【作者】 季孟波；

【导师】 魏子栋；

【作者基本信息】 重庆大学 ， 物理化学， 2006， 硕士

【摘要】 质子交换膜燃料电池正极生成水如不及时排除，将淹没电极催化层，阻止氧气进入催化层发生还原反应；取而代之的是H⁺得到电子还原为氢气，由于后者的还原电位比前者低1.23伏，使得正极电位急剧下降，从而产生所谓的“负差效应”。本论文在传统的Pt／C电极中加入一富氧油相——二甲基硅油，从而制备出了一种新型的抗溺水性气体多孔电极。当电极被水淹时，二甲基硅油优异的富氧能力能够拓宽氧的传输通道，从而可为催化层持续供氧以达到消除负差效应的目的。论文首先通过抽滤法制备出了这种抗溺水性气体多孔电极，然后利用电化学测试技术分析了该种电极的电化学性能，并与传统的Pt／C电极进行了对比。实验发现：当电极浸泡在持续供氧的0.5mol L^-1H₂SO₄的电解质溶液中时，抗溺水性多孔电极由于具有优异的动态储氧性能，可为氧还原最大限度的供氧，从而增强了Pt／C电极的抗水淹性能；而在外界不供氧的体系中，抗溺水性气体多孔电极的抗水淹性能也要稍优于传统的Pt／C电极。二甲基硅油的主要优势在于大大拓宽了氧的传质通道，而非它的绝对储氧性能。电极中硅油的含量对于抗溺水性气体多孔电极的氧还原能力会产生较大影响，当电极中oil：Pt=2：1（质量比）时电极的氧还原效果最佳。抗溺水性气体多孔电极的交流阻抗谱检测以及等效电路的解析表明电极的电化学反应主要受氧的扩散所控制：电极在通氧情况下，所有电极在中频区和低频区分别出现了法拉第阻抗和感抗，有硅油的电极在相同极化电位下Z_F要小于没有硅油的；当外界不供氧时，所有电极在中频区和低频区都出现了电容性半圆。随着极化电位逐渐负移，含有硅油的电极的法拉第阻抗逐渐趋近于没有硅油的，而低频区的阻抗却与之有明显的差异；前者与电极反应受电化学极化控制有关，后者可能与硅油对氧中间态吸附物种的相互作用相关。电极微孔中氧浓度充足与否是电极在低频区出现电感还是电容的原因。等效电路模拟结果为：外界有氧供给时是R₁（Q₁[R₂W₁]）（[R₃L₁]Q₂R₄），外界不持续供氧时为R₁（Q₁[R₂W₁]）（Q₂[（R₃Q₃）R₄]）。氮吸附BET比表面分析发现含有硅油的电极的总孔隙体积较小，BJH孔径分布表明硅油主要分布在电极的10nm以下的微孔中。UCE的透气性实验发现，含有硅油的电极氧透过性不如无硅油的通畅。由于含硅油的电极的透气性较差，抗溺水性气体扩散电极中硅油的含量不宜太高，以减小这种不利影响。更多还原

【Abstract】 Water is always produced at the cathode of polymer electrolyte membrane fuel cells （PEMFC）. If water was not removed timely; it would accumulate at the cathode of PEMFC and further flood the cathode. With water accumulation and the cathode flooded, oxygen starvation, and even oxygen depletion would occur at the cathode. In this case, protons H⁺ reduction reaction （PRR） carries on at the cathode of PEMFC rather than oxygen reduction reaction （ORR）. The potential of PRR is 1.23 V less than that of ORR; it would cause a remarkable decline of cathode potential as ORR was replaced by PRR. The output voltage of a single cell with oxygen starvation would be likely reversed. This phenomenon was defined as voltage reversal effect in this paper.Aiming at solving the so-called voltage reversal effect, an oil phase full of abundant oxygen was added into gas porous electrode with Pt/C catalyst to ensure supplying enough oxygen for ORR in the case of oxygen starvation. Therefore, the voltage reversal effect caused by PRR could be avoided at the cathode.The anti-flooding porous electrode was obtained through adding dimethyl-silicon-oil into the catalyst layer, and checked by electrochemical methods. The conventional Pt/C electrode was taken as a baseline for comparison. The following points have been concluded in the case of porous electrode flooded: When the electrode was dipped in an O₂ saturated 0.5 mol L^-1H₂SO₄ electrolyte with supplying oxygen continuously during experiment, the oil in the anti-flooding electrode can offer a reliable oxygen diffusion channel for the ORR because of its excellent capability of oxygen storage. As a result, the oxygen for the ORR on the Pt/C with silicon oil was guaranteed even in the case of the electrode flooded. While in the case of oxygen starvation, the tolerance of the anti-flooding electrode was a bit better than that of the conventional Pt/C electrode. The principal advantage of the dimethyl-silicon-oil was in that it can guarantee the channels for oxygen diffusion instead of its capacity for oxygen storage. The proportion of silicone in the anti-flooding electrode is critical for the ORR. When the ratio of the silicon to Pt is 2:1 in the catalyst layer the anti-flooding electrode showed a better performance.The EIS analysis of the anti-flooding electrode disclosed: in the case of continuous oxygen supply, the Nyquist complex plane has a capacitive loop at middle frequencies and an inductive loop at low frequencies, respectively. The former was ascribed to Faraday resistance （Z_F）, the latter to the relaxation and diffusion of adsorbed oxygenated species. At the same overpotential, the Z_F of the anti-flooding electrode was smaller than that of the conventional electrode. When there was no oxygen offer, on the ORR impedance spectra appeared two capacitive loops at middle frequencies and at low frequencies, respectively, which was different from the case of continuous oxygen offer as above-mentioned. With overpotential increase the Z_F of the anti-flooding electrode was close to that of the conventional Pt/C electrode gradually, but the semicircle at low frequencies had obvious difference between the two electrodes. The conclusion was that limited oxygen through narrow pores gave an inductive arc instead of a capacitive arc. The equivalent circuit was like to R₁（Q₁[R₂W₁]）（[R₃L₁]Q₂R₄） and R₁（Q₁[R₂W₁]）（Q₂[（R₃Q₃）R₄]） in the case of continuous oxygen offer and no oxygen off respectively.The analysis of BET surface area disclosed that the whole pore volume of the anti-flooding electrode was smaller than that of the conventional electrode. According to the BJH pore diameter distribution, the silicone oil was mainly distributed in those pores whose pore diameter was less than 10 nm. The test of oxygen permeating showed that oxygen diffusion is a bit more difficult in the anti-flooding electrode than that in the conventional electrode. Because of this, the content of silicone in the anti-flooding electrode should be controlled within a proper loading.Above these experiments confirmed that the anti-flooding electrode could not only restrict the so-caller "voltage reversal effects" in the case of oxygen starvation, but also enhance the ORR in the case of rich oxygen.更多还原