Design And Performance Study Of Key Electrocatalysts For Hydrogen Applications

In recent years,owing to the excessive use of traditional fossil fuels,environmental pollution and energy crisis have been largely aggravated.Therefore,it is essential to develop clean and efficient renewable energy.Hydrogen has the merits of high energy density and the sole by-product of water,which is a promising energy.Thereinto,fuel cells and electrolytic water splitting are two key technologies in the field of hydrogen energy.Oxygen reduction reaction and hydrogen evolution reaction,as half reaction of fuel cell and electrolytic water splitting,respectively,are very important for hydrogen generation and utilization.And the key factors affecting the oxygen reduction and hydrogen evolution reactions are electrocatalysts.However,there are still a lack of stable and efficient electrocatalysts,due to the influence of the structure,surface chemical properties of the catalysts,and external environment.At present,the commercial catalysts for oxygen reduction and hydrogen evolution are mainly platinum-based catalysts.On the one hand,as the cathodic oxygen reduction reaction with slow dynamics in fuel cells,catalysts with high loading are extremely essential.Nevertheless,the shortcomings of easily poisoning and poor stability of the catalysts greatly limit the large-scale application of fuel cells.On the other hand,the alkaline hydrogen evolution reaction involves an additional water dissociation process with a high kinetic energy barrier,which greatly inhibits the efficiency of the reaction.And even the most active Pt/C catalyst,its alkaline catalytic activity is about two to three times lower than that in acidic conditions.Therefore,the key problems of catalysts for the preparation and conversion of hydrogen energy are studied.This paper firstly designed the catalysts for cathodic oxygen reduction in fuel cells.We selected the nano-porous carbon as target materials,owing to their large specific surface area,excellent ability of electron conduction and electrochemical stability.The catalytic performance of oxygen reduction of heteroatom-doped carbon material is comparable to that of commercial Pt/C.Subsequently,we employed the above-mentioned carbon material to regulate the electronic structure of metal nanoparticles(NPs),thus to significantly enhance their alkaline hydrogen evolution activity.Moreover,we applied the substrate effect to transition metal oxide systems.We constructed the oxide-based composite as an efficient nano-catalyst for alkaline hydrogen evolution reaction.The main results are as follows:(1)In-situ reaction template method was used to prepare a series of nitrogen-doped graphene-like carbon nanosheets(NCNSs).During the pyrolysis process of dicyandiamide,the generated g-C₃N₄ could serve as the template to limit the growth of two-dimensional carbon nanosheets.The resultant carbon nanosheets have a high nitrogen content(5.3-6.8 at.%),of which pyridinic nitrogen and graphitic nitrogen are main nitrogen-containing functional groups.Meanwhile,numerous three-dimensional channels could accelerate the rate of mass and charge transfer during the reaction process.Oxygen reduction reaction tests shown that the half-wave potential(E_1/2)of the NCNS?1:10 is 0.8V_RHE with a four-electron transfer process.In compared to 40%Pt/C,the NCNS?1:10 exhibited high stability and excellent methanol crossover resistance.However,the catalytic activity of the sample is still inferior to that of Pt/C(E_1/2:0.86V_RHE).We further designed the iron and nitrogen co-doped carbon-based catalysts,and further achieved a three-dimensional mesoporous Fe-N-C material.The Fe-N-C material possesses a high surface area of 840.4m²g^-1,which is beneficial to expose more active sites.The Fe-N_x groups originated from the coordination between iron single atoms and nitrogen atoms on the carbon substrate(Fe-N-C)show high catalytic activity for the oxygen reduction reaction.The Fe-N-C exhibited excellent four-electron process in alkaline media with a E_1/2 value of 0.87V_RHE,which even surpasses than 40%Pt/C.In addition,it also shown good durability and methanol crossover resistance.(2)The iron and nitrogen co-doped carbon served as the substrate to control the coordination forms and charge distribution of ruthenium(Ru),thus to optimize its alkaline hydrogen evolution performance.The results show that the Ru anchored on the carbon matrix exists in the form of both single atoms and nanoclusters,while the Fe has a coordination of Fe-N₄.XPS and XANES results reveal that there is a charge transfer between Ru and Fe.DFT calculations and synchrotron radiation fitting results exhibit that Ru single atoms should exist in the coordination form of Ru-N₄,while Ru nanoclusters prefer to connect with the Fe-N₄ moieties.Ru/Fe-N-C shown excellent hydrogen evolution activity under alkaline conditions with an overpotential(?₁₀)of 9m V at the current density of 10mAcm^-2,and a Tafel slope of 28mVdec^-1,as well as a nearly 100%Faradaic Efficiency.The TOF value of Ru/Fe-N-C is also 7.6 times to that of commercial Pt/C(?=25mV).The calculations show that,Ru-N₄ groups could promote the dissociation of water to produce more adsorbed intermediate hydrogen.Then,these adsorbed hydrogens recombine on nearby Ru nanoclusters to generate H₂,and desorb from the catalyst to release the active sites.(3)In order to boost the alkaline hydrogen evolution activity of oxide-based catalysts,K_0.5La_0.5TiO₃ perovskite oxide(KLTO)was used as the substrate,and coupled with ruthenium oxide(RuO₂)NPs.Partial potassium cations on the surface of KLTO were replaced by Ru cations(RKLTO)by using hydrothermal treatment,and simultaneously involving nucleation and growth of NPs on the KLTO substrate.HAADF-STEM images show that the NPs is Ti-doped RuO₂(TRO).EELS spectra results reveal that there are two forms of Ti,one of which exists in the KLTO substrate(Ti⁴⁺),and another in the TRO NPs(Ti³⁺).XPS and XAFS measurements indicate that the valence states of Ru and Ti in TRO/RKLTO are mainly 4+.Under alkaline conditions,the?₁₀ value of TRO/RKLTO catalyst is only 20mV,which is superior than that of Pt/C(34mV),and far exceeds the reported oxide-based hydrogen evolution catalysts as well.On the point of thermodynamics,DFT calculations demonstrate that Ti cations are easily incorporated into the lattice of Ru O₂.And the K ions located in the A sites prefer to be exchanged by Ru ions,owing to its low exchange energy,which is consistent with the TEM results.DFT calculations show that TRO NPs and RKLTO substrates are responsible for water dissociation and hydrogen proton coupling,respectively.Both synergistically enhance the hydrogen evolution performance of TRO/RKLTO catalyst.