Degradation studies of Me-N-C catalysts for the oxygen reduction reaction in fuel cells

Abstract

The increasing demand forrenewable energy along with the requirement of decreasing CO2 emissions is a major challenge for the scientific community. Fuel cells are among the most promising electrochemical devices because of their low operating temperature and high power density. The main advantage of a fuel cell is that electrical power can be produced continuously as long as the fuel supply is provided. Another important advantage is high efficiency. The efficiency of fuel cells is superior to that of combustion engines, particularly at low loads, which makes low-temperature fuel cells (0―100 °C) attractive for automotive propulsion. State of the art catalyst for the anode as well as the cathode is typically based on platinum-supported on carbon. However, the platinum catalyst alone would account for 38-56% of the stack cost [1]. Thus the higher efficiency, in comparison to combustion engines, comes with a higher price that makes the commercialization not competitive now. As a large quantity of the precious metal is required to catalyse the oxygen reduction reaction (ORR), current research is focused on this reaction and especially on the development of alternative non-precious metal catalysts (NPMC). In order for these catalysts to be a commercially viable solution for replacing platinum-based catalysts, they should meet two criteria, improving both activity and stability of these catalysts. Despite, several milestones that have been achieved regarding the activity of these catalysts [2– 5], stability is still relatively poor in comparison to platinum-based systems. This dissertation focuses on the investigation of the stability of non-precious metal catalysts for oxygen reduction reaction mainly in acidic media for application in Proton Exchange Membrane Fuel Cells (PEMFCs) and Direct Methanol Fuel Cells (DMFCs). Α part of this study also deals with performance determination of NPMC in alkaline media, regarding their application in Alkaline Fuel Cells (AFCs). The electrochemical tests were performed with a Rotating Disk Electrode technique. Stability refers to the ability of a system to maintain performance at constant current (or voltage) conditions, while durability refers to the ability to maintain performance following a voltage cycling. First, a systematic study on the impact of the metal centre on durability was conducted. Thirteen MeN-C catalysts were examined with a Start/ Stop (SSC) durability protocol in the potential range of 1.0 V – 1.5 V. Raman spectroscopy was performed before and after the durability tests and a correlation between electrochemical evaluation and Raman spectroscopy in this potential region was found. The carbon oxidation is related to the disintegration of active MeN4 sites that might be initiated by both: the oxidation of the surrounding graphene sheets and by a displacement of the metal out of the N4 plane and this was evidenced by a decrease in the D3 band. Furthermore, a novel synthesis protocol was developed in our group and a Fe-N-C catalyst was optimized with the addition of sulfur(S) in the precursor. With respect to activity the best-off S-added catalyst and the S-free one were then examined for durability under a Load Cycle (LC) protocol (0.6– 1.0 V) in alkaline media. A modification of both catalysts with ionic liquid (IL) was introduced by the group of Professor B. J.M. Etzold within a cooperation framework. The durability of the modified S-free catalyst was found superior to the durability of the non-modified catalyst. In the case of the Sadded catalyst, the IL modification did not further improve its durability. Finally, a third synthesis approach was developed, leading to an active Fe-N-C catalyst also with sulfur in the precursor. The stability of this catalyst was investigated in a DMFC within a research stay abroad project in collaboration with Professor S. Specchia from Politecnico di Torino and subsequently examined by post mortem Mössbauer spectroscopy. This catalyst was further evaluated with a Load Cycle durability protocol and post mortem Raman spectroscopy in our laboratories.