The Second Life of Cobalt MOF: Alternating Magnetic Field‐ Assisted Electrocatalytic Oxygen Evolution Reaction in MOF‐derived Nanoparticles

Abstract

A major challenge in hydrogen production from water electrolysis is the slow kinetics of oxygen evolution (OER). Applying an alternating magnetic field (AMF) to ferromagnetic metal nanoparticles on electrodes has gained attention due to the generation of a thermally activated electrocatalyst, which can boost OER performance. This work studies the influence of external parameters and intrinsic characteristics of carbon‐encapsulated cobalt MOF‐derived nanoparticles deposited onto graphite paper electrodes on the electrocatalytic AMF‐OER coupled process. Specifically, the impact of AMF strength, the electrolyte composition (concentration and cation nature) and cobalt content on the electrocatalytic AMF‐OER performance are thoroughly investigated. Results reveal that AMF significantly boosts OER activity of Co@C‐based electrodes, their enhancement being strongly dependent on the electrolyte composition. Furthermore, both the heating capacity of the herein synthesized catalyst for magnetic hyperthermia and their structural features remain intact after an intense and prolonged electrocatalytic AMF‐OER experiment. No signs of sintering, leaching, or particle size increase, which are typical issues observed when metal nanoparticles are subjected to an intense external magnetic field, have been found. This underscores the high operational stability of this catalyst. These findings provide new insights into thermal AMF‐assisted alkaline water oxidation for developing high‐performance catalysts for enhanced electrocatalysis.

This study investigates the effect of an alternating magnetic field (AMF) on carbon‐supported cobalt nanoparticles for enhanced electrocatalytic performance in the oxygen evolution reaction (OER). Key factors such as AMF strength, electrolyte composition and concentration, and cobalt content are analyzed, revealing significant efficiency improvements. The catalyst demonstrated high stability, maintaining its structure and activity without degradation.

1. Introduction

The use of hydrogen as an alternative green fuel has been proposed in recent years to produce sustainable green energy and reduce fossil fuel consumption.^{[ 1} , ² , ^{3 ]} Electrocatalytic water splitting has turned into the most relevant alternative to generate this molecule, since the required electricity for the process can be derived from renewable energy sources, such as wind and solar technologies.^{[ 4} , ^{5 ]} The main problem of breaking the water molecule lies in the thermodynamically impairing and slowness of the anodic oxygen evolution half‐reaction (OER).^{[ 6} , ^{7 ]} This drawback requires the use of very efficient and stable noble metal‐based electrocatalysts (Ru/Ir‐base materials), making the large‐scale industrial hydrogen production unfeasible due to their high price and scarcity.^{[ 8 ]} Consequently, studying new catalysts based on low‐cost transition metals for OER has been a hot topic lately.

In order to find new catalysts, 3d‐transition metal oxides (TMOs) have taken center stage because of their cost‐effectiveness.^{[ 9} , ¹⁰ , ^{11 ]} Fe‐, Co‐ and Ni‐based oxides are strong enough to activate reactants and not too weak to easily desorb products, according to Sabatier's principle.^{[ 12 ]} Furthermore, their combinations have shown remarkable enhancements in the electrocatalytic performance of water‐splitting reactions.^{[ 13} , ¹⁴ , ^{15 ]} In addition to optimizing the electrocatalyst materials, magnetic field‐enhanced electrocatalysis has recently emerged as an intensification strategy for boosting the overall electrocatalytic performance. In this sense, the application of an alternating magnetic field (AMF) has been discovered as a new tactic to tailor electrocatalytic properties, mainly because the magnetic‐induced heating saves energy consumption, reducing the OER overpotential, and improving the efficiency of the reaction by accelerating the reaction kinetics.^{[ 16} , ¹⁷ , ^{18 ]} In this regard, this magnetic heating effect only affects the surroundings of the magnetic particles, heating reactions locally and avoiding the electrolytic cell corrosion when water splitting takes place at high temperatures.^{[ 19 ]} Beyond electrocatalysis, the use of magnetic fields in electrochemical systems has demonstrated promising results across diverse fields, highlighting the versatility of this approach. For instance, in the biomedical area, magnetic field‐assisted drug delivery has enabled the controlled release of anticancer agents, facilitating the development of efficient and non‐toxic therapeutic strategies.^{[ 20} , ^{21 ]} In chemical engineering, rapid cold‐start processes for ammonia decomposition, critical for ammonia‐powered vehicles, have been enhanced using magnetic heating.^{[ 22 ]} Similarly, in the food industry, AMF freezing systems have been developed to mitigate flavor degradation in frozen cooked rice.^{[ 23 ]} These applications underline the crucial role of ferromagnetic materials, with iron being the most commonly used due to its low cost, chemical stability, and favorable magnetic properties.^{[ 24} , ^{25 ]} Nonetheless, cobalt‐based ferromagnetic materials have shown superior performance in oxygen evolution reaction (OER) electrocatalysis across various media, particularly under alkaline conditions.^{[ 26 ]} Actually, it has been demonstrated that the real active species are the in situ formed Co(oxy)hydroxides, which generate the active oxygen ligand through their di‐µ‐oxo Co‐Co bridges, improving the OER kinetics.^{[ 27 ]} Recently, it has been reported that the application of an constant external magnetic field to a ferromagnetic Co(oxy)hydroxide catalysts can improve their OER performance due to the generation of parallel spin arrangement of electrocatalysts conducting paramagnetic triplet oxygen molecules, that reduces the kinetic barrier of the oxygen‐oxygen bond of the metal oxo intermediate.^{[ 28} , ²⁹ , ^{30 ]} In this sense, the reconstruction of ferromagnetic cobalt oxides to realize the spinning pinning effect in its corresponding oxyhydroxide has been revealed to be the key step of the process. Therefore, considerable efforts are focused on the development of profitable strategies for controllable surface reconstruction of oxides.^{[ 31 ]} Nevertheless, Co(oxy)hydroxides do not usually show ferromagnetic ordering and this requires the application of high magnetic fields to align their spins in a hostile environment, such as the water electrolysis systems.^{[ 32 ]} A recent work led by Gao highlights the effects of the distinct magnetic properties of Co, CoO, and Co₃O₄ electrocatalysts on their catalytic behavior for the OER under an AMF.^{[ 33 ]} This study concludes that the spin in the reconstructed paramagnetic layer formed on the surface of ferromagnetic Co is strongly pinned by the ferromagnetic matrix and that ferromagnetic Co exhibits a pronounced magnetic heating effect under an AMF. This is evidenced by the ferromagnetic Co electrocatalyst, which demonstrates the most significant magnetic field promotion effect, achieving the highest magnetic current density of 15.6 mA cm⁻² compared to the other two electrocatalysts. Additionally, the Co electrocatalyst required a substantially lower overpotential of 243 mV to develop a current of 10 mA cm⁻² when an AMF of 4.32 mT is applied, compared to that required in the absence of AMF. Furthermore, the magnetic field promotes the in situ reconstruction, facilitating the formation of highly catalytically active species on the surface of electrocatalysts, as described for the ferromagnetic/paramagnetic CoFe₂O₄@CoBDC core‐shell structure used in the electrochemical OER.^{[ 34 ]} However, this reconstruction process also introduces challenges, including undesirable phenomena such as sintering, leaching and catalyst deactivation.^{[ 35} , ^{36 ]} The most important challenge in AMF‐assisted catalysis is the development of highly robust and stable catalysts able to maintain the structural integrity of both the heating agent and the electrocatalytic active sites. This is crucial, as AMF‐induced sintering and structural reorganization resulting from the elevated temperatures can compromise the catalytic performance.^{[ 37 ]} To the best of our knowledge, prior studies on electrocatalysis under AMF conditions have largely neglected post‐performance structural analyses of the electrocatalysts. Moreover, the role of the electrolyte solution composition, including both its concentration and the nature of its alkali cation and, on the AMF‐OER coupled process is unexplored.

In this context, and building upon previous cited studies, cobalt‐based nanoparticles derived from well‐defined precursors, when supported on conductive carbon and subjected to an AMF, should exhibit enhanced OER activity. In the AMF‐OER coupled reaction, various factors may influence the catalytic performance by modulating localized magnetic heating and spin alignment effects during catalysis. Based on this hypothesis, we aim to investigate how parameters such as cobalt loading, AMF strength, and electrolyte composition impact the overall electrocatalytic behavior. These insights are expected to provide a rational foundation for designing robust and efficient AMF‐assisted electrocatalysts. With this purpose, we report the novel synthesis of a family of 2D‐CoMOFs/C materials, having different MOF content supported on carbon, as precursors for the development of cobalt‐encapsulated nanoparticles supported on carbon through rapid pyrolysis. This synthetic strategy is particularly effective for the creation of highly robust catalysts that resist leaching in demanding reactions, such as hydroformylation, amine formation, and/or sintering during the reverse water‐gas shift (RWGS) reaction under an AMF at temperatures exceeding 600 °C.^{[ 38} , ³⁹ , ^{40 ]} The metal loading of Co@C catalysts is controlled during the synthesis of 2D‐CoMOFs@C by adjusting the ratio of carbon to MOF precursor during the growth process. Furthermore, we developed graphite paper electrodes modified with the MOF‐derived cobalt nanoparticles supported on carbon (Co@C) to assess the influence of external parameters and intrinsic characteristics of the catalyst on the electrocatalytic AMF‐OER coupled reaction. Specifically, the impact of AMF strength, electrolyte composition and cobalt metal content in the catalyst on the electrocatalytic AMF‐OER performance has been thoroughly investigated. Finally, the operational stability and robustness of the electroactive materials are evaluated, demonstrating their suitability for practical applications.

2. Results and Discussion

2.1. Synthesis and Characterization of MOF‐Derived Cobalt Catalyst

Cobalt nanoparticles encapsulated in carbon (Co@C) were synthesized following a two‐step procedure from a well‐defined 2D‐CoMOF ^{[ 41 ]} precursor supported on carbon (2D‐CoMOF/C). Briefly, the corresponding quantities of reagents were added to a stainless‐steel autoclave, which was sealed for 9 days at 150 °C in solvothermal, dynamic conditions. Then, the resulting 2D‐CoMOF/C material was filtered, dried and pyrolyzed (800°C, 2 h, 25 °C min⁻¹, 20 mL min⁻¹ N₂), leading to uniform cobalt nanoparticles supported on carbon (Co@C) The metal content of this material was determined by X‐ray fluorescence (XRF) to be 13.3 wt% Co (13‐Co@C) (Scheme 1 ).

Scheme 1.

Synthetic procedure of the Co@C catalysts.

Power X‐ray diffraction patterns (PXRD) (Figure 1a) show the characteristic peaks of 2D‐CoMOF in the carbon‐supported MOF material (2D‐CoMOF/C), highlighting the correct formation of the MOF in the presence of carbon. However, after the pyrolysis process, these characteristic peaks of the pristine MOF (from 8° to 40°) disappear. Instead, noticeable graphitic carbon signals at 22° ^{[ 42 ]} and four peaks corresponding to face‐centered cubic (fcc) Co⁰ nanoparticles can be observed at 44°, 51°, 76° and 92° (JCPDS: 00‐015‐0806).^{[ 43 ]} In agreement with the PXRD results, the XANES spectra (Figure 1b) reveal that the local environment of Co remains unchanged after supporting the precursor on carbon, consistently maintaining its oxidation state at +2, as it can be noticed by comparing the spectra with that of the Co(Ac)₂ reference. After the pyrolysis process, the cobalt in the material is almost entirely reduced to metallic Co⁰, as indicated by a spectrum closely resembling that of the Co foil reference. However, the slightly higher rising edge intensity observed in the pyrolyzed sample compared to the foil suggests partial oxidation of the Co nanoparticles. Raman spectra (Figure 1c) also show the disappearance of the bands associated with the ligands of the organometallic framework from the initial to the final material. In the 2D‐CoMOF, the Raman bands associated with the axially coordinated pyridine ligands (768, 1010, and 1283 cm⁻¹), the bipyridine (774, 1022, and 1278 cm⁻¹) and the carboxylic groups (1289, 1426, 1546, and 1615 cm⁻¹) are visible. In the case of carbon‐supported material, 2D‐CoMOF/C, the intensity of the peaks decreases due to the presence of this support, which contains overlapping bands with those of the catalyst. The maintenance of bands associated to the pyridine (768 cm⁻¹) and bipyridine (1010 cm⁻¹) can be observed for this precursor. It is noticeable that the ones related to the carboxylate groups appear less intense due to the Raman bands associated with the Vulcan carbon. These can be clearly seen in the final nanoparticulate 13‐Co@C material (1340 and 1590 cm⁻¹),^{[ 44 ]} where none of the ligand bands can be discerned.

Figure 1.

a) Powder X‐ray diffraction patterns of 2D‐CoMOF, 2D‐CoMOF/C and 13‐Co@C; b) normalized XANES absorption spectra at the Co K‐edge for 2D‐CoMOF, 2D‐CoMOF/C and 13‐Co@C (for comparative purposes, references such as Co° foil and Co(OAc)₂ have also been included); c) Raman spectra of 2D‐CoMOF, 2D‐CoMOF/C and 13‐Co@C; d) FESEM image of 2D‐CoMOF/C; e) STEM image and NPs size histogram 13‐Co@C; f) HRTEM micrographs of 13‐Co@C. Lattice spacings are highlighted in yellow; g) Magnetization hysteresis loop of 13‐Co@C at RT; h) Temperature profile at 282 kHz and 20 mT of 13‐Co@C (20 mg mL⁻¹) and KOH 0.1 M.

Field emission scanning electron microscopy (FESEM) of the supported MOF shows the presence of the typical hexagonal crystals of the 2D‐CoMOF interacting with the Vulcan carbon on the surface (Figure 1d). The material after the thermal treatment was studied through Scanning Transmission Electron Microscopy (STEM). STEM images show the formation of spherical and well‐distributed Co nanoparticles onto the carbon support. The average particle size is 13 ± 3 nm for the 13‐Co@C material (Figure 1e). In addition, close‐up high‐resolution TEM (HRTEM) images of one single nanocrystal show lattice fringes with d spacings of 2.04 Å, which could be ascribed to (111) plane of Co fcc (Figure 1f) in good agreement with the X‐ray diffraction data.^{[ 43 ]}

Figure 1g illustrates the magnetic properties of 13‐Co@C with the resulting hysteresis loops, while Table S1, Supporting Information includes the saturation magnetization (M_S) and coercive field (H_C) values. The magnetic properties of this catalyst reveal a typical ferromagnetic behavior, with a M_S of 26 Am² kg⁻¹, a remnant magnetization (M_R) of 7.3 Am² kg⁻¹, yielding an M_R/M_S ratio of ≈0.28 and a coercivity value of 25 kA m⁻¹. By comparing the M_S value of the sample to that of bulk cobalt (162 Am² kg⁻¹),^{[ 45 ]} it is possible to estimate the cobalt loading. In this regard, a value of 16% was calculated, which is similar to or even slightly higher than the Co content calculated by XRF, indicating that the degree of oxidation is relatively low. The low M_R/M_S ratio in comparison to the theoretical value of 0.5 for randomly oriented single‐domain particles described by the Stoner–Wohlfarth model suggests that, while the material retains a measurable portion of its magnetization after the external field is removed, there is a fraction of superparamagnetic nanoparticles (<5 nm) that likely contributes to lowering the M_R. The measured H_C of 25 kA m⁻¹ is consistent with non‐interacting cobalt nanoparticles of ≈10 nm in diameter evenly dispersed in a matrix^{[ 46 ]} and larger than H_c of bulk cobalt (a few tens of oersteds) probably due to surface effects.^{[ 47 ]} The heating performance of the 13‐Co@C was evaluated in a KOH 0.1 M solution under an AMF with a frequency of 282 kHz and an amplitude of 20 mT (Figure 1h). At a concentration of 20 mg mL⁻¹, the sample exhibited a temperature increase of ≈5.32 °C, corresponding to an initial heating rate of 0.0153 °C s⁻¹ during the first 30 s of AMF exposure. This resulted in a calculated specific absorption rate (SAR) of 3.20 W g⁻¹. Comparing the results with bibliography data, we found SAR values under similar magnetic field conditions close to those obtained for Co@C nanocomposites^{[ 48 ]} containing nanoparticles with a diameter ranging from 1 to 25 nm prepared by combustion chemical vapor deposition (CCVD). It should be mentioned that it is not straightforward to compare the heating efficacy for a material because of the field dependence of the SAR ∼ H² and the particle size effect, being maximum for particles with crystallite diameters ≈12–14 nm, and about one order of magnitude lower for 8 nm in diameter particles.^{[ 49 ]} In fact, although the bulk temperature of the medium does not reach significantly high values, it is essential to consider that the surface of the 13‐Co@C particles may experience localized heating, creating “hot spots.” These hot spots can significantly elevate the temperature at the particle‐liquid interface, enhancing catalytic activity or other thermal processes. It should be mentioned that blank experiments using only KOH 0.1 M under the same AMF conditions were also performed, showing a temperature increase of 1.22 °C. This rise in temperature is attributed to Joule heating within the conductive KOH medium slightly contributes a secondary heating effect, as the induced eddy currents mobilize dissolved ions and generate further heat.^{[ 50 ]}

2.2. Electrocatalytic AMF‐OER Coupled Process in Co@C‐based Electrodes

Thermally activated electrocatalysts can significantly increase the performance of electrolysis. Bearing in mind that the heating capacity of nanoparticles for magnetic hyperthermia depends on external parameters, such as the frequency and the intensity of the applied magnetic field, as well as on the intrinsic characteristics of the nanoparticles (size, shape, material, agglomeration state and even on properties of the dispersion medium),^{[ 51 ]} we tested the magnetic field‐assisted thermocatalytic OER performance of the herein constructed electrodes by using materials with different metal content in different electrolyte solutions. First, we focused on the effect of the AMF strength on the electrocatalytic OER activity at KOH 0.1 M. For this purpose, we deposited a composite of 13‐Co@C with Nafion onto a graphite paper electrode, whose metallic connection to the external circuit was located far away from the action of the magnetic field (Figure S1, Supporting Information). Figure S3, Supporting Information shows the voltammetric response of the so‐modified electrode, which displays the typical voltammetric features of α‐cobalt (II) hydroxide.^{[ 52 ]} It is characterized by a pair of voltammetric waves located at ≈0.30 V (in the anodic scan) and 0.17 V (in the cathodic scan) vs. Ag/AgCl/NaCl sat. associated with the Co(II)/Co(III) redox conversion. The concomitant increase of the anodic current for potentials higher than 0.55 V is attributed to the electrocatalytic OER.

Influence of the Magnetic Field Strength‐ The effect of the AMF strength on the electrocatalytic OER performance is revealed by the linear scan voltammograms (LSV) recorded under different magnetic field strengths applied from 1.52 V vs. RHE, which corresponds to the OER onset potential (Figure 2a). A significant improvement in the OER performance is clearly noticeable when AMF is applied. It results in a decrease of the electrocatalytic overpotential at 10 mA by 40, 60, 83, and 100 mV for 28.8, 40.3, 49.1, and 57.1 mT, respectively. This significant enhancement of the electrocatalytic OER (up to 14% reduction in the overpotential) is attributed to the magnetic hyperthermia heating related to Néel relaxation and generated from the spin polarization flip under AMF.^{[ 53} , ^{54 ]} In addition to this magnetothermal effect, other effects should be considered to contribute to a lesser extent to the OER activity improvement such as the magnetohydrodynamic effect, which improves diffusion, the magnetoresistance effect, which modifies the electrical conductivity of the catalysts, and the spin selectivity effect that reduces the kinetic barrier for the breaking of the oxygen−oxygen bond.^{[ 18} , ³¹ , ⁵⁴ , ⁵⁵ , ⁵⁶ , ^{57 ]}

Figure 2.

The effect of the magnetic field intensity on the electrocatalytic AMF‐OER coupled process in a graphite paper electrode coated with 13‐Co@C catalyst. a) Linear sweep voltammograms recorded at 5 mV s⁻¹ under variable AMF intensities; b) Thermally activated electrocatalytic current by applying an external AMF at variable field intensities extracted from (a); c) Baseline subtracted chronoamperograms registered at 1.80 V for successive magneto‐pulses at the indicated AMF intensities applied. The pulse time was 10 s; d) Increase of the OER electrocatalytic current (black points) and the temperature (bars) as a function of the field strength of the external AMF extracted from (c); e) Background subtracted chronoamperograms registered at 1.80 V of the consecutive magneto‐pulses with increasing field intensity. The pulse time was 10 s employing 13‐Co@C catalyst (aquamarine) and carbon paper without catalyst (black); f) A percentage current increase of the OER electrocatalytic current as a function of the applied external AMF extracted from (e). Other experimental conditions: aqueous KOH 0.1 M solution at room temperature.

The thermally activated electrocatalytic current obtained by subtracting the polarization curves acquired at variable AMF from that obtained in the absence of a magnetic field is depicted in Figure 2b. Interestingly, the instantaneous current increase upon switching on the AMF is consistent with the pronounced local heating of the 13‐Co@C catalyst.^{[ 58 ]} Then, sweeping to higher potential while applying a constant AMF reveals a gradual increase of the thermocatalytic current, being steeper as the field intensity rises. The latter is in good agreement with the strong dependence of the heating power of the metallic NPs on external parameters, such as the frequency and intensity of the applied magnetic field.^{[ 51 ]} Besides, we note a global temperature increase in the cell solution during the voltammogram acquisition, which is higher as the magnetic field strength increases. Specifically, the solution temperature increase ΔT was 1, 3, 5, and 6 °C for an AMF strength of 28.8, 40.3, 49.1 and 57.1 mT, respectively (Figure 2b). This effect may be attributed to the dispersion medium owing to the heat‐generating capability of dissolved ions (see above).^{[ 59 ]} However, a negligible temperature increase of 1°C was measured when the highest AMF intensity of 57.1 mT was applied to the KOH 0.1 M solution for 1 min in the same experimental set‐up, but without the composite deposited on the working electrode. Thus, the observed temperature changes in the solution mainly arise from the heating induced by the localized hyperthermia of the cobalt NPs. Figure S4, Supporting Information shows a comparison of the polarization curves obtained at different temperatures and at various AMF strengths. As expected, OER performance improves as the bulk electrolyte temperature increases. However, the effect of the bulk temperature on electrocatalytic improvement appears to be less significant than the effect of the localized thermal impact caused by applying an AMF. Indeed, similar OER polarization curves are achieved by increasing the bulk temperature to 50 °C (which implies a ΔT of 30 °C) or by applying an AMF of 40.3 mT (which implies a ΔT of 5 °C). Therefore, the enhancement of electrocatalytic OER when an AMF is applied can mostly be attributed to localized magnetic heating in the vicinity of the catalytically active sites. Furthermore, the influence of the bulk temperature on the Co(II)/Co(III) redox wave has also been assessed (Figure S5, Supporting Information), showing a quasi‐reversible behavior in the entire range of temperature tested, whose peak potential separation decreases as temperature increases. In addition, a linear increase of the midpoint potential E_1/2 with temperature was observed.

For more direct evidence of the thermally activated electrocatalytic OER by AMF, we performed a chronoamperometric study by recording i–t curves at a fixed potential with on‐off switching of the magnetic field source (hereafter referred to as magneto‐pulse). First, we recorded the chronoamperometric response at an OER potential of 1.80 V for the 13‐Co@C‐based electrode after being subjected to four successive magneto‐pulses with an AMF switched on‐off time of 10 s. Before switching on the AMF, the electrode was kept at a fixed potential up to a stable baseline current was reached. For comparative purposes, the chronoamperometric current intensity obtained with different AMF intensities was subtracted from the initial baseline electrocatalytic current intensity (Figure 2c). Instantaneous enhancement of the electrocatalytic OER current is evidenced upon switching on the AMF, whose extent strongly depends on the field intensity, with a current increase (Δi) of 21, 27, 34, 45% for an AMF strength of 28.8, 40.3, 49.1, and 57.1 mT, respectively. Subsequent switching off the AMF results in a decrease in the current back to a value higher than the initial baseline, and this effect increases progressively with the number and field strength of the magneto‐pulses (Figure 2c). The increase of this basal intensity after the four magneto‐pulses can be attributed to the collateral increase of the electrolyte solution temperature driven by the Co‐NPs of the electromagnetic materials, as illustrated in Figure 2d. Note that the OER current enhancement is reproducible for the four consecutive magneto‐pulses and the ΔT values after the four pulses were 1, 2, 3 and 4 °C for an AMF strength of 28.8, 40.3, 49.1, and 57.1 mT, respectively. Moreover, the gradual change of the AMF‐assisted current, lasting over several seconds for both the current increase upon field application and the subsequent decrease when the AMF was off (Figure S6, Supporting Information), suggests heating as the primary source of the magneto‐induced enhancement.^{[ 60} , ^{61 ]} Indeed, the exponential relationship observed between the AMF‐assisted electrocatalytic current increments and the field strength (Figure 2d) clearly supports^{[ 62 ]} that the catalytic enhancement is primarily due to thermal effects on the electroactive surface.

Additional detailed testing of the effect of AMF field intensity on catalytic current enhancement performed by recording consecutive magneto‐pulses of variable AMF within the range 20.6–63.3 mT (Figure 2e,f), reveals an exponential dependence of Δi on AMF strength in the 0–50 mT interval, after which it reaches a constant value of ~ 40%. This limiting Δi value is consistent with a saturation of the magnetization of 13‐Co@C, which ultimately is related to their maximum heating power for hyperthermia. In addition, this finding indicates that the electrocatalytic OER current enhancement arises from a pronounced local heating effect by the 13‐Co@C modified electrode rather than by a bulk heating effect in the electrolyte solution (Figure 2e). As a test of consistency, we quantified the effect of bulk heating on Δi by acquiring a chronoamperometric pulse at 1.80 V while continuously and progressively increasing the temperature from 20 to 50°C (see Figure S7, Supporting Information). Similar increases in the OER electrocatalytic current (Δi) values were achieved with a bulk temperature increment of ΔT = 20 °C (that corresponds to an electrolyte temperature of 40°C) or with the application of an AMF of 40.3 mT, which results in a very low bulk temperature increment of ΔT = 2°C (Figure S7a, Supporting Information). Based in these results, we can conclude that the contribution of the bulk electrolyte thermal effect to the magnetoelectrocatalytic current is negligible; therefore, it is primarily driven by the localized thermal effect likely induced by “hot spots” on the electrode surface.

While direct experimental evidence of such “hot spots” remains scarce, multiple studies support their existence through indirect measurements and modelling. For example, Niether et al. showed that FeC–Ni nanoparticles under a 300 kHz, 48 mT field reduced the HER overpotential by 100 mV, an effect equivalent to a 200 °C local increase, even though the measured bulk temperature only rose by 5 °C.^{[ 58 ]} Similarly, Zhan et al. demonstrated a ≈146% HER enhancement with Pt nanoparticles supported on Fe₃O₄/C under AMF, where COMSOL simulations confirmed the formation of localized thermal fields extending hundreds of nanometers.^{[ 63 ]} Furthermore, Ovejero et al. directly measured surface‐localized temperatures in iron oxide nanoparticles using fluorescent thermoprobes, confirming temperature spikes at the nanoparticle surface far beyond the bulk medium.^{[ 64 ]} These studies collectively suggest that magnetic nanoparticles can act as nanoscale heaters, creating transient high‐temperature microenvironments that accelerate electrochemical processes without significantly affecting the bulk electrolyte. This effect is likely operative in our Co/C catalyst, wherein the ferromagnetic cobalt nanoparticles absorb AMF energy and induce catalytic “hot spots” at their surface. To complement this hypothesis, macroscopic thermal imaging was conducted (Figure S2, Supporting Information). The results clearly show that the area containing the electrode exhibits a significantly higher temperature increase under AMF exposure compared to the surroundings. Although this method cannot resolve nanometric gradients, it provides indirect evidence that heat is concentrated in specific regions of the material. These findings are illustrated in Figure S2, Supporting Information, where thermal images before and after AMF activation reveal strong local heating at the sample location, and the corresponding time‐dependent temperature curve shows a rapid and sustained increase (ΔT ≈ 55 °C in 30 s for an electrode with 13 wt% Co). Taken together, these data support the presence of magnetic heating‐driven hot spots that enhance catalytic performance in our system. In addition, comparing the energy consumption of magnetic induction heating with conventional heating reveals that the most relevant difference lies in heating times (Figure S7, Supporting Information). For instance, to achieve an Δi of ≈25%, the energy consumed by localized magnetic induction heating is ≈180 times more energy‐efficient than conventional bulk heating during the heating ramp (more details ESI).

Influence of the Electrolyte Composition‐ On the other hand, it is well recognized that the composition of the electrolyte solution has a strong influence on the electrocatalytic OER activity.^{[ 65} , ^{66 ]} However, the electrolyte effect on the AMF‐assisted electrocatalytic reactions has been poorly addressed. Accordingly, we assessed the electrolyte composition effect on the electrocatalytic performance of our 13‐Co@C‐based electrode. The cyclic voltammograms obtained in the absence of AMF for different KOH concentrations (Figure 3a) reveal a similar pattern, with a pronounced voltammetric wave associated with the Co(II)/Co(III) redox conversion. Increasing the solution pH by raising the OH⁻ concentration resulted in the anodic peak potential of Co(II)/Co(III) shifting toward more negative potentials with a slope of ≈−60 mV pH⁻¹, which is typical of a redox conversion involving PCET with the same number of exchanged protons and electrons. Additionally, the voltammetric branch associated with the electrocatalytic oxidation of water shifts in a similar manner to the cobalt voltammetric wave with the solution pH, suggesting that cobalt‐mediated electrocatalysis of OER occurs via a series of PCET steps. This is consistent with the literature, which reports that OER Co‐based electrocatalysts often operate through redox cycles involving multiple redox conversions coupled with proton transfers. In these cycles, PCET is crucial for the efficiency and stability of the catalysts.^{[ 67 ]}

Figure 3.

The effect of the KOH concentration on the electrocatalytic AMF‐OER coupled process in a graphite paper electrode coated with 13‐Co@C catalyst. a) Cyclic voltammograms recorded at 50 mV s⁻¹; b–e) OER and OER‐AMF of 40.3 mT linear sweep voltammograms recorded at 5 mV s⁻¹; f) Thermally activated electrocatalytic current by applying external AMF of 40.3 mT extracted from (b–e).

Their corresponding OER polarization curves obtained in the absence of AMF show a significant improvement of the electrocatalytic OER kinetics with increasing KOH concentration, evidenced by the diminution of the Tafel slopes (Figures S8 and S9, Supporting Information). It has been reported that the OER process mediated by cobalt‐based catalysts is strongly pH‐dependent, where the oxidative deprotonation process to generate the catalytic species for the OER is favored in more alkaline media.^{[ 68} , ⁶⁹ , ^{70 ]} Notably, the AMF‐activated electrocatalytic current obtained from the polarization curves at 40.3 mT (Figure 3b–f) is enhanced as the pH increases. Interestingly, the bulk temperature increases after the LSV acquisitions are also influenced by the ion concentration, being larger as increases the ion concentration. Thus, their improved electrocatalytic current can be interpreted as a direct consequence of the thermal‐induced improvement of the OER electron transfer kinetics. However, the contribution of magnetohydrodynamics induced by a higher ion concentration to the enhanced electrocatalytic current should also be considered. This phenomenon was also confirmed with the temperature profiles of 13‐Co@C in KOH media at concentrations ranging from 0.1 to 1 M measured at 282 kHz and 20 mT (Figure S10, Supporting Information).

Chronoamperometric experiments were also performed at four fixed AMF strengths and four different solution pHs (Figure 4a–d). It is observed that the OER electrocatalytic current increment becomes more significant as both KOH concentration and AMF strength increase. In addition, the baseline OER currents after amperometric experiments are higher as increasing both the AMF strength, the solution pH and the number of pulses, which is attributed to their corresponding bulk electrolyte temperature increases (Figure 4e). Moreover, irrespective of the KOH concentration, an exponential relationship between the OER electrocatalytic current increment and the magnetic field strength is evidenced (Figure 4f, Figure S11, Supporting Information). Thus, we conclude that the thermal effect may be the primary source of OER kinetic enhancement at the different electrolyte solutions, which is more remarkable at higher KOH concentrations. This effect has not been considered in previous works, where KOH 1 M is usually employed.^{[ 33} , ³⁴ , ^{58 ]} The higher impact of the AMF on the electrocatalytic current as the KOH concentration increases can be also explained as a response to changes in the interface between the electrolyte and the electrocatalytic surfaces, being the increase in the temperature of the bulk electrolyte solution the indirect measurement of the mentioned synergetic effect between the electrolyte and the electromagnetic material. The role of the AMF field intensity on the catalytic current enhancement is analyzed by recording consecutive magneto‐pulses of variable AMF at different KOH concentrations (Figure 4f–h). This study reveals that, irrespective of the KOH concentration, a maximum ∆i value of 40% is obtained (Figure 4h). In addition, this maximum value is achieved at a similar AMF intensity, indicating that the intrinsic heating power of the 13‐Co@C catalyst is not affected by the electrolyte concentration.

Figure 4.

The effect of the KOH concentration on the electrocatalytic AMF‐OER coupled process in a graphite paper electrode coated with 13‐Co@C catalyst. a–d) Baseline‐subtracted chronoamperograms for the successive magneto‐pulses registered AMF intensities. The pulse time was 10 s at the applied potential of 1.80, 1.79, 1.76, and 1.7 V for 0.1, 0.25, 0.5, and 1 M KOH solution, respectively; e) Their corresponding temperature change as a function KOH concentration and AMF intensities; f) Baseline‐subtracted chronoamperograms registered at the indicated applied potential of the consecutive magneto‐pulses with increasing field intensity. The pulse time was 10 s at the applied potential of 1.80, 1.79, 1.76, and 1.7 V for 0.1, 0.25, 0.5, and 1 M KOH solution, respectively; g) Their corresponding temperature change as a function KOH concentration; h) their percentage current increases of the OER electrocatalytic current as a function of the applied external AMF extracted from (f).

Furthermore, the possible dependence of the OER activity with the electrolyte cation under AMF has also been investigated. Representative cyclic and linear voltammograms obtained for the 13‐Co@C‐based electrode at different alkaline mediums in the absence of AMF are depicted in Figure 5a,b. It can be observed that the nature of the alkaline cation has an impact on the OER activity of the 13‐Co@C material. The electrocatalytic current is shifted to lower potentials upon increasing the size of the cation (Li⁺ < Na⁺ < K⁺). A similar cation effect has been previously reported,^{[ 65 ]} but a detailed mechanism is still under discussion in the OER field. One of the most prominent hypotheses suggests that a cation–intermediate interaction enhances the OER process, either through non‐covalent interactions^{[ 71 ]} between the hydrated alkali metal cation and adsorbed OH⁻ species or through interactions between the superoxo intermediate of the OER and the alkali cation.^{[ 72 ]} Additionally, it has been proposed that cation intercalation during the oxygen evolution reaction induces structural restructuration of the electrocatalyst, effectively modifying the oxidation states of the metal active sites. This, in turn, optimizes the adsorption strength of oxygen intermediates, thereby enhancing OER activity. In addition, it has been postulated that cations can interact strongly with the solvent in the solvation layer, which positively alters the microenvironment where OER occurs. Indeed, it is proposed that the lower degree of H‐bonding of interfacial OH^– results in improved OER kinetics.^{[ 73 ]} Moreover, from DFT calculations, it has been proposed a decrease in the OER activity with the increase of the effective size of the electrolyte cation because of cation overcrowding near the negatively charged electrode surface.^{[ 66 ]} Recently, it has been proposed that alkali metal cations change the OER activity simply by affecting the local pH values.^{[ 74 ]} However, the fact that the Co(II)/Co(III) peak potential and electrocatalytic current branch shift in opposite directions with the cation size, rules out this hypothesis for the present system.

Figure 5.

The effect of the alkali metal cation on the electrocatalytic AMF‐OER coupled process in a graphite paper electrode coated with 13‐Co@C catalyst. a) Cyclic voltammograms recorded at 50 mV s⁻¹; b) Linear sweep voltammograms recorded at 5 mV s⁻¹; c) Thermally activated electrocatalytic current by applying external AMF of 40.3 mT; d–g) Baseline subtracted chronoamperograms for the successive magneto‐pulses registered at the indicated applied potential and AMF intensities; h) Background subtracted chronoamperograms of the consecutive magneto‐pulses with increasing field intensity. The pulse time was 10 s at the applied potential of 1.80 V for 0.1 M solution of KOH and NaOH, and 1.82 V for 0.1 M LiOH solution; i) A percentage current increase of the OER electrocatalytic current as a function of the applied external AMF extracted from (h).

The effect of the electrolyte cation on the polarization curve under the application of an AMF of 40.3 mT is depicted in Figure 5c, where both the negative shift and the enhancement of the electrocatalytic OER current follow the sequence: Li⁺ < Na⁺ ~ K⁺. Besides, the effect of the AMF intensity in the chronoamperometric experiments carried out at the different alkaline solutions reveals that the AMF‐induced current increments are strongly influenced by the nature of alkali cation (Figure 5d–g and Figure S12, Supporting Information) following a similar sequence Li⁺ < Na⁺ ~ K⁺. This cation dependence of the AMF‐OER current improvement may be attributed to a different AMF field sensitivity of the electrode‐electrolyte interphase. Accordingly, the larger positive impact on the OER activity by applying AMF as the cation size increases is consistent with favored activation parameters associated with the catalytic process involving the cobalt intermediate species. In this case, the current intensity increments (∆i) obtained from the successive magneto‐pulses with a gradual increase of the AMF strength at different alkaline electrolytes (Figure 5h,i), were found to reach a maximum of ≈28%, ≈34% and 40% for LiOH, NaOH and KOH electrolyte solutions, respectively. This indicates that the intrinsic heating power of the cobalt nanoparticles is influenced by the alkali metal cation, which may be related to specific interactions between the cations and the catalyst. It has been postulated that the alkali metal cation interacts with the catalytically active oxyhydroxide species, CoOOH, modulating the strength of the Co‐O bond, thereby leading to the favorable adsorption capacity of oxygen‐contained intermediates during the OER process.^{[ 75 ]} Thus, one can hypothesize a specific cation‐dependent modulation of the activation parameters during the thermally activated OER process upon AMF application. Besides, the cation nature also plays a key role in increasing the bulk electrolyte solution temperature beyond the effect of the magnetic pulse. This effect is more pronounced for K⁺ and Na⁺ compared to Li⁺ and may be attributed to an improvement of the electrolyte‐electrocatalyst interface, which depends on the cation interaction with the applied magnetic field at the same OH⁻ concentration and electromagnetic material. Additionally, the temperature increase under the magnetic effect is less significant in the absence of electroactive material (see Table S2, Supporting Information). Therefore, the interplay between electrolyte composition and electroactive materials influences the achieved temperature at both the electroactive surface and the bulk electrolyte, with K⁺ demonstrating the most notable effect.

Influence of the Cobalt Content‐ To study the effect of internal properties of the nanoparticles, such as size, metal loading and spatial distribution on the AMF‐assisted electrocatalytic OER, two additional catalysts have been synthesized (more information in SI). These two additional catalysts have a cobalt loading of 5.8 wt% and 9.2 wt% (6‐Co@C and 9‐Co@C, respectively). PXRD shows the same diffraction peaks as 13‐Co@C, ascribed to fcc Co NPs (Figure S13, Supporting Information). In addition, HRTEM images of one single nanocrystal show lattice fringes with d spacings of 1.85 Å and 2.04 Å, which could be ascribed to (200) and (111) planes of fcc Co NPs (Figure S14, Supporting Information) in good agreement with the X‐ray diffraction data. STEM images show the formation of spherical and well‐distributed Co nanoparticles on the carbon support. The average particle size increases slightly as the Co content in the materials rises, with values of 9.1 ± 4.6 nm for 6‐Co@C, 10.4 ± 3.2 nm for 9‐Co@C, and 13.1 ± 3.3 nm for 13‐Co@C (Figure S14, Supporting Information). The XANES spectra (Figure S15, Supporting Information) confirm the formation of metallic Co nanoparticles. However, an increase in the whiteline intensity is observed as the Co concentration in the sample decreases. This phenomenon could be attributed to a higher proportion of surface atoms with less saturated electronic states compared to bulk materials or larger nanoparticles. To validate this hypothesis, two additional analyses were performed. First, a linear combination fitting of the XANES spectra, which indicates a small but increasing presence of oxidized surface Co as the cobalt content in the sample decreases (Tables S3 and S4, Supporting Information). Second, an EXAFS fitting, which reveals a reduction in the percentage of bulk atoms, estimated through the first shell coordination number (Figure S16 and Table S5, Supporting Information), as the cobalt concentration decreases. The ratio between surface and bulk atoms of Co provides an indirect measurement of the average nanoparticle size.^{[ 76 ]} The combination of the two analyses confirms that a higher Co loading is associated with slight nanoparticle growth.

The magnetic properties of the two additional Co@C samples, with cobalt loadings of 5.8% and 9.2%, were also investigated and compared to the magnetic behavior of the sample with 13.3% cobalt loading. Figure S17a, Supporting Information shows the corresponding hysteresis loops, and Table S1, Supporting Information summarizes the key magnetic parameters (M_S, H_C and M_R/M_S) for all three samples. A clear trend is observed for M_S values, increasing with cobalt loading, from 4.4 Am² kg⁻¹ for 5.8%, to 8.3 Am² kg⁻¹ for 9.2%, and to 26.1 Am² kg⁻¹ for 13.3%, consistent with the increasing cobalt content estimated by XRF. However, the M_R/M_S ratio only changes from 0.23 for 5.8%, to 0.28 for 9.2%, and to 0.32 for 13.3% and H_C shows a slight increase from 24 kA m⁻¹ at 5.8% to 30 kA m⁻¹ at 9.2%, and reaching 35 kA m⁻¹ at 13.3%, which is also indicative of a slight variation in particle size with cobalt loading. The heating performance of the Co@C samples with cobalt loadings of 5.8% and 9.2% was evaluated at a concentration of 20 mg mL⁻¹ under an AMF of 282 kHz and 20 mT (Figure S17b, Supporting Information). The samples exhibited temperature increases of 1.65 and 2.05 °C, respectively. This means that the magnetic induction heating efficiency of the Co@C composite under the applied AMF can be adjusted with the amount of magnetic material present in the composite.

The electrochemical characterization for the electrocatalytic AMF‐OER coupled process of a graphite paper electrode coated with Co@C materials having different cobalt content was also performed. The cyclic voltammograms measured in the absence of AMF (Figure 6a) exhibit a similar pattern, where the voltammetric wave for the Co(III)/Co(II) redox conversion becomes more pronounced as the Co metal loading increases, which is consistent with a rise of the surface concentration of electroactive cobalt centers. In fact, the population of electroactive Co sites, calculated from the faradaic charge under the baseline‐corrected voltammetric wave,^{[ 52 ]} was 33.0, 3.4, and 2.8 nmol for the electrodes coated with the 13.3, 9.2 and 5.8 wt% Co materials, respectively. As expected, this increase in the number of redox‐active Co sites results in a higher electrocatalytic OER current at a fixed potential (Figure 6a,b). However, we noticed that there is not a direct linear correlation between the electrocatalytic OER current (at 1.80 V) and the electroactive population of cobalt centers, indicating that not all electroactive cobalt centers that can exchange electrons with the support participate in the catalytic reaction.

Figure 6.

The effect of the cobalt loading of the catalyst on the electrocatalytic AMF‐OER coupled process in a graphite paper electrode coated with the indicated Co@C catalyst. a) Cyclic voltammograms recorded at 50 mV s⁻¹; b) Linear sweep voltammograms recorded at 5 mV s⁻¹; c) Thermally activated electrocatalytic current by applying external AMF of 40.3 mT; d–g) Baseline subtracted chronoamperograms of the successive magneto‐pulses registered at 1.80 V and the indicated AMF intensities. The pulse time was 10 s; h) Background subtracted chronoamperograms registered at 1.80 V of the consecutive magneto‐pulses with increasing field intensity. The pulse time was 10 s; i) A percentage current increase of the OER electrocatalytic current as a function of the applied external AMF extracted from (h).

In addition, the OER current enhancement induced by the AMF obtained from the chronoamperometric experiments was found to increase with the cobalt content (Figure 6c–g and Figure S18, Supporting Information), in good agreement with the previously estimated heating capabilities of cobalt NPs (see above). Interestingly, a similar trend was observed in the variation of the magneto‐current normalized by the cobalt mass content with the AMF strength, indicating a similar degree of magnetic anisotropy in the nanoparticle ensemble for the different composites deposited on the electrodes (Figure S18, Supporting Information).^{[ 77 ]} This result well matches the very similar average particle size observed for the different metal‐loading catalysts.^{[ 78} , ^{79 ]} Furthermore, considering that the Néel relaxation is the dominant mechanism for hyperthermia in small particles, the similar degree of magnetic anisotropy obtained is consistent with a homogeneous distribution of the nanoparticles through the carbonaceous matrix with minimized inter‐particle interactions. Comparable conclusions can be extracted from the similar variation of the relative AMF‐assisted OER current increments (∆i) obtained for the different metal loading catalysts from the magneto‐pulse recorded with increasing AMF strength (Figure 6h,i).^{[ 78} , ^{79 ]} Furthermore, the cobalt concentration in the electroactive phase directly influences the solution temperature at fixed KOH concentration. These findings highlight the roles of the electromagnetic materials and electrolyte composition in the temperature increase of the solution. This study supports the idea that, beyond its conventional role in the OER process, the electromagnetic material under a magnetic field plays two roles: acting as an electroactive phase and facilitating magnetic harvesting. Meanwhile, the electrolyte composition improves thermal conductivity from the electroactive surface to the solution with a minor contribution to magnetic induction harvesting.

Operational Performance and Catalyst Stability‐ Finally, from a practical point of view, we performed short galvanostatic electrolysis with consecutive 40.3 mT AMF switch on‐off cycles for 90 s with short magneto‐pulse (3 s). The influence of external and internal parameters on the AMF‐assisted electrocatalytic OER was evaluated. Specifically, we assessed the effect of electrolyte concentration, the nature of its alkali cation and the metal loading content on the thermally AMF‐assisted OER electrolysis (Figure 7a–c). Interestingly, irrespective of the reaction conditions, the chronoamperograms reveal reproducible magneto‐pulses along the different OER experiments, indicating that the localized heating capabilities of the NPs remain intact during the whole electrolysis. This is consistent with the abovementioned homogeneous distribution of nanoparticles in the carbonaceous matrix as well as with the high thermal stability of the NPs‐modified electrode. These findings showcase the high stability of the NPs in operating under thermally AMF‐assisted alkaline water oxidation. Additionally, the exponential growth of the baseline electrocatalytic OER current in the absence of AMF during the electrolyses is observed, which is ascribed to the indirect bulk electrolyte heating effect in agreement with what was observed with longer magneto‐pulses. We estimated the charge passed during the experiments to quantify the AMF‐induced OER current improvement during the electrolysis. Likewise, to disentangle the contribution of the thermal effects owing to either the localized NP hyperthermia or bulk electrolyte indirect heating, we separately calculated the charge under the magneto‐pulses and the remaining charge arising from the current baseline increase (see Figure S19, Supporting Information).

Figure 7.

(Left panels) Chronoamperograms of the AMF on‐off short galvanostatic OER electrolysis at 40.3 mT at the indicated applied potential of a graphite paper electrode coated with 13‐Co@C catalyst as a function of a) KOH concentration with 13‐Co@C catalyst; c) Metal alkali cation with 13‐Co@C catalyst. The experiment time was 130 s at the applied potential of 1.80 V for 0.1 M solution of KOH and NaOH, and 1.82 V for 0.1M LiOH solution and e) the Co metal loading at KOH 0.1 M. The experiment time was 130 s at the applied potential of 1.80, 1.79, 1.76, and 1.7 V for 0.1, 0.25, 0.5, and 1 M KOH solution, respectively. (Right panels) b, d, f) Their corresponding charge increments arise from both NP localized and electrolysis bulk thermal effects (see Figure S18, Supporting Information).

The charge increments observed in the different electrolytic experiments are depicted in Figure 7b,d,f. Interestingly, the contribution of bulk heating to the charge increments is lower than that of NPs localized AMF‐induced hyperthermia, except in electrolysis performed at high electrolyte concentrations (≥0.5 M). Moreover, the influence of localized heating by cobalt NPs under varying reaction conditions strongly supports the results obtained in the preparative experiments. Consequently, the charge increment induced by thermally activated OER current increases with both the electrolyte concentration and the cobalt metal loading on the electrode (Figure 8 ). In addition, AMF‐assisted electrolysis performance improves with the increasing size of the alkali metal cation of the electrolyte, following the trend Li⁺ < Na⁺ ≈ K⁺.

Figure 8.

Schematic illustration of the synergistic effects of the CoNPs–K⁺ interaction and concentration on the enhancement of the electrocatalytic OER activity under an AMF.

Based on the findings of this study, the enhancement of OER activity arises from the collective effects of cobalt nanoparticle loading, [CoNPs], pH and the nature and concentration of alkaline cation, [K⁺], as illustrated in Figure 8. Indeed, the results suggest that the plausible cation intercalation from the electrolyte can play a key role in modulating the interfacial structure and thus the activity of the electrocatalyst. This structural reorganization, which enhances catalytic performance, is further amplified under magnetic induction. In particular, K⁺ ions are known to influence the magnetic behavior of certain materials, including molecular magnets, by delaying magnetic relaxation processes.^{[ 80 ]} This property likely extends to the CoNP‐based catalyst system under AMF, facilitating localized magnetic heating. At the same time, increased CoNP loading increases the number of both active catalytic sites and heating agents, thus leading to a greater increase of the AMF‐induced electrocatalytic current. Altogether, these effects create a synergistic enhancement of OER activity driven by the interplay between the number of catalytically active sites, electrolyte composition, and magnetic field application (see Figure 8).

Finally, the structural and compositional stability of the 13‐Co@C material after the AMF‐OER coupled process was also assessed. Interestingly, an analysis of the magnetic properties of the 13‐Co@C‐modified electrode after a long AMF‐OER coupled experiment reveals that the magnetic properties (M_S, H_C and M_R) remain unchanged, as evidenced by the magnetic hysteresis loops recorded post‐experiment (Figure S20a, Supporting Information). Similarly, temperature profiles directly measured for the carbon paper electrode modified with 13‐Co@C, before and after the AMF‐OER coupled process, do not show significant variation (Figure S20b, Supporting Information). This observation is further corroborated using different spectrochemical and microscopy techniques. Powder X‐ray diffraction patterns indicate that the predominant species after catalysis remains Co fcc, as evidenced by the presence of peaks at 44, 51, and 76° (Figure S21, Supporting Information).

Additionally, post‐experiment STEM images of the catalytic composite show that the Co nanoparticles on the carbon support retain their spherical shape and uniform distribution without agglomeration and that the average particle size remains constant for the 13‐Co@C material (Figure S22a, Supporting Information). In addition, HRTEM images of one single nanocrystal display lattice fringes with d spacings of 2.05 Å, which could be ascribed to (111) planes of Co fcc NPs (Figure S22b, Supporting Information) in good agreement with the PXRD data. Moreover, a plane related to CoO_x is observed on the surface of the nanoparticles, indicating some degree of metal oxidation. To gain further insight into the surface composition, X‐ray photoelectron spectroscopy (XPS) was performed on the 13‐Co@C composite both before and after AMF‐OER coupled process. The relatively thick carbon layer surrounding the nanoparticles, as observed in HRTEM images, resulted in low signal intensity, which limited the reliability of quantitative fitting. Nevertheless, qualitative comparisons of the spectra provided insights consistent with the previous findings. In the XPS spectra of the electrode measured before the AMF‐OER coupled process (Figure S23, Supporting Information), the Co 2p binding energy region exhibits multiple components, suggesting a mixture of metallic cobalt and cobalt oxides on the surface. The main peak, centered at ≈778.8 eV, corresponds to metallic cobalt (Co⁰), accompanied by a broader shoulder in the 780–781 eV range, characteristic of cobalt oxides. After the AMF‐OER coupled process, the XPS spectra of the electrode (Figure S23, Supporting Information) show that the Co° contribution to the main peak is no longer detectable, indicating a more oxidized surface on the 13‐Co@C catalyst.

3. Conclusions

In this work, we have thoroughly shown the influence of different external parameters and intrinsic properties of novel MOF‐derived cobalt nanoparticles supported on a carbon matrix, on the magnetic field‐assisted electrocatalytic oxygen evolution reaction. Our findings reveal that increasing the AMF strength significantly enhances the electrocatalytic OER activity, which is attributed to the magnetic hyperthermia heating effect, driven by Néel relaxation and spin polarization flip under AMF. Additionally, to the best of our knowledge, we evidenced for the first time, the impact of electrolyte composition (KOH concentration and the nature of the electrolyte cation) on the electrocatalytic AMF‐OER process, revealing a strong dependence on the solution pH and the alkali cation size (Li⁺ < Na⁺ < K⁺). Furthermore, the study of the effect of intrinsic properties of the cobalt nanoparticles, such as size, metal loading, and spatial distribution on the electrocatalytic AMF‐OER process reveals that a higher cobalt content led to a superior OER activity, which is consistent with a higher number of redox‐active Co sites and a greater magnetic hyperthermia heating. Additionally, a similar degree of magnetic anisotropy was observed, indicating a homogeneous distribution of the nanoparticles through the carbonaceous matrix with minimized inter‐particle interactions. Most importantly, the present electrodes displayed highly operational stability for the electrocatalytic AMF‐OER coupled process, evidenced by the absence of significant leaching, sintering, or particle size increase. Overall, our results demonstrate the real and effective potential application of electrocatalytic AMF‐assisted OER, providing new insights into the interplay between magnetic, thermal, and electrochemical effects in electrocatalysis. Moreover, this work underscores the stability and efficiency of the MOF‐derived 13‐Co@C material as catalyst for electrocatalytic AMF‐OER process, paving the way for further advancements in magnetically enhanced water oxidation technologies.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

del Rio‐Rodríguez J. L., Gutiérrez‐Tarriño S., Márquez I., et al. “The Second Life of Cobalt MOF: Alternating Magnetic Field‐ Assisted Electrocatalytic Oxygen Evolution Reaction in MOF‐derived Nanoparticles.” Small 21, no. 33 (2025): 21, 2503871. 10.1002/smll.202503871

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.