Effect of Annealing on HER Performance of Unsupported Ni-Mo Transition Metal Sulfide Electrocatalyst
Résumé
Transition metal sulfides (TMSs) have emerged as strong candidates for hydrogen production due to their good electrical conductivity, layered structures, and abundance of catalytically active sites.1 This makes them promising materials for green hydrogen production, particularly in anion exchange membrane water electrolysis (AEMWE) systems operating at low hydroxide concentrations.2, 3 In this study, Ni₀.₅Mo₀.₅Sₓ electrocatalysts were synthesized via a simple, environmentally friendly hydrothermal method using diethyldithiocarbamate as the sulfur source. The resulting materials were characterized by XRD, Raman spectroscopy, SEM, EDS, and ICP-OES. To investigate the effect of crystallinity on HER performance, the as-prepared material was subjected to an annealing treatment at 350 °C under argon atmosphere. All samples showed catalytic activity toward the hydrogen evolution reaction (HER). However, the annealed material (Ni₀.₅Mo₀.₅Sₓ-AN) exhibited a fivefold decrease in HER performance, indicating that thermal treatment negatively impacts catalytic efficiency in Ni-Mo-based TMS systems. This decline is further reflected in the Tafel slope, which increased by approximately 100 mV dec⁻¹ after annealing. The increase in Tafel slope suggests a change in the reaction kinetics, often associated with decreased density of active sites or altered surface chemistry.4 Notably, XRD and Raman analyses indicate increased crystallinity post-annealing, which likely reduces the number of catalytically active edge sites and surface defects that are typically more abundant in amorphous or poorly crystalline structures. These edge sites are known to facilitate efficient HER kinetics. Despite this, the best-performing sample (Ni₀.₅Mo₀.₅Sₓ-HT) displayed a Tafel slope comparable to that of Pt/C under similar alkaline conditions (73 mV dec-1) 5 , highlighting Ni₀.₅Mo₀.₅Sₓ-HT as a cost-effective alternative to noble metal catalysts for alkaline HER applications. Figure 1 - (a) Uncompensated polarization curves for HER in 0.1 M KOH at 1600 rpm; (b) Tafel plot derived from iR-compensated polarization curves; (c, d, e, f) SEM images for Ni₀.₅Mo₀.₅Sₓ-HT and Ni₀.₅Mo₀.₅Sₓ-AN materials References: 1. S. Ali, S. U. H. Bakhtiar, A. Ismail, P. M. Ismail, S. Hayat, A. Zada, X. Wu, A. N. Alodhayb, M. Zahid, F. Raziq, J. Yi and L. Qiao, Coordination Chemistry Reviews, 2025, 523, 216237. 2. A. Ali, I. Hussain, I. Hameed, M. A. Khan and J.-J. Shim, Journal of Alloys and Compounds, 2024, 1003, 175568. 3. W. U. Mulk, A. R. A. Aziz, M. A. Ismael, A. A. Ghoto, S. A. Ali, M. Younas and F. Gallucci, International Journal of Hydrogen Energy, 2024, 94, 1174-1211. 4. Y. Luo, L. Tang, U. Khan, Q. Yu, H.-M. Cheng, X. Zou and B. Liu, Nature communications, 2019, 10, 269. 5. S. Zhou, W. Cao, L. Shang, Y. Zhao, X. Xiong, J. Sun, T. Zhang and J. Yuan, Nature Communications, 2025, 16, 1849.