The use of magnetic fields as external stimuli to improve the kinetics of electrochemical reactions is attracting substantial attention, given their potential to reduce energy losses. Despite recent reports showing a positive effect on catalytic performance upon applying a magnetic field to a working electrode, there are still many uncertainties and a lack of experimental evidence correlating the presence of the magnetic field to the electrocatalytic performance. Here, we present a combination of electrochemical and spectroscopic tools that demonstrate how the presence of an external magnetic field alters the reaction mechanism of the electrocatalytic oxygen evolution reaction (OER), accelerating the overall performance of a Ni4FeOx electrode. Complementary experimental evidence has been gathered supporting the participation of this microscopic magnetic field effect. Electrochemical impedance spectroscopy (EIS) points to a speed-up of the intrinsic reaction kinetics, independent of other indirect effects. In the same direction, the spectro-electrochemical fingerprint of the intermediate species that appear during the electrocatalytic cycle, as detected under operando conditions, indicates a change in the order of the reaction as a function of hole accumulation. All these experimental data confirm the direct influence of an external magnetic field on the reaction mechanism at the origin of the magnetically enhanced electrocatalytic OER.

Magneto-electrochemical effects are becoming of high interest as a means to escape the linear relationships that limit electrocatalytic processes.1 The effect of dc magnetic fields on mass transport2 or of ac magnetic fields to achieve heating locally3–5 has been proposed to improve the engineering of electrolyzers.6,7 More recently, the direct impact of an applied dc magnetic field to accelerate and/or modify the microscopic mechanism of electrochemical reactions is becoming a hot trend in the field. Magnetic field enhancement was first found in the oxygen evolution reaction (OER), where a significant kinetic improvement (>100% increase in current density) at constant cell potential was reported under an applied external magnetic field.8 Consistent results have been found for different OER catalysts,9–13 both when their intrinsic magnetic phases are tuned14–17 and when the magnetic fields are generated at the electrode surface using different magnetic setups, including the use of electromagnets and permanent magnets.18,19 Therefore, the origin of the phenomenon remains unclear and may differ in accordance with the magnetic nature of the catalytic phase and the electrolyte.

A major problem in most of these works is the difficulty in distinguishing between direct mechanistic phenomena,20 where the chemical reaction rates are affected by the magnetic fields, and indirect phenomena that may co-exist, such as Lorentz and Kelvin forces, local heating, or other indirect effects that magnetic fields may favor.21,22 In addition, the theoretical interpretation of the origin of a magneto-electrocatalytic enhancement is not necessarily supported by direct experimental evidence since available data may be attributed to a variety of microscopic origins, whereas the computational models only tackle these effects separately. Some authors point toward a spin-polarization effect in strongly correlated electrons.9,23–25 Others relate the magnetic enhancement to the alignment of radical species on the surface of the catalyst, which facilitates spin-aligned radical coupling.8,26,27 Magnetoelectric effects have been claimed in multiferroic catalysts,28,29 and some metal oxides have also shown magnetic field-induced structural phase transitions.30 

In a remarkable work, Ren et al. were able to relate the magnetic enhancement to the disappearance of domain walls in a ferromagnetic material,31 suggesting that single-domain ferromagnetic particles do not require an external magnetic field to reach maximum performance.32 In addition, magnetic enhancement has also been observed in antiferromagnetic or paramagnetic catalysts,33 where the actual synergy must have been driven by an entirely different origin. Furthermore, other researchers have observed unexpected effects on the electrochemical reactivity of small molecules, including the hydrogen evolution reaction (HER)28 or the oxygen and CO2 reduction reactions (ORR and CO2RR).34–38 In summary, despite the rapid growing amount of experimental data available, the origin of this phenomenon is still far from being understood. In part, this is due to the lack of a systematic approach to study the fundamental origin of this effect, with available data originated from reactions with very different mechanisms and involving a wide range of materials.

In this study, we investigate the effect of magnetic fields on electrocatalytic OER using a model, well-defined electrode, Ni4FeOx. In order to gather direct information regarding the mechanism of the reaction, we utilized two powerful techniques. Electrochemical Impedance Spectroscopy (EIS) provides information related to the kinetics and limitations of each distinct process during catalysis, being able to distinguish between internal resistance, charge transfer, charge accumulation, and chemical reactions. Complementarily, spectroelectrochemistry provides access to the population of electronic states, the oxidation states, and the concentration changes of the catalysts by probing their absorption fingerprints and quantifying them through the well-known Lambert–beer law.39 According to the data, the magnetic field favors a larger accumulation of active NiOOH species leading to faster OER kinetics at the electrode surface.

To systematically study the effect of a magnetic field on the OER catalysis, we employed a model nickel iron oxide, synthesized by Physical Vapor Deposition (PVD) followed by thermal annealing (for more detailed information, please refer to the Methods section). This mixed-oxide material, denoted as Ni4FeOx, essentially comprises a NiO matrix decorated with finely dispersed Fe–Ni spinel nano-islands on its surface, with a composition ratio of ∼82 at. % Ni and 18 at. % Fe (see Figs. S1–S4 and Table S1 for the detailed structural and chemical characterization of the electrocatalyst). Our choice of this catalyst, renowned as a standard catalyst for OER, is rooted in its established electrocatalytic performance.40–42 It is also a magnetically active catalyst, with paramagnetic centers and a mixture of ferro- and antiferromagnetic interactions depending on the local structure, but with positive magnetization under an applied magnetic field.43 Moreover, we have meticulously ensured a uniform film thickness of ∼75 nm (Fig. S3), a key parameter in our experimental strategy to allow for transmission spectroscopy experiments.

The electrochemical characterization of our Ni4FeOx films in alkaline environments when subjected to an external magnetic perturbation (see supplementary material, note 1, and Figs. S5 and S6 for details on the electrochemical setup) is shown in Fig. 1. Before the relevant experiments, a preconditioning step was systematically executed on all electrodes. It consisted of 50 successive cyclic voltammetry (CV) cycles [Fig. S7(a)], followed by >5 linear sweep voltammetry (LSV) runs at a slow scan rate to reach consistent, repeatable, and stable electrocatalytic behavior [Fig. S7(b)], so as to discard any dynamic effects. Once this stationary electrocatalytic stage is achieved, it is apparent from Fig. 1 that when the magnetic field is applied (HON), we observed a minor reduction in the onset potential, ∼20 mV, compared to the behavior in the absence of the magnetic field (HOFF), signifying an enhancement in the catalytic region for the OER. This catalytic enhancement translates to an enhanced current density, with a discernible increment of ∼20% of current density above 1.6 V vs RHE. This result is consistent with our prior research on NiOx, confirming the effectiveness of magnetic fields for catalytic improvement.8 

FIG. 1.

Electrochemical characterization. Linear sweep voltammetry (LSV) data for the Ni4FeOx films without (HOFF, blue data) and with (HON, red data) the presence of an applied magnetic field of <30 mT in KOH 1M. Inset: Zoom-in of the LSV at the characteristic Ni(OH)2 to NiOOH redox peak to show the effect of an increased formation of the NiOOH phase upon HON conditions.

FIG. 1.

Electrochemical characterization. Linear sweep voltammetry (LSV) data for the Ni4FeOx films without (HOFF, blue data) and with (HON, red data) the presence of an applied magnetic field of <30 mT in KOH 1M. Inset: Zoom-in of the LSV at the characteristic Ni(OH)2 to NiOOH redox peak to show the effect of an increased formation of the NiOOH phase upon HON conditions.

Close modal

Notably, in both experimental conditions, with and without the magnetic field, the redox waves in the range of 1.35 V vs RHE to 1.50 V vs RHE are prominently featured (see Fig. 1, inset), attributed to the oxidation of both α and β Ni(OH)2 to γ and β NiOOH phases.44 Interestingly, when the magnetic field is turned on (HON conditions), a more intense redox peak is observed, indicating a larger formation of both γ and β NiOOH phases. More quantitatively, under conditions of HON, a ∼6-fold increase in the past charge, calculated as the area under the redox peak,45 from 0.036 to 0.218 C, is observed (Fig. S8), suggesting that the magnetic field facilitates the formation and accumulation of NiOOH. Although it is out of the scope of this paper, we note that qualitatively, a larger proportion of the β-NiOOH phase is produced (∼1.47 V vs RHE) under HON conditions.44 We also observe that this transition takes place rather preferentially after removing the magnetic field, suggesting a possible facet reconstruction. However, this apparent more permanent change caused by the presence of the magnetic field does not seem to lead to improved catalytic activity (see Fig. S9 and associated discussion), supporting the observation that under catalytic conditions, a facilitated accumulation of catalytically active NiOOH species under the magnetic field takes place.

The magnetocurrent effect on the electrochemical activity of the Ni4FeOx films was then studied by electrochemical impedance spectroscopy (EIS). Figure 2(a) shows the Nyquist response of the Ni4FeOx film held around the onset of the catalytic current, i.e., 1.5 V vs RHE. Two semicircles can be observed in this figure, indicating that different processes occur on different timescales under both HOFF and HON conditions. These two processes are present throughout the entire studied potential window (1.2–1.8 V vs RHE; see Fig. S10). From Fig. 2(a), it can be observed that under HON, both the high- and the low-frequency resistances [R1 and RCT, respectively, in the equivalent circuit shown in Fig. 2(a), inset] decrease significantly.

FIG. 2.

Electrochemical impedance spectroscopy (EIS) analysis. (a) Nyquist plot measured by EIS and selected equivalent circuit employed to fit the experimental data; (b) series and film resistance (RS and R1, respectively, left axis) and film capacitance (C1, right axis) fitted to the circuit shown in Fig. 2(a), inset; and (c) charge transfer resistance (RCT, left axis) and surface capacitance (CS, right axis), compared to the LSV (second right axis) of the Ni4FeOx film without (HOFF, blue data) and with (HON, red data) the presence of the magnetic field (<30 mT) in KOH 1M (pH ≈ 14).

FIG. 2.

Electrochemical impedance spectroscopy (EIS) analysis. (a) Nyquist plot measured by EIS and selected equivalent circuit employed to fit the experimental data; (b) series and film resistance (RS and R1, respectively, left axis) and film capacitance (C1, right axis) fitted to the circuit shown in Fig. 2(a), inset; and (c) charge transfer resistance (RCT, left axis) and surface capacitance (CS, right axis), compared to the LSV (second right axis) of the Ni4FeOx film without (HOFF, blue data) and with (HON, red data) the presence of the magnetic field (<30 mT) in KOH 1M (pH ≈ 14).

Close modal

The respective resistances and capacitances of both high- and low-frequency processes can be calculated by fitting the Nyquist data [Figs. 2(a) and S10] to the equivalent circuit depicted in Fig. 2(a), inset. First, the series resistance (RS), which accounts for all the electrical processes external to the Ni4FeOx layer (including the contribution of the electrolyte, contacts, and connection wires), appears to be invariant as a function of the application of the magnetic field and the applied potential, 19.00 ± 0.40 Ω for HOFF vs 18.99 ± 0.30 Ω for HON conditions, especially when catalytic processes occur [from ∼1.5 V vs RHE, see Fig. 2(b), empty circles, left axis]. This result suggests that Lorentzian forces generated by magnetohydrodynamic effects are negligible with the magnetic fields applied in our study.46–48 Conversely, Fig. 2(b) shows that the high-frequency (HF) resistance (R1) decreases almost monotonically as a function of the applied potential. R1 is typically associated with the bulk transport in semiconductors and/or to the solid interface with the electrode support, both affected by the applied electric field. This parameter decreases upon turning the magnetic field ON [HON, red squares in Fig. 2(b)], suggesting easier transport of carriers. On the other hand, the HF capacitance (C1), shown in Fig. 2(b) filled circles, exhibits a peak between 1.40 and 1.65 V vs RHE, which mostly coincides with the potential range where Ni(OH)2 is oxidized to NiOOH.11,49

Figure 2(c) reports the low-frequency processes. The charge transfer resistance [RCT, squares data in Fig. 2(c)] appears to decrease exponentially from highly resistive values to 10 Ω resistance values, close to RS around the catalytic onset potential (∼1.5 V vs RHE), as observed previously for similar Ni-based OER electrocatalysts.50 The RCT remains constant under HOFF conditions after the onset potential, correlating with the linear increase in the current density with the applied potential observed in the LSV. However, the RCT in HON conditions further decreases, reaching a minimum value of ∼1.70 V vs RHE. On the other hand, the surface capacitance (CS) behavior appears to be different when the magnetic field is applied. From Fig. 2(c), two surface capacitance peaks are evident under HON conditions compared to a unique broad one for HOFF from 1.50 to 1.80 V vs RHE. This surface capacitance, under both HOFF and HON conditions, appears to peak at ∼1.6 V vs RHE, coinciding with the potential at which the RCT becomes insensitive to the applied potential. At potentials more positive than 1.60 V vs RHE, the surface capacitance values decrease. A similar behavior has been observed in other Ni-based electrocatalysts49,51–53 and Cu-based electrocatalysts for hydrogen evolution.54 As a result of the increased catalytic activity, the species built up at lower potentials are consumed faster. Additionally, the formation of gas bubbles decreases the electrochemical surface area of the electrocatalytic film.

Finally, the second CS peak observed under HON conditions, peaking at 1.42 V vs RHE, appears to follow the formation of the NiOOH species previously mentioned and observed in Ni-based electrodes.50 Upon comparing both CS values without and with the magnetic field (HOFF and HON, respectively), a ∼5-fold increase is observed upon turning ON the magnetic field (from 0.14 to 0.70 mF cm−2), matching the calculated increase of ∼6-fold in the past charge from the LSV in Fig. 1. This result supports the hypothesis of a facilitated formation and accumulation of NiOOH at the electrode surface upon turning ON the magnetic field.

To further study the effect of a magnetic field on the electrochemical response of the Ni4FeOx electrodes, steady-state spectroelectrochemistry (SEC) was used to monitor the kinetics of the OER rate determining step (RDS). As such, UV–Vis spectra [see Figs. S11(a) and S11(b)] were taken during chronoamperometric measurements being performed [see Figs. S11(c) and S11(d)] as a function of an increasing applied potential. Figure 3(a) shows the average current densities vs voltage curve, i.e., steady-state LSV, of the Ni4FeOx electrodes. This figure shows that the faradaic current is positively affected by the magnetic field, resulting in an increase of ∼30% magnetocurrent at 1.9 V vs RHE [red and blue empty circles in Fig. 3(a)].

FIG. 3.

Spectroelectrochemical analysis of the OER. (a) Steady-state differential optical density of the optically absorbing species at 500 nm (left axis) compared to the steady-state LSV (right axis) and (b) rate law analysis, log–log plot of the current density as a function of the density of highly Ni oxidized species (*) of the Ni4FeOx film without (HOFF, blue data) and with (HON, red data) the presence of the magnetic field in KOH 1M.

FIG. 3.

Spectroelectrochemical analysis of the OER. (a) Steady-state differential optical density of the optically absorbing species at 500 nm (left axis) compared to the steady-state LSV (right axis) and (b) rate law analysis, log–log plot of the current density as a function of the density of highly Ni oxidized species (*) of the Ni4FeOx film without (HOFF, blue data) and with (HON, red data) the presence of the magnetic field in KOH 1M.

Close modal

The steady-state differential optical density shown in Fig. 3(a) exhibits two distinct changes upon increasing the applied potential. Between 1.40 and 1.57 V vs RHE, the differential optical density initially appears to sharply increase up to ∼1.50 V vs RHE to further plateau. This increase in the differential optical density is observed at the Ni(OH)2 oxidation potential range observed in Fig. 1 and, therefore, it is assigned to the formation of NiOOH, which is consistent with the observations of Francàs et al. in NiFeOOH systems.55,56 Interestingly, this redox peak is centered at 1.45 V vs RHE under both HOFF and HON conditions. This suggests that the free energy change (ΔGO − ΔGOH) of the Ni(OH)2 oxidation is insensitive to the magnetic field.57–59 Furthermore, as observed in Fig. 3(a), the differential optical density is considerably higher under HON conditions, supporting the hypothesis of a kinetically facilitated accumulation of NiOOH at the electrode surface, as observed in Fig. 2. This increased density of surface NiOOH formed is responsible for the increased capacitive peak in LSV and EIS analysis, suggesting that, under a magnetic field, the surface density of NiOOH grows larger because the electrical path to reach this state is more readily accessible.

A further increase in the differential optical density is observed at potentials more positive than 1.6 V vs RHE in Fig. 3(a), corresponding to further oxidized Ni species that are catalytically active toward OER.56 Although a full structural assignment is out of the scope of this paper, these highly oxidized species have been discussed to be assigned to NiIII–O·, μ-oxo-NiIV–NiIV, or FeIV(=O)–NiIII–O intermediates,55,60 which for simplicity will be referred to herein as *. We note that the spectra of these * species accumulated at the surface of our electrode exhibit a broad absorption centered at 500 nm at potentials more positive than 1.55 V vs RHE for HOFF and 1.52 V vs RHE for HON conditions, as shown in Fig. S12. Interestingly, these spectra do not exhibit significant differences, suggesting that the nature of the * species is equivalent under both HOFF and HON conditions. These spectra are further used to monitor the surface density of * species and perform the rate law analysis in Fig. 3(b) and discussed next. It is apparent from Fig. 3(a) that the larger extension of surface NiOOH under the influence of the magnetic field yields an increased surface density of * in the resting state but does not change their chemical nature.61–63 This increased population of * in the resting state due to a larger density of active NiOOH sites is also observed upon monitoring the open circuit potential (OCP) concomitant to the optical signal at 500 nm immediately after stopping bulk electrolysis (see Fig. S13 and associated discussion). Remarkably, the different resting states of HON and HOFF yield a striking difference in the kinetics of the OER by reducing the reaction order of the rate determining step (RDS), as observed in Fig. 3(b), which is discussed next.

To further study the influence of the magnetic field on the OER mechanism, the differential optical density data from the highly oxidized Ni species was used in combination with the steady-state current density data acquired during the spectroelectrochemistry measurements. Considering the change in the accumulated surface oxidized species upon HON conditions, a kinetic “population” model is used herein to build rate law analyses (J = kWO × mΔODβ, where mΔOD is directly proportional to the density of surface *). These analyses have been used to determine key kinetic parameters of (photo)electrochemical reactions such as reaction rate constants (kWO) and orders of reactions (β).39, Figure 3(b), blue dots, shows that the OER transitions from an approximate first order to a third order of reaction when no magnetic field is present (HOFF). Interestingly, a similar transitioning behavior of the OER had been observed previously in photoelectrodes such as α-Fe2O3 and BiVO4,64,65 with the third order being assigned to a multi-redox mechanism where three holes undergo equilibrium with the active sites before the RDS is overcome.66 Therefore, we propose that the NiIII–OO–NiIII intermediate63 at the RDS is formed by the accumulation of 3 * species at the active sites. Strikingly, Fig. 3(b), red dots, shows that, followed by a larger accumulation of NiOOH under HON conditions, the RDS only requires the equilibrium of 2 * with the active sites. As such, we propose that under HON conditions, a faster reaction pathway, i.e., from third to second order RDS, most likely arises from a larger accumulation of * species at the electrode surface. This faster OER kinetics, under HON conditions, is also observed upon monitoring the optical signal associated with * species [see Fig. S13(c) and associated discussion]. A reduction in the reaction order using the same (photo)electrocatalyst had been observed when changing the water for hole scavengers, such as methanol, ethanol, or hydrogen peroxide,65,67,68 and when reducing the pH from 14 to 7 in NiFeOOH electrocatalysis (from fourth to second order) and in TiO2 photoanodes (from third to second order) for OER.69 Interestingly, highly efficient M0.1Ni0.9O, where M = Mn, Co, Fe, and Zn, all exhibit a second order of reaction toward OER.70 In our Ni4FeOx electrodes, this reduction in the order of reaction from third to second order upon HON is consistent with the faster reaction kinetics and improved performance observed in Ni4FeOx electrodes.

In summary, our work demonstrates that the stabilization of a different phase under a magnetic field at potentials close to the OER wave results in a higher density of pre-catalytic domains (NiOOH) under HON. The consequence is that the dependence (reaction order) is two, which is lower than in the system without stimuli. The reason is that the catalytically active NiOOH species formation corresponds to the first step in the activation of Ni(OH)2 in the catalytic cycle; without stimuli, the NiOOH domains are generated at the potential of OER and, therefore, this is one of the steps in the cycle that contributes to the counting in the reaction order. When the magnet is on, this step is out of the cycle as active extensive domains are formed at lower potentials than the OER onset; therefore, this elementary step is out of the cycle, and the dependence on the number of * to be accumulated is smaller, resulting in less demanding kinetics and faster processes.

We have gathered multiple and complementary data to investigate, at the microscopic level the effect of an applied magnetic field, on the mechanism of electrocatalytic OER using a Ni–Fe oxo-hydroxide electrode in an alkaline electrolyte. Electrochemically, this electrode exhibits enhanced performance once a magnetic field is applied. Higher current densities are achieved at lower overpotentials after the onset of OER, found at ≈1.35 V vs RHE. With the help of EIS and spectroelectrochemical (SEC) data, one can discard any indirect effect participating in the observed enhancement. Neither mass transport nor magnetotransport issues were found to contribute to this process. According to our results, the magnetic field favors a change in the resting state of the material prior to starting the reaction. EIS data show how the electron transfer event, typically associated with the catalytic process, is faster, showing lower resistance under a magnetic field. SEC data, which gives direct information about the reaction orders demonstrating different values with or without magnetic field. This is direct experimental evidence about the influence of the magnetic field on the interplay between the reaction mechanism and the resting state of the catalyst.

Looking at the latter, it is remarkable how the order of the reaction changes with respect to the hole population. This suggests that the magnetic field is pushing a faster mechanism to become dominant by changing the domains of the pre-catalytic material before the onset potential for OER. The most plausible hypothesis is probably related to the concentration of active sites participating in this mechanism, which is minor in the absence of the magnetic field and is increased at the cost of those participating in the magnetic field-less dominant mechanism.

In addition to the direct effect on the reaction mechanism, we also observed in EIS analysis a faster and energetically easier accumulation of oxidized reactive sites on the surface of the catalyst before the limiting reaction occurs. This is an additional feature that enhances the electrocatalytic performance under a magnetic field. Therefore, our results confirm the presence of a direct influence of the magnetic field in the catalysis, which may be exploited to improve the performance of OER catalysts in alkaline conditions, and highlight the complexity of analyzing the microscopic principles for this phenomenon, which acts upon different reaction intermediates.

Finally, we would also like to highlight that the observed effect of the magnetic field on the catalytic surface may be accompanied by additional magnetic effects, such as those that could originate from spin pinning71 imposed by the ferromagnetic Ni4FeOx bulk structure below the reaction sites.

Fluorine-doped Tin Oxide (FTO) commercial substrates were employed as electrode supports. These substrates underwent a meticulous cleaning process that involved an initial 5-min sonication in soapy water, followed by rinsing and a subsequent treatment with acetone and sonication. Finally, a stream of nitrogen was used to ensure the complete drying of the substrate. In the subsequent step, the FTO substrates were placed inside a physical vapor deposition chamber (ATC Orion 8HV from AJA International Inc.), where a 50 nm film of a NiFe alloy (81/19 wt. %) was grown at a deposition rate of 3 Å/s. This process was conducted under a RF power supply with a power of 200 W, all within an argon flow of 20 SCCM and a pressure of 3 mTorr. The next phase involved an annealing process, performed in an air atmosphere with a tubular oven (Nabertherm, C 530–1200 °C). The annealing temperature was set at 550 °C for 45 min. The purpose of this step was to oxidize the alloy into a Ni–Fe oxide film, which was subsequently used in the electrochemical procedures.

Powder x-ray diffraction (PXRD) data were recorded with a Bruker D8 Advance Series equipped with a VANTEC-1 PSD3 detector. A JEOL F200 TEM ColdFEG operated at 200 kV was used for transmission electron microscopy characterization. TEM images were acquired with a Gatan OneView camera, a CMOS-based and optical fiber-coupled detector of 4096 × 4096 pixels. The Gatan Digital Micrograph program was used to process the TEM images. STEM images (1024 × 1024 pixels) were recorded from the JEOL bright-field (BF) and high-angle annular dark-field (HAADF) detectors using the Gatan DigiScan3 scanning unit with a camera length of 200 mm. The quasi-parallel STEM mode for the localized Nano-Beam Diffraction (NBD) patterns was aligned following the procedure by Plana-Ruiz et al.72 Samples were inserted in a JEOL beryllium double-tilt holder for energy dispersive x-ray spectroscopy (EDS). STEM-EDS mapping was recorded from an EDS Centurio detector (silicon drift) with an effective area of 100 mm2 and 133 eV of energy resolution. STEM-EDS maps (512 × 512 pixels) were processed with the JEOL Analysis software. Simulated electron diffraction patterns were obtained from the ReciPro software.73 The surface morphologies of the films were investigated by high resolution field emission scanning electron microscopy with a focused Ga ion beam (FESEM-FIB, Zeiss Supra), and the chemical analysis was performed using an FESEM Inspect F50 with an EDS detector (EDAX).

All electrochemical experiments were conducted under ambient conditions using a Bio-Logic VMP3 multichannel potentiostat, employing a three-electrode configuration. For all electrochemical experiments, the electrolyte solution consisted of 1M KOH (pH 13.9), a Pt mesh counter electrode, and a Hg/HgO (1M KOH) reference electrode. The working FTO/Ni4FeOx electrode was prepared as described earlier, with a geometric surface area of 1 cm2. All recorded potentials were referenced against the Hg/HgO electrode and converted to the RHE reference scale using the formula ERHE = EHg/HgO + 0.100 + 0.059 pH (V) (see Fig. S14 for calibration). Current densities were calculated based on the geometrical surface area of the electrodes. Onset potential was estimated as the potential where the current density becomes non-zero (≈0.1 mA cm−2). Electrochemical impedance spectroscopy (EIS) was conducted using a standard three-electrode cell, encompassing a frequency range from 100 kHz to 0.1 Hz, with 10 data points per decade under different voltage conditions, following the same 0.025 V steps. The EIS data were used to estimate the Ohmic drop (iR drop) for all electrochemical data, utilizing the electrochemical impedance spectroscopy (EIS) technique within the Bio-Logic interface for its compensation. Spectroelectrochemical (SEC) experiments were run during chrono-amperometry (CA) tests. CA tests were conducted at intervals of 10 min (scan duration in spectrophotometer) with voltage steps of 0.025 V, spanning the range from 0.00 to 0.9 V vs Hg/HgO.

SEC experiments, which involve measuring optical absorption as a function of applied potential, were conducted using a UV–vis–NIR spectrophotometer (PerkinElmer 1050). The obtained data was typically presented in the form of spectro-electrochemical difference spectra (ΔO.D.). These spectra are generated by subtracting a reference spectrum, often taken at the Open Circuit Potential (OCP) or the onset of water oxidation, from the absorption data acquired under specific conditions of interest. For a comprehensive understanding of this technique, readers are referred to the detailed explanation provided by Pastor et al.74 

The supplementary material describes the electrochemical setup. Detailed microscopic data include elemental analysis, SEM, and TEM images. Powder x-ray diffraction data. Description of the magnetic field generated by the permanent magnets in the electrochemical set-up.

The authors acknowledge the support from MCIN/AEI/10.13039/501100011033/ and “ERDF A way of making Europe” through Project Nos. RED2022-134508-T (CAT&SCALE), PID2021-124796OB-I00, and PID2020-116093RB-C41; and from the Generalitat de Catalunya (Grant No. 2021SGR1154). ICIQ is supported by the Ministerio de-Ciencia e Innovación through the Severo Ochoa Excellence Accreditations Grant Nos. CEX2019-000925-S (MCIN/AEI) and CEX2021-001214-S, and by the CERCA Program/Generalitat de Catalunya. S.G. and C.A.M. acknowledge funding from the Generalitat Valenciana through Grant No. APOSTD/2021/251 fellowship and from the University Jaume I through Project No. UJI-B2020-50. M.G.T. acknowledges the support of a fellowship from "La Caixa" Foundation (ID 100010434). The fellowship code is LCF/BQ/11980046. The HRTEM instrumentation was partially funded by the operative program FEDER Catalunya 2014-2020 (IU16-015844).0.3.

The authors have no conflicts to disclose.

C.A.M. and F.A.G.-P. contributed equally to this work.

C. A. Mesa: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). F. A. Garcés-Pineda: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). M. García-Tecedor: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). J. Yu: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). B. Khezri: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Supervision (supporting); Writing – review & editing (supporting). S. Plana-Ruiz: Data curation (supporting); Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). B. López: Formal analysis (supporting); Methodology (supporting); Software (supporting); Validation (supporting); Visualization (supporting); Writing – review & editing (supporting). R. Iturbe: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Software (supporting); Supervision (supporting); Writing – review & editing (supporting). N. López: Conceptualization (supporting); Writing – review & editing (equal). S. Gimenez: Conceptualization (supporting); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Supervision (supporting); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). J. R. Galan-Mascaros: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (lead); Methodology (equal); Resources (lead); Supervision (lead); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material and are openly available in the Zenodo repository at https://doi.org/10.5281/zenodo.8413863.75 

1.
S.
Luo
,
K.
Elouarzaki
, and
Z. J.
Xu
, “
Electrochemistry in magnetic fields
,”
Angew. Chem., Int. Ed.
61
,
e202203564
(
2022
).
2.
L. M.
Monzon
and
J.
Coey
, “
Magnetic fields in electrochemistry: The Kelvin force. A mini-review
,”
Electrochem. Commun.
42
,
42
45
(
2014
).
3.
C.
Niether
,
S.
Faure
,
A.
Bordet
,
J.
Deseure
,
M.
Chatenet
,
J.
Carrey
,
B.
Chaudret
, and
A.
Rouet
, “
Improved water electrolysis using magnetic heating of FeC-Ni core-shell nanoparticles
,”
Nat. Energy
3
,
476
483
(
2018
).
4.
W.
Zeng
,
Z.
Jiang
,
X.
Gong
,
C.
Hu
,
X.
Luo
,
W.
Lei
, and
C.
Yuan
, “
Atomic magnetic heating effect enhanced hydrogen evolution reaction of Gd@MoS2 single-atom catalysts
,”
Small
19
,
2206155
(
2022
).
5.
X.
Gong
,
Z.
Jiang
,
W.
Zeng
,
C.
Hu
,
X.
Luo
,
W.
Lei
, and
C.
Yuan
, “
Alternating magnetic field induced magnetic heating in ferromagnetic cobalt single-atom catalysts for efficient oxygen evolution reaction
,”
Nano Lett.
22
,
9411
9417
(
2022
).
6.
S.
Yuan
,
C.
Zhao
,
X.
Cai
,
L.
An
,
S.
Shen
,
X.
Yan
, and
J.
Zhang
, “
Bubble evolution and transport in PEM water electrolysis: Mechanism, impact, and management
,”
Prog. Energy Combust. Sci.
96
,
101075
(
2023
).
7.
X.
Jiang
,
Y.
Chen
,
X.
Zhang
,
F.
You
,
J.
Yao
,
H.
Yang
, and
B. Y.
Xia
, “
Magnetic field-assisted construction and enhancement of electrocatalysts
,”
ChemSusChem
15
,
e202201551
(
2022
).
8.
F. A.
Garcés-Pineda
,
M.
Blasco-Ahicart
,
D.
Nieto-Castro
,
N.
López
, and
J. R.
Galan-Mascaros
, “
Direct magnetic enhancement of electrocatalytic water oxidation in alkaline media
,”
Nat. Energy
4
,
519
525
(
2019
).
9.
X.
Ren
,
T.
Wu
,
Y.
Sun
,
Y.
Li
,
G.
Xian
,
X.
Liu
,
C.
Shen
,
J.
Gracia
,
H.
Gao
,
H.
Yang
, and
Z. J.
Xu
, “
Spin-polarized oxygen evolution reaction under magnetic field
,”
Nat. Commun.
12
,
2608
(
2021
).
10.
C.
Hunt
,
Z.
Zhang
,
K.
Ocean
,
R. P.
Jansonius
,
M.
Abbas
,
D. J.
Dvorak
,
A.
Kurimoto
,
E. W.
Lees
,
S.
Ghosh
,
A.
Turkiewicz
,
F. A.
Garcés Pineda
,
D. K.
Fork
, and
C. P.
Berlinguette
, “
Quantification of the effect of an external magnetic field on water oxidation with cobalt oxide anodes
,”
J. Am. Chem. Soc.
144
,
733
739
(
2022
).
11.
Y.
Zhang
,
P.
Guo
,
S.
Li
,
J.
Sun
,
W.
Wang
,
B.
Song
,
X.
Yang
,
X.
Wang
,
Z.
Jiang
,
G.
Wu
, and
P.
Xu
, “
Magnetic field assisted electrocatalytic oxygen evolution reaction of nickel-based materials
,”
J. Mater. Chem. A
10
,
1760
1767
(
2022
).
12.
P.
Guo
,
Y.
Zhang
,
F.
Han
,
Y.
Du
,
B.
Song
,
W.
Wang
,
X.
Wang
,
Y.
Zhou
, and
P.
Xu
, “
Unveiling the coercivity-induced electrocatalytic oxygen evolution activity of single-domain CoFe2O4 nanocrystals under a magnetic field
,”
J. Phys. Chem. Lett.
13
,
7476
7482
(
2022
).
13.
L.
Lin
,
R.
Xin
,
M.
Yuan
,
T.
Wang
,
J.
Li
,
Y.
Xu
,
X.
Xu
,
M.
Li
,
Y.
Du
,
J.
Wang
,
S.
Wang
,
F.
Jiang
,
W.
Wu
,
C.
Lu
,
B.
Huang
,
Z.
Sun
,
J.
Liu
,
J.
He
, and
G.
Sun
, “
Revealing spin magnetic effect of iron-group layered double hydroxides with enhanced oxygen catalysis
,”
ACS Catal.
13
,
1431
1440
(
2023
).
14.
Y.
Ma
,
T.
Wang
,
X.
Sun
,
Y.
Yao
,
H.
Chen
,
G.
Wu
,
C.
Zhang
, and
Y.
Qin
, “
Enhanced oxygen evolution of a magnetic catalyst by regulating intrinsic magnetism
,”
ACS Appl. Mater. Interfaces
15
,
7978
7986
(
2023
).
15.
P. K.
Sharma
,
M.
Pramanik
,
M. V.
Limaye
, and
S. B.
Singh
, “
Magnetic field-enhanced oxygen evolution in YMn1−xCrxO3 (x = 0, 0.05, and 0.1) perovskite oxides
,”
J. Phys. Chem. C
127
,
16259
16266
(
2023
).
16.
Y.
Cao
,
L.
Gao
,
Z.
Lai
,
C.
Wang
,
Y.
Yao
,
X.
Zhu
, and
Z.
Zou
, “
Constructing spin pathways in LaCoO3 by Mn substitution to promote oxygen evolution reaction
,”
Appl. Phys. Lett.
119
,
163902
(
2021
).
17.
Y.
Miao
,
Q.
Huang
,
D.
Wen
,
D.
Xie
,
B.
Huang
,
D.
Lin
,
C.
Xu
,
W.
Zeng
, and
F.
Xie
, “
One-pot synthesis of NiFe nanoarrays under an external magnetic field as an efficient oxygen evolution reaction catalyst
,”
RSC Adv.
13
,
4249
4254
(
2023
).
18.
X.
Qin
,
J.
Teng
,
W.
Guo
,
L.
Wang
,
S.
Xiao
,
Q.
Xu
,
Y.
Min
, and
J.
Fan
, “
Magnetic field enhancing OER electrocatalysis of NiFe layered double hydroxide
,”
Catal. Lett.
153
,
673
681
(
2023
).
19.
S.
Jiang
,
F.
Chen
,
L.
Zhu
,
Z.
Yang
,
Y.
Lin
,
Q.
Xu
, and
Y.
Wang
, “
Insight into the catalytic activity of amorphous multimetallic catalysts under a magnetic field toward the oxygen evolution reaction
,”
ACS Appl. Mater. Interfaces
14
,
10227
10236
(
2022
).
20.
L.
Luo
,
L.
Xu
,
Q.
Wang
,
Q.
Shi
,
H.
Zhou
,
Z.
Li
,
M.
Shao
, and
X.
Duan
, “
Recent advances in external fields-enhanced electrocatalysis
,”
Adv. Energy Mater.
13
,
2301276
(
2023
).
21.
T. A.
Butcher
and
J. M. D.
Coey
, “
Magnetic forces in paramagnetic fluids
,”
J. Phys.: Condens. Matter
35
,
053002
(
2023
).
22.
H.
Zheng
,
Y.
Wang
,
J.
Xie
,
P.
Gao
,
D.
Li
,
E. V.
Rebrov
,
H.
Qin
,
X.
Liu
, and
H.
Xiao
, “
Enhanced alkaline oxygen evolution using spin polarization and magnetic heating effects under an AC magnetic field
,”
ACS Appl. Mater. Interfaces
14
,
34627
34636
(
2022
).
23.
T.
Wu
and
Z. J.
Xu
, “
Oxygen evolution in spin-sensitive pathways
,”
Curr. Opin. Electrochem.
30
,
100804
(
2021
).
24.
C.
Biz
,
M.
Fianchini
, and
J.
Gracia
, “
Strongly correlated electrons in catalysis: Focus on quantum exchange
,”
ACS Catal.
11
,
14249
14261
(
2021
).
25.
L.
Li
,
J.
Zhou
,
X.
Wang
,
J.
Gracia
,
M.
Valvidares
,
J.
Ke
,
M.
Fang
,
C.
Shen
,
J.
Chen
,
Y.
Chang
,
C.
Pao
,
S.
Hsu
,
J.
Lee
,
A.
Ruotolo
,
Y.
Chin
,
Z.
Hu
,
X.
Huang
, and
Q.
Shao
, “
Spin-polarization strategy for enhanced acidic oxygen evolution activity
,”
Adv. Mater.
35
,
2302966
(
2023
).
26.
Q.
Huang
,
S.
Xie
,
J.
Hao
,
Z.
Ding
,
C.
Zhang
,
H.
Sheng
, and
J.
Zhao
, “
Spin-enhanced O–H cleavage in electrochemical water oxidation
,”
Angew. Chem., Int. Ed.
62
,
e202300469
(
2023
).
27.
A.
Cao
and
J. K.
Nørskov
, “
Spin effects in chemisorption and catalysis
,”
ACS Catal.
13
,
3456
3462
(
2023
).
28.
D.
Kim
,
I.
Efe
,
H.
Torlakcik
,
A.
Terzopoulou
,
A.
Veciana
,
E.
Siringil
,
F.
Mushtaq
,
C.
Franco
,
D.
Von Arx
,
S.
Sevim
,
J.
Puigmartí-Luis
,
B.
Nelson
,
N. A.
Spaldin
,
C.
Gattinoni
,
X.-Z.
Chen
, and
S.
Pané
, “
Magnetoelectric effect in hydrogen harvesting: Magnetic field as a trigger of catalytic reactions
,”
Adv. Mater.
34
,
2110612
(
2022
).
29.
D.
Shao
,
T.
Wu
,
X.
Li
,
X.
Ren
, and
Z. J.
Xu
, “
A perspective of magnetoelectric effect in electrocatalysis
,”
Small Sci.
3
,
2300065
(
2023
).
30.
X.
Lyu
,
Y.
Zhang
,
Z.
Du
,
H.
Chen
,
S.
Li
,
A. I.
Rykov
,
C.
Cheng
,
W.
Zhang
,
L.
Chang
,
W.
Kai
,
J.
Wang
,
L.
Zhang
,
Q.
Wang
,
C.
Huang
, and
E.
Kan
, “
Magnetic field manipulation of tetrahedral units in spinel oxides for boosting water oxidation
,”
Small
18
,
2204143
(
2022
).
31.
X.
Ren
,
T.
Wu
,
Z.
Gong
,
L.
Pan
,
J.
Meng
,
H.
Yang
,
F. B.
Dagbjartsdottir
,
A.
Fisher
,
H.-J.
Gao
, and
Z. J.
Xu
, “
The origin of magnetization-caused increment in water oxidation
,”
Nat. Commun.
14
,
2482
(
2023
).
32.
J.
Ge
,
X.
Ren
,
R. R.
Chen
,
Y.
Sun
,
T.
Wu
,
S. J. H.
Ong
, and
Z. J.
Xu
, “
Multi-domain versus single-domain: A magnetic field is not a must for promoting spin-polarized water oxidation
,”
Angew. Chem., Int. Ed.
62
,
e202301721
(
2023
).
33.
R. C.
Oglou
,
T. G. U.
Ghobadi
,
F. S.
Hegner
,
J. R.
Galan-Mascaros
,
N.
López
,
E.
Ozbay
, and
F.
Karadas
, “
Manipulating intermetallic charge transfer for switchable external stimulus-enhanced water oxidation electrocatalysis
,”
Angew. Chem., Int. Ed.
62
,
e202308647
(
2023
).
34.
H.
Bai
,
J.
Feng
,
D.
Liu
,
P.
Zhou
,
R.
Wu
,
C. T.
Kwok
,
W. F.
Ip
,
W.
Feng
,
X.
Sui
,
H.
Liu
, and
H.
Pan
, “
Advances in spin catalysts for oxygen evolution and reduction reactions
,”
Small
19
,
2205638
(
2023
).
35.
Q.
Xue
,
Y.
Wang
,
M.
Jiang
,
R.
Cheng
,
K.
Li
,
T.
Zhao
, and
C.
Fu
, “
Engineering electronic spin state of a CoNi alloy for an efficient oxygen reduction reaction
,”
ACS Appl. Energy Mater.
6
,
1888
1896
(
2023
).
36.
C. C.
Lin
,
T. R.
Liu
,
S. R.
Lin
,
K. M.
Boopathi
,
C. H.
Chiang
,
W. T.
Tzeng
,
W. H. C.
Chien
,
H. S.
Hsu
,
C. W.
Luo
,
H. Y.
Tsai
,
H. A.
Chen
,
P. C.
Kuo
,
J.
Shiue
,
J. W.
Chiou
,
W. F.
Pong
,
C. C.
Chen
, and
C. W.
Chen
, “
Spin-polarized photocatalytic CO2 reduction of Mn-doped perovskite nanoplates
,”
J. Am. Chem. Soc.
144
,
15718
15726
(
2022
).
37.
Y.
Zhao
,
X.
Wang
,
X.
Sang
,
S.
Zheng
,
B.
Yang
,
L.
Lei
,
Y.
Hou
, and
Z.
Li
, “
Spin polarization strategy to deploy proton resource over atomic-level metal sites for highly selective CO2 electrolysis
,”
Front. Chem. Sci. Eng.
16
,
1772
1781
(
2022
).
38.
P.
Wang
,
Y.
Qu
,
X.
Meng
,
J.
Tu
,
W.
Zheng
,
L.
Hu
, and
Q.
Chen
, “
A strong magnetic field alters the activity and selectivity of the CO2RR by restraining C–C coupling
,”
Magnetochemistry
9
,
65
(
2023
).
39.
C. A.
Mesa
,
E.
Pastor
, and
L.
Francàs
, “
UV–Vis operando spectroelectrochemistry for (photo)electrocatalysis: Principles and guidelines
,”
Curr. Opin. Electrochem.
35
,
101098
(
2022
).
40.
J.
Li
,
Y.
Zhu
,
W.
Chen
,
Z.
Lu
,
J.
Xu
,
A.
Pei
,
Y.
Peng
,
X.
Zheng
,
Z.
Zhang
,
S.
Chu
, and
Y.
Cui
, “
Breathing-mimicking electrocatalysis for oxygen evolution and reduction
,”
Joule
3
,
557
569
(
2019
).
41.
C.
Roy
,
B.
Sebok
,
B.
Scott
,
E. M.
Fiordaliso
,
J. E.
Sørensen
,
A.
Bodin
,
D. B.
Trimarco
,
C. D.
Damsgaard
,
P. C. K.
Vesborg
,
O.
Hansen
,
I. E. L.
Stephens
,
J.
Kibsgaard
, and
I.
Chorkendorff
, “
Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeOxHy
,”
Nat. Catal.
1
,
820
829
(
2018
).
42.
M.
Gong
and
H.
Dai
, “
A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts
,”
Nano Res.
8
,
23
39
(
2015
).
43.
A.
Mandziak
,
J.
de la Figuera
,
S.
Ruiz-Gomez
,
G. D.
Soria
,
L.
Pérez
,
P.
Prieto
,
A.
Quesada
,
M.
Foerster
, and
L.
Aballe
, “
Structure and magnetism of ultrathin nickel-iron oxides grown on Ru(0001) by high-temperature oxygen-assisted molecular beam epitaxy
,”
Sci. Rep.
8
,
17980
(
2018
).
44.
R. L.
Doyle
,
I. J.
Godwin
,
M. P.
Brandon
, and
M. E. G.
Lyons
, “
Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes
,”
Phys. Chem. Chem. Phys.
15
,
13737
13783
(
2013
).
45.
L. J.
Enman
,
M. S.
Burke
,
A. S.
Batchellor
, and
S. W.
Boettcher
, “
Effects of intentionally incorporated metal cations on the oxygen evolution electrocatalytic activity of nickel (oxy)hydroxide in alkaline media
,”
ACS Catal.
6
,
2416
2423
(
2016
).
46.
Y.
Zhang
,
C.
Liang
,
J.
Wu
,
H.
Liu
,
B.
Zhang
,
Z.
Jiang
,
S.
Li
, and
P.
Xu
, “
Recent advances in magnetic field-enhanced electrocatalysis
,”
ACS Appl. Energy Mater.
3
,
10303
10316
(
2020
).
47.
Y.
Liang
,
M.
Lihter
, and
M.
Lingenfelder
, “
Spin-control in electrocatalysis for clean energy
,”
Isr. J. Chem.
62
,
e202200052
(
2022
).
48.
P.
Vensaus
,
Y.
Liang
,
J.-P.
Ansermet
,
G. J. A. A.
Soler-Illia
, and
M.
Lingenfelder
, “
Enhancement of electrocatalysis through magnetic field effects on mass transport
” (
2023
).
49.
S.
Corby
,
M. G.
Tecedor
,
S.
Tengeler
,
C.
Steinert
,
B.
Moss
,
C. A.
Mesa
,
H. F.
Heiba
,
A. A.
Wilson
,
B.
Kaiser
,
W.
Jaegermann
,
L.
Francàs
,
S.
Gimenez
, and
J. R.
Durrant
, “
Separating bulk and surface processes in NiOx electrocatalysts for water oxidation
,”
Sustainable Energy Fuels
4
,
5024
5030
(
2020
).
50.
J.
Noguera-Gómez
,
M.
García-Tecedor
,
J. F.
Sánchez-Royo
,
L. M.
Valencia Liñán
,
M.
de la Mata
,
M.
Herrera-Collado
,
S. I.
Molina
,
R.
Abargues
, and
S.
Gimenez
, “
Solution-processed Ni-based nanocomposite electrocatalysts: An approach to highly efficient electrochemical water splitting
,”
ACS Appl. Energy Mater.
4
,
5255
5264
(
2021
).
51.
A.
Bucci
,
M.
García-Tecedor
,
S.
Corby
,
R. R.
Rao
,
V.
Martin-Diaconescu
,
F. E.
Oropeza
,
V. A.
de la Peña O'Shea
,
J. R.
Durrant
,
S.
Giménez
, and
J.
Lloret-Fillol
, “
Self-supported ultra-active NiO-based electrocatalysts for the oxygen evolution reaction by solution combustión
,”
J. Mater. Chem. A
9
,
12700
12710
(
2021
).
52.
J.
Zhang
,
W.
Gong
,
H.
yin
,
D.
Wang
,
Y.
Zhang
,
H.
Zhang
,
G.
Wang
, and
H.
Zhao
, “
In situ growth of ultrathin Ni(OH)2 nanosheets as catalyst for electrocatalytic oxidation reactions
,”
ChemSusChem
14
,
2935
2942
(
2021
).
53.
F. A.
Garces-Pineda
,
H.
Chuong Nguyën
,
M.
Blasco-Ahicart
,
M.
García-Tecedor
,
M.
de Fez Febré
,
P.
Tang
,
J.
Arbiol
,
S.
Giménez
,
J. R.
Galan-Mascaros
, and
N.
López
, “
Push-pull electronic effects in surface-active sites enhance electrocatalytic oxygen evolution on transition metal oxides
,”
ChemSuschem
14
,
1595
1601
(
2021
).
54.
R.
Fernández-Climent
,
J.
Redondo
,
M.
García-Tecedor
,
M. C.
Spadaro
,
J.
Li
,
D.
Chartrand
,
F.
Schiller
,
J.
Pazos
,
M. F.
Hurtado
,
V.
de la Peña O’Shea
,
N.
Kornienko
,
J.
Arbiol
,
S.
Barja
,
C. A.
Mesa
, and
S.
Gimenez
, “
Highly durable nanoporous Cu2−xS films for efficient hydrogen evolution electrocatalysis under mild pH conditions
,”
ACS Catal.
13
,
10457
10467
(
2023
).
55.
L.
Trotochaud
,
S. L.
Young
,
J. K.
Ranney
, and
S. W.
Boettcher
, “
Nickel–iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation
,”
J. Am. Chem. Soc.
136
,
6744
6753
(
2014
).
56.
L.
Francàs
,
S.
Corby
,
S.
Selim
,
D.
Lee
,
C. A.
Mesa
,
R.
Godin
,
E.
Pastor
,
I. E. L.
Stephens
,
K.-S.
Choi
, and
J. R.
Durrant
, “
Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts
,”
Nat. Commun.
10
,
5208
(
2019
).
57.
D. A.
Kuznetsov
,
B.
Han
,
Y.
Yu
,
R. R.
Rao
,
J.
Hwang
,
Y.
Roman-Leshkov
, and
Y.
Shao-Horn
, “
Tuning redox transitions via inductive effect in metal oxides and complexes, and implications in oxygen electrocatalysis
,”
Joule
2
,
225
244
(
2018
).
58.
I. C.
Man
,
H.
Su
,
F.
Calle-Vallejo
,
H. A.
Hansen
,
J. I.
Martínez
,
N. G.
Inoglu
,
J.
Kitchin
,
T. F.
Jaramillo
,
J. K.
Nørskov
, and
J.
Rossmeisl
, “
Universality in oxygen evolution electrocatalysis on oxide surfaces
,”
ChemCatChem
3
,
1159
1165
(
2011
).
59.
Y.
Zhou
and
N.
López
, “
The role of Fe species on NiOOH in oxygen evolution reactions
,”
ACS Catal.
10
,
6254
6261
(
2020
).
60.
J.
Gallenberger
,
H.
Moreno Fernández
,
A.
Alkemper
,
M.
Li
,
C.
Tian
,
B.
Kaiser
, and
J. P.
Hofmann
,
Catal. Sci. Technol.
13
,
4693
4700
(
2023
).
61.
R. D. L.
Smith
,
C.
Pasquini
,
S.
Loos
,
P.
Chernev
,
K.
Klingan
,
P.
Kubella
,
M. R.
Mohammadi
,
D.
González-Flores
, and
H.
Dau
, “
Geometric distortions in nickel (oxy)hydroxide electrocatalysts by redox inactive iron ions
,”
Energy Environ. Sci.
11
,
2476
2485
(
2018
).
62.
L.
Bai
,
S.
Lee
, and
X.
Hu
, “
Spectroscopic and electrokinetic evidence for a bifunctional mechanism of the oxygen evolution reaction
,”
Angew. Chem., Int. Ed.
60
,
3095
3103
(
2021
).
63.
S.
Lee
,
Y.-C.
Chu
,
L.
Bai
,
H. M.
Chen
, and
X.
Hu
, “
Operando identification of a side-on nickel superoxide intermediate and the mechanism of oxygen evolution on nickel oxyhydroxide
,”
Chem Catal.
3
,
100475
(
2023
).
64.
F.
Le Formal
,
E.
Pastor
,
S. D.
Tilley
,
C. A.
Mesa
,
S. R.
Pendlebury
,
M.
Grätzel
, and
J. R.
Durrant
, “
Rate law analysis of water oxidation on a hematite surface
,”
J. Am. Chem. Soc.
137
,
6629
6637
(
2015
).
65.
Y.
Ma
,
C. A.
Mesa
,
E.
Pastor
,
A.
Kafizas
,
L.
Francàs
,
F.
Le Formal
,
S. R.
Pendlebury
, and
J. R.
Durrant
, “
Rate law analysis of water oxidation and hole scavenging on a BiVO4 photoanode
,”
ACS Energy Lett.
1
,
618
623
(
2016
).
66.
C. A.
Mesa
,
L.
Francàs
,
K. R.
Yang
,
P.
Garrido-Barros
,
E.
Pastor
,
Y.
Ma
,
A.
Kafizas
,
T. E.
Rosser
,
M. T.
Mayer
,
E.
Reisner
,
M.
Grätzel
,
V. S.
Batista
, and
J. R.
Durrant
, “
Multihole water oxidation catalysis on haematite photoanodes revealed by operando spectroelectrochemistry and DFT
,”
Nat. Chem.
12
,
82
89
(
2020
).
67.
C.
Bozal-Ginesta
,
R. R.
Rao
,
C. A.
Mesa
,
X.
Liu
,
S. A. J.
Hillman
,
I. E.
Stephens
, and
J. R.
Durrant
, “
Redox-state kinetics in water-oxidation IrOx electrocatalysts measured by operando spectroelectrochemistry
,”
ACS Catal.
11
,
15013
15025
(
2021
).
68.
C. A.
Mesa
,
A.
Kafizas
,
L.
Francàs
,
S. R.
Pendlebury
,
E.
Pastor
,
Y.
Ma
,
F.
Le Formal
,
M. T.
Mayer
,
M.
Grätzel
, and
J. R.
Durrant
,
J. Am. Chem. Soc.
139
,
11537
11543
(
2017
).
69.
A.
Kafizas
,
Y.
Ma
,
E.
Pastor
,
S. R.
Pendlebury
,
C. A.
Mesa
,
L.
Francas
,
F.
Le Formal
,
N.
Noor
,
M.
Ling
,
C.
Sotelo-Vazquez
,
C. J.
Carmalt
,
I. P.
Parkin
, and
J. R.
Durrant
, “
Water oxidation kinetics of accumulated holes on the surface of a TiO2 photoanode: A rate law analysis
,”
ACS Catal.
7
,
4896
4903
(
2017
).
70.
R. R.
Rao
,
S.
Corby
,
A.
Bucci
,
M.
García-Tecedor
,
C. A.
Mesa
,
J.
Rossmeisl
,
S.
Giménez
,
J.
Lloret-Fillol
,
I. E. L.
Stephens
, and
J. R.
Durrant
, “
Spectroelectrochemical analysis of the water oxidation mechanism on doped nickel oxides
,”
J. Am. Chem. Soc.
144
,
7622
7633
(
2022
).
71.
T.
Wu
,
X.
Ren
,
Y.
Sun
,
S.
Sun
,
G.
Xian
,
G. G.
Scherer
,
A. C.
Fisher
,
D.
Mandler
,
J. W.
ager
,
A.
Grimaud
,
J.
Wang
,
C.
Shen
,
H.
Yang
,
J.
Gracia
,
H.
Gao
, and
Z. J.
Xu
, “
Spin pinning effect to reconstructed oxyhydroxide layer on ferromagnetic oxides for enhanced water oxidation
,”
Nat. Commun.
12
,
3634
(
2021
).
72.
S.
Plana-Ruiz
,
J.
Portillo
,
S.
Estradé
,
F.
Peiró
,
U.
Kolb
, and
S.
Nicolopoulos
, “
Quasi-parallel precession diffraction: Alignment method for scanning transmission electron microscopes
,”
Ultramicroscopy
193
,
39
51
(
2018
).
73.
Y.
Seto
and
M.
Ohtsuka
, “
ReciPro: Free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools
,”
J. Appl. Cryst.
55
,
397
410
(
2022
).
74.
E.
Pastor
,
F.
Le Formal
,
M. T.
Mayer
,
S. D.
Tilley
,
L.
Francàs
,
C. A.
Mesa
,
M.
Grätzel
, and
J. R.
Durrant
, “
Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface
,”
Nat. Commun.
8
,
14280
(
2017
).
75.
C. A.
Mesa
,
F. A.
Garcés-Pineda
,
M.
García-Tecedor
,
J.
Yu
,
B.
Khezri
,
S.
Plana-Ruiz
,
B.
López
,
R.
Iturbi
,
N.
López
, and
J. R.
Galan-Mascaros
(
2023
). “Experimental evidence for the direct influence of external magnetic fields on the mechanism of the electrocatalytic oxygen evolution reaction,”
Zenodo
.https://doi.org/10.5281/zenodo.8413862

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