Single-atom catalysts have emerged as a transformative approach in the hydrogen evolution reaction due to their exceptional atomic efficiency and tunable electronic structures. Unlike bulk or nanoparticle catalysts, SACs maximize the utilization of each active site, offering near-100% atomic dispersion. This characteristic is particularly advantageous for HER, where the reduction of protons to hydrogen gas demands precise control over adsorption energetics. The electronic properties of SACs are governed by the coordination environment of the isolated metal atom, which can be systematically adjusted to optimize hydrogen binding energy, a critical descriptor of HER activity.

The synthesis of SACs requires precise control to prevent aggregation and ensure uniform anchoring of single metal atoms on suitable supports. Common methods include atomic layer deposition, wet-impregnation followed by high-temperature stabilization, and pyrolysis of metal-organic frameworks. For instance, platinum single atoms anchored on nitrogen-doped graphene exhibit a coordination environment of Pt-N4, which significantly alters the d-band center compared to bulk Pt, leading to enhanced HER kinetics. The turnover frequency for such SACs can exceed 10,000 h−1 under acidic conditions, rivaling conventional Pt/C catalysts while using a fraction of the precious metal.

Support materials play a pivotal role in stabilizing SACs and modulating their electronic properties. Graphene derivatives, particularly those doped with heteroatoms like nitrogen or sulfur, provide strong anchoring sites and facilitate charge transfer. Recent advances have demonstrated that MXenes, a class of two-dimensional transition metal carbides and nitrides, offer superior electrical conductivity and abundant surface functional groups for stabilizing single atoms. For example, Mo2TiC2Tx MXene-supported single-atom cobalt catalysts exhibit HER overpotentials as low as 80 mV at 10 mA cm−2 in alkaline media, outperforming many nanoparticle-based alternatives.

Another breakthrough involves the use of defective carbon substrates, where vacancies or edge sites act as traps for single metal atoms. These defects create localized electronic states that can fine-tune the interaction between the metal center and hydrogen intermediates. Single-atom nickel embedded in defective graphene has shown remarkable HER activity due to the optimized Gibbs free energy of hydrogen adsorption, achieving a turnover frequency of 3,200 h−1 in neutral pH electrolytes.

The stability of SACs under operational conditions remains a key challenge, particularly in acidic or high-current-density environments. Strategies such as encapsulating single atoms within graphitic layers or employing covalent bonding to robust supports have improved durability. For instance, single-atom iridium on tungsten oxide supports retains over 90% of its initial activity after 50 hours of continuous HER operation, attributed to the strong Ir-O-W interfacial bonding.

Recent studies have also explored non-precious metal SACs to reduce costs while maintaining performance. Iron and cobalt single atoms coordinated with nitrogen and carbon have demonstrated competitive HER activity, with overpotentials below 150 mV in alkaline solutions. These systems benefit from the synergistic effects between the metal center and the surrounding ligands, which facilitate proton adsorption and recombination.

The mechanistic understanding of SACs in HER has advanced significantly through in-situ spectroscopic techniques and density functional theory calculations. Operando X-ray absorption spectroscopy reveals dynamic changes in the oxidation state and coordination geometry of single atoms during catalysis, providing insights into the active sites' nature. For example, single-atom ruthenium on carbon nitride undergoes reversible Ru-N bond elongation under applied potential, directly correlating with enhanced HER activity.

Future directions in SAC development for HER include the exploration of dual-atom catalysts, where paired metal atoms may offer cooperative effects, and the integration of SACs with advanced electrolytes to further reduce overpotentials. The scalability of synthesis methods and the long-term stability under industrial conditions remain critical areas for research.

In summary, single-atom catalysts represent a paradigm shift in HER electrocatalysis, combining maximal atomic efficiency with precisely engineered electronic structures. Advances in support materials, such as graphene, MXenes, and defective carbons, have enabled unprecedented control over catalytic performance. While challenges in stability and scalability persist, the continued refinement of synthesis techniques and mechanistic understanding positions SACs as a cornerstone of next-generation hydrogen production technologies.