Introducing complementary active materials in tandem as heterostructures of various shapes and sizes can significantly improve the physicochemical properties of the developed materials. Recently, the use of amorphous materials in conjunction with crystalline moieties as highly efficient electrocatalysts for water-splitting electrolyzers has surged rapidly. Excellent bifunctional activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can be achieved from amorphous–crystalline-based materials owing to their remarkably conductive crystalline phase as well as their long-range disordered amorphous phase. In addition, the high specific surface area, disordered structure, and enhanced structural stability can accommodate various defects and strains that develop during in situ electrochemical reactions. Consequently, amorphous materials can be used as both surface-confined and volume-confined electrocatalysts. Although crystalline materials have better electronic properties as well as higher thermal and mechanical stability, they suffer from limited activity due to only surface-confined electrocatalysis. In this regard, the judicious integration of crystalline and amorphous active materials suitable for the OER and HER can tremendously enhance their bifunctional electrocatalytic activities owing to their synergistic effects. The variety in the choice of amorphous and crystalline counterparts presents ample opportunities for further exploration of the development of amorphous–crystalline heterostructured materials with unique properties. However, considering the recent massive developments in the understanding and protocols for fabricating such heterostructures, a critical review is vital for further advancement in this direction. Although some review papers have focused only on amorphous materials, including their synthesis methods, properties, and applications, no critical review that provides an overview emphasizing amorphous–crystalline heterostructures for electrocatalytic water-splitting applications has been published. In this regard, this review aims to present comprehensive details on the recent developments in the fabrication of different amorphous–crystalline heterostructures, the choice of the composition of each component, and the resulting physicochemical properties for the OER, HER, and overall water-splitting. It provides an in-depth understanding of amorphous–crystalline material synthesis protocols, their selection rationale, and their targeted physicochemical properties. This review also provides thought-provoking ideas and long-term perspectives for future research.

The interface engineering includes intricate procedures for the development of heterostructures and heterojunctions, modifying the composition at the interface, and optimizing the interfacial area to enhance H₂ catalytic performance.

Alkaline water electrolysis is an efficient technical pathway for producing high‐purity green hydrogen (H₂). However, rational design and fabrication of efficient electrocatalysts are essential for energy conversion. Herein, MoO₂ nanoclusters on N/Mo dual‐doped ReS₂ nanosheets (MoO₂@N/Mo–ReS₂) develops through a hydrothermal and CVD‐nitridation process. This novel strategy leads to modifying the electronic properties of metastable ReS₂ through metal/nonmetal doping, heterostructure formation, and basal plane activation, thus increasing the number of electrochemically active sites. The MoO₂@N/Mo–ReS₂ catalyst is effective at hydrogen‐adsorption, has a low energy barrier for water dissociation, and exhibits high electrical conductivity, as demonstrated by density functional theory (DFT) studies. The optimal MoO₂@N/Mo–ReS₂ heterostructure shows exceptional endurance at low overpotentials of −93 and 249 mV, respectively, and catalytic activity for the evolution of both H₂ and oxygen (O₂) at a current density of 10 mA cm⁻² in an alkaline electrolyte. The performance of the MoO₂@N/Mo–ReS₂ electrolyzer is 1.54 V at 10 mA cm⁻², which is comparable to a commercial Pt/C||RuO₂ (1.56 V at 10 mA cm⁻²) electrocatalyst. This study offers a promising strategy for the development of scalable and efficient electrocatalysts, aiming to enhance their suitability for energy applications.

Introducing secondary heteroatoms and simultaneous in situ surface modification can enhance electrocatalysts by affecting their porosity for adjusting electrochemically active surface area (ECSA), number of active sites, and electronic properties, thus mitigating the sluggish kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. Here, mesoporous 3D heterostructures of boron‐doped cobalt phosphide@nitrogen‐doped carbon nanosheet network arrays are successfully grown on Ni foam as free‐standing bifunctional electrocatalysts with controlled phosphorous levels (B‐CoP _x @NC/NF, x = 0.25, 0.5, and 1). Boron doping induces the Co active sites to bind O* and OOH* intermediates. Meanwhile, an optimal phosphorous content also leads to ideal adsorption strength at each reaction step, satisfying the Sabatier principle well. The optimal B‐CoP _0.5 @NC/NF requires low overpotentials of 248 mV for OER and 95 mV for HER with long‐term stability. The B‐CoP _0.5 @NC/NF (+/−) electrolyzer exhibits a low cell potential of 1.59 V at 10 mA cm ⁻² for overall water‐splitting, with superior activity compared to the RuO ₂ /NF(+)//20%Pt/NF(−) electrolyzer at high current densities above 50 mA cm ⁻² . Such exceptional bifunctional activities are attributed to the modulated electronic structure, lower charge‐transfer resistance, higher ECSA, and inductive effect of B‐doping, thus boosting both OER and HER activities in alkaline media.