Trends in competing oxygen and chlorine evolution reactions over electrochemically formed single-atom centers of MXenes

Abstract

Single-atom catalysts (SACs) have garnered widespread attention in the catalysis community due to their ability to catalyze transformations relevant to energy conversion and storage with high activity and selectivity and maximum atomic efficiency. Although considerable efforts are being made to develop synthetic routes for SACs based on non-noble metal atoms, the state-of-the-art SACs are largely based on rare Pt-group metals. MXenes, a new class of two-dimensional materials, offer the exciting possibility of synthesizing single-atom centers with structural similarity to archetypical SACs and without the need for scarce metal atoms such as Pt or Ir. Instead of a dedicated synthetic protocol, only a sufficiently large anodic electrode potential is required to enable the activation of the MXene basal plane by surface oxidation, and the as-formed single-atom centers are sufficiently stable under anodic bias. The electrochemically formed single-atom centers of MXenes based on surface reconstruction differ significantly from previous studies based on SAC sites obtained by doping with foreign metal atoms. In the present work, we demonstrate that the in situ formed single-atom centers of MXenes can be effectively used to catalyze energy conversion processes relevant to the chemical industry. By combining electronic structure theory calculations and descriptor-based analysis, we determine activity and selectivity trends in competing oxygen and chlorine evolution reactions and derive activity and selectivity trends for a noble metal-free electrochemical synthesis of gaseous chlorine. Our results indicate that electrochemically formed single-atom centers of two-dimensional materials can play a crucial role for the development of next-generation catalysts for sustainable energy.

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Introduction

MXenes are two-dimensional transition-metal carbides and nitrides with the general formula M_n+1X_nT_x, where M represents a transition metal (Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, or Mn), X is either carbon or nitrogen, and T denotes surface adsorbates (*O, *OH, and *F, among others).^1,2 Since their discovery by Gogotsi and co-workers in 2011,³ this class of materials has attracted interest in energy conversion and storage due to their excellent electronic conductivity, large surface area, and hydrophilic surfaces combined with their low cost due to the use of non-scarce metals.⁴ The application of MXenes even goes beyond the energy sector and includes medical platforms and devices.⁵

In the field of electrocatalysis, two-dimensional materials^6,7 including MXenes are discussed as a potential replacement for traditional Pt-group catalysts.^8–15 The suitability of this class of materials for catalyzing electrochemical transformations was demonstrated back in 2016 by Vojvodic and co-workers,¹⁶ who reported that the basal planes of the MXene Mo₂CT_x are catalytically active toward the hydrogen evolution reaction. In the following years, numerous DFT-based studies investigated the elementary steps of (electro-)catalytic processes on the basal planes of MXenes, with a focus on reactions occurring under cathodic conditions.^17–21 While the MXene basal plane could represent a suitable active site motif for cathodic polarization, the MXene surface reconstructs in contact with water under anodic polarization:^22,23 a surface metal atom is pulled out of the basal plane, and the resulting site is somewhat reminiscent of a single-atom catalyst (cf.Fig. 1a). Only recently, it has been demonstrated that this motif formation through surface oxidation is potential dependent,²⁴ and the as-formed single-atom centers (SACs)²⁵ are active for the oxygen evolution reaction (OER) — 2 H₂O → O₂ + 4 H⁺ + 4 e⁻, U_OER⁰ = 1.23 V vs. RHE (reversible hydrogen electrode). A clear challenge in (electro-)catalysis is therefore to activate the MXene surface by surface oxidation to enable the formation of the SAC-like sites, although this activation process should not lead to degradation of the material, which would be the case at large anodic potentials. A recent theoretical work for Ti₂CT_x based on ab initio molecular dynamics demonstrated that the SAC-like motif is stable at applied electrode potentials up to U = 1.76 V vs. RHE,²⁶ which provides a sufficient potential range beyond the equilibrium potential of the OER to catalyze electrochemical transformations under anodic bias. This suggests that the electrochemically formed SAC-like motif of MXenes in the homologous series of M₂X-SAC is a promising candidate for anodic conversion reactions due to its stability under anodic polarization.

Fig. 1 (a) Single-branch MXene-SAC motif with a single intermediate adsorbed at the out-of-plane metal atom, represented by M_SAC-*O. (b) Double-branch MXene-SAC motif with two intermediates adsorbed at the out-of-plane metal atom, represented by M_SAC-*O-*OH. Blue, brown, red, and white spheres denote metal, carbon or nitrogen, oxygen, and hydrogen atoms, respectively.

In addition to the OER, there is interest in the formation of gaseous chlorine, which is realized industrially by chlor-alkali electrolysis:^27–29 there, the chlorine evolution reaction (CER) — 2 Cl⁻ → Cl₂ + 2e⁻, U⁰_OER = 1.36 V vs. SHE (standard hydrogen electrode) — takes place at the anode, the selectivity of which is impaired by the competing OER. While the introduction of single-atom^30–32 or atomically-dispersed catalysts as anode materials has opened new avenues to direct the selectivity in the competing CER and OER toward the desired product Cl₂,³³ the state-of-the-art catalysts for the CER still rely on Pt-group metals.^34–36 Mixed-metal oxides based on RuO₂, IrO₂, and TiO₂ are combined in dimensionally stable anodes for industrial electrolysis, and the most prominent SAC for the CER refers to a single platinum site doped on a carbon nanotube developed by Joo and co-workers.³⁷

In the present manuscript, we suggest the application of electrochemically formed SAC-like sites on the MXene basal plane as a new sustainable pathway for selective chlorine formation to overcome the dependence on scarce platinum group metals. To this end, we report activity and selectivity trends of competing CER and OER over SAC-like sites of twelve different MXenes. All computational details for the application of density functional theory (DFT) are summarized in the following section, while section 2 of the ESI† compiles the relevant SAC-like structures for the investigated M₂XT_x MXenes with ABC stacking³⁸ under applied bias.

Computational details

In this work, we apply electronic structure calculations in the density functional theory (DFT) framework as implemented in the Vienna Ab initio Simulation Package (VASP),^39–41 using the Perdew–Burke–Ernzerhof (PBE) exchange correlation functional⁴² combined with Grimme's D3 scheme to account for dispersion effects.⁴³ Core electron effects on the valence electron density are taken into account by the projector augmented wave (PAW) approach.⁴⁴ Valence electron density is expanded in a plane wave basis set with a kinetic energy cutoff of 440 eV. Structural relaxation is systematically achieved through energy minimization, and the total energy convergence and maximum force threshold is set to 10⁻⁶ eV and 0.01 eV Å⁻¹, respectively. For the integration of the reciprocal space, we employ a 5 × 5 × 1 Γ-centered grid within the Brillouin zone.

To ensure physical isolation of the MXene layers along the direction perpendicular to the surface, a vacuum region with a thickness of at least 12 Å is included in all our models. We have performed test calculations for all the adsorbate species observed under chlorine evolution (CER) and oxygen evolution (OER) reaction conditions (all intermediate structures are listed in eqn (1)–(10) or Fig. 3 of the main text), and it turns out that spin polarization changes adsorption energies by less than 0.02 eV. This is the line with previous works,⁴⁵ reporting that spin polarization is not relevant to functionalized MXenes surfaces. For a thorough computational benchmark of the single-atom centers (SAC) formed on MXenes, we refer to our recent work in which we tested different levels of theory.⁴⁶ There, we also investigated the electronic structure of the SAC-like motif and found that the formation of the SAC motif does not cause any change in the metallic character of functionalized MXenes.⁴⁶

Besides electronic energies, E_DFT, we determine the vibrational frequencies of adsorbate species on the MXene surface in the harmonic approximations, by means of DFT calculations, building and diagonalizing the corresponding block of the Hessian matrix, with elements computed as finite difference of analytical gradients. This allows for the calculation of the zero-point energy and entropy of the reaction intermediates. While the entropic contribution consists of the sum of translational, rotational, and vibrational contributions, for adsorbate species we only use the vibrational frequencies to determine an entropic correction. The equations to calculate zero-point energy (ZPE) and vibrational entropy (S) based on the vibrational frequencies are as follows:

In the above equation, k_B, h, v_i, n, and T denote the Boltzmann constant, Planck's constant, frequency of vibration, total number of frequencies, and temperature in Kelvin. Zero-point energy and entropic corrections are needed to derive free energies according to the following relation:

G = E_DFT + E_ZPE − TS (3)

In the remainder of this article, we discuss free-energy changes, ΔG, for the formation of adsorbate species on the MXene-SAC motif. Note that the free-energy changes obtained from the free energies of eqn (3) refer to U = 0 V vs. SHE (standard hydrogen electrode) and pH = 0, which is denoted as ΔG(0). We will make use of this nomenclature in section 4 of the ESI† when we introduce the computational hydrogen electrode (CHE) approach⁴⁷ to determine potential-dependent free-energy changes to describe the elementary steps of the CER and OER.

A possible source of error in the chosen PBE + D3 level of theory is the existence of localized d-electrons, which GGA functionals tend to excessively delocalize. This can be avoided by making use of hybrid functionals including a fraction of nonlocal Fock exchange such as PBE0 or HSE06, or through the addition of the somehow empirical onsite two-electron repulsion term U leading to the basis of the PBE + U approach. However, one must advert that the choice of the contribution of Fock exchange in hybrid functionals, and also the range separation parameter in HSE06, or the value for the U parameter in PBE + U remain open issues as discussed elsewhere.⁴⁸ Considering that the main goal of this study is to discuss activity and selectivity trends for single-atom centers of MXenes based on the calculation of adsorption free energies, one can safely compare the results obtained using the PBE + D3 level of theory as the main interest refers to free-energy differences rather than absolute values. A discussion of the electronic structure of the SAC-like site of MXenes is provided in section 1 of the ESI.†

Results and discussion

In a previous work, we found that the electrochemically formed SAC-like site of MXenes — the one-branch MXene-SAC motif (cf.Fig. 1a) — is able to catalyze the OER and CER with reasonable electrocatalytic activity, whereas the MXene basal plane is inactive for both processes.⁴⁶ Previous work by Pacchioni and co-workers^49,50 provides evidence that the surface chemistry of single-atom catalysts often differs from that of traditional bulk materials, as the coordination of intermediates adsorbed to the single-atom site is reminiscent to the coordination of ligands in organometallic chemistry. This can give rise to unconventional intermediates and unconventional reaction mechanisms,⁵¹ and — despite the fact that the electrochemically formed SAC-like sites of MXenes are not typical single-atom catalysts even if they show structural similarity — we witness a similar observation for the MXene-SAC motif. With sufficient anodic bias, it is possible that a second adsorbate is stabilized at the single-atom center, which we refer to as the two-branch MXene-SAC motif (cf.Fig. 1b). This finding is also confirmed by the application of ab initio molecular dynamics simulations with explicit water molecules⁴⁶ (cf. Fig. S5 in section 2 of the ESI†).

Using thermodynamic considerations in a Pourbaix-like approach,^45,52–54 we determine the stability region of the two-branch MXene-SAC motif depending on the metal atom of M₂X. While a detailed analysis can be found in section 3 of the ESI,†Fig. 2 illustrates the electrode potential at which the one- and two-branch MXene-SAC motifs are in electrochemical equilibrium. We infer that the two-branch MXene-SAC motif is energetically favored over the one-branch MXene-SAC motif at electrode potentials relevant for the CER and OER; that is, U ≥ 1.40 V vs. RHE. Therefore, we investigate the elementary steps of both anodic processes at the two-branch SAC motif and choose a target potential of U = 1.40 V vs. RHE for analysis purposes. In this context, we assume that one of these branches catalyzes the OER, whereas the other branch is responsible for the CER (cf.Fig. 3).

Fig. 2 Equilibrium potential, U_eq, of the single-branch (cf.Fig. 1a) and double-branch (cf.Fig. 1b) MXene-SAC motifs. At potentials exceeding the specified U_eq value, the double-branch MXene-SAC motif is energetically favored over the single-branched one, indicating the prevalence of the double-branch MXene-SAC motif under oxygen evolution and chlorine evolution reaction conditions. Please note that Ti₂N is the only MXene among the investigated materials with U_eq > 1.23 V and therefore the data point for Ti₂N is not shown in this plot.

Fig. 3 Schematic representation of the oxygen evolution reaction (OER) over the double-branch MXene-SAC motif via two different pathways: (a) mononuclear mechanism, (b) Walden-type mechanism. The competing chlorine evolution reaction is investigated in each step of the OER at the second branch of the SAC site.

OER is a four proton-coupled electron transfer process, in which different adsorbates, including the *OH, *O, and *OOH intermediates are formed. Similar to single-atom catalysts, the MXene-SAC motif facilitates the stabilization of unconventional OER intermediates,⁵⁵ including η₁–*OO(H) or η₂–*OO(H), which can become part of the catalytic cycle.⁴⁶ While the η₂–*OO(H) intermediate is energetically favored in the case of the one-branch MXene-SAC motif,⁴⁶ we observe that only the η₁–*OO(H) adsorbate is formed in the case of the two-branch MXene-SAC motif, which we attribute to steric hindrance between the intermediates at the SAC-like site. Therefore, we assess the elementary steps of the OER by the traditional mononuclear mechanism (cf.eqn (1)–(4)) or by a Walden-type description^56–58 including the η₁–*OO(H) intermediate (cf.eqn (5)–(8)):

M_SAC–*O + H₂O → M_SAC–*O–*OH + (H⁺ + e^–), ΔG_1a (4)

M_SAC–*O–*OH → M_SAC–*O–*O + (H⁺ + e⁻), ΔG_2a (5)

M_SAC–*O–*O + H₂O → M_SAC–*O–*OOH + (H⁺ + e⁻), ΔG_3a (6)

M_SAC–*O–*OOH → M_SAC–*O + (H⁺ + e⁻) + O₂, ΔG_4a (7)

M_SAC–*O–*OH → M_SAC–*O–*O + (H⁺ + e⁻), ΔG_1b (8)

M_SAC–*O–*O + H₂O → M_SAC–*O–*OOH + (H⁺ + e⁻), ΔG_2b (9)

M_SAC–*O–*OOH → M_SAC–*O–*OO + (H⁺ + e⁻), ΔG_3b (10)

M_SAC–*O–*OO + H₂O → M_SAC–*O–*OH + (H⁺ + e⁻) + O₂, ΔG_4b (11)

We note that during the formation of the different intermediate species in the OER, the CER can take place on the second branch of the SAC motif. Therefore, in each elementary step of the OER in Fig. 3, the CER cycle is indicated at the other branch. In this context, we describe the CER by means of a Volmer–Heyrovsky mechanism:⁵⁹

M_SAC–*O–*X + Cl⁻ → M_SAC–*OCl–*X + e⁻, ΔG_Vol (12)

M_SAC–*OCl–*X + Cl⁻ → M_SAC–*O–*X + e⁻ + Cl₂, ΔG_Hey (13)

In eqn (9) and (10), *X denotes an arbitrary OER intermediate. The free-energy changes of eqn (1)–(10) are calculated using the computational hydrogen electrode approach, and we refer to section 4 of the ESI† for details (cf. Tables S1 and S2†). Knowledge of the OER and CER free-energy changes enables determination of the activity descriptor G_max(U) (cf. Table S3† in section 4 of the ESI†),^60,61 which is a potential-dependent measure for the electrocatalytic activity based on the energetic span model.⁶² A definition of this descriptor for the competing reaction channels of the OER — G_max^OER(U) — and CER — G_max^CER(U) — is provided in section 4 of the ESI,† and the energetics of the different mechanisms of eqn (1)–(10) is discussed at U = 1.40 V vs. RHE in section 5 of the ESI† (cf. Fig. S6–S17†).

Since the OER can proceed via different reaction mechanisms, we first determine the energetically preferred mechanistic description. This is achieved by comparing the G_max^OER(U = 1.40 V) values for the mononuclear (cf.eqn (1)–(4)) and Walden-type (cf.eqn (5)–(8)) mechanisms for the twelve different M₂X-SAC structures, as shown in Fig. 4. While for the one-branch MXene-SAC motif (cf.Fig. 1a) a Walden-type description is energetically favored over the conventional mononuclear mechanism,⁴⁶ a different situation is encountered with the two-branch MXene-SAC motif (cf.Fig. 1b): a few MXenes follows the traditional mononuclear mechanism, while for some materials (Ta₂C, V₂C, V₂N) both pathways can proceed, and for other materials (Ti₂C, Ti₂N, Zr₂N) the Walden pathway is preferred due to a lower G_max^OER(U = 1.40 V) value. This finding suggests that the presence of the second branch modulates the surface chemistry of the SAC site, and this might also have implications for CER activity, evaluated by determining G_max^CER(U) based on to eqn (9) and (10), and CER selectivity, which is discussed next.

Fig. 4 Comparison of G_max^OER(U) values for the mononuclear and Walden mechanisms (cf.Fig. 3) of twelve double-branch MXene-SAC motifs at U = 1.40 V vs. RHE to identify the preferred OER mechanism.

Knowledge of the activity descriptors G_max^OER(U = 1.40 V) and G_max^CER(U = 1.40 V) for the M₂X-SAC structures allows the determination of the CER selectivity following previous works on this topic:⁶³

G_sel(U) = G_max^OER(U) − G_max^CER(U) (14)

The results are summarized in Fig. 5, where we provide an activity-selectivity map for the competing CER and OER over two-branch MXene-SAC at U = 1.40 V vs. RHE. Considering that we calculated the energetics for twelve different MXene-SAC motifs and that there are four different possibilities for the CER for each structure (cf.Fig. 3), we arrive at a total of 48 different surface states, which are grouped according to their activity and selectivity in the CER. Relating to selectivity, we distinguish between highly selective (CER selectivity = 1) and non-selective (CER selectivity = 0) motifs, while for CER activity we use G_max^CER(U = 1.40 V) < 0.50 eV or G_max^CER(U = 1.40 V) ≥ 0.50 eV as a threshold criterion to identify active and inactive surface states, respectively. Note that the selection of these criteria follows previous works on the same topic.⁴⁶

Fig. 5 Activity-selectivity map for the competing chlorine evolution (CER) and oxygen evolution (OER) reactions over two-branch MXene-SAC at U = 1.40 V vs. RHE. (a) Classification of four regions with different CER activity and selectivity. (b) Categorization of 48 different structures based on the double-branch MXene-SAC motif (cf.Fig. 1b) according to their CER activity and selectivity.

Fig. 5 shows that about 48% (23 out of 48) of all surface states considered are located in the most relevant region with high CER selectivity and high activity (region 1). It becomes clear that in particular the presence of the non-conventional *OO adsorbate on the OER branch enables selective CER, which underlines the similarity of the MXene-SAC motif to archetypal single-atom catalysts in terms of their chemical reactivity. Almost 30% (14 out of 48) of all surface states considered are found in region 2, indicating highly selective CER but reduced CER activity. In particular, the *OH and *OOH adsorbates on the OER branch facilitate selective CER (motif in region 1 or region 2), although the presence of *OH and *OOH is often detrimental to high CER activity (motif in region 2). This is in contrast to the *O and *OO adsorbates, which lead to high CER activity (motif in region 1). On the other hand, the presence of the *O and *OO adsorbates at the OER branch can also result in low CER activity and selectivity (motif in region 4) compared to the *OH and *OOH intermediates. Overall, the selectivity is only in favor of OER over CER in about 23% (11 of 48) of all surface states considered. While the above analysis refers to U = 1.40 V vs. RHE, we refer to section 5 of the SI for a potential-dependent analysis of the CER selectivity (cf. Fig. S18†). There, we demonstrate that the general trends discussed for U = 1.40 V vs. RHE (cf.Fig. 5) are not affected in the potential regime where the MXene-SAC is reported to be stable.²⁶

Regarding the metal atom in the twelve different MXene-SAC motifs, we emphasize that there are seven MXenes — V₂C, Nb₂C, Ta₂C, V₂N, Zr₂N, Nb₂N, and Ta₂N — that maintain high CER selectivity throughout the catalytic cycle regardless of the adsorbate on the OER branch. On the other hand, for the other six MXenes — Ti₂C, Hf₂C, Ti₂N, Hf₂N, V₂N, and Mo₂N — we observe that some of the surface states are selective for the CER, while others favor the OER. This limits the application of the electrochemically formed SAC-like sites of the latter MXenes for selective CER, while especially group V-based MXenes (V₂X, Nb₂X, and Ta₂X) appear as promising candidates for experimental testing.

Finally, we comment on the structural properties of the double-branch MXene-SAC motif (cf.Fig. 1b) with regard to previous studies in the literature. As shown by previous work, it is important to consider the stacking and oxygen coverage of MXenes to properly describe thermal catalytic and electrocatalytic processes.⁶⁴ Single-atom catalysis on the oxygen-covered surface of MXenes has been largely realized by the doping with foreign metal atoms.^65,66 While single atoms on the surface of MXenes have shown to be a realistic description for thermal catalytic processes at the solid/gas interface,⁶⁷ a different situation is encountered in electrocatalysis, where the solid/liquid interface causes reconstruction of the oxygen-covered surface of MXenes under formation of the MXene-SAC motif (cf.Fig. 1). While the use of simplified SAC models based on doping with foreign metal atoms is still widely used in the theoretical description of electrocatalytic processes,^14,15 these models are likely not tenable under the harsh anodic conditions of CER and OER due to the reconstruction of the MXene surface. Therefore, the reported MXene-SAC motif not only refrains from rare noble metal atoms to enable efficient and selective catalysis similar to the actual functioning of SAC, but also is a better representation of MXenes in an electrochemical environment.

Conclusions

In summary, we have provided trends in the competing CER and OER at two-branch MXene-SAC sites by using electronic structure theory calculation in the DFT framework coupled with a descriptor-based analysis. Although previous works have outlined the application of MXenes in the form of composite catalysts with transition-metal oxides for selective CER or seawater splitting,^68–70 we demonstrate herein that in situ formed SAC sites of group V-based MXenes (V₂X, Nb₂X, and Ta₂X) are potential candidates for CER. While single-atom catalysts based on traditional synthesis routes are considered a game changer for selective chlorine evolution in slightly acidic media,^71–74 we propose the application of electrochemically formed SAC-like sites based on low-cost two-dimensional materials such as MXenes. This could help to overcome the dependence on rare precious metals, such as Pt and Ir in SAC catalysts or Ru and Ir in conventional heterogeneous catalysts, for energy conversion processes relevant to the chemical industry.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. F., S. R., D. S., and K. S. E. thank the Ministry of Culture and Science of the Federal State of North Rhine-Westphalia (NRW Return Grant) for financial support to carry out this study. K. S. E. acknowledges funding by the RESOLV Cluster of Excellence, funded by the Deutsche Forschungsgemeinschaft under Germany's Excellence Strategy, EXC 2033-390677874, RE-SOLV. F. V. and F. I. acknowledge the Spanish Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (AEI) MCIN/AEI/10.13039/501100011033 and, as appropriate, by “European Union Next Generation EU/PRTR”, through grant PID2021-126076NB-I00, la Unidad de Excelencia María de Maeztu CEX2021-001202-M granted to the ITQCUB and, in part, from COST Action CA18234 and Generalitat de Catalunya 2021SGR00079 grant. F. V. is thankful for the ICREA Academia Award 2023 ref. Ac2216561. L.M. thanks the China Scholarship Council (CSC) for financing her PhD (CSC202108390032).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02220g

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