Studies On Direct Borohydride-hydrogen Peroxide Fuel Cell Anodic Nanocatalysts

Direct borohydride-hydrogen peroxide fuel cell （DBHFC） has been extensivelystudied for many advantages, such as high fuel efficiency, easily to transport andconveniently to storage, no catalytic poisoning, and environmentally friendly.Moreover, DBHFC has high theoretical open circuit voltage （3.01V）, high theoreticalcapacity （5.67Ah g-1） and energy convert efficiency （91%）. The anode electrocatalystis the key component in DBHFC. In present, almost all DBHFCs use Pt, Au and Pd astheir anode electrocatalysts. However, the high cost of the noble metals inhibits thecommercialization of DBHFC.In this thesis, a series of bimetallic nanocatalysts and hollow nanospherecatalysts were synthesized. Through studying the chemical compositions, structures,physical and electrochemical performance of these synthesized electrocatalysts, weare going to seek a way to not only enhance the catalytic activity of Pt, Au and Pdbased anode electrocatalysts, but also decrease their cost. The main contents of thisthesis contain four parts:（1） The Pt/C nanocatalyst was prepared using a NaBH₄reduction method. Cyclicvoltammetry （CV） and linear sweep voltammetry （LSV） were used to study theeffects of BH₄-concentration and OH-concentration on the electrochemical propertiesof BH₄-electrooxidation on Pt/C electrode. It showed that both BH₄-and OH-concentrations influenced the results of electrochemical characterization significantly,and the optimized operations were0.1M NaBH₄+3M NaOH solution.The influence of operation conditions on DBHFC performance were studied indetail and the results showed that the operation conditions had great influences on theperformances of DBHFC. The optimized catalyst loading was4.5mg cm^-2and thebest content of Nafion on the anode was7wt.%. Combined with the most applicationsurroundings of DBHFC, the optimized anolyte was1.0M NaBH₄+3M NaOHsolution, and the optimized catholyte was2M H₂O₂+0.5M H₂SO₄at ambienttemperature in the DBHFC with Pt/C anode and cathode catalysts system. Short-termstability of DBHFC was tested by monitoring cell voltage change with the workingtime during the galvanostatic operation at20mA cm^-2and25°C, and the resultsshowed that DBHFC presented initial high performance even at low temperature andmaintained a relatively stable performance for a period of about17h with a little decay of cell voltage during the test period.（2） Pt-Cu/C, Au-Cu/C and Pt-Co/C nanocatalysts with various molar ratios wereprepared using the NaBH₄reduction method, and the electrocatalytic activities of theas-prepared catalysts were measured by electrochemical characterization and fuel celltest. It was showed that the bimetallic catalysts exhibited better catalytic performancecompared with the monometallic catalysts, and Pt₅₀Cu₅₀/C, Au₆₇Cu₃₃/C andPt₆₇Co₃₃/C exhibited the highest catalytic activity among Pt-Cu/C, Au-Cu/C andPt-Co/C catalysts, respectively. The maximum power densities for the DBHFCs usingPt₅₀Cu₅₀/C, Au₆₇Cu₃₃/C and Pt₆₇Co₃₃/C as anode at25°C were approximate42.8mWcm^-2,19.9mW cm^-2,71.6mW cm^-2,51.8mW cm^-2and79.7mW cm^-2, respectively.The sequence of catalytic activity of these anode catalysts was Pt₆₇Co₃₃/C＞Pt₅₀Cu₅₀/C＞Au₆₇Cu₃₃/C＞Pt/C＞Au/C. The XPS results showed that the Pt（0）content in the Pt/C catalyst （58.1%） is relatively lower than that in the Pt₆₇Co₃₃/Ccatalyst （61.5%）, which confirmed the electron-withdrawing effect of Pt from theneighbouring transition metal atoms in the bimetallic catalysts and result in theoxide-cleaning action of transition metal addition. The electron effect might be theprimary reason for the higher electrocatalytic activity of the bimetallic catalyststowards the BH₄-electrooxidation.（3） Carbon supported Pt hollow nanosphere catalyst （HN-Pt/C） and carbonsupported Pd hollow nanosphere catalyst （HN-Pd/C） used as the anode electrocatalystfor DBHFC were prepared by employing cobalt nanoparticles as sacrificial templatesat room temperature in aqueous solution. TEM and HR-TEM results showed thatmany nanoparticles assembled together and formed the hollow spherical shell. Thecenters of the nanospheres were brighter than the edges, indicating that thenanosphere was actually a hollow spherical structure. The electrochemically activesurface area （ECSA） of HN-Pt/C and HN-Pd/C were higher than that of carbonsupported Pt solid nanoparticles （SN-Pt/C） and carbon supported Pd solidnanoparticles （SN-Pd/C）, respectively. The HN-Pt/C and HN-Pd/C catalysts showedhigher catalytic activity for BH₄-electrooxidation than that of SN-Pt/C and SN-Pd/C.In addition, the intrinsic catalytic activities of HN-Pd/C and SN-Pd/C catalysts weresimilar. As a result, the much higher electrocatalytic activities of HN-Pt/C andHN-Pd/C catalysts could be attributed to the increase in the ECSA of the hollownanosphere catalysts. The maximum power densities of the DBHFCs using HN-Pt/Cand HN-Pd/C as anode catalysts at20°C were54.4mW cm^-2and48.4mW cm^-2, respectively, compared with the DBHFCs using SN-Pt/C (37.8mW cm^-2) andSN-Pd/C (36.0mW cm^-2) as anode catalysts, the maximum power densities ofDBHFCs with hollow nanosphere catalyst anode increased43.9%and34.4%,respectively.（4） The influence of potential scan rate, BH₄-concentration, and temperature forBH₄-electrooxidation on Pt/C and Pt₆₇Co₃₃/C catalyst electrodes was studied in detail.It was found that the process of BH₄-electrooxidation on both catalysts’ surface wasirreversible with the reaction order of1. The peak current density of BH₄-oxidationincreases with the increase of potential scan rate, BH₄-concentration, and temperature.Some kinetic parameters were obtained by caculation:（a） The total number ofelectrons exchanged n for BH₄-oxidation on Pt/C and Pt₆₇Co₃₃/C electrodes were2.3and2.6, respectively.（b） The transfer coefficient α for BH₄-electrooxidation on Pt/Cand Pt₆₇Co₃₃/C electrodes were0.20and0.24, respectively.（c） The standard rateconstant ksfor BH₄-oxidation on Pt/C and Pt₆₇Co₃₃/C catalysts were0.41cm s-1and10.17cm s-1, respectively.（d） The reaction rate constant kffor BH₄-oxidation on Pt/Cand Pt₆₇Co₃₃/C catalysts were6.82×10-4s-1and1.10×10-3s-1, respectively.（e） Theapparent activation energy Eafor BH₄-oxidation on Pt/C and Pt₆₇Co₃₃/C catalystswere24.54kJ mol-1and19.49kJ mol-1at-0.2V,23.83kJ mol-1and19.69kJ mol-1at-0.1V, respectively, indicating better electrocatalytic activity of Pt₆₇Co₃₃/C for BH₄-electrooxidation.