High porosity silica xerogels prepared by a particulate sol gel route: pore structure and proton conductivity
High porosity silica xerogels prepared by a particulate sol gel route: pore structure and proton conductivity
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DOI：
10.1016/s0022-3093(01)00815-8
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发表时间：
2001-09
影响因子：
3.5
通讯作者：
M. T. Colomer;M. Anderson
中科院分区：
文献类型：
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作者：
M. T. Colomer;M. Anderson
二氧化硅干凝胶较低的成本和较高的亲水性使其成为质子交换膜燃料电池（PEMFC）中全氟磺酸聚合物膜的潜在替代品。为此，我们需要获得具有高孔隙率的微孔或微孔和介孔二氧化硅干凝胶。与使用原硅酸四乙酯（TEOS）作为前体的二氧化硅聚合物悬浮液相反，使用由颗粒制备微米（<2 nm）和微孔和介孔二氧化硅干凝胶（2<d孔径≤10 nm）。这项工作采用了两种不同堆积密度的技术：（1）通过调节胶凝前的 pH 值（pH 5、6 和 8）来控制溶胶中的聚集程度，以及（2）具有不同平均粒径的溶胶混合物（在酸和碱催化反应下形成的颗粒）。研究了所得干凝胶的质子电导率与温度和相对湿度（RH）的函数关系。在250~550℃的煅烧温度范围内实现了高孔容、高孔隙率和小孔径的SiO2干凝胶。煅烧后的干凝胶在整个煅烧温度范围内均表现出微孔性或微孔性和中孔性。通过混合溶胶（摩尔比：酸/碱=4.8），在 250 °C 的烧成温度下，孔隙率可达到 54.7±0.1%。根据 EMF 测量，电传输是由此类材料中的质子引起的。所研究的干凝胶的质子电导率随测量温度线性增加。观察到电导率与相对湿度的 S 形依赖性，在 58% 至 81% 相对湿度之间出现最大增加。在整个温度和相对湿度范围内，具有低孔隙率（40.8±0.1%）和平均孔径小于2.0 nm的干凝胶表现出比具有较高孔隙率和较高平均孔径的干凝胶更低的质子电导率值。当具有最高电导率的二氧化硅干凝胶在 pH 1.5 下处理时，在 81% RH 和 80 °C 下，该性能从 2.84×10−3±5.11×10−5S/cm 增加到 4.0×10−3±7.2×10−5S/cm。这表明这些材料的表面位点密度对电导率有很大影响。在相同的温度和相对湿度条件下，所实现的质子电导率值比 Nafion 低不到一个数量级。
The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). For that purpose, we need to obtain micro or micro and mesoporous silica xerogels with a high porosity. The preparation of micro (<2 nm) and micro and mesoporous silica xerogels (2<dpore size⩽10 nm ) from particulate as oppossed to polymeric suspensions of silica using tetraethyl orthosilicate (TEOS) as precursor is used. Two techniques of varying packing density have been performed in this work: (1) Control of the aggregation degree in the sol by adjusting its pH before gelation (pH 5, 6 and 8) and (2) Mixture of sols with a different average particle size (particles formed under acid and base catalyzed reactions). Proton conductivity of the obtained xerogels was studied as a function of temperature and relative humidity (RH). High pore volume, high porosity and small pore size SiO2xerogels have been achieved in the calcination temperature range from 250 to 550 °C. The calcined xerogels showed microporosity or micro and mesoporosity in the whole range of calcination temperatures. By mixing sols (molar ratio: acid/base=4.8) porosities up to 54.7±0.1% are achieved, at 250 °C of firing temperature. According to EMF measurements, electrical transport is due to protons in this kind of materials. The proton conductivity of the studied xerogels increased linear with measured temperature. A S-shaped dependence of the conductivity with the RH was observed with the greatest increase noted between 58% and 81% RH. Xerogels with a low porosity (40.8±0.1%) and an average pore size less than 2.0 nm showed lower values of proton conductivity than that of xerogels with a higher porosity and a higher average pore size in the whole range of temperature and RH. When silica xerogels, with the highest conductivity, are treated at pH 1.5, that property increased from 2.84×10−3±5.11×10−5S/cm to 4.0×10−3±7.2×10−5S/cm, at 81% RH and 80 °C. It indicates that the surface site density of these materials has a strong effect on conductivity. Proton conductivity values achieved are less than one order of magnitude lower than that of Nafion, under the same conditions of temperature and RH.