# Electrocatalysis of organics for electrolysis and/or fuel cells: Some thoughts on using the ratio of forward to backward peak current as a measure of electrocatalyst efficiency and/or poisoning

Yaovi Holade, Teko Napporn, Kouakou Boniface Kokoh

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Yaovi Holade, Teko Napporn, Kouakou Boniface Kokoh. Electrocatalysis of organics for electrolysis and/or fuel cells: Some thoughts on using the ratio of forward to backward peak current as a measure of electrocatalyst efficiency and/or poisoning. Current Opinion in Electrochemistry, 2025, 49, pp.101625. 10.1016/j.coelec.2024.101625. hal-04850004

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# Electrocatalysis of organics for electrolysis and/or fuel cells: Some thoughts on using the ratio of forward to backward peak current as a measure of electrocatalyst efficiency and/or poisoning

Yaovi Holade,<sup>1,2,3</sup> Teko W. Napporn<sup>2,4</sup> and Kouakou Boniface Kokoh<sup>2,4*</sup>

**Corresponding author:** Kouakou Boniface, Kokoh (boniface.kokoh@univ-poitiers.fr)

## Abstract

In the current context of sustainability, the selective electrocatalytic transformation of biomass-derived organic substances into value-added products should offer vast design possibilities for power generation or the electrosynthesis of fuels and commodity chemicals. In this contribution, we have examined a number of concepts concerning the electrocatalysis of organic molecules for which noble metals cannot be excluded from the electrocatalyst composition without compromising the significant energy savings promised in electrolyzers (up to 50% for H₂ co-production compared to conventional water electrolysis). The widespread practice of using the ratio of forward peak current to backward peak current as a measure of activity, anti-poison capacity or removal of adsorbed poisons or intermediates is unsuitable based voltammetry and spectroelectrochemical analysis.

## Keywords

electrocatalysis, electrosynthesis, electroconversion, biomass, biofuels, bio-based chemicals, cyclic voltammetry, spectroelectrochemistry, ion exchange membrane.

<footnote n="1"><sup>1</sup>Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, 34090 Montpellier, France</footnote>

<footnote n="2"><sup>2</sup>French Research Network on Hydrogen (FRH2), Research Federation No. 2044 CNRS, BP 32229, Nantes CEDEX 3 44322, France</footnote>

<footnote n="3"><sup>3</sup>Institut Universitaire de France (IUF), 75005 Paris, France</footnote>

<footnote n="4"><sup>4</sup>Université de Poitiers, IC2MP UMRCNRS 7285, 86073 Poitiers, France</footnote>
Introduction

Historically, alongside organic electrosynthesis [1,2], the study of the catalytic electrooxidation of organic compounds such as methanol, ethanol, formic acid, glycerol and oligosaccharides, was driven by demand for fuel cells (FCs), since these liquid fuels offer a handling advantage over H₂, which is still more than 90% derived from fossil fuels [3-6]. The growth over the last five years is exponentially greater, because in addition to hydrogen evolution reaction (HER), an efficient anode is critical for other energy-intensive cathodic reactions such as CO₂ reduction reaction (CO2RR) and N₂ reduction reaction (N2RR) for the electrosynthesis of value-added compounds in electrolysis cells (ECs) [7-13]. In fact, the kinetics of OER being at least an order of magnitude slower than HER, substituting OER with organic compounds having thermodynamically lower oxidation potentials than water activation should enable up to 50% electricity savings compared to traditional water electrolysis [7-9,14-16]. For ECs, achieving high selectivity in the oxidation of organics at consequent current densities (0.2-2 A cm⁻²) [17] and moderate cell voltage is challenging when noble metals (Pt, Au, Pd, etc.) are excluded from the electrocatalyst composition. For organic-fueled FCs (ethanol, glycerol, etc.), anode electrocatalysts based on the so-called “abundant and cheaper” metals (M = Ni, Ag, Fe, Mn, etc.) have not yet guarantee lower anode’s potential than that of the cathode (oxygen reduction reaction, ORR), generally operating between 0.8 and 1.0 V vs RHE (reversible hydrogen electrode). Hence, for both ECs and FCs purpose, a compromise in terms of the anode’s potential, selectivity, and current density can be provided by low noble metal(s) loading electrocatalysts, which may include, in addition to abundant metals, either Pt for its dehydrogenating properties at low potential, Au for improved stability and/or selectivity, or Pd which behaves as an intermediate between Pt and Au.

Since a number of reviews have extensively dealt with the advantages and disadvantages of many organic electrochemical reactions [1,2,16,18,19], it seems redundant to us to repeat these arguments yet again. So, in this contribution, we concentrate on certain aspects of fundamental understanding of organic molecules’ electrocatalysis. We discuss the interesting open question of whether the ratio of forward peak current to backward peak current, *I*<sub>f</sub>*I*<sub>b</sub>, can be used as a metric for the efficiency/poisoning of electrocatalysts, a widespread practice [20-25].
Organics electrooxidation paired to HER, NRR, CO2RR, etc.

Before tackling the question of peak current ratios, we first discuss the scope regarding the electrooxidation of organics. Figure 1a suggests that the electrooxidation of organics (glycerol, glucose, etc.) should lower the cell voltage and thus improve the energy efficiency of electrolysis, since the relationship between the required electricity and the cell voltage is $W[\text{kWh/kg(H}_2\text{)] = 26.59 \times U_{cell}[\text{V}]$ [14,15], wherein $U_{cell} = E_{(anode)} - E_{(cathode)}$. Therefore, $E_{(anode)}$ should ideally be low; this is a challenge for noble-metal-free electrode materials which so far catalyze the oxidation of organic compounds at potentials above 1 V vs RHE. For high electrode potential of 1.4-1.7 V vs RHE [26-28], the carbon-carbon bond rupture is frequent, in addition the competition with OER, hence a small gain in energy efficiency at industrial level current densities of 0.2-2 A cm$^{-2}$. Under acidic media, $CO_2$ can be the final product of the organics electrooxidation, provided that it is sent back to cathode [7,8,10,19], however, the situation is complicated in alkaline media where the carbonation and/or precipitation is detrimental to the electrolyzer performance [29,30].

Since the electrooxidation of an organic molecule is a proton-coupled electron transfer process (PCET, influenced by the pKa of the organic molecule [31]), the kinetics may be slower than that of OER. For example, while the theoretical oxidation potential of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is about 0.3 V vs RHE [32], lower than the potential for OER (1.23 V vs RHE), an electrode potential of 1.3-1.6 V vs RHE was necessary to 0.36 A cm$^{-2}$ at Cr-Ni(OH)$_2$/nickel foam electrocatalysts (faradaic efficiency of 90-98% [27,28]). Different characterization has revealed that the needed concomitant reduction of Ni(III) to Ni(II) as the electron acceptor to promote the HMF dehydrogenation occurs within the MOOH (M = Co-Fe-Mo-Cr) region of 1.3-1.5 V vs RHE [27,28,33]. This indirect electrooxidation is driven by the active redox center MOOH/M(OH)$_2$ [34]. The electrochemical performance can be augmented by transition metals phosphides or sulfides where the electron density transfer from partially and positively charged metal centers $M^{\delta+}$ (M = Ni, Co, etc.) to partially and negatively charged $X^{\delta-}$ (X = P, S) promote the dehydrogenation [28,35]. The three fundamental driving forces being the long-range geometric lattice deformation, the synergy (multiple metals), and the short-range electronic charge transfer effects [36]. Furthermore, the governing mechanism for electrode materials comprising noble metals (direct electrooxidation sites) and non-noble metals (indirect electrooxidation sites) remains unraveled when the anode potential is above 1.2-1.4 V vs RHE, where MOOH/M(OH)$_2$ electrochemistry predominates.
To consider the above applications, we question in the following a common practice of basing
the catalytic activity of electrode materials on the $I_f/I_b$ ratio.

Can $I_f/I_b$ be used as a measure of efficiency/poisoning of electrocatalysts?

Typically, the cyclic voltammogram (CV) of Pt in an electrolyte in the presence of methanol, formic acid, glycerol, etc., shows at least one peak during the forward scan and the same during the backward scan. We note that Pt, Pd and Au are metals for which a straightforward discriminating observation is possible (Figure 1b for Pd [20]). On Ni, for example, the behavior [33] is similar to bioelectrocatalysts [37-39], i.e. there is no clear presence of a peak during the backward scan. It has been claimed that [20-25], an increase of $I_f/I_b$ indicates: (i) that the electrooxidation performance of the intermediates is enhanced (Figure 1c) as well as a better anti-poisoning ability [20,22], (ii) a superior electrocatalytic activity [21]. Some reports attributed the forward scan peak to organics electrooxidation and the backward scan peak to the removal of carbonaceous species generated in the forward scan [23]. Another explanation would be that the reduction of metal oxides during the backward scan liberates the surface that was oxidized during the forward scan, allowing the electrooxidation of new reactant molecules, thus producing an oxidation peak [3,40,41]. The two underlying questions are then, as there is a (main) peak during the anodic scan (forward) and a peak during the cathodic scan (backward) for the CV of electrooxidation of organics on a metallic electrocatalyst (for instance Pt, Au, Pd): (i) What exactly is electrooxidized during the backward scan? (ii) Can $I_f/I_b$ be used as a metric of efficiency and/or does $I_f/I_b$ formally assess the poisoning tolerance of electrocatalysts? To answer these questions, a careful analysis of the electrochemical signature is necessary (Figures 2a-d & 3a) together with spectroelectrochemistry (Figure 3b-d) put forward by the authors of the present paper.

A comparative analysis of CVs in the absence and presence of the reactive organic molecule provides the first tentative answers, Figure 2a (Au) and Figure 2b (Pt) for the electrooxidation of formic acid [3]. Firstly, the peak during the anodic scan in the presence of the substrate begins to decrease when reaching the region of metal oxide formation. Secondly, the onset of the cathodic scan peak in the presence of the substrate coincides with the start of the reduction of metal oxides to metal in the absence of the substrate. So, intuitively, the simplest explanation would be to postulate that the new voltammetric wave during cathodic scanning results from the electrooxidation of new substrate molecules on the freshly released active sites. This
hypothesis seems logical, given that the CV joins the blank one at low potential range. We note
that the *I*(*E*) response to an excitation *E*(*t*) being the sum of all current contributions, an
oxidation current does not mean that there is no concomitant reduction. The CV profile on bio-
electrocatalysts (Figure 3a) does not show any significant difference because the nature of the
active sites does not change like PGMs-based electrodes. So, one can conclude that the origin
of the cathodic scan peak depends on the kinetics of the metal oxides reduction, which is in
agreement with similar observations during the ORR [42,43].

The CV and first-principles density functional theory (DFT) results in Figure 2c-d show
that the cathodic direction starts in the region where the electrocatalyst surface is covered by
PdO species, which *d*-band center (ε<sub>d</sub>, relative to the Fermi level) is shifted away from the Fermi
level compared to Pd(OH)<sub>x</sub> and Pd surfaces [42]. This results into lower activity according to
Nørskov theory [44], the closer the position of ε<sub>d</sub> toward the Fermi level, the stronger the
interactions with adsorbates. So, the kinetics of noble metal-based oxides reduction, the surface
coverage with hydroxyl species as well as the concentration of the reactant might be the driving
force behind the second wave of oxidation during the backward scan. Assuming that the theory
of “strongly adsorbed poisons” is true, and reasoning by the absurd, we have hypothesized that
[41], using spectroelectrochemistry in single potential alteration infrared reflectance
spectroscopy (SPAIRS, CV coupled to FTIRS), the bands characteristic of the oxidation
products of the anodic scan will be absent when doing the opposite (Figure 3b). For the forward
scan from 0.05 to 1.40 V vs RHE (Figure 3c), the increase in current is accompanied by the
synchronous appearance of IR bands (upwards for reactant consumption and downwards for
product formation, mainly gluconate, detailed assignment in ref. [41]). Now, going from 1.40
to 0.05 V vs RHE (Figure 3d) to unequivocally determine the nature of the process that marks
the return peak, we can see that the SPAIRS shows the appearance of the same bands. The
increase in band intensity is therefore indicative of increasing gluconate production, in phase
with the increase in current. These results therefore invalidate the widespread explanation that
the backward peak (Figure 2b) is essentially due to the oxidation of “poisons formed during the
forward (strongly adsorbed intermediates)”. Conclusively, the origin of the peak observed in
the reverse voltammetric wave during the oxidation of an organic molecule on a noble metal-
based electrocatalyst is not only the result of the oxidation of “possible poisons formed during
the forward scan” but also the oxidation of substrate molecules on the active sites released by
the reduction of metal oxides and the elimination of other organic intermediates.
It is worth pointing out that using fast-scan CVs (except in molecular electrochemistry to mitigate irreversibility situations) to probe the organics oxidation kinetics should be used with caution and not straightforwardly to conclude on steady-state faradaic processes because of the significant capacitive response. Potential, catalyst, electrolyte, coverage, etc. dictate the steps of PCET, i.e. the nature of the bond that breaks first [28,45-50]. Whereas this knowledge is essential to guide the design of efficient electrode materials, existing studies are divergent, some proposing that the organic molecule approaches the catalytic surface via the OH group [28,45-47] while others [48,49] suggest that among C-H, O-H and C-C bonds, the C-H bond is the easiest to break. For gold, we have shown that the increased population of hydroxylated species at high potentials favors C-H dehydrogenation while the metallic surface favors O-H dehydrogenation at lower potentials together resulting in an oxide-like gold surface [50].

## Conclusions and outlook

In this contribution, we have reviewed some of the basic concepts of electrocatalysis of organic molecules on a metal surface, which have been controversially explained. The presence of an intense oxidation peak during the backward scan of electrooxidation of an organic compound on noble metal-based electrocatalysts is being considered a guarantee of efficiency (anti-poisoning, superior electrocatalytic activity, removal of carbonaceous species generated in the forward scan), in contrast to the seminal models. It can be concluded that the backward peak (negative scanning of the electrode potential) results from the electrooxidation of substrate molecules on the active sites released by the reduction of metal oxides and possible adsorbed organic intermediates. Consequently, the ratio of forward peak current to backward peak current ($I_f/I_b$) should not be always used to gauge the efficiency/poisoning of electrocatalysts as it widespread practiced. Finally, the open question of the first electron transferred, that is, which of the C-H, O-H and C-C bonds breaks first, is still debated and synergetic theory-experiment studies could provide more realistic models.

## Declaration of competing interest

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 paper.
**Data availability**

Data will be made available on request.

**Acknowledgements**

This work received funding from French National Research Agency (ANR-22-CE43-0004, ANR-10-LABX-05-01 LabEx CheMISyst), European Union (ERDF), “Région Nouvelle-Aquitaine”, and “Grand Angoulême”. This work pertains to the French government program “Investissements d’Avenir” (EUR INTREE, reference ANR-18-EURE-0010). Prof. K.B. Kokoh is grateful to the “Fondation Poitiers Université” and EDF for their financial support (R-2024-37) within his green chemistry projects.

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This work compares the CV of Au, Pt and Pd bulk electrodes in the NaOH electrolyte in the absence and presence of glucose, gluconic acid and sorbitol. The formation of surface metal oxides (MOₓ, M = Au, Pt, Pd) considerably reduces the oxidation current corresponding to organic substances. During the cathodic scan (backward scan), the reduction of these oxidized surface MOₓ into metallic surface leads to the reactivation of the surface, resulting in an oxidation peak that is dependent on a number of parameters, such as scan rate, MOₓ reduction rate, and surface coverage by adsorbates formed at high potentials.

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**KB: Electrocatalytic and electroanalytic investigation of carbohydrates oxidation on gold-based nanocatalysts in alkaline and neutral pHs. *J Electrochem Soc* 2018, **165**:H425-H436. https://doi.org/10.1149/2.0311809jes.

This work leverages spectroelectrochemistry to demonstrate for the first time that the oxidation peak during the reverse sweep (also known as the backward sweep or cathodic sweep) corresponds to the electrooxidation of fresh reagent molecules on freshly released metalloid active sites, following the concomitant reduction of metal oxides formed at high potential

42. Wang Q, Guesmi H, Tingry S, Cornu D, Holade Y, Minteer SD: **Unveiling the pitfalls of comparing oxygen reduction reaction kinetic data for Pd-based electrocatalysts without the experimental conditions of the current-potential Curves. *ACS Energy* Lett 2022, **7**:952-957. https://doi.org/10.1021/acsenergylett.2c00181.

This work establishes that, owing to the difference in the surface state of Pd as a function of potential between 0.3 and 1 V vs RHE, the non-disclosure (possibly unintentional) when recording the polarization curves of ORR is very problematic. The anodic scanning (low to high potentials) leads to a kinetic current density that is 4-15 times larger than the cathodic scanning (high to low potentials).

43. Shinozaki K, Zack JW, Richards RM, Pivovar BS, Kocha SS: **Oxygen reduction reaction measurements on platinum electrocatalysts utilizing rotating disk electrode technique: I. Impact of impurities, measurement protocols and applied corrections. *J Electrochem Soc* 2015, **162**:F1144-F1158.
https://doi.org/10.1149/2.0551512jes.
44. Jeff Greeley, Jens K. Nørskov, Mavrikakis M: Electronic structure and catalysis on metal surfaces. *Annu Rev Phys Chem* 2002, **53**:319-348.
https://doi.org/10.1146/annurev.physchem.53.100301.131630.

45. Wei L, Sheng T, Ye J-Y, Lu B-A, Tian N, Zhou Z-Y, Zhao X-S, Sun S-G: Seeds and potentials mediated synthesis of high-index faceted gold nanocrystals with enhanced electrocatalytic activities. *Langmuir* 2017, **33**:6991-6998.
http://dx.doi.org/10.1021/acs.langmuir.7b00964.

46. Ishimoto T, Kazuno H, Kishida T, Koyama M: Theoretical study on oxidation reaction mechanism on Au catalyst in direct alkaline fuel cell. *Solid State Ionics* 2014, **262**:328-331. https://doi.org/10.1016/j.ssi.2013.10.020.

47. Caglar A, Düzenli D, Onal I, Tezsevin I, Sahin O, Kivrak H: A comparative experimental and density functional study of glucose adsorption and electrooxidation on the Au-graphene and Pt-graphene electrodes. *Int J Hydrogen Energy* 2020, **45**:490-500. https://doi.org/10.1016/j.ijhydene.2019.10.163.

48. Liu B, Greeley J: Decomposition pathways of glycerol via C–H, O–H, and C–C bond scission on Pt(111): A density functional theory study. *J Phys Chem C* 2011, **115**:19702-19709. http://dx.doi.org/10.1021/jp202923w.

This work leverages the first-principles density functional theory (DFT) to interrogate the decomposition of glycerol on Pt(111) via the comparative scission of C–H, O–H and C–C bonds. The dehydrogenation via activation of the C–H bond is thermochemically favored, followed by the O–H bond. Both dehydrogenation transition state energies (C–H and O–H) are much lower than the corresponding transition state energy for C–C bond cleavage.

49. Garcia AC, Kolb MJ, van Nierop y Sanchez C, Vos J, Birdja YY, Kwon Y, Tremiliosi-Filho G, Koper MTM: Strong impact of platinum surface structure on primary and secondary alcohol oxidation during electro-oxidation of glycerol. *ACS Catal* 2016, **6**:4491-4500. http://dx.doi.org/10.1021/acscatal.6b00709.

This work shows that for the first dehydrogenation step of glycerol oxidation on Pt(111) and Pt(100) single crystal electrodes, carbon-bonded intermediates resulting from the removal of hydrogens bonded to carbon atoms (C–H bond cleavage) are always significantly more stable than oxygen-bonded intermediates resulting from O–H bond cleavage. The energy difference is about 0.2 eV for the Pt(100) electrode and at least 0.5 eV for the Pt(111) electrode.

50. Holade Y, Guesmi H, Filhol J-S, Wang Q, Pham T, Rabah J, Maisonhaute E, Bonniol
** V, Servat K, Tingry S, et al.: Deciphering the electrocatalytic reactivity of glucose anomers at bare gold electrocatalysts for biomass-fueled electrosynthesis. *ACS Catal* 2022, **12**:12563-12571. https://doi.org/10.1021/acscatal.2c03399.

This work suggests that in the absence of hydroxyl functions on the gold surface (acid and neutral pH, low electrode potentials), glucose is adsorbed by deprotonation of the O-H group bound to the anomeric carbon (α-anomer) after the first proton-coupled electron transfer (PCET). This produces a more stable oxidized-like gold surface intermediate than the carbon-bonded intermediate resulting from the dehydrogenating PCET of hydrogen directly bonded to the anomeric carbon (C-H bond cleavage, β-anomer).
**Figure 1.** (a) Illustrative half-cell polarization curves of different electrosynthesis scenarios pairing a process at the negative electrode (cathode, −) with one at the positive electrode (anode, +). (b) CVs recorded at 50 mV s⁻¹ in Ar-saturated 0.5 M KOH + 1 M ethanol (room temperature); Reprinted and adapted from Ref. [20], Copyrights 2023, Elsevier B.V. (c) Trend in the ratio of the forward peak current density (I_f) to the backward peak current density (I_b), I_f/I_b (or j_f/j_b), from CVs recorded at 50 mV s⁻¹ for different electrocatalysts in N₂-saturated 1 M KOH + 1 M ethanol; Reprinted and adapted from Ref. [22], Copyrights 2024, The Royal Society of Chemistry.

**Figure 2.** (a) CVs of bulk Au in 0.5 M H₂SO₄ (80 °C, 20 mV s⁻¹) in the absence (solid lines) and presence of 10 mM HCOOH (dotted lines) and (b) CVs of bulk Pt in 0.5 M H₂SO₄ (80 °C, 20 mV s⁻¹) in the absence (solid lines) and presence of 10 mM HCOOH (dotted lines); Reprinted and adapted from Ref. [3], Copyright 1987, Elsevier Inc. (c) Impact of the anodic/cathodic direction on the CVs recorded at the disk and ring of a RRDE in the absence and in the presence of O₂ (1 M KOH, 1600 rpm, 10 mV s⁻¹). (d) First-principles Density Functional Theory (DFT) calculated local densities of states (LDOSs) of the Pd 4d bands in the three modeled systems (insets of (c)): (A) pristine Pd(111), (B) hydroxylated Pd(111), and (C) oxidized Pd(111); Reprinted and adapted from Ref. [40], Copyright 2022, American Chemical Society.

**Figure 3.** (a) CVs at 10 mV s⁻¹ of bioelectrode (broader substrate glucose oxidase, bGOx; red dashed line is the control bioelectrode without the redox mediator) in citrate/phosphate buffer (pH 6.5, 0.2 M) in the absence (dashed lines) and in the presence of 100 mM of glucose (solid lines); Reprinted and adapted from Ref. [35], Copyright 2022, American Chemical Society. (b-d) Spectroelectrochemistry experiments (0.1 M NaOH, 50 mM glucose, Temperature: 22 ± 2 °C) on Au₈₀Pt₂₀/C nano-electrocatalyst: (b) CV at 1 mV s⁻¹, (c) SPAIRS in the anodic direction and plotted every 0.1 V (except 0.05 and 0.1 V vs RHE) for an initial electrode potential of 0.05 V vs RHE, and (d) SPAIRS in the cathodic direction and plotted every 0.1 V (except 0.05 and 0.1 V vs RHE) for an initial electrode potential of 1.40 V vs RHE; Reprinted and adapted from Ref. [39] under a CC BY-NC-ND 4.0 license, Copyright 2018, The Author(s), Published by ECS.
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3</figcaption></figure>