Synthesis And Electrocatalytic Behavior Of Catalysts In-situ Supported On Current Collector Toward NaBH₄ Oxidation And H₂O₂ Reduction

Direct borohydride-hydrogen peroxide fuel cell(DBHPFC)is a new type of liquid fuel cell directly using sodium borohydride(NaBH₄)solution as anode fuel and hydrogen peroxide(H₂O₂)solution as cathode oxidant.Compared to traditional hydrogen-oxygen fuel cells,oxygen is not essential for DBHPFC as oxidant,making it suitable for use in oxygen-free environments such as underwater and space.As important components of fuel cells,selecting cathode and anode is significant for the cell performance.Noble metal and their alloys are mostly used catalysts and conventional electrode materials are commonly prepared by slurry coating technique.The use of noble metals and binders will not only increase the electrode cost,but the wrapping of binders to the noble metals also reduce the utilization of noble metals and the electrode conductivity,which drives the research of three-dimensional electrode materials without binders.In this work,Ni foam is mainly used as the current collector and facile modifications are applied to increase the specific surface of Ni foam for better supporting effect.Meanwhile,a Ti plate with unique nanorod-array structure is developed as a new-type current collector with excellent stability in acidic solution.Transition metal oxides and noble metals are in-situ prepared on the designed current collector.The catalytic performance of these composite electrodes toward NaBH₄electrooxidation and H₂O₂electroreduction are systematically tested as follows.Homogeneous Co₃O₄nanosheets were prepared on the surface of Ni foam by a facile hydrothermal method.After that,the Co₃O₄nanosheets modified Ni foam(Co₃O₄@Ni foam)electrode was immersed into a solution of NaBH₄for 4 h to obtain the Co/Co₃O₄@Ni foam electrode.In alkaline solution,the Co/Co₃O₄@Ni foam electrode shows better catalytic activity than Co₃O₄@Ni foam electrode toward H₂O₂reduction which can be attributed to the improved conductivity after metal Co was formed on the surface of Co₃O₄.In addition,the unique structure of Co/Co₃O₄was used to support and disperse noble metal Pt.Pt Co/Co₃O₄@Ni foam was synthesized through a galvanic displacement process.The Pt Co/Co₃O₄@Ni foam electrode exhibit good catalytic performance for the oxidation of NaBH₄due to the good dispersion of Pt and the synergy effect of Pt,Co and Co₃O₄.Compared to the Pt directly prepared on Ni foam(Pt@Ni foam)electrode,the current density of NaBH₄oxidation on Pt Co/Co₃O₄@Ni foam electrode increased about 3 times.CuNi alloy with three-dimensional nanoneedle structure was prepared on Ni foam by a designed hydrothermal method for the dispersion of noble metals.Similarly,galvanic displacement was also applied to form Pd on the surface of CuNi nanoneedles and finally obtained the ternary CuNi Pd modified Ni foam(CNP)electrode.By modulating the time of galvanic displacement,a series of CNP electrodes were obtained.According to the electrochemical test results,the CNP electrode with a galvanic displacement time of 15 s exhibits the best performance and the mass percentage of Pd was only 12.1%.Due to the support and disperse effect of CuNi nanoneedles,the CNP electrode shows good catalytic activity and selectivity toward NaBH₄oxidation.The activation energy of NaBH₄oxidation on the CNP was calculated to be 18.42 k J mol^-1and the transfer electron number reached 4.9which mean that NaBH₄can be easily oxidized on the CNP electrode.Due to the unique structure and electrical conductivity,reduced graphene oxide(rGO)was employed to modify Ni foam and a new current collector of rGO modified Ni foam(RN)was obtained by a facile hydrothermal method.On the surface of RN,rGO nanosheets stack with each other forming a honeycomb structure with plenty of channels.The existence of these channels is expected to increase the specific surface area of the current collector.Pd nanoparticles were subsequently deposited on the surface of RN and formed the Pd modified RN(PRN)electrode for H₂O₂electroreduction.The unique structure of RN allows the uniform dispersion of Pd nanoparticles and successfully avoid the agglomeration.Electrochemical test results reveal that the PRN electrode not only possesses larger electrochemical active surface but also better catalytic activity than the electrode prepared by directly depositing Pd nanoparticles on Ni foam(PN).In 2.0 mol L^-1Na OH and 0.5 mol L^-1H₂O₂,the current density on PRN electrode is 1.4 times as that on PN electrode under the same condition.Meanwhile,the supporting effect of RN substrate for transition metal oxides was also investigated.Co₃O₄modified RN(CRN)electrode was also prepared by hydrothermal method due to the catalytic effect of the conversion between Co²⁺and Co³⁺on H₂O₂reduction.The prepared Co₃O₄nanosheets insert into the original honeycomb channel formed by rGO which further increases the specific surface area of the electrode.And more pathways for electron transfer are provided by the cross-linking structure of Co₃O₄nanosheets and rGO nanosheets which efficiently accelerates the reaction of H₂O₂reduciton on the CRN electrode.Compared to Ni foam,the special stability of Ti plate makes it can be used in both alkaline and acidic solution.However,its small specific surface area restricts its direct application as a current collector.In order to break this limitation of Ti plate,Ti nanorod arrays were in-situ prepared on the surface of Ti by an etching treatment.The obtained Ti nanrod-array substrate(Ti NRs)can be used for both NaBH₄oxidation and H₂O₂reduction(in acidic solution).Pd nanoparticles were deposited on Ti NRs and obtained a Pd modified Ti NRs(Pd@Ti NRs)electrode.According to the structural characterization,the deposited Pd failed to form a layer cover the Ti NRs but distributed on the surface of Ti nanorods.The prepared Pd@Ti NRs electrode shows both good catalytic activity and stability toward H₂O₂reduction in acid solution.The higher open circuit potential obtained in acid solution than in alkaline solution will contribute to a higher cell voltage in practice.At the same time,the application of Ti NRs for NaBH₄oxidation was investigated.Au as a noble metal has good catalytic activity and specific selectivity for NaBH₄oxidation.A composite electrode of Au modified Ti NRs(Au@Ti NRs)was also synthesized by the electrodeposition method.The three-dimensional structure of Ti NRs can efficiently avoid the agglomeration of Au nanoparticles and help to expose a large electrochemical active surface area of Au.The activation energy of NaBH₄oxidation on the Au@Ti Ps electrode was only 6.16 k J mol^-1which is much lower than most reported values.