Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2025 Feb 11;64(7):3242–3255. doi: 10.1021/acs.inorgchem.4c04619

Elucidation of the Activity and pH Stability Limits of Polyoxometalate-Intercalated Layered Double Hydroxide Nanocomposites toward Water Oxidation Catalysis

Joaquín Soriano-López †,*, Javier Quirós-Huerta †, Álvaro Seijas-Da Silva †, Ramón Torres-Cavanillas †,‡, Eduardo Andres-Garcia †, Gonzalo Abellán †, Eugenio Coronado †
PMCID: PMC11863369  PMID: 39933709

Abstract

graphic file with name ic4c04619_0009.jpg

The inclusion of water oxidation active polyoxometalates (POMs) inside layered materials is a promising strategy to increase their catalytic efficiency while overcoming their fragility under homogeneous conditions. In this sense, intercalation of POMs in the interlaminar space of layered double hydroxides (LDHs), formed by positively charged brucite-type inorganic layers, is a very interesting strategy that is gaining attention in the field. Despite their huge potential, there is a lack of accurate characterization of the materials, especially after their use as water oxidation catalysts under pH conditions in which the POM counterpart has been demonstrated to be unstable (strong alkali media). For this reason and as a proof of concept, we have intercalated the well-known [Co4(H2O)2(PW9O34)2]10– POM (Co4-POM) in the lamellar space of the Mg2Al-LDH, to study its catalytic capabilities and stability. Remarkably, the nanocomposites show improved water oxidation efficiencies with excellent stability in close-to-neutral media compared with the water-insoluble cesium salt of Co4-POM or commercial Co3O4. However, thorough postcatalytic characterization of the nanocomposites demonstrates that the polyoxotungstate framework of the POM suffers from hydrolytic instability in strong alkali conditions, leading to the formation of a mixed-valence cobalt(II/III) oxide in the interlayer space of Mg2Al-LDH. This work highlights the importance of accurately assessing the fate of the catalytic POM after the catalytic reaction, especially when conditions are employed outside the pH stability window of the POM, which can lead to erroneous conclusions and mistaken catalytic activities.

Short abstract

Electrocatalytic oxygen evolution employing a cobalt-substituted polyoxometalate intercalated in a layered double hydroxide: Improving the efficiency and setting the stability limits.

1. Introduction

Climate change and global warming caused by the use of fossil fuels to produce energy urges us to rapidly change the energy production paradigm to environmentally friendly, efficient, and cost-effective alternatives.1,2 Hydrogen has the potential to contribute to this goal as the energy vector in a carbon-free energy system.3 The baseline technology for green hydrogen production is water electrolysis, which uses as inputs electricity powered from renewable resources and pure water to produce hydrogen and oxygen through the water splitting reaction.4 In this scheme, the water oxidation half-cell reaction, also known as the oxygen evolution reaction (OER), represents the bottleneck for the development of new technologies and energy storage concepts.5 This is due to its high thermodynamic requirements and sluggish kinetics, which typically require the use of scarce and geolocated elements that drastically limit the wide deployment of novel energy technologies.6 Therefore, there is an urgent need to develop and optimize new OER nanomaterials based on earth-abundant elements for achieving sustainable hydrogen production via electrolysis.7

The OER capabilities of transition-metal-substituted polyoxometalates (POMs) have been studied during the last years since they offer high stability under strong oxidizing conditions and rich intrinsic redox chemistry.8 Particularly interesting are cobalt-substituted POMs (Co-POMs), which are nowadays known to be genuine robust molecular OER electrocatalysts displaying excellent activities.9−11 Thanks to their high chemical amenability it was possible to study structure–reactivity relationships that contributed to pinpoint the contribution of each of the different moieties that compose their structures.12−18 Nevertheless, water electrolysis technologies require the immobilization of the molecular Co-POM into solid supports resulting in hybrid catalysts working in the solid state, which in turn can increase the stability and OER efficiency of the molecular counterpart.19 Different strategies have been used in this area focused on the electrostatic interaction between the negatively charged Co-POM and positively charged 3-dimensional supports, for instance, mesoporous carbon nitride or zeolitic imidazolate frameworks.20,21 In this respect, layered materials such as layered double hydroxides (LDHs) can play a crucial role in the immobilization of Co-POMs since they provide positively charged surfaces in between the layers that can accommodate the Co-POMs to form self-assembled POM/LDH nanocomposites via established electrostatic and hydrogen bond interactions.22,23 The laminar nature of the LDH provides additional advantages for the Co-POMs heterogenization since they result in OER nanomaterials with high surface area and good mechanical strength and chemical stability, as well as an enhancement of charge separation by minimizing the path channels in which electrons have to migrate during the electrocatalytic reaction.24 Importantly, the confinement effect created at the LDH interlayer space prevents the leaching of the POM species into the reaction media, thus overcoming one of the biggest drawbacks of electrostatically supporting POMs on 3D and 2D surfaces.25

Therefore, the synergistic effects that appear at the POM/LDH interface can boost the stability and catalytic activity of the hybrid nanomaterials compared with their constituents separately. This characteristic has made POM/LDH nanocomposites very attractive nanomaterials for diverse catalytic reactions. For instance, Song and co-workers reported the intercalation of a Zn-substituted Dawson-type POM (ZnP2W17) into a Mg3Al-suberic LDH precursor leading to the formation of the ZnP2W17/Mg3Al hybrid, which showed superior catalytic efficiency and selectivity toward the sulfoxidation of various sulfides compared to that of the POM or LDH precursors separately.26 Miras and colleagues prepared a POM/LDH hybrid containing the Zn4–Weakley sandwich POM and the Mg2Al-Tris LDH (Tris = Tris(hydroxymethyl)aminomethane). The Zn4/Mg2Al nanocomposite demonstrated efficient bifunctional catalysis for cascade reactions involving the oxidation of benzyl alcohol to benzaldehyde promoted by the POM species in the presence of H2O2. The reaction then occurs through a Knoevenagel condensation step between the benzaldehyde and ethyl cyanoacetate at the basic sites of the LDH counterpart, which lead to the production of benzylidene ethyl cyanoacetate with excellent activity and selectivity.27 Surprisingly, the use of POM/LDH nanocomposites as OER materials is far less explored, whereby only a few examples can be found in the literature. Wang and co-workers intercalated the OER inactive Keggin-type POM [PW12O40]3– into a Ni2Fe-LDH to boost its OER performance.28 This POM/LDH nanocomposite showed superior OER performance under strong basic conditions than the bare Ni2Fe-LDH. However, postcatalytic characterization of the compound is highly required in order to clearly address the role and fate of the Keggin POM during turnover conditions, since it is well known that the Keggin anion is only stable below pH 2, it forms lacunary species at pH values between 2 and 9, and it completely dissolves above pH 9.29 Yu and Liang intercalated different transition metal monosubstituted Keggin POMs into MgAl-Tris LDH.30 OER electrocatalytic tests showed that the hybrid bearing the Co-POM showed good electrocatalytic properties at neutral pH values, whereas postcatalytic characterization of the recovered materials suggests that the Co-POM maintained its structure during the water oxidation reaction with most of the initial Co(II) ions appearing in higher oxidation states, most likely as Co(III) ions. Interestingly, the authors demonstrated that carrying out the experiments at pH 10 leads to deactivation of the hybrid nanomaterial due to the poor hydrolytic stability of this Co-POM at the mentioned pH. The Tris ligand though, bearing three primary alcohols, can be easily oxidized under the strong oxidative environment created during the OER measurements, which will mask the real current density assigned to OER.31,32 Therefore, quantification of the oxygen evolved in these experiments would be very interesting to clarify the Faradaic efficiency of the system. More recently, Ni and co-workers reported the bifunctional electrocatalytic activity for OER and HER of a hybrid POM/LDH system composed of the phospho-molybdate Wells–Dawson-type POM (P2Mo18) and the ZnFe-LDH directly formed on a Nickel foam electrode.33 Again, the role and fate of the POM need to be carefully assessed in these structures since it is well known that P2Mo18 possesses a very low hydrolytic stability in solution regardless of the pH of the media.34,35 Hence, it would not be surprising that the POM structure is not maintained during electrocatalytic tests performed in strong alkali media. This highlights the importance of selecting the proper working conditions that allow us to define the boundaries of action of the electroactive material and to study in detail the synergies that appear at the molecular and 2D material interface. This knowledge will be pivotal for the rational design of novel hybrid nanomaterials with improved catalytic capabilities and durability.

In this work, we report the preparation of Co4/Mg2Al nanocomposites through the intercalation of the OER active POM [Co4(H2O)2(PW9O34)2]10– (Co4-POM) in the lamellar space of a Mg2Al LDH (Mg2Al-LDH) (Scheme 1). Employing different reaction conditions allowed us to obtain the nanocomposites with increasing amounts of the intercalated POM. These Co4/Mg2Al nanocomposites were tested as OER materials under different working conditions, from neutral to strong alkali media. Our electrocatalytic results show that the OER activity does not scale with the quantity of Co4-POM present in the materials, hence permitting to optimize the OER efficiency of the system by employing just the required amount of POM. Different ex situ and in situ characterization methods performed on the Co4/Mg2Al nanocomposites demonstrate the fate of the Co4-POM as part of the nanocomposites in the different reaction conditions. We clearly observe that the Co4-POM is not stable under strong basic conditions, where Co(II) ions are expelled from the POM framework leading to the Mg2Al-LDH with Co(II) ions present in the lamellar space, which are partially oxidized to Co(III) under the OER applied potential. It is interesting to mention that in the latter scenario, the W and P atoms conforming to the POM are lost almost completely from the hybrids. On the other hand, the material is stable in neutral media, while a possible partial decomposition of Co4-POM occurs at pH 9 where Co4-POM and related species may coexist in the hybrids during OER conditions.

Scheme 1. Schematic Representation of the Anion Exchange Process for the Fabrication of the Co4/Mg2Al Nanocomposites.

Scheme 1

Open in a new tab

2. Experimental Details

2.1. Synthesis of K10[Co4(H2O)2(B-α-PW9O34)2]·44.2H2O (Co4-POM)

This well-known polyoxometalate was prepared by applying an optimized literature method.1 Na2WO4·2H2O (33.0 g, 100 mmol) and Na2HPO4 (1.57 g, 11.1 mmol) were dissolved in 100 mL of H2O, and the pH was adjusted to 6.5 through addition of glacial acetic acid. Then, a solution containing Co(CH3COO)2·4H2O (5.48 g, 22 mmol) dissolved in 50 mL of H2O was added to this mixture. The resulting dark-purple solution was refluxed for 2 h. Thereafter, the mixture was filtered twice to remove a residual purple precipitate. The resulting solution was saturated with 3 g of potassium acetate, filtered again, and allowed to cool at room temperature. Rhombic, purple crystals of Co4-POM were collected by filtration and washed with water, methanol, and acetone. The sample was dried in air. The compound was characterized by FT-IR (Figure S1), the POM composition was calculated by elemental analysis (ICP-MS) (Table S1), and the water content was calculated by thermogravimetric analysis (TGA) (weight loss: 13.46%). MW = 5918.04 g/mol.

2.2. Preparation of Cs8.3K1.7[Co4(H2O)2(B-α-PW9O34)2]·14H2O (CsCo4-POM)

CsCo4-POM was achieved by metathesis reaction following an established literature method2 and characterized by FT-IR (Figure S1) and by elemental analysis (ICP-MS) (Table S1).

2.3. Synthesis of Mg2Al-Cl-LDH (Mg2Al-LDH)

This LDH was prepared by the urea hydrolysis method under hydrothermal conditions. In a first step, 250 mM aqueous solutions of MgCl2·6H2O and AlCl3·6H2O were prepared separately employing Milli-Q H2O and stored at 4 °C. Then, 36 mL of the MgCl2 solution and 18 mL of the AlCl3 solution were mixed in a Teflon vial, and the final volume was increased to 90 mL with the addition of 36 mL of Milli-Q H2O. To this solution, 2.365 g (39.4 mmol) of urea was added, and the resulting mixture was stirred for 5 min. The Teflon vial was placed in the autoclave and the reaction was carried out in an oven at 140 °C for 24 h. After cooling down to room temperature, the obtained Mg2Al-LDH was recovered by centrifugation, washed with Milli-Q H2O until the resulting washing waters had a neutral pH, and then washed with EtOH, and dried under vacuum for 24 h. Infrared spectrum of the as-synthesized Mg2Al-LDH indicates that in addition to chloride, carbonate anions have also been incorporated into the LDH interlayer space (Figure S2). The carbonate was removed from the structure by anion exchange. For this, we prepared first in a round-bottom flask a 1 M NaCl aqueous solution using 1 L of Milli-Q H2O to which HCl was added until a final concentration of 3.3 mM Then, the solution was degassed with N2. Thereafter, 1 g of the as-synthesized Mg2Al-Co3-LDH was dispersed in the aqueous solution, the round-bottom flask was sealed, and the suspension was stirred for 12 h under a flow of N2. This step was repeated twice to completely exchange the carbonate by chloride anions, as confirmed by FT-IR (Figure S2). Finally, the Mg2Al-Cl-LDH was recovered by centrifugation, washed three times with Milli-Q H2O, and then with EtOH, and dried under a vacuum overnight.

2.4. Synthesis of MgCoAl-LDH

This LDH was prepared similarly to Mg2Al-LDH following the urea hydrolysis method under hydrothermal conditions. Note that we have selected a concentration of Co ions of 10% in the synthetic procedure based on the quantity of cobalt found in the Co4/Mg2Al_3 nanocomposite. First, 250 mM aqueous solutions of MgCl2·6H2O, CoCl2·6H2O, and AlCl3·6H2O were prepared separately employing Milli-Q H2O and stored at 4 °C. Then, 30.6 mL of the MgCl2 solution, 5.4 mL of the CoCl2 solution, and 18 mL of the AlCl3 solution were mixed in a Teflon vial, and the final volume was increased to 90 mL with the addition of 36 mL of Milli-Q H2O. To this solution, 2.365 g (39.4 mmol) of urea was added and the resulting mixture was stirred for 5 min. The Teflon vial was placed in the autoclave, and the reaction was carried out in an oven at 110 °C for 24 h. After cooling down to room temperature, MgCoAl-LDH was obtained as a pale pink solid that was recovered by centrifugation, washed with Milli-Q H2O until the resulting washing waters had a neutral pH, then it was washed with EtOH, and dried under vacuum for 24 h. FT-IR spectrum and PXRD pattern of the as-synthesized material are shown in Figure S3. The diffraction peaks correspond to the main basal reflections expected for a hydrotalcite-like material.

2.5. Fabrication of the Hybrid Co4/Mg2Al Nanocomposites

2.5.1. Preparation of Co4/Mg2Al_1

A 5 mM Co4-POM aqueous solution was prepared by dissolving 148.5 mg of Co4-POM in 5 mL of Milli-Q H2O. The solution was then filtered using a Nylon syringe filter, transferred into a vial, and degassed with Ar for 20 min. Then, 10 mg of Mg2Al-LDH was added and the vial was sealed and sonicated for 20 min. After this, the solution was stirred while Ar was passed through the gas space of the vial using a syringe for 42 h. Finally, the hybrid nanocomposite was collected by centrifugation, washed three times with Milli-Q H2O, then with EtOH, and dried under vacuum overnight.

2.5.2. Preparation of Co4/Mg2Al_2

A 5 mM Co4-POM aqueous solution was prepared by dissolving 148.5 mg of Co4-POM in 5 mL of Milli-Q H2O. The solution was then filtered using a Nylon syringe filter, transferred into a vial, and degassed with Ar for 20 min. Then, 10 mg of Mg2Al-LDH was added and the vial was sealed and sonicated for 60 min. After this, the solution was stirred while Ar was passed through the gas space of the vial using a syringe for 42 h. Finally, the hybrid nanocomposite was collected by centrifugation, washed three times with Milli-Q H2O, then with EtOH, and dried under vacuum overnight.

2.5.3. Preparation of Co4/Mg2Al_3

8.0 mg of Mg2Al-LDH was placed in a vial to which 4 mL of formamide was added. Then, the vial was sealed and purged with Ar for 30 min and placed in the ultrasound bath for 20 min. Separately, 118.8 mg of Co4-POM were dissolved in 4 mL of Milli-Q H2O to obtain a 5 mM solution, which was also purged with Ar for 30 min. Thereafter, the Co4-POM solution was added dropwise to the dispersed Mg2Al-LDH, and under stirring, a syringe was pierced through the septum of the sealed vial. After the complete addition of the Co4-POM solution, Ar was passed through the gas space of the vial and left under stirring for 20 h. Finally, the hybrid nanocomposite was collected by centrifugation, washed three times with Milli-Q H2O, then with EtOH, and dried under vacuum overnight. Note: to ensure the stability of Co4-POM in the H2O:formamide medium, we have recorded the UV–vis of a 2.5 mM solution of Co4-POM in H2O:formamide (1:1 v/v) and compared to an aqueous 2.5 mM solution of Co4-POM. The absence of a precipitate in the mixture and the UV–vis show that Co4-POM is stable in the H2O:formamide mixture (Figure S4).

2.6. Electrode Preparation

Nafion inks were prepared by dispersing 2 mg of each sample and 1 mg of carbon black in 0.5 mL of a 1:1 (H2O:EtOH) solution and 10 μL of a Nafion 117 solution (∼5%). Then, the mixture was sonicated for 2 h to obtain a homogeneous suspension. Aliquots of 5 μL were drop-cast on the glassy carbon electrodes (already polished with 1.0 and 0.05 μm alumina powder) and dried at room temperature, whereas aliquots of 20 μL were drop-cast on the FTO electrodes (washed by sequentially sonicating them in water and then in isopropanol for 3 min each, and dried with a nitrogen flow) and dried at 60 °C.

2.7. Electrochemical Characterization

The electrochemical measurements were performed in a typical three-electrode setup using either a glassy carbon rotating disk electrode (RDE-GC, 0.07 cm2) for cyclic voltammetry (CV) and linear sweep voltammetry (LSV) or a fluorine-dope tin oxide (FTO, 0.25 cm2) for chronoamperometry as working electrode. The setup was completed by a Ag/AgCl (3 M KCl) double-junction reference electrode and a Pt foil as the counter electrode. Three different buffered solutions were employed in this work: (i) sodium phosphate (0.1 M) buffer at pH 6.9 with NaNO3 (1 M) as electrolyte (NaPi); (ii) sodium borate (0.1 M) buffer at pH 9 with NaNO3 (1 M) as electrolyte (NaBi); and (iii) KOH (1 M) at pH 14.2. Cyclic voltammetry and linear sweep voltammetry were performed using an Autolab PGSTAT 128N potentiostat/galvanostat. Chronoamperometric measurements were carried out in a Gamry 1000E potentiostat/galvanostat. The uncompensated resistance value was calculated using the utility implemented in the Gamry 1000E potentiostat/galvanostat prior to each LSV measurement by applying a constant OER overpotential of 300 mV. Ohmic drop (iR) of the LSV curves was then compensated (100%) during the data treatment using the formula: Ereal = Eapp – iR. Prior to each experiment, the buffer solutions were degassed with Ar for at least 30 min. Before the LSV measurements were conducted, CV at a scan rate of 100 mV/s was performed as a preconditioning step of the materials until no substantial changes were observed (typically 20 cycles). LSVs were performed at a scan rate of 1 mV/s and a rotation of 1600 rpm, which was controlled with an Autolab RDE motor controller. LSV data was used for Tafel analyses. Chronoamperometry measurements were carried out in an H-cell where the working and reference electrodes were separated from the counter electrode by a glass frit (P0). For these experiments the applied overpotentials were 700 mV, 600 mV, and 500 mV when using NaPi at pH 6.9, NaBi at pH 9, and KOH at pH 14.2, respectively. All of the measurements were repeated at least three times to ensure reproducibility of the results.

The Nernst equation was employed to calculate the thermodynamic potential for water oxidation (E0H2O/O2) at each pH used:

2.7. 1

All applied potentials (Eapp) were converted to the NHE reference scale using ENHE = EAg/AgCl + 0.210 (V). The overpotentials were calculated by subtracting the thermodynamic water oxidation potential (E0H2O/O2) from Eapp as

2.7. 2

The current densities were calculated based on the geometrical surface area of the electrodes. Nevertheless, the intrinsic catalytic activity was calculated based on the estimated electrochemical surface area (ECSA) value for each material. ECSA was estimated from the relationship of the current intensity associated with the charging of the double-layer capacitance (Cdl) during cyclic voltammetry with the scan rate. The applied potential was ±50 mV around the open-circuit potential of each electrode and the scan rates were 50, 100, 150, 200, 300, 400, and 500 mV/s. The Cdl was calculated from ΔI (Ia – Ic) versus scan-rate plot, where the slope = 2 × Cdl. Anodic and cathodic current values were taken from the CVs at the open-circuit potential value. The estimated ECSA was then obtained by dividing Cdl by the specific capacitance (Cs) from which the “typical” value of 40 μF/cm2 was employed.6

The obtained current densities were set to 0 mA/cm2 at 0 mV overpotential to remove any possible contribution of capacitance to the electrocatalytic current. The onset potentials were estimated from the intersection point between the tangent lines of the Faradaic current at 0.2 mA/cm2 and the non-Faradaic current. Herein, all potentials are given versus NHE, unless otherwise stated.

3. Results and Discussion

3.1. Fabrication and Characterization of the Nanocomposites

For this study, we have selected the Mg2Al-LDH for the intercalation of Co4-POM for different reasons: (i) it is one of the most studied LDHs, and its synthetic protocols as well as its properties are well studied; (ii) platelet-like microcrystals with well-defined hexagonal shape and with large, homogeneous sizes can be obtained using the urea hydrolysis method that will allow for a more facile characterization of the POM/LDH nanocomposites; and (iii) Mg2Al-LDH does not contain catalytically active metal sites, and therefore, we can safely assume that the OER activity observed by the nanocomposites arises just from the POM counterpart. Therefore, we synthesized the Mg2Al-LDH following the urea hydrolysis method under hydrothermal conditions,36 which yielded hexagonal platelet-like microcrystals of ca. 2 μm in diameter (Figure S5). This synthetic method typically yields carbonate intercalated LDHs, as it was confirmed by FT-IR spectroscopy (Figure S2), and due to the high affinity of LDHs to carbonate, which may preclude the proper intercalation of POMs, it had to be exchanged by chloride anions. This anion exchange was carried out by dispersing the Mg2Al-LDH in a 1 M NaCl aqueous solution containing HCl 3.3 mM.37 Note that this step was performed at least twice to ensure proper removal of the carbonate from the lamellar space. Thereafter, a second anion exchange was performed to prepare three different Co4/Mg2Al nanocomposites by dispersing the LDH in water for 20 and 60 min—samples Co4/Mg2Al_1 and Co4/Mg2Al_2, respectively, or in formamide—sample Co4/Mg2Al_3—followed by the dropwise addition of an aqueous solution of Co4-POM.

Characterization of the Co4/Mg2Al nanocomposites confirms the successful intercalation of the POM in the lamellar space of the LDH. The PXRD pattern of pristine Mg2Al-LDH displays the main basal reflections expected for a hydrotalcite-like material, whereby the position of the (003) and (006) peaks depend on the size of the interlayer anion (Figure 1a). The (003) peak, which is attributed to interlayer reflections, appears at a 2θ value of 11.54°. This corresponds to a basal space distance (dBS) of 7.67 Å; considering a layer thickness of 4.8 Å,38 the interlayer space distance is 2.87 Å. In the case of the Co4/Mg2Al nanocomposites, the (003) peak is shifted to lower values leading to an increased basal space of 16.67 Å, which corresponds to a lamellar space distance of 11.87 Å. This value is in good agreement with the diameter of the short axis of this Weakley sandwich-type POM (with dimensions of 1.0–1.5 nm),39 and confirms that the Co4-POM has been intercalated into the interlayer space of the Mg2Al-LDH with the C2h symmetrical axis parallel to the host layers. Moreover, this orientation favors the hydrogen-bonding interactions between the hydroxyl groups of the LDH layer and the terminal oxygen atoms in the POM structure. Comparing the PXRD patterns of the three different Co4/Mg2Al hybrids we can see that the (003) peak of the pristine Mg2Al-LDH is still present for the intercalations performed in water (Co4/Mg2Al_1 and Co4/Mg2Al_2), suggesting only a partial anion exchange in these samples. Moreover, this peak slightly shifted to 11.45°, which corresponds to a gallery height of 7.74 Å and may indicate the presence of carbonate anions adsorbed during the anion exchange reaction. Therefore, these samples alternate layers intercalated with POM with layers intercalated with chloride and carbonate in the nanocomposite. On the contrary, PXRD of Co4/Mg2Al_3 only shows a small shoulder of the (003) peak of the pristine LDH at somewhat lower values (10.7 Å), indicating a higher ability of the formamide to exfoliate the LDH slabs resulting in a more successful anion exchange reaction.

Figure 1.

Figure 1

Open in a new tab

(a) Powder X-ray diffraction patterns of the hybrid Co4/Mg2Al nanocomposites compared to that of the Mg2Al-LDH. (b) TEM and EDX analysis of the hybrid Co4/Mg2Al_3 nanocomposite.

Figure S6 shows the FT-IR spectrum of pristine Co4-POM displaying peaks at 934, 877, 753, and 697 cm–1, which can be assigned to the vibrations of W-Ot, W-Oc-W, and W-Oe-W (t, terminal; c, corner-sharing; e, edge-sharing), respectively. In the case of the Co4/Mg2Al nanocomposites, the W-Ot and W-Oc-W bands appear slightly red-shifted as a result of the electrostatic interaction and hydrogen bonds formed between the POM framework and the LDH layers. Interestingly, those bands associated with the P–O vibrations of the POM heteroatom at ca. 1032 cm–1 do not show any shift after POM intercalation, thus indicating that the interaction of the POM with the LDH occurs purely through the W addenda atoms. The FT-IR spectrum of the pristine Mg2Al-LDH shows absorption bands in the 350–800 cm–1 range assigned to O–M–O vibrations of the hydrotalcite-like layers of the LDH. These bands also appear shifted in the POM-LDH samples, again indicating an interaction between the POM and the LDH structure and overlap with the W-Oe-W vibrations of the POM framework. Finally, the band appearing at about 1360 cm–1 in the Co4/Mg2Al nanocomposites indicates the presence of carbonate anions that may have been incorporated during the POM intercalation step.

The elemental compositions of the Co4/Mg2Al nanocomposites and that of the corresponding isolated components were analyzed by ICP-MS (Table S1). The expected Mg:Al ratio of 2:1 is maintained for all of the samples, although Co4/Mg2Al_3 showed a slightly higher concentration of Al(III) ions. This may be caused by the loss of a minimal amount of Mg(II) ions from the LDH structure when dispersing it in formamide. Nevertheless, this did not affect the morphology of the resulting hybrids. On the other hand, we observed in some samples a slightly higher Co ion concentration than the expected values. We ascribe this to an intrinsic artifact of the measurement, since during the digestion process in acid a small portion of the dissolved W(VI) ions may precipitate to form WO3, thus leading to an increase of the Co:W and P:W ratios. The ICP-MS results allowed us to calculate the amount of Co4-POM that has been intercalated in Mg2Al-LDH employing three different methods. As expected, the quantity of Co4-POM present in each nanocomposite increases as Co4/Mg2Al_1 < Co4/Mg2Al_2 < Co4/Mg2Al_3, with calculated LDH:POM ratios of [Mg0.66Al0.34(OH)2]:[(Co4)0.004], [Mg0.67Al0.33(OH)2]:[(Co4)0.006], and [Mg0.64Al0.36(OH)2]:[(Co4)0.026], respectively.

The morphology of the samples was examined with transmission electron microscopy (TEM) images. Figure 1b shows that the initial hexagonal shape of pristine Mg2Al-LDH is retained after the POM intercalation. Note that we could also find partially broken hexagonal crystals produced during the sonication of the LDH to obtain the swollen phase. Additionally, electron diffraction patterns indicate that the high crystallinity of the LDH is partially lost in the hybrid nanomaterials (Figure S5), as already observed in the PXRD patterns, since restacking of the slabs from the swollen phase does not lead to perfectly aligned layers, showing a pronounced turbostratic disorder. In addition, EDX mapping of the Co4/Mg2Al_3 sample confirms the presence of all of the expected elements and indicates that the Co4-POM is well dispersed across the lamellar space of the LDH, Figure 1b.

The successful intercalation was also confirmed by adsorption–desorption isotherm measurements (carbon dioxide; 273 K), where the hybrid nanomaterials show an increased specific surface area with respect to the LDH alone (Figure S7). The presence of POMs evidences a better separation between layers, promoting CO2 adsorption in the layered material. Moreover, one can clearly see that the hybrids obtained with the formamide method resulted in substantially higher enhancement of the specific surface area compared with that obtained with the water method. Although POM content can be calculated, analyzing its distribution along the layers can be a milestone to understand its application potential: a high %POM can derive in steric impediments, hindering the accessibility to the nanomaterial.

The thermal stability of the samples was unveiled by thermogravimetric differential thermal analysis (TG-DTA) in the 25–700 °C range. Figure S8 shows the TG-DTA of Mg2Al-LDH in air, in which the first weight loss can be ascribed to the loss of water molecules. This occurs in two steps that are assigned to the loss of surface water molecules at 50–100 °C and the loss of interlayer water molecules at 100–150 °C. The material is stable up to 300 °C where it is then dehydroxylated at 300–395 °C and loses the chloride anions at 395–480 °C. Thereafter, the LDH collapses into an amorphous mixed oxide.

TG-DTA of the POM-LDH samples prepared in water is somehow similar to that of Mg2Al-LDH, although we can find some meaningful differences. The release of surface and interlayer water molecules is slightly slower and occurs up to 250 °C. The dehydroxylation of Co4/Mg2Al_1 and Co4/Mg2Al_2 occurs in the 300–385 °C range. Finally, the fourth weight loss associated with the loss of chlorine anions is smaller than in the pristine Mg2Al-LDH since most of them have been replaced by Co4-POM. It is interesting to mention that Co4/Mg2Al_2 has a higher total weight loss than Co4/Mg2Al_1. Looking closely at the TG analysis, we can observe that the main difference in the weight loss compared with Co4/Mg2Al_1 arises from the fourth step. This is contrary to what was expected given that Co4/Mg2Al_2 has a higher amount of POM molecules. However, the elemental analysis indicates that this sample also has a higher amount of chlorine anions that will account for a higher mass loss before the structure collapses. Following our analysis, we can see that for Co4/Mg2Al_3 the loss of surface and interlamellar water molecules behaves similarly to that of the hybrids prepared in water. However, an extra weight loss step can be observed between 205 and 245 °C. The latter can be assigned to the degradation of formamide molecules present in the interlayer space that are trapped during the anion exchange reaction. In addition, the dehydroxylation seems to occur in two different stages, the first one occurring in the 290–335 °C range, which has been previously assigned to Al–OH dehydroxylation, followed by a second step between 335 and 390 °C attributed to simultaneous Mg–OH dehydroxylation and decarbonation.40

Figure 2 shows a comparison of the XPS data of the freshly made materials. Note that only the data corresponding to Co4/Mg2Al_3 is shown. The same Co 2p, W 4f, and P 2p bands can be seen in the hybrid, further supporting the finding that the Co4-POM has been successfully incorporated into the lamellar space of the Mg2Al-LDH. Interestingly, the appearance of a new band in the Al edge of the Co4/Mg2Al_3 nanocomposite suggests that the presence of the Co4-POM may modify the electronic structure of Al in the LDH. In fact, this behavior is not surprising since one would expect a stronger interaction of the Co4-POM with the trivalent Al ions in the LDH since these are the metals that carry the positive charge in the LDH layer. In addition, we observe in the O 1s edge of the Co4/Mg2Al_3 nanocomposite the increase in the intensity of terminal M-O species compared to that of Mg2Al-LDH, which can be assigned to the terminal W=O moieties of the Co4-POM.

Figure 2.

Figure 2

Open in a new tab

XPS data of the as-synthesized Mg2Al-LDH, Co4-POM, and Co4/Mg2Al_3 samples.

3.2. Electrocatalytic OER Evaluation of the Co4/Mg2Al Nanocomposites

The electrocatalytic OER activity of the Co4/Mg2Al nanocomposites was evaluated with the aim of first, studying the effect of the Co4-POM in the LDH compared with the water-insoluble CsCo4-POM salt and Co3O4; second, determining the optimal amount the Co4-POM to maximize the OER activity of the composite among the tested samples; third, to assess the influence of the working media and the nature of the catalytic species in the POM-based hybrid since hydrolysis of Co4-POM may lead to Co(II)aq leached in a solution that forms self-assembled CoOx species under working conditions acting as heterogeneous electrode-bound OER active species.41,42 Hence, we performed the OER experiments using three different electrolytes: (i) sodium phosphate (NaPi) at pH 6.9; (ii) sodium borate (NaBi) at pH 9; and (iii) potassium hydroxide at pH 14.2.

In this regard, linear sweep voltammetry was used to evaluate the OER electrocatalytic activity of the nanocomposites. The polarization curves with and without the iR correction can be seen in Figure S9. Figure S10a shows the results obtained using the NaPi buffer at pH 6.9. As expected, Mg2Al-LDH (catalyst-free, blank) does not display any OER activity. On the contrary, the Co4/Mg2Al nanocomposites exhibit an important OER activity, which increases with the amount of Co4-POM intercalated, whereby only Co4/Mg2Al_3 shows an improved OER activity compared to that of CsCo4-POM. The reason for this small improvement resides in the total amount of Co4-POM present in each electrode. Therefore, considering that all of the samples share the same OER catalytic species, this result is not surprising since there is a larger quantity of POM per gram of sample in CsCo4-POM than in Co4/Mg2Al_3. In order to allow for a fair comparison that permits us to study the intrinsic activity of each sample, we have normalized the obtained current densities per nmol of Co present in each electrode, although only two of the four Co atoms in the POM structure can act as active sites. Figure 3a demonstrates the superior OER performance of the Co4/Mg2Al nanocomposites, outperforming that of CsCo4-POM. Therefore, we can unequivocally say that the efficiency of Co4-POM has been substantially improved in our nanocomposites. Interestingly, we can also observe that the hybrid with the highest Co4-POM loading does not yield the highest level of the OER activity. This feature suggests that high POM loadings in the lamellar space of the LDH may preclude part of the active sites from being reached by water molecules resulting in a decrease of the efficiency. In our case, the Co4/Mg2Al_2 sample leads to the highest OER efficiency. Furthermore, we have also compared the OER activity of the hybrids with that of Co3O4 under the same experimental conditions and considering the number of nanomoles of Co employed. The results clearly demonstrate that Co3O4 cannot attain the OER efficiency displayed by our Co4/Mg2Al nanocomposites considering the amount of Cobalt present in each experiment.

Figure 3.

Figure 3

Open in a new tab

Electrocatalytic OER performance of the hybrid Co4/Mg2Al nanocomposites compared to that of CsCo4-POM and of commercial Co3O4 considering the nmols of Co employed in each experiment performed in (a) sodium phosphate (0.1 M) buffer with NaNO3 (1 M) at pH 6.9; (b) sodium borate (0.1 M) buffer with NaNO3 (1 M) at pH 9; and (c) potassium hydroxide (1 M) at pH 14.2. (d) Calculated Tafel slopes of all of the Co4-POM-containing samples.

The calculated overpotentials displayed by the Co4/Mg2Al nanocomposites decrease when increasing the Co4-POM loadings into the LDH, reaching a minimum value of 548 mV in Co4/Mg2Al_3, which is 19 mV lower than that for CsCo4-POM (see Table S2 and Figure S11). Additionally, analysis of the Tafel behavior displayed by the materials shows rather high Tafel slopes in all cases with values between 194 and 316 mV/dec (Figures 3d and S10a and Table S2). The Tafel slope depends solely on the rate-determining step of the catalytic reaction independently of the quantity of active sites present in the material.43 Hence, these high Tafel slopes indicate that the catalytic OER is limited by a diffusion process and electron transfer.31 It is further striking that CsCo4-POM typically displays a much lower Tafel slope in the range of 70–80 mV/dec as part of a modified carbon paste electrode.18,44 This value contrasts with the Tafel slope of 275 mV/dec obtained for CsCo4-POM in the present case. Hence, this result suggests that the observed high Tafel slope originated, at least in part, by the use of Nafion ink.

Following our study, we then performed the OER analysis using a NaBi buffer at pH 9. Under these working conditions, Co4/Mg2Al_3, Co4/Mg2Al_2, and CsCo4-POM display similar OER activities and superior to that shown by Co4/Mg2Al_1 (Figure S10b). Moreover, Co4/Mg2Al_3 and Co4/Mg2Al_2 show an overpotential of 452 mV, whereas the overpotential for CsCo4-POM is 464 mV (Table S2 and Figure S11). The calculated Tafel slopes are close to 90 mV/dec, which are much closer to the expected values previously reported for Co4-POM and indicate a rate-determining step limited by a competition between a chemical and an electron-transfer steps (Figures 3d and S10b and Table S2). As previously observed, a comparison of the OER activity taking into account the nanomoles of Co employed in each electrode shows the superior OER efficiency of the Co4/Mg2Al nanocomposites compared to that of CsCo4-POM, with the Co4/Mg2Al_2 hybrid showing again the best OER activity (Figure 3b). Additionally, a comparison with the OER activity displayed by Co3O4 highlights the superior OER efficiency of our hybrid nanomaterials.

Finally, we analyzed the OER activity of the Co4/Mg2Al systems under strong alkali conditions by using KOH at pH 14.2. It was previously reported that Co4-POM in solution undergoes hydrolytic decomposition above pH 10 leading to the formation of the corresponding oxide species.39 Nevertheless, Co-POMs can be stabilized by the proper selection of the environment in which they are dispersed to perform the electrocatalytic tests, for instance with the use of carbon paste.11,45 Therefore, given the superior OER efficiency displayed by the nanocomposites compared to CsCo4-POM and Co3O4, it seems reasonable to test our materials under basic conditions and analyze whether the Mg2Al-LDH component is able to protect the Co4-POM from hydrolytic decomposition.

Figure S10c shows a clear change in the OER behavior of the nanocomposites under basic conditions compared to the above-described results since none of them could perform a better catalytic activity than CsCo4-POM. In fact, Co4/Mg2Al_3, and Co4/Mg2Al_2 display an overpotential of 370 mV, which is substantially higher than the value of 322 mV shown by CsCo4-POM (Table S2 and Figure S11). Moreover, we can observe a change in the rate-determining step of the catalytic reaction limited by a chemical step, most likely the O–O bond formation. This is exemplified by the change in the Tafel slope with values between 51 and 56 mV/dec (Figures 3d and S10c and Table S2). Additionally, the comparison of the results when considering the nanomoles of Co employed in each experiment also shows differences in the trends previously observed. In this case, CsCo4-POM shows a better initial OER efficiency than the nanocomposites. However, when the applied overpotential reaches approximately 450 mV, Co4/Mg2Al_1 and Co4/Mg2Al_2 display better performances (Figure 3c). The origin of this change in the OER activity may be originated by a change in the reaction kinetics displayed by the materials that depend on the applied potential, i.e., a change in the Tafel behavior. This causes a decrease of the OER kinetics of CsCo4-POM at high overpotentials that is more pronounced than that observed in the hybrids. Finally, even though the nanocomposites do not improve the OER activity of CsCo4-POM, we can see that the Co3O4 OER efficiency is much lower than those of Co4-POM-containing samples.

Overall, these results show the activity of Co4-POM per nanomole of Cobalt employed in each material, which is somehow related to the mass activity of the catalysts. On the other hand, in order to reflect the intrinsic activity of the catalysts we have also normalized the recorded current intensities by the electrochemical surface area (ECSA) of the Co4/Mg2Al nanocomposites and CsCo4-POM (Figure S12). In this case, the results show that at pH 6.9 and 9 Co4/Mg2Al_3 delivers a higher OER activity than the other tested materials. It is interesting to see that Co4/Mg2Al_1 and Co4/Mg2Al_3 possess a similar estimated ECSA value, even though the amount of Co4-POM in the Co4/Mg2Al_3 nanocomposite is higher. This suggests that not all of the cobalt active sites in Co4/Mg2Al_3 are electrochemically active but could aid in improving the OER performance of neighboring active POMs. This is an interesting effect that would require advanced synchrotron radiation facilities to study with more detail the factors influencing the intrinsic activity of Co4-POM in the confined space of the LDH gallery. Moreover, the improvement in the OER intrinsic activity of Co4/Mg2Al_3 compared to that of CsCo4-POM points to an enhancement of the catalytic capabilities of Co4-POM due to confinement effects. It is striking to see that in strong basic conditions, all of the materials display a substantial increase in the ECSA value. Moreover, the intrinsic OER activity of CsCo4-POM surpasses that displayed by the Co4/Mg2Al nanocomposites. These results suggest that Co4-POM is not the true catalyst in alkali media due to the low hydrolytic stability of the Keggin moiety but can rather be considered as a precatalyst material.

It is important to note that under these strong alkali conditions, we could observe the appearance of a redox event during the preconditioning CV cycles of the Co4/Mg2Al nanocomposites centered at 0.29 V (Figure S13). This redox event can be attributed to the Co(II)/Co(III) redox pair due to Co leaching from the Co4-POM framework, which would indicate that Mg2Al-LDH does not possess the ability to stabilize the Co4-POM. This feature, together with the change in the observed OER activity trend, made us ponder whether the Co4-POM is the true active species or, on the contrary, hydrolytic decomposition of the POM under strong alkali media leads to the formation of the corresponding OER catalytically active CoOx species during electrocatalytic working conditions.

3.3. Stability of the Co4/Mg2Al Nanocomposites

To unravel the pH-dependent stability of the nanocomposites, we have performed a battery of ex situ and in situ tests employing the Co4/Mg2Al_3 hybrid since it contains the highest amount of Co4-POM; thus, it will aid in the evaluation of the fate of the catalyst.

We first analyzed the stability of Co4/Mg2Al_3 by simply immersing the material in the three buffer solutions under stirring for 72 h. Thereafter, the hybrid was recovered, washed, and dried in air. Powder X-ray diffraction patterns of the recovered Co4/Mg2Al_3 show that the structure of Co4/Mg2Al_3 is maintained at pHs 6.9 and 9 (Figure 4a). However, an obvious change occurred at pH 14.2, in which the structure of the original Mg2Al-LDH was recovered with no accountable sign of Co4-POM present in the lamellar space of the LDH. Additionally, the FT-IR spectra seem to confirm this behavior since we could still observe the corresponding bands of Co4-POM at pH 6.9 and 9, whereas they are not present at pH 14.2 (Figure 4b). Interestingly, we could observe an extra broad band around 1080 cm–1 in the recovered Co4/Mg2Al_3 sample at pH 6.9 that is assigned to the intercalation of phosphates in the LDH that have been adsorbed from the NaPi buffer. In fact, this behavior is not surprising given the good phosphate adsorption properties of LDHs.46 The morphology and composition of these samples were analyzed by TEM and EDX analyses (Figure 5). TEM images show that the platelet-like hexagonal crystallites are partially broken and assembled into bigger aggregates, whereas EDX mapping indicates that the Co4-POM is retained in the nanocomposites at 6.9 and 9. However, at pH 14.2 the Co4-POM is clearly unstable, as confirmed by the enormous decrease of W and P content from the hybrids. In other words, at pH 6.9 and 9, the W:Co:P ratios are maintained with respect to the freshly made Co4-POM, whereas the concentration of Co is 1 order of magnitude higher than that of W and P when the sample was immersed in KOH at pH 14.2 (see Table 1), clearly indicating the hydrolytic decomposition of Co4-POM. This hydrolytic decomposition is further supported by the XPS data.

Figure 4.

Figure 4

Open in a new tab

Powder X-ray diffraction patterns (a) and FT-IR spectra (b) of the Co4/Mg2Al_3 nanocomposite collected after being dispersed in the different buffer solutions for 72 h.

Figure 5.

Figure 5

Open in a new tab

TEM images and EDX mapping of the hybrid Co4/Mg2Al_3 nanocomposite collected after being dispersed in (a) NaPi buffer at pH 6.9, (b) NaBi buffer at pH 9, and (c) KOH solution at pH 14.2 for 72 h.

Table 1. Molar Ratio Extracted from the Elemental Analysis (EDX) Performed on the Recovered Co4/Mg2Al_3 Samples after Being in Suspension in the Buffers for 72 h.

    Co4/Mg2Al_3 Co4/Mg2Al_3 Co4/Mg2Al_3
molar ratio expected NaPi, pH 6.9 NaBi, pH 9 NaOH, pH 14.2
Mg:Al 2:1 2.00:1.34 2.00:1.10 2.00:0.93
W:Co:P 18:4:2 18.00:4.19:9.95a 18.00:4.57:1.96 18.00:167.14:11.57
Open in a new tab
a

The high quantity of phosphorus found in this sample arises from the adsorption of phosphates from the NaPi buffer solution by the LDH counterpart.

High-resolution XPS of the component elements clearly shows the substantial loss of W and P atoms from the hybrid, but the Co atoms seem to remain in the lamellar space of Mg2Al-LDH appearing as Co(II) ions (Figure 6). In addition to this, the O 1s XPS spectrum shows a decrease in the deconvoluted peak at 530.69 eV assigned to M=O terminal oxygens of the Co4-POM framework.

Figure 6.

Figure 6

Open in a new tab

XPS data of the hybrid Co4/Mg2Al_3 nanocomposite collected after being dispersed in the different buffer solutions for 72 h.

The ex situ characterization already provides strong evidence that Co4-POM may be stable at close-to-neutral pH values but suffers from hydrolytic decomposition in strong basic conditions. The remaining question is whether they suffer the same fate under the OER working conditions. For this reason, we conducted long-term stability tests by performing chronoamperometry measurements at the three different pH values studied above (Figure S14). For these experiments, we employed fluorine-doped tin oxide (FTO) working electrodes to foster a more facile postcatalytic characterization of the Co4/Mg2Al_3 nanocomposite directly on the surface of the electrode, since the recovery of the catalyst from the Nafion ink is a challenging task and may result in the transformation or complete loss of the OER active material during the process. Unfortunately, these measurements did not deliver stable current densities since the constant evolution of oxygen gas bubbles would damage the deposited ink, resulting in the partial detachment and loss of the material from the surface of the electrode. Nevertheless, we could still perform in situ XPS measurements of the material that remained attached to the surface of the FTO electrodes after 24 h chronoamperometry measurements (Figure 7). In this case, we observe a decrease of the typical satellite peaks of Co(II), which indicates a partial Co oxidation leading to a mixture of Co(II)/(III), especially at pH 9 and 14.2. Again, the loss of W and P atoms at pH 14.2 also occurs under working conditions, and to our surprise, we can also see the partial loss of P atoms at pH 9, as suggested by the decrease in the relative intensities of such elements with respect to the rest of the elements present in the same measured sample. This may indicate a partial hydrolytic decomposition of Co4-POM at pH 9, whereby different Co4-POM-related species may be present in the lamellar space of the Mg2Al-LDH. This feature can also be seen in the XPS spectra of the O 1s in which the deconvoluted peak assigned to the terminal oxygens of the POM framework (W = O) has shifted at pH 9 to lower binding energies with respect to the experiment performed at pH 6.9, whereas it is completely absent at pH 14.2. In addition, we identified further differences with respect to the ex situ XPS measurements regarding the stability of Mg2Al-LDH. Therefore, under OER working conditions at pH 14.2, the high-resolution Al 2p XPS edge shows a decrease in the overall quantity of Al atoms present in the nanocomposite, whereas the O 1s XPS can only be deconvoluted in one broad peak centered at 532.54 eV that can be assigned to Al(OH)x species formed from the Al(III) ions leached from the LDH structure.47 This indicates that even being synthesized in basic media, the stability of the LDH under harsh oxidizing conditions may not be sufficient in the long term.

Figure 7.

Figure 7

Open in a new tab

Postcatalytic XPS data of the recovered Nafion inks containing the hybrid Co4/Mg2Al_3 nanocomposite after 24 h of chronoamperometric measurements. Note that the valence region of the Nafion polymer shows a strong overlap with the W 4f edge (Figure S15). These fitted bands are shown in yellow in the W 4f data plot.

To further understand the stability of Co4-POM in the nanocomposites during turnover conditions at the different pH values explored, we performed ICP-MS analysis of the recovered electrolytes after chronoamperometric measurements and compared the results with those of the fresh electrolytes. In Table 2, we can see a general increase in the concentration of the compositional elements of the nanocomposites after chronoamperometry at pH 6.9. This indicates that the nanocomposite has been detached from the FTO electrode during the measurement, which is reasonable since we could clearly see the loss of the material during the oxygen bubbles generation. However, we cannot completely rule out the possibility of some Co4-POM being lost from the nanocomposite since this is a typical issue found when POMs are electrostatically deposited in a substrate. Interestingly, at pH 9 we observe a pronounced increase in phosphorus concentration in the electrolyte after the chronoamperometry, which is directly aligned with the results obtained with XPS. This feature again indicates that Co4-POM may partially decompose at pH 9 whereby different Co4-POM-related species may coexist at the same time. Further analysis may be of interest, for instance, employing synchrotron-based radiation, to fully understand the decomposition pathway observed under these conditions. Moreover, we can observe a decrease in the Al concentration after the experiments, which could be assigned to a precipitation of Al(III) ions leached from the LDH structure in the form of Al(HO)x and, therefore, not quantified in the ICP-MS measurement. Finally, analyzing the results obtained in alkali conditions, we can see again the leaching of Al(III) ions from the LDH, as suggested by XPS, and the increase in the concentration of P and W ions, indicating a low hydrolytic stability of the Keggin moiety of the POM under strong alkali conditions. Note that the decrease in Co ion concentration from 3.4 in the fresh KOH buffer to 3.1 in the recovered KOH buffer after chronoamperometry may arise from the error in the measurement, which is ±0.2 ppb in each case.

Table 2. Elemental Analysis (ICP-MS) of the Three Electrolytes before and after Chronoamperometric Measurements.

  Mg (ppb) Al (ppb) Co (ppb) P (ppb) W (ppb)
NaPi fresh 31.2 201 0.9 3.1 × 106 38.3
NaPi post-OER 519 300 73 3.1 × 106 3498
NaBi fresh 34 114 0.2 54 84.6
NaBi post-OER 200 94,9 4.6 6151 2034
KOH fresh 1.1 5748 3.4 178 53,9
KOH post-OER 4.1 7648 3.1 901 1154
Open in a new tab

From the above discussion, it is clear that the POM framework in the Co4/Mg2Al nanocomposite is not stable in alkali conditions and that the applied bias may have an effect on the nature of the new species form, as we could see by comparing the XPS data of the ex situ and the in situ materials. The effect of the applied bias on the formation of these novel species can also be seen from the OER electrocatalytic activity displayed by both materials. LSV curves in Figure S16 show that the Co4/Mg2Al_3-derived material formed in situ displays a higher OER activity than the ex situ-formed material with Tafel slopes of 52 and 90 mV/dec, respectively. In addition, since the Co ions leached from the POM counterpart could be immobilized in the Mg2Al-LDH layers, we have compared the OER activity displayed by the Co4/Mg2Al_3-derived material formed in situ at pH 14.2 with that of MgCoAl-LDH. Figure S17 demonstrates that even if some of the leached Co ions have been immobilized in the LDH layer, the observed OER activity arises from the cobalt oxide species formed in the interlayer space of the LDH.

4. Conclusions

In this work, we have employed three different methods for the intercalation of Co4-POM in the lamellar space of Mg2Al-LDH, which resulted in the fabrication of three Co4/Mg2Al nanocomposites with different amounts of Co4-POM. Characterization of the hybrids confirmed the successful intercalation of the POM and allowed us to quantify the amount of Co4-POM present in each sample. Thereafter, we studied the electrocatalytic OER activity of the Co4/Mg2Al nanocomposites employing different buffer solutions from neutral pH to alkali media. We observe that the Co4/Mg2Al_2 sample performs an overall better OER activity than the other samples tested when considering the number of mols of Co4-POM present in each hybrid. Interestingly, Co4/Mg2Al_2 does not contain the highest amount of Co4-POM among the three studied samples, which indicates that higher loadings of the POM do not benefit the OER activity, possibly due to the decrease of channels in the lamellar space of the LDH from which water has to penetrate to reach the POM. Additionally, comparison with the parent, water-insoluble cesium salt of the POM (CsCo4-POM), and with Co3O4 demonstrates the superior OER efficiency of the Co4/Mg2Al nanocomposites per moles of Co atom. This improvement has been attributed to better dispersion of the POM over the surface of a 2D material such as the LDH, thus increasing the number of active sites (i.e., Co–OH2 sites) readily accessible for the water molecules. It is also interesting to mention that using formamide to obtain the swollen phase of the LDH delivers the Co4/Mg2Al_3 nanocomposite with the highest amount of intercalated Co4-POM; still, this sample does not show the highest OER efficiency. As mentioned before, the Co4/Mg2Al_2 nanocomposite, for which water was employed to obtain the swollen phase of the LDH, shows the highest OER efficiency among the samples tested in this work. Therefore, we provide an environmentally friendly method for the fabrication and optimization of POM/LDH nanocomposites with high catalytic efficiencies without the use of organic solvents and easily oxidizable organic ligands, such as the Tris ligand. Interestingly, this method can be extrapolated to a variety of catalytic reactions beyond the energy production area.

In the quest to study the fate of the Co4-POM OER catalyst under working environment that can aid in the understanding of the present POM/LDH nanocomposite, and other systems reported by other groups, we have performed a battery of ex situ and in situ characterization techniques. Our results indicate that Co4-POM is stable in neutral media, but the POM structure is completely lost in strong alkali media. In the latter, the W and P ions that compose the POM framework are almost completely expelled from the lamellar space of the LDH leading to Co(II) ions intercalated in the LDH. Interestingly, these Co ions can be partially oxidized under an applied potential and possibly form a layered mixed-valence Co oxide. This is an intriguing behavior that could open a new avenue for the fabrication of van der Waals heterostructures formed by alternating layers of different metal oxides and with interest to the wider research community. The instability issues shown by the intercalated Co4-POM in strong alkali media are in fact not surprising since it was already demonstrated that this POM suffers from hydrolytic stability at pH values higher than 10. Moreover, we could also identify a decrease of the Al content in the LDH under strong basic conditions that can compromise the stability of the hybrids. Finally, the scenario at pH 9 does not seem to be trivial, and further synchrotron-based characterization techniques are envisioned to accurately assess the observed behavior. In this case, ex situ characterization agrees well with the intercalated Co4-POM being stable; however, in situ XPS data suggests a partial decomposition of the POM, whereby Co4-POM and related species may coexist in the lamellar space of the LDH.

Our results demonstrate that the Keggin framework of the Co4-POM intercalated in the lamellar space of an LDH suffers from hydrolytic stability issues when the pH value of the solution surpasses the pH window of stability of the POM in aqueous solution. This behavior can be extended to other polyoxotungstates and polyoxomolybdates that have been employed for the fabrication of similar hybrid systems toward water splitting. Therefore, we emphasize that a careful study of the stability of the materials under working conditions is of utmost importance to accurately assess the OER activity, to identify the fate of the catalysts, and, if so, to identify the true OER active species.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c04619.

  • Instrumentation and methodology, materials and reagents, and additional physical and electrochemical characterization (PDF)

Author Contributions

J.S.-L. conceived the project and carried out the syntheses, chemical characterization, and electrochemical measurements together with J.Q.-H. A.S.-D.S. assisted in the synthesis and characterization of the LDHs under the supervision of G.A. R.T.-C. analyzed the XPS data. E.A.-G. performed and analyzed the adsorption/desorption measurements. J.S.-L. prepared the manuscript; all authors contributed to discussions throughout the project and the final editing of the manuscript. All authors have given approval to the final version of the manuscript.

J.S.-L. and G.A. acknowledge the funding from Generalitat Valenciana through the Plan Gen-T of Excellence (CDEIGENT/2021/037 and CIDEGENT/2018/001). R.T.-C. thanks the Generalitat Valenciana for his APOSTD Fellowship (CIAPOS/2021/269). E.A.-G thanks Spanish MICINN for a Margarita Salas fellowship (MS21-035). This study forms also part of the Advanced Materials Program and was supported by MICIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana. This work was supported by the EU (ERC AdG Mol-2D 788222 and ERC Proof of Concept Grant 2D4H2 No. 101101079), the Spanish MCIN (PID2022-143297NB-I00, PID2020-117152RB-I00, TED2021-131347B-I00), and Unit of Excellence “Maria de Maeztu” CEX2019-000919-M. A.S.-D.S. thanks the Universidad de Valencia for an “Atracció de talent” predoctoral grant.

The authors declare no competing financial interest.

Notes

A preprint version of this manuscript was uploaded to ChemRxiv.48

Special Issue

Published as part of Inorganic Chemistryspecial issue “Forum on Polyoxometalate and Metal-Oxo Chemistry”.

Supplementary Material

ic4c04619_si_001.pdf (2.2MB, pdf)

References

  1. Galán-Mascarós J. R. Photoelectrochemical solar fuels from carbon dioxide, water and sunlight. Catal. Sci. Technol. 2020, 10, 1967–1974. 10.1039/C9CY02606A. [DOI] [Google Scholar]
  2. Eisenberg R.; Gray H. B.; Crabtree G. W. Addressing the challenge of carbon-free energy. Proc. Natl. Acad. Sci. U.S.A. 2020, 117, 12543–12549. 10.1073/pnas.1821674116. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. van der Spek M.; Banet C.; Bauer C.; Gabrielli P.; Goldthorpe W.; Mazzotti M.; Munkejord S. T.; Røkke N. A.; Shah N.; Sunny N.; Sutter D.; Trusler J. M.; Gazzani M. Perspective on the hydrogen economy as a pathway to reach net-zero CO2 emissions in Europe. Energy Environ. Sci. 2022, 15, 1034–1077. 10.1039/D1EE02118D. [DOI] [Google Scholar]
  4. McKone J. R.; Lewis N. S.; Gray H. B. Will solar-driven water-splitting devices see the light of day?. Chem. Mater. 2014, 26, 407–414. 10.1021/cm4021518. [DOI] [Google Scholar]
  5. Craig M. J.; Coulter G.; Dolan E.; Soriano-López J.; Mates-Torres E.; Schmitt W.; García-Melchor M. Universal scaling relations for the rational design of molecular water oxidation catalysts with near-zero overpotential. Nat. Commun. 2019, 10, 4993 10.1038/s41467-019-12994-w. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. McCrory C. C. L.; Jung S.; Ferrer I. M.; Chatman S. M.; Peters J. C.; Jaramillo T. F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137, 4347–4357. 10.1021/ja510442p. [DOI] [PubMed] [Google Scholar]
  7. Roger I.; Shipman M. A.; Symes M. D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 2017, 1, 0003. 10.1038/s41570-016-0003. [DOI] [Google Scholar]
  8. Gao D.; Trentin I.; Schwiedrzik L.; González L.; Streb C. The reactivity and stability of polyoxometalate water oxidation electrocatalysts. Molecules 2020, 25, 157 10.3390/molecules25010157. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Vickers J. W.; Lv H.; Sumliner J. M.; Zhu G.; Luo Z.; Musaev D. G.; Geletii Y. V.; Hill C. L. Differentiating homogeneous and heterogeneous water oxidation catalysis: confirmation that [Co4(H2O)2(α-PW9O34)2]10- is a molecular water oxidation catalyst. J. Am. Chem. Soc. 2013, 135, 14110–14118. 10.1021/ja4024868. [DOI] [PubMed] [Google Scholar]
  10. Natali M.; Bazzan I.; Goberna-Ferrón S.; Al-Oweini R.; Ibrahim M.; Bassil B. S.; Dau H.; Scandola F.; Galán-Mascarós J. R.; Kortz U.; Sartorel A.; Zaharieva I.; Bonchio M. Photo-assisted water oxidation by high-nuclearity cobalt-oxo cores: Tracing the catalyst fate during oxygen evolution turnover. Green Chem. 2017, 19, 2416–2426. 10.1039/C7GC00052A. [DOI] [Google Scholar]
  11. Blasco-Ahicart M.; Soriano-López J.; Carbó J. J.; Poblet J. M.; Galán-Mascarós J. R. Polyoxometalate electrocatalysts based on earth-abundant metals for efficient water oxidation in acidic media. Nat. Chem. 2018, 10, 24–30. 10.1038/nchem.2874. [DOI] [PubMed] [Google Scholar]
  12. Lv H.; Song J.; Geletii Y. V.; Vickers J. W.; Sumliner J. M.; Musaev D. G.; Kögerler P.; Zhuk P. F.; Bacsa J.; Zhu G.; Hill C. L. An exceptionally fast homogeneous carbon-free cobalt-based water oxidation catalyst. J. Am. Chem. Soc. 2014, 136, 9268–9271. 10.1021/ja5045488. [DOI] [PubMed] [Google Scholar]
  13. Soriano-López J.; Musaev D. G.; Hill C. L.; Galán-Mascarós J. R.; Carbó J. J.; Poblet J. M. tetracobalt-polyoxometalate catalysts for water oxidation: Key mechanistic details. J. Catal. 2017, 350, 56–63. 10.1016/j.jcat.2017.03.018. [DOI] [Google Scholar]
  14. Soriano-López J.; Song F.; Patzke G. R.; Galán-Mascarós J. R. Photoinduced oxygen evolution catalysis promoted by polyoxometalate salts of cationic photosensitizers. Front. Chem. 2018, 6, 302. 10.3389/fchem.2018.00302. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Martin-Sabi M.; Soriano-López J.; Winter R. S.; Chen J.-J.; Vilà-Nadal L.; Long D.-L.; Galán-Mascarós J. R.; Cronin L. Redox tuning the weakley-type polyoxometalate archetype for the oxygen evolution reaction. Nat. Catal. 2018, 1, 208–213. 10.1038/s41929-018-0037-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Haider A.; Bassil B. S.; Soriano-López J.; Qasim H. M.; Sáenz de Pipaón C.; Ibrahim M.; Dutta D.; Koo Y.-S.; Carbó J. J.; Poblet J. M.; Galán-Mascarós J. R.; Kortz U. 9-Cobalt(II)-containing 27-tungsto-3-germanate(iv): Synthesis, structure, computational modeling, and heterogeneous water oxidation catalysis. Inorg. Chem. 2019, 58, 11308–11316. 10.1021/acs.inorgchem.9b01495. [DOI] [PubMed] [Google Scholar]
  17. Tao M.; Yin Q.; Kaledin A. L.; Uhlikova N.; Lu X.; Cheng T.; Chen Y.-S.; Lian T.; Geletii Y. V.; Musaev D. G.; Bacsa J.; Hill C. L. Structurally precise two-transition-metal water oxidation catalysts: Quantifying adjacent 3d metals by synchrotron X-radiation anomalous dispersion scattering. Inorg. Chem. 2022, 61, 6252–6262. 10.1021/acs.inorgchem.2c00446. [DOI] [PubMed] [Google Scholar]
  18. Soriano-López J.; Steuber F. W.; Mulahmetović M.; Besora M.; Clemente-Juan J. M.; O’Doherty M.; Zhu N.-Y.; Hill C. L.; Coronado E.; Poblet J. M.; Schmitt W. Accelerating water oxidation – a Mixed Co/Fe polyoxometalate with improved turnover characteristics. Chem. Sci. 2023, 14, 13722–13733. 10.1039/D3SC04002J. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Cherevan A. S.; Nandan S. P.; Roger I.; Liu R.; Streb C.; Eder D. Polyoxometalates on functional substrates: Concepts, synergies, and future perspectives. Adv. Sci. 2020, 7, 1903511 10.1002/advs.201903511. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Wu J.; Liao L.; Yan W.; Xue Y.; Sun Y.; Yan X.; Chen Y.; Xie Y. Polyoxometalates immobilized in ordered mesoporous carbon nitride as highly efficient water oxidation catalysts. ChemSusChem 2012, 5, 1207–1212. 10.1002/cssc.201100809. [DOI] [PubMed] [Google Scholar]
  21. Mukhopadhyay S.; Debgupta J.; Singh C.; Kar A.; Das S. K. A Keggin polyoxometalate shows water oxidation activity at neutral pH: POM@ZIF-8, an efficient and robust electrocatalyst. Angew. Chem., Int. Ed. 2018, 57, 1918–1923. 10.1002/anie.201711920. [DOI] [PubMed] [Google Scholar]
  22. Li T.; Miras H. N.; Song Y. F. Polyoxometalate (POM)-layered double hydroxides (LDH) composite materials: Design and catalytic applications. Catalysts 2017, 7, 260. 10.3390/catal7090260. [DOI] [Google Scholar]
  23. Stamate A.-E.; Pavel O. D.; Zavoianu R.; Marcu I.-C. Highlights on the catalytic properties of polyoxometalate-intercalated layered double hydroxides: A review. Catalysts 2020, 10, 57. 10.3390/catal10010057. [DOI] [Google Scholar]
  24. Ullah I.; Munir A.; Haider A.; Ullah N.; Hussain I. Supported polyoxometalates as emerging nanohybrid materials for photochemical and photoelectrochemical water splitting. Nanophotonics 2021, 10, 1595–1620. 10.1515/nanoph-2020-0542. [DOI] [Google Scholar]
  25. Proust A.; Matt B.; Villanneau R.; Guillemot G.; Gouzerh P.; Izzet G. Functionalization and post-functionalization: A step towards polyoxometalate-based materials. Chem. Soc. Rev. 2012, 41, 7605. 10.1039/c2cs35119f. [DOI] [PubMed] [Google Scholar]
  26. Liu K.; Yao Z.; Song Y.-F. Polyoxometalates hosted in layered double hydroxides: Highly enhanced catalytic activity and selectivity in sulfoxidation of sulfides. Ind. Eng. Chem. Res. 2015, 54, 9133–9141. 10.1021/acs.iecr.5b02298. [DOI] [Google Scholar]
  27. Liu K.; Xu Y.; Yao Z.; Miras H. N.; Song Y. Polyoxometalate-intercalated layered double hydroxides as efficient and recyclable bifunctional catalysts for cascade reactions. ChemCatChem 2016, 8, 929–937. 10.1002/cctc.201501365. [DOI] [Google Scholar]
  28. Xue X.; Yu F.; Li J.-G.; Bai G.; Yuan H.; Hou J.; Peng B.; Chen L.; Yuen M.-F.; Wang G.; Wang F.; Wang C. Polyoxometalate intercalated NiFe layered double hydroxides for advanced water oxidation. Int. J. Hydrogen Energy 2020, 45, 1802–1809. 10.1016/j.ijhydene.2019.11.038. [DOI] [Google Scholar]
  29. Gumerova N. I.; Rompel A. Polyoxometalates in solution: speciation under spotlight. Chem. Soc. Rev. 2020, 49, 7568–7601. 10.1039/D0CS00392A. [DOI] [PubMed] [Google Scholar]
  30. Yu L.; Liang Q. A fully noble-metal-free electrocatalyst based on a cobalt-polyoxometalate immobilized in a layered double hydroxide for water oxidation at neutral pH. New J. Chem. 2022, 46, 3073–3077. 10.1039/D1NJ05630A. [DOI] [Google Scholar]
  31. Soriano-López J.; Elliott R.; Kathalikkattil A. C.; Ako A. M.; Mulahmetović M.; Venkatesan M.; Schmitt W. Bioinspired water oxidation using a Mn-oxo cluster stabilized by non-innocent organic Tyrosine Y161 and Plastoquinone mimics. ACS Sustainable Chem. Eng. 2020, 8, 13648–13659. 10.1021/acssuschemeng.0c03379. [DOI] [Google Scholar]
  32. Soriano-López J.; Elliott R.; Kathalikkattil A. C.; Ako A. M.; Schmitt W. Tuning the catalytic water oxidation activity through structural modifications of high-nuclearity Mn-oxo clusters [Mn18M] (M = Sr2+, Mn2+). Water 2021, 13, 2042. 10.3390/w13152042. [DOI] [Google Scholar]
  33. Gautam J.; Wang P.; Chishti A. N.; Ma Z.; Liu Y.; Jiang X.; Zha J.; Zhang L.-N.; Diao G.; Wei Y.; Ni L. Heterointerface engineering of hierarchically assembling polyoxometalate on zinc/iron layered double hydroxide nanosheet as a remarkable bifunctional electrocatalyst for overall water splitting. J. Power Sources 2023, 581, 233502 10.1016/j.jpowsour.2023.233502. [DOI] [Google Scholar]
  34. Pettersson L.; Andersson I.; Oehman L. O. Multicomponent polyanions. 39. Speciation in the aqueous hydrogen ion-molybdate(MoO42-)-hydrogenphosphate(HPO42-) system as deduced from a combined Emf-phosphorus-31 NMR study. Inorg. Chem. 1986, 25, 4726–4733. 10.1021/ic00246a028. [DOI] [Google Scholar]
  35. Briand L. E.; Valle G. M.; Thomas H. J. Stability of the phospho-molybdic Dawson-type ion P2Mo18O626– in aqueous media. J. Mater. Chem. 2002, 12, 299–304. 10.1039/b106769a. [DOI] [Google Scholar]
  36. Okamoto K.; Iyi N.; Sasaki T. Factors affecting the crystal size of the MgAl-LDH (layered double hydroxide) prepared by using ammonia-releasing reagents. Appl. Clay Sci. 2007, 37, 23–31. 10.1016/j.clay.2006.10.008. [DOI] [Google Scholar]
  37. Liu Z.; Ma R.; Osada M.; Iyi N.; Ebina Y.; Takada K.; Sasaki T. Synthesis, anion exchange, and delamination of Co–Al layered double hydroxide: Assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies. J. Am. Chem. Soc. 2006, 128, 4872–4880. 10.1021/ja0584471. [DOI] [PubMed] [Google Scholar]
  38. Newman S. P.; Jones W. Synthesis, characterization and applications of layered double hydroxides containing organic guests. New J. Chem. 1998, 22, 105–115. 10.1039/a708319j. [DOI] [Google Scholar]
  39. Goberna-Ferrón S.; Soriano-López J.; Galán-Mascarós J. R.; Nyman M. Solution speciation and stability of cobalt-polyoxometalate water oxidation catalysts by X-ray scattering. Eur. J. Inorg. Chem. 2015, 2015, 2833–2840. 10.1002/ejic.201500404. [DOI] [Google Scholar]
  40. Theiss F. L.; Ayoko G. A.; Frost R. L. Thermogravimetric analysis of selected layered double hydroxides. J. Therm. Anal. Calorim. 2013, 112, 649–657. 10.1007/s10973-012-2584-z. [DOI] [Google Scholar]
  41. Stracke J. J.; Finke R. G. Distinguishing homogeneous from heterogeneous water oxidation catalysis when beginning with polyoxometalates. ACS Catal. 2014, 4, 909–933. 10.1021/cs4011716. [DOI] [Google Scholar]
  42. Folkman S. J.; Soriano-Lopez J.; Galán-Mascarós J. R.; Finke R. G. Electrochemically driven water-oxidation catalysis beginning with six exemplary Cobalt polyoxometalates: Is it molecular, homogeneous catalysis or electrode-bound, heterogeneous CoOx catalysis?. J. Am. Chem. Soc. 2018, 140, 12040–12055. 10.1021/jacs.8b06303. [DOI] [PubMed] [Google Scholar]
  43. Kushner-Lenhoff M. N.; Blakemore J. D.; Schley N. D.; Crabtree R. H.; Brudvig G. W. Effects of aqueous buffers on electrocatalytic water oxidation with an Iridium oxide material electrodeposited in thin layers from an organometallic precursor. Dalton Trans. 2013, 42, 3617. 10.1039/c2dt32326e. [DOI] [PubMed] [Google Scholar]
  44. Azmani K.; Besora M.; Soriano-López J.; Landolsi M.; Teillout A.-L.; de Oliveira P.; Mbomekallé I.-M.; Poblet J. M.; Galán-Mascarós J.-R. Understanding polyoxometalates as water oxidation catalysts through Iron vs. Cobalt reactivity. Chem. Sci. 2021, 12, 8755–8766. 10.1039/D1SC01016F. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Arens J. T.; Blasco-Ahicart M.; Azmani K.; Soriano-López J.; García-Eguizábal A.; Poblet J. M.; Galán-Mascarós J. R. Water oxidation electrocatalysis in acidic media with Co-containing polyoxometalates. J. Catal. 2020, 389, 345–351. 10.1016/j.jcat.2020.06.006. [DOI] [Google Scholar]
  46. Keyikoglu R.; Khataee A.; Yoon Y. Layered double hydroxides for removing and recovering phosphate: Recent advances and future directions. Adv. Colloid Interface Sci. 2022, 300, 102598 10.1016/j.cis.2021.102598. [DOI] [PubMed] [Google Scholar]
  47. Seijas-Da Silva A.; Sanchis-Gual R.; Carrasco J. A.; Oestreicher V.; Abellán G.; Coronado E. Boosting the supercapacitive behavior of CoAl layered double hydroxides via tuning the metal composition and interlayer space. Batteries Supercaps 2020, 3, 499–509. 10.1002/batt.201900223. [DOI] [Google Scholar]
  48. Soriano-López J.; Quirós-Huerta J.; Seijas-Da Silva A.; Torres-Cavanillas R.; Andres-Garcia E.; Abellán G.; Coronado E.. Elucidation of the activity and pH stability limits of polyoxometalate-intercalated layered double hydroxide nanocomposites towards water oxidation catalysis, 2024. https://doi.org/10.26434/chemrxiv-2024-99gt2. [DOI] [PMC free article] [PubMed]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ic4c04619_si_001.pdf (2.2MB, pdf)

Articles from Inorganic Chemistry are provided here courtesy of American Chemical Society

RESOURCES