Preparation And Properties Of H₂O₂ Electroreduction Catalysts

Hydrogen peroxide?H₂O₂?has replaced oxygen and been regarded as a liquid oxidant for fuel cell because of its high volume energy density,quick reaction kinetics,easy storage and transport.By far,H₂O₂ has been applied for metal semi-fuel cell,direct NaBH₄-H₂O₂ fuel cell,direct H₂O₂-H₂O₂ fuel cell,direct methanol-H₂O₂ fuel cell,and so on.The electroreduction of H2O2 is a two-electron process,which possesses lower reaction activation energy and higher cathode potential than the electroreduction of O₂ via a four-electron process.Besides,liquid H₂O₂ doesn't need to be stored in high pressure vessel,which can therefore reduce the system weight and simplify the structure of the fuel cell.However,there are also several problems for using H₂O₂ as oxidant.Firstly,the recent catalysts can not only catalyze the direct reduction of H₂O₂,but also possess catalytic activity toward H₂O₂ decomposition,leading to the decrease of energy density.Secondly,the preparation technology of traditional catalysts is complex and high cost.Thirdly,the catalytic activities of traditional catalysts also need to be improved to meet the actual application requirements.Thus,it is highly significant to prepare catalysts with simple preparation technology,low cost,high catalytic activity and high selectivity.In the first part of this work,chemical vapor deposition method is applied to obtain C@TiO₂ nanowire-arrays on Ti plate with three-dimentsional?3D?structure and then Pt is electrodeposited on the C@TiO₂ nanowire-arrays to build Pt modified C@TiO₂nanowire-arrays?Pt/C@TiO₂?electrode.The formed Pt exists in a form of nanoparticle and the size of the nanoparticle is around 23.4 nm.Besides,the loading mass of Pt is very low,which is only 0.0631 mg cm^-2.In alkaline medium,the current density of 450 mA cm^-2 for H₂O₂ reduction on Pt/C@TiO₂ electrode is obtained due to the unique structure,which shows obvious superiority than commercial Pt/C electrode.In the second part,to investigate the effect of alkaline and acidic medium on the performance of the catalysts,we prepare the Au modified C@TiO₂ nanowire-arrays electrode?Au/C@TiO2?by electrodeposition and then investigate their catalytic activity in alkaline and acidic media.Results show that the current density on the Au/C@TiO₂ electrode is only 30mA cm^-2 in alkaline medium,while in acidic medium the current density reaches up to 300mA cm^-2,demonstrating that the as-prepared exhibits better electrochemical performance in the acidic medium.Meanwhile,in the acidic medium,the open circuit potential of the Au/C@TiO2 electrode is more positive than that in alkaline medium,which is very beneficial to increase the working voltage of the whole fuel cell.In the third part,in order to reduce the cost of catalysts,MnO₂ as one of the transition metal oxides with low cost and good catalytic activity toward H2O2 reduction is chosen as an active components,which is electrodeposited on the first-prepared C@TiO2 nanowire-arrays to obtain MnO2 modified C@TiO2 nanowire-arrays?MnO2/C@TiO2?electrode.The deposited MnO₂ shows a nanosheet-like structure,which can increase the surface area of the electrode to provide more reaction active sites.In alkaline medium,the current density of 275 mA cm^-2for H2O2 reduction on the MnO2/C@TiO2 electrode is obtained,which can be attributed to the3D structure and the good ability of MnO₂ to break O-O bond of O₂.In order to simplify the preparation process of the catalyst and reduce the cost of the electrode,8B pencil and A4 paper are used to prepare novel graphite-modified A4 paper substrate to support NiAg double metal catalysts.Ni layer is formed on the surface of 8B-A4substrate by electrodeposition and Ag is coated on the Ni layer by galvanic replacement.According to the electrochemical tests,the NiAg@C/A electrode exhibits excellent catalytic activity for H2O2 electroreduction.It can be seen that the current density on the NiAg@C/A electrode is 640 mA cm^-2 at 70?in the electrolyte consisted of 0.6 M H₂O₂ and 1 M NaOH.Furthermore,NiPt@C/A electrode is also synthesized by displacing the surface Ni with metal Pt to study the catalytic activity of double metal catalysts for H2O2 reduction.In the H2O2solution with low concentration,the current density of 375 mA cm^-2 on the NiPt@C/A electrode is still obtained,revealing the good performance of NiPt@C/A electrode.