Supportless Pt and PtPd nanotubes as electrocatalysts for oxygen-reduction reactions
Supportless Pt and PtPd nanotubes as electrocatalysts for oxygen-reduction reactions
复制标题
DOI：
10.1002/anie.200700894
复制
发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Yan, Yushan
中科院分区：
文献类型：
--
作者：
Chen, Zhongwei;Waje, Mahesh;Yan, Yushan
近年来，电催化剂的耐久性已成为质子交换膜燃料电池（pemfc）商业化之前必须解决的最重要问题之一。[1,2]耐久性问题在发生氧还原反应（ORR）的阴极尤为严重。目前，应用最广泛的阴极催化剂体系是以小纳米颗粒（2-5 nm）形式支撑在无定形碳颗粒聚集体（Pt/C）上的铂。Pt/C催化剂的耐久性较差，反映在燃料电池运行过程中铂电化学表面积（ECSA）随着时间的推移而迅速而显著地损失。[1,2]已经讨论了铂ECSA在阴极上损失的机制[1,2]，可以总结如下：1)由于碳载体的腐蚀导致电接触中铂纳米粒子的损失，2)铂纳米粒子的溶解和再沉积或奥斯特瓦尔德成熟，3)表面能最小化驱动的铂纳米粒子聚集，4)铂纳米粒子在聚合物电解质中的溶解和随后的可溶性Pt2+的迁移，以及氢从阳极通过质子交换膜交叉的最终化学还原。碳腐蚀问题可以通过使用更耐腐蚀的催化剂载体来缓解，例如石墨化碳材料或碳纳米管但是，没有有效的解决办法来处理其他三个机制。我们开发耐用电催化剂的方法是开发无支撑铂纳米管（PtNTs）和铂合金纳米管（例如铂-钯合金纳米管（PtPdNTs））作为阴极催化剂。由于其在多个长度尺度上的独特尺寸组合，PtNTs和PtPdNTs可以通过其纳米级的壁厚提供高铂表面积（支持信息中的图S1和S2），而无需高表面积支撑（例如，炭黑）。同时，由于其微米级的长度，这些材料具有消除或显著减少上述四种降解途径的潜力：首先，ptnt不需要支撑，因此消除了支撑腐蚀问题。其次，与铂纳米颗粒（0D纳米结构）相比，PtNTs的微米长度（1D纳米结构）使得PtNTs在燃料电池运行过程中不易溶解、奥斯特瓦尔德成熟和聚集。此外，PtNTs和PtPdNTs与碳纳米管一样，具有各向异性的形态，可以提高质量传递和催化剂利用率如果组装得当（如立方体或六角形紧密堆积并垂直排列在离子膜上），它们还可以形成薄的催化剂层（如在PtNT壁厚为2 nm时为0.5 μm，铂负载为0.2 mgcmÀ2；图S3），进一步改善催化剂层内的传质特性。我们合成了PtNTs和PtPdNTs（直径50 nm，长5-20 μm，壁厚4 - 7 nm），并测试了它们作为pemfc中ORR催化剂的适用性。PtNTs是由Xia及其同事开发的银纳米线（AgNWs）的电替换反应合成的。[5,6] AgNWs采用多元醇法合成，随后在水溶液中与Pt (CH3COO) 2回流加热。经酸处理和热处理后，离心收集产物。AgNWs的直径（图1A， B）和长度（图S4）分别约为40nm和10 μm。的
Electrocatalyst durability has been recently recognized as one of the most important issues that must be addressed before the commercialization of proton exchange membrane fuel cells (PEMFCs).[1, 2] The durability problem is particularly severe in the cathode, where the oxygen-reduction reaction (ORR) occurs. At present, the most widely used cathode catalyst system is platinum in the form of small nanoparticles (2–5 nm) supported on amorphous carbon-particle aggregates (Pt/C). The poor durability of the Pt/C catalyst is reflected by a fast and significant loss of platinum electrochemical surface area (ECSA) over time during fuel cell operation.[1, 2] The mechanisms for the loss of platinum ECSA at the cathode have been discussed [1, 2] and can be summarized as follows: 1) loss of platinum nanoparticles from the electrical contact because of corrosion of the carbon support, 2) platinum dissolution and redeposition or Ostwald ripening of the platinum nanoparticles, 3) platinum-nanoparticle aggregation driven by surface-energy minimization, and 4) platinumnanoparticle dissolution and subsequent migration of the soluble Pt2+ species within the polymer electrolyte and the eventual chemical reduction by hydrogen crossed-over from the anode through the proton-exchange membrane. The carbon-corrosion problem can be alleviated by the use of a more corrosion-resistant catalyst support, for example, graphitized-carbon materials or carbon nanotubes.[3] No effective solutions, however, exist for addressing the other three mechanisms. Our approach to a durable electrocatalyst is to develop supportless platinum nanotubes (PtNTs) and platinum-alloy nanotubes (eg, platinum–palladium-alloy nanotubes (PtPdNTs)) as the cathode catalyst. Because of their unique combination of dimensions at multiple length scales, PtNTs and PtPdNTs can provide high platinum surface area by their nanometer-sized wall thickness (Figures S1 and S2 in the Supporting Information) without the need for a high-surface-area support (eg, carbon black). At the same time, these materials have the potential to eliminate or significantly reduce all of the four degradation pathways discussed above as a result of their micrometer-sized length: First, PtNTs do not require a support, and thus the supportcorrosion problem is eliminated. Second, the micrometersized length of the PtNTs (1D nanostructure) makes the PtNTs less vulnerable to dissolution, Ostwald ripening, and aggregation during fuel cell operation than the platinum nanoparticles (0D nanostructure). Additionally, the PtNTs and PtPdNTs, like carbon nanotubes, have an anisotropic morphology that can improve mass transport and catalyst utilization.[3] And if properly assembled (eg, cubic or hexagonal close-packing and vertical alignment on the nafion membrane), they can also lead to a thin catalyst layer (eg, 0.5 μm at a PtNT wall thickness of 2 nm and a platinum loading of 0.2 mgcmÀ2; Figure S3), further improving the mass-transfer characteristics within the catalyst layer.[4]We have synthesized PtNTs and PtPdNTs (50 nm diameter, 5–20 μm long and 4–7 nm wall thickness) and tested their suitability as catalysts for ORR in PEMFCs. PtNTs were synthesized by a galvanic replacement reaction of silver nanowires (AgNWs) developed by Xia and co-workers.[5, 6] The AgNWs were synthesized using a polyol method and subsequently heated at reflux with Pt (CH3COO) 2 in an aqueous solution. After acid and heat treatment, the product was collected by centrifugation. The diameter (Figure 1A, B) and length (Figure S4) of AgNWs are about 40nm and 10 μm, respectively. The