Synthesis And Performance Of The Electrocatalyst Based On Layered Double Hydroxide And MXene

Hydrogen is not only an important industrial raw material,but also an efficient secondary clean energy,which is of great significance for China and even the world to develop a low-carbon economy,improve the ecological environment and ease the energy crisis.Electrochemical water splitting is regarded as one of the most efficient and clean ways for hydrogen production,which has taken advantages of the abundant raw materials,environmental friendly,production of high purity hydrogen,low carbon emission and efficient use of clean energy power generation.The process comprises two half reactions:the oxygen evolution reaction(OER)at the anode and hydrogen evolution reaction(HER)at the cathode.Efficient electrocatalyst is highly desirable to overcome the high energy barrier of OER and HER,reduce energy consumption of water electrolysis process and promote hydrogen production efficiency.Due to the lack of efficient and stable catalysts for OER in acidic media,the alkaline water-splitting electrolyzers is more valuable for widespread applications.Precious metal based electrocatalysts show the best activity towards water electrolysis.However,their widespread application is still limited by unaffordable cost.Therefore,developing economical and efficient non-precious metal electrocatalysts is significant to improve hydrogen production efficiency and promote the development of hydrogen economy.To this end,this study focuses on the design and synthesis of active alkaline water-splitting electrocatalysts by taking advantage of synergistically electronic,structural and chemical coupling of highly OER active LDH nanostructure and highly conductive,hydrophilic and surface active MXene nanosheets.The main results are as follows:A highly active OER electrocatalyst with mesoporous nanostructures and rich electrochemical active surface area has been fabricated by in-situ assembling the NiFe-LDH nanoplates network on 2D Ti3C2-MXene nanosheets through chemical liquid-phase deposition.In this structure,MXene serves as conductive substrate,which can enhance electronic transmission capacity of the nanohybrids and inhibit the aggregation of LDH nanosheets.The LDH is the active phase for catalyzing OER,and it can also effectively inhibit the aggregation of MXene caused by van der Waals force.The DFT calculation shows that strong electronic coupling between LDH and MXene not only guarantees robust structure of the hybrid material,but also accelerates the Ni2+/Ni3+,4+redox processes for better OER activity and enhances the O binding strength.As a result,the NiFe-LDH/Ti3C2-MXene nanohybrid is demonstrated to outperform the RuO2 and graphene/LDH based catalyst for alkaline OER in terms of substantially reduced overpotential(ηj=10＝298 mV)and superior durability.A highly active and bifunctional 3D free-standing electrode is fabricated by in-situ assembling the amorphous NiFe-LDH nanoplates network on 3D MXene frame.The 3D MXene frame with highly hydrophilic property is utilized to enhance the adsoprtion/activation of the water molecules on the electrocatalytic interface,which accelerates the Volmer step of HER.The strong electronic coupling between the LDH and MXene can enhance the binding strength between catalyst and OOH*.The 3D interconnected macroporous skeleton facilitates fast accessibility to the electrolyte and rapid gas release upon vigorous electrocatalytic reactions.Free-standing electrode design further minimizes the electrode polarization and the loss of the electrochemical active surface area caused by using insulating binder in traditional casting-made way.The NiFe-LDH/MXene/NF can be directly used as electrocatalytic electrodes for overall water-splitting.An alkaline electrolyzer using this electrocatalytic electrodes as both the anode and cathode can be steadily driven at the low cell voltage of 1.75 V to achieve the commercially required high current density of 500 mA cm-2 with high Faradaic efficiency(98%)and excellent durability(over 200 h for 100 mA cm-2).Their performance also outperforms the Pt/C-RuO2 couple.Amorphous La-doped NiFe-LDH nanosheets coupled 3D vertically aligned porous MXene(v-MXene)frame has been constructed.By optimizing the composition and the hierarchically pore structure of integrated electrode,the performance of the LDH/MXene based composites for alkaline HER can be further enhanced.This unique 3D vertically aligned MXene frame with rich porous structure can expose more active sites,provide larger electrochemical active surface area and facilitate the mass/charge transport.The incorporation of La dopants in NiFeLa-LDH may alter the micromorphology and generate the strong electron interactions within the metal center.As a result,the electrocatalytic performance of prepared NiFeLa-LDH/v-MXene/NF is further enhanced.Only a lower cell voltage of 1.71 V voltage is needed to achieve the current density of 500 mA cm-2 in an alkaline electrolyzer with NiFeLa-LDH/v-MXene/NF electrodes and the working life can extend to 400 h with 100%Faraday efficiency.