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Study On Nano-Palladium And Palladium-Ruthenium As Catalysts For H2O2 Electroreduction

Posted on:2009-01-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:L M SunFull Text:PDF
GTID:1101360275977262Subject:Materials science
Abstract/Summary:PDF Full Text Request
Fuel cells using hydrogen peroxide as oxidant have been studied as underwateror space power sources recently. Hydrogen peroxide has several advantages asfuel cell cathode oxidants. Firstly, hydrogen peroxide is liquid, much denser thana gas phase oxidant, such as oxygen. Its handling, storage and controllable feedingto a fuel cell are easy. Secondly, the two-electron direct reduction of hydrogenperoxide has a lower activation barrier and thus a faster kinetics than thefour-electron reduction of oxygen. Thirdly, the electroreduction of liquidhydrogen peroxide at the cathode of a fuel cell occurs in a solid/liquid two-phasereaction zone, while oxygen electroreduction requires a solid/liquid/gasthree-phase region. The two-phase reaction zone is readily realizable and muchsteady during fuel cell operation than the three-phase region. However, there aretwo major problems for hydrogen peroxide as fuel cell cathode oxidant. One isthat the current electrocatalysts not only catalyze the direct electroreduction ofhydrogen peroxide, but also catalyze its chemical decomposition, resulting in thereduction of energy density of fuel cell. The other one is that the activity ofcurrent electrocatalysts remains to be enhanced. Therefore, the development ofelectrocatalysts with high activity and selectivity for the direct reduction ofhydrogen peroxide is necessary.Electrocatalytic reduction of H2O2 on Pd nanoparticles in acidic medium wasinvestigated by linear potential sweep method using an Au rotating disk electrodecovered with Pd nanoparticles. TEM and XRD analysis indicated that the Pdnanoparticles were spheres with mean particle size of around 20 nm and have highcrystallinity. The kinetics study shows that H2O2 reduction on Pd nanoparticles isfirst order with respect to H2O2 and the zero order with respect to proton. Theapparent activation energy for electroreduction of H2O2 on Pd nanoparticles was determined to be around 27.6 kJ/mol. The reaction proceeds via a two electronprocess. Electrolyte anions significantly affect hydrogen peroxide reductionactivity, and the activity decreases in the order ClO4- > HSO4- > Cl-, which isconsistent with the increasing adsorption bond strength of the anions.The catalytic activity of Ru nanoparticles for H2O2 electroreduction in acidicmedium was investigated. XRD and TEM measurements showed that the meanparticle size of Ru is around 10 nm and Ru oxides existed in Ru nanoparticles.The limiting current density of H2O2 reduction on Ru nanoparticles was muchsmaller than that on Pd nanoparticles. However, the onset potential of H2O2reduction on Ru nanoparticles was 0.35 V higher than that on Pd nanoparticles.The catalytic behavior of Ru nanoparticles was independent with the increasingadsorption bond strength of the anions. The existence of oxides on the surface ofRu nanoparticle lowered the catalytic current density but increased the onsetpotential.A Pd/C (20wt.%) electrocatalyst was prepared by a adjusted pH chemicalreduction method. XRD and electrochemical characterization (CV) showed thatthe particle size of Pd/C was about 8.9 nm. And it has enhanced activity for H2O2reduction reaction. Effects of various conditions for preparation of Pd/C oncatalytic activity were investigated. The pretreatment to the C support and thethermal treatment of Pd/C in N2 enhanced the catalytic activity of Pd/C. Pd-Rusupported on XC-72 with different Pd-Ru ratio were prepared by adjusted pHchemical reduction method. The optimal Pd-Ru ratio was 1:1. XRD, TEM andelectrochemical characterization (CV) showed that homogeneous PdRu particleswith a mean particle size of 7.5 nm were deposited on carbon support. Theoptimal PdRu/C catalyst showed higher H2O2 reduction activity than Pd/C in RDEtests, which may be attributed to the larger surface area and the formation ofPd-Ru alloy. Mg-H2O2 semi fuel cells with PdRu/C as cathode catalyst showed higherperformance than that using Pd/C as cathode catalyst. A peak power density of105 mW/cm2 at the cell current density of 75 mA/cm2 was obtained for the cellusing PdRu/C as cathode catalyst operating at 25℃. The anode fuel and cathodeoxidizer was 40 g/L NaCl, 0.4 mol/L H2O2+0.1 mol/L H2SO4+40 g/L NaCl,respectively. The electrolyte flow rate is 50 mL/min. A peak power density of 80mW/cm2 at the current density of 70 mA/cm2 was obtained for the cell using Pd/Cas cathode catalyst at the same operating conditions. The open circuit voltage ofthe cell using PdRu/C as cathode catalyst is 0.2 V higher than that using Pd/C ascathode catalyst. Constant current discharge at 50 mA/cm2 tests indicated that cellwith PdRu/C cathode catalyst exhibited higher stability than that with Pd/Ccathode.
Keywords/Search Tags:nano-Palladium, nano-Ruthenium, H2O2 electroreduction, kinetics, Mg-H2O2 semi fuel cell
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