Synthesis And Application Of Nickel/Cobalt Sulfides/Oxides For Energy Conversion

Energy,environmental and ecological problem are the most serious problems in human civilization and the economy.To improve the living environment and restore the natural ecological balance is an inevitable requirement for the sustainable development of human beings in the future.Electrocatalytic reactions,such as water splitting,hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,oxygen reduction reaction（ORR）,metal-air batteries and so on,have been widely researched by many workers for their promising application in sustainable development,which can be attributed to their high efficiency,clear and environmental friendly.Howeverr,their performance are seriously decreased for limited electrocatalysts with poor activity or stability.Transition metal sulfides（TMS）and oxides（TMO）are very famous compounds in our life,while they show electrocatalytic activity for those electrocatalytic reactions,but the performance is not very good for application.Recently,studys demonstrate that interface structure has a great function for enhancing the catalytic performance,while optimizing the electronic structure also can increase the electrocatlytic performance.Therefore,we developed some works and fabricated several catalysts with unique structure and high electrocatalytic activity.The works are as follows:（1）we have synthesized transition-metal（Cu,Fe,Co）doped ultrathin NiS₂ nanosheets,in which Co-NiS₂ NSs with optimized e_g¹ electron configuration show prominent HER performance,such as an overpotential of 80 mV at current density of 10 mA cm^-2 and long-term stability（90 h）in alkaline HER electrocatalysis.Notablely,at an overpotential of 100 and 200 mV,Co-NiS₂ NSs give TOFs of 0.55 and 4.1 s^-1 for HER better than many previously reported noble-metal-free HER catalysts.Meanwhile,DFT calculations reveal that the surface Co dopants induce longranged sensitization on upshifting Ni-3d bands high lying near the EF,paving a nearly barrier-free electron-transfer for HER under alkaline conditions.（2）Using in situ electrochemical reaction strategy,we fabricated surface oxygen vacancies dominated cobalt–nickel sulfide interface porous nanowires（NiS₂/CoS₂–O NWs）,which can be used as the air–cathode for fabricating high-performance rechargeable Zn–air batteries with a very high open-circuit voltage of 1.49 V.NiS₂/CoS₂–O NWs fabricated Zn–air batteries show stable rechargeable performance more than 30 h at the current densities of 3 and 5 mA cm^-2.In particularly,we carefully designed the self-driven water splitting device using NiS₂/CoS₂–O NWs as anode and cathode,powered by the two-series-connected NiS₂/CoS₂–O NWs fabricated portable Zn–air batteries,which shows advanced concept of efficient energy conversion from chemical energy to electric energy and clear fuel gas（H₂）.（3）We prepared N and Cu co-doped CoS_1.97 nanowires(N-CuCoS_1.97 NWs)with boosting activity for OER and Zn–air battery,which can be attributed to intrinsic metallic and abundant defects,while construction of lattice oxygen in N-CuCoS_1.97 NWs can further improve its activity and stability for OER.DFT results also evident that lattice oxygen constructed surface of N-CuCoS_1.97 NWs has more favorable OER energetic profiles and absorption for reaction intermediate.More importantly,N-CuCoS_1.97 NWs fabricated Zn-air batteries not only show excellent rechargeable and mechanical stability,but also display an amazing performance in flexible and wearable devices,which enriched the commercial prospect of Zn-air battery.（4）We synthesized hybride structure based on NiCo₂O₄ nanowires,and then coated a thin Co_0.57Ni_0.43 layer oxide on the surface obtained NiCo₂O₄/Co_0.57Ni_0.43LMOs interface structure,as a highly efficient bifunctional catalyst for both OER and HER.The overall water splitting performance of NiCo₂O₄/Co_0.57Ni_0.43LMOs is the better among all the reported nanowire array electrodes,while the excellent water splitting performance can be attributed to the porous nanowire array electrode,oxygen vacancies and interface structure,leading to strong structural stability,and improved mass/charge transport.Therefore,NiCo₂O₄/Co_0.57Ni_0.43LMOs can serve as a promising noble-metal-free catalyst as both anode and cathode for splitting water into H₂ and O₂,which have promising potential application in overall water splitting.（5）Combined with NiO and CoN compound,we synthesized porous NiO/CoN interface NWs with both oxygen vacancies and the strongly interconnected nanointerface,as efficient air-cathode for Zn-air batteries.The NiO/CoN PINWs fabricated Zn-air batteries with high open-circuit potential（1.46 V）,high energy density(945 Wh kg^-1),high power density(79.6 mW cm^-2)and a high stability（500 min）,which can be attributed to their porous NW structure,the synergistic effect of the high oxidation capability for cobalt and oxygen vacancies,and the strongly interconnected nanointerface between NiO and CoN domains.Moreover,the solid rechargeable Zn-air batteries with the NiO/CoN PINWs air-cathode also exhibit a higher open-circuit potential of 1.335 V and an excellent cycling stability.The three-series solid batteries can drive a timer to work continuously for more than 12 h.（6）We have prepared the hybrid Ni-C-N NSs that are<2 nm thin and chemically stable.The Ni-C-N NSs show the metallic characters and can serve as a new kind of robust catalyst for the HER in 0.5 M H₂SO₄,or 1.0 M KOH,or 1.0 M PBS（pH=7）.The catalytic behavior of the Ni-C-N NSs is similar to that of the common Pt catalyst and it can work well in all pH values for more than 70 h without obvious current drop.Our work demonstrates that it is indeed possible to hybridize NiC with Ni₃N to enhance HER activity and stability to a level that is comparable to Pt.Judging from the overall catalytic performance,we can conclude that our Ni-C-N NS catalyst is one of the most active non-Pt catalysts ever reported.