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

点击翻译按钮获取中文摘要

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.