Figure S1. Size distributions of (a) MoP, (b) Co-1.5 h-MoP, and (c) Ni-1.5 h-MoP NPs.

Figure S2. TEM images and size distributions of (a, d) CoMoP and (b, c, e) NiMoP NPs synthesized by one-step processes. The CoMoP NPs are quasi-spherical in shape and 2.47 ± 0.39 nm in size, on average, while the NiMoP NPs show an irregular size and shape morphology: (b) polyhedral NPs (15.1 ± 2.1 nm) and (c) cluster-like shapes (1.99 ± 0.44 nm). Since the decomposition temperature and rate are different for each precursor, controlling the products using the one-step synthetic method, where all chemical reactions occur simultaneously, was difficult.

Figure S3. FFT patterns of the MoP NPs obtained from HR-TEM images and corresponding lattice distances.

Figure S4. STEM images (inset) and EDS line-scan profiles of (a) Co-1.5 h-MoP and (b) Ni-1.5 h-MoP NP along the lines highlighted in the STEM images.

Figure S5. XRD patterns of (a) Co-, and (c) Ni-incorporated MoP NPs with varying cation addition reaction times (0.5-12 hours). The lower patterns represent hexagonal MoP (PDF 03-065-6024), orthorhombic Co₂P (PDF 03-065-2381), and hexagonal Ni₂P (PDF 00-003-0953). TEM images of (b) Co-, and (d) Ni-12 h-MoP NPs as references. The insets show the particle size histograms of the samples.

Figure S6. (a) STEM image and corresponding elemental maps of the Co-1.5 h-MoP NPs showing (b) Co, (c) Mo, and (d) P.

Figure S7. (a) STEM image and corresponding elemental maps of the Ni-1.5 h-MoP NPs showing (b) Ni, (c) Mo, and (d) P.

Figure S8. XRD patterns of the MoP (lower), Co- (middle), and Ni-1.5 h-MoP (upper) NPs after heat treatment for surfactant ligand removal.

Figure S9. CV curves for (a) MoP, (b) Co-1.5 h-MoP, and (c) Ni-1.5 h-MoP NPs at varying scan rates (20-100 mV s⁻¹).

Figure S10. (a) Fitted curves and (b) Bode diagrams obtained from the EIS data and equivalent circuit shown in Figure 3E.

Figure S11. HER polarization curves of (a) MoP, (b) Co-1.5 h-MoP, and (c) Ni-1.5 h-MoP before and after 500 and 1000 potential cycles.

Figure S12. TEM images of (a) MoP, (b) Co-, and (c) Ni-1.5 h-MoP after 2 hours of durability testing.

Figure S13. CV curves for (a) Co-3 h-MoP, (b) Co-7 h-MoP, (c) Co-7 h-MoP, (d) Ni-3 h-MoP, (e) Ni-7 h-MoP, and (f) Ni-12 h-MoP NPs at varying scan rates (20-100 mV s⁻¹).

Figure S14. HER polarization curves for MoP and the (a) Co- and (b) Ni-incorporated MoP samples reacted for 1.5 to 12 hours normalized by their electrochemically active surface areas.

Table S1. SEM-EDS-determined Co, Mo, and P atomic percentage and Mo-to-P ratios for MoP and the Co- and Ni-incorporated MoP NPs after various cation addition reaction times.

Table S2. Summarizing the HER performance of Mo-, Co-, and Ni-based catalysts.

Table S3. SEM-EDS-determined atomic percentages of M, Mo, and P in MoP and Co- and Ni-1.5 h-MoP NPs after 2 hours of durability testing.

Table S4. Fitted Mo 3d and P 2p XPS peak areas of the various oxidation states of MoP, Co-, and Ni-1.5 h-MoP.