燃料电池中的游离四价铈离子对铂和碳载体的腐蚀影响
Detrimental effect of Ce4+ ion on the Pt/C catalyst in polymer electrolyte membrane fuel cells
Seongmin Yuk
Dong Wook Lee
Kah-Young Song
Sungyu Choi
Dong-Hyun Lee
Gisu Doo
Jonghyun Hyun
Jiyun Kwen
Jun Young Kim
Hee-Tak Kim
Abstract
Ce-based materials have been widely used asa radical scavenger to improve the chemical durability of the membrane inpolymer electrolyte fuel cells; however, any negative effects from Ce ions onpower performance have not been reported as an issue before except for ionomercontamination by Ce3+. Herein, we propose and experimentally demonstratethe detrimental effect of Ce4+ on the Pt/C catalyst. Due to the strongoxidation power of Ce4+, Pt and carbon are subjected to oxidativedecomposition under contact with Ce4+. Based on the electrochemical andspectroscopic analyses of the Pt/C stored in Ce3+ and Ce4+ electrolyte, Pt2+dissolution and carbon corrosion by Ce4+ are verified. As a result, theactive surface area and oxygen reduction reaction activity of Pt/C areconsiderably reduced after storage in the Ce4+ electrolyte. Additionally, atthe membrane electrode assembly level, it is demonstrated that the Ce4+leached out from the CeO2 causes a significant power performance decay alongwith the loss of the active surface area and an increase in the charge transferresistance. The newly-found detrimental effect of Ce4+ on the Pt/C catalystemphasizes the importance of preventing Ce4+ dissolution to improve themembrane durability by Ce-containing radical scavengers.
理论背景:
Ce4+ has a strong oxidation power, represented by its high standard redox potential (Ce4+ + e- → Ce3+) which is higher than 1.5 V vs. reversible hydrogen electrode (RHE)
实验
The stability of the Pt/C under contact with Ce3+ or Ce4+ was assessed by storing the Pt/C rotating disc electrode in 5 mM Ce3+ or Ce4+-containing 0.5 M H2SO4 electrolyte for 20 h and tracking the changes in the electrochemical properties of the electrode with the storage
Scheme 1. Pt/C degradation scenario by regenerative Ce3+/4+ redox reaction
Ce4+和H2O2、∙OOH、Pt、C、H2似乎成了五个竞争反应。
Fig. 1. (a) CV curves recorded in N2-saturated 0.5 M H2SO4 solution and (b) ORR polarization curves in O2 saturated 0.5 M H2SO4 solution before/after immersing the Pt/C in the 5 mM Ce3+ solution. (c) CV curves recorded in N2-saturated 0.5 M H2SO4 solution and (b) ORR polarization curves in O2-saturated 0.5 M H2SO4 solution before/after immersing the Pt/C in the 5 mM Ce4+ solution.
Fig. 2. UV–vis spectra (5 times diluted) of 5 mM Ce4+ solution after immersing (a) Nafion membrane (b) Pt/C for 20 h at 80 C. XPS spectra of the Pt/C after immersing in 0.5 M H2SO4 as a blank solution, 5 mM Ce3+ and 5 mM Ce4+ electrolyte for 20 h at 80C: (c) C1s and (d) Pt4f spectra.
As shown in the UV spectra of the Ce4+ electrolytes (Fig. 2 (a)), the absorbance from Ce4+ was nearly unvaried after the storage with Nafion, indicating that Nafion has a high oxidation stability enough to withstand the oxidation power of Ce4+.
这个推论是不正确的。
Ce4+ + H2O2 → Ce3+ + ∙OOH + H+
Ce4+ + ∙OOH → Ce3++ O2 + H+
这两个反应能发生,Ce4+的氧化性比H2O2、∙OOH强,Nafion能被H2O2、∙OOH攻击降解,不能被Ce4+攻击降解有些说不过去。Nafion有没有降解没有做任何表征,也没有溶液中完全无Ce3+的检测数据和Ce3+的空白样品谱峰。这个对比试验可以说明相比于Nafion,铂和碳动力学上更容易被Ce4+氧化。
Fig. 3. The IV polarization curves recorded at 80C and 100% RH for (a) control-MEA and (b) CeO2-MEA. The EIS curves recorded at 80C, 100% RH and 0.1 A cm-2 for (c) control-MEA and (d) CeO2-MEA. The CV curves for (e) control-MEA and (f) CeO2-MEA.
a MEA containing CeO2 nanoparticles at the interfaces of the membrane and catalyst layer (CeO2-MEA) was prepared and was immersed in hot water (80C) for 7 days to induce the dissolution of Ce4+ from CeO2.
一方面有氢气在论文谈到的这个问题似乎不是什么问题,但是另一方面有氢气在H2O2、∙OOH对膜的破坏依然存在。如果Ce4+这么可怕,丰田MIRAI 20万公里实车耐久测试后膜电极中铈的空间分布、价态原位分析文献报道膜电极中也有高比例的Ce4+。
这篇文章说的问题其实就是在证实理论背景中的那句话。
文章中并未明确是哪种三价铈和四价铈的物种,可能各种物种还有显著差异。
Conclusions
Although Ce-based materials are widely usedas a radical scavenger due to their high radical scavenging activity, only thecontamination effect of Ce3+ on the ionomer has been studied so far, and anynegative effects of Ce4+ on the catalyst have not been reported. In this work,we discovered the oxidative degradation of Pt/C by Ce4+. Under contact with Ce4+ions, the ECSA and ORR activity of the Pt/C were considerably reduced.According to the UV–vis and XPS spectra, Ce4+ in the electrolyte not onlyoxidizes the carbon support but also accelerates Pt2+ dissolution, resulting inthe irreversible degradation of the Pt catalyst. At the single cell level, thedissolution of Ce4+ caused a power performance loss, ECSA loss, and increase inthe charge transfer resistance. Therefore, together with the contamination onthe ionomer phase by Ce3+, the detrimental effect of Ce4+ on the Pt/C has to beconsidered when designing chemically durable MEAs with Ce-containing radical scavengers.