Integrated MXene and metal oxide electrocatalysts for the oxygen evolution reaction: synthesis, mechanisms, and advances

Abstract

Electrochemical water splitting is a promising approach to produce H₂ through renewable electricity, but its energy efficiency is severely constrained by the kinetically slow anodic oxygen evolution reaction (OER), which uses about 90% of the electricity in the water-splitting process due to its multistep proton (H⁺)-coupled electron (e^-) transfer process, high overpotential (η), and low energy efficiency. Therefore, the quest for efficient, sustainable, and cost-effective electrocatalysts for hydrogen production through water electrolysis has intensified, highlighting the potential of two-dimensional (2D) MXenes. MXenes have emerged as a promising class of materials characterized by excellent stability, hydrophilicity, and conductivity. However, challenges such as low oxidation resistance, facile stacking, and the absence of intrinsic catalytically active sites limit their performance. This review thoroughly explores various synthesis methods for MXenes and their integration with transition metal oxides (TMOs) to tackle the challenges and enhance catalytic activity. The review also delves into advanced strategies for structural tuning of MXenes and TMOs, such as termination engineering, heteroatom doping, defect engineering, and the formation of heterojunctions. The integration of MXenes with TMOs has addressed the current limitations of MXenes and significantly boosted OER activity. By considering these structural tuning parameters and limitation factors, researchers can gain insights into the design principles and optimization strategies for MXene- and TMO-integrated electrocatalysts. The review concludes with a summary of the key findings and an outlook on future research directions, emphasizing the unexplored potential and innovative approaches that could further advance the field of electrocatalytic water splitting.

Fig. 1. Electrochemical water splitting cell. Reproduced with permission from ref. . Copyright 2023, Elsevier.

Fig. 2. OER mechanism in (a) acidic and (b) alkaline media. Reproduced with permission from ref. . Copyright 2022, Elsevier.

Fig. 3. (a) The placement of the key components of the MAX phase on the periodic table of elements, and (b) arrangements and compositions of several MXenes, including M₂XT_x, M₃X₂T_x, and M₄X₃T_x. Reproduced with permission from ref. . Copyright 2021, Elsevier.

Fig. 4. Overview of various kinds of MXenes, their methods of production, and the corresponding timeframes. Reproduced with permission from ref. . Copyright 2023, Elsevier.

Fig. 5. (a) Diagram illustrating the procedure for the synthesis of MXene Ti₃C₂T_x using HF (to etch Ti₃AlC₂). Reproduced with permission from ref. . Copyright 2024, Elsevier. (b) The XRD patterns of rGO integrated Ti₃C₂ MXene. Reproduced with permission from ref. . Copyright 2022, Wiley. (c) Illustration depicting the molten salt shielding MXene production technique. Reproduced with permission from ref. . Copyright 2023, Wiley. The SEM graph shows the (d) MS-Ti₃C₂T_x MXene and (e) e-MS-Ti₃C₂T_x (gathered by filtering). Reproduction with permission from ref. . Copyright 2022. ACS. (f) Schematic diagram showing the step-by-step method of creating Ti₂AlC and electrochemically etching MXene (E-Ti₂CT_x). Reproduced with permission from ref. . Copyright 2022, Wiley.

Fig. 6. (a) Schematic illustration of alkali etching for Mo₂C MXene production. Reproduced with permission from ref. . Copyright 2023, SciOpen. The TEM and HR-TEM images of the Ti₃C₂T_x@Al–NaOH MXenes: (b and c) 25 M, (d) 30 M, and (e) 35 M. Reproduced with permission from ref. . Copyright 2024, Elsevier. (f and g) The XPS survey spectra of Ti₃C₂T_x@Al–NaOH, as well as the high-resolution spectra of Ti 2p. Reproduced with permission from ref. . Copyright 2024, Elsevier.

Fig. 7. (a) The process of obtaining Ti₃C₂T_x MXene from Ti₃AlC₂via the microwave-assisted hydrothermal technique. Reproduced with permission from ref. . Copyright 2022, Elsevier. (b) The XRD spectra illustrating the conversion of the Ti₃AlC₂ MAX phase to Ti₃C₂T_x (MXene). Reproduced with permission from ref. . Copyright 2022, Elsevier. (c) The BET curve and UV findings (inset) of Ti₃C₂T_x. Reproduced with permission from ref. . Copyright 2022, Elsevier. (d) Schematic diagram of ionic liquid etching of Ti₃C₂ MXene. Reproduced with permission from ref. . Copyright 2023, Elsevier.

Fig. 8. Graphic illustration for different strategies for tuning MXene-based TMOs to enhance electrocatalytic performance, including termination engineering, defect engineering, heteroatom doping, and heterojunction formation.

Fig. 9. (a) Diagram for the F-Co₃O_4−x electrocatalyst's gas-phase fluorination process. (b) SEM of the F-Co₃O_4−x electrocatalyst. (c) HR-TEM image of the F-Co₃O_4−x electrocatalyst. (d) OER polarisation curves and (e) Tafel plots for the electrocatalysts F-Co₃O_4−x, Co₃O_4−x, and Co₃O₄. (f) Schematic diagram of an OWS device. (g) OWS polarization curves of the Co₃O₄‖Co₃O₄, Co₃O_4−x‖Co₃O_4−x and F-Co₃O_4−x‖F-Co₃O_4−x and commercial RuO₂‖Pt/C couples, and (h) CP measurements of the F-Co₃O_4−x electrocatalyst at 100 mA cm⁻², with the SEM image displayed in the inset following 100 h of the OER. Reproduced with permission from ref. . Copyright 2021, Elsevier.

Fig. 10. (a) Diagrammatic representation of the LDH/H-Ti₃C₂T_x catalyst's synthesis processes, (b) field emission (FESEM) image, (c) TEM, (d) high-angle annular dark-field imaging and scanning transmission electron microscopy (HAADF-STEM), and (e) HRTEM of the LDH/H-Ti₃C₂T_x nanoarchitecture. The LDH/H-Ti₃C₂T_x nanoarchitecture's OER performance. (f) Representative LSV curves, (g) histograms of required overpotentials and (h) Tafel plots of LDH/H-Ti₃C₂T_x with varying LDH contents, H-Ti₃C₂T_x, Ti₃C₂T_x, pure LDH and RuO₂ electrodes in 1 M KOH solution. Reproduced with permission from ref. . Copyright 2023, Elsevier.

Fig. 11. (a) Schematic illustration of the fabrication of the S-NiFe₂O₄@Ti₃C₂@NF hierarchical network structure. Reproduced with permission from ref. . Copyright 2019, Elsevier. (b) Diagram illustrating the method for creating CoFe-LDH/MXene hybrids, (c) SEM image of CoFe-LDH/MXene nanohybrids, (d) LSV curves of samples, and (e) Tafel plots of samples. Reproduced with permission from ref. . Copyright 2019, Elsevier.

Muhammad Nazim Lakhan

Abdul Hanan

Yuan Wang

Hiang Kwee Lee

Hamidreza Arandiyan