Crystal Structure and Morphology-Controlled Synthesis of Co_1-xMn_xP Nanocrystals and Their Composition-Dependent...

An important obstacle to long-term hydrogen sustainability is the lack of efficient and stable non-noble-metal catalysts for hydrogen generation through water electrolysis. Cobalt phosphides have emerged as earth-abundant catalysts for the hydrogen evolution reaction (HER), and its activity can be augmented by admixing synergistic elements to produce heteroatom-doped catalysts. Herein, we report an integrated computational and experimental study leading to the synthesis of Co_1-xMn_xP nanocrystals (NCs) displaying superior activity and stability for the alkaline HER compared to the benchmark Pt/C catalyst at higher current densities (j ≥ -35 mA/cm²). Density functional theory calculations predicted that Mn doping modulates the hydrogen adsorption energies (ΔG_H) of orthorhombic CoP toward thermoneutral values. Accordingly, a series of Co_1-xMn_xP NCs (x = 0.038-0.169) with control over structure, morphology, and composition was produced via colloidal synthesis. Physical characterization of Co_1-xMn_xP NCs revealed an orthorhombic structure, pseudospherical morphology, and average diameters of ∼5.7-10.2 nm. The incorporation of Mn caused significant modulation of the electronic structure prompting a decrease in Co(2p) and P(2p) binding energies, suggesting an increase in electron density on both surface sites. Among NCs investigated, Co_0.909Mn_0.091P composition displayed the highest HER activity with an overpotential (η_-10) of 136.29 mV at j = -10 mA/cm², consistent with composition-dependent ΔG_H studies. With a Tafel slope of 65.77 mV/dec, Co_0.909Mn_0.091P NCs showed similar kinetics to the Pt/C catalyst (62.31 mV/dec), indicating the Volmer-Heyrovsky HER mechanism. The highest-performing Co_0.909Mn_0.091P NCs showed a prominent increase in electrochemically active surface area and significantly lower charge transfer resistance compared to parent CoP NCs. The Co_0.909Mn_0.091P NCs showed exceptional stability in alkaline media compared to CoP NCs and commercial Pt/C catalysts. Co_0.923Mn_0.077P, Co_0.909Mn_0.091P, and Co_0.831Mn_0.169P compositions displayed superior HER activity and stability compared to monometallic CoP NCs suggesting that dopant-induced compositional and surface modification is an effective strategy for designing high-efficiency, durable nanostructures for numerous heterogeneous (electro)catalytic studies.