# Ni-Based Catalysts for 5-Hydroxymethylfurfural Electrooxidation Coupled with Hydrogen Production

I. Garduno-Ibarra, Z. Yan, S. Ebrahim, E. Baranova, J. Gonzalez-Cobos, M.

Prevot, P. Vernoux

► To cite this version:

I. Garduno-Ibarra, Z. Yan, S. Ebrahim, E. Baranova, J. Gonzalez-Cobos, et al., Ni-Based Catalysts for 5-Hydroxymethylfurfural Electrooxidation Coupled with Hydrogen Production. ChemElectroChem, 2025, 10.1002/celc.202500067. [https://hal.science/hal-05086010](https://hal.science/hal-05086010)

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Submitted on 7 Nov 2025

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# Ni-Based Catalysts for 5-Hydroxymethylfurfural Electrooxidation Coupled with Hydrogen Production

Itzcóatl Rafael Garduño-Ibarra, Zhigang Yan, Sayed Ahmed Ebrahim, Elena Baranova, Jesús González-Cobos, Mathieu Prévot, and Philippe Vernoux*

This review presents a comprehensive analysis of Ni-based catalysts for the co-electrolysis of H₂O and 5-hydroxymethylfurfural (HMF) under alkaline conditions, enabling the co-production of low-carbon hydrogen and 2,5-furandicarboxylic acid (FDCA), a key biobased platform chemical. First, recent advances in elucidating the mechanism of HMF electrooxidation (HMFOR) to FDCA on Ni are examined. Next, an in-depth evaluation of the

HMFOR performance of various Ni-based catalysts is provided, highlighting the effects of doping or combining Ni with transition metals such as Fe, Co, Cu, and Mn, as well as multimetallic compositions. Finally, HMFOR activity is compared across recent studies to identify key trends and propose research directions for scaling this technology to an industrial level.

## 1. Introduction

Alkaline water electrolysis (AWE) is a promising technology for large-scale production of low-carbon hydrogen. It leverages transition metal oxides, with nickel-based materials standing out as one of the most effective and cost-efficient alternatives to precious metals. Additionally, nickel is abundant in the earth's crust. As any other 3d transition metal, nickel undergoes several electronic transitions, allowing the formation of various active sites for both half reactions in water electrolysis: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In alkaline conditions, the activity of Ni-based structures toward HER is significantly improved by the interaction of the nickel's 3d-orbital with a heteroatom to form phosphides, nitrides, sulfides, or carbides.[1] It is also well known that a small amount of iron (1 ppm) can significantly boost nickel performance toward OER.[2] For both HER and OER, various nickel structures and interactions with other metals have been extensively studied.[3] However, regardless of the material used to composite with nickel, the active phase for OER is formed in operando. In the case of nickel,[4] the active sites have been identified as the high-valent hydroxyl oxide (Ni^{II}OOH), which is formed in

operando through the electrochemical oxidation of Ni^{II}(OH)₂. Furthermore, this Ni²⁺/Ni³⁺ redox couple is also known to elicit nucleophile oxidation reactions of organic compounds exhibiting hydroxyl, aldehyde, and amino groups.[5] This characteristic has drawn greater interest in recent years for its potential to enable the simultaneous electrolysis of water and biomass. This process allows for the co-production of value-added platform chemicals at the anode and hydrogen at the cathode through HER.

Biomass-derived furanic compounds are considered viable substitutes for fossil feedstock in the chemical industry.[6,7] Among the most interesting of these molecules, 5-hydroxymethylfurfural (HMF) stands out for its potential as valuable platform chemical.[6] HMF is obtained from the dehydration of simple sugars, mainly fructose, resulting in an unsaturated furan ring with hydroxymethyl and formyl substituents, which makes it readily susceptible to transformation into a wide variety of compounds. In particular, the HMF oxidation reaction (HMFOR) can yield 2,5-furandicarboxylic acid (FDCA), the most promising HMF oxidation product, as it can serve as a monomer for the production of biobased polyamides, polyesters and polyurethanes as well as in the pharmaceutical industry.[8] For instance, FDCA can replace the petroleum-derived terephthalic acid, used in the synthesis of polyethylene terephthalate (PET),[9] to produce polyethylene furanoate (PEF), a biobased polymer. PEF outperforms PET in thermal stability, mechanical strength and gas barrier properties for both O₂ and CO₂, owing to the presence of the furan ring.[10-12] In addition, this sustainable polymer has better recyclability and biodegradability than PET. Consequently, PEF is regarded as an attractive alternative to the fossil-based PET for various applications, including packaging (e.g., plastic bottles[12]), textiles, and electronics. The growing demand for FDCA due to its widespread application requires the development of economically and environmentally viable production pathways. Among various HMF oxidation processes, co-electrolysis of water and HMF in alkaline conditions (Equation (1)) on Ni-based catalysts offers multiple benefits. The Gibbs free energy at 298 K for the overall HMF/H₂O co-electrolysis (65.62 kJ mol⁻¹) is significantly lower

I. R. Garduño-Ibarra, Z. Yan, J. González-Cobos, M. Prévot, P. Vernoux
Université Claude Bernard Lyon 1, CNRS

IRCELYON

UMR5256

Villeurbanne F-69100, France

E-mail: philippe.vernoux@ircelyon.univ-lyon1.fr

S. A. Ebrahim, E. Baranova

Department of Chemical and Biological Engineering

Centre for Catalysis Research and Innovation (CCRI)

Nexus for Quantum Technologies (NexQT)

University of Ottawa

Ottawa, Ontario K1N 6N5, Canada

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© 2025 The Author(s). ChemElectroChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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than that of water electrolysis (237.129 kJ mol⁻¹). Consequently, the production of 1 mole of H₂ only requires 22 kJ instead of 237 kJ in the water electrolysis process. In other words, the thermodynamic onset potential for HMFOR is expected to be around 0.113 V<sub>NHE</sub><sup>[13]</sup> compared to 1.23 V<sub>NHE</sub> for the OER. However, literature reports indicate that the average onset potential for HMFOR on Ni-based catalysts is much higher, ≈1.3 V<sub>RHE</sub>. This overpotential arises due to the aforementioned Ni(II)/Ni(III) redox transition (Ni(OH)₂ to NiOOH), which drives the oxidation process and occurs only above 1.3 V<sub>RHE</sub>. In contrast, efficient OER typically has an onset potential in the range of 1.45–1.60 V<sub>RHE</sub>,<sup>[14]</sup> resulting in a relatively narrow HMFOR potential window of about 150–300 mV. For instance, Hauke et al.<sup>[15]</sup> used operando

differential electrochemical mass spectrometry to monitor oxygen production from a NiFe-based catalyst as a function of the applied potential in a 0.1 M KOH electrolyte, both with and without HMF. Their results showed that the OER was suppressed up to 1.7 V<sub>RHE</sub> as a function of HMF concentration, enabling selective HMFOR without OER between 1.4 and 1.6 V<sub>RHE</sub> above 5 mM HMF. The authors suggested that the strong adsorption of HMF onto the NiFe catalytic active sites is at the origin of the inhibition of OER. Still, the lower overpotential required for HMF/H₂O co-electrolysis compared to H₂O electrolysis offers a significant advantage. It can reduce the energy input needed for hydrogen production and co-produce value-added chemical intermediates such as FDCA, thereby potentially lowering production costs of

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Itzcóatl Rafael Garduño-Ibarra Ibarra obtained his PhD in Environmental Engineering from the University of Ottawa in 2023, for his work on biohydrogen production from brewery sludges, combining the use of bioelectrochemical systems to boost energy recovery. As a postdoctoral fellow at CNRS-IRCELYON (2023–2025), he participated in the development of nickel-based electrodes for the co-electrolysis of biomass-derived molecules and water. He has also collaborated in other works involving alkaline water electrolysis.

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Zhigang Yan is currently a PhD candidate at the Institute for Research on Catalysis and Environment of Lyon under the supervision of Dr. P. Vernoux and Dr. J. Gonzalez-Cobos. His research is focusing on development of Ni-based electrocatalysts for biomass valorization and green hydrogen production.

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Sayed Ahmed Ebrahim is a PhD candidate in the Department of Chemical and Biological Engineering at the University of Ottawa, Canada. His research, in collaboration with the Clean Energy Innovation (CEI) research center at the National Research Council of Canada, focuses on developing catalysts for converting biomass and organic waste into sustainable fuel and chemicals.

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Elena Baranova is a Professor in the Department of Chemical and Materials Engineering at the Gina Cody School of Engineering, Concordia University. Prior to joining Concordia, she was a Professor and Co-Director of the Nexus for Quantum Technologies Institute at the University of Ottawa. She previously held an NSERC Postdoctoral Fellowship and worked as a Research Associate at the National Research Council (NRC), before joining the uOttawa as a tenure-track Assistant Professor in 2008. She completed her Ph.D. at École Polytechnique Fédérale de Lausanne (EPFL). Her research focuses on electrochemical energy conversion and storage, water electrolysis, electrochemical promotion, and electrocatalysis.

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Jesús González-Cobos received his Ph.D. in Chemical Engineering at the University of Castilla-La Mancha (Spain) in 2015, under the supervision of Prof. J. L. Valverde and A. de Lucas-Consuegra. Then he worked for 4 years in the group of Prof. J. R. Galán-Mascarós at the Institute of Chemical Research of Catalonia (Spain). In 2020, he joined the group of Dr P. Vernoux at the Institute for Research on Catalysis and Environment of Lyon (France) and in 2023 he became a Junior Professor Chair at CNRS. His research is focused on heterogeneous catalysis, electrocatalysis and operando surface analysis.

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Mathieu S. Prévot obtained a PhD in Chemistry from EPFL in 2017 for his work on metal oxide photoelectrodes. After a postdoctoral appointment at UC Berkeley (2017–2019), during which he studied the electrocatalytic and water-harvesting properties of metal-organic frameworks, he joined the CNRS, where he has been a tenured researcher since 2020. His research work currently aims at advancing electrocatalytic and photoelectrochemical approaches for carbon dioxide and biomass valorization, in the context of the defossilization of the chemical and energy sectors. His expertise spans material design and characterization to (photo)electrochemical device assembly and testing.

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Philippe Vernoux is a scientist and Research Director at CNRS, based in Lyon, France. With a PhD in Electrochemistry (1998) and a Habilitation in Catalysis (2006), Dr. Vernoux has made significant contributions to several domains, including solid-state electrochemistry, low-temperature electrolysis and environmental catalysis. He has authored more than 150 refereed papers and filed 21 patents. Dr. Vernoux's leadership extends to prominent roles such as coordinating the European Pathfinder project Elobio (www.elobio.cnrs.fr) and heading the CARE (Catalytic and Atmospheric Reactivity for the Environment) group at the Institute for Research on Catalysis and Environment of Lyon.
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low-carbon hydrogen. Furthermore, the occurrence of OER can also cause damages to catalytic layers, such as surface shedding, due to the generation of oxygen bubbles.[16] However, suppressing the OER while selectively oxidizing HMF remains a significant challenge, as the higher Ni oxidation state necessary for HMFOR often occur at potentials overlapping with those of OER.[17,18]

There is still much to elucidate about HMFOR on Ni-based electrocatalysts. As will be described earlier, HMFOR exhibits a complex mechanism that depends on the applied potential and pH. Furthermore, HMF (and biomass in general) is unstable in strongly basic solutions. While Ni-based catalysts are renowned for their stability under the harsh conditions of conventional AWE at pH 14, their performance in HMF electrolysis at lower pH (typically below 13) presents a challenge, as the activity and the stability of the $Ni^{2+}/Ni^{3+}$ redox couple diminishes.[19] Therefore, optimal operating conditions must be established to ensure both HMF conversion efficiency and nickel catalyst stability. Likewise, there is the challenge of finding robust and cost-effective catalysts supports. Catalysts are often supported on carbon-based materials (e.g., Vulcan carbon, paper, and graphene). However, these supports may be susceptible to corrosion under alkaline conditions at high potentials, typically above $2V_{RHE}$ at pH 13 according to Möller et al.[20] As HMFOR typically occurs below $1.7V_{RHE}$ at the anode, this risk is anticipated to be minimal under HMF/H$_2$O co-electrolysis. Nickel structures and composites can be also directly deposited or grown on nickel foam (NF) or Cu foam (CF), but scarce literature is found regarding other high-performance materials for OER, such as stainless steel. This review addresses the recent developments on Ni-based catalysts for co-electrolysis of HMF and water coupled with hydrogen production

in alkaline media. The aforementioned challenges and considerations will be critically discussed, from HMF stability and its oxidation mechanisms to bimetallic interactions and the type of support employed. This review summarizes recent advances in the nanostructure and composition of Ni-based catalysts aimed at enhancing HMFOR performance at the lab scale. It emphasizes the combination of Ni with commonly used transition metals such as Fe, Co, Cu, and Mn, as well as multimetallic compositions. The main objective of these studies is to develop HMFOR catalysts that are cost-efficient, sustainable, active, selective, and stable.

# 2. Ni-Based Catalysts

## 2.1. HMFOR Mechanism

HMF serves as a versatile, carbon-neutral platform for synthesizing a variety of valuable chemicals.[6] It contains six carbons, derived from the fructose-, but only half of its oxygen content. The HMF/H$_2$O co-electrolysis (Figure 1a) to produce FDCA while simultaneously generating hydrogen is the focus of numerous recent academic studies.[21–24] This growing interest stems from the mild operating conditions of co-HMF/H$_2$O electrolysis, which occurs at low temperatures (<40 °C) and atmospheric pressure. Moreover, transition metal-based catalysts like Ni can be used, whereas industrial heterogeneous catalysts typically rely on noble metals and require high pressures (up to 100 bar) and temperatures (100–300 °C). Additionally, a recent life cycle analysis concluded that HMF/H$_2$O co-electrolysis has a lower environmental impact than the thermochemical route.[8]

<figure><img src="image_12.png" /><figcaption>Figure 1. a) Schematic of a flow electrolysis cell for simultaneous FDCA and H$_2$ generation. b) Reaction pathways for FDCA production by HMFOR.</figcaption></figure>
The HMF/H₂O co-electrolysis process in alkaline media is described by the Equation (1)–(3).

Overall reaction

$$C_6H_6O_3 + 2H_2O \rightarrow C_6H_4O_5 + 3H_2 \quad (1)$$

Cathode reaction

$$6H_2O + 6e^- \rightarrow 3H_2 + 6OH^- \quad (2)$$

Anode reaction

$$C_6H_6O_3 + 6OH^- \rightarrow C_6H_4O_5 + 4H_2O + 6e^- \quad (3)$$

As can be inferred, HMFOR pathways are highly dependent on the pH of the solution. This affects the catalyst states participating in the reaction, leading to different possible mechanisms, as recently reviewed.[25] The most commonly described mechanism of HMFOR on Ni-based catalysts in strong alkaline conditions is the indirect oxidation mechanism (IOM). In this mechanism, the applied potential serves as the driving force to generate NiOOH[26] (Ni³⁺) active species, from Ni(OH)₂ (Ni²⁺). The formation of these high-valence catalytic species occurs at relatively high potentials, between 1.35 and 1.55 V<sub>RHE</sub>, typically in 1.0 M KOH alkaline media (see Table 1). HMF is oxidized by Ni³⁺ species, reducing it back to Ni²⁺ in the process. The oxidation mechanism requires the prior hydration of the functional groups in an activation process induced by OH⁻ ions. The C–H/O–H bond formed from each functional group is then deprotonated at the active sites of the electrode.[4] NiOOH species, containing electrophilic oxygen, act as oxidizing reactants through a rate-limiting hydrogen atom transfer of the α-hydrogen of the alcohol group to Ni³⁺ cations in NiOOH, following the known mechanism initially proposed by Fleischmann et al.[27] These pioneering studies focused on alcohol oxidation. However, since aldehydes in solution exist in equilibrium with their hydrate, 1,1-geminal diol (Figure 1b), which is also an alcohol, it can be inferred that this indirect mechanism applies to aldehyde oxidation as well, as supported by several studies.[4,28] Once the applied potential is sufficient to trigger a fast electrooxidation of Ni(OH)₂ to NiOOH, the rate of HMF oxidation becomes potential independent and follows the Langmuir–Hinshelwood mechanism, due to a competitive adsorption between HMF and OH⁻ ions on NiOOH active sites.[29] The maximum achieved current density is also related to the proton-coupled electron transfer (PCET) process in which NiOOH is reduced back to Ni(OH)₂, coupled with HMF dehydrogenation, and eventually to the competition of HMFOR with OER.[30,31] Fleischmann et al.[32] also reported that the PCET is the rate-determining step while further oxidation steps to produce carboxylic acid are very fast. Nevertheless, HMF can also be electrooxidized at lower potentials than those required for the oxidation of Ni²⁺. This process, known as the direct oxidation mechanism (DOM),[29] occurs approximately between 1.2 and 1.35 V<sub>RHE</sub>, coinciding with the oxidation of HMFOR intermediates discussed below. Unlike the IOM, the DOM at lower potentials depends solely on the HMF coverage on Ni²⁺, consistent with a Eley–Rideal mechanism.

There are two identified HMF-to-FDCA oxidation pathways. The first route begins with the deprotonation of the hydroxyl substituent that results in the formation of an aldehyde, yielding 2,5-diformylfuran (DFF). The second pathway affects the aldehyde group of HMF and entails a succession of Cannizzaro reactions, starting with the hydration of the formyl substituent of HMF forming dihydroxymethylfuran (DHMF). DHMF is further oxidized following a catalytic-mediated deprotonation process, forming the carboxyl group in HMFCA. In the same manner, FDCA is finally synthesized after the oxidation of 5-formyl-2-furancarboxylic acid (FFCA), derived either from DFF or HMFCA (Figure 1b).

As can be intuited, HMFOR pathways are highly dependent on the pH of the solution, affecting the Ni oxidation states (both the oxidation of Ni²⁺ to Ni³⁺ and the regeneration to Ni²⁺), the deprotonation rates of HMF, the competition with OER, and also the equilibrium between aldehydes and their geminal diol states. Therefore, the pH-dependence of the oxidation routes and their rate-determining step (RDS) is quite complex and not clearly described in the literature. It was experimentally observed that HMFOR preferentially follows the route HMF → HMFCA → FFCA → FDCA at pH above 13, while DFF is produced more significantly as pH decreases,[33] although it has been observed the concurrence of both pathways at pH 13.[34,35] Bender et al.[35] claim that FFCA → FDCA is the RDS, when DFF is accumulated throughout the HMFOR process conducted at pH 13. In contrast, it is commonly seen in the reported literature that HMFCA → FFCA is the RDS in processes at pH higher than 13.[33,36] For instance, in situ/operando spectroscopic characterizations at 1 M KOH (pH = 14) have shown that HMFCA formation is directly linked to the applied potential, starting as early as 1.18 V<sub>RHE</sub> and becoming stronger between 1.35 and 1.38 V<sub>RHE</sub>.[37,38]

Recent studies have proposed an alternative mechanism for alcohol oxidation to aldehyde, involving a hydride transfer from the carbon at the α-position of the alcohol (or geminal diol) group of HMF to Ni⁴⁺ active sites.[4,39] The formation of these highly oxidizing Ni⁴⁺ species occurs at potentials higher than those required for Ni³⁺. Since the production and regeneration of the Ni⁴⁺ strongly influence the oxidation rate, this mechanism has been termed the potential-dependent (PD) route or hybrid mechanism, distinguishing it from the IOM (Figure 2a). It has been proposed, mainly from DFT calculations,[33] that the oxidation degree of Ni in NiOOH active phase is not exclusively 3⁺, but rather a mixture of 3⁺ and 4⁺. The structure of NiOOH active phases is complex, as various structural modifications can occur during charging and discharging processes, such as those observed in cyclic voltammetry experiments.[40] The most active NiOOH species toward both OER and organic molecules oxidation is the γ-NiOOH phase, a more disordered structure than β-NiOOH. γ-NiOOH is formed from the oxidation of α-Ni(OH)₂ or by overcharging β-Ni(OH)₂. The presence of these Ni⁴⁺ species at high oxidation potentials was indirectly deduced from the appearance of a second oxidation peak during linear sweep voltammetry (LSV) experiments (Figure 2a) of a NiOOH film at pH 13.[4,35] In addition, Kang et al.[41] observed two reduction peaks on the cyclic voltammograms of monolayer Ni(OH)₂ nanosheets in 6 M KOH, attributed to the reduction of both Ni⁴⁺ and Ni³⁺. In the same study,[41] in situ Raman spectroscopy evidenced a blue-shift
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<table><caption>**Table 1.** Electrocatalytic performance, achieved at 25 °C in a H-cell, of different Ni-based electrode materials for FDCA production from HMF electrooxidation. F: foam, NF: Ni foam, CF: Cu foam, CP: carbon paper, GC: glassy carbon, CC: carbon cloth and G: graphite (The bold characters were used to highlight some results and experimental data).</caption><thead><tr><th>Catalyst</th><th>Preparation method</th><th>Anode support</th><th>Electrolyte</th><th>[HMF]</th><th>Overpotential HMFOR versus OER @ current density</th><th>Potential (V vs RHE)</th><th>FDCA yield</th><th>FE<sub>FDCA</sub></th><th>FDCA production rate</th><th>References</th></tr><tr><td></td><td></td><td></td><td></td><td>[mM]</td><td>[mV] @ [mA cm<sup>-2</sup>]</td><td></td><td>[%]</td><td>[%]</td><td>[g m<sup>-2</sup> h<sup>-1</sup>]</td><td></td></tr></thead><tbody><tr><td colspan="11" style="text-align:center; font-weight:bold;">Ni catalysts at pH &lt; 14</td></tr><tr><td>Thick NiOOH film<sup>[34]</sup></td><td>Electrodeposition</td><td>Tin oxide</td><td>0.1 M KOH</td><td>5</td><td>-</td><td>1.47</td><td>96.2</td><td>96.0</td><td>-</td><td>[60]</td></tr><tr><td>Ni<sub>3</sub>N</td><td>Hydrothermal + annealing in NH<sub>3</sub></td><td>GC</td><td>0.1 M KOH</td><td>50</td><td>-</td><td>1.47</td><td>92.0</td><td>-</td><td>-</td><td>[34]</td></tr><tr><td>NiOOH</td><td>Electrodeposition</td><td>CP</td><td>0.1 M KOH</td><td>5</td><td>-</td><td>1.36</td><td>99.4</td><td>&gt;99</td><td><b>38.8</b></td><td>[58]</td></tr><tr><td>NiOOH</td><td>Electrodeposition</td><td>G rod</td><td>0.5 M LiOH + 0.5 M LiCl (pH = 13.7)</td><td>10</td><td>150 @ 5</td><td>1.50</td><td>94.0</td><td>88.0</td><td><b>66.5</b></td><td>[124]</td></tr><tr><td colspan="11" style="text-align:center; font-weight:bold;">Ni Catalysts at pH 14</td></tr><tr><td>NiO/NiOOH</td><td>Electrodeposition</td><td>Tin oxide</td><td>1 M LiOH</td><td>5</td><td>-</td><td>1.50</td><td>95.0</td><td>98.0</td><td>-</td><td>[56]</td></tr><tr><td>NiOOH</td><td>Hydrothermal</td><td>NF</td><td>1 M KOH</td><td>10</td><td>240 @ 20</td><td>1.40</td><td>98.9</td><td>99.4</td><td>-</td><td>[125]</td></tr><tr><td>NiOOH@C</td><td>Sol-gel + calcination + acid etching</td><td>NF</td><td>1 M KOH</td><td>10</td><td>-</td><td>1.45</td><td>98.2</td><td>98.5</td><td><b>76.6</b></td><td>[33]</td></tr><tr><td>Ni@C-450</td><td>Microwave-assisted DES + pyrolysis</td><td>CC</td><td>1 M KOH</td><td>10</td><td>250 @ 50</td><td>1.34</td><td>97.7</td><td>97.4</td><td>-</td><td>[126]</td></tr><tr><td>NiB<sub>x</sub></td><td>Electroless plating</td><td>NF</td><td>1 M KOH</td><td>10</td><td>-</td><td>1.43</td><td>99</td><td>99.5</td><td>-</td><td>[127]</td></tr><tr><td>NiS<sub>x</sub>/β-Ni(OH)<sub>2</sub></td><td>Electrodeposition + impregnation + sulfidation</td><td>NF</td><td>1 M KOH</td><td>10</td><td>240 @ 20</td><td>1.41</td><td>98</td><td>98.3</td><td><b>51.1</b></td><td>[17]</td></tr><tr><td>S-Ni@C-600</td><td>Hydrothermal + calcination + carbonization</td><td>CC</td><td>1 M KOH</td><td>10</td><td>240 @ 40</td><td>1.47</td><td>95.7</td><td>96</td><td><b>102.5</b></td><td>[128]</td></tr><tr><td>NiS<sub>x</sub>/Ni<sub>2</sub>P</td><td>Electrodeposition + calcination + basic etching + phosphorization</td><td>CC</td><td>1 M KOH</td><td>10</td><td>204 @ 20</td><td>1.46</td><td>98.5</td><td>95.1</td><td>-</td><td>[129]</td></tr><tr><td>66Ni3N@C</td><td>Hydrothermal + nitridation</td><td>NF</td><td>1 M KOH</td><td>10</td><td>240 @ 50</td><td>1.45</td><td>98.0</td><td>99.0</td><td>-</td><td>[37]</td></tr><tr><td colspan="11" style="text-align:center; font-weight:bold;">NiFe catalysts</td></tr><tr><td>NiFe-LDH</td><td>Hydrothermal</td><td>CP</td><td>1 M KOH</td><td>10</td><td>180 @ 20</td><td>1.23</td><td>98</td><td>99.4</td><td><b>15.3</b></td><td>[68]</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>1.33</td><td>98.0</td><td>98.6</td><td><b>102</b></td><td>[68]</td></tr><tr><td>dNiFe-LDH</td><td>Hydrothermal with ZnCl<sub>2</sub> + KOH etching</td><td>CP</td><td>1 M KOH</td><td>10</td><td>250 @ 10</td><td>1.48</td><td>96.8</td><td>84.5</td><td><b>5</b></td><td>[69]</td></tr><tr><td>Ni(OH)<sub>2</sub>/NiFeP</td><td>Electrodeposition</td><td>NF</td><td>1 M KOH</td><td>10</td><td>&le;150 @ 20</td><td>1.435</td><td>99.4</td><td>94.6</td><td>-</td><td>[70]</td></tr><tr><td>FeNi-40</td><td>Aerosol-assisted chemical vapor deposition</td><td>NF</td><td>1 M KOH</td><td>10</td><td>240 @ 50</td><td>1.32</td><td>95.3</td><td>96.42</td><td>-</td><td>[130]</td></tr><tr><td colspan="11" style="text-align:center; font-weight:bold;">NiCo catalysts</td></tr><tr><td>Ni<sub>2</sub>P/NiCo<sub>2</sub>S<sub>4</sub></td><td>Chemical deposition + phosphorization</td><td>NF</td><td>0.1 M KOH</td><td>10</td><td>-</td><td>1.52</td><td>92.4</td><td>95.4</td><td>-</td><td>[76]</td></tr><tr><td>CoNi-LDH</td><td>Corrosion</td><td>NF</td><td>1 M KOH</td><td>10</td><td>240 @ 50</td><td>1.425</td><td>97.4</td><td>97.3</td><td>-</td><td>[85]</td></tr><tr><td>Co<sub>2</sub>S<sub>8</sub>@Ni<sub>3</sub>S<sub>2</sub></td><td>Hydrothermal + sulfidation</td><td>NF</td><td>1 M KOH</td><td>10</td><td>190 @ 10</td><td>1.4</td><td>&approx;95</td><td>89.04</td><td>-</td><td>[84]</td></tr><tr><td>Co<sub>3</sub>O<sub>4</sub>-NiO-500</td><td>Co-precipitation + calcination</td><td>GC</td><td>1 M KOH</td><td>10</td><td>-</td><td>1.45</td><td>83.3</td><td>89.5</td><td>-</td><td>[82]</td></tr><tr><td>H-0.3Co-Ni<sub>3</sub>S<sub>2</sub></td><td>H<sub>2</sub>O-assisted Hydrothermal</td><td>NF</td><td>1 M KOH</td><td>10</td><td>-</td><td>1.424</td><td>97.6</td><td>97.6</td><td>-</td><td>[16]</td></tr><tr><td>Cu<sub>x</sub>S@NiCo-LDH</td><td>Chemical oxidation + sulfidation + electrodeposition</td><td>CF</td><td>1 M KOH</td><td>10</td><td>-</td><td>1.32</td><td>&gt;99.0</td><td>&gt;99.0</td><td><b>494.5</b></td><td>[28]</td></tr><tr><td colspan="11" style="text-align:center; font-weight:bold;">NiCu catalysts</td></tr><tr><td>Cu</td><td>Electrodeposition</td><td>Cu foil</td><td>0.1 M KOH</td><td>5</td><td>-</td><td>1.62</td><td>96.4</td><td>95.3</td><td><b>14</b></td><td>[86]</td></tr><tr><td>CuO nanoflower</td><td>Chemical treatment</td><td>CF</td><td>0.1 M KOH</td><td>10</td><td>-</td><td>1.62</td><td>99.6</td><td>99.3</td><td><b>413</b></td><td>[87]</td></tr></tbody></table>
<figure><img src="image_14.png" /></figure>

<table><caption>**Table 1. Continued.**</caption><thead><tr><th>Catalyst</th><th>Preparation method</th><th>Anode support</th><th>Electrolyte</th><th>[HMF]</th><th>Overpotential HMFOR versus OER @ current density<br>[mM]</th><th>Potential (V vs RHE)</th><th>FDCA yield</th><th>FE<sub>FDCA</sub> [%]</th><th>FDCA production rate [%]</th><th>References</th></tr><tr><td></td><td></td><td></td><td></td><td></td><td>[mV] @ [mA cm<sup>-2</sup>]</td><td></td><td></td><td></td><td>[g m<sup>-2</sup> h<sup>-1</sup>]</td><td></td></tr></thead><tbody><tr><td>Cu nanoparticles</td><td>Pulse electrodeposition</td><td>Ti plate</td><td>0.1 M KOH</td><td>4.2</td><td>–</td><td>1.47</td><td>94</td><td>94.1</td><td><strong>3.5</strong></td><td>[66]</td></tr><tr><td>Cu/NiOOH-SOx</td><td>Hydrothermal</td><td>NF</td><td>1 M KOH</td><td>20</td><td>202 @ 100</td><td>1.4</td><td>≈100</td><td>≈100</td><td><strong>312</strong></td><td>[95]</td></tr><tr><td>NiCu nanotubes</td><td>Electrodeposition</td><td>NF</td><td>1 M KOH</td><td>20</td><td>–</td><td>1.423</td><td>≈100</td><td>96</td><td><strong>150</strong></td><td>[92]</td></tr><tr><td>NiCu</td><td>Electrodeposition</td><td>NF</td><td>1 M KOH</td><td>50</td><td>290 @ 200</td><td>1.45</td><td>99.51</td><td>99.67</td><td><strong>416</strong></td><td>[96]</td></tr><tr><td>NiCu</td><td>Impregnation</td><td>CP</td><td>1 M KOH</td><td>5</td><td>–</td><td>1.45</td><td>93.3</td><td>94.4</td><td><strong>11</strong></td><td>[131]</td></tr><tr><td>Ni(OH)<sub>2</sub>-Cu<sub>2</sub>O</td><td>Hydrothermal</td><td>NF</td><td>1 M KOH</td><td>100</td><td>222 @ 400</td><td>1.424</td><td>>85</td><td>>93</td><td><strong>1 990</strong></td><td>[102]</td></tr><tr><td>CuO-Ni(OH)<sub>2</sub> (1:3)</td><td>Microwave-assisted DES + refluxing</td><td>CP</td><td>1 M KOH</td><td>10</td><td>–</td><td>1.48</td><td>99.8</td><td>98.4</td><td>–</td><td>[100]</td></tr><tr><td>Ni<sub>0.75</sub>Cu<sub>0.25</sub>O<sub>x</sub></td><td>Soaking</td><td>NF</td><td>1 M KOH</td><td>50</td><td>198 @ 300</td><td>1.4</td><td>>99.5</td><td>>99.5</td><td><strong>259</strong></td><td>[101]</td></tr><tr><td>Cu(OH)<sub>2</sub>@Cedoped Ni(OH)<sub>2</sub></td><td>Chemical oxidation + electrodeposition</td><td>Cu mesh</td><td>1 M KOH</td><td>10</td><td>145 @ 100</td><td>1.45</td><td>98.5</td><td>97.9</td><td><strong>75</strong></td><td>[132]</td></tr><tr><td>Cu(OH)<sub>2</sub>/NiOOH</td><td>Electrodeposition</td><td>Cu foil</td><td>0.1 M NaOH</td><td>5</td><td>–</td><td>1.5</td><td>99.4</td><td>98.5</td><td><strong>14</strong></td><td>[99]</td></tr><tr><td>Cu(OH)<sub>2</sub>/NiOOH</td><td>Electrodeposition</td><td>Cu nanoF</td><td>0.1 M NaOH</td><td>5</td><td>–</td><td>1.40</td><td>>95</td><td>>95</td><td><strong>19</strong></td><td>[99]</td></tr><tr><td colspan="11" style="text-align: center; font-weight: bold;">NiMn catalysts</td></tr><tr><td>UTNiMn-LDH</td><td>Co-precipitation</td><td>NF</td><td>1 M KOH</td><td>10</td><td>200 @ 40</td><td>1.52</td><td>99.7</td><td>92.7</td><td>–</td><td>[112]</td></tr><tr><td>Ni<sub>1</sub>Mn<sub>5</sub>-LDH</td><td>Hydrothermal</td><td>NF</td><td>1 M KOH</td><td>10</td><td>–</td><td>1.40</td><td>94.7</td><td>>97</td><td>–</td><td>[113]</td></tr><tr><td>Mn<sub>0.2</sub>NiS</td><td>Hydrothermal</td><td>G felt</td><td>1 M KOH</td><td>100</td><td>300 @ 100</td><td>1.5 – 1.7 (500 mA cm<sup>-2</sup>)</td><td>97.6</td><td></td><td><strong>4 570</strong></td><td>[114]</td></tr><tr><td>Mn-Ni<sub>2</sub>P</td><td>Microwave assisted deep eutectic solvent + phosphidation</td><td>CC</td><td>1 M KOH</td><td>10</td><td>200 @ 20</td><td>1.43</td><td>98.0</td><td>97.8</td><td><strong>30.6</strong></td><td>[115]</td></tr><tr><td>MnNi(OH)<sub>2</sub></td><td>Hydrothermal</td><td>CP</td><td>1 M KOH</td><td>50</td><td>266 @ 20</td><td>1.45</td><td>98.5</td><td>98.5</td><td><strong>54.9</strong></td><td>[111]</td></tr><tr><td colspan="11" style="text-align: center; font-weight: bold;">Multimetallic catalysts</td></tr><tr><td>NiFe-LDH/CoCH</td><td>Two-step hydrothermal</td><td>NF</td><td>1 M KOH</td><td>5</td><td>~ 80 @ 200</td><td>1.58</td><td>98.6</td><td>98.1</td><td><strong>77</strong></td><td></td></tr><tr><td>NiCoFe-LDH</td><td>Hydrothermal/calcination</td><td>NF</td><td>1 M KOH</td><td>20</td><td>157 @ 10</td><td>1.50</td><td>98.7</td><td>98.0</td><td><strong>78</strong></td><td>[133]</td></tr><tr><td>NiCoFe-LDH</td><td>Hydrothermal</td><td>CP</td><td>1 M NaOH</td><td>10</td><td>20 @ 10</td><td>1.54 at 55 °C</td><td>84.9</td><td>90</td><td><strong>464</strong></td><td>[117]</td></tr><tr><td>NiCoFe-LDH</td><td>Impregnation + hydrothermal vulcanization</td><td>NF</td><td>1 M KOH</td><td>10</td><td>125 @ 10</td><td>1.45</td><td>94.8</td><td>94.7</td><td>–</td><td>[118]</td></tr><tr><td>FeP-NiMoP<sub>2</sub></td><td>Hydrothermal + phosphorization</td><td>Fe-Ni F</td><td>1 M KOH</td><td>10</td><td>142 @ 100</td><td>1.40</td><td>99.2</td><td>99</td><td>–</td><td>[119]</td></tr></tbody></table>

of the NiOOH characteristic bands (Eg and A1g modes) when the applied potential was larger than 1.36 V. This was attributed to hydrogen desorption from NiOOH. Similar in situ Raman investigations,[33] recently performed on a Ni@C electrode in 1 M KOH solution containing 10 mM HMF, have shown the same blue-shift of NiOOH characteristic bands above 1.43 V<sub>RHE</sub> (Figure 2b), which was also attributed to the formation of Ni<sup>4+</sup> species evolved in the partially deprotonated NiOOH<sub>x</sub> (x < 1) species. The formation and regeneration of Ni<sup>4+</sup> from NiOOH<sub>x</sub> (x < 1) species, such as γ-NiOOH, strongly depends on the alkalinity of the solution, where the higher the pH, the faster the rates of both organic oxidation and Ni(OH)<sub>2</sub> deprotonation (Ni<sup>2+</sup> → Ni<sup>3+</sup> → Ni<sup>4+</sup>).[39] Although energetically unfavorable, the deep oxidation of Ni<sup>2+</sup> → Ni<sup>4+</sup> may occur gradually in a spontaneous manner, notably during the deprotonation of the hydrous NiO/NiOOH layer

toward negatively charged surfaces (NiO<sup>-</sup> and NiOO<sup>-</sup>) at high potentials, but it can also be promoted by exogenous factors (e.g., doping with metals like Co).[42]

## 2.2. Performance of Ni-Based Catalysts for HMFOR

HMFOR performances of Ni-based catalysts in alkaline conditions are typically measured at pH 14 in 1 M KOH or NaOH and to a lesser extent at pH 13 in 0.1 M KOH or NaOH (Table 1 and 2). Typically, chronoamperometry (CA) experiments are running at fixed applied potentials between 1.3 and 1.6 V<sub>RHE</sub> in a divided electrochemical batch H-cell equipped with a polymeric membrane until the HMF conversion becomes complete or close to 100%, i.e., after a few hours of polarization. The investigated concentrations of HMF are small, in the range 5–100 mM (Table 1
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 2.** a) CVs recorded on a Ni(OH)₂ film at pH 13 in KOH with and without HMF. PD: Potential-Dependent. Scan rate: 10 mV s⁻¹. Reproduced with permission.[35] Copyright 2022, John Wiley and Sons. b) Operando Raman spectra of Ni@C electrode after nine cycles at varying potentials in 1 M KOH solution containing 10 mM HMF, demonstrating the blue-shift from 475 to 479 cm⁻¹ and from 555 to 559 cm⁻¹, indicating an increase in valence from Ni³⁺ to Ni⁴⁺. Reproduced with permission.[33] Copyright 2024, Elsevier. c) Degradation of a 50 mM HMF solution in presence of different KOH concentrations without any catalyst and applied potential. 100% corresponds to the initial mass of HMF. Reproduced with permission.[44] Copyright 2023, Royal Society of Chemistry. d,e) Product distribution and LSV at different pH (pH 13 in 0.1 M KOH, pH 12 in 0.01 M KOH + 0.09 M KCIO₄, pH 11 in 0.001 M KOH + 0.099 M KCIO₄) on a thin-film Ni catalyst. Reproduced with permission.[50] Copyright 2024, American Chemical Society. f) HPLC chromatograms for the non-electrochemical reaction of DFF obtained without applying potential over A-Co-Ni2P catalyst. Reproduced with permission.[36] Copyright 2024, Nature. g) ATR-FTIR spectra of 0.1 M DFF and 0.1 M HMFCA without applying potential as a function of time and reference spectra of HMF, DFF, HMFCA, FFCA and FDCA. Reproduced with permission.[36] Copyright 2024, Nature.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

<table><caption>**Table 2.** Electrocatalytic performance of different Ni-based electrode materials for HMFOR coupled with HER. NF: Ni foam, CF: Cu foam, CoF: Co foam, CP: carbon paper, GF: graphite felt (The bold characters were used to highlight some results and experimental data).</caption><thead><tr><th rowspan="2">HMFOR // OER<br>catalysts</th><th rowspan="2">Anode preparation<br>method</th><th rowspan="2">Support</th><th rowspan="2">Electrolyte</th><th>[HMF]</th><th>Current<br>density</th><th>Cell<br>Voltage</th><th>FDCA<br>yield</th><th>FE<sub>FDCA</sub></th><th>FEH<sub>2</sub></th><th>Cell</th><th>FDCA<br>production<br>rate</th><th rowspan="2">References</th></tr><tr><th>[mM]</th><th>[mA cm<sup>-2</sup>]</th><th>[V]</th><th>[%]</th><th>[%]</th><th>[%]</th><th>[g m<sup>-2</sup> h<sup>-1</sup>]</th></tr></thead><tbody><tr><td>NiCo//NiCo</td><td>Corrosion +<br>electrochemical activation</td><td>CoF</td><td>1 M KOH</td><td>100<br>(BHMF)</td><td>100</td><td>1.64</td><td>95</td><td>92</td><td>100</td><td>H-cell</td><td>-</td><td>[48]</td></tr><tr><td>Co-Ni<sub>x</sub>P@C//<br>Co-Ni<sub>x</sub>P@C</td><td>Pyrolysis using a deep<br>eutectic liquid precursor</td><td>Ni-Co F</td><td>1 M KOH</td><td>10</td><td>&lt;40</td><td>1.50</td><td>100</td><td>96.8</td><td>&gt;99.0</td><td>H-cell</td><td>151.4</td><td>[83]</td></tr><tr><td>NiCo<sub>2</sub>@MoO<sub>2</sub> //<br>NiCo<sub>2</sub>@MoO<sub>2</sub></td><td>Hydrothermal + reducing<br>thermal treatment</td><td>NF</td><td>1 M KOH</td><td>10</td><td>35</td><td>1.4</td><td>98.6</td><td>99.4</td><td>99.1</td><td>H-cell</td><td>133</td><td>[120]</td></tr><tr><td>Ni<sub>2</sub>P/NiCo<sub>2</sub>S<sub>4</sub> //<br>Ni<sub>2</sub>P/NiCo<sub>2</sub>S<sub>4</sub></td><td>Chemical deposition +<br>phosphorization</td><td>NF</td><td>0.1 M KOH</td><td>5</td><td>10</td><td>2.158<br>1.60<sup>a)</sup></td><td>89.1</td><td>86.8</td><td>96.1</td><td>H-cell</td><td>-</td><td>[76]</td></tr><tr><td>CoFe@NiFe-<br>LDH // Pt</td><td>Hydrothermal +<br>electrodeposition</td><td>NF</td><td>1 M KOH</td><td>10</td><td>30</td><td>1.40</td><td>99.8</td><td>99.8</td><td>99.8</td><td>H-cell</td><td>155.8</td><td>[13]</td></tr><tr><td>Rh-SA/NiFe-<br>LDH // Rh-SA/<br>NiFe-LDH</td><td>Hydrothermal +<br>Zn etching</td><td>CP</td><td>1 M KOH</td><td>50</td><td>100</td><td>1.48</td><td>-</td><td>96.8</td><td>99.5</td><td>H-cell</td><td>-</td><td>[121]</td></tr><tr><td>NiFe // Pt</td><td>Electrodeposition</td><td>NF</td><td>0.33 M KOH</td><td>100</td><td>-</td><td>3.2</td><td>94.35</td><td>77.29</td><td>-</td><td>2 L<br>CSTR</td><td>250</td><td>[53]</td></tr><tr><td>NF // NF</td><td>-</td><td>-</td><td>0.1 M<br>Na<sub>2</sub>SO<sub>4</sub></td><td>1180</td><td>-</td><td>2.5<br>(40 A)<br>and 2.1<br>(10 A)</td><td>70</td><td>84</td><td>-</td><td>Flow<br>cell</td><td>375</td><td>[46]</td></tr><tr><td>Ni<sub>x</sub>B//NF</td><td>NaBH<sub>4</sub> reduction +<br>annealing in Ar</td><td>NF</td><td>1 M KOH</td><td>10</td><td>50</td><td>1.45<sup>a)</sup></td><td>98.5</td><td>100</td><td>-</td><td>Flow<br>cell</td><td>5.7</td><td>[38]</td></tr><tr><td>Mn<sub>0.2</sub>NiS/-</td><td>Hydrothermal</td><td>GF</td><td>1 M KOH</td><td>100</td><td>-</td><td>1.68<sup>a)</sup></td><td>94.7</td><td>91.3</td><td>-</td><td>Flow<br>cell</td><td>44 000</td><td>[114]</td></tr><tr><td rowspan="2">NiFe(-Cl)-<br>LDH//Pt-C@CP</td><td rowspan="2">Microwave assisted<br>hydrothermal + anion<br>exchange</td><td>NF</td><td>0.1 M KOH</td><td>10</td><td>-</td><td>1.56</td><td>100</td><td>99</td><td>-</td><td>Flow<br>cell</td><td>154</td><td>[15]</td></tr><tr><td></td><td></td><td>1 M KOH</td><td>10</td><td>-</td><td>1.56</td><td>100</td><td>100</td><td>-</td><td>Flow<br>cell</td><td>412</td><td>[15]</td></tr><tr><td>Ni<sub>0.75</sub>Cu<sub>0.5</sub>O<sub>2</sub>//<br>Ni<sub>4</sub>Mo/MoO<sub>2</sub></td><td>Soaking</td><td>NF</td><td>1 M KOH</td><td>200</td><td>400</td><td>2.07</td><td>99.0</td><td>-</td><td>-</td><td>Flow<br>cell</td><td>830</td><td>[101]</td></tr><tr><td>CuS/CF//Pt/NF</td><td>Chemical oxidation +<br>sulfidation</td><td>CF</td><td>1 M KOH</td><td>10</td><td>≈20</td><td>1.6</td><td>97</td><td>78</td><td>-</td><td>Flow<br>cell</td><td>18</td><td>[89]</td></tr><tr><td>Co-Ni<sub>2</sub>P//Pt-Ti</td><td>Hydrothermal +<br>phosphorization</td><td>NF</td><td>1 M KOH</td><td>100</td><td>115</td><td>1.8</td><td>-</td><td>-</td><td>100</td><td>Flow<br>cell</td><td>-</td><td>[36]</td></tr><tr><td>Co<sub>2</sub>S<sub>8</sub>@Ni<sub>3</sub>S<sub>2</sub>//<br>Co<sub>2</sub>S<sub>8</sub>@Ni<sub>3</sub>S<sub>2</sub></td><td>Hydrothermal +<br>in situ sulfidation</td><td>NF</td><td>1 M KOH</td><td>10</td><td>50</td><td>1.6</td><td>-</td><td>93.0</td><td>&gt;99.0</td><td>Flow<br>cell</td><td>72.5</td><td>[84]</td></tr></tbody></table>

and 2). The most commonly used conditions correspond to 10 mM HMF in a 1 M KOH anolyte. However, as already mentioned, HMF is unstable in these alkaline electrolytes and can rapidly degrade through various chemical reactions, with the intensity of these reactions depending on both the pH and the HMF concentration. Consequently, these degradation reactions may have a significant impact on the HMFOR performance of Ni-based catalysts. However, these HMF degradation reactions are often overlooked or even ignored in most publications. This is particularly the case for strongly basic conditions (pH > 13) and high HMF concentrations where a fast chemical oxidation of HMF occurs via the Cannizzaro reaction (4).

$$2\text{HMF} + \text{OH}^- \rightarrow \text{HMFCA} + \text{DHMF} \quad (4)$$

The Cannizzaro reaction is a base-mediated disproportionation of aldehydes to their carboxylic acids and alcohol,

producing DHMF (also called 2,5-bis(hydroxymethyl)furan (BHMF)) and HMFCA from HMF.[43] In ambient conditions, 50 mM HMF in 1 M KOH, Krebs et al.[44] found a loss of HMF, due to the Cannizzaro reaction, of about 20% without any applied potential and catalyst after only 1 h (Figure 2c). In higher KOH concentrations, the HMF loss becomes larger than 80% after 4 h (Figure 2c). A lower HMF concentration at pH 14 can slightly limit this HMF degradation, which nevertheless remains at 10%[45] after only 1 h for 5 mM HMF and can reach 70% after 24 h for 6 mM HMF.[46] However, this HMF chemical degradation, via the Cannizzaro reaction, is not particularly detrimental for the HMFOR process as DHMF and HMFCA, the two products of this reaction, are stable in alkaline conditions and can be further selectively electrooxidized to FDCA[44,47,48] Nevertheless, measuring the intrinsic HMFOR activity of a catalyst at high pH over periods longer than 1 h during

<footnote>a) anode potential versus RHE.</footnote>
<figure><img src="image_18.png" /></figure>

chronoamperometries is inevitably affected by the parasitic
Cannizzaro reaction.

One solution to overcome this HMF degradation issue could
be to operate under less alkaline conditions, at pH ≤ 13. Indeed,
lowering the pH enhances the stability of HMF. For instance,
Latsuzbaia et al.[46] have shown that a high concentration of
HMF (713 mM) is stable for 6 h at pH 12.5. On the other hand,
a low concentration of OH⁻ inhibits the kinetics of HMFOR.[46,49]
According to Zhou et al.[34] who investigated a Ni₃N catalyst, this
could be due to the fact that strong alkaline conditions in 1 M
KOH can promote the adsorption of HMF on the electrode sur-
face. Consequently, this highly basic environment could acceler-
ate the electrooxidation of 50 mM HMF to FDCA through the
intermediate production of HMFCA, compared to a less alkaline
0.1 M KOH anolyte which promotes the DFF pathway. More
recently, Prajapati et al.[50] demonstrate that HMFOR performan-
ces of a NiOOH thin film strongly decline as the bulk pH value
decreases from 13 to 11. Besides, the product distribution signifi-
cantly depends on the pH during CA at 0.8 V<sub>SHE</sub> and 5 C charge
passed. Indeed, the faradaic efficiency toward FDCA (FE<sub>FDCA</sub>)
drops with the pH in favor of the production of DFF and FFCA
(Figure 2d). The pH also affects the oxidation step of Ni²⁺ to
Ni³⁺, which is shifted to more positive potentials as the pH
decreases (Figure 2e). Moreover, lowering the pH further to a
value of 10 completely inhibits the HMFOR.[46] Therefore, a pH
value between 12 and 13 could be a good compromise for
HMF/H₂O co-electrolysis. In this pH range, the kinetics of the
Cannizzaro reaction become lower but HMF can be also
degraded to dark brown colored and insoluble solid humins.[44,51]
Unfortunately, these polymeric species of HMF cannot be electro-
oxidized to FDCA.[44] At pH 13, the HMF degradation to humins
after 10 h[52] of 5 mM HMF in 0.1 M NaOH was found to be 18% at
25 °C reaching 80% at 50 °C. Consequently, the formation of these
insoluble humins can significantly reduce the FDCA yield during
long time experiments, essentially for temperatures higher than
30 °C. This generation of insoluble solid humins in the electrolyte
can also strongly complexify the separation and purification pro-
cesses of FDCA. The solubility of FDCA is also pH dependent. In
pure water, its solubility is only 6.4 mM L⁻¹. Chakthranont et al.[53]
demonstrated that fully dissolving 100 mM FDCA at 25 °C requires
a minimum KOH concentration of 0.33 M. At lower KOH concen-
trations, such as 0.1 M KOH at pH 13, FDCA may crystallize on the
electrode, leading to significant deactivation of the catalyst.
Finally, the base-catalyzed dehydration of HMF in water can also
produce formate and lactate[54] via ring-opening decomposition.
For instance, this reaction has been observed at 100 °C, 40 bar,
and pH above 11.[55] More recently, Gouda et al.[56] have evi-
denced, by ¹H NMR, a significant formation of formate in an alka-
line solution of 5 mM HMF at pH 14 at room temperature, which
can reach 0.3 mM after 25 h. In 0.33 M KOH and 100 mM HMF,
Chakthranont et al.[53] suggested that the main HMF degradation
pathway (10.7% after 1 h at 25 °C) involves ring opening rather
than the Cannizzaro reaction. Interestingly, the same study sug-
gested a first-order HMF degradation rate that could be strongly
hindered by a high electrolytic HMFOR current.

The instability of DFF can also be an issue as it can undergo a
rapid transformation of its aldehydes leading to FFCA or even

back to HMF, as recently demonstrated by Chen et al.[36] who fol-
lowed the electrochemical and non-electrochemical oxidation
reaction of DFF and HMFCA separately in 1.0 M KOH using a
Co-doped Ni phosphide as catalyst. When no potential was
applied, the reaction of 0.1 M DFF monitored by ATR-FTIR showed
an increase in the signal for the corresponding peak of C—H
bonds at 1397 cm⁻¹, indicating the accumulation of aldehyde
groups. The HPLC analysis not only confirmed the formation
of HMF and FFCA but also revealed the increasing concentrations
of HMFCA and FDCA over the course of 30 min (Figure 2f). On the
contrary, HMFCA showed no chemical reactivity without any
applied potential (Figure 2g). These results are in agreement with
the study of Chakthranont et al.[53] who found a significant deg-
radation rate of 10 mM DFF after 1 h at 25 °C in 0.33 M KOH of
around 40.5% while those of HFMCA and FFCA were much lower
(2.7% and 1.4%, respectively). It is therefore advisable to avoid the
formation of DFF in favor of the HMFCA pathway, which is pro-
moted at high pH.

Table 1 summarizes recent HMFOR performances reported on Ni-based catalysts prepared through various methods. The goal of most of these studies was to optimize the nanostructure by achieving a high concentration of dispersed active NiOOH sites (generally formed in situ during polarization) combined with excellent electronic conductivity to facilitate electron transfer. Typically, a Ni precursor was deposited onto a Ni foam (NF) or a carbon substrate via electrodeposition or hydrothermal synthesis. When Ni is combined with a heteroatom such as N, S, B, or P, additional steps like calcination are often necessary, adding complexity to the synthesis process and making large-scale production more challenging (Table 1). All these studies report very high FE<sub>FDCA</sub> as well as high FDCA yields (Table 1 and 2). In the vast majority of cases, these two metrics are larger than 90% and therefore are not very relevant for comparing HMFOR performances. The overpotential difference between HMFOR and OER, which defines the HMFOR potential window, is around 200–240 mV at pH 14 whatever the nanostructure of the Ni catalyst. The effect of alkali cation (Li⁺, N⁺, K⁺ and Cs⁺) in alkaline electrolytes on the competition between HMFOR and OER on Ni was also investigated at pH 14.[56] It was found that small cations can enlarge the HMFOR potential window, especially when traces of iron are present in the catalyst. The FE<sub>FDCA</sub> at 1.5 V<sub>RHE</sub> of 5 mM HMF in 1 M alkaline electrolyte with trace of iron followed the trend: Li⁺ (90%) > Na⁺ (88%) > K⁺ (85%) > Cs⁺ (<6%). Garcia et al.[57] established a direct correlation between the size of alkali metal cation (M) and the OER activity of Ni-based catalyst. This correlation was attributed to the enhanced stability of NiO-O-M intermediate by larger cations, such as Cs. Furthermore, the presence of Fe reduces the covalent character of the Ni—O bonds, promoting the adsorption of oxygen intermediates and thereby enhancing the OER. Therefore, the use small alkali cations such as Li, along with the absence of any Fe traces as in the ref. [56], can shift the OER onset potential and widen the HMFOR potential. The utilization of LiOH electrolyte instead of KOH could be a solution to slightly enlarge the HMFOR potential window.

More discriminating parameters to compare the HMFOR per-
formances than FE<sub>FDCA</sub> could be the FDCA production rate
obtained at a given applied anodic potential. This FDCA
<figure><img src="image_19.png" /></figure>

production rate, expressed in g of produced FDCA per m² of anode and per hour, was only calculated for the publications where the following data are available: anode geometric area, volume of the anolyte, and HMF conversion at the end (or when reaching 100%) of a CA and FDCA yield. Surprisingly, few data are reported at pH 13 (0.1 M KOH), where the HMF degradation is less pronounced. However, Lu et al.[58] have obtained outstanding performances inside a H-cell by using NiOOH nanosheets, simply prepared by electrodeposition on a carbon paper. For a quite low applied potential of 1.36 V<sub>RHE</sub>, 5 mM HMF was fully and selectively oxidized to FDCA within 1 h. The FDCA yield was found to be 99.4%, indicating that, in these conditions, the HMF degradation was negligible. The FDCA production rate of 38.8 g m⁻² h⁻¹, is slightly smaller than the values reported at pH 14 (Table 1) but was achieved at a smaller applied potential. Furthermore, the specific energy consumption was calculated to be 1.47 kWh kg⁻¹ of FDCA with a 95.3% H₂ Faradaic efficiency at the cathode.

At pH 14, the FDCA production rate, calculated for a HMF con-
centration of 10 mM converted within a small-volume anode
compartment (10–25 mL), increases almost linearly with the
applied potential, ranging from 51.1 to 102 g m⁻² h⁻¹ as the
potential rises from 1.41 to 1.47 V<sub>RHE</sub>. These results indicate that
HMFOR activity is more influenced by the applied potential than
by the Ni catalyst nanostructure. This also underlines the out-
standing HMF activity achieved at pH 13 and low potential
(1.36 V<sub>RHE</sub>) of NiOOH nanosheets.[58]

However, the instability of HMF and DFF poses a major chal-
lenge to scaling up HMF/H₂O co-electrolysis under industrially rel-
evant conditions, such as high HMF concentrations and large
processing volumes. For example, Zhou et al.[⁵⁹] recently reported
that the FDCA selectivity of a Ni₂P catalyst supported on Ni foam,
measured in a batch reactor during CA at 1.5 V<sub>RHE</sub>, dropped
sharply from 92.9% to 46.6% as the HMF concentration increased
from 10 to 200 mM. This dramatic decline in FDCA selectivity was
attributed to non-Faradaic degradation reactions, as evidenced
by the significant formation of insoluble humins and HMFCA
(Equation (4)). However, by using the same high HMF concentra-
tion of 200 mM, a much higher FDCA selectivity of 91.3% was
found in a continuous flow reactor with an estimated carbon loss
of less than 8%. This demonstrates that implementing continuous
electrochemical flow cells could address the challenge by
enabling operation with short residence times for HMF-based
alkaline solutions, thereby greatly minimizing HMF degradation.
A pioneering continuous HMF/H₂O alkaline exchange membrane
(AEM) co-electrolyser prototype with an integrated product sep-
aration unit for FDCA production was developed and tested by
Latsuzbaia et al.[³⁸] in 2018 (Table 2). The anodic compartment
was fed with a high-concentration HMF solution (15 wt%,
≈1180 mM) mixed with 0.1 M Na₂SO₄ in water. Sodium sulfate
was added to enhance electrolyte conductivity, and the anolyte
pH was maintained at 12 through continuous NaOH injections.
The electrolysis process utilized two electrochemical reactors
in series, each equipped with a stack of two nickel foam anodes
(400 cm² per anode). The first electrolyzer operated galvanostati-
cally at 40 A (with a voltage limit of 2.5 V), while the second oper-
ated at 10 A (with a voltage limit of 2.1 V). A continuous flow of

0.2 L h⁻¹ of the HMF solution was introduced into the first electro-
lyser, achieving ≈70% HMF conversion to FDCA. The product
stream was then directed to the second electrolyser at an
increased flow rate of 0.35 L h⁻¹ to account for water produced
during HMF electrooxidation. Complete HMF conversion was
achieved at the outlet of the second electrolyser, with a stable
FDCA concentration maintained for at least 10 h. This first dem-
onstration of continuous HMF/H₂O co-electrolysis in electrochem-
ical flow reactors achieved a production rate of 30 g h⁻¹
(375 g h⁻¹ m⁻² based on anode surface area) of FDCA with a
Faradaic efficiency of 84% (Table 2). However, the FDCA yield
was limited to 70%, likely due to HMF degradation into humins
and/or the formation of intermediate oxidation products. This
finding highlights the need to develop more active Ni-based cat-
alysts to the electrooxidation of HMF within a short contact time
to avoid the HMF degradation. An increasingly explored
approach to improve HMFOR activity is the modification of Ni
structures by doping with other transition metals, in order to
reduce the onset potential of the metallic active species and/
or delay the OER reaction. This will be further discussed in the
following sections for each of the most common transition metal,
such as Fe, Co, Ni, and Mn as well as multimetallic Ni-based
materials.

3. Ni Doped with Non-Noble Metals

3.1. Ni-Fe Bimetallic Catalysts

Nickel catalysts containing iron have been extensively considered for HMFOR. Even though FeOOH is poorly active as a catalyst for HMF conversion (vs NiOOH or CoOOH),<sup>[60]</sup> Fe has an important effect as Ni dopant. The impact of iron on nickel structures is indeed of great relevance given its ubiquity in catalyst precursors, materials and electrolytes. Multiple studies confirm that iron traces, even in the order of a few ppm, can be incorporated to NiOOH structures and enhance their activity toward OER.<sup>[61-63]</sup> Operando spectroelectrochemical techniques have contributed to a better understanding of the effect of iron on charge accumulation in NiOOH active sites, which correlates with accelerated water oxidation kinetics.<sup>[64]</sup> DFT studies have also shown that iron doping decreases the overpotential of OER intermediates.<sup>[65]</sup> However, the role of iron remains a matter of debate when it comes to HMFOR.<sup>[56]</sup> Iron traces in electrolytes, even at the nanomolar scale, can negatively impact HMFOR performance by shifting the nickel redox peak toward OER. Similarly, Zhou et al.<sup>[66]</sup> confirmed, by in situ surface-enhanced Raman spectroscopy (SERS) on a Ni film electrodeposited on an Au/Ti support, that Fe<sup>3+</sup> suppresses HMFOR on Ni, while promoting water oxidation. Cyclic voltammetry (CV) experiments also revealed that the charge of the Ni(OH)<sub>2</sub> oxidation peak increased along the CV cycles due to the activation of subsurface Ni atoms and more importantly decreased in presence of Fe<sup>3+</sup> impurities (<b>Figure 3a</b>). The authors claimed that the activity suppression is the result of the restrictive effect of iron, impeding the transformation of Ni(OH)<sub>2</sub> into its active NiOOH phase (<b>Figure 3b</b>).
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 3.** a) Evolution of the charge of the Ni oxidation peak during cyclic voltammetry experiments with and without 1 ppm Fe³⁺ in 0.1 M KOH, b) Schematic illustration of the Ni surface reconstruction in 0.1 M KOH with and without 1 ppm Fe³⁺ during HMFOR. (a,b) reproduced with permission.[66] Copyright 2022, Elsevier. c) LSV curve of NiFe LDH and Ni(OH)₂ for HMFOR and OER with a scan rate of 10 mV s⁻¹ in 1 M KOH with and without 10 mM of HMF. Reproduced with permission.[68] Copyright 2018, American Chemical Society. d) LSV curves NiFe LDH, NiFeZn LDH and d-NiFe LDH in 1 M KOH and 10 mM HMF. Reproduced with permission.[69] Copyright 2022, American Chemical Society. e) Schematic representation of the 2 L CSTR with a jelly-roll configuration. f) Variation of FDCA yield, faradaic efficiency and HMF conversion as a function of flow rate using 100 mM HMF in 0.33 M KOH in the large-scale CSTR operated at 100 A (33.33 mA cm⁻²). Reproduced with permission.[53] Copyright 2023, Elsevier.</figcaption></figure>

Nevertheless, the incorporation of iron has drawn the attention of researchers, especially in the case of Ni layered double hydroxide (LDH) catalysts. Seo et al.[67] explored the optimal iron content in layered Ni(OH)₂ prepared by a low-temperature microwaved synthesis, ranging the doping of iron between 0 and 5 at%. The authors found that, above 0.4 at% Fe, the activity toward OER begins to decrease, while HMFOR performance drops with any introduction of iron. In contrast, NiFe LDHs[68] directly synthetized on a carbon paper by a simple hydrothermal route, exhibited outstanding HMFOR performances at very low potentials in a H-cell despite a low-onset potential at 1.37 V$_{RHE}$ toward OER (Figure 3c). These NiFe nanosheets, containing a high surface concentration of Fe (around 15 wt% according to XPS analysis, initial molar ratio Ni/Fe = 1/3), are very active for HMFOR reaching

a FDCA production of 15.3 and 102 g m⁻² h⁻¹ during CA at 1.23 and 1.33 V$_{RHE}$, respectively (Table 1). Furthermore, the HMFOR performances of this electrocatalyst were found to be stable along four consecutive electrolysis cycles at 1.33 V$_{RHE}$ with only a slight decrease of HMF conversion from 98% to 93%. These results seem to confirm that the nanostructure in the form of nanosheets of the active Ni phase appears to be particularly effective for HMFOR, in agreement with the finding of Lu et al.[58] Qi et al.[69] used a similar hydrothermal route than Liu et al.[68] to prepare nanosheets of NiFe LDHs and NiFeZn LDHs directly deposited on carbon paper. The idea was to etch out Zn, in a strong basic solution, to form cationic defects in the structure, noted as d-NiFe LDHs. However, the HMFOR performances of these NiFe LDHs and d-NiFe LDHs samples, measured in similar
<figure><img src="image_22.png" /></figure>

conditions (Table 1) than Liu et al.[68] were found to be much poorer (Figure 3c,d). The HMF electrooxidation only started above 1.475 V<sub>RHE</sub> and at 1.48 V<sub>RHE</sub>, the FDCA production rate of d-NiFe LDHs was 3 times smaller than the one measured on the NiFe LDHs prepared by Liu et al.[68] at only 1.23 V<sub>RHE</sub>. These observations show that modifying a detail during the catalyst preparation and/or the HMFOR test protocol can significantly impact the results. Kauke et al.[15] also investigated bimetallic-LDH catalysts combining Ni with Fe, Mn, Co, and V. The materials were directly grown on NF using a microwave-assisted hydrothermal route, and their HMFOR performances were tested in a single electrolyzer cell at 1.56 V<sub>cell</sub> with a Pt-based cathode in 0.1 M KOK with 10 mM HMF. Among the tested catalysts, NiFe and NiV catalysts, with a Ni:X molar ratio of 3/1 and 1.5/1, respectively, exhibited the highest performance. This enhancement was attributed to the synergy between Ni and V/Fe dual surface sites. The HMFOR activity of NiFe-LDH was further improved through anion exchange, where of CO<sub>3</sub><sup>2-</sup> was replaced with Cl<sup>-</sup>. This modification facilitated the activation of the α-LDH interlayer phase, promoting its transition to the γ-LDH active structure. High FDCA production rates, with nearly 100% FDCA yields and Faradaic efficiencies, were achieved with 10 mM HMF in both 0.1 M KOH and 1 M KOH at a cell voltage of 1.56 V, with a Pt-based cathode (Table 1).

Other Ni/Fe heterostructures proposed like those studied by Luo et al.[70] revealed interesting insights on HMFOR mechanism. Electrodeposited NiP and NiFeP structures on Ni foam (and on fluorine-doped tin oxide (FTO)) were found to easily give way to the in situ formation of active amorphous Ni(OH)<sub>2</sub> layers, which later evolve into NiOOH, a potentials higher than 1.36 V<sub>RHE</sub>. The insertion of Fe in the NiP structure, with Ni/Fe ratios in the range 2:1-1:2, enhances the HMFOR activity. In situ Raman spectroscopy experiments reveal that the HMFOR active sites are NiOOH species rather than Fe species. This study speculated that the synergy between Fe and Ni could promote the existence of high-valence Ni<sup>4+</sup> and Ni<sup>3+</sup> active species, without clear insights and in contradiction with other literature studies. Recent research has demonstrated that FeNi nanowires, specifically the FeNi-40 catalyst composed of 52.4 at% of Fe and 47.6 at% of Ni, which was prepared using aerosol-assisted chemical vapor deposition can trigger HMFOR from 1.32 V<sub>RHE</sub> (Table 1). However, the HMFOR potential window (240 mV at 50 mA cm<sup>-2</sup>) is comparable to that of typical Ni-based catalysts, suggesting that the OER onset was also shifted to lower values.

NiFeOOH electrodes were recently utilized in a 2 L continuous stirred-tank reactor (CSTR) for the continuous production of FDCA.[53] Ni and Fe precursors (with a Ni/Fe molar ratio of 95/5) were prepared by pulse electrodeposition on NFs to achieve a nanosheet morphology. The cathodes consisted of Pt coated on NFs prepared by electroless deposition. Three pairs of large anodes and cathodes were alternately stacked, separated only by a plastic net to prevent short circuit and minimize the electrode distance. These electrodes were rolled into a jelly-roll configuration, with Pt-based cathodes inside and NiFeOOH-based anodes outside (Figure 3e), providing a total active surface area of 3000 cm². To limit HMF degradation, an aqueous mixture of 150 mM HMF and 1 M KOH (flow ratio 2:1) was prepared just before being introduced into the CSTR (Figure 3e). This

large-scale continuous batch electrochemical cell operated at 33.3 mA cm⁻² with 100 mM HMF in 0.33 M KOH, reaching a high total current of 100 A. However, the cell voltage was relatively high (around 3.2 V), with nearly half of this value attributed to the ohmic resistance. As a result, the CSTR was continuously cooled down with a water circulation to maintain a constant temperature of around 27 °C. Additionally, the batch cell was sparged with a flow of oxygen to mitigate the undesired electrochemical reduction of HMF on the Pt/NF cathode while also significantly inhibiting the HER. Figure 3f displays the impact of different flow rates on HMFOR performance. A low flow rate (50 mL min⁻¹) resulted in a low FE<sub>FDCA</sub> due the competition with the OER, attributed to HMF transport limitations to the electrode surface. A higher flow rate (100 mL min⁻¹) solved this issue but decreased the FDCA yield most likely due to a shorter reactant contact time. The best compromise was found at an intermediate flow of 85 mL min⁻¹. However, the FE<sub>FDCA</sub> of only 77.3% remained lower that the values achieved in membrane electrode assembly electrolyzers, although the FDCA production rate was comparable to that reported by Hauke et al.[15] despite the much higher cell voltage (Table 1). Nevertheless, this study demonstrates the possibility to operate NiFe-based anodes to produce high-purity FDCA (97.7%) in a large-scale continuous batch cell.

While a few studies suggested that iron may positively influence HMFOR activity on NiOOH-based catalysts, such effects appear limited to specific nanostructures. The majority of studies indicate that Fe doping has a detrimental impact on the HMFOR performance of Ni-based catalysts. Furthermore, Fe can promote the OER activity of Ni, which can significantly lower the FDCA Faradaic efficiency. Therefore, it appears essential to minimize iron contamination in HMFOR processes involving Ni whenever possible.

## 3.2. Ni-Co Bimetallic Catalysts

Cobalt has been intensively investigated for the oxidation of small organic molecules,[71,72] and OER.[73,74] Cobalt is after nickel the most studied transition metal for HMFOR. It is also the most widely used transition metal for doping Ni-based catalysts for HMFOR. Cobalt alone is capable to fully electrooxidize HMF to FDCA.[49] Analogously to all Ni-based catalysts discussed so far under strong alkaline conditions, both Co alone and NiCo bimetallic materials promote FDCA production via the HMFAA pathway, with negligible traces of DFF observed. Cobalt atoms in the surface of catalysts exhibit a strong affinity to adsorb the C and O atoms of the formyl group, promoting its transformation into a carboxyl group, whereas the oxidation of the hydroxyl group by cobalt is highly unlikely, as evidenced by experimental and DFT analysis by Zhao et al.[75] As described by Hung et al.[76] in milder alkaline environments, the DFF route could also be expected, although the impact of chemically derived intermediates under lower pH conditions has to be clarified. For instance, Cai et al.[77] did not find any trace of DFF after 4 h of electrolysis with 10 mM HMF in 0.1 M KOH. Either way, it is agreed that the oxidation of the formyl group is a potential-dependent reaction driven by the Co<sup>2+</sup>/Co<sup>3+</sup> redox couple. In fact, HMFAA can be
<figure><img src="image_23.png" /></figure>

formed as early as 1.1 V<sub>RHE</sub>.[49] Likewise, Kang et al.[78] observed a decrease of cobalt coordination number during HMFOR by XANES analysis, describing that cobalt participates in the oxidation by the reduction of Co<sup>3+</sup> to Co<sup>2+</sup>. However, HMFOR kinetics are significantly faster with the species Co<sup>4+</sup>, so complete oxidation of HMF happens at higher potentials.[49] Therefore, while it is widely recognized that NiCo interactions typically shifts the Ni<sup>2+</sup>/Ni<sup>3+</sup> transition to lower onset potentials, this does not always translate into a better performance for HMFOR, even if it means a lower overpotential for OER.[30] Based on the different Tafel slope trends observed, the kinetic response of Ni-based catalysts decreased with the introduction of Co in some cases,[77,79,80] while in others, it improved.[75,78,81] Notwithstanding, a positive effect of Co doping commonly reported is a higher electrochemically active surface area (ECSA), generally determined by the double layer capacitance method[16,78–81] and lower charge transfer resistance, determined by EIS.[16,80,81]

A diverse range of Co-doping Ni-based materials has been explored, from simple nanoparticle-based catalysts to complex composites (sulfides, phosphides, as well as metal-organic framework (MOFs)). The most common electrode material to support NiCo catalysts is NF[16,75,77,78,81] although in some cases a carbon support was employed.[79,80] Likewise, any electrolyte other than 1.0 M KOH is rarely used, but some exceptions have been reported under milder conditions. Such is the case of the work conducted by Gao et al.[81] who studied the performance of NiO, Co<sub>3</sub>O<sub>4</sub>, and different mixed oxides with different Ni to Co ratios (Ni<sub>x</sub>Co<sub>3-x</sub>O<sub>4</sub>), synthesized by a hydrothermal method. The authors observed the growth of smooth NiO nanosheets and assembled Co<sub>3</sub>O<sub>4</sub> nanowires on NF (Figure 4a,e). However, in the mixed oxide catalysts, i.e., Ni<sub>2</sub>CoO<sub>4</sub>, Ni<sub>1.5</sub>Co<sub>1.5</sub>O<sub>4</sub>, and Ni<sub>1</sub>Co<sub>2</sub>O<sub>4</sub> (Figure 4b–d), the increase in cobalt doping led to the formation of nanowires onto the edges of the nanosheets, obtaining a fully covered nanowire structure on Ni<sub>1</sub>Co<sub>2</sub>O<sub>4</sub> (Figure 4d). In consequence, this catalyst showed a higher surface area with respect to the others, visually noticeable by microscopy and further verified by ECSA measurements. This is one of the main reasons for the improved electrocatalytic performance, producing 10 mA cm<sup>-2</sup> at 1.35 V<sub>RHE</sub> with 10 mM HMF in 0.1 M KOH, closely followed by Ni<sub>1.5</sub>Co<sub>1.5</sub>O<sub>4</sub> (Figure 4f). With respect to product yield, Ni<sub>1</sub>Co<sub>2</sub>O<sub>4</sub> led to over 90% yield to FDCA at full HMF conversion and 1.55 V<sub>RHE</sub>. Similar Co<sub>3</sub>O<sub>4</sub>/NiO heterogeneous interfaces were recently synthesized using a simple co-precipitation method followed by calcination at 500 °C, resulting in porous nanocubes.[82] The interfacial effect in the Co<sub>3</sub>O<sub>4</sub>NiO catalyst was shown to enhance HMFOR performance in 10 mM HMF with 1 M KOH compared to pure Co<sub>3</sub>O<sub>4</sub> and NiO. The Co<sub>3</sub>O<sub>4</sub>/NiO composite shifted the onset of Ni<sup>2+</sup>/Ni<sup>3+</sup> oxidation potential to lower values (1.22 V<sub>RHE</sub> versus 1.3 V<sub>RHE</sub> for NiO), accelerating the conversion of FFCA to FDCA. Overall, the study demonstrated that the NiO/Co<sub>3</sub>O<sub>4</sub> synergy promotes the oxidation of aldehyde to carboxylic groups. However, at 1.45 V<sub>RHE</sub>, the HMFOR performance of the Co<sub>3</sub>O<sub>4</sub>dash; NiO catalyst resulted in a relatively low FDCA yield (83.33%) compared to pure Ni catalysts under similar conditions (Table 1). This discrepancy, not discussed in the paper, may be attributed to the competitive OER at this potential and/or HMF degradation at this high pH.

Cai et al.[77] described the performance of various catalysts synthesized from a terephthalic acid (BDC)-based MOF, comparing Ni alone, NiCo, NiFe, and NiMn in 0.1 M KOH, with and without 10 mM HMF. The BDC MOF-derived structures were supported on a NF. In the presence of HMF, NiCoBDC demonstrated better performance than the rest of the MOF-derived catalysts (Figure 4g). However, the highest FDCA yield (99%) was obtained at 1.55 V<sub>RHE</sub>, with a FE<sub>FDCA</sub> lower than 80%. Besides, by XPS measurements, the authors elucidated the promoting effect of Co. This was attributed to a change in the local electronic structure of Ni<sup>2+</sup>. See, for instance, the left- and right shifts in the peaks in the Ni 2p region (Figure 4h) and O 1s region (Figure 4i), respectively. The introduction of Co<sup>2+</sup> into the sites of Ni<sup>2+</sup> in the NiBDC crystal cell could lead to a π-donation of the unpaired electrons in Ni<sup>2+</sup> π-orbital between them and promoting the charge transfer from Ni<sup>2+</sup> to Co<sup>2+</sup> (Figure 4j). Thus, the formation of high valence nickel species could be directly related with the enhanced performance of Co-doped Ni catalysts for HMF electrooxidation.

More recently, Hung et al.[76] achieved a FE<sub>FDCA</sub> above 90% under the same electrolyte conditions (10 mM HMF in 0.1 M KOH), composed of nickel cobalt sulfide nanosheets (NCS) and Ni<sub>2</sub>P crystals on NF (denoted as NCSP). The preparation method consisted on growing the NCS nanosheets from NiCo<sub>2</sub>S<sub>4</sub> on the support through a low-temperature chemical bath, followed by vapor-phase phosphorization treatments (Table 1), varying the synthesis pressure. XRD analysis confirmed the formation of Ni<sub>2</sub>P crystals over the NF and the NCS nanosheets, which in turn exhibited a poor crystallinity structure. The 2p XPS spectra revealed that Ni was present in the Ni<sup>2+</sup> and Ni<sup>3+</sup> oxidation degrees, while Co existed as Co<sup>2+</sup> and Co<sup>3+</sup>. Ni<sup>8+</sup> was also found but only in the Ni<sub>2</sub>P structures. In the presence of 10 mM HMF, the best performing catalyst, NCSP150 (corresponding to the material treated at 150 torr during the phosphorization), attained 10 mA cm<sup>-2</sup> at 1.43 V<sub>RHE</sub>. After 3 h of electrolysis at 1.52 V<sub>RHE</sub>, 99% of HMF was converted and the observed FE<sub>FDCA</sub> reached 95%, with an FDCA selectivity of 92.5% (Table 1). Since the actual pH of the electrolyte was around 12.7, the authors argued that both HMF oxidation pathways were elicited, conclusion inferred by the selectivity found for each intermediate molecule by chromatography (roughly, 3.1% FFCA, 1.7% DFF, and 1.7% HMFCA). The co-generation of hydrogen was also evaluated in an H-cell using the same NSCP150 material as both the anode and cathode. The Faradaic efficiency for HER was 98.5% without HMF. During HMF/H<sub>2</sub>O co-electrolysis with 5 mM HMF, NCSP150 maintained a high FE of 96.1% for HER. However, the FE<sub>FDCA</sub> dropped to 86.8% at an anode potential of 1.6 V<sub>RHE</sub> (Table 2), suggesting the occurrence of OER. The cell voltage was 2.12 V at 10 mA cm<sup>-2</sup>, around 153 mV lower than conventional water electrolysis under the same 0.1 M KOH electrolyte. Ni phosphides were also recently investigated in a study[83] that reported the synthesis of a Co-NixP@C catalyst. This catalyst was prepared via one-step pyrolysis at 450 °C of a deep eutectic liquid precursor impregnated onto Ni-Co foam (Ni/Co ratio 9:1). The resulting material was a bifunctional catalyst comprising a 3D interconnected nanowire network coated with a thin layer of oxygen-containing graphitic carbon. This carbon layer enhanced conductivity and protected the
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 4.** SEM images of nanostructured NixC<sub>3</sub>@xO<sub>4</sub> catalysts on 3D NF: a) NiO, b) Ni<sub>2</sub>CoO<sub>4</sub>, c) Ni<sub>1.5</sub>Co<sub>1.5</sub>O<sub>4</sub>, d) NiCo<sub>2</sub>O<sub>4</sub>, and e) Co<sub>3</sub>O<sub>4</sub>. f) Comparison of the HMF oxidation LSV curves for five NixC<sub>3</sub>@xO<sub>4</sub> catalysts. Reprinted with permission.<sup>[81]</sup> Copyright 2018, John Wiley and Sons. g) FDCA yield rate and FE of NiBDC, NiCoBDC, NiFeBDC and NiMnBDC for 1 h. h) Ni 2p and i) O 1s of specimens. j) Schematic representation of the electronic coupling between Ni and Co in NiBDC-NF and NiCoBDC-NF. Reproduced with permission.<sup>[77]</sup> Copyright 2020, Royal Society of Chemistry. k) *I*-t curves of stepwise electrolysis. l) Comparison of the HMF conversion, FDCA yield and FE<sub>FDCA</sub> between constant-potential HMFOR electrolysis and stepwise electrolysis. H-0.3Co-Ni<sub>3</sub>S<sub>2</sub> in 30 mM HMF with 1 M KOH. Reproduced with permission.<sup>[16]</sup> Copyright 2020, Elsevier.</figcaption></figure>

Co-NixP nanowires from leaching, while the nanowire network significantly increased the number of active sites. In a symmetric H-cell (same catalyst both at the anode and cathode, Table 2), fed with 10 mM HMF in 1 M KOH, the catalyst achieved an FDCA production rate of 151.4 g m⁻² h⁻¹, coupled with a FE<sub>FDCA</sub> of 96.8% at a cell voltage of 1.5 V. This production rate exceeds those reported for pure Ni-based catalysts (Table 2) and remained stable over six successive electrolysis cycles at 1.5 V. Additionally, the

low cell voltage demonstrates the exceptional activity of the Co-NixP@C catalyst for both HMFOR and HER. This high performance was attributed not only to the increased number of active sites but also to its "superaerophobic/superhydrophilic" surface properties, which could facilitate efficient release of H₂ bubbles at the cathode. Using a similar material (a Co-doped Ni₂P catalyst deposited on NF), Chen et al.<sup>[36]</sup> achieved an extremely high current density of 812 mA cm⁻² at 1.5 V<sub>RHE</sub> with 100 mM HMF
<figure><img src="image_26.png" /></figure>

in 1 M KOH. The HMF oxidation kinetics were remarkably fast, with complete conversion of 100 mM HMF to FDCA in just 6 min, thus avoiding the HMF degradation. In a MEA (Membrane Electrode Assembly) electrolyser, the same electrode consistently delivered 115 mA cm⁻² at a cell voltage of 2 V for over 16 h, coupled with a 100% FE for H₂ at the cathode (Table 2). The authors estimated that HMF-H₂O co-electrolysis reduces electricity consumption by 16% compared to AWE for the same volume of hydrogen produced. However, the study did not specify the FDCA yield or faradaic efficiency.

Another recent example of HMFOR coupled with HER was also conducted using a heterogeneous structure based on cobalt and nickel sulfides nano-heterojunction on Ni foam (denoted as Co₉S₈@Ni₃S₂/NF).[⁸⁴] The material was synthesized by a simple hydrothermal method, first reacting a cobalt precursor with urea, followed by the sulfidation by Na₂S·9H₂O to achieve a nanowire-like morphology. XPS, EIS, and DFT calculations revealed a suitable H⁺ adsorption energy, promoting a deeper interaction with organics than the catalysts consisting only on cobalt (Co₉S₈/NF) and nickel sulfides (Ni₃S₂/NF). The maximum FDCA yield was attained between 1.35 and 1.45 V<sub>RHE</sub> in 1.0 M KOH. At 1.4 V<sub>RHE</sub>, 10 mM HMF can be fully converted in 1 h with an FDCA yield of around 95% (Table 2). When coupling the reaction with HER in a flow cell equipped with an anion exchange membrane using Co₉S₈@Ni₃S₂/NF both at the anode and the cathode, a cell voltage of 1.61 V was sufficient to generate a current density of 50 mA cm⁻², instead of 1.73 V with pure water splitting. Further more, the hydrogen production was sixfold higher than that generated in the HER||OER system.

Deng et al.[²⁸] explored the activity of Ni-Co LDH nanosheets electrodeposited onto a Cu foam. The Cu foam was pretreated by chemical oxidation, followed by immersion in Na₂S to form CuₓS nanorarrays which can enhance both conductivity and surface area. The resulting catalyst demonstrated high activity for HMFOR in 10 mM HMF with 1M KOH, reaching optimal performance with a Ni/Co ratio of 3 in the NiCo LDH. This catalyst achieved a current density of 87 mA cm⁻² at 1.3 V<sub>RHE</sub>, which is 5.8 times higher than that of Ni-LDH supported on the same CuₓS foam. At only 1.32 V<sub>RHE</sub>, the FDCA production rate reached an impressive 494.5 g m⁻² h⁻¹ (Table 1), one of the highest reported to date, with nearly 100% FDCA yield and Faradaic efficiency. The authors claim that the HMF degradation in such strong alkaline conditions is minimized by the fast oxidation process as all the HMF is fully converted to FDCA within 35 min. The catalyst's exceptional activity was sustained over five successive electrolysis cycles at 1.32 V<sub>RHE</sub>. However, SEM analysis revealed significant morphological changes in the NiCo-LDH after HMFOR testing, with the layer becoming denser and thicker. Furthermore, ICP-OES analysis detected severe Cu leaching of 11.03 mg L⁻¹, highlighting the importance of conducting extended durability tests beyond a few hours to fully validate catalyst stability. Interestingly, the same CuₓS@NiCo-LDH catalyst also demonstrated activity for HER, even in the presence of 10 mM HMF, achieving a current density of 10 mA cm⁻² at an overpotential of just 107 mV. Recently, NiCo-LDH was grown on a NF using a corrosion method,[⁸⁵] where the NF was immersed in a seawater solution containing the Co precursor. This scalable

and sustainable preparation method resulted in active electrocatalysts, exhibiting an HMFOR onset potential below than 1 V<sub>RHE</sub> in 1 M KOH with 10 mM HMF, attributed to the introduction of Co. Optimal performances were achieved at a relatively low potential of 1.425 V<sub>RHE</sub>, with an FDCA yield and Faradaic efficiency exceeding 97% (Table 1) and a carbon balance of 97.4%, indicating a low HMF loss despite the strong alkaline conditions. The same corrosion method was employed to prepare a NiCo catalyst supported on a Co foam (CoF).[⁴⁸] This material was used in a membrane-less symmetrical H-cell both at the anode and the cathode to perform the electrooxidation of BHMF to FDCA and the HER, respectively. As previously discussed, BHMF, with its stable diol structure, offers a viable alternative to HMF for FDCA production. The NiCo@CoF catalyst achieved full conversion of 100 mM BHMF in 1 M KOH after within ≈110 min at a relatively low cell voltage of 1.64 V, with a high FE<sub>FDCA</sub> of around 92% (Table 2).

Zhang et al.[¹⁶] have investigated NiCo sulfide synthesized on an NF by hydrothermal process with the assistance of H₂O₂ (denoted as H-0.3Co-Ni₃S₂). The morphology of this catalyst features nanoflowers distributed on nanosheets, resulting in a large active surface area. The authors demonstrated through XPS that the addition of Co shifts the Ni 2p peak to a higher binding energy, indicating a decrease in Ni electronic density. This can facilitate the oxidation of Ni²⁺ to Ni³⁺ active sites. In the electrolysis of 10 mM HMF in 1.0 M KOH, current densities of 10 and 200 mA cm⁻² were obtained at rather low potentials, i.e., 1.31 and 1.38 V<sub>RHE</sub>, respectively. At 1.424 V<sub>RHE</sub>, a 97.6% FDCA yield was obtained after converting 98.2% of 10 mM HMF (Table 1). To mitigate HMF degradation in strongly alkaline solutions, the authors proposed a novel approach called the stepwise electrolysis strategy. This method involves the successive application of decreasing potentials. Initially, at the maximum HMF concentration where OER is more easily suppressed, the highest potential is applied to maximize the current and HMF conversion rate, thereby reducing both electrolysis time and HMF degradation (Figure 4k). As the HMF concentration diminishes, and before the onset of OER, the potential is progressively lowered to sustain HMF electrolysis (Figure 4k). The improved FE<sub>FDCA</sub> observed compared to constant-potential electrolysis confirms the effectiveness of this stepwise strategy in avoiding OER (Figure 4l). However, implementing this approach in a flow electrochemical cell, where the HMF concentration is continuously adjusted, remains challenging. A similar material Co₉S₈@Ni₃S₂, grown on NF via a hydrothermal method following a sulfidation step, was tested in a AEM electrolyzer, at both the anode and cathode.[⁸⁴] After six cycles at a constant cell voltage of 1.6 V<sub>RHE</sub>, the system maintained a good stability, generating 45.6 L H₂ h⁻¹ m⁻² and 72.5 g m⁻² h⁻¹ of FDCA with FE of 100% and 93%, respectively (Table 2). The FDCA production rate is in the same range than values reported for pure Ni anode materials (Table 1). However, it can be increased up to 215 g m⁻² h⁻¹ for a cell voltage of 1.8 V, but the FE<sub>FDCA</sub> decreased to 85%.

In summary, recent studies on NiCo-based materials have shown that Ni and Co can work in synergy, cobalt mainly boosting the formation of intermediates, while Ni increases the current density, accelerating the reaction rate towards FDCA. Worth noting, however, is that depending on the catalyst features, the last
<figure><img src="image_27.png" /></figure>

reaction steps may occur at potentials as high as well into the OER region, compromising the effectiveness of the process.

### 3.3. Ni-Cu Bimetallic Catalysts

Cu alone can catalyze HMFOR.[66,86-88] Some recent studies have shown that Cu deposited on Cu foil or foam can selectively electrooxidize HMF into FDCA with very high Faradaic efficiencies in alkaline electrolyte conditions (Table 1). Moreover, promising results were achieved in an AEM electrolyzer operating in a one-pass flow mode.[89] A Cu foam-supported Cu sulfide, in situ electrochemically converted to Cu oxide, was used in the anodic compartment. The anolyte consisted of a mixture of two solutions: 20 mM HMF and 2.0 M KOH. These were mixed immediately before being introduced into the anodic compartment to minimize HMF degradation. For a cell voltage of 1.6 V, corresponding to an anode potential close to 1.52 V<sub>RHE</sub>, the average production rate of FDCA was found to be 18 g m<sup>-2</sup> h<sup>-1</sup> and stable for 60 h (Table 2). However, the FE<sub>FDCA</sub> was only 78%, suggesting the occurrence of the competitive OER. Regarding the mechanism of the HMFOR reaction in alkaline media on Cu, additional in situ/in operando experiments are clearly necessary as no consensus is existing in the literature on the nature of the active Cu species. Recent in situ Raman experiments suggested the indirect electrochemical-chemical HMF oxidation pathway triggered by the electrochemical oxidation of Cu(OH)<sub>2</sub> to Cu<sup>II</sup> species, described as the active species.[89] On the other hand, another study dealing with in situ SERS experiments concluded that CuO and Cu<sup>II</sup> species, identified as CuO<sup>2-</sup>, are mainly active for OER, while Cu(OH)<sub>2</sub> would be the main active species for HMFOR.[66] In contrast, Lie et al.[90] proposed, from cyclic voltammetry experiments, XPS and XAS analysis, that CuO is most likely the HMFOR active sites after prolonged reaction time as CuO is more thermodynamically stable than Cu(OH)<sub>2</sub> in alkaline solutions.[91] The best HMFOR performances of Cu-based catalysts, in terms of FDCA production rate (Table 1), are typically achieved at potentials 100-200 mV higher than those of Ni-based catalysts. In addition, the possible dissolution in alkaline condition of Cu(OH)<sub>4</sub><sup>2-</sup> above 0.6-0.7 V<sub>RHE</sub> and of CuO<sup>2-</sup> species above 1.6 V<sub>RHE</sub> may cause stability issues. It is interesting to note, however, that some studies take advantage of this dissolution property via a dissolution/precipitation mechanism to tailor specific surface Cu nanostructure, such as dendritic Cu crystals[86] or nanotubes.[92] All in all, the relatively good activity of Cu for HMFOR and its cost around two times smaller than Ni justify to combine Cu with Ni in order to reduce the Ni content and create synergies.

A series of studies on Ni-Cu bimetallic catalysts have been recently published, using various preparation methods (chemical, electrodeposition, and hydrothermal), to deposit different Ni-Cu nanostructures either on Ni foam or Cu mesh, foam, or foil (Table 1). All these studies emphasize that Cu species can modulate the electronic properties of Ni catalysts and modify their electrochemical properties. Cu typically exhibits a +2 valence state in alkaline conditions (Cu(OH)<sub>2</sub> or CuO) and does not readily form +3 valence state like Ni at high anodic potentials. This can result in metal active site in mixed Ni-Cu catalysts with different

coordination geometries, which can alter the HMF adsorption[90] or even facilitate the formation of active NiOOH species.[93] For instance, Zhu et al.[94] have deposited a NiO-CuO composite layer on NF by a hydrothermal route followed by a calcination step at 400 °C, aiming to maximize the interface between NiO and CuO nanosheets to promote the charge transfer between the two oxides. This NiO-CuO heterostructure exhibited better HFMOR activity than individual NiO and CuO electrodes (Figure 5a). This synergy was attributed to the electron transfer from NiO to CuO, able to reduce Cu<sup>2+</sup> and produce Cu<sup>+</sup>-V<sub>o</sub> sites. These latter would be efficient to adsorb OH<sup>-</sup> species, thereby releasing more NiO sites for HMF adsorption. Noteworthy is the electronic configuration of Cu<sup>2+</sup> (3d<sup>9</sup>) sites, providing Lewis acid properties and thus the ability to adsorb HMF. This property is also mentioned in some studies[95] to explain the Ni-Cu synergy for HMFOR. The higher HMF adsorption capacity of NiO-CuO composite compared to individual NiO and CuO was demonstrated by Zhu et al.[94] using OCP measurements before and after the injection of HMF (Figure 5b). Furthermore, the poor activity of CuO sites for the deprotonation of OH<sub>ads</sub> to O<sub>ads</sub> can decrease the OER activity. This property of copper, confirmed by other recent studies,[93,96,97] is very important because it could enlarge the potential window of HMFOR, which was found to reach 200 mV at 100 mA cm<sup>-2</sup> on the NiO-CuO composite,[94] while a value up to 290 mV at 200 mA cm<sup>-2</sup> was reported by Chen et al.[96] (Table 1). Yang et al.[98] also explored the effect of the Ni/Cu interface on the HMFOR activity by preparing a Ni(OH)<sub>2</sub>/Cu(OH)<sub>2</sub> heterostructure on a Cu foam using a simple wet-chemical method. They confirmed by coupling DFT calculations with XPS, ultraviolet photoelectron microscopy, and XANES analysis that electron transfer can occur at the Ni(OH)<sub>2</sub>/Cu(OH)<sub>2</sub> interface but in the opposite direction, from Cu(OH)<sub>2</sub> to Ni(OH)<sub>2</sub>, than that proposed by Zhu et al.[94] at the NiO-CuO interface. The Ni(OH)<sub>2</sub>/Cu(OH)<sub>2</sub> heterostructure exhibits an outstanding HMFOR performance, reaching 1300 mA cm<sup>-2</sup> at 1.5 V<sub>RHE</sub> in the presence of 1.0 M KOH and 50 mM HMF. This performance significantly outperforms that of the individual Cu(OH)<sub>2</sub> and Ni(OH)<sub>2</sub> catalysts (Figure 5d). This impressive activity was attributed to a faster mass transfer of HMF at the catalyst-electrode interface. Indeed, this study highlighted the impact of rigid hydrogen bonds formed by H<sub>2</sub>O and OH<sup>-</sup> at the catalyst/electrolyte interface under high positive potentials, which can strongly hinder HMF diffusion to the surface catalytic sites. In situ Raman spectroscopy experiments and molecular dynamics simulations disclosed that the electric field generated at the Ni(OH)<sub>2</sub>/Cu(OH)<sub>2</sub> interface reduces both the number and the strength of hydrogen bonds of the interfacial water, thereby mitigating HMF mass transfer limitations (Figure 5e). Another notable characteristic of Cu is its distinct catalytic oxidizing properties compared to Ni. By using in situ XAS experiments, Woo et al.[99] demonstrated the higher ability of Cu(OH)<sub>2</sub> species than NiOOH sites for the oxidation of aldehyde to carboxylic acid, while the NiOOH surface is more efficient for the oxidation of alcohol to aldehyde. This difference in reactivity can explain the synergy observed in NiOOH-Cu(OH)<sub>2</sub> mixed electrodes, prepared by electrodeposition (Table 1), whose HMFOR performance performed in 0.1 M NaOH is significantly higher than that of Cu(OH)<sub>2</sub> and NiOOH electrodes.
<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /><figcaption>**Figure 5.** a) ECSA normalized LSV curves of HMFOR on NiO-CuO composite/NF and individual Cu(OH)₂ and Ni(OH)₂ catalysts, b) OCP measurements on the same catalysts in 1.0 M KOH solution before and after 10 mM HMF injections, c) LSV curve of HMFOR and OER for NiO-CuO/NF, HMFOR: 10 mM HMF + 1 M KOH, OER: 1 M KOH. Reprinted with permission.[94] Copyright 2024, Elsevier. d) ECSA normalized LSV curves of Ni(OH)₂/Cu(OH)₂ heterostructure and individual Cu(OH)₂ and Ni(OH)₂ catalysts, e) Number of hydrogen bonds at different electrode-electrolyte interfaces calculated by molecular dynamics simulation. Reprinted with permission.[98] Copyright 2024, Royal Society of Chemistry. f) LSV curves of Ni₁-xCux(OH)₂ catalysts in presence of 5 mM HMF in 1 M KOH. Reprinted with permission.[93] Copyright 2021, Royal Society of Chemistry.</figcaption></figure>

The performance of mixed electrodes is even better when the proportion of Cu(OH)₂ on the surface is high, as the oxidation process of the aldehyde to carboxylic acid (HMF to HMFCA or DFF to FFCA, FFCA to FDCA) must occur twice as often as the oxidation of alcohol to aldehyde (HMF to DFF or HMFCA to FFCA) (Figure 1). DFT calculated free energy diagram for HMFOR at 1.42 V<sub>RHE</sub> on a Cu-based Prussian blue analog catalyst confirmed the ability of CuO to oxidize aldehyde groups with a low activation energy.[90] Curiously, the impact of Cu content in mixed Ni-Cu catalysts on HMFOR performance has been little studied. We highlight the study by Zhang et al.[93] which suggests that HMFOR activity depends on the Cu content, peaking at a Ni/Cu molar ratio close to 8 (Figure 5f). The study explored Ni<sub>1-x</sub>Cu<sub>x</sub>(OH)₂ (x = 0, 0.05, 0.1, and 0.2) catalysts, prepared by a co-precipitation method and deposited on carbon paper, mainly composed of Ni(OH)₂ nanosheets decorated with amorphous Cu(OH)₂ islands. Best HMFOR performances in 1.0 M KOH with 5 mM HMF were achieved at 1.45 V<sub>RHE</sub> on Ni<sub>0.9</sub>Cu<sub>0.1</sub>(OH)₂ nanosheets in which the Ni/Cu molar ratio determined by ICP was 8.35. Recently, Wang et al.[100] prepared various CuO-Ni(OH)₂ heterostructure nanosheets with Ni/Cu molar ratios ranging from 2 to 4, using a microwave-assisted deep eutectic solvent (DES) method and a subsequent refluxing in KOH. The material with a Ni/Cu molar ratio of 3 achieved nearly 100% of FDCA yield and FE<sub>FDCA</sub> at 1.48 V<sub>RHE</sub> (Table 1), confirming the excellent selectivity of Cu-doped Ni catalyst for FDCA. Recently, Ding et al.[101] confirmed that a Ni/Cu molar ratio of 3 is optimal. They used a straightforward preparation method by soaking NF directly into a solution containing Ni²⁺ and Cu²⁺ precursors. The resulting Ni<sub>0.75</sub>Cu<sub>0.25</sub>Oₓ material achieved excellent HMFOR performances (Table 1) in a H-cell

coupling very high current densities (up to 1 A cm⁻² at 1.42 V<sub>RHE</sub> with 100 mM HMF in 1 M KOH) and nearly 100% FE<sub>FDCA</sub>.

We attempted to compare the catalytic activities for HMFOR of various mixed Ni-Cu-based catalysts described in the literature by calculating the FDCA production rate expressed in g m⁻² h⁻¹. FDCA selectivity and FE<sub>FDCA</sub> are consistently close to 100% across all studies (at least >90%), rendering them less critical for comparison (Table 1). It is tricky to compare the HMFOR activities from one study to another because the operating conditions vary such as the HMF concentration, the anolyte volume, and the nature of the membrane or separator between the anodic and cathodic compartments. Moreover, the mass of deposited catalyst is not considered in these estimates as it is often not measured or specified. Nevertheless, the Table 1 allows to identify trends, such as the fact that the FDCA production rate is highly dependent on the HMF concentration. The highest FDCA production rate was obtained for the highest HMF concentration (100 mM) on a catalyst composed of Cu₂O-doped amorphous Ni(OH)₂ needle-like nanowires prepared via an hydrothermal method.[102] An impressive FDCA production rate of 1990 g m⁻² h⁻¹ was achieved at 1.424 V<sub>RHE</sub> in 1 M KOH although the probable degradation of HMF is not taken into account but can explain the low FDCA yield of 85%. We also noted that the activity measured for a low HMF concentration, 5 mM, is quite similar in the range 11–19 g m⁻² h⁻¹, regardless of the catalyst or the nature of the support. This underlines that the HMFOR activity is primarily limited by the HMF chemisorption step on the catalyst and/or by HMF mass transport limitations at the electrolyte/interface as pointed out by recent publications.[98,103] A recent study investigated the use of Ni-Cu catalysts in a continuous-flow AEM electrolyzer. The setup
<figure><img src="image_30.png" /></figure>

employed Ni<sub>0.75</sub>Cu<sub>0.25</sub>O<sub>x</sub>/NF at the anode and Ni<sub>1</sub>Mo/MoO<sub>2</sub> at the cathode, both with sizes of 5 × 5 cm. A single pass of 200 mM HMF in 1 M KOH with a flow of 0.58 mL min<sup>-1</sup> was nearly fully converted to FDCA at a constant current of 400 mA cm<sup>-2</sup> (cell voltage of 2.07 V). The FDCA production rate reached 830 g m<sup>-2</sup> h<sup>-1</sup> (Table 2) and remained stable over 100 h, despite significant Cu leaching detected by ICP-MS.

### 3.4. Ni–Mn Bimetallic Catalysts

The use of manganese is also a subject of intense study, mainly inspired by the natural occurrence of water-oxidizing complexes containing this metal in their catalytic clusters.[104] Manganese alone can easily adopt a wide variety of oxide/hydroxide structures, some of which are electrochemically inert. These inactive phases appear with ease upon cycling in alkaline conditions, rapidly compromising the capacity of Mn-based electrodes compared to other transition metal oxides.[105,106] However, when incorporated into NiOOH structures, manganese contributes to the exposure of more active sites for HER, promoting better absorption of hydrogen on the surface.[107,108] Mn–Ni interactions generally exhibit less charge transfer resistance, eliciting better performances for both HER and OER, and in consequence lower overpotentials are observed.[109,110] This bifunctional feature is very attractive for co-electrolysis of water and other organics as well. A recent publication investigated the impact of anionic and cationic vacancies in Ni(OH)<sub>2</sub> catalysts on HMFOR performances.[111] Both materials were prepared hydro-thermically on carbon paper (Table 1). The Ni(OH)<sub>2</sub> with anionic vacancies (av-Ni(OH)<sub>2</sub>/CP) was produced by Mn doping as confirmed by electron paramagnetic resonance (EPR) spectroscopy. The catalyst with cationic vacancies (cv-Ni(OH)<sub>2</sub>/CP) was doped with Cu, which was later etched out using an NH<sub>4</sub>OH solution, thereby creating the cationic vacancies in the electrode. The remaining content of Mn in the av-Ni(OH)<sub>2</sub>/CP, around 6 at%, was evenly distributed along the material. The av-Ni(OH)<sub>2</sub>/CP electrode demonstrated a superior performance as a result of the interaction with the Mn, which induced a larger oxygen-vacancies content (45.37%) estimated by XPS analysis, significantly higher than cv-Ni(OH)<sub>2</sub>/CP (28.9%) and Ni(OH)<sub>2</sub>/CP without defects (31.0%). This is in line with the expanded Ni(OH)<sub>2</sub> lattice upon doping with Mn observed in XRD and further STEM observations. These oxygen vacancies are supposed to improve the HMF adsorption capacity. In H-cell tests with 50 mM HMF in 1.0 M KOH, the catalyst reached 20 mA cm<sup>-2</sup> at 1.4 V<sub>RHE</sub>, that is 266 mV lower than OER (Table 1). The maximum HMF conversion of 99.0% occurred at 1.45 V<sub>RHE</sub> after 140 min (58 C), with an FDCA selectivity of 99.5% and a FE<sub>FDCA</sub> of 98.5%. However, the FDCA production rate was within the same range as those observed for pure Ni-based electrodes (Table 1). Additionally, in this case also, the HMF degradation was not taken into account despite the high HMF concentration and pH. As in the case of iron, NiMn catalysts display promising results in the form of LDH structures. Yang et al.[112] studied LDHs and ultrathin LDHs (UT-LDHs) of both NiMn and NiFe prepared by a co-precipitation method to achieve near single layer LDHs with a 2–3 nm average thickness (molar Ni/Mn

ratio = 3) deposited on NF. The authors found that NiMn UT-LDHs enabled a HMFOR working potential window (Table 1) of 200 mV at 40 mA cm<sup>-2</sup> instead of only 64 mV for NiFe UT-LDH. In addition, higher HMF conversion, FE<sub>FDCA</sub> and FDCA yield were observed compared to NiFe structures (Table 1). Moreover, the NiMn UT-LDHs attained FE<sub>FDCA</sub> larger than 95% in a relatively wide window potential (1.37–1.47 V<sub>RHE</sub>). Using in situ XANES, the authors characterized the oxidation states of Ni and Mn during OER and HMFOR (Figure 6a). In 1.0 M KOH, the valence state of Ni in NiMn UT-LDHs increased with the applied potential, rising from 1.65 at OCV up to 3.84 at 1.55 V<sub>RHE</sub>, confirming the formation of Ni<sup>4+</sup> species during OER. Simultaneously, the oxidation degree of Mn increased from 3.1 at OCV up to 3.98 at 1.45 V<sub>RHE</sub>. In the presence of 10 mM HMF, the Ni oxidation state was stable around 2.2 up to 1.65 V<sub>RHE</sub>, while the Mn valence increased. This suggests that the deprotonation kinetics at Mn–O sites is faster than that of HMFOR, delaying the onset of OER compared to other catalysts. Conversely, no significant deprotonation occurs at Fe–OH sites in the presence of HMF. This observation further supports the lower performance of NiFe-based materials in HMFOR applications. Bao et al.[113] tested different Ni-to-Mn ratios on LDHs. The best performing LDH catalyst was found at a 1:5 molar ratio (Ni<sub>1</sub>Mn<sub>5</sub> LDH), directly synthesized on Ni foam. An elongation of the Ni(OH)<sub>2</sub> lattice was found compared to the (003) crystal plane of pure α-Ni(OH)<sub>2</sub> XRD pattern. This was caused by the presence of Mn<sup>3+</sup> and Mn<sup>4+</sup> species, as confirmed by XPS. In consequence, faster charge transfer kinetics occurred in comparison to single Ni LDHs, which can promote the oxidation of Ni<sup>II</sup> to Ni<sup>III</sup>. In the presence of 10 mM HMF, Ni<sub>1</sub>Mn<sub>5</sub> LDHs/NF promoted the oxidation through the HMFCA route at 1.4 V<sub>RHE</sub> (1.0 M KOH), attaining a FE<sub>FDCA</sub> about 97% and a very high FDCA yield of 95% within only 1 h reaction. After five consecutive cycles, Li et al.[114] found that the most efficient Ni:Mn molar ratio in Mn-doped NiS nanosheets supported on graphite felt (GF) is 10:1. This Mn<sub>0.2</sub>NiS/GF catalyst was prepared from 0.2 mmol of Mn precursor and 2 mmol of Ni precursor in a one-step hydrothermal process, achieving a final content of Mn in the nanosheets of 4.7 wt%. Preliminary tests in a three-electrode H-cell revealed a remarkable HMFOR working potential window of around 300 mV at 100 mA cm<sup>-2</sup> (Table 1). Furthermore, Mn<sub>0.2</sub>NiS/GF exhibited an outstanding FDCA production rate of 4570 g m<sup>-2</sup> h<sup>-1</sup> under a constant current of 500 mA cm<sup>-2</sup> for 100 mM HMF in 1.0 M KOH. Complete HMF conversion was achieved in just 20 min, effectively minimizing its degradation. During the chronopotentiometry, the potential increased from 1.49 to 1.78 V<sub>RHE</sub>. Mn<sub>0.2</sub>NiS/GF also demonstrated a stability for about ten consecutive cycles. EIS analysis confirmed not only a significantly lower charge transfer resistance for the Mn<sub>0.2</sub>NiS/GF in the presence of HMF vs OER but also against the undoped electrodes. In addition, DFT simulations suggest that new adsorption sites for HMF molecules might be formed on the Mn moiety of the structure. The performance of Mn<sub>0.2</sub>NiS/GF was finally evaluated in a scaled-up continuous-flow electrolyzer, with a 10 cm<sup>2</sup> Mn<sub>0.2</sub>NiS/GF anode while the nature of the cathode material was not specified. The reaction was conducted with 100 mM HMF in 1.0 M KOH controlling the current density at 500 mA cm<sup>-2</sup> with a flow rate of 200 mL min<sup>-1</sup>. LSVs in the flow cell reactor show a lower overpotential of around
<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 6.** a) Optimal liner combination fitting result of the X-ray absorption near-edge structure (XANES) spectra and average oxidation state of the Ni and Mn K-edge of NiMn UT LDHs catalyst in 1 M KOH with and without 10 mm KOH. Reproduced with permission.[112] Copyright 2023, John Wiley and Sons.
b) LSV curves of Mn0.2NiS/GF in 100 mL of 1 M KOH with or without 100 mM HMF in a continuous-flow electrolyser. c) Anode potential and charge passed for HMFOR by Mn 0.2NiS/GF in 100 mL of 1 M KOH with 100 mm HMF. d) Concentration changes of HMF and its oxidation products during HMF electrooxidation at 500 mA cm⁻². e) FDCA selectivity, yield, and FE obtained by the Mn0.2NiS/GF for 5 consecutive cycles of HMFOR in a continuous-flow electrolyser. Reprinted from permission.[114] Copyright 2023, John Wiley and Sons.</figcaption></figure>

≈180 mV of HMFOR versus OER (Figure 6b). Notwithstanding, the process exhibited an outstanding performance and stability under this very high current corresponding to stable anode potential of 1.68 V<sub>RHE</sub> (Figure 6c) The FDCA production rate was as high as 44 kg m<sup>-2</sup> h<sup>-1</sup> (Table 1).

After 20 min, the HMF was completely consumed, achieving a 98.3% selectivity for FDCA, a 97.6% yield of FDCA, and an FE<sub>FDCA</sub> of

94.2% (Figure 6d). These results suggest that, despite the high anode potential, the OER was effectively suppressed or negligible by using 100 mM HMF. The exceptional HMFOR performance remained nearly stable over five consecutive cycles (Figure 6e), with only a slight decrease in FDCA yield and selectivity. This is an important result, as operating at high HMF concentrations and current densities (typically >500 mA cm⁻²) is essential for
<figure><img src="image_33.png" /></figure>

efficient FDCA production at an industrial scale. In the same vein as above, Xu et al.[115] investigated the HMFOR performances of Mn-doped Ni₂P catalysts with three initial atomic ratios of Ni/Mn salts of 3, 5, and 7. Catalysts were prepared by a microwave-assisted deep eutectic solvent method followed by a phosphidation step at 350 °C. It was found that the Mn content strongly impacted the microstructure, the sample Mn-5Ni2P (with a Ni/Mn at. ratio of around 5, measured by EDS) exhibiting a unique morphology of Ni₂P nanocrystal-decorated amorphous nanosheets. This catalyst demonstrated stronger adsorption capacity for HMF at open-circuit potential in agreement with Li et al.[114] DFT simulations suggested that Mn sites can modify the electronic properties of Ni sites and then provide new adsorption sites for HMF. Best HMFOR performances were achieved on Mn-5Ni₂P (Table 1). The FDCA production rate at 1.43 V<sub>RHE</sub> was 30.6 g m⁻² h⁻¹, i.e., in the same range than pure Ni catalysts and much lower than that of Mn<sub>0.2</sub>NiS prepared by the hydrothermal route.[114]

All these reports highlight the promising impact of Mn doping on the HMFOR activity of Ni-based catalysts. Several studies suggest that Mn doping enhances the adsorption capacity of HMF, promotes the electrooxidation of Ni²⁺ to Ni³⁺, and delays the OER onset due to the rapid deprotonation rate at Mn–O sites. However, the optimal at. Ni/Mn ratio remains undefined, with studies covering a wide range from 0.2[112] to 15.39.[111] The stability of manganese in alkaline media is another critical factor that must not be overlooked. Even at pH 13, repeated cycling of Mn surfaces into the OER potential region and back to reducing conditions leads to an irreversible fraction of Mn oxides that can no longer be reduced.[116] This unreduced fraction increases with the number of cycles. Additionally, according to Rabe et al.[116] the Mn dissolution starts at pH 13 from 1.37 V<sub>RHE</sub>, while significant dissolution take place near the OER onset potential. This makes Mn unsuitable as a standalone HMFOR catalyst. However, the incorporation of Mn into the Ni structure may mitigate its dissolution. Systematic studies are essential to evaluate the long-term stability of NiMn catalysts. Furthermore, the operating potentials for HMFOR must be carefully controlled and as low as possible (ideally below 1.37 V<sub>RHE</sub>) to prevent or minimize activity loss.

### 3.5. Multimetallic Ni-Based Catalysts

Recently, more complex electrocatalyst compositions, involving three or more metals, have been investigated for the HMFOR. The Ni-Co-Fe system, well-known for its excellent performance in alkaline OER, has been reported by several groups to exhibit improved performance for HMFOR over bimetallic systems. An early 2020 study by Zhang et al.[117] describes how hydrothermally grown NiCoFe LDHs nanosheets can efficiently drive the oxidation of HMF. When immersed in a 1.0 M NaOH electrolyte containing 5 mM of HMF, a current density of 10 mA cm⁻² was reached at 1.53 V<sub>RHE</sub> on NiCoFe LDH. This represented a 20 mV cathodic shift compared to the same electrode immersed in 1 M NaOH alone, showing a narrow HMFOR working potential window, most likely due to the presence of Fe (Table 1). Interestingly, the Tafel slope measured on NiFeCo changed from

140 to 68 mV dec⁻¹ upon addition of 5 mM of HMF to 1.0 M NaOH, revealing faster oxidation kinetics. Moreover, increasing HMF concentration resulted in a noticeable decrease in overpotential while increasing the temperature expectedly increased the kinetics of HMF conversion. At 55 °C in the presence of 10 mM of HMF, a current density of 30 mA cm⁻² was reached with a 110 mV cathodic shift compared to an HMF-free electrolyte. Finally, on this NiCoFe LDH electrocatalyst, HMF was converted to FDCA at 55 °C with a relatively low 85% selectivity at 1.54 V<sub>RHE</sub>. The calculated FDCA production at 55 °C and 1.54 V<sub>RHE</sub> (464 g m⁻² h⁻¹, Table 1) is similar than the values reported at 25 °C on some NiCu catalysts.[87,96] In addition, the low FDCA selectivity is likely attributed to fast HMF degradation at 55 °C, a phenomenon that was not investigated in this study. A subsequent study by Xie et al.[13] demonstrated the excellent performance of hierarchical CoFe@NiFe LDH for alkaline HMFOR. This electrocatalyst was generated in a two-step fashion: first a layer of CoFe LDH was grown hydrothermally on NF, then a second layer of NiFe was electrodeposited on top of it to form the final electrode. When tested as anode in 1 M KOH, CoFe@NiFe generated 10 mA cm⁻² at 1.35 V<sub>RHE</sub> while the same current density was achieved at 1.31 V<sub>RHE</sub> upon addition of 10 mM of HMF. For comparison, a potential of 1.34 V<sub>RHE</sub> was required in the latter conditions on a reference CoFe LDH anode. This improved activity was primarily attributed to the increased specific surface area brought by the growth of the NiFe layer on the surface of the CoFe flakes. This electrocatalyst was further tested for HMFOR in 1.0 M KOH + 10 mM HMF at different applied potentials. At quite low potential, 1.34 V<sub>RHE</sub>, at full HMF conversion, a mixture of FDCA, HMFCA, DFF and FFCA was obtained. The main products were identified as FDCA (selectivity of 61.4%) followed by HMFCA. In contrast, at 1.40 V<sub>RHE</sub>, at full HMF conversion, a selectivity of nearly 100% towards FDCA was obtained (Table 2). Finally, pushing the potential more anodically toward 1.50 V<sub>RHE</sub> resulted in a significant competition with the OER, where at full HMF conversion, more than half the charges passed through the electrocatalysts corresponded to O₂ evolution. In all cases, the amount charges passed through the anode were found to correspond to the production of H₂ at the cathode (a Pt sheet) with a 100% FE. Strikingly, the authors calculated that when the anode was polarized at 1.40 V<sub>RHE</sub> in the presence of 10 mM HMF (Table 2), H₂ was generated at the counter-electrode at a rate of 468 µmol h⁻¹, i.e. 18 times higher than what was generated in the absence of HMF at the same applied potential (26 µmol h⁻¹). This led them to estimate that, when coupled with HMFOR (10 mM HMF), H₂ can be generated with an energy cost of 35.89–40.14 kWh kg⁻¹ in the potential range 1.34–1.50 V, while the corresponding energy cost was calculated to be 37.52–45.45 kWh kg⁻¹ in HMF-free 1.0 M KOH. Furthermore, we calculated that the FDCA production rate at 1.4 V<sub>RHE</sub> in the H-cell was quite high (155.8 g m⁻² h⁻¹) but not significantly higher than values reported on NiFe-LDH catalyst at lower potentials in the study of Liu et al.[68] (Table 2). Using a combination of XPS, XAS and Raman spectroscopy, the authors established that metal centers in the +III oxidation state are the active species. Through a comparison with NiFe LDH, CoFe LDH and FeOOH, they further showed that Ni³⁺ is the main active species for the HMFOR, while Co³⁺ helps reducing the overpotential. The
<figure><img src="image_34.png" /></figure>

role of Fe is not further discussed, except for the confirmation that FeOOH alone is electrocatalytically inactive for the HMFOR. More recently, sulfidized CoNiFe LDH nanoflowers (termed FeCoNi-S) were prepared by Lin et al.[118] through the sulfidation of a zeolitic imidazolate framework ZIF-68 exhibiting Co, Fe, and Ni metal sites and impregnated on NF. XPS analysis revealed that the presence of sulfur resulted in an increased electron density on the metal sites, compared to a reference CoNiFe LDH, suggesting favorable behavior towards oxidation reactions, including the HMFOR. To confirm this hypothesis, the authors tested the HMFOR performance in 1 M KOH + 50 mM HMF. Once again, FeCoNi-S showed improved performance compared to the sulfidized monometallic and bimetallic counterparts CoS, CoNi-S and FeCo-S. Indeed, FeCoNi-S produced 10 mA cm⁻² at an applied potential of 1.38 V<sub>RHE</sub>, corresponding to a 125 mV cathodic shift compared to the OER. This improved performance was primarily attributed to an increased charge density on Ni, as revealed by DFT calculations, which facilitated HMF oxidation. The HMF conversion was predominantly selective toward FDCA, though competition with the OER increased at anodic potentials higher than 1.45 V<sub>RHE</sub> (Table 1). A NiFeCo-LDH catalyst was recently grown on NF by using a hydrothermal route, followed by a calcination at 300 °C for 2 h. This catalyst achieved full HMF conversion at 1.5 V<sub>RHE</sub> in 20 mM HMF and 1 M KOH with a very high FE<sub>FDCA</sub> (Table 1). The FDCA production rate was less than 100 g m⁻² h⁻¹, comparable to values for similar hydrothermally synthesized materials. In situ Raman spectroscopy revealed that β-NiOOH acts as the active center, while the FeCo co-doping helps lower the reaction energy barrier and enhances HMF activation.

Besides the well-known Co-Fe-Ni system extensively studied in alkaline OER and also applied to HMFOR (as shown through the aforementioned reports), an increasing number of innovative Ni-including multimetallic compositions have been explored in recent years. For instance, Yang et al.[119] reported a highly efficient FeP-NiMoP₂ anode material for HMFOR. First, FeMo oxide was generated on NiFe foam (FNF) hydrothermally, followed by a phosphorization at 500 °C to afford FeP-NiMoP₂/FNF. This material displayed a nanowire morphology (5 µm × 150 nm), while STEM analysis evidenced the presence of a FeP-NiMoP₂ core-shell structure. Furthermore, XPS analysis showed that the material exhibited electron-rich Ni and Fe and electron-deficient Mo, after an electron transfer occurred between the metals. FeP-NiMoP₂/FNF was tested as anode for HMFOR in 1.0 M KOH + 10 mM HMF. Once again, a significant cathodic shift was observed in the voltammetry upon addition of HMF: 132 mV at 10 mA cm⁻² and 142 mV at 100 mA cm⁻². At 1.40 V<sub>RHE</sub>, FeP-NiMoP₂/FNF achieved the conversion of HMF to FDCA with 100% selectivity and faradaic efficiency. Post-mortem XPS analysis showed that surface phosphorus was replaced by oxygen under operating conditions, leading to an oxide surface layer. This was confirmed by the detection of leached phosphorus in the electrolyte. DFT modeling revealed that electron-deficient Mo favored the adsorption of HMF and that Mo was the active site in FeP-NiMoP₂.

This promotional effect of MoO₂ was also reported by Wu et al.[120] who prepared NiCo₂ nanowires doped by MoO₂ on

NF via a straightforward hydrothermal route followed by a reducing treatment at 500 °C. This NiCo₂@MoO₂/NF catalyst demonstrated higher activity both for HMFOR and HER compared to NiCo₂/NF and MoO₂/NF prepared using the same method. This highlights the synergistic interactions between MoO₂ and NiCo₂. The pH dependence analysis of HMFOR and the study of deuterium kinetic isotope effects revealed that MoO₂ enhances the proton transfer and facilitates the cleavage of C—H/O—H bonds during HMFOR. When used as both the cathode and anode in a two-electrode H-cell equipped with an anion-exchange membrane, the NiCo₂@MoO₂/NF catalyst achieved excellent performances at 1.4 V with 10 mM HMF in 1 M KOH, yielding nearly 100% Faradaic efficiency for both FDCA and H₂ (Table 2). The FDCA production rate of 133 g m⁻² h⁻¹ was comparable to rates reported on NiCo- and NiCoFe-based catalysts (Table 2) but was achieved at one of the lowest cell voltages.

In 2023, Zeng et al.[121] designed and studied a NiFe LDH system onto which isolated Rh atoms were anchored, labeled Rh-SA/NiFe LDH (Figure 7). The rationale given for the development of this complex architecture was the need for generating separated but synergetic active sites for the cooperative adsorption of OH* and HMF intermediates at the surface of the electrocatalyst (as opposed to more traditional Ni-based surfaces where both OH* and HMF competitively adsorb on the same site). This material was generated by, first, growing Rh single atoms on NiFeZn LDH hydrothermally, followed by etching of the Zn in alkaline conditions. XAS analysis revealed that the Rh sites were first coordinated to five oxygen atoms (Rh-O₅ sites), with a secondary coordination sphere involving Ni and Fe ions. It was further established by XPS that a significant electron transfer occurred from Ni to Rh through the Ni—O—Rh bond. The HMFOR performance of this material was studied in 1.0 M KOH + 50 mM HMF. While pristine NiFe LDH showed an onset potential of 1.31 V<sub>RHE</sub>, loading it with Rh-O₅ sites (1.25 wt% Rh) resulted in an impressive cathodic shift leading to an onset at 1.20 V<sub>RHE</sub>. Importantly, Rh/C exhibited no activity for HMFOR. The inclusion of Rh further led to a decrease in Tafel slope from 120 mV dec⁻¹ to 55 mV dec⁻¹. Moreover, this electrocatalyst afforded the conversion of HMF to FDCA with a 99.8% selectivity and a 98.5% faradaic efficiency, although the applied potential was not specified. Quasi-operando ATR-FTIR revealed that HMF oxidation likely proceeded through a mechanism in which HMF is oxidized by an adsorbed hydroxyl intermediate (rather than by the Ni³⁺ site directly). Operando XAS further showed that the Rh structure remained stable under operating conditions, and that electron transfer from Ni to Rh led to the stabilization of Ni—OH sites. Additionally, no shrinkage of Ni—O bonds towards Ni—C bonds was observed when polarizing the material, suggesting that HMF was not adsorbed on Ni sites. On the contrary, replacement of Rh—O by Rh—C bonds was observed, indicating the probable adsorption of HMF on Rh sites. Thus, the authors propose a mechanism through which adsorbed HMF on Rh reacts with electrophilic OH<sub>ads</sub> on Ni to form oxidized products. Finally, stability comparison between Rh/Ni LDH and Rh/NiFe LDH pointed toward the stabilizing power of Fe in the trimetallic structure. This electrocatalyst was also operated in H-cell as an anode for HMFOR (with 50 mM HMF) and at the cathode for HER. This resulted in very low cell voltages of 1.32,
<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /><figcaption>**Figure 7.** a) Polarization curves obtained on NiFe LDH, NiFe nanoporous mesh LDH (NiFe NMLDH), NiFe NMLDH coated with Rh single atoms (Rh-SA/NiFe NMLDH) and Rh/C in 1 M KOH + 50 mM. b) Concentrations versus charge plot of HMF, FDCA, and the intermediates at various electrolysis charges (applied potential not provided) c) Fitting curve of EXAFS spectra. Inset, showing the optimized atomic model of Rh-SA/NiFe NMLDH, Ni (gray), Fe (brown), Rh (yellow), O (red), and H (pink). d) Schematic illustration of the HMFOR mechanism over Rh-SA/NiFe NMLDH. Reprinted with permission.[121] Copyright 2023, American Chemical Society.</figcaption></figure>

1.42, and 1.48 V to reach current densities of 10, 50, and 100 mA cm⁻² respectively, with faradaic efficiencies of 100% and 97% for H₂ and FDCA respectively. The cell was finally successfully operated for 100 h at about 15 mA cm⁻² without notable loss of performance and FE<sub>FDCA</sub> higher than 90% without exploring the HMF degradation.

Overall, while most metallic electrocatalysts for HMFOR remain variations of the Ni-Co-Fe LDH system, recent advances in more complex, atomically precise hierarchical architectures are paving the way for significantly enhanced HMFOR activity on nickel-based materials. For instance, doping Ni-based catalysts with single atoms of noble metals like Rh could significantly reduce the potential required for HMFOR, far below the OER potential onset, thereby enhancing FE<sub>FDCA</sub>.

## 4. Summary and Outlook

This review offers a detailed examination of recent intensive research on the nanostructure and composition of Ni-based

catalysts, enhanced with transition metals, for the selective electrooxidation of HMF to FDCA in alkaline conditions. Several challenges hinder the industrial application of this reaction. The first drawback is the competition between HMFOR and OER, which typically occurs above 1.45–1.5 V<sub>RHE</sub>, depending on the electrolyte pH and HMF concentration. This competition can lower FE<sub>FDCA</sub>, impacting the energy efficiency of the process. Therefore, the scientific community's goal is to develop Ni-based catalysts that operate effectively below 1.5 V<sub>RHE</sub>. This is challenging because the Ni(II)/Ni(III) redox transition, which drives HMFOR, only occurs above 1.3 V<sub>RHE</sub>. A detailed analysis of recent studies (Table 1) in H-cells with low HMF concentrations indicates that doping or combining Ni with transition metals like Co, Cu, Mn, or Fe, does not significantly alter the HMFOR potential window. However, synergies with Co, Cu, and Mn can enhance efficiency by increasing the concentration of NiOOH active sites or creating additional active sites that work synergistically with Ni. Only a few catalysts have demonstrated the ability to fully convert HMF below 1.4 V<sub>RHE</sub> with high FDCA yields.[28,58,68] These catalysts feature an open morphology, such as nanosheets or nanowires
<figure><img src="image_37.png" /></figure>

dispersed on a 3D porous conductive support. This review also
attempts to establish an activity-composition relationship by
comparing FDCA production rates of recent developed Ni-based
catalysts (Table 1 and 2). At pH 14 and below 1.5 V<sub>RHE</sub>, most cata-
lysts exhibit FDCA production rates lower than 100 g m<sup>-2</sup> h<sup>-1</sup>.
Notably, catalysts combining Ni with Cu demonstrate FDCA pro-
duction rates much larger than 100 g m<sup>-2</sup> h<sup>-1</sup>, ranging from 150
to 1990 g m<sup>-2</sup> h<sup>-1</sup> [28,92,95,95,96,101,102] These findings highlight the
potential of Ni-Cu materials for developing very active HMFOR
catalysts. However, durability tests extending beyond a few hours
are scarcely reported, despite being essential for validating cata-
lyst stability, especially true for Ni-Cu and Ni-Mn materials, as Cu
and Mn are prone to dissolution in strongly basic electrolytes.

The second challenge is the instability of HMF, which can
polymerize into insoluble humins, leading to a net loss of HMF
in the process. To prevent humin formation, HMF must be rapidly
converted to FDCA, which remains stable under alkaline condi-
tions. This can be achieved using a continuous-flow electrolyser
by operating with short residence times and HMF concentrations
suitable for industrial applications. Promising results have been
recently obtained with $Mi_{0.2}NiS^{[114]}$ and $NiCuO_x^{[101]}$ catalysts
implemented in AEM electrolysers. However, high HMF concen-
trations can damage the polymeric membrane of the AEM elec-
trolyzers,[15] and FDCA can diffuse from the anode to the
cathode.[29] A promising alternative is to develop membrane-less
continuous-flow electrolyzers with a small gap between the
anode and cathode. This design minimizes cell volume and
optimizes the electrode area-to-electrolyte volume ratio. Since
no gas molecules are generated at the anode during HMF/H₂O
co-electrolysis, there is no theoretical need to separate the cathodic
reaction, where hydrogen is produced, from the anodic reaction.
The absence of oxygen in the process could reduce exposure to
highly corrosive conditions, enhancing device durability compared
to alkaline water electrolyzers. This membrane-less approach could
also significantly lower system costs, as alkaline membranes are
typically expensive and short-lived.[122] However, membrane-less
electrolyzers may experience lower voltage efficiencies due to
higher IR losses in the electrolyte.[123] Another challenge is the
HER cathode's tolerance to organic molecules and its inactivity
toward the electroreduction of HMF and its oxidation intermedi-
ates and products. The feasibility of a zero-gap membrane-less
electrolyzer for HMF/H₂O co-electrolysis was recently demon-
strated by Zhou et al.[59] who manufactured nine stacked mem-
brane-less cells. Each cell consisted of CoOOH electrodeposited
on NF as the anode and NF as the cathode. This stack was fed with
high concentrations of HMF (300–800 mM), achieving single-pass
conversions greater than 95%. However, FE<sub>FDCA</sub> did not exceed
80%, likely due to the presence of OER at the anode. However,
combining this membrane-less electrolyzer approach with high-
performance Ni-based materials holds significant promise for scal-
ing up HMF/H₂O co-electrolysis technology to industrial levels.

Acknowledgements

This work is part of the EU-ELOBIO project. The project ELOBIO
received funding from the European Union's Horizon Europe

EIC-2021-PATHFINDERCHALLENGES-01-04, under grant agree-
ment N° 101070856. Views and opinions expressed are however
those of the author(s) only and do not necessarily reflect those of
the European Union or European Innovation Council. Neither the
European Union nor the granting authority can be held respon-
sible for them. The authors also acknowledge "the PhD Mobility
Program in France (MDF) provided by the University of Ottawa,
Canada".

**Conflict of Interest**

The authors declare no conflict of interest.

Keywords: 5-hydroxymethylfurfural • electrolysis • furandicarboxylic acid • hydrogen • Ni

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Manuscript received: February 14, 2025

Revised manuscript received: April 11, 2025

Version of record online: