Water-soluble nickel and iron salts for hydroxymethylfurfural (HMF) and water oxidation: the simplest precatalysts?

Abstract

Electrochemical production of large-scale chemicals and fuels is critical to reaching carbon neutrality. However, the required anodic oxidation reactions, namely the oxygen evolution reaction (OER) or the oxidation of organics into value-added products, suffer from large overpotentials. To address this challenge, researchers have been widely investigating non-water-soluble (pre)catalysts to operate in the aqueous electrolyte. On the contrary, in this work, we approach a rapid, easy, and green carbon cloth electrode preparation using merely water-soluble nitrate precursors and ethanol as chemicals and no heating steps. The drop-coated, water-soluble transition metal salts reconstruct rapidly into the respective oxyhydroxides under OER conditions, with the oxyanion acting as a beneficial sacrificial reagent. This approach is shown herein for nickel(–iron) catalysts and their successful application for the OER (220 mV overpotential at 10 mA cm⁻², long-term stability of 40 h at 100 mA cm⁻²) and the oxidation of 5-hydroxymethylfurfural (HMF, quantitative faradaic efficiency). We compare both reactions with both electrodes closely and find that the iron-free sample is more active for the HMF oxidation in regimes where mass transport is not the main limiting factor. We anticipate that this simple electrode preparation approach can find wide application in electrocatalysis and beyond.

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Introduction

Fossil fuels and petrochemicals must be replaced by carbon-neutral alternatives to stop climate change and close the carbon cycle.¹ The most promising alternatives are hydrogen and hydrocarbons produced electrocatalytically through water or carbon dioxide reduction, respectively.^2–4 These (cathodic) reduction reactions require protons and electrons that are supplied by their (anodic) oxidative counterpart. This anodic counterpart can be the oxygen evolution reaction (OER) or an organic oxidation reaction (OOR, hybrid water splitting).¹ The OER has the advantage that only water is required as substrate; thus, it can be applied on a large scale.^1,5 However, the OER produces no value-added compound. The advantage of organic oxidation reactions is that they require thermodynamically less energy and can be value-added, e.g., 5-hydroxymethylfurfural (HMF) oxidation can yield 2,5-furandicarboxylic acid (FDCA), a biomass-derived, value-added precursor for sustainable polymers.^4,6,7 However, the demand for organic oxidation products is much smaller than for hydrogen and hydrocarbon fuels.¹ Thus, the OOR can only partly replace the OER, and both reactions are crucial.¹ Nevertheless, both reactions still suffer from large overpotentials, substantially lowering the efficiency of electrolysers.¹ Thus, new catalysts that combine a high activity with a fast, green, cheap, and straightforward synthesis must be developed.

So far, a prerequisite for OER and OOR (pre)catalysts has been that they are non-water-soluble, as water is the main component of the electrolyte, leading to the potential loss of the electrodes’ catalyst coating. Therefore, various non-water-soluble compounds have been investigated, such as transition metal (oxy)hydroxide,^8–10 borides,¹¹ pnictides,¹² chalcogenides,^13,14 alloys,¹⁵ or intermetallics.^16–20 These materials comprise an active site transition metal, such as iron, cobalt, or nickel, and additional constituting elements. Under alkaline OER conditions, iron, cobalt, and nickel form oxyhydroxides, and the additional constituting elements oxidise, forming oxyanions.^19,21–24 These formed oxyanions leach into the electrolyte.^13,23 Thus, the additional constituents act as a sacrificial reagent, helping to create a high surface area, defect-rich, and catalytically more active iron/cobalt/nickel oxyhydroxide phase.^13,19 Considering the inevitable and beneficial reconstruction process of the applied precatalysts, we projected that the simplest, commercially available transition metal salts, such as nitrates/sulfates/acetates, could also be suitable precatalysts, despite their water solubility, as they could rapidly reconstruct to the respective non-soluble oxyhydroxides while the anion acts as a beneficial sacrificial reagent.²⁵ Furthermore, such species can be dissolved and then drop-coated, enabling a simple, rapid, and reliable deposition. Such cheap, water-soluble, readily available precatalysts could enormously simplify synthesising catalysts with various transition metals, accelerating electrocatalyst development in general.

Nickel and nickel–iron are among the promising non-noble metals for the alkaline OER and OOR, making them suitable to test our water-soluble precatalyst approach.^6,15,26–31 Purely nickel-based systems are usually mediocre OER catalysts as they too strongly bind the first OER intermediate, *OH formed in the nickel(II) oxidation peak.^32–34 The addition of iron substantially improves the OER activity by shifting the *OH formation to higher energies/potentials, optimising the OER intermediate binding strengths.³² For nickel- and nickel–iron-based catalysts, the OER and OOR share this first *OH reaction intermediate.^32,35,36 Thus, these systems can be bifunctional for both processes. While the superiority of nickel–iron-based systems for the OER is unambiguous,³⁷ it is controversial which system performs better for the OOR.^{35,36,38–40} Some of the best OOR performances have been reported for nickel–iron-based systems.^38–41 However, other reports showed that monometallic, nickel-based systems could outperform bimetallic nickel–iron ones due to an easier formation of the *OH intermediate and a better suppression of the OER, an unwanted side reaction during OOR.^{6,32,35,36,42,43} Thus, another report comparing nickel- and nickel–iron-based systems closely for the OER and OOR is valuable.

Herein, we used simple nickel and iron nitrate precursors that we dissolved in ethanol and drop-casted on a metal-free electrode substrate, carbon cloth. This approach leads to the deposition of the respective nitrates. Under OER conditions, these nitrates reconstruct within seconds into the respective oxyhydroxides. The oxyhydroxide-loaded electrodes can be rapidly prepared, their loading straightforwardly controlled, their synthesis requires no additional chemicals besides ethanol, and they achieve an excellent OER (overpotential of 220 mV at 10 mA cm⁻²) and OOR performance (quantitative faradaic efficiency). Furthermore, we find that the nickel electrode performs better than the nickel–iron one for the OOR as it can sustain a quantitative faradaic efficiency until higher potentials and forms the reactive *OH intermediate more easily. However, in regimes where mass transport is strongly limiting, nickel- and nickel–iron-based systems can show the same conversion rate and the kinetic superiority of the monometallic nickel sample can be missed, explaining previous observations.

Results and discussion

Drop coating of the water-soluble nickel and iron nitrate salts

To realise the drop-coating of the water-soluble precursors, we choose carbon cloth as substrate, as it is transition-metal-free, cheap, conducting, comparably inert, has widely been applied for anodic oxidation reactions already, has a reasonably high surface area, and can soak in solutions.⁴⁴ As transition metal salt, we choose nickel and iron nitrate, as they are cheap, widely applied, readily available, and can be obtained in high purities. Ethanol proved to be more suitable for drop-coating than water as it evaporates more quickly, and both nitrate salts are readily soluble inside. In conclusion, this approach requires only the electrode substrate, a pipette, metal precursors, ethanol, and not a single additional chemical, binder, or machinery.²⁵ Furthermore, the drop coating can be performed within seconds, and no heating step is required, making this approach sustainable.⁴⁵ Additionally, in contrast to most electrodeposition approaches,^46,47 the loading can be controlled straightforwardly without using quartz microbalances, and this approach should be suitable for various transition metals, as it just requires a soluble precursor.

Fig. 1 shows in detail how the electrodes were prepared following these considerations. One electrode was drop-coated using pure nickel nitrate solution (NiNO₃-CC) and the other using a nickel–iron nitrate solution in a 4 : 1 molar ratio (NiFeNO₃-CC). NiNO₃-CC and NiFeNO₃-CC were characterised by powder X-ray diffraction (PXRD, Fig. S1) infrared (Fig. S2) and Raman spectroscopy (Fig. S3†) showing that nickel(–iron) nitrate hexahydrate formed on the carbon cloth after drying at ambient conditions.

Fig. 1 Schematic illustration of the electrode preparation. Commercial carbon cloth (CC) was activated through acid treatment (HNO₃). Ni(NO₃)₂ and a mixture of 80% Ni(NO₃)₂ and 20% Fe(NO₃)₃ was drop coated onto the carbon cloth and the substrate was dried under air. Subsequently, the electrodes were activated at 1.63 V_RHE in 1.0 M KOH for 30 minutes to ensure full reconstruction into the respective oxyhydroxides.

Reconstruction to nickel(–iron) oxyhydroxides

The next step is the reconstruction of the transition metal nitrate precursors into the respective oxyhydroxides. Here, it is essential to note that the nitrate salts are water-soluble. However, in alkaline conditions, (oxy)hydroxides are thermodynamically most stable and should form over time in accordance with the Pourbaix diagrams.⁴⁸ Thus, to enable a complete and rapid reconstruction with beneficial nitrate leaching, an anodic potential of 1.63 V_RHE was immediately applied when the electrodes entered the 1 M KOH electrolyte (pH 13.89).⁴⁹ The current responses (Fig. S4†) show an initial peak that we assign to the precatalyst oxidation, followed by a relatively stable current that we assign to the OER. The precatalyst oxidation completes within seconds. To ensure that the material reached a steady state, we kept it for 30 min under the OER potential.

The electrodes reconstructed for 30 min were washed with demineralised water and then characterised. These electrodes are called NiNO₃-CC-OER and NiFeNO₃-CC-OER, depending on the drop-coated transition metal. The PXRDs of both electrodes contain no sharp peaks (Fig. S1†), revealing that the crystalline nitrates have been reconstructed into X-ray amorphous phases. The infrared spectra of these amorphous phases are comparable to those of nickel(–iron) oxyhydroxides with intercalated carbonate (Fig. S2†).⁵⁰ In- and ex situ Raman data (Fig. 2a and d) further supports the formation of oxyhydroxides as the in situ Raman data shows the typical bending δ(M^III-O) 470 cm⁻¹ and stretching ν(M^III-O) 566 cm⁻¹ vibrations of Ni(Fe)OOH. The formation of such intercalated oxyhydroxides during OER has been widely reported in the literature.^{12,23,29,51,52} For both phases, SEM-EDX mappings (Fig. 2a and b, for EDX spectra see Fig. S5 and S6†) disclose that nickel, (iron), oxygen, and potassium are homogeneously distributed. As reported previously, the potassium is most likely intercalated into the oxyhydroxide structure.^12,23,29 For both compounds, transmission electron microscopy (TEM, Fig. 2c and f) shows porous, unordered, sheetlike morphologies. Even though both compounds did not show diffraction peaks in our PXRD setup, the selected area electron diffraction (SAED) pattern reveals that both are nanocrystalline. The diffraction rings of the SAED are typical for transition metal oxyhydroxides formed during the OER.^12,53–56 The fast Fourier transforms of high-resolution TEM images confirm the lattice distances from the SAED, and the images show around 2 nm large crystallites, explaining the absence of diffraction in the PXRD. Thus, through the reconstruction of the water-soluble nitrates, porous, nanocrystalline, layered oxyhydroxides have formed.

Fig. 2 (a) and (d) in situ Raman spectra of NiNO₃-CC/NiFeNO₃-CC and NiNO₃-CC-OER/NiFeNO₃-CC-OER (see Fig. 1 for sample abbreviations) at 1.10 V_RHE and 1.55 V_RHE, showing the formation of oxyhydroxides. The vertical dashed lines mark the prominent vibrations of the [MO₆] octahedra. The spectra of the as prepared electrodes can be found in Fig. S3.† (b) SEM image of NiNO₃-CC-OER and SEM/EDX mapping of NiNO₃-CC-OER showing a homogeneous distribution of nickel and oxygen as well as the incorporation of potassium from the electrolyte during activation. The EDX spectra can for the mapping can be found in Fig. S5† (c) TEM images of NiNO₃-CC-OER. (e) SEM image of NiFeNO₃-CC-OER and SEM/EDX mapping of NiFeNO₃-CC-OER showing a homogeneous distribution of nickel, iron, and oxygen as well as the incorporation of potassium from the electrolyte during activation. The EDX spectra can for the mapping can be found in Fig. S6† (f) TEM images of NiFeNO₃-CC-OER.

Electrocatalytic oxygen evolution reaction (OER)

First, a loading study (0.025–4.0 mg cm⁻², Fig. 3a) concerning the OER activity at 400 mV overpotential (η) was conducted for NiNO₃-CC-OER and NiFeNO₃-CC-OER. For both, it shows that increasing the loading to 2 mg cm⁻² leads to a substantial rise in the current density. Therefore, a loading of 2 mg cm⁻² have been used throughout this manuscript. For loadings above 2 mg cm⁻², the current density quickly reaches a plateau, indicating a limited utilisation of the additionally loaded catalyst. Normalising the current density per loaded transition metal confirms this limited utilisation, as it keeps decreasing at higher loadings (yellow oval in Fig. 3a).

Fig. 3 Electrochemical data, all measurements were conducted in 1.0 M KOH in a three-electrode setup with platinum as the counter electrode with Hg/HgO reference. See Fig. 1 for the naming of the samples. (a) Loading study of NiNO₃-CC-OER (green) and NiFeNO₃-CC-OER (blue) per geometric current density (solid lines with squares) with normalisations per loaded metal (dashed lines with triangles) and per redox active sites (dotted lines with circles) as determined in Fig. S7.† (b) Cyclic Voltammetry of NiNO₃-CC-OER and NiFeNO₃-CC-OER at 5 mV s⁻¹ with and without 50 mM HMF. (c) Tafel plots of NiNO₃-CC-OER and NiFeNO₃-CC-OER acquired through steady-state measurements. (d) 40-hour Stability measurement of NiNO₃-CC-OER and NiFeNO₃-CC-OER through chronopotentiometry at 100 mA cm⁻². (e) Potential versus current density through steady-state CA. “Faradaic efficiency” is the current portion that contributes to HMFOR at a given potential in per cent, assuming the OER current is identical with and without HMF. (f) CA data of a HMF bulk oxidation at 1.43 V_RHE with 600 rpm stirring conversion of HMF to FDCA on the right y-axis.

To understand this decrease in activity per loaded transition metal, we determined the number of redox-active sites, an often-used descriptor for OER active site availability.^28,35,53 The redox activity has been measured by a previously suggested potential step method (Fig. S7†) and used to normalise the current density.^57,58 The normalised current decreases initially but reaches a plateau at high loadings (3 mg cm⁻²) together with the geometric current density (red ovals in Fig. 3a). Thus, higher loadings lead to almost no new redox active sites and simultaneously to no increase in the geometric current density. Prerequisites for redox activity and OER availability of a site are electrolyte access and electric connection to the anode. For high loadings, the additionally loaded nickel/iron sites do not fulfil these two criteria and thus cannot participate in electrocatalysis.

We note here that double-layer capacitance measurements by cyclic voltammetry (CV) cannot be applied for these materials, as their conductivity is insufficient at potentials below the redox peak.^{24,30,37,59,60} For our system, previously suggested impedance-based method were not successful, as the suggested simulation models cannot reproduce our data.^59,60

The electrocatalytic properties towards the OER in 1 M KOH were examined for both NiNO₃-CC-OER and NiFeNO₃-CC-OER. The CV reverse scans shown in Fig. 3(b) reveal an overpotential at 10 mA cm⁻² (η₁₀) of 290 mV for NiNO₃-CC-OER and 220 mV for NiFeNO₃-CC-OER. Both values were confirmed by chronopotentiometry measurements, maintaining these potentials for 30 minutes (Fig. S8†). These overpotentials compare favourably with previously reported ones (Table S1†). Furthermore, the incorporation of iron shifts the metal oxidation feature to higher potentials, overlapping with the onset of the OER, which has been reported previously.^28,35,61 The contribution of carbon cloth was also investigated by CV, revealing current densities below 1 mA cm⁻² at relevant potentials (Fig. S9†). Steady-state Tafel analysis was conducted to get insights into the kinetics of the different electrodes. Fig. 3c reveals that the estimated Tafel slope of the NiNO₃-CC-OER electrode is 56 mV dec⁻¹ and thus significantly larger than for NiFeNO₃-CC-OER, which is 32 mV dec⁻¹. Such a difference has already been reported for iron incorporation. It is consistent with the substantially better OER performance of the nickel–iron oxyhydroxides, where most likely edge-site, bimetallic, adjacent nickel–iron sites are responsible for the OER.^29,30,54

We also examined the long-term stability to ensure that the simple drop-coating preparation method yielded stable electrodes. Fig. 3d reveals excellent stability over 40 hours at 100 mA cm⁻². After this stability test, SEM images (Fig. S10 and 11†) with elemental mappings still show a homogeneous distribution of nickel, iron, oxygen, and potassium. Furthermore, no significant change in the morphology compared to our investigations after 30 min (Fig. 2b and e) can be seen. The XRD pattern after the 40 h stability test reveals that the materials remain amorphous (Fig. S12†).

Electrocatalytic hydroxymethylfurfural (HMF) oxidation

Besides the OER, we also investigated the electrocatalytic capabilities of our drop-coated NiNO₃-CC-OER and NiFeNO₃-CC-OER electrodes towards the conversion of 5-hydroxymethylfurfural (HMF) to furan dicarboxylic acid (FDCA), a promising sustainable biopolymer building block. First, cyclic voltammetry was conducted in 1 M KOH electrolyte without stirring and with the addition of 50 mM HMF (Fig. 3b). For a NiNO₃-CC-OER, without HMF, a nickel(II) oxidation peak onsets at ∼1.34 V_RHE; with HMF, larger current onsets around the same potential. This additional current is assigned to the oxidation of HMF.^35,38,62 In the nickel(II) oxidation peak nickel(II) is oxidised to nickel(III) and Ni^III+δ-O^{−II+(1−δ)} (around 1.6 electrons per nickel are transferred and oxygen is redox non-innocent).^{28,35,54,63,64} Two different mechanisms for the HMFOR have been proposed in the literature, depending on the applied reaction conditions and catalysts (see Fig. S13†)^{6,36,40,62,65} (i) indirect oxidation, where the metal acts as a redox mediator that changes its valence state, (ii) and direct oxidation, which is potential dependent and directly oxidises HMF involving Ni^III+δ-O^{−II+(1−δ)}. Nickel-based catalysts in strongly alkaline electrolytes (pH > 13) are known to favour the indirect oxidation pathway. Ni(OH)₂ is oxidised to NiOOH, which is a potential-dependent step. The subsequent HMF oxidation, however, is influenced by the number of active sites that can be reduced back to Ni(OH)₂ and the intrinsic catalytic activity of the catalyst.^{6,36,40,62,65} The enlarged oxidation peak in the forward scan of the HMF containing CVs is consistent with this hypothesis, as NiOOH is reduced to Ni(OH)₂ by HMF several times in the timespan of one CV resulting in multiple oxidations of the same Ni(OH)₂ site enlarging the nickel(II) oxidation peak. Contrary to this, the reduction peak in the back scan is less pronounced because the nickel(III) is reduced through the indirect HMF oxidation catalytic cycle, rather than the applied potential.⁶ For NiFeNO₃-CC-OER, a similar behaviour can be observed, but the currents assigned to the oxidation peak and HMF oxidation are significantly smaller.

Fig. 3c also includes steady-state Tafel plots of the two electrodes in HMF containing 1 M KOH. HMF oxidation begins at an overpotential below 25 mV for NiNO₃-CC-OER, while for NiFeNO₃-CC-OER, it starts at 107 mV. The curves have a more complex shape than the ones in HMF-free electrolyte and lack a long linear range that could be used to unambiguously assign a Tafel slope for the HMF oxidation.

Before a closer look at the competition between the OER and HMF oxidation is taken, we investigated the effect of substrate concentration and stirring speed on the HMF oxidation. In this regard, we conducted chronoamperometry measurements (1.41 V_RHE) with different stirring speeds for our system. Interestingly, without stirring, NiNO₃-CC-OER does not outperform NiFeNO₃-CC-OER as both electrodes only exhibit current densities under 20 mA cm⁻², although one would have assumed a better performance for NiNO₃-CC-OER from the CVs. However, higher stirring speeds significantly increase the current density, whereby the effect is four times stronger for NiNO₃-CC-OER, increasing the current density from 17 mA cm⁻² to 100 mA cm⁻² (Fig. S14†). For NiFeNO₃-CC-OER, even with rigorous stirring speeds, the current density does not increase to more than 40 mA cm⁻². The results highlight that mass transport limitations play a vital role for the catalytic turnover. In our unstirred system, these mass transport limitations are the decisive factor. Thus, the intrinsic activity of the catalyst cannot be measured and two catalysts with different intrinsic activities might perform similarly. However, at faster stirring speeds, the electrocatalytic activity of NiFeNO₃-CC-OER is limiting the overall performance, while NiNO₃-CC-OER's current density is substantially higher. We suggest that these mass transport limitations might be the reason why some reports have missed the superior activity of purely nickel-based systems compared to nickel–iron-based ones.

When HMF oxidation is the desired reaction, OER is an unwanted side reaction. However, at sufficiently large potential, both processes can co-occur. To investigate these two competing reactions as a function of potential, we have acquired steady-state data (3 min chronoamperometry for each data point) in stirred, HMF-free and HMF-containing 1 M KOH electrolyte (Fig. 3e). Only catalytic processes (HMF oxidation and OER) contribute to the current density of the steady-state measurements.⁶⁶ This steady-state data can be used to determine a potential at which only HMF oxidation and little OER takes place. For NiNO₃-CC-OER and NiFeNO₃-CC-OER, the HMF onset is around 1.3 V_RHE, and the current density increases with positive potential. Then, at higher potential, the OER starts to interfere, the range between can be considered suitable for selective HMF oxidation. For NiNO₃-CC-OER, this interference is less due to its 70 mV higher OER overpotential, enabling selective HMF oxidation at higher potentials and thus higher conversion rates. Additionally, using the steady-state data, we calculated and added to Fig. 3e a “faradaic efficiency” for HMF oxidation under the estimation that the OER turnover is the same with and without HMF. Consequently, Fig. 3e can be used to estimate the trade-off between turnover and faradaic efficiency for the HMF oxidation as a function of the anode potential.

Following these investigations, we choose to perform a bulk HMF oxidation with stirring and at a potential of 1.43 V_RHE in 1.0 M KOH with 50 mM HMF. While higher HMF concentrations are beneficial for the current density, the reaction time will also increase. As HMF is known to be unstable under strongly alkaline conditions,⁶⁷ a quick, complete conversion is vital for high yields. Furthermore, at the chosen potential, both catalysts should have a good selectivity towards HMF oxidation (Fig. 3e). NiNO₃-CC-OER passed the charge necessary for 100% theoretical conversion in almost half the time (170 min) of NiFeNO₃-CC-OER (300 min), as can be seen from Fig. 3f. Both reactions start at reasonable high current densities of 120 mA cm⁻² and 50 mA cm⁻², respectively, which steadily decline until the necessary charge was passed, due to less substrate availability. The reaction mixture was investigated by ¹H-NMR before (Fig. S15) and after (Fig. S16 and S17†) the bulk oxidation. An internal standard was used to quantify the products and calculate the faradaic efficiencies for each catalyst. NiNO₃-CC-OER fully converted HMF into FDCA with a faradaic efficiency of 98%, while NiFeNO₃-CC-OER could only achieve 88%, indicating that the OER significantly contributed to the current density. The results fit our other findings and support that purely nickel-based systems are superior for HMF oxidation compared to those with iron incorporated.

Conclusions

We have demonstrated a rapid, simple and sustainable approach for synthesising water-soluble precursors that can be rapidly reconstructed into highly active catalysts for the oxygen evolution reaction (OER) and HMF oxidation. By drop-coating commercially available nickel and iron nitrate precursors onto carbon cloth electrodes, we achieved the deposition of the respective nitrates, which subsequently reconstructed rapidly into amorphous nickel–iron oxyhydroxide phases under OER conditions. The resulting nanocrystalline oxyhydroxide electrodes exhibited excellent OER performance, with an overpotential of 220 mV at 10 mA cm⁻².

The incorporation of iron in the nickel-based system shifted the oxidation of the *OH intermediate to higher energies, optimising the OER intermediate binding strengths and enhancing the OER activity. Furthermore, the nickel-based system demonstrated superior performance in the oxidation of HMF, sustaining quantitative faradaic efficiency until higher potentials and facilitating the formation of the reactive *OH intermediate. Our findings shed light on the performance comparison between nickel-based and nickel–iron-based systems for both the OER and HMFOR. While nickel–iron-based systems showed clear superiority in the OER, the OOR performance was worse. Previous reports have suggested that monometallic nickel systems could outperform bimetallic nickel–iron systems due to easier *OH intermediate formation and a better suppression of the OER during HMFOR. However, in regimes where mass transport limitations are significant, both systems may exhibit similar conversion rates, potentially overlooking the kinetic superiority of the monometallic nickel sample.

The drop-coating method utilising water-soluble precursors offers several advantages, including simplicity, a wide application range, quickness, and cost-effectiveness. It eliminates the need for additional chemicals and enables precise control over catalyst loading. Moreover, this approach can be extended to various transition metals by utilising soluble precursors, thereby accelerating the development of novel (multimetallic) catalysts for electrocatalytic conversions.

Author contributions

J. N. H. initiated the idea and developed it together with T. K. and P. W. M. T. K. prepared the electrodes, performed all electrocatalytic measurements, analysed the data. IR and TEM were measured and analysed by J. N. H. SEM experiments were conducted by I. M. K. L. and I. Z conducted in situ Raman experiments. T. S. helped in the investigation. T. K. and J. N. H. wrote the first draft. P. W. M. discussed the results, commented on the manuscript, and supervised the project.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Funded by the German Federal Ministry of Education and Research (BMBF) in the framework of the project Catlab (03EW0015A/B). P. W. M. acknowledges the support from the BMBF project “PrometH2eus”, 03HY105C. The authors thank Prof. Dr Matthias Driess for the infrastructure.

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Footnotes

† Electronic supplementary information (ESI) available: Containing all experimental procedures and characterisation details as well as additional data. See DOI: https://doi.org/10.1039/d3gc02119j

‡ These authors contributed equally to this work.

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