Crystal Structure and Morphology-Controlled Synthesis of Co _{1– <i>x</i>} Mn _<i>x</i> P Nanocrystals and Their Composition-Dependent Electrocatalytic Activity for the Hydrogen Evolution Reaction

High Resolution Image Download MS PowerPoint Slide 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– x Mn x P 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/cm 2 ). 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– x Mn x P NCs (x = 0.038–0.169) with control over structure, morphology, and composition was produced via colloidal synthesis. Physical characterization of Co 1– x Mn x P 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 0.909 Mn 0.091 P composition displayed the highest HER activity with an overpotential (η –10 ) of 136.29 mV at j = −10 mA/cm 2, consistent with composition-dependent ΔG H studies. With a Tafel slope of 65.77 mV/dec, Co 0.909 Mn 0.091 P NCs showed similar kinetics to the Pt/C catalyst (62.31 mV/dec), indicating the Volmer-Heyrovsky HER mechanism. The highest-performing Co 0.909 Mn 0.091 P NCs showed a prominent increase in electrochemically active surface area and significantly lower charge transfer resistance compared to parent CoP NCs. The Co 0.909 Mn 0.091 P NCs showed exceptional stability in alkaline media compared to CoP NCs and commercial Pt/C catalysts. Co 0.923 Mn 0.077 P, Co 0.909 Mn 0.091 P, and Co 0.831 Mn 0.169 P 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.