Investigation Of Electrochemical Properties Of Hydrogen Storage Alloys-based Hybrid Materials

Energy crisis,environment problem and global climate change etc.have been great challenges for the past few years,it is highly desirable for people to explore green technologies to alleviate these problems.As green technologies,new energy vehicles have attracted much attention.And the power sources of new energy vehicles are the electrochemical energy storage/conversion devices,including nickel metal hydride battery?Ni-MH?,lithium-ion battery?Li-ion?and fuel cell,etc.Ni-MH battery has attracted extensive attentions and studies attributing to the excellent safety,consistency and environmental friendliness.Although it has been applied in portable electric devices,power tools and new energy vehicles etc.However,its power density could not satisfy the high-power applications,such as electrical vehicles and military devices.For Ni-MH batteries,the key point which determine the high rate dischargeability?HRD?is the performance of its negative material—hydrogen storage alloys?HSAs?.Although researchers have developed several methods to modify the HRD performance of HSAs in recent years,such as element substitution,annealing treatment,surface treatment,etc.It is still urgent to explore new types of materials to further improve the performance of hydride electrode.We have designed and prepared new types of hybrid materials to improve the overall electrode electrochemical performances by means of optimizing the experimental plan,modifying preparation methods.As examples,we make the composites of HSAs with nanoporous metal or graphene,which has large specific surface area and excellent electrical conductivity,to improve the HRD performance;we fabricate the composite of HSAs with Co3O4 which owns superior electrocatalytic activity to modify the overall electrochemical properties of MH electrode;furthermore,we prepare the Co₃O₄/N-doped graphene composite based on the synergistic effect,which enhances the overall electrochemical performance of Ni-MH battery.This thesis expands these contents and are mainly divided into the following three parts:1.The design of HSAs/nanoporous metal hybrid electrode and its applications in Ni-MH batteries.There is a large internal resistance in MH electrode due to the traditional fabrication method,which induces large polarization in the electrochemical reactions.Here we make the composite of HSAs with three-dimensional bicontinous nanoporous Ni?NPNi act as a current collector?,which reduces the electrode internal resistance and electrochemical polarization because of the seamless integration of NPNi with HSAs.Such a unique hybrid architecture not only enhances charge transfer between NPNi and HSAs,but also facilitates rapid diffusion of hydrogen in alloy bulk.The electrode shows enhanced HRD performance with the capacity retention rate reaching 44.6%at a discharge current density of 3000 mA g^-1,which is 2.4 times that of the bare HSAs?18.8%?.The developed HSAs/nanoporous metals hybrid structure exhibits great potential to be candidate as electrode in high-performance Ni-MH batteries towards applications in new energy vehicles.2.The design of HSAs/graphene hybrid electrode and its applications in Ni-MH batteries.A top-down synthesis method is adopted to yield the composite of HSAs and graphene.It is believed that the high conductivity of graphene accelerates the charge transfer rate;and the high specific surface area of graphene increases electrochemical active sites,both of which are beneficial to the adsorption of ion and the proceeding of electrochemical reactions.Moreover,the unique interconnected graphene sheets among HSAs reduces the internal resistance and electrochemical polarization.The electrochemical capacity retention could reach 51.3%at a discharge current density of 3000 mA g^-1,which is almost 4 times that of the bare HSAs electrode?13.5%?.The design and synthesis of HSAs/graphene hybrid electrode could also be extended to other energy storage devices to enhance their electrochemical performances.3.The applications of Co₃O₄ and its composite in improving the overall electrochemical performances of Ni-MH batteries.Co₃O₄ is an effective catalyst to accelerate the electrochemical reaction rate of MH electrode.However,the low utilization efficiency has become a big challenge for direct adding Co₃O₄ powders into the alloys with mechanical mixing.Here we in situ grown nanosheet-shaped Co3O4 on the alloy surface to fabricate the Co3O4/HSAs composite,which not only improves the catalytic activity and utilization efficiency of Co₃O₄ on the electrochemical reaction kinetics,but also decreases the internal resistance and polarization due to the seamless integration of Co₃O₄ with HSAs.Compared with bare HSAs,the fabricated composite shows larger maximum discharge capacity,326.37 vs.302.62 mAh g^-1,and enhanced HRD performance with larger discharge capacity(59.01 vs.40.88 mAh g^-1)at a current density of 3000 mA g^-1.The synergistic effect between Co3O4 and N-doped graphene has been considered to synthesis the Co₃O₄/N-doped graphene hybrid.Then we combine it with HSAs.The as-fabricated hybrid material exhibits the following advantages:?1?a conductive substrate of N-doped graphene for uniformly dispersing or anchoring Co3O4 and thus enhancing the conductivity and electrocatalytic activity of Co₃O₄;?2?Co₃O₄ suppresses the re-stacking of N-doped graphene nanosheets during electrochemical cycling;and?3?the unique three-dimensional integrated porous structure of Co₃O₄/N-doped graphene hybrid material is beneficial to the efficient ion and electron pathways and also short transport distances.The hybrid material improves the HRD performance without sacrificing the discharge capacity.An ultra-high capacity of 223.1 mAh g^-1 is achieved at a current density of 3000 mA g^-1,which is 3.2 times that of the commercial alloy electrode(68.7 mAh g^-1).The developed hybrid material shows great potential for practical applications in high-power energy storage devices.