School of Materials Science and Engineering, Nanyang Technological University , 50 Nanyang Avenue, 639798 Singapore.
J Am Chem Soc. 2014 Apr 2;136(13):4954-64. doi: 10.1021/ja411268q. Epub 2014 Mar 25.
We have developed for fuel cells a novel proton exchange membrane (PEM) using inorganic phosphotungstic acid (HPW) as proton carrier and mesoporous silica as matrix (HPW-meso-silica) . The proton conductivity measured by electrochemical impedance spectroscopy is 0.11 S cm(-1) at 90 °C and 100% relative humidity (RH) with a low activation energy of ∼14 kJ mol(-1). In order to determine the energetics associated with proton migration within the HPW-meso-silica PEM and to determine the mechanism of proton hopping, we report density functional theory (DFT) calculations using the generalized gradient approximation (GGA). These DFT calculations revealed that the proton transfer process involves both intramolecular and intermolecular proton transfer pathways. When the adjacent HPWs are close (less than 17.0 Å apart), the calculated activation energy for intramolecular proton transfer within a HPW molecule is higher (29.1-18.8 kJ/mol) than the barrier for intermolecular proton transfer along the hydrogen bond. We find that the overall barrier for proton movement within the HPW-meso-silica membranes is determined by the intramolecular proton transfer pathway, which explains why the proton conductivity remains unchanged when the weight percentage of HPW on meso-silica is above 67 wt %. In contrast, the activation energy of proton transfer on a clean SiO2 (111) surface is computed to be as high as ∼40 kJ mol(-1), confirming the very low proton conductivity on clean silica surfaces observed experimentally.
我们为燃料电池开发了一种新型质子交换膜（PEM），使用无机磷钨酸（HPW）作为质子载体，介孔硅作为基质（HPW-介孔硅）。通过电化学阻抗谱测量的质子电导率在 90°C 和 100%相对湿度（RH）下为 0.11 S cm(-1)，活化能低至约 14 kJ mol(-1)。为了确定与 HPW-介孔硅 PEM 中质子迁移相关的能量以及确定质子跳跃的机制，我们报告了使用广义梯度近似（GGA）的密度泛函理论（DFT）计算。这些 DFT 计算表明，质子转移过程涉及分子内和分子间质子转移途径。当相邻的 HPWs 靠近（小于 17.0 Å）时，HPW 分子内质子转移的计算活化能较高（29.1-18.8 kJ/mol），而氢键沿质子转移的分子间质子转移势垒较低。我们发现，HPW-介孔硅膜内质子迁移的总势垒由分子内质子转移途径决定，这解释了为什么当 HPW 在介孔硅上的重量百分比高于 67wt%时，质子电导率保持不变。相比之下，在清洁的 SiO2（111）表面上质子转移的活化能计算高达约 40 kJ mol(-1)，证实了实验上观察到的清洁硅表面极低的质子电导率。
