The improvement of carbon quantum dots on the manganese-nickel phosphide for electrocatalytic hydrogen evolution reaction*

Transition-metal phosphides are good electrocatalysts for hydrogen evolution reaction due to their high catalytic efficiency and low cost. Carbon quantum dots (CQDs) deposited on top of the surface could make the phosphides even better for HER by increasing the number of active sites and reducing the charge transfer resistance. Here we adopt a simple method to synthesize CQDs-modified manganese-nickel phosphide (CQDs/Mn_xNi_5-xP₄) for efficient and stable HER activity using inexpensive raw materials. In 0.5 M H₂SO₄, CQDs/Mn_xNi_5-xP₄ requires a low overpotential of only 31 mV to achieve a current density of 10 mA cm^-2, as well as having a low Tafel slope of 41.0 mV dec^-1, a large exchange current density of 1.753 mA cm^-2, and good stability, making it comparable with the best transition-metal-based catalyst available. Moreover, CQDs/Mn_xNi_5-xP₄ also displays high activity and stability in alkaline solution, revealing that the ancillary role played by CQDs could be beneficial in both acidic and alkaline conditions. Based on our results, we believe that CQDs have great potential to be applied to other materials with various morphologies and structures for designing high-performance HER catalysts.