Controlling product selectivity is essential for improving the efficiency of multi-product reactions. Electrochemical water oxidation is a reaction of main importance in different applications, e.g., renewable energy schemes and environmental protection, where H2O2 and O2 are the two principal products. In this Communication, the product selectivity of electrochemical water oxidation was controlled by making use of the chiral induced spin selectivity (CISS) effect at mesoporous-TiO2 on the molecule-modified Au substrate. Our results show a decrease in H2O2 formation when using chiral hetero-helicene molecules adsorbed on the Au substrate. We propose a mechanism for this kinetic effect based on the onset of CISS-induced spin polarization on the Au–helicene chiral interface. We also present a new tunable substrate to investigate the CISS mechanism.

Product selectivity is a critical performance indicator that holds paramount importance in catalytic systems. The optimization of selectivity toward the desired product is of utmost necessity, particularly in multi-product reactions, such as CO2 reduction and electrochemical water oxidation.1,2 The latter is a reaction of main importance in different renewable energy schemes. In particular, the oxygen evolution reaction [OER, Eq. (1)] is a critical process in electrocatalytic water splitting for hydrogen production or artificial photosynthesis.3 However, a competing oxidation reaction exists, the production of H2O2 [Eq. (2)]. While this is a valuable chemical with a wide range of industrial applications,4,5 its generation is often undesirable in certain practical contexts, e.g., in an electrolyzer. This is primarily due to its propensity to adsorb onto the catalyst, reducing the overall reaction rate and potentially diminishing its stability,6,7 or to corrode other materials within the electrolytic system. Understanding the reaction pathway to favor one or another product remains essential for optimizing these applications.
(1)
(2)

The OER has sluggish kinetics, mainly attributed to the need to transfer four electrons. This leads to a high overpotential in water splitting applications. When high potentials are applied to produce oxygen, the formation of H2O2 can become a competing reaction. Recently, an additional possible cause of the increased overpotential to achieve the OER related to spin has been introduced. The ground state of the oxygen molecule is a triplet state, while that of water is a singlet state. Thus, a spin change needs to occur during the reaction, and these spin-restrictions can increase the overpotential. The formation of H2O2, on the other hand, requires the transfer of only two electrons and does not involve a spin conversion, as H2O2 is a singlet in its fundamental state.2,8

It was recently reported that a preferred orientation of electron spin can be found in chiral molecules when charges pass through, referred to as chiral induced spin selectivity (CISS) effect.9–11 This phenomenon can play a paramount role in chemical reactions where spin multiplicity changes are involved.11 Taking into account the different spin states of the products in the water oxidation, the CISS effect can thus have an influence on its product selectivity, favoring the formation of O2 over H2O2 by providing spin-aligned electrons. Hence, using a chiral molecule as a spin polarizer could reduce the amount of H2O2 formed, as was already seen in different (photo-) electrochemical reactions.6,12

Along the H2O2 hindered formation, the CISS effect has been shown to enhance the O2 production and double the OER activity of state-of-the-art NiFe-based catalysts.13,14 However, the latter predominantly favor the OER and are unable to produce H2O2 in great quantities,15–18 thus limiting their suitability as a model for investigating CISS-induced changes in the water oxidation pathways. In addition, the lack of tunability further restricts the possibility for examining such phenomena. Moreover, how a chiral molecule delivers spin-polarized electrons to the catalyst surface (e.g., if the chiral molecule is placed between the catalyst and the substrate, or if it polarizes electrons in its vicinity) requires further investigations.

In this work, we present a new tunable electrode that can be used to quantitatively test the CISS effect in electrochemical water oxidation. To achieve this, we constructed a hybrid electrode comprising an Au substrate coated with a mesoporous TiO2 thin film. The selection of TiO2 as our electrode material stems from its ability to generate H2O2 in substantial quantities,4 as well as its increasing application in H2O2 production.2,19 Mesoporous TiO2 thin films can be produced with high surface areas in a controllable manner and can act as a versatile platform, as pore sizes and wall materials can be easily tuned.20–22 The porous structure of the TiO2 allows uncovered areas at the Au substrate to bond the molecules of interest. Here, we quantitatively compared the selectivity of H2O2 formation during the electrochemical water oxidation reaction by using chiral and achiral molecule-modified electrodes. The molecules were added using a near-effortless and non-destructive deposition method on the exposed Au substrate to determine the lower limit of the CISS effect.

Helicenes are organic molecules with axial chirality, high racemization barriers, and a rigid structure.23,24 These make them ideal molecules for electrode modification especially when incorporating heteroatoms in the helicene framework, to provide heterohelicenes,25,26 or when attaching functional groups that preserve the chirality of the molecule and stably bind to metal surfaces.25 

Mesoporous TiO2 thin films were prepared on Au (111) substrates by evaporation-induced self-assembly. Figure 1 shows the mesoporous structure obtained, with a highly porous and well-ordered Im3m structure with a pore diameter of ∼8 nm. This allowed the deposition of small molecules within the pores. Enantiopure (M)-bis(thiadiazole)-[8]helicene (helicene hereafter),13 a chiral molecule capable of binding to the gold substrate, was used. Indeed, in a previous recent study,13 some of us chirally functionalized gold electrodes, coated with NiOx catalysts, with bis(thiadiazole)-[8]helicene and its shorter analog thiadiazole-[7]helicene.27 Their achiral analog, 2,1,3-benzothiadiazole, has the same interaction with the surface but lacks the chiral helicene structure (Fig. 2).

FIG. 1.

Highly ordered mesoporous TiO2 film on Au(111): Top-view SEM and AFM images. The top and bottom insets in the AFM image show the height profile and the fast Fourier transform, respectively.

FIG. 1.

Highly ordered mesoporous TiO2 film on Au(111): Top-view SEM and AFM images. The top and bottom insets in the AFM image show the height profile and the fast Fourier transform, respectively.

Close modal
FIG. 2.

Scheme of the TiO2/Au electrodes and molecules used in this work: 2,1,3-benzothiadiazole (achiral) and (M)-bis(thiadiazole)-[8]helicene (chiral).

FIG. 2.

Scheme of the TiO2/Au electrodes and molecules used in this work: 2,1,3-benzothiadiazole (achiral) and (M)-bis(thiadiazole)-[8]helicene (chiral).

Close modal

Based on scanning tunneling microscopy (STM) measurements of ultra-thin mesoporous TiO2 films after helicene deposition (Fig. 1 of the supplementary material), it appears that most of the molecules are located in the pores, and the TiO2 surface is uncovered. This observation is consistent with the high affinity of the thiadiazole group to the gold substrate13 and its low affinity to the TiO2 walls. It is likely that helicene molecules can only be stably adsorbed on the Au substrate. Considering the self-assembled packing structure that we have reported for (M)-bis(thiadiazole)-[8]helicene on gold surfaces13 and that the pore has a diameter of 8 nm and a height of ∼5 to 7 nm, we estimate that there are ∼25 molecules per pore.

Figure 3 displays the measured current during a 40-min electrochemical reaction at a potential of 3.0 V vs RHE. The current obtained in bare electrodes and those modified with helicene or benzothiadiazole is similar, which could be an indication that the active surface area does not change significantly. The total current accounts for the water oxidation reactions (OER and H2O2 formation) and possibly some Au oxidation. The CVs of the different studied samples also show a similar behavior (Fig. 2 of the supplementary material). Nonetheless, the amount of H2O2 produced differs significantly, with a decrease of ∼33% in the number of moles formed in helicene-modified electrodes compared to benzothiadiazole-modified ones. The same trend was observed in thicker TiO2 samples (∼60 and 200 nm thickness, Fig. 3 of the supplementary material). The effect of the chiral molecules is stronger when the film thickness is lower (see Methods and Fig. 4 of the supplementary material for further information on H2O2 quantification). Even if we could not quantify the oxidation state of the molecules after OER for this system, according to our previous work, the helicene molecules are stable on the Au surface after electrolysis.13 Moreover, if oxidation of the helicene molecules by highly oxidant intermediates28 generated on TiO2 were to occur, it would be reasonable to expect a similar situation with the control benzothiadiazole molecule. The use of this control molecule allows us to assign the observed reduction on the H2O2 production to the presence of chiral molecules.

FIG. 3.

(a) Chronoamperometry of bare TiO2 and helicene or benzothiadiazole-modified electrodes. The noise is caused by bubble formation and release during the measurement. The inset shows the charge transferred after 40 min. (b) Amount of H2O2 produced for each type of electrode.

FIG. 3.

(a) Chronoamperometry of bare TiO2 and helicene or benzothiadiazole-modified electrodes. The noise is caused by bubble formation and release during the measurement. The inset shows the charge transferred after 40 min. (b) Amount of H2O2 produced for each type of electrode.

Close modal

The reduction in H2O2 formation with the chiral molecules was not observed in TiO2 samples deposited on FTO, or in bare Au substrates (Tables 1 and 2 of the supplementary material). Thus, it is likely that the molecules are not directly affecting the active sites at the TiO2 surface (i.e., where the reaction occurs), but their presence at the Au substrate affects the reaction happening on TiO2.

It is also interesting to note that although the molecules are not located between the substrate and the catalyst, or at the active sites on the catalyst, the CISS effect is anyway observed. This is evidence of CISS in a totally new and different configuration from previous studies, leading to new insights into the mechanism of the CISS effect.

It has been suggested that the chiral molecules act as spin filters when electrons pass through them.10 Some other studies indicated that a chiral molecule polarizes the spin of electrons in its vicinity.11,29–31 In this work, we used a molecule-catalyst configuration that differs from the catalyst–chiral molecule–substrate sandwich configuration, where the current goes through the chiral molecules. Here, the helicene molecules are adsorbed on the areas of Au surface that are not covered by TiO2. The catalytic activity is still predominant at TiO2 surface sites, regardless of the fact that the spin polarization interfacial activity seems to take place at the gold surface. Our results suggest that the spin polarization is somehow transmitted to the TiO2 surface.

Although the mechanism is not totally clear, it has been demonstrated both theoretically and experimentally that spin polarization occurs as a consequence of electron transfer, electron transport, and bond polarization through chiral centers. Based on this result, we suggest that the formation of the bond between the chiral molecule and the gold surface leads to spin polarization32,33 in the Au substrate surrounding the molecules, subsequently affecting the kinetics of the reaction taking place at the catalysts supported by the same Au substrate (Fig. 4), or that the spin-polarized electrons are transmitted through the molecules in close proximity to the TiO2 walls, effectively acting as a bridge between the Au and TiO2.

FIG. 4.

Illustration of the CISS effect on the helicene-modified mesoporous TiO2 on the Au substrate. The chiral molecules could polarize the spin of electrons traveling through the gold (red areas) and turn the TiO2 into a spin-polarized state, which can then favor the formation of triplet O2 over H2O2 production.

FIG. 4.

Illustration of the CISS effect on the helicene-modified mesoporous TiO2 on the Au substrate. The chiral molecules could polarize the spin of electrons traveling through the gold (red areas) and turn the TiO2 into a spin-polarized state, which can then favor the formation of triplet O2 over H2O2 production.

Close modal

To explain the spin polarization around the molecules, we could assume that the nanoscale Au surface exposed to the electrolyte becomes ferromagnetic, as was observed for small Au nanoparticles.34–36 This would lead to a generation of spin polarization at the interface of chiral molecules and gold. In the context of understanding the magnetic behavior of gold nanoparticles coated with organic molecules linked through N or S atoms, the transition from diamagnetism to ferro- and ferri-magnetism was mediated by precursor states called spin-polarized singlets (SPS), that is, singlet states corresponding to systems with an even number of electrons where both the total spin and the total sum of local spin densities around atomic centers vanish, but the individual local spin densities were not zero. The TiO2 cavities in the nanoscale could have the right order of magnitude to provoke similar effects in our system.

The transfer of spin polarization information to the active sites can then occur in the vicinity of the TiO2/Au/molecule interface, as this type of behavior has been found in other systems where the spin polarization transfer occurs effectively with a distance of the order of the coherence length.37 Even though this transfer of polarization is essential for the observed kinetic effect, we cannot rule out the possibility that the Au–helicene chiral interface can provide an alternative kinetic path to the predominant one associated with the TiO2 sites. We are currently advancing further work to elucidate this point.

Considering that the accessible Au surface is limited by the pore size of the TiO2 mesoporous structure, this hybrid substrate offers an intriguing opportunity to serve as a customizable platform for probing the limits of the CISS effect within confined systems. Tuning this model system could provide valuable insights into fundamental inquiries arising from this investigation, such as the optimal pore sizes and distances, and the minimal quantity of chiral molecules required for discernible shifts in product selectivity trends. Further studies could assist in the mechanism understanding, as it is not yet clear whether the TiO2 surface is influenced by Au polarization caused by the chiral molecules, or if the change in selectivity is the result of changing the way reaction intermediates adsorb on TiO2 because the pores act as a grid of spin-polarized centers.

In summary, these results introduce a new tunable model system to study the CISS effect on the H2O2 formation on hybrid nanostructured electrodes. The use of chiral helicene molecules is an exceptionally efficient way to optimize product selectivity and energy efficiency in electron spin-dependent electrocatalysis applications. Moreover, the mesoporous structure can act as a versatile platform, as pore sizes and wall materials can be easily tuned, not only for electrocatalysis but TiO2 also can be explored as a photoelectrocatalyst for this reaction, allowing for further studies of the CISS effect in nanostructured materials. This work is a first step in the direction of designing tunable substrates for CISS studies in electrocatalytic interfaces.

See the supplementary material for experimental methods and supplementary data.

M.L. and N.A. acknowledge the Swiss Academies of Arts and Sciences (SATW) and the French Ministry of Foreign Affairs for the financial support through the bilateral Germaine de Staël (PHC Project No. 47931VB) project. P.V. acknowledges the Federal Commission for Scholarships for Foreign Students (FCS) for the Swiss Government Excellence Scholarship. P.V. and G.J.A.A.S.-I. acknowledge the financial support of CONICET and UNSAM. V.M. acknowledges the support of the Keck Foundation and the National Science Foundation for projects in quantum biology.

The authors have no conflicts to disclose.

Priscila Vensaus: Conceptualization (equal); Data curation (lead); Formal analysis (lead); Funding acquisition (supporting); Validation (lead); Writing – original draft (lead). Yunchang Liang: Conceptualization (equal); Data curation (equal); Formal analysis (supporting); Funding acquisition (supporting); Methodology (equal); Supervision (supporting); Validation (supporting); Visualization (supporting); Writing – original draft (supporting). Nicolas Zigon: Investigation (supporting); Resources (supporting); Writing – review & editing (supporting). Narcis Avarvari: Conceptualization (supporting); Funding acquisition (equal); Writing – review & editing (supporting). Vladimiro Mujica: Validation (supporting); Writing – review & editing (supporting). Galo J. A. A. Soler-Illia: Supervision (equal); Writing – review & editing (supporting). Magalí Lingenfelder: Conceptualization (lead); Funding acquisition (lead); Investigation (supporting); Supervision (lead); Validation (lead); Writing – review & editing (lead).

The data that support the findings of this study are available within the article and its supplementary material.

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Supplementary Material