The scarcity of platinum (Pt) and its high cost relative to its excellent electrocatalytic activity necessitate atomic-level engineering to maximize performance while minimizing Pt usage. This research focuses on addressing this challenge by designing and synthesizing ultrasmall, atomically precise Pt nanoclusters. The hydrogen oxidation reaction (HOR) is a crucial process in fuel cells, and efficient, durable, and CO-tolerant HOR catalysts are essential for their widespread adoption. Current Pt-based catalysts often suffer from low mass activity, poor stability, and susceptibility to CO poisoning. This study aims to overcome these limitations by utilizing atomically precise Pt6 nanoclusters stabilized by triphenylphosphine ligands. The precise control over the size and structure of these nanoclusters is expected to optimize their electronic properties and catalytic performance, leading to improved HOR activity, stability, and CO tolerance compared to conventional Pt/C catalysts and other Pt-based materials like single atoms and larger nanoparticles. The investigation involves experimental synthesis and characterization, electrochemical testing to evaluate HOR performance, and density functional theory (DFT) calculations to understand the underlying mechanisms.

The literature extensively documents the need for improved Pt-based electrocatalysts for the hydrogen oxidation reaction (HOR), particularly in alkaline media. Numerous studies have explored different strategies to enhance HOR performance, including alloying Pt with other metals, supporting Pt nanoparticles on various materials, and designing single-atom catalysts. However, challenges remain in achieving high mass activity, long-term stability, and CO tolerance. Recent advancements in the synthesis of atomically precise metal nanoclusters offer a promising avenue for developing highly efficient electrocatalysts. These clusters possess well-defined structures and compositions, allowing for precise control over their electronic and catalytic properties. Studies on other metal nanoclusters have demonstrated their superior performance compared to their larger nanoparticle counterparts. This study builds upon this foundation by investigating the potential of atomically precise Pt6 nanoclusters for HOR catalysis.

The study employed a multi-faceted methodology combining synthesis, characterization, electrochemical measurements, and theoretical calculations. **Synthesis:** Pt6 nanoclusters (Pt6NCs) were synthesized using a solution-phase method involving the reduction of chloroplatinic acid (H2PtCl6·6H2O) in the presence of triphenylphosphine (PPh3) as a stabilizing ligand and borane-tert-butylamine complex (TBAB) as a reducing agent. Control experiments were performed to synthesize Pt single atoms (Pt1SAs) and Pt nanoparticles (PtNPs) using similar methods. These catalysts were deposited on carbon black (Vulcan XC-72R) to form Pt6NCs/C, Pt1SAs/C, and PtNPs/C. A high-temperature calcination treatment of Pt6NCs/C was done to remove the ligands to produce Pt6NCs/C-550. **Characterization:** The synthesized materials were thoroughly characterized using various techniques including UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Thermogravimetric analysis (TGA) was used to determine the Pt loading. **Electrochemical Measurements:** The HOR catalytic activity was evaluated using a standard three-electrode system with a rotating disk electrode (RDE) coated with the synthesized catalysts. Linear sweep voltammetry (LSV) was performed in H2-saturated 0.1 M KOH solution at different rotation rates to determine the kinetic current density and mass activity. Cyclic voltammetry (CV) was used to obtain Tafel plots and estimate the exchange current density. Accelerated durability tests (ADT) were conducted using repetitive CV scans to evaluate the catalysts’ stability. Chronoamperometry was used to assess the long-term stability under continuous operation. CO-tolerance was evaluated by introducing diluted CO into the H2-saturated electrolyte during chronoamperometric tests. **Computational Studies:** Density functional theory (DFT) calculations were performed to investigate the electronic structure of the Pt6NCs, Pt1SAs, and PtNPs. The calculations were used to determine the binding energies of H, OH, and CO, providing insights into the HOR mechanism and CO tolerance. The d-band center was calculated to understand the influence of the ligand on the adsorption energies.

The study yielded several key findings: 1. **Synthesis of Atomically Precise Pt6 Nanoclusters:** The researchers successfully synthesized atomically precise Pt6 nanoclusters stabilized by triphenylphosphine ligands. ESI-MS confirmed the presence of Pt6(PPh3)4Cl6 clusters. 2. **Superior HOR Performance:** Pt6NCs/C exhibited significantly enhanced HOR activity compared to Pt/C, Pt1SAs/C, and PtNPs/C. The mass-specific activity of Pt6NCs/C was approximately 9.1 times higher than that of commercial Pt/C (Pt/Ccom). The area-specific exchange current density (jos) was also substantially higher for Pt6NCs/C. 3. **Enhanced Stability and Durability:** Pt6NCs/C demonstrated superior durability compared to Pt/Ccom. Accelerated durability tests showed significantly less degradation of Pt6NCs/C after 2000 cycles. Chronoamperometry confirmed the long-term stability of Pt6NCs/C, retaining 98.3% of its activity after 10 hours of continuous operation. 4. **High CO Tolerance:** Pt6NCs/C exhibited remarkably high CO tolerance compared to Pt/Ccom. The catalyst maintained a substantial portion of its activity even under CO contamination. DFT calculations suggested a lower CO adsorption energy for Pt6NCs. 5. **Ligand Effect:** The triphenylphosphine ligand played a crucial role in enhancing the catalytic performance. Removal of the ligand through thermal treatment resulted in significantly reduced activity and CO tolerance. 6. **DFT Calculations and Mechanism:** DFT calculations revealed that the PPh3 ligand tuned the electronic structure of the Pt6NCs, leading to a downshift of the d-band center. This resulted in weaker binding of H and CO, explaining the improved HOR activity and CO tolerance. The Gibbs free energy of H adsorption on Pt6NCs was near-optimal for HOR.

The results of this study demonstrate that atomically precise Pt6 nanoclusters offer a significant advancement in HOR electrocatalysis. The superior performance of Pt6NCs/C in terms of activity, durability, and CO tolerance can be attributed to the synergistic effects of the precise atomic structure and the unique role of the triphenylphosphine ligand. The ligand's influence on the electronic structure and the adsorption energies of key intermediates (H, OH, and CO) is a crucial factor contributing to the observed enhancements. This research underscores the importance of ligand engineering in designing high-performance electrocatalysts. The findings challenge the conventional understanding that ligands merely serve as stabilizing agents and highlight their active participation in catalytic processes. This research provides a fundamental understanding of the structure-activity relationship in Pt-based HOR catalysts and offers guidance for designing future high-performance electrocatalysts for energy conversion applications.

This study successfully synthesized and characterized atomically precise PPh3-stabilized Pt6 nanoclusters, showcasing their superior performance in alkaline HOR catalysis. The combination of experimental and theoretical findings elucidates the crucial role of the ligand in optimizing the electronic structure and adsorption energies, leading to enhanced activity, durability, and CO tolerance. Future research could explore other ligand systems to further enhance performance and investigate the scalability and cost-effectiveness of this approach for practical applications in fuel cells.

While the study demonstrates the superior performance of Pt6NCs/C, several limitations exist. The synthesis method may not be easily scalable for industrial production. Long-term stability testing under more rigorous conditions is warranted. The DFT calculations rely on model structures, and further characterization to confirm the precise structure of the synthesized clusters in the working environment is desirable. The effect of PPh3 ligand degradation during long-term operation should be further explored.