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（LTFCs）在向清洁能源转变的过程中，尤其是在交通运输领域，将发挥重要作用，因此受到越来越多的关注。然而，氧还原反应（ORR）的低效率是该技术普及的主要障碍。用于阴极ORR的现有技术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PEMFC价格的55%，是燃料电池价格昂贵的主要原因。此外，由于供应量减少，铂价格在过去几十年中持续上涨。铂回收可能是一种有价值的技术，以实现更可持续地使用这种贵金属。然而，这将带来额外的成本，而且不可能实现完全再循环，这仍将导致全球铂供应量的减少。除了需要高且昂贵的铂负载之外，Pt基催化剂仍然需要300 mV的大的过电位以克服ORR的动力学势垒。因此，开发高活性、耐久性和地球资源丰富的ORR催化剂是LTFCs在汽车工业中大规模应用的特权。广泛的研究工作已经应用于无金属和无贵金属催化剂的开发。一些最有前途的不含贵金属的催化剂是基于碳负载的过渡金属/氮。此类ORR催化剂是使用廉价的前体合成的，含有多种过渡金属，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。已经报道了一些Co和Fe基催化剂在碱性电解质中提供比Pt/C更高的活性和稳定性。然而，尽管对不含贵金属的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大大低于铂基催化剂和在碱性条件下表现显著的催化剂，由于在这些条件下氮中心的低活性，人们认识到可以通过增加比表面积（SSA）来提高催化活性。和活性位点的覆盖率。然而，由于传质限制，在数十纳米的深度处，催化位点变得无活性，这限制了可以通过增加表面积来实现的活性改善。为了进一步提高ORR效率，研究人员最近尝试修改电解质、氧气和固体催化剂之间的三相点界面。离子液体已被用于调节催化剂表面的疏水性，以防止产出水阻塞活性位点，增加催化剂附近的氧的溶解度和传输性能，并促进质子传导性。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显示出改进的催化性能。尽管是一种非常有前途的技术，但在离子液体存在下增加活性背后的机制需要更好地理解。已经提出，离子液体增加氧溶解度，增加催化剂表面上的反应物浓度。其他研究人员认为，ORR活性的增加主要是由于非活性物质覆盖率的降低，这些物质是稳定的中间体，可以限制催化活性，或者是由于表面疏水性的增加，从而改善了采出水的去除。

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.