NiFe-LDH/Carbon-Based Composites For Electrocatalytic Alkaline Water And Seawater Oxidation

The conventional energy crisis forces people to get rid of the dependence on fossil fuels with limited reserves and explore clean energy with development potential.Hydrogen energy is regarded as a new secondary energy that can replace fossil energy.Water electrolysis is a attractive alternative technology for hydrogen production.Currently,water electrolysis technology is highly dependent on the feed of high-purity fresh water,which largely hinders its commercial development in various environment.In this case,seawater electrolysis has received more and more attention due to the inexhaustible reserves of ocean(occupying about 96.5%of the water reserve on the earth).However,the complex chemical ions of seawater brings great difficulties to deal with ionic diffusion,side reactions and anode corrosion/poison during the electrolysis of seawater.Actually,the most direct technical obstacle to seawater electrolysis stems from the competition between the oxygen evolution reaction(OER)and the chlorine oxidation reaction(ClOR)on the anode.In theory,another crucial challenge of seawater electrolysis cannot be ignored:inorganic impurities and organic microorganisms can cause catalyst poisoning.Accordingly,it is of great significance to design a novel catalyst to provide efficient and stable OER activity for seawater electrolysis in a mild electrochemical environment.Based on the rational design and development of electrocatalysts,two types of nickel-iron hydrotalcite/carbon-based composites were successfully prepared by a simple hydrothermal synthesis method for efficient electrocatalytic OER.The main research contents of this paper are as follows:1 Construct 3D NiFe-LDH/GF for electrocatalytic alkaline freshwater splitting and seawater oxidationRationally design NiFe layered double hydroxide(LDH)to grow in situ hydrothermally on three-dimensional(3D)graphite felt(NiFe-LDH/GF)into a 3D nanosheet arrays,which serves as an efficient catalyst for promoting OER.In 1.0 M KOH electrolyte,NiFe-LDH/GF requires an ultralow overpotential of 214 mV to deliver a geometric current density of 50 mA cm-2.In addition,despite the complex environment of real seawater,NiFe-LDH/GF still effectively promotes alkaline seawater(1.0 M KOH+seawater)electrolysis.The NiFe-LDH/GF electrode at 50 mA cm-2 shows an overpotential of 297 mV,which is much less than 480 mV,well avoid triggering chlorine oxidation reaction(ClOR).More importantly,NiFe-LDH/GF shows good durability at 50 mA cm-2 within 50 h of OER catalysis testing and delivers a Faradaic efficiency of nearly 100%in the electrocatalysis of OER.2 N-doped carbon dots coupled NiFe-LDH hybrids for electrocatalytic oxygen evolution in alkaline water and seawaterAlthough NiFe-LDH/GF is superior for OER electrolysis,the high specific surface area of the substrate GF plays a non-negligible role in the catalytic activity of the material.However,to enhance the intrinsic activity of the material itself,we selectively construct hybrid structure for efficient each individual component utilization.In this chapter,we we propose the in situ growth of nitrogen-doped carbon dots strongly coupled NiFe-LDH on Ni foam(N-CDs/NiFe-LDH/NF)can efficiently facilitate alkaline seawater oxidation.Benefiting from the high exposure of metal active sites,the M-N-C bonds generated by hybridization,and the optimized electronic structure in the composite constituents,N-CDs/NiFe-LDH/NF exhibits excellent OER performance with an ultralow overpotential of 260 mV to deliver the geometric current density of 100 mA cm-2 and a Tafel slope of as low as 43.4 mV dec-1 in 1.0 M KOH.More importantly,the N-CDs/NiFe-LDH/NF electrode at 100 mA cm-2 shows overpotentials of 285 mV and 273 mV,respectively,by utilizing 1.0 M KOH with 0.5 M NaCl and 1.0 M KOH with 1.0 M NaCl as the simulated seawater,which are much less than 480 mV,well avoid triggering chlorine oxidation reaction(CIOR).Notably,despite the complex environment of real seawater,N-CDs/NiFe-LDH/NF still effectively promotes alkaline seawater(1.0 M KOH+seawater)electrolysis with a lifetime longer than 50 and 20 hours,respectively,in 1.0 M KOH and alkaline seawater electrolytes.The investigation result reveals that M-N-C bonding generated between N-CDs and NiFe-LDH intrinsically optimize the charge transfer efficiency,further promoting the OER kinetics.