This review has examined the advancements and challenges in the development of transition metal-based electrocatalysts for the alkaline water splitting reaction in the last decade.

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.

Green hydrogen production via electrocatalytic water splitting paves the way for renewable, clean, and sustainable hydrogen (H₂) generation. H₂ gas is produced from the cathodic hydrogen evolution reaction (HER), where the reaction is catalyzed primarily from Pt-based catalysts under both acidic and alkaline environments. Lowering the loading of Pt and the search for alternative active catalysts for HER is still an ongoing challenge. Two-dimensional MXenes are effective supports to stabilize and homogenously distribute HER-active electrocatalysts to boost the HER performance. Factors involved in the effectiveness of MXenes for their role in HER include transition metal types and termination groups. Recently, tailoring the conditions during the synthesis of MXenes has made it possible to tune the morphology of MXenes from multilayers to few layers (delaminated), formation of porous MXenes, and those with unique crumpled and rolled structures. Changing the morphology of MXenes alters the surface area, exposed active sites and accessibility of electrolyte materials/ions to these active sites. This review provides insight into the effects of varying morphology of MXenes towards the electrocatalytic HER activity of the MXene itself and MXene composites/hybrids with HER-active catalysts. Synthesis methods to obtain the different MXene morphologies are also summarized.