可用于燃料电池的掺杂型二氧化铈：酸中溶解度低、过氧分解速度快、自由基选择性适中
Doped Ceria Nanoparticles with Reduced Solubility and Improved Peroxide Decomposition Activity for PEM Fuel Cells

Andrew M． Baker
S． Michael Stewart
Kannan P． Ramaiyan
Dustin Banham
Siyu Ye 叶思宇
Fernando Garzon
Rangachary Mukundan
Rod L． Borup

Abstract

      Ceria nanoparticles （NPs） have unique catalytic properties which make them suited to scavenge degrading radical species and their precursor peroxides during PEM fuel cell operation． However， in the acidic environment of the fuel cell， ceria dissolves and the resulting cations migrate within the MEA， causing performance and durability losses． In this work， ex situ testing was used to evaluate the peroxide decomposition， selectivity towards radical generation， and solubility of Gd， Pr， and Zr－doped ceria NPs over a range of crystallite sizes and dopant levels． These doped materials exhibit better peroxide scavenging activity and dissolution resistance than undoped ceria． In these materials， activity is largely governed by increased surface area due to high internal porosity at smaller crystallite sizes compared to undoped ceria． Of the compounds tested， ceria NPs doped with 15 at％ Zr （10 nm） and 5 at％ Pr （17 nm） exhibited greater dissolution resistance than undoped ceria． Stabilization of the former doped NPs is attributed to crystallite agglomeration， while the increased stability of the latter is proposed to be due to its internally－porous， mesoscale structure suggested by its sorption isotherm． Both materials are more dissolution－resistant and active peroxide decomposers compared to undoped ceria but exhibit increased byproduct radical generation．

Figure 1． （a） UV–vis spectra for calibration standards of various Ce3＋／Ce4＋ concentrations and （b） the corresponding calibration curves．

近紫外波段，大于320nm是逐渐进入可见光波长，小于250nm是远紫外光区。这个方法用于评价粒子在硫酸溶液中的溶解速度。an average deviation of ±10％

Figure 2． Peroxide decomposition activity of （a） undoped ceria and （b） CGO， （c） CPO， and （d） CZO with 5 and 15 at％ dopants as a function of crystallite size obtained from XRD．

Ce1−xGdxO2−δ （CGO）、Ce1−xPrxO2−δ （CPO）、Ce1−xZrxO2−δ（CZO）

共沉淀法制备纳米颗粒

根据上图的机理，6－羧基荧光素造成的荧光越弱，说明自由基含量越低，自由基猝灭剂的活性越强。

CeO2粒子较大时活性较强，而其他掺杂型CeO2在某个粒子范围内具有较强的猝灭活性。

Figure 4． BET surface area as a function of XRD crystallite size for selected undoped and doped ceria NPs plotted from the data in Table I． The dotted line is the ideal surface area （SAideal） calculated using Eq． 7．

Figure 5． Dissolution rate as a function of （a） crystallite size and （b） BET
surface area for the various doped and undoped ceria samples after stirring for 72 h 1 M H2SO4 at 50 °C．

Figure 6． Pore volume vs surface area for the various doped and undoped ceria samples plotted from the data in Table I． The dashed line is linear fit to all samples aside from the 17 nm CPO－5 and the commercial ceria．

Figure 7． N2 sorption isotherms for （a） undoped ceria， （b） CPO－5， （c） CZO－15， and （d） commercial ceria．

Figure 8． Peroxide reactivity and radical generation rates shown as a
function of dissolution rate for the most stable NPs in liquid．

文中给出了商品化Commercial ceria nanopowder （Sigma Aldrich）， with a specified crystallite size of ＜50 nm，实测是30nm，55－60m2／g， 溶解速度60mg／hr，BJH孔容0．12cm3／g（图6坐标单位错了），很遗憾没有和反应相关的数据。和商品化催化剂比较是很令人兴奋的事情。

Conclusions

In this work， Gd， Pr， and Zr－doped ceria NPs were synthesized in
various compositions and sizes and evaluated against undoped ceria
for their suitability as PEM fuel cell additives． Their peroxide
decomposition activity and radical selectivity were measured in ex
situ experiments． The most active and selective doping compositions，
5 at％ Pr and 15 at％ Zr were selected for measurement of
morphological properties and solubility． All the samples showed
consistent exponentially－increasing solubility with decreasing crystallite size． However， when assessed vs surface area， certain
compositions were distinct， favorable outliers， implicating unique
stabilization mechanisms． Pore volume per surface area is significantly
enhanced in these samples， which corresponds to a proposed
mesopore structure without pore condensation， according to BET．
This structure could be more dissolution－resistant， while high
catalytic activity is maintained through its high surface area due to
internal porosity． Agglomeration of NPs is also proposed as a
stabilization mechanism， however， it is not clear how break－up of
agglomerates during catalyst layer ink preparation would impact
their resulting scavenging abilities and solubilities．

Of the most stable compounds， the 10 nm 15 at％ Zr－doped and
17 nm 5 at％ Pr－doped ceria samples showed the most optimal
performance in terms of stability and activity， while exhibiting
slightly higher byproduct radical generation compared to undoped
ceria． In the liquid test conditions employed here， however， typical
concentrations in fuel cells would dissolve ∼1–2 orders of magnitude faster than fuel cell lifetime targets． This motivates the analysis
of radical scavenging NPs in MEAs operated under realistic fuel cell
conditions， to assess their durability enhancement effect and
solubility．

要读燃料电池文献读了半天竟然是篇催化文献。但是也学到了很多东西，荧光实验从来没有听说过。

溶解实验设计的很务实，总不能纳米粒子溶解后去称量纳米粒子剩多少吧。

要说疑问还是有：

纳米粒子溶解后在SEM下会出现粒子体积变小么？

是选择性晶面溶解么？

掺杂的科学机理如果有DFT的佐证可能会更有意思。

这些材料在膜材料中的应用阴性结果和阳性结果这些放在一起会是一篇更加成功的文章。

不理解燃料电池膜和MPL中的铈的迁移是以直接溶解的方式进行的么？燃料电池微孔层中添加氧化铈纳米粒子对COCV工况耐受性的影响 附论文指瑕说是（整个膜电极内部浓度均一化了），而丰田MIRAI 20万公里实车耐久测试后膜电极中铈的空间分布、价态原位分析似乎说明不是（浓度没有均一化，电极内部仍然存在很大的铈梯度）。