Preparation Of Ultrathin Hydrotalcite Nanosheet-carbon Composites And Their Electrocatalytic Properties

The oxygen evolution reaction(OER),oxygen reduction reaction(ORR) and hydrogen evolution reaction(HER) in the process of storage and conversion of hydrogen energy are one of the most important issues because of the slow dynamics and high energy barriers.Currently,standard catalysts are all precious metal catalysts,which increases the cost of energy storage and conversion.Among the OER catalysts,the low cost and effective-catalytic of hydrotalcite-like nanosheets(LDHNS) composites have become the focus of research.However,there are many issues in synthesis of LDHNS compounds,for example,the steps of obtaining LDHNS composites are complicated,the active sites of LDHNS is covered,and the performance needs to be further improved.Therefore,it is an urgent problem to explore a simple and easy to synthesize LDHNS composite with ultrathin LDHNS.It is an effective strategy to enhance catalyst active by exposing more active sites and strengthening the synergistic effect of LDHNS and carbon materials.In view of this,three ultrathin LDHNS complexes were constructed in situ growth and used for electrocatalysis.The main contents are as follows:(1)NiFe layered hydrotalcite-like nanosheets on reduced oxide graphene(NiFe-LDHNS/rGO) was synthesized in situ growth in formamide aqueous solution followed by simple chemical reduction.The ultrathin LDHNS in the composite occur in single-or multi-layer forms on rGO matrix with a thickness of 0.8-3 nm.Because more active sites were exposed,NiFe-LDHNS/rGO exhibits a large electrochemically active area and a low charge transfer resistance.As an OER catalyst,NiFe-LDHNS/rGO only requires an overpotential of 254 mV to achieving current density of 10 mA/cm~2 in 0.1 M KOH.It also demonstrates the robust long-time electrochemical durability.When catalyzing ORR,the onset potential is 0.85 V and the ultimate diffusion current density arrive-4.1 mA/cm~2.The ultra-thin LDHNS complex can effectively enhance the electrocatalytic activity of OER and ORR..(2)Ultrathin CoFe layered double hydroxide nanosheets on oxidized carbon nanotube surface(LDHNS/OCNT) was synthesized by a facile bottom-up method in formamide aqueous solution.The preparation strategy results in the ultrathin nature of hydrotalcite with less than 4 nm thickness.Since the ultra-thin structure of LDHNS increases the more exposed active sites and has a stronger synergy with oxidized carbon nanotubes,LDHNS/OCNTs exhibits excellent OER catalytic activity:the onset potential is 1.41 V,the overpotential at a current density of 10 mA/cm~2 is only 250 mV in 0.1 M KOH.The Tafel slope is 48 mV/dec and the electrochemically active area is 1.31 mF/cm~2,confirming the faster mass transfer rate.Moreover,LDHNS/OCNT also exhibits excellent stability.However,the catalytic activity is not ideal when catalyzing ORR.(3)CuO doped Ultra-thin NiFe hydrotalcite-like nanosheets/reduced oxide graphene nanocomposite(CuO-NiFe-LDHNS/rGO)was directly synthesized by bottom-up in-situ synthesis in 50% aqueous solution of formamide at room temperature followed by reduction of hydrazine and heat.Because of more edge active sites of LDHNS caused by CuO doping and the ultra-thin structure and enhanced synergistic effect,its catalytic performance in OER and HER was enhanced.As an OER catalyst,an overpotential of a current density of 10 mA/cm~2 was 256 mV in 0.1 M KOH.Compared with NiFe-LDHNS/rGO,the overpotentials reaching current densities of 10 and 100 mA/cm~2 were reduced by 2 mV and 33 mV,respectively.The Tafel slope is 15.3 mV/dec and the equivalent potential is 232Ω,confirming the faster kinetics of the reaction.As a HER catalyst,an overpotential of a current density of-10 mA/cm~2 in 0.1 M KOH was 320 mV.The results show that the LDHNS lattice defects caused by CuO doping and the increased edge active sites effectively improve the catalytic properties of the materials for OER and HER.