Efficient Hydrogen Evolution from Ru-Doped MoSe₂ Nanoflowers Catalyst

Two dimensional layered materials, such MoS₂, WS₂ and MoSe₂, are efficient catalyst electrode material for electrochemical hydrogen evolution reaction (HER). In the present study, Ru-doped MoSe₂ hierarchical 3D nanoflowers were synthesized using colloidal method and the HER activity of the catalyst was investigated as a function of the Ru substitution percentage (3.6%, 7.4% and 11.4%). The samples showed flower-like morphology with size spanning in the range 200-500 nm. X-ray diffraction (XRD) pattern revealed that the Ru-doped MoSe₂ nanoflowers were crystallized in the 2H-hexagonal crystal structure. Optical excitons A and B show a gradual blue shift and gradual increase of optical bandgap with higher Ru content. The change in the physical properties originates from additional electrons donated to the MoSe₂ skeleton by the Ru. In addition, the higher number of charge carriers contributed to the conductance of the nanoflowers. Therefore, the Ru-doped MoSe₂ nanoflowers showed improved electrochemical HER activity (in 0.5 M H₂SO₄) with increased Ru doping, decreasing the necessary overpotential from η ≈ 234 mV for the undoped structures to η ≈ 143 mV for a substitution of 11.4%. The calculated Tafel slopes for MoSe₂ and Ru doped- MoSe₂ varied in the range of 85-91 mV/dec. The variation of the Tafel slope from that of a Pt/C (~33 mV/dec), indicates a change of mechanism, i.e. adopting the Volmer–Heyrovsky mechanism with the Volmer discharge reaction as the rate-limiting step in contrast to the Volmer–Tafel pathway for the Pt/C. Impedance measurements verified that the Ru-doped MoSe₂ exhibited lower charge transfer resistance. The R_ct of 210 Ω for undoped MoSe₂ has decreased to 66 Ω for a substitution of 11.4%. The use of Ru demonstrates that with a suitable electron donor, a significant enhancement of the catalytic performance is achieved.