indicate, a higher ratio leads to a higher defect degree of carbon present in the system. The defects would lead to altering the local electronic structure to improve the exposure of surface-active sites.^40,41

2.1 Acidic oxygen evolution reaction measurement

The electrochemical measurement details are given in ESI, section 3.† Electrochemical measurements for the oxygen evolution reaction (OER) were obtained in an acidic medium of 0.5 M H₂SO₄. Initially, the electrocatalyst (2:1) was stabilized by performing multiple cyclic voltammetric cycles. After stabilization, linear sweep voltammetry (LSV) curves of Cat 1, 2:1, bare GS (graphitic sheet), and RuO₂ were recorded at a scan rate of 10 mV s⁻¹ in 0.5 M H₂SO₄. The LSV voltammogram is shown in Fig. 5a, which indicates that the 2:1 nanocomposite possesses improved onset and overpotential as compared to other supplementary catalysts. More specifically, Cat 1, 2:1 nanocomposite, bare GS (graphitic sheet), and RuO₂ were able to reach a current density of 10 mA cm⁻² with an overpotential of 365, 185, 539, and 258 mV, respectively. The overpotential of Cat 1 was significantly improved after compositing it with rGO at 2:1, which indicates that Cat 1 was precisely activated by rGO towards acidic OER. The LSV performance of all the prepared nanocomposites in different ratios was checked and compared (Fig. S11†). It was found that the (2:1) ratio shows the lowest overpotential because of the synergistic effect between the polyoxometalate and reduced graphene oxide indicating the ratio being the best-optimized amount for the formation of the POM–rGO nanocomposite (Table S2†). The corresponding Tafel slopes (Fig. 5b) for Cat 1, 2:1 nanocomposite, RuO₂, and GS were also calculated and found to be 361, 252, 349, and 431 respectively which indicates that the 2:1 composition of an electrocatalyst appreciably improves the reaction kinetics for OER at lower pH. The overpotentials and Tafel slopes of all the electrocatalysts are compared via a bar graph in Fig. 5c. EIS Nyquist studies were carried out at an OCP of 0.56 V and in the frequency range of 0.1–100 kHz to check the charge transfer resistance (R_ct) for Cat 1 and 2:1 nanocomposite, which indicates the superconductive nature of our synthesized nanocomposites. The inset shows the fitted single-phase equivalent circuit diagram with the constant phase element (Q), solution resistance (R_s), and R_ct (Fig. 5d). The R_ct value was found to be 0.48 ohms. To evaluate the capacity of the formed double layer of the catalyst, the non-faradaic region was chosen to perform the CV at multiple scan rates (given in ESI Fig. S12 and Fig. S13†). The intrinsic activity of the catalyst is evaluated using C_dl, apparent turnover frequency (TOF) (Fig. 6b) and ECSA normalized LSV (Fig. S14†). Fig. 5e indicates the C_dl plot for Cat 1 and 2:1 nanocomposite, with the C_dl values of 34.15 and 63.8 mF cm⁻² respectively which can be one of the major causes for the better activity of the prepared POM–rGO (2:1) nanocomposite. The acidic OER stability is one of the bottleneck issues. The elevated C_dl value indicates the increased accessibility to active sites on the 2:1 nanocomposite material as compared to Cat 1.⁵⁸ The optimized concentration of rGO with Cat 1 leads to enhanced double-layer capacitance (C_dl) of the 2:1 catalyst significantly. The formation of a multiple charged layer in the non-faradaic region of the OER window signifies the conductive nature of the material. The acidic OER stability is one of the bottleneck issues for commercialization and overall water splitting at low pH for H₂ production. To examine the stability, chronoamperometric analysis was performed (Fig. 5f) for more than 24 hours at a higher current density of 100 mA cm⁻², which is appreciable in comparison to the recently reported catalyst. Thus, enhancing the stability to a larger extent as compared to the pure polyoxometalates, which are otherwise highly soluble and unstable in an acidic medium, was achieved in this work by compositing a pure POM with a carbon-based active material i.e., reduced graphene oxide. The inset shows the LSV curve of the 2:1 nanocomposite, pre- and post chronoamperometry study suggesting a minute elevation in the overpotential after 24 hours of stability test (Fig. 5f). Most of the reported catalysts for acidic water oxidation include noble metals like Ru and Ir in their composition. These metals suffer from poor stability under low pH conditions. A comparison of the 2:1 nanocomposite with various reported catalysts for acidic OER is given in Table S1.† The electrochemical activity of the POM–rGO nanocomposite (2:1) is compared with that of other reported electrocatalysts in acidic medium in Fig. 5g.^42,60–73

Faradaic efficiency was calculated by comparing the amount of O₂ evolved experimentally to the amount of O₂ evolved theoretically. The faradaic efficiency was found to be 99.45% (Fig. 6a). Also, the intrinsic activity of Cat 1 and the 2:1 nanocomposite was investigated by analyzing its turnover frequency (TOF) (Fig. 6b). The TOF value came out to be 0.023 s⁻¹, and 0.043 s⁻¹ for Cat 1 and 2:1 respectively. All electrochemical examination indicates that the POM was precisely activated by the rGO in an optimized ratio, to make acidic OER feasible. Nevertheless, this comparison can be applied to enhance the durability and effectiveness of alternative POM-based substances in catalyzing water oxidation under acidic conditions.

2.2 Post-OER characterization

In order to evaluate the stability of an electrocatalyst, it is necessary to characterize the morphology and valence state. To examine the catalyst post-OER, we have performed PXRD, FESEM, TEM, EDX, XPS, and Raman characterization for the 2:1 nanocomposite. The sheet-like morphology is retained even after the electrochemical stability experiment shown in Fig. S16a.† The presence of all elements post-OER can be seen through the FESEM elemental mapping and TEM EDX mapping shown in Fig. S16b and Fig. S17† respectively. Further post-OER XPS analysis clearly shows a favorable positive shift in the binding energy of Cu 2p and a negative shift in Mo 3d spectra (Fig. 7c and d), as compared to the pre-OER binding energies, which might be due to the oxidation of Cu and reduction of Mo⁶⁺ to Mo⁴⁺ during the OER process.¹⁶

HR-TEM was performed which exhibits the d-spacing of 0.25 nm corresponding to Cu(OH)₂ for the (111) plane (JCPDS 01-080-0656) along with the inverse FFT pattern shown in Fig. 7a. From these results, it can be assumed that Cu is acting as an active site for the oxygen evolution reaction here. However, in PXRD we have found no significant peaks, which implies that the structure became amorphous on the surface during the OER stabilization (Fig. S15†).⁷⁴ Thereafter post OER Raman spectroscopy was also performed. The spectra showed three sharp D-band, G-band, and 2-D graphene peaks at 1350, 1577.5, and 2710 respectively (Fig. 7b).⁷⁵ A plausible mechanism for electrocatalytic oxygen evolution is shown in Fig S17.† To evaluate the dissolution rate of Cu and Mo, ICP-MS was performed after 24 hours of stability. It revealed that the electrolyte contains 0.13 ppm of Cu and 0.79 ppm of Mo maintaining the almost 1:8 ratio of Cu–Mo even after 24 hours of stability. Cat 1 with 2:1 POM to rGO ratio displayed better OER activity, while rGO contributed to enhanced conductivity. However, when rGO composition was further increased (as in the 1:1 composite), it may have covered the active sites of Cat 1. However, when rGO composition was further increased (as in the 1:1 composite), it may have covered the active sites of Cat 1 leading to higher work function. On the other hand, in a 4:1 composite (with more POM composition), the conductivity decreased. Consequently, it was inferred that the 2:1 POM-to-rGO composition displayed the best synergistic effect, resulting in lower work function and improved oxygen evolution reaction (OER) activity compared to other ratios.

3. Conclusion

In this work, the work function of polyoxometalate (POM) was optimized for efficient electrochemical water oxidation at low pH by compositing it with reduced graphene oxide (rGO) for the first time. The compositing method of POM was sustainable and performed by manual grinding at room temperature. The specific POM used in this study was an octamolybdate solid-cluster, {Cu(pz)₄}₂[Mo₈O₂₆]·2H₂O. The compositing ratio was optimized to 2:1 (POM:rGO) to achieve a lower work function and enhanced oxygen evolution reaction (OER) activity. The 2:1 composite demonstrated superior electrocatalytic performance, exhibiting an overpotential of 185 mV in an acidic medium. Additionally, it maintained stability for over 24 hours at a high current density of 100 mA cm⁻². The focus of this study was to activate the POM by compositing it with rGO through a sustainable method, resulting in groundbreaking electrocatalytic activity and stability at low pH. This research proposes a novel approach to engineering POM for use as an acidic water oxidation catalyst through environmentally friendly techniques.

Author contributions

The completion of this article involved contributions from all the authors. All the authors have given their consent for the final manuscript.

Data availability

Raw data for this article, including PXRD, FESEM, XPS, electrochemical results etc. are available from the corresponding author Monika Singh (monika@inst.ac.in).

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

HB, PS, and AKD acknowledge INST for PhD fellowships. MS is grateful to INST for financial and infrastructural support. MS thanks SERB-DST for the project EEQ/2022/000149 and CSIR-EMR project 01(3064)21-EMR-II for funding. We also thank DST: SR/FST/CSII-07/2014 and the Institute of Eminence (IOE) grant from the University Grants Commission (UGC-Ministry of Human Resource and Development, India) for funding the single-crystal diffractometers at the Department of Chemistry, IIT Delhi.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr02645d

This journal is © The Royal Society of Chemistry 2024