# Local pH engineering to impact electrocatalysis

Maxime Decker, Quentin Lenne, Jalal Ghilane, Carlos M Sánchez-Sánchez

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Maxime Decker, Quentin Lenne, Jalal Ghilane, Carlos M Sánchez-Sánchez. Local pH engineering to impact electrocatalysis. Current Opinion in Electrochemistry, 2025, 53, pp.101728. 10.1016/j.coelec.2025.101728. hal-05357785

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Review Article

Local pH engineering to impact electrocatalysis

Maxime Decker<fnref n="1" />, Quentin Lenne<fnref n="2" />, Jalal Ghilane<fnref n="2" /> and Carlos M. Sánchez-Sánchez<fnref n="1" />

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Local pH refers to the pH gradient developed within the diffusion layer of the electrode, which can deviate significantly from the bulk value due to proton consumption or generation during electrocatalytic reactions. The proton availability is often a thermodynamic or kinetic limiting factor during electrocatalytic reactions involving proton-electron transfer as the determining step. Thus, controlling local pH can effectively impact both reaction selectivity and activity. In this short review, we present recent advances and strategies that emerged to effectively tune the local pH and impact on different electrocatalytic reactions, such as CO₂ reduction reaction (CO₂RR), electrochemical nitrate reduction (ENR), O₂ reduction reaction, (ORR) and ethanol oxidation reaction (EOR). The catalyst engineering approach through microenvironment modification, tuning mass transport conditions by catalyst size and porosity, as well as by pulsed potential electrolysis are the strategies described here.

Addresses

Corresponding author: Sánchez- Sánchez, Carlos M. (carlos.sanchez@sorbonne-universite.fr)

Current Opinion in Electrochemistry 2025, 53:101728

This review comes from a themed issue on **Electrocatalysis (2025)**

Edited by **Angel Cuesta Ciscar** and **Tharangattu N. Narayanan**

For complete overview about the section, refer **Electrocatalysis (2025)**

Available online 9 July 2025

https://doi.org/10.1016/j.coelec.2025.101728

2451-9103/© 2025 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Introduction

Electrocatalysis plays a crucial role in energy conversion and storage technologies, enabling efficient and sustainable chemical transformations. It is fundamental to processes such as water splitting for hydrogen production, fuel cells for clean energy generation, and carbon dioxide conversion to mitigate greenhouse gas accumulation. By enhancing reaction kinetics at the

electrode surface, electrocatalysts improve energy efficiency and reduce overpotentials, making electrochemical processes feasible in large-scale applications. Synthesis and engineering of materials have been extensively explored to improve electrocatalysts efficiency [1,2]. However, the performance of electrocatalytic reactions is also determined by the local microenvironment at the electrocatalyst-electrolyte interface [3], in addition to the intrinsic activity of electrocatalytic materials. In electrochemistry, molecules or ions approaching a metallic or semiconducting electrode can undergo the addition or removal of electrons by following a redox reaction [4]. This can involve only electrons such as chloride electrooxidation reaction (CER) or both electrons and protons, which are often together in a proton-coupled electron transfer (PCET). PCET plays a critical role and is a fundamental mechanism in various electrocatalytic processes, governing key reactions in energy conversion and storage systems including water splitting, oxygen reduction, hydrogen production, and carbon dioxide conversion. PCET refers to the concerted or sequential transfer of electrons and protons in a reaction, ensuring charge balance and facilitating chemical transformations at electrode interfaces. Notably, protons and electrons are very often transferred together at the electrode-electrolyte interface to drive the formation of specific products, highlighting the critical role of proton availability in activity and selectivity. Unlike bulk pH, which is controlled by the overall solution composition, local pH refers to the pH in the immediate vicinity of the electrode surface. This localized pH can deviate significantly from the bulk value due to proton consumption or generation during electrochemical reactions that impact reaction pathways and rates, product selectivity, and catalyst stability. Figure 1a displays the acidic local pH developed within the diffusion layer at the anode due to PCET oxidation reactions and the alkaline local pH developed within the diffusion layer at the cathode due to PCET reduction reactions. Local pH has begun to garner interest over the years in electrocatalysis due its influence on many aspects of the electrocatalytic reactions. Firstly, the reaction kinetics is impacted because local pH determines the amount of protons available near the electrode; a lower local pH increases proton concentration, which might impact in the speciation and stability of the reactant. Secondly, it also can alter the selectivity and/or mechanism pathway by

<footnote n="1">¹ Sorbonne Université, CNRS, Laboratoire Interfaces et Systèmes Electrochimiques (LISE), 4 Place Jussieu, F-75005 Paris, France</footnote>

<footnote n="2">² Université Paris Cité, CNRS UMR 7086, Laboratoire ITODYS, 15 Rue Jean-Antoine de Baïf, F-75013, Paris, France</footnote>
<figure><img src="image_4.png" /><figcaption>Figure 1</figcaption></figure>

Schematic representation of (a) local pH gradients formed in the electrolytic cell during electrocatalytic reactions involving proton-electron transfer and (b, c and d) different strategies to control local pH on electrocatalytic reactions: (b) controlling the micro environment of the electrode-electrolyte interface, (c) controlling mass transport phenomena and (d) modulating applied potential to enrich the diffusion layer in specific chemical species.

favoring certain reaction intermediates and inducing the degradation of others [5,6]. Finally, it can directly control the activity by affecting either the number of active sites on the electrocatalyst or its stability (corrosion and degradation processes). For those reasons, it appears that controlling the local pH is an effective approach to impact on the different electrocatalytic reactions involving PCET steps. Figure 1b–d display a schematic

representation of the different strategies presented and discussed herein for controlling the local pH, which are modulating the polarity and charge at the electrode surface (Figure 1b), controlling the mass transport by using porous catalyst to induce a confinement effect (Figure 1c) and modulating the applied potential to enrich the diffusion layer with specific chemical species (Figure 1d).
Several electrocatalytic reactions, such as CO₂ reduction reaction (CO₂RR), electrochemical nitrate reduction (ENR), H₂ evolution reaction (HER), O₂ reduction reaction (ORR), O₂ evolution reaction (OER) and ethanol oxidation reaction (EOR) involve PCET steps. Table 1 summarizes selected examples where the local pH modulation has been implemented for tuning these electrocatalytic reactions, classified by the desired product in each case and the type of local pH engineering strategy used. The main strategies considered in Table 1 include tuning catalyst porosity or chemical nature, the interfacial microenvironment, and the electrolysis conditions or the nature of the electrolyte. In some cases, concomitant electrocatalytic reactions

might happen together at both the cathode (CO₂RR and HER, ENR and HER) and the anode (CER and OER) due to the proximity of their thermodynamic potentials. However, those reactions might not exhibit the same H⁺/e⁻ or OH⁻/e⁻ ratio and thus, local pH represents a very interesting tool for selectively enhancing or inhibiting one of those concomitant reactions. For instance, the anodic concomitant reactions CER (where H⁺ are not involved) and OER (1H⁺/1e⁻ ratio) represent an easy example to understand how the variation in local pH impacts the thermodynamics. OER is a pH-dependent reaction, but CER is a pH-independent reaction. Thus, only the standard thermodynamic potential of OER is impacted by the acidic local pH reached at the

<caption>**Table 1**</caption>

Some selected examples of electrocatalytic reactions tuned by local pH.

<table><thead><tr><th>Acronym</th><th>Electrocatalytic reaction</th><th>Desired product</th><th>Ratio (H<sup>+</sup>/e<sup>-</sup>) or (OH<sup>-</sup>/e<sup>-</sup>)</th><th>Local pH strategies<sup>a</sup></th><th>Ref</th></tr></thead><tbody><tr><td>CER</td><td>Cl<sup>-</sup> &rarr; -2e<sup>-</sup> &rarr; 1/2 Cl<sub>2</sub></td><td>Cl<sub>2</sub></td><td>-</td><td>-</td><td>[12]</td></tr><tr><td>CO<sub>2</sub>RR</td><td>CO<sub>2</sub> &rarr; 8e<sup>-</sup>, 8H<sup>+</sup> &rarr; CH<sub>4</sub> &rarr; CO &rarr; 8e<sup>-</sup>, 8H<sup>+</sup> &rarr; CH<sub>3</sub>COOH &rarr; CO &rarr; 8e<sup>-</sup>, 8H<sup>+</sup> &rarr; C<sub>2</sub>H<sub>4</sub> &rarr; CO &rarr; 8e<sup>-</sup>, 8H<sup>+</sup> &rarr; CH<sub>3</sub>CH<sub>2</sub>OH &rarr; CO</td><td>CH<sub>4</sub><br>CH<sub>3</sub>COOH<br>C<sub>2</sub>H<sub>4</sub><br>CH<sub>3</sub>CH<sub>2</sub>OH<br>HCOOH</td><td>1<br>1<br>1<br>1</td><td>A, B, D<br>B, D<br>A, B, D, E<br>A, B, D, E<br>A, B, E</td><td>[13-16]<br>[17,18]<br>[13,15,17-22]<br>[13,15,17,18,21-23]<br>[14,23-27]</td></tr><tr><td>ENR</td><td>NO<sub>3</sub><sup>-</sup> &rarr; 2e<sup>-</sup>, 2OH<sup>+</sup> &rarr; NO<sub>2</sub><sup>-</sup> &rarr; 3e<sup>-</sup>, 4OH<sup>+</sup> &rarr; 1/2 N<sub>2</sub> &rarr; 6e<sup>-</sup>, 7OH<sup>+</sup> &rarr; NH<sub>3</sub></td><td>CO<br>N<sub>2</sub><br>NH<sub>3</sub></td><td>1<br>1.2<br>1.125</td><td>A, C, E<br>D<br>D</td><td>[7,28-34]<br>[35]<br>[35]</td></tr><tr><td>HER</td><td>H<sup>+</sup> &rarr; 1e<sup>-</sup> &rarr; 1/2 H<sub>2</sub></td><td>H<sub>2</sub></td><td>1</td><td>E</td><td>[9,36,37]</td></tr><tr><td>ORR</td><td>O<sub>2</sub> &rarr; 2e<sup>-</sup>, 2H<sup>+</sup> &rarr; H<sub>2</sub>O<sub>2</sub> &rarr; 4e<sup>-</sup>, 4H<sup>+</sup> &rarr; 2H<sub>2</sub>O</td><td>H<sub>2</sub>O<sub>2</sub><br>H<sub>2</sub>O</td><td>1<br>1</td><td>A, B, E<br>A, B, E</td><td>[38-43]<br>[38-43]</td></tr><tr><td>OER</td><td>2OH<sup>+</sup> &rarr; -2e<sup>-</sup> &rarr; 1/2 O<sub>2</sub></td><td>O<sub>2</sub></td><td>1</td><td>E</td><td>[36,44]</td></tr><tr><td>EOR</td><td>1/2 CH<sub>3</sub>CH<sub>2</sub>OH &rarr; -6e<sup>-</sup>, 6OH<sup>+</sup> &rarr; CO<sub>2</sub></td><td>CO<sub>2</sub></td><td>1</td><td>B</td><td>[45]</td></tr></tbody></table>

CER, chloride electrooxidation reaction; CO₂RR, CO₂ reduction reaction; ENR, electrochemical nitrate reduction; OER, ethanol oxidation reaction; HER, H₂ evolution reaction; OER, O₂ evolution reaction; ORR, O₂ reduction reaction.

ª Local pH engineering strategies on different electrocatalytic reactions: A (Porous catalyst), B (Chemical microenvironment), C (Catalyst nature), D (Pulsed potential electrolysis), and E (Electrolyte engineering).
anode surface, inhibiting OER and playing a crucial role in CER efficiency. In addition to this, the cathodic concomitant reactions ENR to $NH_3$ (9$OH^−$/8$e^−$) and HER (1$H^+$/1$e^−$) represent another clear example to understand how the relative activity of these two reactions on the cathode impact on the alkaline local pH reached at the cathode surface since each reaction consumes a different amount of protons per electron. During electrocatalytic reactions, a broad range of parameters affect the local pH, such as the $H^+$/e$^−$ ratio, the nature of the catalyst [7] and its morphology [8], applied current density or potential, buffering capacity [9,10] of the supporting electrolyte, and mass transfer rate [11] at the electrode surface.

Recent advancements in sensing techniques have underscored the importance of modulating the pH near the electrode surface to influence the activity and selectivity of various electrocatalytic reactions. Different techniques have been proposed to monitor the local pH, which are mainly based on scanning probe techniques such as scanning electrochemical microscopy (SECM) [46,47], scanning ion conductance microscopy (SICM), scanning ion-selective technique (SIET), or spectroscopic technics (fluorescence [39], RAMAN [43], Fourier transform infrared (FTIR) [48]), as well as rotating ring-disc electrode (RRDE) [49] and have been recently reviewed [50]. However, the description of localized pH measurements is out of the scope of the present work. In particular, modulating the local pH can be achieved by manipulating different parameters that affect the interfacial microenvironment based on either catalyst or electrolyte engineering approaches. This review mainly focuses on catalyst engineering strategies as they have garnered more attention and yielded a wider choice of recent innovative methods. Therefore, the following sections review recent studies on tuning the local pH as an effective strategy for controlling activity and/or selectivity on a large variety of electrocatalytic reactions.

## Catalyst engineering for impacting local pH

The local pH is significantly influenced by the electrode-electrolyte interface, meaning the electrode surface plays a crucial role in generating the pH gradient. Several strategies exist to finely tune the electrode properties. Notably, the modification of catalyst properties by grafting or simply depositing organic compounds onto the electrode surface, such as polymers [19,41,51] or thin films [52,53]. One effective approach is the change of the chemical microenvironment of the catalyst by adding a polymeric binder on the electrode surface and more precisely varying the nature of the polymer binder that serves for the catalyst's immobilization on the current collector. This not only alters interactions with the electrolyte but also introduces new active sites that can buffer the local pH [33]. Another way to tune

the electrocatalyst without altering its composition is by adjusting its geometry to influence mass transport at the interface. Different strategies to modify these factors are explored in the following sections.

### Chemical microenvironment on the electrode/ electrolyte interface

One key parameter in the chemical microenvironment is the electrode surface polarity, which can be tuned by surface modification at the electrode, as it directly influences the amount of water present near the electrode surface and, consequently, proton availability [21]. Recently, Chang et al. demonstrated that the choice of polymeric binder during electrode preparation, which controls proton availability in the interface, can significantly impact product selectivity in $CO_2RR$ [14]. Using a carbon-supported copper nanoparticle catalyst, these authors compared two polymeric binders with different wettability characteristics: poly(acrylic acid) (PAA, containing –COOH groups), which is hydrophilic, and poly(vinylidene difluoride) (PVDF, containing –$CF_2$ groups), which is hydrophobic. Their results showed that the faradaic efficiency (FE) for $CH_4$ production was approximately 1.6 times higher for the Cu-PVDF electrode compared with the Cu-PAA electrode at an applied potential of –1.4 V vs. RHE. In contrast, the FE for HCOOH increased by 1.4–1.7 times for the Cu-PAA electrode under the same conditions. Two key factors were proposed to explain these differences. Firstly, hydrophilic surfaces accumulate more protons and tend to enhance the HER; however, in this potential range, adsorbed hydrogen on the surface is preferentially utilized for HCOOH formation rather HER. Secondly, density functional theory (DFT) calculations indicated that the functional groups on PVDF influenced the binding energies of key intermediates, strengthening the interaction between Cu and $CO_2$ reduction intermediates, ultimately altering the selectivity on the $CO_2RR$. Another way to tune the proton availability in the interface by the choice of a polymeric binder is based on tuning electrostatic interactions. Y.-Y. Li et al. reported supported iron phthalocyanine (FePc) catalyst mixed with different charged binders to effectively enhance ORR performance in acidic medium by tuning the local pH [43]. Figure 2a,b show the chemical structure of both studied ionomers. Figure 2a shows the Nafion binder, where the negatively charged sulfonic groups allow protons to be easily transported from the bulk to the electrode surface. This ensures high proton availability for the ORR process (Figure 2c). In contrast, Figure 2b shows positively charged quaternary ammonium polysulphone (QAPS) ionomer, where the protons are repulsed by the quaternary ammonium groups, with results in a rise of interfacial pH detected by Raman spectroscopy and water molecules at the electrode-electrolyte interface acting as the proton source (Figure 2d). This alkaline local pH improves
<figure><img src="image_5.png" /><figcaption>Figure 2</figcaption></figure>

(a, b) The chemical structure of (a) Nafion ionomer and (b) QAPS ionomer; (c, d) schematic diagram of local microenvironment near FePc electrode with (c) Nafion as binder, (d) QAPS as binder, different interface pH values are achieved due to strong electrostatic interaction of charged groups during ORR, (e) ORR polarization curves with Nafion (blue) and QAPS (red) as binder, (f) peak intensity ratio (CO₃²⁻)/(HCO₃⁻) from *in situ* SHINERS versus electrolysis time highlighting local pH changes. Adapted from [43] (CC-BY 4.0 2022 Elsevier B.V.). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) ORR, O₂ reduction reaction; QAPS, quaternary ammonium polysulphone; SHINERS, shell-isolated nanoparticle-enhanced Raman spectroscopy.

ORR activity and stability on the FePc catalyst. Polarization curves revealed that the half-wave potential for the positively charged QAPS-modified electrode shifted by 104 mV (Figure 2e) toward lower overpotential compared with the negatively charged Nafion-modified electrode, indicating much higher ORR activity. Additionally, the mass activity of FePc-QAPS increased significantly, ranging from 7.3 to 18.5 times higher than that of the FePc-Nafion electrode. This enhancement is attributed to differences in local pH associated with the microenvironment created by electrostatic interactions at the electrode-electrolyte interface. Local pH evaluation was monitored by *in situ* shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) (Figure 2f) and consists of measuring the intensity ratio between CO₃²⁻ and HCO₃⁻ Raman signals at the electrode surface, which depends on the pH value.

## Catalyst porosity and size effect

Mass transport is a key parameter when it comes to local pH, and tuning either the electrocatalyst size (nanoparticles with different diameters) or its geometry, such as porous catalysts, is an interesting approach to induce specific gradient of protons near the electrode surface

without modifying the catalyst composition [54]. This is of particular interest for poorly selective reactions as it has been reported for CO₂RR and ethanol electrooxidation. Providing high efficiency catalysts for CO₂RR to C₂⁺ products is a challenging task due to the high energy necessary to form the C–C bond. Notably, the selectivity between C₁ (CO, CH₄ ...) and C₂⁺ (C₂H₄, C₂H₅OH ...) products in CO₂RR was revealed as pH-dependent. As a result, alkaline electrolytes have been widely used to enhance selectivity for C₂⁺ products in CO₂RR. Considering this effect at the local scale, mass transport within the diffusion layer near the electrode surface can be adjusted by controlling the packing density of nanoparticles that form the catalyst, which is influenced by their size-to-volume ratio. This explains the different ratio of C₁ and C₂⁺ products from CO₂RR on catalytic Cu nanoparticles of different sizes (25–80 nm) from a mass transport perspective, which was not initially considered [55]. Additionally, porous catalysts further impact the diffusion layer by creating confined spaces that disrupt mass transport of chemical species. Both the reactant and the product have to move through a narrow pathway, which can lead to local depletion or accumulation of protons resulting in
amplified pH gradients. Given its strong influence on local pH, porosity on the electrocatalyst has been explored in the recent years and remains one of the most effective approaches for tuning reaction selectivity [56,57]. Fan et al. has shown that using three-dimensional ordered porous cuprous oxide cuboctahedra (referred as 3DOP Cu₂O–CO) a high local alkaline pH is induced, enhancing C–C bond coupling in CO₂RR, while suppressing the competing HER [17]. As a proof of concept, these authors compared catalytic performances of nonporous cuprous oxide cuboctahedra (Cu₂O–CO) to porous 3DOP Cu₂O–CO. Over six different applied potentials ranging from –1.2 V to –1.7 V vs RHE, 3DOP Cu₂O–CO consistently exhibits higher FE(C₂+) than Cu₂O–CO, with a maximum of 73.4 % obtained at –1.4 V vs RHE. To investigate the CO₂RR mechanism, the authors performed finite element method simulations, which revealed that in porous catalysts, the accumulation of C₁ products within the cavities promotes the formation of C₂+ products. In contrast, for nonporous Cu₂O–CO, C₁ intermediate species diffuse out of the electrode, leading to the production of monocarbon products. Local pH was examined using *in situ* Raman spectroscopy by following the signals associated with HCO₃ and CO₃²⁻, showing higher local alkalinity triggered by the 3DOP nanoarchitecture. The pH increased from 10.8 to 11.2 as the potential shifted from 0 to –0.5 V vs. RHE. In contrast, the local pH for the nonporous catalyst remained below 10.8. Based on the role of catalyst

porosity as well, Kim et al. investigated the effect of hollow structured Pt–Rh catalysts on EOR [45]. These authors prepared different Pt₁-xRhx hollow nanospheres by a template-assisted synthesis to obtain different particle sizes (250 nm or 350 nm) with different porous nanostructures as schematically presented in Figure 3a. Complete oxidation by C–C bond splitting and C₁ products’ formation (CO₂, CO...) represent the desired pathway in EOR. However, it is in competition with incomplete oxidation to C₂ products such as acetaldehyde (CH₃COH) and acetic acid (CH₃COOH), which are governed by the high coverage of OH adsorbed at the electrode surface reached in alkaline media. Thus, Pt₀.₅Rh₀.₅ hollow catalytic particles with larger diameter (350 nm) exhibit FE(CH₃COOH) of 77.7 % and FE(C₁ products) as low as 9.5 %. In contrast, hollow particles of smaller diameter (250 nm), with a similar mass loading on the electrode, exhibit a significant increase in FE for C₁ products (38.3 %), meanwhile the FE(CH₃COOH) decreased to 53.4 %. The FE of different products as a function of hollow particle size is displayed in Figure 3b. This change in EOR selectivity is attributed to the different mass transport conditions due to the confinement effect in the catalyst porosity. Narrowing the pore size hinders the OH⁻ diffusion from the bulk electrolyte to the electrode–electrolyte interface and allows the accumulation of H⁺ generated during EOR, which creates a local acidic environment decreasing the OH coverage at the electrode surface and favoring the EOR to C₁ products as schematically illustrated in Figure 3a.

<figure><img src="image_6.png" /><figcaption>**Figure 3**</figcaption></figure>

Schematic illustration of OH⁻ mass transport conditions toward the active sites for EOR on different porous structures of hollow Pt₁-xRhx electrocatalyst, (b) faradaic efficiency (FE) of acetic acid, acetaldehyde, and C₁ products from EOR as a function of the mass loading and hollow particle size of electrocatalysts. Reproduced with permission from [45]. Copyright 2023 American Chemical Society. EOR, ethanol oxidation reaction.
<figure><img src="image_7.png" /><figcaption>Figure 4</figcaption></figure>

Effect of potential pulse characteristics on FE (stacked bars) and total current density (markers) during pulsed CO₂ electrolysis. (a) Impact of pulse amplitude, (b) pulse width, (c) more cathodic potential limit, and (d) less cathodic potential limit. Adapted with permission from [13]. Copyright (2021) American Chemical Society. FE, faradaic efficiency.

This strategy of tuning the local pH by modifying the catalyst porosity allows decoupling the electrode-electrolyte conditions from the bulk electrolyte properties.

## Pulsed potential for impacting local pH

A different alternative to impact on the pH gradient formed in the electrode-electrolyte interface is the pulsed potential electrolysis, which is based on periodically switching the applied potential during electrolysis between two different values. Notably, CO₂RR concentrates most of the efforts using this approach either by oxidative [16] or reductive potential pulsing [13,15,21,35]. In particular, recent studies from A. T. Bell group [13,15,21] show that choosing two distinct potentials within the CO₂RR activity potential window and both negative enough to avoid Cu electrode surface modification can effectively impact on the selectivity of the CO₂RR. Pulsed potential CO₂ electrolysis on bare Cu electrode oscillating between -0.8 V and -1.15 V vs RHE for 10 s each exhibits an increase in FE (C₂⁺) from 60 % to 81 % compared with static electrolysis at -1.15 V vs RHE [21]. Furthermore, FE(H₂) decreases from 22 % to 8 % with this pulsed electrolysis. These authors investigated the local pH generated by a concentration-dependent Tafel kinetics model. They concluded that during pulse at the most cathodic potential, the local pH rises immediately as the result of a

higher current generated and then a rapid generation of OH⁻ is produced. Meanwhile, during the pulse at the less cathodic potential (lower current generated), the CO₂ molecules replenished the electrode-electrolyte interface because -0.8 V vs RHE displays almost no activity for CO₂RR. Since the rate of CO₂ consumption is slower than the increase in local pH, it induces a transient state of high local pH and high concentration of CO₂ favoring C₂⁺ products as soon as the applied potential is back to -1.15 V vs RHE. Figure 4 displays the FE of H₂, C₁, and C₂⁺ products from CO₂RR, the amplitude of the potential pulse during electrolysis and the total current density achieved for a large set of pulsed potential electrolysis. A pulsed potential strategy has been also applied to other electrocatalytic reactions such as ENR [35] impacting on the local pH as well as on ENR selectivity and competition ENR vs. HER.

## Conclusions and outlook

The pH gradient developed within the diffusion layer of the electrode significantly impacts the selectivity of different electrocatalytic reactions involving a proton-electron transfer. Different strategies based on catalyst engineering, which allow modifying the chemical environment by tuning hydrophobicity, electrostatic interactions or mass transport conditions at the electrode-electrolyte interface are presented. Moreover, variations in applied current density or pulsed
potential during electrolysis are also explored. These strategies represent a useful tool to control the local pH gradient and intentionally modify the reaction thermodynamics and/or kinetics. In particular, poorly selective reactions such as CO₂RR, ORR, EOR, and ENR, which exhibit multiple reaction pathways with both pH-dependent and independent rate-determining steps, allow boosting specific products by tuning the local pH at the electrode-electrolyte interface. By highlighting the recent advances in these various strategies to impact on local pH during electrocatalytic reactions, we hope that higher consideration on this phenomenon can stimulate the development of electrolytic system with higher performances to face the different challenges that electrocatalysis might encounter.

## 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.

## Acknowledgements

The authors acknowledge the support provided by the Centre National de la Recherche Scientifique (CNRS) and the French National Research Agency (ANR) through the project pHSELECT (ANR-22-CE04-0006-01).

## Data availability

No data was used for the research described in the article.

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