# Electrocatalysis at the polarised interface between two immiscible electrolyte solutions

Alonso Gamero-Quijano, Grégoire Herzog, Pekka Peljo, Micheál Scanlon

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Alonso Gamero-Quijano, Grégoire Herzog, Pekka Peljo, Micheál Scanlon. Electrocatalysis at the polarised interface between two immiscible electrolyte solutions. Current Opinion in Electrochemistry, 2023, pp.101212. 10.1016/j.coelec.2023.101212. hal-03972006

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Review Article

# Electrocatalysis at the polarised interface between two immiscible electrolyte solutions

Alonso Gamero-Quijano<fnref n="1" />, Grégoire Herzog<fnref n="2" />, Pekka Peljo<fnref n="3" /> and Micheál D. Scanlon<fnref n="4" />

<figure><img src="image_2.png" /></figure>

## Abstract

Electrocatalysis at the interface between two immiscible electrolyte solutions (ITIES) is an emerging field of research, which allows the separation of reactants according to their lipophilicity. Electrocatalysts of various natures (noble metals, carbon-based and inorganic nanomaterials, enzymes, and supramolecular ensembles) are assembled at the ITIES, either spontaneously or following the application of an interfacial Galvani potential difference. While primarily used for the electro-catalysis of the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), recent work has focused on the electrocatalysis of the oxygen evolution reaction (OER) and the electrocatalytic oxidation of elemental sulfur ($S_8$) and an organosulfur compound. Protocols to compare electrocatalytic performances at the ITIES call for careful data analysis and a detailed knowledge of the catalyst's morphological parameters (e.g., active surface area and catalyst loading). However, standardisation of such protocols at the ITIES has yet to be implemented and is required to allow better comparison of the results from individual biphasic systems.

## Addresses

Corresponding authors: Gamero-Quijano, Alonso (daniel.gamero@ua.es); Herzog, Grégoire (gregoire.herzog@cnrs.fr); Peljo, Pekka (pekka.peljo@utu.fi); Scanlon, Micheál D. (micheal.scanlon@ul.ie)

## Keywords

Electrocatalysis, Polarised liquid-liquid interfaces, Interface between two immiscible electrolyte solutions, Hydrogen evolution reaction, Oxygen reduction reaction, Oxygen evolution reaction.

## Introduction

Biphasic systems allow the electrocatalysis of reactants which are separated according to their lipophilicity [1]. Catalytic nanomaterials and supramolecular films can be assembled at the polarised Interface between Two Immiscible Electrolyte Solutions (ITIES) spontaneously, or by the application of an external driving force, e.g., in the form of an interfacial Galvani potential difference ($\Delta_w^\phi$) [2–7]. Furthermore, electrochemistry at the ITIES allows direct probing of the electrocatalytic activity of the nanomaterials or films free from the influence of an underlying support [8].

The two main electrocatalytic processes investigated at the ITIES to date, using metallic or inorganic nanomaterials, are of the energy-related oxygen reduction reaction (ORR) [9–15] and hydrogen evolution reaction (HER) [16–34]. The electrocatalytic oxidation of elemental sulfur ($S_8$) and the organosulfur compound 2,5-dimercapto-1,3,4-thiadiazole (DMcT) by gold nanoparticles (AuNPs) [35,36], photoelectrocatalysis of the oxygen evolution reaction (OER) [37–43], HER [44–49], and ORR [50–57] by a series of diverse strategies, and bioelectrocatalysis of the ORR using the redox-active protein cytochrome *c* (Cyt *c*) [58,59] at the ITIES have also been reported. In this review, recent progress for each of these biphasic electrocatalytic processes is overviewed. An in-depth discussion is also provided on the need for stricter standard protocols for electrocatalysis at the ITIES, to allow better comparison of the results from individual biphasic systems. Electrocatalysis of the ORR at the ITIES using molecular catalysts, such as cobalt or iron porphyrins and phthalocyanines dissolved in the aqueous phase, is not discussed in detail herein, as a detailed description of this research is provided in a recent review by Opallo et al. [60].

**Current Opinion in Electrochemistry** 2023, 38:101212

This review comes from a themed issue on **Electrocatalysis**

Edited by **Galina A. Tsirlina** and **Elena R. Savinova**

For a complete overview see the **Issue** and the **Editorial**

Available online 19 January 2023

https://doi.org/10.1016/j.coelec.2023.101212

2451-9103/© 2023 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/).

<footnote n="1">¹ Department of Physical Chemistry, University of Alicante (UA), E-03080, Alicante, Spain</footnote>

<footnote n="2">² Université de Lorraine, CNRS, LCPME, F-54000 Nancy, France</footnote>

<footnote n="3">³ Research Group of Battery Materials and Technologies, Department of Mechanical and Materials Engineering, Faculty of Technology, University of Turku, 20014 Turun Yliopisto, Finland</footnote>

<footnote n="4">⁴ The Bernal Institute and Department of Chemical Sciences, School of Natural Sciences, University of Limerick (UL), Limerick V94 T9PX, Ireland</footnote>
Electrocatalysis at the ITIES by metallic, carbon-based, or inorganic nanomaterials

In the liquid|liquid interface configuration, both the ORR and HER require: (i) a lipophilic electron donor dissolved in the organic phase, which is typically a ferrocene derivative (ferrocene Fc, dimethylferrocene DiMFc, or decamethylferrocene DcMFc) or tetrathiafulvalene; and (ii) protons available from the aqueous phase and oxygen dissolved in either phase to diffuse freely towards the 2D or 3D assemblies of catalysts. Although the interfacial electron transfer (IET) reaction is thermodynamically possible between the oxidising species dissolved in the aqueous phase (H+ or O2) and the electron donor present in the organic phase, the reaction kinetics are greatly improved by the presence of nanocatalysts in the ITIES. Thanks to Fermi level equilibration, conducting nanoparticles (NPs) adsorbed at the ITIES allow direct electron transfer by acting as bipolar electrodes and facilitating catalytic reactions [12,61]. Catalytically improved electron transfer through an AuNP film was demonstrated through the investigation of electron transfer between lipophilic ferrocenium cation/ferrocene and hydrophilic ferrocyanide redox couples (Figure 1a(i)) [62]. The peak-to-peak separation was greater than 90 mV in the absence of a catalyst (Figure 1a(ii)) but decreased down to 70 mV when the AuNP film was present at the ITIES (Figure 1a(iii)).

Catalysts made of noble metals (e.g., Au [10,12], Pt [14], and Pd [10]) or purely carbon-based (in situ generated reduced graphene oxide [11], graphene [13] and even lithium-ion battery waste [15]) were mainly used for the ORR, whereas a larger selection of inorganic materials were tested for the HER: MoS₂ [16,17,27,28,31,32] Mo₂C [18,19], Cu [21,25,34], Cu₂CoSnS₄ [22], Cu₂WS₄ [23], CoS [24], black phosphorus [33], NiS [26], W₂S [20] and W-based ternary metal sulfides [29]. These catalysts were adsorbed as such at the ITIES or were supported onto carbon-based materials, e.g., carbon nanotubes [17,19,24–26] and reduced graphene oxide [27,34]. Carbon-based nanomaterials, on which the catalysts were immobilised, helped to efficiently transfer electrons between the lipophilic electron donor and the hybrid materials (and not exclusively from the catalytically active site) leading to an increased HER rate (Figure 1b).

Over the past decade, the study of single NPs by their
collision with microelectrodes has been exploited to size
NPs, determine their shape, and provide insights into
NP agglomeration or aggregation phenomena [63,64].
Such single-entity electrochemical measurements at a
micron-sized ITIES were demonstrated by Stockmann
et al. [14] by recording current spikes, that were
rationalised by a COMSOL 2D simulation, and attrib-
uted to the electrocatalytic ORR upon impact of Pt NPs

at the ITIES with ferrocene acting as the electron donor (Figure 1c). A major advantage of this approach to single-entity electrochemistry is the more facile and reproducible preparation of microITIES, supported at the tip of a micropipette filled with either the aqueous or organic phase, *versus* microelectrodes [14,65]. Nevertheless, single-entity electrochemistry remains underexplored at microITIES *versus* microelectrodes due to the relatively small community of researchers actively working on this topic presently.

Suárez-Herrera et al. used a film of AuNPs at the ITIES to act as a catalytic surface onto which O₂ and elemental sulfur (S₈) can adsorb [35]. Electrocatalytic oxidation of S₈ by O₂ was then achieved by providing an electro-chemical driving force through polarisation of the ITIES and tuning the Fermi level of the interfacial AuNPs by the adsorption of aqueous anions (Figure 1d). This system mimics biochemical sulfur oxidation under ambient conditions with the ITIES being the ideal environment to oxidise S₈, overcoming the incompatible solubilities of the hydrophobic reactants (O₂ and S₈) and hydrophilic products (H⁺, SO₃²⁻, SO₄²⁻, etc.). In a separate study, Suárez-Herrera et al. also used a film of AuNPs at the ITIES to electrocatalyse the oxidation of the organic soluble organosulfur molecule 2,5-dimercapto-1,3,4-thiadiazole (DMcT) by O₂, yielding a floating interfacial AuNP/poly(DMcT) film by oxidative electrosynthesis [36]. Raman spectroscopy strongly suggested the AuNP/poly(DMcT) film is surface-enhanced Raman spectroscopy (SERS)-active and a coordination polymer of poly(DMcT) with AuNPs.

**Photoelectrocatalysis at the ITIES**

To date, all photoelectrocatalytic OER experiments at the ITIES have been performed with butyronitrile as the organic solvent due to its stability under strongly oxidising conditions [37–43]. Using a film of BiVO₄ photocatalyst with a specific hyperbranched structure at the ITIES, Rastgar et al. added [Co(bpy)₃](PF₆)₃ to the organic phase with the dual role of acting as the redox-active oxidant, scavenging conduction band electrons from BiVO₄ upon illumination and thereby enhancing the separation efficiency of photogenerated charge carriers, and as a scanning electrochemical microscopy (SECM) redox probe [37,38]. The latter permits the kinetics of the photoinduced interfacial electron transfer (IET) reaction to be probed by recording SECM feedback approach curves using a microelectrode that approaches the ITIES from the organic side and re-oxidises the photo-generated [Co(bpy)₃]²⁺ (Figure 2a(i)). Additional SECM feedback approach curves directly detected the O₂ evolved upon oxidation of water by the valence band holes of BiVO₄ during illumination [38]. By tuning the applied Δ₀Wφ by varying the concentration ratio of the potential determining perchlorate anion (ClO₄⁻) in each phase, increased rates of [Co(bpy)₃]²⁺
<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>

Electrocatalysis at the ITIES by metallic, carbon-based, or inorganic nanomaterials at the ITIES. (a) (i) Images of AuNP films prepared at a polarisable water|α,α,α-trifluorotoluene (TFT) interface in a four-electrode electrochemical cell using AuNPs with mean diameters of 12 nm (left panel) and 38 nm (right panel). (ii), (iii) Cyclic voltammograms of interfacial electron transfer between organic soluble ferrocenium cation/ferrocene and aqueous soluble ferri-ferrocyanide redox couples, with various ratios between Fe²⁺ and Fe³⁺ investigated, both (ii) in the absence and (iii) in the presence of an interfacial AuNP nanofilm. The scan rate used was 10 mV s⁻¹. (b) Schematic of the biphasic HER in the presence of a CNT/Mo₂C nanocomposite adsorbed at a polarisable liquid|liquid interface. The electron donor, decamethylferrocene (DcMFc), may inject electrons anywhere on the carbon support and not necessarily directly at the Mo₂C active site. (c) Schematic of a Pt NP impact leading to a measurable current spike at a micron-sized ITIES due to the electrocatalysis of the biphasic ORR using ferrocene as the electron donor. (d) An overview of the electrochemically driven biphasic sulfur oxidation reaction under aerobic conditions when the interfacial Galvani potential difference (Δφ) is polarised negatively, positively, or equal to the potential of zero charge (PZC). (a) is reproduced with permission from ref. [62]. Copyright 2015 American Chemical Society. (b) is reproduced from ref. [19] with permission from the Royal Society of Chemistry. Copyright 2013. (c) is reproduced with permission from ref. [14]. Copyright 2017 Wiley-VCH. (d) is reproduced from ref. [35] with permission from Elsevier. Copyright 2022.
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>

**Photoelectrocatalysis at the ITIES.** (a) (i) Schematic of scanning electrochemical microscopy (SECM) approach curves for the photoelectrocatalytic O₂ evolution reaction (OER) using a film of BiVO₄ photocatalyst at the ITIES and organic soluble [Co(bpy)₃]²⁺ as both a sacrificial oxidant and SECM redox probe. (ii) Photocurrent transients of increased magnitude upon reduction of O₂ at a microelectrode positioned in the organic phase at a fixed distance away from the ITIES as the applied ΔWφ was tuned to less positive potentials using perchlorate anions (ClO₄⁻) as the potential determining ion (PDI); the [ClO₄⁻]ₐq/[ClO₄⁻]₀rg. ratios are 1:1, 1:2 and 1:10 for the transients labelled 1, 2 and 3, respectively. (b) (i) Composition of a biphasic cell used to evaluate the effect of polarisation of an aqueous|butyronitrile (BCN) interface on the photoelectrocatalytic OER by tetrathiafulvalene (TTF), TTF⁺⁺ and tetrafluoroborate anions (BF₄⁻) molecular assemblies adsorbed on the surface of Pt microparticles. (ii) O₂ produced in the biphasic cells described in (b) (i) with BF₄⁻, tetrakis(pentafluorophenyl)borate anions (TB⁻) or bis(triphenylphosphoranylidene)ammonium cations (BA⁺) as the PDI. The O₂ evolved was quantified by sampling the headspace of septum-sealed glass vials using a lock-in syringe with a push-pull valve and injecting the sample on to a gas chromatography-mass spectroscopy (GC-MS) instrument. (c) (i) Optical images of the formation of a film of ZnTPPc interfacial nanostructures at an aqueous|z,α,α-trifluorotoluene (TFT) interface with the pH of the aqueous phase precisely matching the pKa of the carboxyphenyl-substituents (pH 5.8). (ii) Comparison of photocurrent transients due to the photoelectrocatalytic O₂ reduction reaction (ORR) measured in the presence of a film of ZnTPPc interfacial nanostructures with and without LED illumination, at an applied ΔWφ of 0.5 V with decamethylferrocene as the organic electron donor. The supporting organic electrolyte was bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate (BATB). (a) is reproduced from ref. [38]. Copyright 2017 American Chemical Society. (b) is reproduced from ref. [42] with permission from Elsevier. Copyright 2022. (c) is reproduced with permission from ref. [57]. Copyright 2020 American Chemical Society.
oxidation and O₂ reduction were recorded as the applied Δw₀φ shifted to less positive potentials. These observations may be due to an increased thermodynamic driving force for the photoelectrocatalytic OER (i.e., following Butler-Volmer theory), more efficient electron-hole separation, or an increased surface coverage of BiVO₄. The photoelectrocatalytic effect of the BiVO₄ nanoparticles was further demonstrated by obtaining photo-current transients upon reduction of O₂ at a microelectrode positioned in the organic phase at a fixed distance away from the ITIES (Figure 2a(ii)) [38]. Additionally, the formation and decay of the OH• intermediates during the OER at the surface of the BiVO₄ nanoparticles was quantified *in situ* by a surface interrogation mode of SECM involving the approach of a microelectrode to a micropore orifice that supported the BiVO₄ modified ITIES [39].

Girault and co-workers employed *in situ* self-assembled p-type semiconductors, consisting of tetrathiafulvalene (TTF), TTF•⁺ and BF₄⁻ or PF₆ molecular assemblies adsorbed on the surface of Co₃O₄ or Pt nanoparticles, as an electron acceptor/photosensitiser redox OER photo-electrocatalyst at the ITIES (Figure 2b(i)) [40–42]. Initial studies in a non-polarisable fully miscible water/ acetonitrile system were hampered by protonation of the molecular assemblies, triggering competing reactions such as the HER and ORR, and preventing electrochemical recycling of TTF•⁺ [40]. However, polarisation of an ITIES formed between water and butyronitrile using the potential determining BF₄⁻ anion increased the efficiency of the photoelectrocatalytic OER by enhancing the separation of the photoproducts (protons and TTF) by pumping protons to the aqueous phase [42]. Polarisation at the negative extreme, using the bis(triphenylphosphoranylidine)ammonium cation (BA⁺), or positive extreme, using the tetrakis(pentafluorophenylborate) anion (TB⁻), of the potential window yielded sub-optimal efficiency (Figure 2b(ii)). With BA⁺, the TTF•⁺ was extracted to the aqueous phase and the resultant TTF/TTF•⁺Cl⁻ assemblies formed were less photocatalytic, while with TB⁻ a stable “ionosome” emulsion [66] formed that scattered the light. Deposition of the TTF/TTF•⁺BF₄⁻ molecular assembly on a carbon electrode in the anodic compartment of a water-splitting H-cell permitted the electrochemical recycling of the electron acceptor TTF•⁺ in a water|butyronitrile emulsion [41]. Girault and co-workers also reported OER at the ITIES simply using photo-excitable fluorinated 7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) aggregates as an electron acceptor [43]. In the proposed mechanism, four photo-excited TCNQF₄⁺ radical cations stacked in an aggregate oxidise water to release O₂ and four protons, with the latter neutralising the TCNQF₄²⁻ dianions to form H₂TCNQF₄ [43].

Photoelectrocatalysis of the HER at the ITIES has been achieved using organic soluble osmocene [44],

decamethylosmocene [45] and decamethylruthenocene [46–49]. Based on their standard redox potentials, none of these metallocenes reduce protons pumped across a positively polarised ITIES spontaneously. However, all act as photo-redox catalysts and evolve H₂ to varying degrees *via* the corresponding hydride species when illuminated. The ability to generate H₂ with weak reductants makes these molecules highly suited for integration into the cathodic compartment of a biphasic H-cell as the oxidised form can be reduced at very positive redox potentials, thereby requiring less electrochemical driving force [67].

Photoelectrocatalysis of the ORR at the ITIES has been achieved by introducing photoactive porphyrin molecules to the aqueous phase [50,51,54] and by assembling either porphyrin-based supramolecular nanostructures [57] or bare [52,55,56] and dye-sensitised [53] colloidal TiO₂ semiconductor NPs at the ITIES. Aqueous soluble porphyrins, for example zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrins (ZnTPPc), act as a photo-redox catalyst at the ITIES. Upon illumination, the excited state is reductively quenched by IET from an organic soluble ferrocene-derivative. The ground state of the porphyrin is then regenerated by binding O₂ and reducing it. Only porphyrins present at the ITIES are photoelectrocatalytically active and therefore the efficiency of the photoelectrocatalytic ORR, as monitored by the magnitude of photocurrent transients recorded at the ITIES, is limited by the interfacial surface coverage of the porphyrin molecules. By optimally tuning the pH of the aqueous phase to match the pKa of the carboxyphenyl-substituents on ZnTPPc, Scanlon and co-workers have self-assembled ZnTPPc into films of highly ordered supramolecular nanostructures at a polarisable aqueous|TFT interface (Figure 2c(i) [57,68–70]). Due to the huge increase in interfacial porphyrin surface coverage, the multilayers of interfacial nanostructures are more efficient light harvesters, with the photocurrent due to the photoelectrocatalytic ORR increasing an order of magnitude compared to studies with ZnTPPc simply dissolved in the bulk aqueous phase (Figure 2c(ii)) [57].

Colloidal TiO₂ semiconductor NPs can be electrostatically assembled at ITIES by tuning the aqueous pH and applied Δw₀φ [52,55]. Upon illumination, O₂ is photo-electrocatalytically reduced by the conduction band electrons of TiO₂, while a ferrocene-derivative is oxidised by interfacial hole-transfer across the ITIES mediated by surface OH• radicals. Plana et al. developed a methodology involving sonication and the use of relatively high aqueous electrolyte concentrations to prepare thick interfacial films of coalesced TiO₂ NPs at the ITIES [56]. Despite the mesoporous nature of the interfacial TiO₂ film, cyclic voltammograms indicated that the ITIES retained a characteristic molecular sharpness. Using this setup, incident photon-to-current
efficiencies (IPCE) of 75% were reported [56], far
beyond the ca. 1% achieved for ITIES modified with
sub-monolayer coverages of photoactive porphyrin mol-
ecules or TiO₂ NPs. In an early contribution to this field,
Fermín et al. demonstrated that photocurrent responses
originating from the photoelectrocatalytic OER and
associated oxidation of a ferrocene-derivative can be
extended into the visible region by dye sensitisation of
TiO₂ NPs assembled at the ITIES [53].

Bioelectrocatalysis at the ITIES

Polarised liquid|liquid interfaces provide the required fluidic, amphipathic environment for redox protein adsorption, and can replicate the electrostatic and/or hydrophobic interactions required to mimic the biological membrane [71]. Furthermore, the electric fields of up to 10⁹ V·m⁻¹ experienced by peripheral membrane proteins are replicated at the ITIES without the need to introduce any solid electrode architectures [72]. In this context, the electrocatalytic activity of the model protein cytochrome c (Cyt c) towards the ORR has recently been demonstrated at the ITIES [58,59]. Cyclic voltammograms with Cyt c in an aqueous phosphate buffer solution and the reductant DcMFc in an organic electrolyte solution show two voltammetric signals, one reversible and the other irreversible (Figure 3a). The irreversible signal at positive potentials is attributed to the electrocatalytic ORR, while the reversible peak at negative potentials is attributed to the ion transfer of DcMFc⁺ cations produced during O₂ reduction [58]. This bioelectrocatalytic IET reaction was facilitated at positive potentials due to electrostatic and hydrophobic interactions between Cyt c and tetrakis(penta-fluorophenyl)borate (TB⁻) organic electrolyte anions in the mixed solvent region. UV/vis spectroscopy in total internal reflection (TIR-UV/vis) configuration demonstrated that Cyt c adsorbs and is partially denatured at a positive applied Δw₀φ (ca. 0.5 V). In contrast, no adsorbed Cyt c species were detected at negative applied Δw₀φ [58]. These findings were supported by Molecular dynamic (MD) computational modelling, where positive potentials promoted interactions between cationic lysine residues that surround the active site of Cyt c and TB⁻ (Figure 3b). The consequences of these interactions are two-fold: (i) causing a conformational change of the adsorbed Cyt c from its native state that exposes the redox-active heme to O₂ and (ii) ensuring the partially exposed heme-pocket is orientated perpendicular to the liquid|liquid interface to favour interfacial electron transfer with DcMFc. The intensity of the irreversible ORR signal decreases with each cycle (Figure 3a) as Cyt c becomes progressively more denatured and redox inactive due to its continued interactions with the organic solvent and TB⁻. The electrochemical behaviour of Cyt c at a positively polarised ITIES mimics Cyt c's *in vivo* peroxidase activity in the inner mitochondrial membrane at the onset

of apoptosis in which reactive O₂ species (ROS; such as
H₂O₂) are reduced at the heme [58]. Specifically, the
dual biological roles of the anionic phospholipid cardi-
olipin in this process, as a disrupter of the tertiary
structure of Cyt *c* and sacrificial oxidant, are played by
TB⁻ and DcMFc, respectively (Figure 3c).

Cyt c's electrocatalytic activity towards the ORR at the
ITIES is enhanced in the presence of denaturing agents
such as guanidinium chloride (GdmCl) and urea
(Figure 3d) [59]. The magnitude of this enhancement is
correlated with physicochemical changes to the liquid-
liquid interface and, most importantly, conformational
changes to the adsorbed Cyt c specific to each chaot-
rope. MD modelling revealed that the distinct Cyt c
unfolding mechanism with GdmCl leads to a more open
heme pocket structure than when urea is added, and
thus GdmCl shows a greater enhancement in electro-
catalytic activity towards the ORR [59]. Additionally,
Gdm⁺ cations penetrate the interface, creating a more
hydrophobic environment that increases the interfacial
concentration of DcMFc in the mixed solvent region
and thereby enhances IET. Conversely, urea partially
impedes access of the organic solvent to the water phase
and hinders IET.

Normalising electrocatalytic performance at
the ITIES

As outlined in the previous sections, over the years,
many different catalyst materials have been investigated
for the HER and ORR at polarised liquid|liquid in-
terfaces. Unfortunately, there has been very little dis-
cussion in the field on how to normalise and compare the
electrocatalytic performance of different materials at
the ITIES. Experiments have been performed in vastly
different experimental conditions, with very different
catalyst amounts. Also, catalysts may be added as a
powder (e.g., [16,27]) or formed *in situ* by reduction from
the precursors (e.g., [9,21]). Therefore, to allow better
comparison of the results, we believe that stricter
standards are required and recently suggested best
practises for reporting electrocatalytic performance of
nanomaterials should also be applied for electrocatalytic
work at the ITIES [73]. One of the recommendations
was to report the turnover frequency, but this is chal-
lenging to estimate as typically the number of active
sites per area is not known. Unlike for catalysts attached
on an electrode surface, even estimation of the elec-
trochemically active surface area of catalyst dispersed
in solution or attached at the ITIES remains a challenge.
As a compromise, we suggest at least normalising the
reaction rates by the mass or the molar amount of the
catalyst. As an example, we have analysed the data on
the biphasic HER presented in the literature in so-
called “shake-flask experiments” (Figure 4). The latter
involved DcMFc dissolved in a 1,2-dichloroethane phase
reacting under stirring with protons transferred to the
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>

**Bioelectrocatalysis at the ITIES.** (a) Cyclic voltammetry with 10 µM Cyt c in a phosphate buffer solution (60 mM Na₂HPO₄ and 20 mM KH₂PO₄ at pH 7) and 500 µM DcMFc in a solution of 5 mM bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate (BATB) organic electrolyte in TFT under aerobic conditions. The scan rate used was 20 mV s⁻¹. (b) Representative molecular dynamics snapshots showing the orientation of the Cyt c heme active site at a positively polarised liquid/liquid interface. The Lys residues on the surface of Cyt c are shown as green ball and sticks, while the BA⁺ and TB⁻ ions from the electrolyte in the TFT organic phase are shown as blue and red ball and sticks, respectively. (c) Schematic of the biomimetic electrified aqueous/organic interface at which DcMFc and TB⁻ anions activate Cyt c for reduction of reactive O₂ species. The electrons are represented by green circles. (d) Cyclic voltammetry of 10 µM Cyt c and 1 mM DcMFc under aerobic conditions, as described in part (a), with and without 6 M urea or GdmCl. The scan rate was 20 mV s⁻¹. (a), (b) and (c) are reproduced with permission from ref. [58]. Copyright 2021 American Association for the Advancement of Science (AAAS). (d) is reproduced from ref. [59] with permission from the Royal Society of Chemistry. Copyright 2022. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

aqueous phase after positive polarisation of the liquid-|liquid interface by distribution of lithium tetrakis(-pentafluorophenyl)borate (LiTB; initially dissolved in the organic phase) between the phases.

For analysis of the data extracted from the literature [9,16–19,21,31,32,34], we have assumed that first-order kinetics with respect to the DcMFc concentration is followed initially, and the catalyst affects the rate constant of the reaction. This assumption is not strictly true, as some of the catalysts have shown different kinetics [18], but it serves as a first approximation.

Apparent first-order reaction rate constants (unit of 1/s) have been extracted for all the catalysts and in the absence of a catalyst (Figure 4a), and the rate constants have been normalised by the mass of the metal (mg/L; Figure 4b), or the concentration of the catalyst (mmol/L; Figure 4c) and divided into catalysts deposited on high-surface-area carbon, as nanomaterials, as catalysts formed by electrodeposition and as microparticles.

The reaction rates depend on the number of active sites, so nanostructuring of the catalysts leads to enhanced normalised reaction rates. Additionally, deposition of the
<figure><img src="image_6.png" /><figcaption>Figure 4</figcaption></figure>

**Normalising electrocatalytic performance at the ITIES.** (a) Apparent first-order reaction rate constants ($k_{app}$ with a unit of 1/s) extracted in the absence and presence of the catalysts Mo₂C, MoS₂, metallic (Pt, Pd and Cu) nanoparticles, W₂C and WC, either deposited on high-surface-area carbon, as nanomaterials, as catalysts formed by electrodeposition, or as microparticles. The rate constants were normalised (b) by the mass (mg/L) of the metal ($k_{app}/m_{metal}$ with a unit of L/s/mg) and (c) the concentration (mmol/L) of the active catalyst ($k_{app}/c_{catalyst}$ with a unit of L/s/mmol). The data was extracted from references [9,16–19,21,31,32,34] and is available on Zenodo (https://doi.org/10.5281/zenodo.7400926) to download.

catalyst on a high-surface-area carbon leads to the highest normalised rates. This is because both hydrogen evolution and DcMFc oxidation need to take place at the

redox electrocatalyst at the same rate. The overpotential for each reaction depends on the area available for the reactions, as explained in a recent review [8]. For supported catalysts, the area available for DcMFc oxidation taking place at the support is much higher than for the HER taking place mostly at the catalyst. For unsupported particles, the areas available for both reactions can be considered similar. Therefore, the driving force will be higher on the supported catalysts, leading to higher rates.

Analysis of only the reaction rates can lead to misinterpretation of the activities. For example, Mo₂C micro-particles show very different reaction rates because the catalyst concentration varied between 25 µM and 1 mM, but this difference disappears when the activities are normalised either by catalyst mass or concentration. Concentration normalised data shows slightly higher activity for Mo₂C than for MoS₂ while the difference is smaller for metal mass normalised data, as the carbide contains twice the mass of the metal.

The data also shows high variability in the reaction rates measured in the absence of catalyst. This might indicate that some chemicals, flasks or even stir bars [74] may very easily cause contamination. Therefore, the practise to compare acceleration to the uncatalysed reaction can lead to inaccurate conclusions. We highlight that carefully cleaned glassware and stir bars should be used for these experiments.

## Conclusions and outlook

The diversity of reactions electrocatalysed at the ITIES remains low, with the overwhelming majority of studies pertaining to the HER and ORR, followed by more challenging OER studies [60]. Scope clearly exists to electrocatalyse other reactions that may benefit from a biphasic environment, as shown by the electrocatalytic oxidation of S₈ [35], or lead to the direct electrosynthesis of thin films of nanocomposites (encompassing the interfacial electrocatalyst), as shown by the electrocatalytic oxidation of DMcT by AuNPs [36]. A particularly important reaction that is conspicuous by its absence in the literature is the carbon dioxide reduction reaction (CO₂RR) at the ITIES. A key challenge limiting the efficiency of the (photo)electrochemical CO₂RR is the low solubility of CO₂ in water at standard conditions (~35 mM). However, organic solvents permit higher CO₂ solubility and enable different reaction products, including value-added C₂+ hydrocarbons like oxalate [75]. Furthermore, aprotic organic solvents eliminate the competing HER, an arduous task in aqueous electrolytes [76]. Thus, biphasic systems may simultaneously harness desirable attributes of both the aqueous phase (proton source) and organic phase (inhibit H₂ evolution, increase access to C₂+ hydrocarbons, reservoir of CO₂) towards the CO₂RR. To date,
biphasic CO2RR systems have been studied using water-miscible organic solvents, e.g., acetonitrile [75,77], immiscible aqueous|organic emulsions [78] and supercritical CO2|ionic liquid biphasic systems [79–81]. However, the benefits of polarisation of an ITIES to drive the biphasic (photo)electrochemical CO2RR towards selective C2+ hydrocarbon production have never been explored. Additionally, nitrogen reduction reactions should also be explored [82].

SECM is a powerful methodology to probe the redox kinetics of any (photo)electrocatalyst as a function of the applied $Δ_0^Wφ$, as clearly demonstrated by the series of articles by Rastgar et al. to photoelectrocatalyse the OER by an interfacial film of $BiVO_4$ [37–39]. Thus, the further development of SECM as a key methodology to combine with (photo)electrocatalysis at the ITIES will allow the measurement of the “true” (photo)electro-catalytic activity of specific materials without interfering effects arising from interactions of the (photo)electro-catalyst with an underlying solid electrode surface, e.g. contact resistance [8].

The field of bioelectrocatalysis at the ITIES is still in its infancy. Thus, studies with proteins beyond the model Cyt c system, such as oxidases [83] and dehydrogenases [84], are required to clarify the limitations and advantages of this new research topic. Furthermore, major scope exists to modify the liquid|liquid interface with phospholipid monolayers to enhance the biocompatibility of the ITIES and optimise IET with proteins. Electrochemical activation of a proteins’ enzymatic activities at the ITIES will open exciting new avenues to realise biofuel cell and artificial photosynthesis-based energy conversion and storage, drug screening for medical applications, and (bio)sensing.

Two-phase shake flask methods offer a quick and simple method to test the activity of powdered catalysts for different reactions. For a more careful comparison of the catalysts, the active surface area should be known. For supported catalysts, the loading of the catalyst will also have a significant effect on the measured rates. Additionally, methods to evaluate the stability of the catalysts should be developed, for example, based on electrochemical setups [67].

## 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

The data extracted from references [9,16–19,21,31,32,34] to prepare Figure 4 is available on Zenodo (https://doi.org/10.5281/zenodo.7400926) to download.

## Acknowledgments

M.D.S. acknowledges funding from the European Research Council through a Starting Grant (agreement no. 716792). A.G.-Q. acknowledges funding received from an Irish Research Council (IRC) Government of Ireland Postdoctoral Fellowship Award (grant no. GOIPD/2018/252) and a Marie Skłodowska-Curie Postdoctoral Fellowship (grant no. MSCA-IF-EF-ST 2020/101018277). P.P. acknowledges funding from the European Research Council through a Starting Grant (agreement no. 950038), the Academy Research Fellow funding and project funding by the Academy of Finland (Grants No. 315739 and 334828).

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