Volume 11, Issue 3 e202300483
Research Article
Open Access

Oxidative Depolymerisation of Kraft Lignin: From Fabrication of Multi-Metal-Modified Electrodes For Vanillin Electrogeneration via Pulse Electrolysis To High-Throughput Screening of Multi-Metal Composites

Ann Cathrin Brix

Ann Cathrin Brix

Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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Dr. Olga A. Krysiak

Dr. Olga A. Krysiak

Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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Ieva A. Cechanaviciutè

Ieva A. Cechanaviciutè

Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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Gal Bjelovučić

Gal Bjelovučić

Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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Dr. Lars Banko

Dr. Lars Banko

Chair for Materials Discovery and Interfaces, Institute for Materials, Faculty of Mechanical Engineering, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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Prof. Dr. Alfred Ludwig

Prof. Dr. Alfred Ludwig

Chair for Materials Discovery and Interfaces, Institute for Materials, Faculty of Mechanical Engineering, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

ZGH, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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Prof. Dr. Wolfgang Schuhmann

Corresponding Author

Prof. Dr. Wolfgang Schuhmann

Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

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First published: 19 December 2023
Citations: 3

Graphical Abstract

Electrocatalytic lignin oxidation: Valorisation of biomass as an alternative anode reaction in water electrolysis can mitigate energy costs by value-added product generation. The targeted lignin oxidation to vanillin suffers from parasitic reactions such as oxygen evolution and vanillin oxidation. Here, pulse electrolysis and scanning droplet cell-assisted performance screening are presented as methods to aid the search for active anodes and prevent overoxidation.

Abstract

The production of green hydrogen may be greatly aided by the use of an alternative anode reaction replacing oxygen evolution to increase energy efficiency and concomitantly generate value-added products. Lignin, a major component of plant matter, is accumulated in large amounts in the pulp and paper industry as waste. It has excellent potential as a source of aromatic compounds and can be transformed into the much more valuable aroma chemical vanillin by electrochemical depolymerisation. We used a flow-through model electrolyser to evaluate electrocatalyst-modified Ni foam electrodes prepared by a scalable spray-polymer preparation method for oxidative lignin depolymerisation. We demonstrate how pulsing, i. e. continuously cycling between a lower and a higher applied current, increases the amount of formed vanillin while improving the energy efficiency. Further, we present a scanning droplet cell-assisted high-throughput screening approach to discover suitable catalyst materials for lignin electrooxidation considering that a suitable electrocatalyst should exhibit high activity for lignin depolymerization and simultaneously a low activity for vanillin oxidation and oxygen evolution. Combining electrosynthesis and electrocatalysis can aid in developing new customised materials for electrochemical processes of potential industrial interest.

Introduction

To counter the adverse effects of climate change, our industry of the near future must not only include a sustainable energy infrastructure but also synthesise chemicals from renewable sources as current fossil fuel-based methods will have to be replaced due to depleting stores and CO2 emissions. Green hydrogen, for example, could be produced via electrochemical means. However, water electrolysis faces a severe challenge as the counter reaction to the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), is often limiting the overall performance due to its sluggish kinetics.1 One approach to mitigate this could be to replace the OER with a reaction that can simultaneously generate value-added products to potentially be run at lower overpotentials and to subsidise the H2 price. Biomass waste valourisation via anodic transformation would be a prime example of such a reaction.2 This may even become the focus for the anodic production of highly value-added fine chemicals with concomitant HER as a valuable byproduct.3 Lignin, a major component of any plant matter, is obtained in large amounts from the pulp and paper industry and has great potential as a source of aromatic compounds such as the well-known aroma chemical vanillin.4, 5 Using electrosynthetic approaches, the transformation of lignin to vanillin has already been explored as a green method that can be run at ambient conditions and avoid using toxic and carcinogenic chemical oxidisers such as nitrobenzenes.6 However, the lignin structure is quite complex and varies based on its plant source and processing method.7

To influence and potentially improve the yield of the target product, an electrocatalytic approach that can run at ambient to moderate conditions using a material with optimal substrate interaction via selective adsorption and therefore efficient depolymerisation may be beneficial.

In literature, different anode materials have been explored. Parpot and coworkers,8 for instance, investigated Ni, Cu, Au, Pt, PbO2 and dimensionally stable anodes for the oxidative depolymerisation of Kraft lignin to vanillin in batch and flow cells and found that the electrode material influences the reaction rates while conversions and yields depend on the applied current density. They observed higher conversions in the flow than in the batch cell. Zirbes et al.9 also conducted a screening of different transition metals using commercial alloys based on Ni, Co, Fe and Ti in batch cells for the oxidation of Kraft lignin at moderate temperatures to obtain vanillin with good selectivity. They found that Co-based materials produced the highest vanillin amounts (1.8 wt%) but corroded while Ni-based alloys were stable with comparably lower yields. Hence, they chose Ni foam for its superior space-time yield at which higher charges could be applied after an activation process in black liquor. With a temperature increase between 25 and 95 °C an almost linear increase in vanillin yield was recorded and maximum yields were obtained in 3 M NaOH. Generally, Ni9, 10 and Co9, 11 are well-known to be active and are abundant and affordable materials. Furthermore, Ir/IrO2-,12, 13 Ti/TiO2-,12, 14 and Pb/PbO2-based anodes15, 16 have often been reported, however significant drawbacks for their use are their high price in the case of Ir, Pb's toxicity which makes it unsuitable for food applications, and the fact that assistance via H2O2 generation is often used in conjunction with high applied potentials which is not in line with the desired direct electrocatalytic oxidation.

Recently, so-called “high-entropy alloys” have gained a lot of interest for numerous (electro)catalytic applications.17 They are usually defined as compositionally complex alloys in the form of solid solutions comprised of at least five elements of approximately equimolar ratio. They benefit from unique properties often surpassing the performance of the individual components.18, 19 The vast multi-metal materials space opened up, illustrated by the many possible elements that can be chosen as well as the variation of their respective ratios, calls for the use of high-throughput techniques for catalyst discovery.20, 21 For example, the scanning droplet cell technique has been well-established for electrocatalytic activity screening for oxygen electrocatalysis and HER.19, 22-25 Another way to influence the yield of the desired product is to avoid follow-up oxidation of a primarily formed target compound. Pulse electrolysis, for instance, is known to improve energy efficiency and change the electrode surface, thus, removing poisoning species.26-31 It may also offer the possibility to tune the selectivity by allowing the intermediates to diffuse away from the working electrode before being further oxidised.

Here, we present the performance testing of modified Ni foam anodes prepared by a spray-polymer method for oxidative lignin depolymerisation in a flow-through electrolyser. We could show that galvanostatic pulse electrolysis is beneficial for improving the vanillin amount formed from lignin and introduce a scanning droplet cell-assisted approach to find suitable materials by not only screening for lignin oxidation activity but also taking competing reactions into account and testing selectivity towards the target product vanillin compared with byproducts acetovanillone and vanillic acid.

Results and Discussion

Electrochemical Activity Screening of Multi-Metal Catalysts for Lignin Oxidation

Following a previous publication from our group featuring a spray-polymer catalyst preparation method (cf. Figure S1),18 a series of four- and five-component multi-metal powder composites were studied for lignin oxidation.

For this purpose, voltammetry was performed using catalyst inks drop-cast on rotating disc electrodes in the absence (Figure 1a) and the presence of approximately 0.6 wt% Kraft lignin in 3 M KOH (Figure 1b). This concentration was chosen based on previous literature accounts favouring low lignin concentrations to avoid dimerisation reactions.32, 33 Zirbes and coworkers33 in particular have found optimal vanillin yields with 0.6 wt% lignin.

Details are in the caption following the image

Rotating disc electrode voltammetry measurements of different four- and five-component multi-metal materials (5 mV s−1, 1600 rpm): a) OER measured in 3 M KOH, b) lignin oxidation reaction measured in the presence of ~0.6 wt% alkali lignin, c) comparison of oxygen evolution and lignin oxidation activities of the selected material CoCuNiFeAl.

It is important to not only evaluate the activity for the reaction of interest itself, which is lignin oxidation in this case, but also for any parasitic reaction which may take place, such as the OER. Thus, it is not sufficient to look merely at the most active material in the presence of lignin (CoCrFeNiMo, Figure 1b, 7 – light blue line) if it exhibits virtually the same performance for the OER, or materials that show lower apparent activity towards lignin oxidation than for the OER (e. g. CoFeNiRuCr, Figure 1a and 1b, 12 – dark green line). Rather it is important to find catalysts that are significantly more active towards lignin oxidation than towards the OER. CoCuNiFeAl (5 – blue line) fits this criterion in the high potential range with significantly higher currents being recorded for lignin oxidation than OER as shown in Figure 1c and Figure S2 which presents the current density ratio of lignin oxidation to OER at maximum applied potential (see also Figure S3). Furthermore, the second cyclic voltammogram (CV 2) does not show any significant deviation from the CV 1, indicating initial stability. As this material is also made up of abundant elements, it was chosen for further investigation.

Performance Evaluation Using a Flow-Through Cell Electrolyser

The use of a flow-through cell enables not only the monitoring of electrocatalytic activity via voltammetric and chronoamperometric or chronopotentiometric techniques but also samples can be taken easily from the anolyte reservoir several times during long-term electrolysis. The cell design developed in our group34, 35 allows for implementing a three-electrode configuration and hence for making the anode the non-limiting electrode.

As shown in Figure 2, the CoCuNiFeAl modified Ni foam working electrode fabricated with the same spray-polymer method (Figure S1) and a Ni wire counter electrode compartment are separated by an anion exchange membrane. An Ag/AgCl/KCl (3 M) reference electrode is installed in the anode compartment. Anolyte and catholyte are recirculated throughout the measurement to enable the accumulation of products. Multiple samples taken over the course of the electrolysis showed that lignin was successfully depolymerised by the progressive discolouration of the anolyte with increasing electrolysis time. This was confirmed by UV-vis measurements, where the absorption intensity decreased with increasing electrolysis time (Figure S4). The two peaks observed in the UV-vis spectra around 280 nm and approximately 370 nm originate from benzene rings substituted by hydroxyl or methoxyl groups36 and conjugated phenolic structures,37 respectively. However, high-performance liquid chromatography (HPLC) results (Figure 2b) do not reflect the formation of expected value-added products such as vanillin and byproducts acetovanillone or vanillic acid but only show their depletion over time, which is supposedly due to overoxidation of the target product. This suggests that vanillin is easily oxidised further at these electrolysis conditions. Interestingly, vanillin is already present in the solution before any potential has been applied (t=0), which may originate from residues in the lignin material after processing. This has also been observed in a previous literature account of Zirbes and coworkers.9 It, therefore, makes more sense to report the amount of vanillin in a sample compared with the amount detected before electrochemistry.

Details are in the caption following the image

Performance test of CoCuNiFeAl/Ni foam at 1.55 V vs. RHE: a) Flow-through cell setup featuring a modified Ni foam working electrode (WE), a Ni wire counter electrode (CE) and an Ag/AgCl/KCl (3 M) reference electrode (RE); b) detection of the products vanillin, acetovanillone and vanillic acid by high-performance liquid chromatography (HPLC). Points are connected by dashed lines as guides for the eye. The inset photograph shows the discolouration of the lignin-containing electrolyte with increasing electrolysis time up to 24 h.

According to the chronoamperogram displayed in Figure S5, CoCuNiFeAl/Ni foam exhibits good stability. An increase in current density is recorded shortly after the start of applying 1.55 V vs. RHE up to approximately 8 h when a stable value of around 32 mA cm−2 is reached.

Due to the highly oxidising conditions, namely high oxidative potentials and strongly alkaline electrolyte, the metal components likely have been transformed into electrocatalytically active oxides(hydroxides). NiOOH and CoOOH, for instance, are well-known as active phases in electrocatalytic oxidations of alcohols and lignin.10, 38

Improving Vanillin Formation by Pulse Electrolysis

One possibility to increase the vanillin yield could be to alleviate the problem of overoxidation by influencing the situation near the working electrode. The reactant lignin is a high-molecular-weight biopolymer that has to diffuse to the working electrode in order to yield the target product vanillin as illustrated in Figure 3a.

Details are in the caption following the image

Pulse electrolysis concept: a) Schematic representation of diffusion paths from the bulk solution (light blue) of e. g. lignin (*structure adapted from Ref. [7]) and vanillin molecules to and from the working electrode through the diffusion layer (yellow, thickness not drawn to scale); b) pulse profile with pulse magnitude (E or j) and duration (▵t) and concentration profile indicated in green.

Applying a pulse method comprised of a continuous cycle of e. g. applying a certain potential E1 or current density j1 for a certain duration ▵t1 and afterwards immediately switching to a second higher potential E2 or current density j2 for a shorter duration ▵t2 without any waiting time in between (cf. Figure 3b) would affect the two molecules very differently as they have different diffusion coefficients related to their significant difference in size. This may give vanillin enough time to diffuse away from the working electrode after its formation and, thus, prevent its further reaction to undesired side products or even CO2.

As shown in Figure 4a, a first try of potentiostatic pulse electrolysis by applying 1.46 V vs. RHE for 5 s immediately followed by 1.56 V vs. RHE for 1 s and repeating this sequence for a total electrolysis time of 4 h did not achieve an improvement over the static electrolysis at 1.55 V vs. RHE. In both cases, the vanillin amount detected after 0.5, 2 and 4 h did not exceed the amount observed before electrochemistry was applied. However, changing to a galvanostatic pulse electrolysis sequence which cycles between 5 mA cm−2 for 5 s and 50 mA cm−2 for 1 s results in a slight increase of vanillin after 0.5 h of electrolysis.

Details are in the caption following the image

Pulsed lignin electrolysis at CoCuNiFeAl/Ni foam: a) Comparison of vanillin formed determined by HPLC for static electrolysis and different pulse electrolysis methods; b) potential plotted against time from galvanostatic lignin pulse electrolysis (5 mA cm−2/5 s, 50 mA cm−2/1 s, 69.5 °C) with inset zoomed-in around 0.5 h to show the potential-time curves of individual pulses.

These applied currents were chosen based on the voltammetry data displayed in Figure S6. The magnitude of the first current density was chosen at a lower value to apply what one may call a “waiting pulse” to allow products to diffuse away from the anode that is higher than the open circuit potential (OCP) to avoid delays by charging effects. For the “reaction pulse”, a second higher current density at a factor of 10 was applied, which is still at not too high potential to avoid the parasitic OER. Elevated applied current densities are expected to improve the performance, although Zirbes and coworkers9 found that moderate applied current densities between 10 and 12.5 mA cm−2 were optimal for increased vanillin yields from lignin. Here, we intentionally apply a higher current density for a shorter duration to achieve efficient bond cleavage for oxidative lignin depolymerisation as visualised and discussed in Figure 3.

Furthermore, with a slightly altered set-up (cf. Figure S7) for electrolysis at increased temperature (reaching almost 70 °C inside the anode compartment), a significant improvement of the detected vanillin amount could be achieved using pulse electrolysis compared to static electrolysis. By increasing the anolyte temperature, the necessary potential is also lowered considerably, namely by 110 mV for the first pulse and 160 mV for the second pulse compared to the same pulse program conducted at room temperature. It has been previously shown, that increased temperature is beneficial for oxidative lignin depolymerisation,33 as the lignin particles unfold which leads to an increased number of accessible reaction sites.39

The inset in Figure 4b gives a closer look at the individual potential-time curves of the pulses by zooming in around 0.5 h. Hence, the progression of the black points recorded during the first pulses is supposedly related to the depletion of the substrate from the diffusion layer as schematically illustrated in Figure 3.

This indicates that pulsing is indeed beneficial for improving the vanillin amount obtained from lignin electrolysis. Additionally, the energy efficiency is increased as a lower potential is applied for a comparably longer time and only short pulses at increased potential are applied compared to static electrolysis.

Scanning Droplet Cell-Assisted Exploration of Multi-Metal Materials for Oxidative Lignin Depolymerisation

Another way to improve the vanillin yield from oxidative lignin depolymerisation is to find a suitable electrocatalyst material. As shown above, multi-metal composites can exhibit favourable properties as electrocatalysts in terms of activity and selectivity. However, when considering the material space that multi-metal oxides open up with millions of possible combinations, one needs a high-throughput method to efficiently explore different metal combinations and compositions rather than the standard RDE screening approach which is much more time-consuming in comparison. The scanning droplet cell technique is well-established to find favourable materials towards hydrogen evolution (HER), oxygen evolution (OER) and oxygen reduction reaction (ORR) as for a single material library made by combinatorial co-sputtering up to hundreds of individual compositions can be evaluated.19-21, 25

Lignin oxidation is much more complex than e. g. oxygen electrocatalysis (OER and ORR) as it competes with the OER but also with further oxidation of the desired product vanillin, as pointed out above. Therefore, it is necessary to adapt the screening criteria to find materials that are not performing very well for OER and vanillin oxidation but are very good lignin oxidation electrocatalysts. This type of selection process is illustrated for the material system Cu−Co−Pd in Figure 5. This materials library was chosen as it previously exhibited good conductivity but low activity towards the parasitic OER. It also includes Co which has previously demonstrated very high activity towards lignin oxidation.9, 11 Figure 5a depicts the OER activity map based on the measured current densities at 1.6 V vs. RHE in 1 M KOH on the thin-film materials library, Figure 5b shows the same with 10 mM vanillin present in the electrolyte, and Figure 5c in presence of about 0.6 wt% lignin. Therein, high activities (current densities at the fixed potential) are indicated with tiles of lighter colour up to yellow while low activities are displayed with darker colours down to dark blue/purple.

Details are in the caption following the image

Comparison of scanning droplet cell measurements of a Cu−Co-Pd thin-film materials library at 1.6 V vs. RHE with different electrolytes: a) 1 M KOH (OER); b) 10 mM vanillin in 1 M KOH (vanillin oxidation); c) 0.6 wt% lignin in 1 M KOH (lignin oxidation). The colour code of the scale bar indicates low activities (current densities) for purple/dark blue tiles and high activities (current densities) at lighter colours with yellow as the maximum. Comparison of voltammograms from the three different reactions for d) point 1 (Cu44Co41Pd15) and e) point 2 (Cu47Co40Pd13), which are marked in c).

Based on the activity maps, two points of interest were chosen (encircled in Figure 5c) to compare the respective voltammograms of the different reactions more closely. The composition measured by energy-dispersive X-ray spectroscopy is included in Figure S8, while the target positions during co-sputtering of a respective metal are indicated in Figures 5a to 5c with the element symbol. Point 1 (Figure 5c) with the composition Cu44Co41Pd15 exhibits higher activity in terms of current density at a fixed potential towards lignin oxidation compared to vanillin oxidation and OER (at 1.6 V vs. RHE: 49.42 μA in the presence of lignin, 27.79 μA in the presence of vanillin, 15.05 μA in 1 M KOH) which is maintained throughout the applied potential range (Figure 5d). Point 2 (Cu47Co40Pd13, Figure 5e), on the other hand, exhibits voltammetry curves that are much closer to each other in the presence of lignin and vanillin, to the point where there is even a cross-over between them at potentials higher than 1.7 V vs. RHE.

Therefore, the composition Cu44Co41Pd15 was chosen for further selectivity investigation. For this purpose, a Ni foam-based electrode was modified with a mixture of polyvinylpyrrolidone (PVP) and the respective metal nitrate salts in the desired ratio and subsequently annealed (cf. Figure S1).18 Using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy quantification (SEM-EDX), it was confirmed that the substrate was covered fully even inside the pores of the Ni foam and the desired composition was obtained (cf. Figure S9 and S10). The obtained electrode was then tested as anode in the flow-through electrolyser using the galvanostatic pulse technique at elevated temperature.

As can be seen in Figure 6a, even lower potentials were recorded for Cu44Co41Pd15/Ni foam than during the first pulsing tests with CoCuNiFeAl/Ni foam. Furthermore, a higher vanillin amount was formed already after half an hour of electrolysis, namely 1.6 μmol (cf. Figure 6b). For direct comparison of the chromatography data (taking the volume change upon sampling into account) we have chosen to present the data in this unit. In literature, it is often reported with respect to the weight of lignin provided in solution. For instance, the Waldvogel group9, 33, 40 has reported 1.4 wt% total vanillin yield from Kraft lignin (MeadWestvaco Indulin AT) with unmodified Ni electrodes at 70 °C. We have observed the strictly electrochemical formation of 0.25 wt% of vanillin here, however, the fact that qualitative information of how much vanillin has been present in solution before electrolysis is, to the best of our knowledge, never reported at these specific conditions makes the truthful comparison with previous literature accounts difficult. Faradaic efficiency is a popular metric for electrocatalytic performance evaluation, however, almost never reported for lignin electrooxidation. This is because for this reaction, it is difficult to determine as the exact structure of lignin is unknown and one would need to determine the stoichiometric coefficients of reactant and product for its calculation. We have included a detailed discussion on this in the Supporting Information.

Details are in the caption following the image

Performance of Cu44Co41Pd15/Ni foam during lignin pulse electrolysis at elevated temperature (ca. 70 °C): a) Potential recorded during pulsed electrolysis, b) product amounts detected via HPLC.

Only very small amounts of the acetovanillone byproduct were detected. While the amount of vanillin decreases, vanillic acid increases progressively with increasing electrolysis time. This indicates that vanillin may be further oxidised to its corresponding acid. In previous static electrolysis measurements (cf. Figure 2) this was not observed, supposedly as all compounds were degraded at a much faster rate. This demonstrates the efficacy of the pulsed electrolysis technique. The combination of identification of a suitable material with the help of the scanning droplet cell technique applied on thin-film materials libraries and product selectivity quantification using pulsed flow-through cell electrolysis proves to be a suitable workflow to discover and test new multi-metal electrocatalysts out of a large number of possible combinations. It demonstrates that one can effectively avoid the time-consuming RDE voltammetry screening in favour of conducting the presented high-throughput method assisted by the scanning droplet cell technique which can naturally be expanded to materials libraries comprised of different numbers of elemental components.

Conclusions

Multi-metal-modified Ni foam-based anodes for oxidative lignin depolymerisation using a flow-through cell electrolyser in combination with a galvanostatic pulsing technique is advantageous over a static electrolysis method in terms of energy efficiency and selectivity. Integrating a scanning droplet cell screening of the catalyst activity opens up the exploration of a vast multi-metal material space to be tested for the transformation of lignin to value-added products such as vanillin by not only screening for lignin oxidation activity but also taking the competing reactions oxygen evolution and vanillin oxidation into account. Preparing a Ni foam-based electrode from the most favourable composition, Cu44Co41Pd15, and applying a galvanostatic pulse method resulted in a significant increase in vanillin amount compared to static chronoamperometry initially shown for CoCuNiFeAl/Ni foam electrodes. This work paves the way for the finding and study of active anodes for oxidative lignin depolymerisation and opens up the possibility of exploring multi-metal (oxide) materials for this transformation. We intend to optimise and refine the pulse electrolysis technique towards vanillin electrosynthesis from the biomass-derived waste product lignin in the future.

Experimental Section

Pre-Screening of Materials Using Rotating Disc Electrode Voltammetry

A three-electrode setup consisting of a Pt wire counter electrode separated by a frit (G5), a double junction Ag/AgCl/KCl (3 M) reference electrode and a glassy carbon working electrode. The latter was modified with catalyst ink by drop-casting 4.8 μL of a catalyst suspension (5 mg mL−1) of 49 : 49 : 2 vol % water:ethanol:Nafion (Sigma-Aldrich) on the 0.1134 cm2 glassy carbon disc after tip sonication (Bandelin Sonopuls) for 3 min (25 % amplitude, 1 s on, 1 s off) and bath sonication for 10 min.

Potentials were applied with an Autolab PGSTAT302N controlled by the NOVA 1.11 software. For oxygen evolution in 3 M KOH impedance spectra were recorded (0 A, 50 μA perturbation current, 100 kHz–100 Hz) followed by potentiodynamic conditioning for 20 cycles between 0 and 0.4 V vs. Ag/AgCl/KCl (3 M) at 100 mV s−1 and 1 cycle between 0 and 0.8 V vs. Ag/AgCl/KCl (3 M) at 5 mV s−1. Then, the electrolyte was changed to 3 M KOH containing ca. 0.6 wt% lignin (alkali, Sigma-Aldrich) and impedance spectra were repeated with the same conditions followed by 2 voltammetric cycles between 0 and 0.8 V vs. Ag/AgCl/KCl (3 M) at 5 mV s−1.

Potentials were reported after conversion to the reversible hydrogen electrode (RHE) following equation (1). Therein, ERHE is the potential vs. RHE, EAg/AgCl/KCl (3 M) is the potential vs. the reference electrode Ag/AgCl/KCl (3 M), Ru is the uncompensated resistance, i is the current and E0Ag/AgCl/KCl (3 M) is the standard potential of the reference electrode (0.21 V.
(1)

Ni Foam-Based Electrode Fabrication and Characterisation

Modification of Ni foam electrode substrates (Goodfellow) was conducted with a spray-polymer technique published elsewhere.18 For this purpose, 5 mmol total metals in the desired percentage ratio were dissolved in 5 mL polyvinylpyrrolidone (PVP, average MW ~55,000, Sigma-Aldrich) solution made by dissolving 0.5 g in distilled water with the help of ultrasonication. The resulting suspension was sprayed onto Ni foam heated to 125 °C with an automated custom-built airbrush-type spray coater dispensing 2 μL every 2 mm with compressed air. Subsequently, the modified Ni foam piece was exposed to an annealing procedure comprised of two steps: First, it was heated with a heating rate of 10 K min−1 under 10 % H2/90 % Ar atmosphere to 800 °C where it was held for 2 h, followed by cooling down under Ar to 250 °C where it was treated for another 2 h in 10 % O2/90 % Ar.

The obtained electrode was characterised using scanning electron microscopy (SEM, FEI Quanta 3D FEG) and energy dispersive x-ray spectroscopy (EDS).

Electrode Testing in a Flow-Through Electrolyser

A three-electrode flow-through cell setup was used for lignin oxidation performance testing. This consists of a modified Ni foam-based anode (Goodfellow, modification procedure described in the previous section), a Ni wire counter electrode and a double junction Ag/AgCl/KCl (3 M) reference electrode. Anode and cathode compartments are separated by an anion exchange membrane (fumatech fumasep FAA-3-PK-130) and electrolytes are continuously recycled at approximately 8 mL min−1 flow rate. For measurements at room temperature, electrolytes are pumped from the reservoir through the peristaltic pump (Spetec) and then pass through the cell. For measurements where the anolyte is heated to 90 °C with a bath heater (Huber CC2), the setup was modified to minimise heat losses between the reservoir and the anode by pumping the electrolyte through the cell first before it passes through the peristaltic pump (see also Figure S1).

Before electrolysis in 3 M KOH (Fisher Scientific, purified with Chelex 100 (Sigma-Aldrich)) galvanostatic impedance spectroscopy was conducted at 0 A (50 μA perturbation current, 100 kHz–100 Hz) followed by potentiodynamic conditioning between 0 and 0.4 V vs. Ag/AgCl/KCl (3 M) at 100 mV s−1 for 20 cycles and 1 cyclic voltammogram (CV) between 0 and 0.8 V vs. Ag/AgCl/KCl (3 M) at 5 mV s−1. Then, the cell was emptied and the anolyte compartment changed to 14 mL 3 M KOH containing 95.55 mg lignin (alkali, Sigma-Aldrich). Galvanostatic impedance was repeated with the same conditions and 2 CVs were recorded in the same potential range (0–0.8 V vs. Ag/AgCl/KCl (3 M)). Then, the respective electrolysis program was applied. For static electrolysis at 1.55 V vs. reversible hydrogen electrode (RHE), the potential vs. reference for chronoamperometry was determined by taking the uncompensated resistance from the impedance measurements and the current from CV following equation (1) which is typically used to calculate the potential vs. RHE solved for EAg/AgCl/KCl (3 M).

Potentiostatic pulse electrolysis was conducted with a repeating sequence of applying 0.4 V vs. Ag/AgCl/KCl (3 M) for 5 s and 0.55 V vs. Ag/AgCl/KCl (3 M) for 1 s without any waiting time in between. For galvanostatic pulse electrolysis, similarly, 4.75 mA (5 mA cm−2, assuming an exposed geometric surface area of 0.95 cm2) was applied for 5 s and 47.5 mA (50 mA cm−2) for 1 s.

Electrolyses were conducted for up to 24 h, during which multiple samples were taken from the anolyte, including one after 5 min of circulating the electrolyte before the electrochemical measurements in the lignin-containing solution, which is indicated as t=0. For UV-vis measurements (Agilent Cary 60), 30 μL of each sample was taken and filled up to 3 mL to achieve 1 : 100 dilution. In a quartz cuvette, spectra were recorded between 200 and 800 nm compared with an ultrapure water background. High-performance liquid chromatography (HPLC) was conducted using a Dionex ICS-5000+ (Thermo Fisher) equipped with a Shimadzu Shimpack GWS 5 μm C18 column which was operated at 25 °C and 1 mL min−1 with eluent comprised of 420 mL methanol (absolute, VWR), 6 mL acetic acid (≥99.8 %, Sigma-Aldrich) filled up to 1 L with ultrapure water. Vanillin (99 %, Sigma-Aldrich), acetovanillone (98 %, Sigma-Aldrich) and vanillic acid (≥97.0 %, Sigma-Aldrich) were calibrated in the concentration range of 0.1 mM to 2 mM. Prior to the measurement, samples were acidified with 45 μL concentrated sulfuric acid (≥95 %, Fisher Scientific), centrifuged at 4000 rpm for 30 min and filtered through a 0.2 μm syringe filter. An example chromatogram is featured in Figure S11.

Scanning Droplet Cell Screening of a Thin-Film Materials Library

The Cu−Co-Pd thin-film materials library was synthesised by combinatorial co-sputtering from individual elemental targets on an oxidised Si wafer (500 nm SiO2) substrate.19 The film thickness was approximately 150 nm. The chemical composition of the material library was measured by energy-dispersive X-ray spectroscopy in a scanning electron microscope with an acceleration voltage of 20 kV. The scanning droplet cell setup consists of a movable electrochemical measurement head equipped with a Pt wire counter electrode and an Ag/Ag/KCl (3 M) reference electrode while the library is connected as the working electrode. Oxygen evolution was measured in 1 M KOH in the potential range between 0 and 0.8 V vs. Ag/AgCl/KCl (3 M) at 10 mV s−1. Vanillin oxidation was measured in the same potential range and with the same scan rate as OER in 10 mM vanillin in 1 M KOH and lignin oxidation similarly in approximately 0.6 wt% lignin (alkali, Sigma-Aldrich). A lower KOH concentration than for the flow-through cell tests was used to avoid electrolyte spreading on the sample surface. Potentials are converted to the RHE scale with equation (1).

Acknowledgments

This work was funded by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the research unit FOR 2982 “UNODE – Unusual Anode Reactions” (413163866). Open Access funding enabled and organized by Projekt DEAL.

    Conflict of interest

    The authors declare no conflict of interest.

    Data Availability Statement

    The data that support the findings of this study are available from the corresponding author upon reasonable request.