MXene-Supported Single-Atom Electrocatalysts

Abstract

MXenes, a novel member of the 2D material family, shows promising potential in stabilizing isolated atoms and maximizing the atom utilization efficiency for catalytic applications. This review focuses on the role of MXenes as support for single-atom catalysts (SACs) for various electrochemical reactions, namely the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR), and nitrogen reduction reaction (NRR). First, state-of-the-art characterization and synthesis methods of MXenes and MXene-supported SACs are discussed, highlighting how the unique structure and tunable functional groups enhance the catalytic performance of pristine MXenes and contribute to stabilizing SAs. Then, recent studies of MXene-supported SACs in different electrocatalytic areas are examined, including experimental and theoretical studies. Finally, this review discusses the challenges and outlook of the utilization of MXene-supported SACs in the field of electrocatalysis.

Keywords: MXenes; catalytic applications; single‐atom catalysts; support; synthesis and characterization.

Figure 1

The periodic table shows elements involved in the formation of MAX phases and MXenes in previous publications. Light blue: M atoms; gray: A atoms; dark green: X atoms; purple: (T) elements; solid circles: SAs discussed in the present work with experiments; dotted circles: SAs discussed in the present work with theoretical calculations.

Figure 2

A) Schematic of the exfoliation process for Ti₃AlC₂ with HF. Reproduced with permission.^{[ 53 ]} Copyright 2011, Wiley‐VCH. B) The reaction between Ti₃AlC₂ and aqueous NaOH solution under different conditions. Reproduced with permission.^{[ 60 ]} Copyright 2018, Wiley‐VCH. C) Schematic of the etching process for Ti₃AlC₂ with molten salt. Reproduced with permission.^{[ 61 ]} Copyright 2019, American Chemical Society. D) Schematic illustration of the etching of Ti₂AlC toward vacancy‐enriched Ti₂CCl_x MXene. Reproduced with permission.^{[ 62 ]} Copyright 2024, Wiley‐VCH. E) Schematic diagram of the synthesis by bottom‐up method. Reproduced with permission.^{[ 65 ]} Copyright 2023, The American Association for the Advancement of Science.

Figure 3

A) Three methods for anchoring single metal atoms on MXenes; B) The advantages of MXenes to support SAs.

Figure 4

A) HAADF–STEM image of Mo₂TiC₂T_x–Pt_SA. B) Magnified HAADF–STEM image of Mo₂TiC₂T_x–Pt_SA and its corresponding simulated image, and illustration of the structure of Mo₂TiC₂T_x–Pt_SA, showing the isolated Pt atoms (circled). C) STEM–EDS elemental mapping of Mo₂TiC₂T_x–Pt_SA nanosheets. Reproduced with permission.^{[ 79 ]} Copyright 2018, Springer Nature. D) Atomic‐resolution HAADF‐STEM image of Ti₂CCl_x MXene. E) Line intensity profiles obtained from the rectangular regions in (D), showed the existence of Ti vacancies. F) Atomic‐resolution HAADF‐STEM image of Zn–Ti₂CCl_x. G) Line intensity profiles obtained from the rectangular regions in (F), showed the existence of single‐atom Zn. H) Atomic‐resolution HAADF‐STEM image, and I) the experimental and simulated HAADF‐STEM images of Ti₂CClx–Pt_SA. Reproduced with permission.^{[ 62 ]} Copyright 2024, Wiley‐VCH. J) HAADF‐STEM image of Ni_SA@N‐Ti₃C₂T_x. K) Corresponding EDS analysis. L) and M) Atomically resolved HAADF‐STEM images. N) 3D atom‐overlapping Gaussian‐function fitting mapping for the g area in (L) illustrating the atomic structure and arrangement of Ni_SA@N‐Ti₃C₂T_x. O) Corresponding profiles of the h area in (M). Reproduced with permission.^{[ 87 ]} Copyright 2024, The Royal Society of Chemistry.

Figure 5

A) Pt L₃‐edge XANES spectra, B) Fourier transforms of Pt L₃, and C) WT Pt L₃‐edge EXAFS spectra for Pt foil, PtO₂, and Ti₂CClx–Pt_SA. Reproduced with permission.^{[ 62 ]} copyright 2024, Wiley‐VCH. D) Pd K‐edge EXAFS spectra of Pd foil, PdO, and 0.5% Pd₁‐Ti₃C₂T_x. E) EXAFS fitting curves for 0.5% Pd₁‐Ti₃C₂T_x. Reproduced with permission.^{[ 85 ]} Copyright 2024, Elsevier. The experimental Ni K‐edge F) XANES and G) EXAFS spectra of Ni SA@N‐Ti₃C₂T_x and counterparts. Reproduced with permission.^{[ 87 ]} Copyright 2024, The Royal Society of Chemistry. H) Normalized operando Ru K‐edge XANES spectra for SA Ru‐Mo₂CT_X under various conditions (applied voltage versus RHE) in 0.5 M K₂SO₄ solution, insert is the magnified image. I) The corresponding FT‐EXAFS spectra derived from (H). J) The oxidation state of Ru and radial distance of the main peak under various conditions. Reproduced with permission.^{[ 92 ]} Copyright 2020, Wiley‐VCH.

Figure 6

A) XPS survey spectra of Ti₂AlC, MS‐Ti₂CCl_x, and Ti₂CCl_x. High‐resolution B) Al 2p, C) Zn 2p, and D) Ti 2p XPS spectra of Ti₂AlC, MS‐Ti₂CCl_x, and Ti₂CCl_x. Reproduced with permission.^{[ 62 ]} Copyright 2024, Wiley‐VCH. E) XPS survey spectra of Cu_SA@N‐Ti₃C₂T_x and Cu_SA@Ti₃C₂T_x, F) Ti 2p spectra, G) C 1s spectra, and H) Cu 2p spectra. Reproduced with permission.^{[ 94 ]} Copyright 2024, Springer Nature.

Figure 7

Raman spectra of A) Mo₂TiC₂T_x‐Pt_SA, B) delaminated Mo₂TiC₂T_x, C) Mo₂TiAlC₂ and Mo₂TiC₂T_x. Reproduced with permission.^{[ 79 ]} Copyright 2018, Springer Nature. D,E) In situ Spectro‐electrochemical Raman spectra of Ti₃C₂T_x. F,G) In situ Raman spectra of Ru_SA@Ti₃C₂T_x. H,I) Schematic diagram of Ru_SA@Ti₃C₂T_x surface under operando HER conditions. Reproduced under the terms of the CC‐BY 4.0 license.^{[ 99 ]} Copyright 2022, Wiley‐ VCH.

Figure 8

A) DFT‐calculated hydrogen adsorption Gibbs free energy of Ti₂CCl_x–Pt_SA and 10% Pt/C for HER. B) DOS of Ti₂CCl_x and Ti₂CCl_x–Pt_SA. Reproduced with permission.^{[ 62 ]} Copyright 2024, Wiley‐VCH. C) The charge density differences of TMs anchored onto NS/NN‐Ti₃C₂O₂ supports. The charge depletion and accumulation are depicted by cyan and yellow, respectively. The isosurface value is 0.005 e/A3. The reaction mechanisms of ECO₂RR to HCOOH under 0 V versus SHE and operando conditions on D) Cr‐NS‐Ti₃C₂O₂, E) Ti‐NN‐Ti₃C₂O₂, F) V‐NN‐Ti₃C₂O₂, and G) Fe‐NN‐Ti₃C₂O₂. Reproduced with permission.^{[ 101 ]} Copyright 2024, Elsevier Ltd.

Figure 9

A) The calculated free‐energy diagrams for H adsorption on M−v−V₂CT₂ SACs. Reproduced with permission.^{[ 113 ]} Copyright 2023, Elsevier Inc. B) The variation in ΔG (H) with the Group number of substitutional impurities in the Nb₄C₃O₂ monolayer. The points corresponding to Nb (representing the pristine Nb₄C₃O₂) and impurities giving rise to the eV are labeled. Reproduced with permission.^{[ 114 ]} Copyright 2024, Springer Nature. C)The calculated ΔGH at the S1 and S2 sites on NM‐Mo₂CO₂ structures. Reproduced with permission.^{[ 115 ]} Copyright 2024, Elsevier B.V. D) Illustration of the synthesis mechanism for Mo₂TiC₂O₂–Pt_SA during the HER process. E) HER polarization curves of carbon paper (CP), Mo₂TiC₂T_x, Mo₂TiC₂T_x–V_Mo, Mo₂TiC₂T_x–Pt_SA and Pt/C (40%), acquired using graphite rod as the counter electrode in 0.5 M H₂SO₄ solution. F) Stability test of Mo₂TiC₂T_x–Pt_SA through potential cycling, before and after 10000 cycles. Inset: chronoamperometry curve of Mo₂TiC₂T_x–Pt_SA and Pt/C. Reproduced with permission.^{[ 79 ]} Copyright 2018, Springer Nature. G) HER polarization curves of Ti₂CT_x, Ti₂CClx, Ti₂CCl_x–Pt_SA, and 10% Pt/C collected in 0.5 M H₂SO₄ electrolyte. H) Tafel plots of Ti₂CT_x, Ti₂CClx, Ti₂CCl_x–Pt_SA, and 10% Pt/C. I) The stability of Ti₂CCl_x–Pt_SA. Reproduced with permission.^{[ 62 ]} Copyright 2024, Wiley‐VCH. J) Schematic illustration of the fabrication of the Co@MXene composites on an example of V₂CT_x; K) HER LSV curves of Co@V₂CT_x, Co@Nb₂CT_x, Co@Ti₃C₂T_x and Pt/C electrodes at 5 mV s⁻¹; L) HER chronopotentiometry response of three listed electrodes at a current density of 10 mA cm⁻². Reproduced under the terms of the CC‐BY 4.0 license.^{[ 117 ]} Copyright 2022, Wiley‐VCH GmbH.

Figure 10

Free energy diagrams of A) Fe‐H₁‐V₂CO₂, B) Co‐H₁‐V₂CO₂, C) Ni‐H₁‐V₂CO₂, D) Co‐V_O‐Nb₂CO₂, E) Ni‐H₂‐Nb₂CO₂, and F) Co‐H₂‐Ta₂CO₂. The steps marked in purple and yellow represent the PDS of the ORR and OER, respectively. Reproduced with permission.^{[ 124 ]} Copyright 2024, Elsevier Inc. G) Schematic illustration of anchoring l‐tryptophan at the surface of Ti₃C₂T_x MXene, followed by fabrication of dual‐atom CoNi‐Ti₃C₂T_x composites. H) OER LSV curves of CoNi‐Ti₃C₂T_x, Ni‐Ti₃C₂T_x, Co‐Ti₃C₂T_x, and RuO₂ electrodes at 5 mV s⁻¹. I) Tafel slopes of CoNi‐Ti₃C₂T_x, Ni‐Ti₃C₂T_x, and Co‐Ti₃C₂T_x electrodes. J) C_dl values of CoNi‐Ti₃C₂T_x, Ni‐Ti₃C₂T_x and Co‐Ti₃C₂T_x. K) OER chronopotentiometry response of CoNi‐Ti₃C₂T_x electrodes at current densities of 10 and 500 mA cm⁻². Reproduced with permission.^{[ 118 ]} Copyright 2024, American Chemical Society.

Figure 11

A) The computed formation energy and dissolution potential of TMs in TM‐NS/NN‐Ti₃C₂O₂. B) Schematic illustration of the reaction mechanism of ECO₂RR toward HCOOH on TM‐NS/NN‐Ti₃C₂O₂. C) Theoretical limiting potential U_L of ECO₂RR toward HCOOH on TM‐NS/NN‐Ti₃C₂O₂. D) Adsorption configurations of *OCHO intermediate and charge density difference on Ti‐NN‐Ti₃C₂O₂ and V‐NN‐Ti₃C₂O₂. The isosurface values are 3 × 10⁻³ e Bohr⁻³. Yellow and cyan regions represent increasing and decreasing electron densities, respectively. E) Projected density of states of catalysts and *OCHO adsorption on catalysts of Cr‐NS‐Ti₃C₂O₂ and Fe‐NN‐Ti₃C₂O₂. Reproduced with permission.^{[ 101 ]} Copyright 2024, Elsevier Ltd. F) HAADF‐STEM image of Cu_SA@N‐Ti₃C₂T_x. The experimental Cu K‐edge G) XANES and H) EXAFS spectra of Cu_SA@N‐Ti₃C₂T_x and counterparts. I) LSV curves of Cu_SA@N‐Ti₃C₂T_x, performed in CO₂‐saturated 0.5 M KHCO₃. J) Potential‐dependent FEs of H₂ and CO for Cu_SA@N‐Ti₃C₂T_x, at different potentials. K) The CO FEs of Cu_SA@N‐Ti₃C₂T_x and Cu_SA@Ti₃C₂T_x at different potentials. L) Jco of Cu_SA@N‐Ti₃C₂T_x and Cu_SA@Ti₃C₂T_x. Reproduced with permission.^{[ 94 ]} Copyright 2024, Springer Nature.

Figure 12

A) Top and side views of the atomic structure of TM/O_v‐MXene. Atom labels: C (white), Ti (blue), O (red), and TM (purple). The screened TM atoms (from Ti to Au) are listed. B) Summary of limiting potentials on TM/O_v‐MXene for NO₃RR. C) Calculated formation energy of TM/O_v‐MXene (TM = 3d to 5d transition metals). Reproduced with permission.^{[ 132 ]} Copyright 2023, Wiley‐VCH. D) Potential‐dependent Faradaic efficiency, NO₃ ⁻ removal, and NH₃ selectivity for Fe_SA/MXene filter. E) Comparison of the highest Faradaic efficiency and NH₃ selectivity for the Fe_SA/MXene filter at different NO₃ ⁻ concentrations. F) The NH₃ yield rate of a Fe_SA/MXene filter in 0.1 M Na₂SO₄ electrolyte with different concentrations of NO₃ ⁻. G) Gibbs free energy diagrams of nitrate reduction to NH₃ and H₂ evolution reaction (the top right) over Fe_SA/MXene and Fe_NP/MXene. Reproduced with permission.^{[ 135 ]} Copyright 2023, American Chemical Society.