Engineering of the triple-point interface, to increase the activity of the oxygen reduction reaction on precious-metal-free catalysts

低温燃料电池(LTFC)正受到越来越多的关注，因为它们有望在向清洁能源的转变中发挥重要作用，特别是在交通运输领域。然而，氧还原反应(ORR)的低效是这项技术渗透的主要障碍。最先进的阴极ORR催化剂是沉积在碳载体上的铂基纳米颗粒。根据美国能源部2007年的一项研究，铂的成本可占质子交换膜燃料电池价格的55%，这是燃料电池价格昂贵的主要原因。此外，由于可获得性减少，铂金价格在过去几十年中显示出持续上涨的趋势。为了实现这种贵金属的更可持续利用，PT回收可能是一项有价值的技术。然而，这将带来额外的成本，而且不可能实现完全回收，仍将导致铂全球供应量的减少。铂基催化剂除了需要高且昂贵的铂负载量外，还需要300 mV的大过电位来克服ORR的动力学障碍。因此，开发高效、耐用、富含稀土的ORR催化剂是LTFC大规模应用于汽车工业的必备条件。无金属和无贵金属催化剂的开发得到了广泛的研究。一些最有前途的无贵金属催化剂是以过渡金属/氮为基础，以碳为载体的。这类ORR催化剂是用廉价的前驱体合成的，这些前驱体含有几种过渡金属，包括Fe、Co、Ni、Cu。在催化合成过程中引入了热解反应，提高了催化剂的稳定性和活性。一些钴和铁基催化剂在碱性电解液中具有比铂/碳更高的活性和稳定性。然而，尽管对无贵金属催化剂进行了广泛的研究和改进，但它们在酸性条件下的活性仍然远远低于在碱性条件下表现显著的铂基催化剂和在低pH条件下表现出较低活性的催化剂，这是因为在这些条件下氮中心的活性较低。人们认识到，通过增加比表面积和活性中心的覆盖率可以提高催化剂的活性。然而，由于传质的限制，在几十纳米的深度，催化中心变得不活跃，这限制了通过增加比表面积来实现的活性提高。为了进一步提高ORR效率，研究人员最近试图修改电解液、氧气和固体催化剂之间的三点界面。离子液体被用来调节催化剂表面的疏水性，以防止产出水堵塞活性中心，增加催化剂附近氧的溶解和传输性能，并促进质子电导。这样得到的催化剂，被称为SCILL(具有离子液体层的固体催化剂)，在酸性和碱性条件下都表现出了更好的催化性能。尽管这是一项非常有前途的技术，但在离子液体存在下提高活性的机理需要更好地理解。有人提出，离子液体增加了氧的溶解度，增加了催化剂表面反应物的浓度。其他研究人员认为，ORR活性的增加主要是由于非活性物种覆盖率的减少，这些非活性物种是稳定的中间体，可以限制催化活性，或者是由于表面疏水性的增加，从而改善了对产出水的去除。

Low-temperature fuel cells (LTFCs) are gaining increasing interest, as they promise to play a major role in the shift to clean energy, particularly in the transportation sector.However, the inefficiency of the oxygen reduction reaction (ORR) presents a major barrier to the penetration of this technology. The state-of-the-art catalysts for the cathodic ORR are platinum-based nanoparticles deposited on a carbon support. According to a 2007 study from the US Department of Energy, the cost of platinum can account for up to 55% of the price of PEMFCs, representing the main cause of fuel cells expensiveness. In addition, platinum price has shown a continuous rise over the past decades, as a result of the decrease in availability. Pt recycling could be a valuable technique to achieve a more sustainable use of this noble metal. However, this would introduce an additional cost, and the impossibility to achieve full recycling would still result in a decrease in Pt global supplies. Besides requiring high and expensive platinum loading, Pt-based catalysts are still in need of a large overpotential of 300mV to overcome the kinetic barrier of the ORR. As a result, LTFCs development is hindered by the inefficiency of the oxygen reduction reaction and by the requirement of noble-metals.Therefore, the development of highly-active, durable and earth-abundant ORR catalysts is a prerogative for LTFCs large-scale applicability in the automotive industry. Extensive research efforts have been applied to the development of metal-free and noble-metal free catalysts. Some of the most promising precious-metal-free catalysts are based on transition metals / nitrogen, supported on carbon. Such ORR catalysts have been synthetized using inexpensive precursors, containing several transitions metals, including Fe, Co, Ni, Cu. Pyrolysis was introduced in the catalysis synthesis procedure, improving both catalyst stability and activity. Some Co and Fe based catalysts have been reported to provide higher activities and stabilities than Pt/C in alkaline electrolyte. However, despite the extensive studies and improvements on precious-metal-free catalysts, their activity in acidic conditions is still considerably lower than that of platinum-based catalysts and catalysts that perform remarkably in alkaline conditions loose activity at low pH, due to the low activity of the nitrogen sites in these conditions.It is recognised that the catalytical activity can be improved by increasing the specific surface area (SSA) and the coverage of active sites. However, due to mass transfer limitations, at a depth of tens of nanometres the catalytic sites become inactive, which limits the activity improvement that can be achieved by the increase in surface area. To further improve the ORR efficiency, researchers have recently attempted to modify the triple point interface, between the electrolyte, oxygen and solid catalyst. Ionic liquids have been used to tune the hydrophobicity of the catalyst surface to prevent produced water from blocking the active sites, increasing the solubility and transport properties of oxygen in the vicinity of the catalyst, and promote the proton conductivity. The so-obtained catalysts, known as SCILL (solid catalyst with ionic liquid layer) have shown improved catalytic performance, both in acidic and alkaline conditions.Despite being a very promising technique, the mechanism behind the increased activity in presence of ionic liquids needs to be better understood. It has been proposed that the ionic liquids increase oxygen solubility, augmenting the concentration of reactant on the catalyst surface. Other researchers have suggested that the increased ORR activity results primarily from the decrease in the coverage of non-active species, which are stable intermediates and can limit the catalytical activity or from the increased hydrophobicity of the surface, resulting in improved removal of the produced water.