【作者】 孙丽美；

【导师】 张密林； 曹殿学；

【作者基本信息】 哈尔滨工程大学 ， 材料学， 2008， 博士

【摘要】 以H₂O₂为阴极氧化剂的燃料电池，已被广泛研究用作水下和空间设备的电源中。H₂O₂作为燃料电池的氧化剂具有很多突出的优点。首先，H₂O₂是液体，储存、运输以及向电池中输送均较方便，不存在严重的安全性问题，而且H₂O₂分解的产物是水和氧气，也不会造成环境污染问题；其次，H₂O₂直接电还原是2个电子反应过程，其直接还原的交换电流密度比氧电还原的大3个数量级；最后，H₂O₂电还原是固-液两相反应，与氧气电还原的固-液-气三相反应相比，两相反应界面区更容易建立且稳定，无需气体扩散层。然而，H₂O₂作为燃料电池氧化剂仍存在两个主要的问题。一是目前的电还原催化剂不仅催化H₂O₂直接电还原，而且同时催化H₂O₂分解，从而导致电池能量密度的下降；二是目前的电催化剂的活性有待提高。因此，研制具有高活性，高选择性的H₂O₂直接电还原催化剂具有极其重要的意义。本论文利用纳米Pd颗粒修饰的Au旋转圆盘电极，通过强制对流条件下的线性电势扫描伏安法，研究了酸性介质中H₂O₂在纳米Pd催化剂上的电还原反应。TEM和XRD表征表明，所用的纳米Pd催化剂呈球形，结晶程度好，粒径分布窄，平均粒径为20 nm左右。动力学研究结果表明，H₂O₂在纳米Pd上电还原反应为2电子转移过程，相对于H₂O₂为一级反应，相对于H⁺为零级反应，其反应的表观活化能为27．6 kJ／mol。纳米Pd对H₂O₂电化学还原反应的催化性能随电解质阴离子吸附力的增强而减弱。酸性介质中，纳米Ru催化剂对H₂O₂电还原反应表现了一定的催化活性。TEM和XRD表征表明，纳米Ru催化剂呈球形，结晶程度不好，表面存在无定型Ru及其氧化物。其粒径分布窄，平均粒径为10 nm左右。H₂O₂在其上的极限催化电流密度较Pd催化剂上低20 mA/cm²，而起始还原电势却高0．35 V。纳米Ru对H₂O₂电化学还原反应的催化性能不随电解质阴离子吸附力的大小而规律的变化。纳米Ru催化剂表面的氧化物越多，H₂O₂在其上的电化学还原反应的极限电流越低，起始还原电势越高。利用调节pH值的化学还原法制备了Pd／C（20 wt．％）催化剂，XRD和电化学表征表明，该Pd／C催化剂的平均粒径较小，约为8．9 nm，线性伏安测试表明其对H₂O₂电还原反应有很好的催化性能。同时考察了不同操作条件对Pd／C催化剂催化活性的影响。碳载体的前处理和N₂条件下的热处理均能提高催化剂的催化活性。利用相同的制备方法，采用共浸渍方式制备了不同原子比例的碳载Pd-Ru催化剂。发现最佳的Pd、Ru比例为1：1，即PdRu／C催化剂。其粒径分布窄，平均粒径为7．5 nm。线性伏安测试表明该催化剂对H₂O₂电还原反应的催化活性高于Pd／C催化剂，这与其较大的电化学表面积和较高的合金化程度有关。Mg-H₂O₂半燃料电池的性能测试表明，以PdRu／C为阴极催化剂的电池性能高于以Pd／C为阴极催化剂的电池。当阳极电解液为40 g／L NaCl溶液，阴极电解液为0．4 mol／L H₂O₂+0．1 mol／L H₂SO₄+40 g/L NaCl混合溶液，电解液流速为50 mL／min，工作温度为25℃，以PdRu／C为阴极催化剂的电池的开路电压为2．2 V，当工作电流为75 ma／cm2时，电池达到最高功率密度为105 mW／cm²；而以Pd／C为阴极催化剂的电池的开路电压约为2．0 V，工作电流为70 mA／cm²时，电池达到最高功率密度仅为80 mW／cm²。电池的恒流放电曲线表明，以PdRu／C为阴极催化剂的电池的稳定性优于以Pd／C为阴极催化剂的电池更多还原

【Abstract】 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.更多还原