# Electrolysis of lignin for production of chemicals and hydrogen

Jésus González-Cobos, Mathieu S Prévot, Philippe Vernoux

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Jésus González-Cobos, Mathieu S Prévot, Philippe Vernoux. Electrolysis of lignin for production of chemicals and hydrogen. Current Opinion in Electrochemistry, In press, 10.1016/j.coelec.2023.101255. hal-04007945

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# Electrolysis of lignin for production of chemicals and hydrogen

Jesús González-Cobos, Mathieu S. Prévot, Philippe Vernoux*

Université de Lyon, Institut de Recherches sur la Catalyse et l'Environnement de Lyon, UMR
5256, CNRS, Université Claude Bernard Lyon 1, 2 avenue A. Einstein, F-69626 Villeurbanne,
France.

* Corresponding author: Vernoux, Philippe (philippe.vernoux@ircelyon.univ-lyon1.fr)

## Keywords

Lignin, Electrolysis, Hydrogen, Oxidation, Kinetics.

## Abstract

Lignin electrolysis offers the possibility of cogenerating valuable aromatic compounds and green H₂ under mild conditions and with a lower energy cost than traditional water electrolysis. In recent years, significant progress has been accomplished towards this objective through the development of active electrocatalysts and the engineering of cell configuration. This short review gives an overview of the anode materials and cells investigated in acidic and alkaline conditions, below and above 100°C and summarizes the main bottlenecks which remain to be addressed. Low-temperature electrolysis could be integrated in biorefineries to convert lignin into higher-value industrial chemicals with co-generation of hydrogen.

## Introduction

Green H₂, generated with renewable energies without CO₂ emissions, will play a key role in the energy transition [1,2]. It is mainly produced by water electrolysis either at low temperature (< 100°C) in alkaline water electrolyzers (AWE) or proton/anion-exchange membrane (PEM/AEM) electrolyzers, or at high temperature (500-1000°C) in solid oxide electrolyzers (SOE) [2,3], all of them producing H₂ and O₂ at their respective cathodes and anodes. However, the oxygen evolution reaction (OER) suffers from thermodynamic and kinetic limitations that must be overcome to make this technology more competitive. One possible solution is to replace the OER by a less-energy intensive reaction, like the oxidation of bio-sourced alcohols [4–11], but the use of these substrates directly competes with food
resources (cropland). Alternatively, this review describes recent advances of the electrolysis of lignin, a highly-abundant natural and chemically-rich polyphenolic polymer (Figure 1a) representing up to 40 wt.% of woody wastes, but currently mostly treated as worthless byproduct: for instance, Kraft lignin, coming from the residues of chemical pulping in paper production, is usually simply incinerated to produce heat [12–14]. Indeed, at the moment, only a small part of lignin waste is valorized into valuable chemicals (e.g. vanillin, vanillic acid, benzene, toluene, xylenes) by emerging processes such as pyrolysis, gasification, or catalytic oxidation and reduction [14–17].

Lignin electrolysis (Figure 1b) combines lignin selective electrooxidation at the anode with green H₂ production at the cathode [18–29], operating under mild and safe conditions (low temperature and atmospheric pressure). Among the pioneering works exploring the direct and indirect use of lignin in fuel cell technology, one can find examples of direct lignin fuel cells operating at low (<100°C) [30] and high (560°C) [31] temperature, both operating at 24 mW.cm⁻². Moreover, indirect lignin fuel cell mediated by a polyoxometalate was reported to deliver 5 mW.cm⁻² at 90°C) [32]. Finally, advanced novel concepts involving lignin electrochemistry are emerging, such as combining lignin electro-oxidation with the hydrogenation of H₂-lean liquid organic hydrogen carriers (LOHC) [33]. In the field of lignin electrolysis, recent research efforts have been classified according to their operation temperature: below 100°C in PEM/AEM-like electrolyzers or at higher temperatures, using innovative materials and cell designs. It should be noted that, from the anodic products point of view, the high variability of the lignin source composition and the wide variety of potential oxidation/depolymerization/re-polymerization reactions, lead to very complex reactants/products distributions that would be the subject of another specific review article.

## Current trends in direct lignin electrolysis for hydrogen production

### i) Low temperature (< 100°C) lignin electrooxidation

Several studies describe the electro-oxidation of pure lignin or black liquor (lignocellulosic effluent with a lignin concentration of about 40%) on noble metal anodes [18,22,23,25,34] in standard batch 3-electrode cells using liquid electrolytes at low temperature (up to 90 °C) and at lower potentials than OER. In general, electrocatalytic performances were found to increase with lignin concentration, temperature and applied voltage. Kraft lignin electrooxidation has been performed in a flow-through AEM electrolyser equipped with a commercial bimetallic PtRu₁ anode supported on carbon cloth [18]. with an onset voltage of ~0.45 V at 80°C, which is much lower than that of water electrolysis (Figure 1c). However, the obtained current densities were below 7 mA cm⁻² at 1 V, highlighting the kinetic limitations of the process, attributed to the difficult C-C cleavage required for lignin depolymerization and the strong deactivation of the anode by adsorbed reaction intermediates and products. These limitations were later confirmed by the study of the electrolysis of 2-Phenoxyethanol, a model molecule exhibiting the β-O-4 linkage [30] representing 50-65% of lignin interunit linkages [34].
<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) Schematic representation of the lignin structure, highlighting monolignols: sinapyl alcohol (in red), guaiacyl alcohol (in blue) and p-coumaryl alcohol (in green) units. Reprinted with permission from Ref. [36]. b) Scheme of a lignin electrolyser. c) Linear sweep voltammetry measured in an AEM electrolyser from 0 to 0.9 V at 80°C; scan rate: 0.5 mV s⁻¹, in an AEM lignin electrolyser with PtRu/C anode and Fumapem FAA-3 membrane; anolyte: 1 M NaOH (black) or 10 g L⁻¹ lignin in 1 M NaOH (red). The non-zero current for water electrolysis as observed in many studies can be due i) the oxidation of the electrode materials, ii) the formation of oxygen- or hydroxyl-surface species from water dissociation (below OER potential), or iii) the charging current. Reprinted with permission from Ref. [18].</figcaption></figure>

Non-noble anode materials such as Ni-based (pure or alloyed with Sn or Co) [20,21,28,29,37] and PbO₂ [28] [38] were also investigated in alkaline media, although lower current densities were obtained, vs. PtRu, at low potentials (e.g., below 2 mA cm⁻² at 1 V on Ni(OH)₂ [21]). According to Table 1, best reported current densities reach 30 mA cm⁻², a value that remain far below the typical specifications of water AEM electrolyzers (from 0.2 to 2 A cm⁻² at 1.4 – 2.0 V) [2,3], tentatively attributed to the limited
lignin depolymerization kinetics and to anode deactivation phenomena. The most notable results, in terms of H₂ production, have been obtained with the NiCo-TiO₂ electrode proposed by Naderi Nasrabadi et al., with 2.3 mL min⁻¹ measured after 24 hours polarization at 1.7 V, corresponding to an energy consumption of 4.1 kWh per Nm³ H₂ [20]. However, at this high potential, OER was significantly more favoured than lignin electrooxidation and any increase of lignin concentration was detrimental due to the blockage of the electrode pores and its catalytic sites. Moreover, Beliaeva et al. operated an AEM cell with a Ni-based anode at 80°C at a voltage as low as 1.1 V [21] and decreased the energy consumption rate to 2.6 kWh per Nm⁻³ H₂, much lower than that required in water electrolyzers (e.g., 4.3 kWh Nm³ H₂ at 1.8 V). Interestingly, even lower energy consumption can be achieved by adding a redox mediator in the electrolyte. This mediator chemically oxidizes the organic substrate in the anolyte and is subsequently electrochemically reoxidized (regenerated) on the anode. Many redox mediators have been reported in the literature for the electrooxidation of lignin and model phenolic compounds, as recently reviewed by Du et al. [39], such as nitroaromatics [40,41], halides [42], polyoxometalates (POMs) [12,43] or transition metal ions [12]. For instance, H₃PMo₁₂O₄₀ [43] and FeCl₃ [12] can oxidize biomass under heating (<100°C) or irradiation (UV) for few hours generating reduced POM and Fe²⁺ species which are electrochemically reoxidized in acidic conditions at 100°C on a noble metal-free anode like a pure graphite felt material. This indirect lignin electrolysis consumes only 0.69 kW h Nm⁻³ H₂ at 0.2 A cm⁻² [12].

The electrooxidation of lignin can produce many different valuable aromatics like vanillin, vanillic acid, sinapic acid or acetovanillone [44]. For instance, the group of Staser [29] observed that Ni and Sn-doped Ni-based anodes can generate vanillin, a high-value chemical, in alkaline media, with a maximum production rate of 300 mg kg lignin⁻¹ min⁻¹ at 1.4 V at RT while at higher voltages the OER predominates. The promoting effect of cobalt on nickel not only enhances the current densities and lowers the onset oxidation potential, as already observed for other electrooxidation reactions [45,46], but also increases the yield of aromatic compounds like vanillin, 3-methylbenzaldehyde and apocynin [37]. However, the separation and recovery of these various products in the anolyte remains a technological bottleneck. One possible strategy is to implement an in-situ selective extraction of low molecular weight products by an aqueous deep eutectic solvent associated with an extractant phase (e.g., methyl isobutyl ketone) to avoid their over-oxidation [47]. Another approach, proposed by Schmitt et al., is based on the absorption of the target fractions by a strongly basic anion exchange resin [48]. Finally, the membrane technology is probably the most promising, due to its compact and modular approach and the avoidance of additional solvent and extractant phase. In this process, a filtration membrane can be integrated, like an AEM, in a 3-chamber electrolyzer [19] (Figure 2a-b) or used downstream of the lignin electrolyzer [49] (Figure 2c) to separate the unreacted lignin (retentate) in the anolyte from the low molecular weight fractions (permeate). For instance, Figure 2d shows the time evolution of the relative molecular weight of the lignin electrooxidation products in an AEM using a Ni-
based anode (indicating enhanced lignin depolymerization with increased electrical current), while Figure 2e shows a typical chromatogram of both retentate and permeate fractions, demonstrating the ability of the ultrafiltration membrane to extract low-molecular weight compounds in the permeate stream [49].

<figure><img src="image_3.png" /></figure>

**Figure 2.** a) Scheme and b) cross section of the setup integrating the alkaline lignin electrolysis and membrane separation processes, where NFM stands for nanofiltration membrane; Reprinted with permission from [19]. c) Scheme of the setup comprising an alkaline lignin electrolyzer and a downstream membrane module; d) relative molecular weight of kraft lignin after electrolysis in an AEM with a Ni-based anode; e) example of size exclusion chromatogram of the retentate and the permeate in configuration c). Reprinted with permission from [49].

## ii) Lignin electrooxidation above 100°C

Operating an electrolyzer at temperatures above 100°C provides advantages such as better oxidation kinetics and more favourable thermodynamic conditions. The group of Hibino at the Nagoya University was the first to explore lignin electrolysis between 100 and 200°C, using a PEM electrolyzer with a $H^+$-conducting $Sn_{0.9}In_{0.1}P_2O_7$-polytetrafluoroethylene (PTFE) membrane [27,50,51]. Lignin was dissolved in $H_3PO_4$ and impregnated on the anode. In a batch cell configuration (Figure 3a), lignin could be electrooxidized from 0.25 V at 200°C on Pt/C and 0.1 V at 150°C on PtFe/C [27], a performant electrocatalyst that also stood out in direct cellulose fuel cells [52]. Faradaic efficiency values of 100% towards $H_2$ evolution on the cathode and 83-91% towards $CO_2$ formation on the anode were determined
by mass spectrometry, regardless of the temperature. C₄-C₇ aromatic and aliphatic molecules were further detected by liquid chromatography-mass spectroscopy after thermal treatment of the lignin-H₃PO₄ solution at 200°C [27]. During continuous operation at 20 mA cm⁻² and 150°C, a voltage of only 0.57 V was required after 30 min using the PtFe/C anode, corresponding to a low electric energy consumption rate of 1.36 kWh per Nm³ H₂ (Table 1), although the energy for heating was not considered.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Schemes of a) batch and b) flow-through cell configurations for intermediate-temperature electrolysis of H₃PO₄-dissolved lignin; c) I-V curves of the electrolysis of various model biomass molecules at 150°C in a batch cell. Reprinted with permission from Ref. [50]. d) Evolution of cell voltage, H₂ and CO₂ production rates with time during continuous electrolysis of newspapers at 175°C in a batch cell for different cell voltages. Anode: carbonyl-group functionalized mesoporous Kerjen black carbon, membrane: Sn<sub>0.9</sub>In<sub>0.1</sub>P<sub>2</sub>O<sub>7</sub>-PTFE, cathode: Pt/C. Reprinted with permission from Ref. [51].</figcaption></figure>

More recently, the same group has investigated the electrochemical oxidation of raw biomass dissolved in H₃PO₄, from model molecules like cellulose, starch, lignin, protein and lipid [50] to more complex lignocellulosic molecules like wastes of bread, cypress, rice chaff [50] and even newspaper [51], both in the previous batch cell configuration (Figure 3a) and in a flow-through configuration equipped with a syringe feeder (Figure 3b). Using a metal-free anode consisting of a carbonyl-functionalized mesoporous carbon, raw biomass was electro-oxidized at cell voltages lower than 0.5 V at 150°C, reaching current densities up to 0.3 A cm⁻² at 1 V and producing mainly CO₂ and NO₂ (from proteins) at the anode (Figures 3c and 3d) [50,51]. Electrochemical performances were dependent on the main functional groups (e.g., carbonyl groups in cellulose and starch were more reactive than carboxyl groups
in proteins) and on the acid-solubility (lignin and lipids being less soluble). These studies demonstrate the possibility of electrolyzing real biomass without any noble metal at moderate temperatures (100-200°C) in acidic media, resulting in lower electric energy consumption (Table 1). However, some bottlenecks remain such as sluggish kinetics, deactivation issues and heating energy requirements. In addition, above 100°C, lignin and other raw biomass are mainly electrooxidized into CO2 (Fig. 3d) to the detriment of value-added chemicals. On the other hand, high-temperature electrolysis in alkaline conditions seem to promote the electrooxidation of lignin into vanillin: for instance, the group of Waldvogel at University of Mainz recently proved that the selective depolymerization of lignin is possible in alkaline media at 160°C using simple Ni foam electrodes in a batch cell configuration at current densities of 5-40 mA cm⁻² (2-3 V) [53]. Vanillin and acetovanillone yields of 2.0-4.2 wt.% and 0.2-1.2 wt.%, respectively, were obtained depending on the lignin source and the operation conditions.

## Conclusions

Low temperature lignin electrolysis has been demonstrated with energy inputs in the 2.5 – 4.5 kWh per Nm³ H₂ range on electrodes made of non-noble metal compounds (Ni, Co, Ti, Pb) in alkaline conditions, with the selective production of H₂ at the cathode and the generation of valuable aromatic molecules at the anode. Furthermore, lignin conversion can be more efficient and less demanding in electrical energy at 150-200°C. Importantly, at these temperatures, lignin is mostly converted in acidic conditions to CO₂, reducing the added value brought by the production of platform aromatic compounds observed at low temperature. However, in alkaline conditions, the generation of aromatics seems to increase with the temperature on Ni-based electrodes. While improvements have been made regarding the energy efficiency of the process, its kinetics remain lower than the ones involved in traditional water electrolysis by several orders of magnitude. If low current densities have been ascribed to the fouling of electrodes and blocking of active sites by lignin, in-depth mechanistic studies are still lacking as well as a thorough analysis of the anodic products obtained in flow-through lignin electrolysis cells. This is partly due to the extreme chemical complexity of the molecule at hand and the wide variety of reaction and side products, requiring the development of advanced in-situ and operando analytic tools to unravel the molecules interaction with catalytic sites under operating conditions. Thankfully, significant progress is currently being made on this front, which gives hope for a rapid improvement in the understanding and efficiency of lignin electrolysis in the coming years.
<table><caption>**Table 1.** Electrocatalytic performance of anodes tested for direct lignin electrolysis for H₂ production.</caption><thead><tr><th>Anode :<br>Catalyst /<br>support</th><th>Cell configuration</th><th>Operation conditions</th><th>T<br>(°C)</th><th>Current<br>density (mA<br>cm<sup>-2</sup>)</th><th>Cell<br>voltage<br>(V)</th><th>Time<br>(h)</th><th>H<sub>2</sub><br>production<br>(mL min<sup>-1</sup>)</th><th>H<sub>2</sub> FE<br>(%)</th><th>Electric energy<br>consumption<br>(kWh Nm<sup>-3</sup>)</th><th>Ref.</th></tr></thead><tbody><tr><td>Ni / Ti</td><td>AEM cell</td><td rowspan="2">Biorefinery lignin (10 g L<sup>-1</sup>) in 1 M NaOH</td><td rowspan="2">RT</td><td>13.1</td><td>1.60</td><td>-</td><td>1.00</td><td>99.0</td><td>3.49</td><td rowspan="2">[29]</td></tr><tr><td>Ni<sub>3</sub>Sn<sub>1</sub> / Ti</td><td>Cathode: Pt<br>Membrane: FFA-3</td><td>16.0</td><td>1.60</td><td>-</td><td>1.20</td><td>99.5</td><td>3.55</td></tr><tr><td rowspan="3">NiCo-TiO<sub>2</sub> (1:3) / Carbon paper</td><td>AEM cell</td><td rowspan="3">Biorefinery lignin (50 g L<sup>-1</sup>) in 1 M NaOH</td><td rowspan="3">RT</td><td>10.0</td><td>1.60</td><td>24.00</td><td>0.74</td><td>99.0</td><td>3.91</td><td rowspan="3">[20]</td></tr><tr><td>Cathode: Pt</td><td>20.0</td><td>1.67</td><td>24.00</td><td>1.49</td><td>100.0</td><td>4.03</td></tr><tr><td>Membrane: FFA-3</td><td>30.0</td><td>1.72</td><td>24.00</td><td>2.26</td><td>101.0</td><td>4.12</td></tr><tr><td rowspan="3">NiCo/TiO<sub>2</sub> / Carbon paper / PbO<sub>2</sub>/MWNTs / Carbon paper</td><td>AEM cell</td><td rowspan="3">Kraft lignin (10 g L<sup>-1</sup>) in 1 M NaOH</td><td rowspan="3">RT</td><td>20.0</td><td>1.98</td><td>0.05</td><td>1.52</td><td>98.5</td><td>4.34</td><td rowspan="3">[28]</td></tr><tr><td>Cathode: Pt</td><td>30.0</td><td>2.15</td><td>0.04</td><td>2.28</td><td>99.4</td><td>4.73</td></tr><tr><td>Membrane: FFA-3-50</td><td>20.0</td><td>1.87</td><td>0.05</td><td>1.52</td><td>98.5</td><td>4.15</td></tr><tr><td rowspan="2">Ti/PbO<sub>2</sub></td><td>Batch AEM cell</td><td rowspan="2">Sodium lignosulfonate (10 g L<sup>-1</sup>) in 1 M NaOH</td><td rowspan="2">60</td><td>0.4</td><td>1.80</td><td>5.00</td><td>0.02<sup>a</sup></td><td>-</td><td>4.31<sup>a</sup></td><td rowspan="2">[39]</td></tr><tr><td>Cathode: Pt<br>Membrane: FFA-3-50</td><td>1.7</td><td>1.10</td><td>1.00</td><td>0.30<sup>a</sup></td><td>-100</td><td>2.63<sup>a</sup></td></tr><tr><td>Ni(OH)<sub>2</sub>/C (22 wt.% Ni) / Carbon cloth</td><td>AEM cell,<br>Cathode: Pt/C<br>Membrane: FAA-3-50</td><td>Kraft lignin (10 g L<sup>-1</sup>) in 1 M KOH</td><td>80</td><td>7.5</td><td>1.50</td><td>1.00</td><td>1.30<sup>a</sup></td><td>-100</td><td>3.59<sup>a</sup></td><td>[21]</td></tr><tr><td rowspan="2">Pt/C (40 wt.% Pt) / PtFe/C</td><td>Batch cell</td><td rowspan="2">H<sub>3</sub>PO<sub>4</sub>-impregnated Lignin (wt. ratio 1:2.8)</td><td rowspan="2">150</td><td>20.0</td><td>0.64</td><td>0.50</td><td>0.28<sup>a</sup></td><td>-100</td><td>1.53<sup>a</sup></td><td rowspan="2">[27]</td></tr><tr><td>Cathode: Pt/C<br>Membrane: Sn<sub>0.9</sub>In<sub>0.1</sub>P<sub>2</sub>O<sub>7</sub></td><td>20.0</td><td>0.57</td><td>0.50</td><td>0.28<sup>a</sup></td><td>-100</td><td>1.36<sup>a</sup></td></tr></tbody></table>

<footnote><sup>a</sup>Values calculated by assuming a H₂ faradaic efficiency (FE) of 100%.</footnote>
**Conflict of interests**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

**Acknowledgments**

The authors thank the European Union's Horizon Europe research and innovation programme under grant agreement No 101070856 ELOBIO for funding.

**References**

* of special interest

** of outstanding interest

[1] M.M. Rashid, M.K. Al Mesfer, H. Naseem, M. Danish, Hydrogen production by water electrolysis: a review of alkaline water electrolysis, PEM water electrolysis and high temperature water electrolysis, Int. J. Eng. Adv. Technol. 4 (2015) 2249–8958.

[2] A. Chapman, K. Itaoka, K. Hirose, F.T. Davidson, K. Nagasawa, A.C. Lloyd, M.E. Webber, Z. Kurban, S. Managi, T. Tamaki, M.C. Lewis, R.E. Hebner, Y. Fujii, A review of four case studies assessing the potential for hydrogen penetration of the future energy system, International Journal of Hydrogen Energy. 44 (2019) 6371–6382.
https://doi.org/10.1016/j.ijhydene.2019.01.168.

** The authors analyzed the current state of hydrogen production technology by comparing different types of electrolyzers.

[3] M. El-Shafie, S. Kambara, Y. Hayakawa, Hydrogen Production Technologies Overview, Journal of Power and Energy Engineering. 7 (2019) 107–154. https://doi.org/10.4236/jpee.2019.71007.

[4] H. Liu, W. Li, Recent advances in paired electrolysis of biomass-derived compounds toward cogeneration of value-added chemicals and fuels, Current Opinion in Electrochemistry. 30 (2021) 100795. https://doi.org/10.1016/j.coelec.2021.100795.

[5] M. Simões, S. Baranton, C. Coutanceau, Electrochemical valorisation of glycerol, ChemSusChem. 5 (2012) 2106–2124. https://doi.org/10.1002/cssc.201200335.

[6] S. Tuomi, A. Santasalo-Aarnio, P. Kanninen, T. Kallio, Hydrogen production by methanol-water solution electrolysis with an alkaline membrane cell, Journal of Power Sources. 229 (2013) 32–35. https://doi.org/10.1016/j.jpowsour.2012.11.131.

[7] A. Caravaca, F.M. Sapountzi, A. de Lucas-Consuegra, C. Molina-Mora, F. Dorado, J.L. Valverde, Electrochemical reforming of ethanol-water solutions for pure H2 production in a PEM electrolysis cell, International Journal of Hydrogen Energy. 37 (2012) 9504–9513.
https://doi.org/10.1016/j.ijhydene.2012.03.062.

[8] A. Caravaca, A. de Lucas-Consuegra, A.B. Calcerrada, J. Lobato, J.L. Valverde, F. Dorado, From biomass to pure hydrogen: Electrochemical reforming of bio-ethanol in a PEM electrolyser, Applied Catalysis B: Environmental. 134–135 (2013) 302–309.
https://doi.org/10.1016/j.apcatb.2013.01.033.
[9] A.T. Marshall, R.G. Haverkamp, Production of hydrogen by the electrochemical reforming of glycerol-water solutions in a PEM electrolysis cell, International Journal of Hydrogen Energy. 33 (2008) 4649-4654. https://doi.org/10.1016/j.ijhydene.2008.05.029.

[10] J.J. Linares, C.C. Vieira, J.B. Costa Santos, M.M. Magalhães, J.R.N. Dos Santos, L.L. Carvalho, R.G.C.S. Dos Reis, F. Colmati, Chapter 4: Electrochemical Reforming of Alcohols, RSC Energy and Environment Series. 2020-Janua (2020) 94-135. https://doi.org/10.1039/9781788016049-00094.

[11] C. Coutanceau, S. Baranton, Electrochemical conversion of alcohols for hydrogen production: a short overview, Wiley Interdisciplinary Reviews: Energy and Environment. 5 (2016) 388-400. https://doi.org/10.1002/wene.193.

** The authors provided a comprehensive review of the electrochemical conversion of alcohols for hydrogen production based on analyses of thermodynamics and kinetics aspects.

[12] X. Du, W. Liu, Z. Zhang, A. Mulyadi, A. Brittain, J. Gong, Y. Deng, Low-Energy Catalytic Electrolysis for Simultaneous Hydrogen Evolution and Lignin Depolymerization, ChemSusChem. 10 (2017) 847-854. https://doi.org/10.1002/cssc.201601685.

* The authors reported the use of H₃PMo₁₂O₄₀ polyoxometalate or FeCl₃ as redox mediators for the low-temperature electrolysis of lignin in a PEM reactor. This is a good example of indirect lignin electrolyser, with an electric energy consumption as low as 0.69 kWh per Nm⁻³ H₂ at 0.2 A cm⁻².

[13] Z.-H. Liu, R.K. Le, M. Kosa, B. Yang, J. Yuan, A.J. Ragauskas, Identifying and creating pathways to improve biological lignin valorization, Renewable and Sustainable Energy Reviews. 105 (2019) 349-362. https://doi.org/10.1016/j.rser.2019.02.009.

[14] H. Wang, Y. Pu, A. Ragauskas, B. Yang, From lignin to valuable products-strategies, challenges, and prospects, Bioresource Technology. 271 (2019) 449-461. https://doi.org/10.1016/j.biortech.2018.09.072.

[15] L. Cao, K.M. Iris, Y. Liu, X. Ruan, D.C.W. Tsang, A.J. Hunt, Y.S. Ok, H. Song, S. Zhang, Lignin valorization for the production of renewable chemicals: state-of-the-art review and future prospects, Bioresource Technology. 269 (2018) 465-475.

[16] J. Zakzeski, A.L. Jongerius, P.C.A. Bruijnincx, B.M. Weckhuysen, Catalytic lignin valorization process for the production of aromatic chemicals and hydrogen, ChemSusChem. 5 (2012) 1602-1609.

[17] A. Kumar, M. Jindal, S. Maharana, B. Thallada, Lignin Biorefinery: New Horizons in Catalytic Hydrodeoxygenation for the Production of Chemicals, Energy Fuels. 35 (2021) 16965-16994. https://doi.org/10.1021/acs.energyfuels.1c01651.

[18] A. Caravaca, W.E. Garcia-Lorefice, S. Gil, A. de Lucas-Consuegra, P. Vernoux, Towards a sustainable technology for H₂ production: Direct lignin electrolysis in a continuous-flow Polymer Electrolyte Membrane reactor, Electrochemistry Communications. 100 (2019) 43-47. https://doi.org/10.1016/j.elecom.2019.01.016.

[19] S. Stiefel, J. Lölsberg, L. Kipshagen, R. Möller-Gulland, M. Wessling, Controlled depolymerization of lignin in an electrochemical membrane reactor, Electrochemistry Communications. 61 (2015) 49-52.

** The authors reported a pioneering 3-chamber reactor design for the simultaneous lignin depolymerization and products separation, where a nanofiltration membrane separates the unreacted lignin (retentate) from the low weight fractions in the anolyte, and the latter are spatially separated from the catholyte by an anion exchange membrane.

[20] M. NaderiNasrabadi, F. Bateni, Z. Chen, P.B. Harrington, J.A. Staser, Biomass-depolarized electrolysis, Journal of the Electrochemical Society. 166 (2019) E317-E322. https://doi.org/10.1149/2.1471910jes.

** The authors tested a NiCo-TiO₂ catalyst in an AEM cell for the electrolysis of a biorefinery lignin at room temperature, by performing the longest-term electrolysis tests (24 h) reported in the field and
quantified one among the highest production rates of pure hydrogen from lignin electrolysis, 2.3 mL min⁻¹, at 30 mA cm⁻² and 1.7 V.

[21] K. Beliaeva, N. Grimaldos-Osorio, E. Ruiz-López, L. Burel, P. Vernoux, A. Caravaca, New insights into lignin electrolysis on nickel-based electrocatalysts: Electrochemical performances before and after oxygen evolution, *International Journal of Hydrogen Energy*. 46 (2021) 35752–35764.
https://doi.org/10.1016/j.ijhydene.2021.01.224.

* The authors tested a C-supported Ni catalyst in an AEM cell for lignin electrolysis at 80°C and noticed two plausible reaction mechanisms depending on the applied voltage (below and above 1.4 V) and the expected Ni active phase (Ni(OH)₂ or NiOOH) and compared the results with those obtained in the electrolysis of water and a model molecule of the β-O-4 linkage.

[22] S.B. Lalvani, P. Rajagopal, Lignin-Augmented Water Electrolysis, *Journal of the Electrochemical Society*. 139 (1992) L1–L2. https://doi.org/10.1149/1.2069212.

[23] S.B. Lalvani, P. Rajagopal, Hydrogen production from lignin-water solution by electrolysis, *Holzforschung-International Journal of the Biology, Chemistry, Physics and Technology of Wood*. 47 (1993) 283–286.

[24] H. Roy Ghatak, Electrolysis of black liquor for hydrogen production: Some initial findings, *International Journal of Hydrogen Energy*. 31 (2006) 934–938.
https://doi.org/10.1016/j.ijhydene.2005.07.013.

[25] H.R. Ghatak, S. Kumar, P.P. Kundu, Electrode processes in black liquor electrolysis and their significance for hydrogen production, *International Journal of Hydrogen Energy*. 33 (2008) 2904–2911. https://doi.org/10.1016/j.ijhydene.2008.03.051.

[26] O. Movil, M. Garlock, J.A. Staser, Non-precious metal nanoparticle electrocatalysts for electrochemical modification of lignin for low-energy and cost-effective production of hydrogen, *International Journal of Hydrogen Energy*. 40 (2015) 4519–4530.
https://doi.org/10.1016/j.ijhydene.2015.02.023.

[27] T. Hibino, K. Kobayashi, M. Nagao, S. Teranishi, Hydrogen Production by Direct Lignin Electrolysis at Intermediate Temperatures, *ChemElectroChem*. 4 (2017) 3032–3036.
https://doi.org/10.1002/celc.201700917.

** The authors explored for the first time the direct lignin electrolysis at intermediate temperatures (100-200°C), by using Pt-based anodes and a H⁺-conducting membrane (Sn<sub>0.9</sub>In<sub>0.1</sub>P<sub>2</sub>O<sub>7</sub>). They also studied the doping effect of Fe in a PtFe anode, finding an onset oxidation potential of only 0.1 V at 150°C. H₂ and CO₂ faradaic efficiencies of 100% and 83-91% were measured, respectively, and the electric energy consumption proved to be much lower than in low-temperature lignin electrolysis (1.4 kWh per Nm⁻³ H₂ at 20 mA cm⁻²).

[28] F. Bateni, M. Naderi, *Nasrabadi, R. Ghahremani, J.A. Staser*, Low-cost nanostructured electrocatalysts for hydrogen evolution in an anion exchange membrane lignin electrolysis cell, *Journal of the Electrochemical Society*. 166 (2019) F1037–F1046.
https://doi.org/10.1149/2.0221914jes.

* The authors explored the use of PbO₂ nanoparticles impregnated on multi-walled carbon nanotubes as anode in an AEM cell for lignin electrolysis and found highly pure hydrogen production at 30 mA cm⁻², although at slightly higher potentials than Ni-based catalysts employed in other works by the same group.

[29] R. Ghahremani, F. Farales, F. Bateni, J.A. Staser, Simultaneous Hydrogen Evolution and Lignin Depolymerization using NiSn Electrocatalysts in a Biomass-Depolarized Electrolyzer, *Journal of the Electrochemical Society*. 167 (2020) 43502.

** The authors studied the effect of Sn-doping in Ni catalysts used in an AEM cell for the electrolysis of a biorefinery lignin at room temperature. An optimum composition of Ni₃Sn₁ led to the highest electrocatalytic performance, which was directly linked to an enhanced electrochemical surface area.
This is also the only study with anodic product quantification using a direct lignin electrolysis cell at low temperature. A maximum vanillin production rate of 18 g kg<sub>lignin</sub> h<sup>-1</sup> was obtained at 1.4 V on Ni<sub>3</sub>Sn<sub>1</sub>.

[30] F. Liu, A. Lusi, H. Radhakrishnan, H. liu, W. li, H. Qin, S. Jiang, X. Bai, H. Shan, Efficient direct lignin fuel cells enabled by hierarchical nickel-iron phosphide nanosheets as an anode catalyst, Sustainable Energy Fuels. 21 (2022) 4866. https://doi.org/10.1039/D2SE00930G

[31] R. Bohn Lima, R. Raza, H. Qin, J. Li, M.E. Lindström, B. Zhu, Direct lignin fuel cell for power generation. RSC Advances. 3 (2013) 5083. https://doi.org/10.1039/C3RA23418E

[32] X. Zhao, J.Y. Zhu, Efficient Conversion of Lignin to Electricity Using a Novel Direct Biomass Fuel Cell Mediated by Polyoxometalates at Low Temperatures, ChemSusChem. 9 (2016) 197. http://dx.doi.org/10.1002/cssc.201501446.

[33] G. Centi, S. Perathoner, C. Genovese, R. Arrigo, Advanced (photo)electrocatalytic approaches to substitute the use of fossil fuels in chemical production. Chem. Commun. 2023. https://doi.org/10.1039/D2CC05132J

[34] K. Beliaeva, M. Elsheref, D. Walden, F. Dappozze, A. Nieto-Marquez, S. Gil, C. Guillard, P. Vernoux, S.N. Steinmann, A. Caravaca, Towards Understanding Lignin Electrolysis: Electro-Oxidation of a β-O-4 Linkage Model on PtRu Electrodes, Journal of The Electrochemical Society. 167 (2020) 134511. https://doi.org/10.1149/1945-7111/abb8b5.

[35] S. Dutta, K.C.-W. Wu, B. Saha, Emerging strategies for breaking the 3D amorphous network of lignin, Catal. Sci. Technol. 4 (2014) 3785-3799. https://doi.org/10.1039/C4CY00701H.

[36] M.S. Karunarathna, R.C. Smith, Valorization of Lignin as a Sustainable Component of Structural Materials and Composites: Advances from 2011 to 2019, Sustainability. 12 (2020) 734. https://doi.org/10.3390/su12020734.

[37] R. Ghahremani, J.A. Staser, Electrochemical oxidation of lignin for the production of value-added chemicals on Ni-Co bimetallic electrocatalysts, Holzforschung. 72 (2018) 951-960.

[38] J. Li, W. Zhou, Y. Huang, J. Gao, Lignin-Assisted Water Electrolysis for Energy-Saving Hydrogen Production With Ti/PbO2 as the Anode, Frontiers in Energy Research. 9 (2021). https://www.frontiersin.org/articles/10.3389/fenrg.2021.762346 (accessed December 8, 2022).

[39] X. Du, H. Zhang, K.P. Sullivan, P. Gogoi, Y. Deng, Electrochemical Lignin Conversion, ChemSusChem. 13 (2020) 4318-4343. https://doi.org/10.1002/cssc.202001187.

** Based on a large bibliography, the authors analyzed the current state of lignin electrochemical conversion, including both direct and indirect routes.

[40] C. Smith, J.H.P. Utley, M. Petrescu, H. Viertler, Biomass electrochemistry: Anodic oxidation of an organo-solv lignin in the presence of nitroaromatics, J Appl Electrochem. 19 (1989) 535-539. https://doi.org/10.1007/BF01022110.

[41] V.L. Pardini, C.Z. Smith, J.H.P. Utley, R.R. Vargas, H. Viertler, Electroorganic reactions. 38. Mechanism of electrooxidative cleavage of lignin model dimers, J. Org. Chem. 56 (1991) 7305-7313. https://doi.org/10.1021/jo00026a022.

[42] W.-J. Gao, C.M. Lam, B.-G. Sun, R.D. Little, C.-C. Zeng, Selective electrochemical CO bond cleavage of β-O-4 lignin model compounds mediated by iodide ion, Tetrahedron. 73 (2017) 2447-2454. https://doi.org/10.1016/j.tet.2017.03.027.

[43] W. Liu, Y. Cui, X. Du, Z. Zhang, Z. Chao, Y. Deng, High efficiency hydrogen evolution from native biomass electrolysis, Energy and Environmental Science. 9 (2016) 467-472. https://doi.org/10.1039/c5ee03019f.

[44] N.D. Fidio, J.W. Timmermans, C. Antonetti, A.M.R. Galletti, R.J.A. Gosselink, R.J.M. Bisselink, T.M. Slaghek, Electro-oxidative depolymerisation of technical lignin in water using platinum, nickel oxide hydroxide and graphite electrodes, New J. Chem. 45 (2021) 9647-9657. https://doi.org/10.1039/D1NJ01037A.
[45] T. Brousse, D. Bélanger, J.W. Long, To Be or Not To Be Pseudocapacitive?, J. Electrochem. Soc. 162 (2015) A5185. https://doi.org/10.1149/2.0201505jes.

[46] X. Cui, W. Guo, M. Zhou, Y. Yang, Y. Li, P. Xiao, Y. Zhang, X. Zhang, Promoting Effect of Co in NimCon (m + n = 4) Bimetallic Electrocatalysts for Methanol Oxidation Reaction, ACS Appl. Mater. Interfaces. 7 (2015) 493–503. https://doi.org/10.1021/am506554b.

[47] D.D. Marino, V. Aniko, A. Stocco, S. Kriescher, M. Wessling, Emulsion electro-oxidation of kraft lignin, Green Chem. 19 (2017) 4778–4784. https://doi.org/10.1039/C7GC02115A.

[48] D. Schmitt, C. Regenbrecht, M. Hartmer, F. Stecker, S.R. Waldvogel, Highly selective generation of vanillin by anodic degradation of lignin: a combined approach of electrochemistry and product isolation by adsorption, Beilstein J. Org. Chem. 11 (2015) 473–480. https://doi.org/10.3762/bjoc.11.53.

[49] S. Stiefel, A. Schmitz, J. Peters, D.D. Marino, M. Wessling, An integrated electrochemical process to convert lignin to value-added products under mild conditions, Green Chem. 18 (2016) 4999–5007. https://doi.org/10.1039/C6GC00878J.

* The authors explored the use of different Ni anodes for the electrolysis of lignin in a setup configuration where the electrolyzer is connected to a membrane module for the continuous separation of the low weight fractions derived from the lignin depolymerization.

[50] T. Hibino, K. Kobayashi, M. Ito, Q. Ma, M. Nagao, M. Fukui, S. Teranishi, Efficient Hydrogen Production by Direct Electrolysis of Waste Biomass at Intermediate Temperatures, ACS Sustainable Chemistry and Engineering. 6 (2018) 9360–9368. https://doi.org/10.1021/acssuschemeng.8b01701.

** The authors explored the use of a functionalized carbon black as anode for the direct electrolysis of a wide variety of raw biomass (cellulose, starch, lignin, protein, lipid, wastes of bread, cypress, rice chaff) in acid media at 100-150°C. An onset cell voltage as low as 0.3 V was reported, with high faradaic efficiencies of c.a. 100% for H₂ production at the cathode and c.a. 90% for CO₂ at the anode.

[51] T. Hibino, K. Kobayashi, M. Ito, M. Nagao, M. Fukui, S. Teranishi, Direct electrolysis of waste newspaper for sustainable hydrogen production: an oxygen-functionalized porous carbon anode, Applied Catalysis B: Environmental. 231 (2018) 191–199. https://doi.org/10.1016/j.apcatb.2018.03.021.

** The authors explored the use of a functionalized carbon black as anode for the direct electrolysis of the cellulose and lignin contained in newspapers at 100-175°C. They found an onset oxidation voltage of c.a. 0.2 V, and an electric energy consumption rate of only 1.3 kWh per Nm³ H₂ at 0.15 A cm⁻².

[52] T. Hibino, K. Kobayashi, P. Lv, M. Nagao, S. Teranishi, High Performance Anode for Direct Cellulosic Biomass Fuel Cells Operating at Intermediate Temperatures, BCSJ. 90 (2017) 1017–1026. https://doi.org/10.1246/bcsj.20170163.

[53] M. Zirbes, L.L. Quadri, M. Breiner, A. Stenglein, A. Bomm, W. Schade, S.R. Waldvogel, High-Temperature Electrolysis of Kraft Lignin for Selective Vanillin Formation, ACS Sustainable Chem. Eng. 8 (2020) 7300–7307. https://doi.org/10.1021/acssuschemeng.0c00162.