Electrochemical decarboxylation of acetic acid on boron-doped diamond and platinum functionalised electrodes for pyrolysis-oil treatment

Electrochemical decarboxylation of acetic acid on boron-doped diamond electrodes (BDD) was studied as a possible means to decrease the acidity of pyrolysis oil. It is shown that decarboxylation occurs without competitive OER on BBD electrodes to form methanol and methyl acetate by consecutive reaction of hydroxyl radicals with acetic acid. The performance is little affected by the applied current density (and associated potential), concentration, and the pH of the solution. At current densities above 50 mA/cm 2 , FEs of 90% towards decarboxylation products are obtained, confirmed by situ electrochemical mass spectrometry (ECMS) investigation showing only small amounts of oxygen formed by water oxidation. Using platinum-modified BDD electrodes, it is shown that ethane-selectivity, the Kolbe product, strongly depends on the shape and geometry of the platinum particles. Using nano-thorn alike Pt particles, a faradaic efficiency of approx. 40 % towards ethane can be obtained, whereas 3D porous platinum nanoparticles showed high selectivity towards OER. Using thin platinum layers, a high FE towards ethane was obtained >70%, which is thickness-independent at layer thicknesses above 20 nm. Comparison with other substrates revealed that BDD is an ideal support for Pt functionalisation giving advantage of stability, high valueable products formation (ethane and methanol). In short, this work provides guidelines for electrode fabrication in the context of the electrochemical upgrading of biomass feedstocks by acid decarboxylation.


Introduction
For a sustainable and circular bioeconomy, biomass has been used as feedstock for various applications including production of chemicals and fuels.Pyrolysis oil obtained from the thermochemical conversion of biomass is a low-value biogenic liquid, consisting of carboxylic acids, sugars, phenolics and lignin.Carboxylic acids are a significant fraction of pyrolysis oil with low energy content (44 MJ/Kg), and their catalytic conversion requires harsh conditions (≥ 225 °C and 60 bar) 1- 3 .Electrochemical treatment of pyrolysis oil enables the conversion of carboxylic acids into hydrocarbons by (non) Kolbe electrolysis.Kolbe chemistry has been first reported by Michael Faraday in 1834 and recently gained attention as potential enabler technology.Thus, product distributions and reaction mechanisms have been analysed.Electrochemical decarboxylation occurs by the discharge of the deprotonated acid (influenced by the electrolyte pH) at potentials > 2.1 VNHE, which is critical for forming alkyl radicals [4][5][6] .Moreover, mechanistic studies suggest that the coverage of carboxylate on the electrode surface inhibits the oxygen evolution reaction 7,8 .The reaction proceeds via acetate adsorption on the electrode surface a. PhotoCatalytic Synthesis Group (PCS-TNW), University of Twente, Drienerlolaan  followed by a proton-coupled electron transfer (PCET) 7 reaction that results in the formation of carboxyl species (Figure S1).The oneelectron transfer decarboxylation process produces carbon dioxide as the by-product along with an alkyl radical.Subsequent alkyl radical conversion depends on the decisive conditions (radical concentration, electrolyte, pH 9 ).Dimerisation results in alkane formation, whereas by disproportionation alkanes and alkenes are formed.The overoxidation of alkyl radicals leads to the formation of carbocations which deprotonate to form alkenes or react with hydroxyl ions to form alcohols via the so-called Hofer Moest reaction.Moreover, the carbocation can react with deprotonated acetate to form the respective ester.Several parameters influence the fate and selectivity of (non)Kolbe electrolysis such as pH, applied current density, solvent, and electrolyte composition 5,10 .The electrode material is also crucial, and platinum electrodes have been reported to be ideal for Kolbe electrolysis at negligible water discharge rates.For Pt electrodes, the formation of a barrier layer consisting of carboxylates is suggested to be essential to prevent water oxidation 9 .Electrode materials such as nickel and gold are considered inactive for (non)Kolbe electrolysis due to the absence of the aforementioned barrier layer.Electrochemical impedance spectroscopy (EIS) has revealed that the pseudo adsorption capacitance of CH3COO -is absent for gold electrodes 11 .Due to an immediate formation of ~30 Å thick Au2O3 (at 0.93 VSCE) 11 upon water discharge, C-C coupling reaction is inhibited.The acetate discharges (~10 Å thick layer) on Pt-O monolayers (~10 Å) 11 at around 2 V, nearby to the OER region 12 : therefore Kolbe electrolysis dominates at potentials higher than OER.Carbon based electrodes show larger activity towards alcohol products, including esters 5 .Thus, the product selectivity of (non)Kolbe electrolysis is highly dependent on the anode material.Various strategies have been proposed to minimize the use of platinum, among others platinised electrodes, platinum nanoparticles(NP) on carbon substrates, nanostructured platinum electrodes, and carbon based electrodes have been used 13- 16 .For Pt-nanoparticle -based electrodes, it has been suggested that the selectivity of the decarboxylation reaction depends on the shape and geometry of the Pt nanoparticles 15 .Yet the activity of platinum nanoparticle electrodes for (non)Kolbe electrolysis has to be confirmed 14,17 and the impact of NP loading or the influence of the substrate material is not fully disclosed.Despite the high activity of platinum electrodes for (non)Kolbe electrolysis, Pt is prone to dissolve at (non)Kolbe conditions as has been shown by Ranninger et al. 18 .In non-aqueous electrolyte, dissolution of approx.13.3 % of a 10 µm thick Pt electrode was estimated (over a span of one year electrolysis of 1 m 2 electrode), whereas a slightly lower material loss of 8.1 % would occur in water-containing electrolytes (Kolbe electrolysis was performed in 1M acetic acid in ethanol based solvent 3 V vs. Fc/Fc + ) 18 .0][21] Particularly, BDD is widely used due to its inertness towards poisoning within a wide potential window and high stability at high current densities [22][23][24][25] .For example, excessive polarisation of BDD for 250 hours in 1M H2SO4 at 1 A/cm 2 shows no sign of etching 26 , confirming the stability of BDD.Generally, BDD electrodes show higher corrosion resistance than Pt electrodes under Kolbe electrolysis conditions and thus extended operation can be achieved 5,27 .Still only a few reports are available related to acetic acid oxidation on BDD 28 , and usually electrooxidation is performed in methanol, sulphate or perchlorate containing electrolytes, while the product distribution is not well described.The decarboxylation mechanism of acetic acid on BDD is considered to occur via the formation of weakly adsorbed hydroxyl radicals or other radicals (such as peroxysulphates) 10 , which oxidise the acetate omitting a direct PCET.A direct electron transfer reaction is only observed for compounds with thermodynamic potentials well below water oxidation and formation of OH radicals.For example, formic acid is reported to be fully oxidised to CO2 via direct DET 29 .Interestingly, BDD is considered unstable at high current densities >50 mA/cm 2 in the presence of alkyl radicals 30 producing dangling bonds by abstraction of OH groups from C-OH functional group.Kashiwada et al 30 reported that corrosion can be effectively prevented at high pH or moderate current densities.Despite the details known about the reaction mechanism of acetic acid on BDD, the impact of the BDD surface morphology has not yet been considered [31][32][33] .In this work, we aim to address the reaction mechanism of (non)Kolbe /indirect oxidation of acetic acid in the absence of foreign anion species and explore the stability of BDD during electrolysis in a flow cell.We show that an indirect oxidation occurs in the presence of BDD and moreover we reveal that functionalisation of the BDD surface with thin platinum films or electrodeposited Pt nanoparticles allows to tune the reaction mechanism towards Kolbe product formation.In comparison to other substrates (Graphite, Nickel foam, Fluorine doped tin oxide FTO), BDD is shown to be an ideal material for functionalisation.In addition, we discuss how the product distribution is affected by current density and pH in a flow cell.

Materials and Methods
Acetic acid (glacial, Reagent Plus®, ≥99 %, Sigma Aldrich), sulphuric acid (reagent grade, 95-98 %, Sigma Aldrich), perchloric acid (reagent grade, 70% Sigma Aldrich), sodium acetate (ACS reagent, ≥99.0 %, Sigma Aldrich), acetone (technical grade, BOOM BV), isopropanol (technical grade, BOOM BV), ethanol (technical grade, BOOM BV), hexachloroplatinic acid, potassium tetrachloroplatinate(II), potassium hexachloroplatinate(IV) were acquired from Sigma Aldrich.Acetic acid was chosen as model compound due to its abundancy in pyrolysis oil 34 .Boron doped diamond electrodes (DiaChem®) with a coating thickness of 15 nm BDD on 2mm thick tantalum substrates and a doping level of 2000-5000 ppm of boron incorporated into the diamond lattice were acquired from Condias Gmbh.The procedure for manufacturing the electrodes is reported in detail elsewhere 35 .Surface cleaning prior to electrochemical testing was performed by washing with MiliQ water (18.2Ωcm) and isopropanol, prior to ultrasonication in MiliQ (18.2 Ωcm) for 10 minutes, anodic polarisation in 1M HClO4 for 30 minutes and subsequent rinsing with MiliQ, following a protocol reported in literature 36 .For comparison Graphite (LFYGY Industry Materials Professional Supplier China), FTO (redox.meSweden) and Nickel foam (IKA, Germany) electrodes were used as electrode and substrate, respectively.Prior to use, graphite was polished with carbide paper and sonicated for 10 minutes in MiliQ.Impurities were removed electrochemically in 1M H2SO4 by potential cycling in between 0.2-1 VRHE at 50 mV/sec for 100 cycles.FTO electrodes were cleaned in an ultrasonic bath in acetone, isopropanol, ethanol and MilliQ for 15 minutes each.Nickel foam was cleaned by ultrasonication in 2M HCl solution for 10 minutes prior ultrasonication in ethanol and MiliQ water for five minutes each.Thin platinum layers (5, 20, 50, 100nm) were deposited on cleaned BDD substrates by sputtering (AJA International USA).Electrodeposition of Pt nanoparticles was carried out with different platinum salt solutions.Porous nanoflower-like platinum nanoparticles were electrodeposited from a solution of 8 mM H2PtCl6 in 0.5 M H2SO4 by applying at constant potential of -0.24 VAg/AgCl for  Kolbe electrolysis of acetic acid/sodium acetate at pH 5 was performed at 25 mA/cm 2 using BDD, graphite, FTO and nickel foam as working electrodes.Figure 1a shows the cyclic voltammograms obtained in acetate solution.Clearly, the lowest onset potential and the highest oxidative currents (39 mA/cm 2 at 3VRHE) are achieved with nickel foam (NiF), which is primarily due to its characteristic high surface area.The onset potential of 1.8 VRHE is related to OER as also confirmed by the high Faradaic efficiency for oxygen obtained during electrolysis (Figure 1b).For BDD, the CV resembles previously reported data in literature 39 , and only above 2.5 VRHE an exponential increase in currents are observed.The CV obtained with FTO electrodes closely resembles the CV obtained with BDD but the increase in current is less steep (attaining 5.7 mA/cm 2 at 3 VRHE).

Results and Discussions (Non)Kolbe electrolysis at various electrodes
Similarly for graphite electrodes, the oxidation current density at high potential is low (5.7 mA/cm 2 at 3 VRHE), contrasting its early current onset.Product selectivity's as depicted by the Faradaic efficiencies in Figure 1b, are clearly electrode material specific.Kolbe electrolysis products are not observed for NiF electrodes.With very high FE towards oxygen of up to 95%, formation of oxides on the nickel surface favour the OER reaction 40 (Figure S3).When using FTO, the OER is also dominant and only small quantities of H2O2 resulting in a faradaic efficiency of 2% were detected, besides oxygen.
Considering that acetic acid oxidation on FTO and nickel foam is inhibited, both materials are inadequate electrode materials for (non)Kolbe electrolysis.Formation of methanol and methyl acetate in equivalent FEs is achieved at graphite electrodes.Thus, overoxidation of the methyl radical to the carbocation leads to Hofer Moest product formation 5 .During chronopotentiometry (Figure S4), the rise in potential within the electrolysis time frame (50 minutes) observed for graphite and FTO electrodes, moreover, suggests that both materials exhibit instability under (non)Kolbe electrolysis conditions.The instability of FTO was reflected in the working electrode potential reaching to ~7 VRHE.This contrasts with the high stability observed for BDD where the electrode potential was stable at 3.6 VRHE (Figure S4).Furthermore, employing BDD electrodes, the electrooxidation of acetic acid is more selective to methanol as compared to graphite electrodes.Considering that the separation of azeotrope methanol and methyl acetate mixtures is challenging, the oxidation on BDD can be considered more efficient 41 .
The stability of BDD electrodes during Kolbe electrolysis was analysed by Raman spectroscopy (Figure S5) and SEM (Figure S6).The BDD coatings were investigated before and after Kolbe electrolysis as shown in Figure 2. The Raman spectras are dominated by bands at 488, 1137, 1216, 1323, and 1147 and 1550 cm -1 respectively in agreement with literature reports 42, 43 .The spectra were taken across random areas on the BDD surface and averaged for analysis (Figure S5).The Raman spectra of the as received BDD electrode confirms a high boron doping, as in general with increasing boron concentration in the diamond lattice the first order diamond phonon line 44 shifts from 1332 cm -1 to lower wavenumber.
Comparison of the BDD surface before and after electrolysis reveals a slight change in the signal of the diamond phonon mode, which shifts from 1312 cm -1 to 1323 cm -1 after electrolysis.In addition, the scattering intensity at 488 cm -1 and 1216 cm -1 decreases corresponds to the weak band due to metallic and non-metallic impurities.
Considering that the signal at 488 cm -1 is due to the boron doping level and vibration modes of boron clusters and pairs 19, 42 , Raman analysis suggest that the concentration of boron in the diamond lattice decreases with electrolysis.Degradation of BDD can occur by either boron leaching or the graphitization of carbon at the grain boundaries.The G band peak observed at 1552cm -1 is due to the bond stretching of sp 2 carbon atoms in ring and chain and is in agreement with the literature 45 .The band present at 1473cm -1 corresponds to the sp 2 carbon, trans-polyacetylene laying in the grain boundaries.Interestingly, due to the high quality of the BDD coating, polymeric carbon species 46 are not prominent before electrolysis as highlighted by the minor signal intensities at 1470 cm - 1 .Still, it is worth noting that the post treatment (see materials and methods) removed graphitized carbon effectively (Figure S6c).
Overall, Raman analysis confirmed that the electrolytic corrosion on BDD is negligible during acetic acid electrolysis at low current densities.Therefore, we conducted experiments all subsequent below 100 mA/cm 2 .The amount of boron content (within the cubic centimetre) can roughly be estimated by the empirical relationship 47 (Equation 1).
[B]=8.44*10 30 x exp(-0.048x W) (cm -3 ) (1) W is the wave number corresponds to peak of Lorentzian component of Raman spectra at 400-600cm -1 .The estimated concentration of boron doping in BDD electrode is found to be 5.66x10 20 cm 3 .We further explored the impact of electrolysis conditions (pH and current density) for the electrooxidation using a divided flow cell (compartment separation was achieved using a Nafion 324 cation exchange membrane, see Figure S2) and an acetic acid/sodium acetate solution as both catholyte and anolyte.The results are shown in Figure 3.The FE is relatively independent on the pH and current density.Methanol is the dominant product, while the consecutive reaction to methyl-acetate is most pronounced at low current densities and moderate pH values (pH 5).At constant flow conditions (60ml/min), the highest accumulative FE towards methanol was achieved at 100 mA/cm 2 .The high selectivity towards methanol is attributed to the high concentration of weakly adsorbed OH radicals, which promote oxidation of concentrated acetic acid/sodium acetate to methanol.
By decreasing the concentration of acetic acid to 0.5 M, the FE towards methanol remains dominant (Figure S7).Thus, the oxidation is not limited by the hydroxyl radical concentration or autoinhibition 39 at 0.5 M acetic acid concentrations.Interestingly, in earlier reports methyl acetate was not detected during acetic acid oxidation on BDD electrodes 28 , so here we report the full product distribution of acetic acid oxidation at BDD electrodes in different conditions for the first time.
Figure 3 The effect of current density on the electrooxidation of 1 M acetic acid/sodium acetate on BDD in a flow cell at pH 5 and the pH effect on FE towards methanol and methyl acetate at 50mA per sqcm in a flow cell at 60 ml/min (catholyte and anolyte flow).

Platinised electrodes
To alter the selectivity from the formation of methanol to Kolbe electrolysis, i.e., ethane formation, and to investigate the behaviour of Pt thin films and nanoparticles on a well-characterized and stable substrate, BDD was functionalised with thin platinum layers by sputtering (5, 20, 50 and 100 nm Pt) and Pt nanoparticles by electrodeposition.For thin film samples the ECSA normalized to the mass deposited (m 2 /gpt) was obtained by integration of the Hupd region from the CVs assuming a monolayer hydrogen charge of 210 µC/cm²Pt (Figure S8b).The ECSA normalized to the loading of the amount of platinum deposited decreases in order of increasing platinum layer thickness (Figure S8c).This is in contrast to the behaviour observed for platinum nanoparticles (Figure S16), but in agreement with literature 48 .Importantly, the inhomogeneous BDD might influence the nucleation and film growth 49 likely leading to a non-uniform coating of the substrate.Indeed, non-normalized ECSA data revealed that surface area (cm 2 ) and roughness factor increase from 5 nm sputtered layers up to 20 nm thin films and only small differences are observed for thick coatings (Figure S9b, c).In shown in Figure 4a, the onset of OER for bare BDD is observed at 2.136VRHE.Considering that for BDD electrodes the onset of OER is usually observed at potentials > 2.3 V 50 , it is speculated that oxygen evolution proceeds via hydrogen peroxide decomposition produced by OH radical recombination as depicted in equation 1-3.This is in contrast to the thin film coated Pt/BDD electrodes (Figure 4b) for which oxygen evolution and the decarboxylation reaction are well-separated in potential.The onset potential for OER is 1.98 VRHE on Pt/BDD, and only at potentials > 2.1 VRHE, Kolbe and indirect oxidation products are detected.Importantly, at the onset of Kolbe product formation, OER is inhibited, being in strong contrast to the profiles observed for bare BDD.In addition, oxygen evolution is decreasing during successive cycling, while Kolbe product formation is more pronounced, as revealed by the intensity of ethane (m/z = 30).Thus, likely a barrier layer builds up 7 during successive cycling enabling a shift in selectivity towards Kolbe.Finally, it is apparent that Kolbe products are primarily produced on Pt/BDD as revealed by the higher currents of the respective signals.Nevertheless, the presence of the methanol and methyl acetate suggest that the BDD surface is not fully covered with Pt and exposed BDD facets enable OH formation and alcohol and ester formation.
Constant current electrolysis of acetic acid was subsequently performed at 25 mA/cm 2 using variable thickness of Pt (Fig 5).For 5 nm Pt on BDD electrodes, initially a high selectivity towards OER is observed and only during extended polarisation, product formation is transitioning to the Kolbe product formation (Figure S11).This is likely due to the formation of ( 110) and (100) facets of Pt (Figure S9a), which are not present during the CV recorded before electrolysis.After 50 minutes constant polarisation, the selectivity of the reaction for the 5 nm thick Pt film shifted primarily to ethane, but is still limited to a FE of ~59% (Figure S10).CVs recorded in 1M acetic acid/sodium acetate shows the presence of infection zone at 2.5VRHE for 20,50,100nm Pt layer, the peak oxidation current of 33.5 mA/cm 2 were achieved with 5nm Pt layer.The absence of inflection zone at the 5nm Pt layer shows the dominancy of OER, which also reflects in the product distribution where upto 80% faradaic efficiency reflects the selectivity of reaction toward OER as the electrolysis starts.For all other electrodes, i.e. with Pt films > 20 nm, instantaneous Kolbe product formation is observed at FE > 70 % (Figure S11c).This is still lower than obtained for Pt foil, where reproducibly a FE towards ethane formation of ~90% is observed (Figure S12).Simultaneously, methyl acetate is detected allowing to close the electron balance.
Thus, it appears that methyl radical recombination is still less favourable for BDD supported Pt, likely due to an insufficient surface radical concentration.Instead, the exposed BDD surface (see Figure S13) allows for formation of methanol (Figure 5f).Likely, some uncovered BDD surface is maintained, in between BDD crystals as suggested by SEM analysis (Figure 5).Considering the interesting product formation transients observed for 5 nm Pt on BDD, and in an attempt to study lower Pt loadings, BDD was functionalised with Pt nanoparticles obtained by electrodeposition (see methods and materials).
The electrodeposited mass of Pt on the BDD is variable due to different deposition time and applied potential.The FE and production rate of the products of each electrodeposited nanoparticle on the BDD were compared to evaluate the selectivity towards Kolbe electrolysis.For porous platinum nanoflowers (ED-A, SEM (see Figure 6a) an average FE toward ethane around 14% (±1.134) was obtained.Initially, the OER dominated and with faradaic efficiencies up to 50% (Figure S17).Nevertheless, during extended electrolysis, OER becomes negligible similar to the trends observed for 5 nm Pt thin films.Post-electrolysis liquid analysis revealed that Please do not adjust margins Please do not adjust margins methanol and methyl acetate were obtained at Faradaic efficiencies of 33%±2.9,7.8%±1.0,respectively.For freestanding, well-dispersed platinum nanoparticles (ED-B, SEM see Figure 6b the FE towards Kolbe products (9.2%±1.5) is low throughout the electrolysis.The OER is suppressed but the FE towards methanol (55.7%±1.8)shows that despite the coverage, the dispersed nanoparticles are not highly selective for Kolbe electrolysis and instead the decarboxylation reaction predominantly occurs at the BDD substrate.With platinum nano thorns (ED-C, SEM see Figure 6c) an ethane selectivity of up to 43%±1.9 was observed, similar to the reported high selectivity towards Kolbe electrolysis when deposited on carbon fibre paper 14 .
With extended electrolysis time the FE decreased to 20% and remained stable.In agreement with the high Kolbe product selectivity, a low FE towards methanol and methyl acetate (46.4% and 9.4% respectively) was observed.Finally, the lowest Kolbe product selectivity was observed for platinum nanocrystals (ED-D, see SEM Figure 6d), Clearly, the structure and shape of Pt particles is of high relevance for Kolbe electrolysis or OER activity as revealed by the high selectivity towards OER on Pt nanoflower structures (ED-A).
For ED-A electrodes a mass-normalised ECSA of 12.4 m 2 pt/gpt with roughness factor of 35.4 were determined.Considering that for ED-C a similar mass-normalized ECSA (13.7 m 2 pt/gpt) at a significantly lower roughness factor (11.6) has been determined, and that for ED-C samples the electrochemical surface area and roughness factor are 5.31 m 2 pt/gpt and 5.6(rf), respectively, it is likely that a high selectivity for Kolbe is achieved for a fine-balance of Pt surface to volume ratio and abundance of low coordinated Pt sites that contribute to an enhanced electrocatalytic activity 51 .To support this hypothesis, BDD modified with platinum nanoflowers were prepared using even higher Pt loadings.Therefore ED-A electrodeposition was carried for 30 minutes which results in fully covered surface without any exposed BDD facets (Figure S18a).The determined ECSA increased to 21.5% at a roughness factor of 60.5, being still intact after electrolysis.Here, OER is the predominant reaction (FE 97%, Figure S19).Eliminating the effect of substrate, the complete inhomogeneous coverage fails to promote Kolbe electrolysis due to exceedingly high roughness factors and porosity, which omits formation of a homogenous acetate barrier layer.It also proves that for promoting the Kolbe reaction, flat surfaces are more favourable.Finally, it is important to note that the loss in activity observed during Kolbe electrolysis is independent of the precise shape of the platinum nanoparticles.This is also in agreement with the ECSA data obtained before and after electrolysis (Figure 6e) that reveal a loss of 80-90% of nanoparticles during electrolysis.Stabilisation of Pt nanoparticles might be achieved by selective modification of the (111) facets of BDD as reported in literature 52 .Facet selective modification will be evaluated in ongoing research.
To disclose the importance of the substrate used for Pt nanoparticle deposition, electrodeposition of nanoflower-like Pt particles (ED-A) has been repeated on different substrates (graphite, FTO, nickel foam).The particles retain the same shape when deposited on graphite and FTO substrate but differences in surface coverage were noticeable (Figure 7a-d).On NiF the same electrodeposition method resulted in the formation of a Pt layer as confirmed by EDX analysis shown in Figure S20).Kolbe electrolysis performed under the same conditions as used for Pt/BDD samples revealed that only for Pt/FTO (Figure 7) electrodes (non)Kolbe product formation was observed.As shown above, bare FTO samples are inactive for Kolbe electrolysis, yet after Pt nanoparticle deposition ethane was detected at a FE of up to 11% at the expense of OER.Moreover, traces of H2O2 were detected.For Pt-modified graphite electrodes, the product distribution is also slightly influenced by the presence of Pt and OER was increased to 5% at similar methanol and methyl acetate formation rates.This shows that the exposed surface of the substrate is greatly influencing the selectivity of Kolbe electrolysis.Finally, Pt was not effectively influencing the efficiency and exclusively OER was taking place.In comparison with bulk platinum electrodes, tailoring of selectivity towards (non)Kolbe electrolysis is not only affected greatly by the size, shape or facets of nanoparticles but also the substrate is greatly influencing the reaction that is predominantly occurring.Therefore, it is important to find a correlation between the coverage of platinum nanoparticle, shape, substrate to design an electrode that efficiently promotes Kolbe electrolysis.

Conclusion
The (non)Kolbe/indirect oxidation was performed on platinum and BDD electrodes in 1 M acetic acid/Na-acetate at pH 5. It is shown that Please do not adjust margins Please do not adjust margins BDD can be used as standalone electrode for alcohols and ester production from carboxylic acid, or as ideal substrate for surface functionalization.The decarboxylation reaction on bare BDD is OHradical mediated, and results in methanol, methyl acetate and ethane production as shown by ECMS.The reaction is slightly limited by the concentration of acetic acid and autoinhibition for concentrated electrolytes (above 1M acetic acid solution).The reaction selectivity can be shifted by tuning the surface of the BDD with a thin layer of platinum or by using electrodeposited platinum nanoparticles.For thicker Pt layers (>20 nm), the reaction is highly selective to Kolbe products and less selective to indirect oxidation products.The use of nanoparticles can alter the selectivity towards (non)Kolbe electrolysis, but still low faradaic efficiencies are retained which decrease over time.The complex shapes and structures of platinum nanoparticles are decisive factors for stability on the substrate and selectivity towards Kolbe electrolysis.Other electrode substrates (FTO, Ni and graphite) are not a suitable for the extreme conditions necessary, and generally promote side reactions instead of Kolbe electrolysis.

Open
Access Article.Published on 05 April 2023.Downloaded on 5/4/2023 10:33:36 AM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online DOI: 10.1039/D3FD00066DPlease do not adjust margins Please do not adjust margins 15 minutes, denoted as ED-A hereafter.Dispersed platinum nanoparticles were obtained from a 1 mM K2PtCl4 in 0.5 M H2SO4 solution during a constant potential treatment (-1 VAg/AgCl for 200 seconds), and labelled as ED-B.Thorn-like platinum nanoparticles(active crystal planes particles grown on 2D substrates37,38 ) were produced in 2.5 mM H2PtCl6 in 0.5M H2SO4 at -0.2 VAg/AgCl for 15 minutes labelled as ED-C.Platinum nanocrystals were produced by applying -0.22 V Ag/AgCl for 15 minutes using a solution of 4.0 mM K2PtCl6 in 0.5 M H2SO4 and labelled as ED-D.The electrochemical decarboxylation reactions were performed in 1 M acetate solution at pH 5 (acetic acid sodium acetate).BDD, Graphite, FTO, NiF (bare), Pt sputtered BDD, and Pt nanoparticles on BDD were used as working electrodes.Platinised/titanium (Magneto Special Anodes B.V.) and Ag/AgCl (3M NaCl ProSense) were used as counter and reference electrodes, respectively.Electrolysis was performed in a custom-made glass cell at a constant stirring rate of 600 rpm with a Helium (99.99%) purge of 30 ml/min.Prior to batch electrolysis, the electrochemical cell and electrode holders were thoroughly rinsed with ethanol, washed five times with MilliQ water and subsequently boiled in MilliQ.Gas analysis during the reaction was performed by online gas chromatography (GC).A flame ionisation detector (FID) and a RTX-1 column (15 mL* 0.32 mm I.D, 5 μm fused silica film thickness at 45 °C) were used for detection of hydrocarbons.CO2, O2 and H2 were detected with a thermal conductivity detector (TCD) coupled with a Carboxen® 1010 column (15 mL* 0.32 mm ID at 70 °C).External calibration was conducted for CH4, C2H4, C2H6, H2, O2, and CO2 with r 2 >0.995.Hofer-Moest and other liquid products (methanol and methyl acetate) were detected by liquid analysis.Aliquots were collected from the electrolyte, diluted 10 times with water, neutralised and detected using a headspace GC-FID (Agilent) using a Zebron 7HG-G013-11 (0.25 μm ID, 250 μm, 40 m) column for product separation.Cyclic voltammetry in the potential range of (0.05 -1.2VRHE) was performed in 1 M H2SO4 solution to determine the electrochemically active surface area (ECSA) of platinised electrodes.CVs were also performed in 1 M acetic acid/sodium acetate solution (0-3 VRHE) before and after electrolysis followed by current interruption (CI) for iR correction.Chronopotentiometry was performed at 25 mA/cm 2 at room temperature and pressure, the measured potentials were converted to RHE scale.The calculations for Faraday efficiency are explained in the supporting information.Flow experiments were conducted using the commercial Condias synthesis cell kit (see FigureS2).Electrochemical mass spectrometry (ECMS by Spectro Inlets, Copenhagen, Denmark) were performed for instantaneous detection of volatile products.Particularly, signals related to methane, ethane, ethylene, methanol, methyl acetate, oxygen and CO2 were recorded during cyclic voltammetry in 1 M acetic acid/sodium acetate electrolyte.A platinum mesh and an Ag/AgCl electrode (sat.KCl, CH Instrument) were used as counter and reference electrodes, respectively.Morphology and surface structure of platinum nanoparticles and BDD electrodes were visualised using scanning electron microscopy performed with a ZEISS MERLIN HR-SEM or a JEOL JSM 7610F, respectively.The stability of the BDD electrodes was analysed by SEM and by a custom-made Raman spectroscopy setup equipped with a 650 nm laser before and after electrolysis.Due to the surface morphology and uneven BDD crystal height, three measurements were taken at different depth level from surface to inside of the sample.The scanned area is 10*10 µm and measurements were performed at 35 mW, for 150 ms at 50 kHz.The Raman spectrometer is equipped with an optical microscope (BX41 Olympus, objective Olymp 40x NA 0.95).

Figure 1
Figure 1 Effect of different electrodes on (non) Kolbe electrolysis (a) CV of BDD, Graphite, FTO, Nickel Foam conducted in 1M acetic acid/sodium acetate at pH 5, (b) Average FE towards (non) Kolbe/Hofer Moest/indirect oxidation products during electrolysis of 1M acetic acid/sodium acetate for 50 minutes in the batch cell at 25mA/cm 2 and pH 5.

Figure 2
Figure 2 Raman spectra of BDD before and after 2 hrs electrolysis in 1 M acetic acid/sodium acetate pH 5 at 25 mA/sqcm.The BDD electrodes used have a concentration of 2000-5000 ppm boron atoms in the lattice .(b)Deconvoluted Raman spectra of BDD reveal the position of the bands in accordance with the existing literature.
To investigate the potential dependent product distribution of Kolbe electrolysis of acetic acid/sodium acetate solution in real-time, an ECMS investigation was performed with BDD and platinized BDD (5 nm sputter Pt) electrodes.EMCS is modern technology compared to conventional DEMS (differential electrochemical mass spectroscopy), listed in SI 27.The mass spectrometry data recorded Open Access Article.Published on 05 April 2023.Downloaded on 5/4/2023 10:33:36 AM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online DOI: 10.1039/D3FD00066D Please do not adjust margins Please do not adjust margins during the CV are shown in Figure 4. Generally, the transient (m/z) signals confirm the presence of (non)Kolbe products.

2 2𝑂𝑂 2 𝑂𝑂 2 →
2 + 2 2  3 Subsequently decarboxylation products are detected.Methanol and methyl acetate are accumulated in the liquid phase during cyclic voltammetry, due to their low vapor pressure.Compared to the signal intensities of CO2, the main Kolbe product ethane is only observed in minor amounts in agreement with the above presented batch and flow cell experiments.Importantly, here we reveal that the onset of the decarboxylation reaction and oxygen evolution as well as the transient product signals are similar.
Open Access Article.Published on 05 April 2023.Downloaded on 5/4/2023 10:33:36 AM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online This journal is © The Royal Society of Chemistry 20xx

Figure 6
Figure 6 SEM images of Pt electrodeposition on BDD electrodes (a) ED-A nanoflowers, (b) ED-B dispersed nanoparticles, (c) ED-C nano thorns, (d) ED-D nanocrystals (i) ECSA (cm 2 ) in 1M H2SO4 before and after (non)Kolbe electrolysis; (j) Average FE towards (non)Kolbe electrolysis/indirect oxidation products from different geometries platinum nanoparticles on BDD at 25 mA/sqcm in 1M acetic acid/sodium acetate (pH 5).The electron balance was not closed which could be due to the formation of H2O2 or other side products (not detected).

Figure 7
Figure 7 SEM images of Pt nanoflowers electrodeposition on different substrate electrodes (a-b) FTO, (c-d) graphite, (e-f) nickel foam; (g) Illustration of predominant reaction on platinum nanoparticles on different substrates.(h) Average FE towards (non)Kolbe electrolysis on platinum nanoflower particles with different substrate at 25 mA/sqcm in 1M acetic acid/sodium acetate (pH 5).