Preparation And Performance Of Iron Phosphide Electrocatalysts For Hydrogen Evolution Reaction

In the current situation where fossil fuels are depleted and carbon emissions have not been effectively solved,hydrogen energy,as a clean,highly efficient and reusable energy carrier,is the most promising energy source in the future and is also a hot topic in the society today.In a variety of hydrogen production methods,electrolysis of water is regarded as an efficient,clean hydrogen production technology.Because of its simple preparation process and high product purity,electrolysis of water is the most promising large-scale hydrogen production technology.However,the decomposition of water into hydrogen molecules and oxygen molecules needs to overcome a large overpotential.The energy consumption of this process is too high,which limits the hydrogen production reaction in water electrolysis.Therefore,the employment of catalysts to reduce energy consumption by reducing overpotential has become the consensus of researchers.At present,Pt is the most active electrocatalyst for hydrogen evolution reaction?HER?,but it is not suitable for large-scale applications due to its limited reserves and high cost.Hence,the search for non-noble metal catalysts that can effectively reduce hydrogen evolution overpotential is of great significance for the large-scale application of water electrolysis technology.Transition metal phosphides?TMPs?have been considered as a promising alternative to Pt-based noble metal hydrogen evolution catalyst due to their superb activity,good electrical conductivity and low cost.However,most of the reported TMPs catalysts are still inferior to Pt.Considering iron is one of the cheapest and abundant metals on the earth,herein,we focused on the preparation and HER catalytic performance of FeP-based HER catalysts.Two FeP-based electrocatalysts were prepared,i.e.,carbon-supported FeP nanocrystals and carbon-enhanced ordered mesoporous FeP.The HER performances of both FeP-based electrocatalysts were enhanced by maximizing the exposure of active sites and increasing the conductivity of the catalysts.Firstly,we investigated the synthesis and catalytic performance of carbon supported FeP ultrafine nanocrystals?FeP/C NCs?.It was successfully prepared by suppressing the growth and aggregation of the crystallites through aborative design and precise control of the synthetic procedure and reaction conditions.The size of the resultant FeP NCs was strictly limited to 10 nm,which provided more active sites for electrocatalytic reactions.Simultaneously,the FeP NCs were well dispersed on commercial carbon black,which facilitated electron transfer during the electrochemical processes.As expected,the electrocatalytic performance of the as-synthesized catalyst was greatly enhanced.It afforded10 mA cm^-22 at overpotential as low as 70 m V,showing a small Tafel slope of 56 mV dec^-11 and a large exchange current density of 0.631 mA cm^-2.Moreover,this synthetic strategy can be employed to prepare other TMP-based nanocrystal HER catalysts,such as CoP/C NCs and Ni₂P/C NCs.Secondly,we investigated the synthesis and catalytic performance of carbon-enhanced ordered mesoporous iron phosphide?meso-FeP?.Ordered mesoporous iron oxide precursor was prepared by using mesoporous silica SBA-15 as template through nanocasting approach.Then it was converted to ordered mesoporous iron phosphide through a following phosphating process.The as-synthesized meso-FeP exhibited a large specific surface area,interconnected mesopores and uniform pore size.These merits endowed the catalyst with high catalytic activity,abundant active sites,high charge/mass transfer efficiency and good electrical conductivity.It exhibited excellent catalytic activity and stability for the HER in both acidic and basic media,requiring overpotentials of 114 and 136 mV to deliver 10 mA cm^-22 current density as well as Tafel slopes of 69 and 68 mV dec^-11 in 0.5 M H₂SO₄ and 1.0 M KOH,respectively,and maintaining its catalytic activity for at least 22 h.