Department of Chemistry, Stanford University, Stanford, California 94305, USA.
Nat Nanotechnol. 2012 May 27;7(6):394-400. doi: 10.1038/nnano.2012.72.
Oxygen reduction reaction catalysts based on precious metals such as platinum or its alloys are routinely used in fuel cells because of their high activity. Carbon-supported materials containing metals such as iron or cobalt as well as nitrogen impurities have been proposed to increase scalability and reduce costs, but these alternatives usually suffer from low activity and/or gradual deactivation during use. Here, we show that few-walled carbon nanotubes, following outer wall exfoliation via oxidation and high-temperature reaction with ammonia, can act as an oxygen reduction reaction electrocatalyst in both acidic and alkaline solutions. Under a unique oxidation condition, the outer walls of the few-walled carbon nanotubes are partially unzipped, creating nanoscale sheets of graphene attached to the inner tubes. The graphene sheets contain extremely small amounts of irons originated from nanotube growth seeds, and nitrogen impurities, which facilitate the formation of catalytic sites and boost the activity of the catalyst, as revealed by atomic-scale microscopy and electron energy loss spectroscopy. Whereas the graphene sheets formed from the unzipped part of the outer wall of the nanotubes are responsible for the catalytic activity, the inner walls remain intact and retain their electrical conductivity, which facilitates charge transport during electrocatalysis.
基于铂或其合金等贵金属的氧还原反应催化剂由于其高活性而在燃料电池中得到常规应用。含铁或钴等金属以及氮杂质的碳载体已被提议用于提高可扩展性和降低成本，但这些替代物在使用过程中通常活性较低和/或逐渐失活。在这里，我们表明，经过氧化和与氨在高温下反应使外壁剥落的少壁碳纳米管可以在酸性和碱性溶液中充当氧还原反应电催化剂。在独特的氧化条件下，少壁碳纳米管的外壁部分被解开，形成附着在内管上的石墨烯纳米片。原子尺度显微镜和电子能量损失光谱表明，石墨烯片中含有源自纳米管生长种子的极少量铁和氮杂质，这有利于催化位点的形成并提高催化剂的活性。在催化剂中，负责催化活性的是来自纳米管外壁解开部分的石墨烯片，而内壁保持完整并保留其导电性，这有利于电催化过程中的电荷传输。
