Non-noble metal catalysts for oxygen reduction reaction in low temperature fuel cells

Polymer electrolyte membrane fuel cells (PEMFC) are electrochemical devices which can directly convert the chemical energy of a fuel (such as hydrogen or a low-molecular weight alcohol) and an oxidant (i.e. oxygen) into electrical energy with high efficiency. Moreover, due their low operating temperature, they are suitable for automotive or portable applications. However, the slow kinetics of oxygen reduction reaction (ORR) requires the use of costly Pt-based catalysts at the cathode in order to obtain the desired power density values. Nevertheless, the cathode is still responsible for the main voltage loss in the cell. The overall objective of the research carried out in this Ph.D. thesis was the development of Pt-free ORR catalysts starting from different carbon, nitrogen and transition metals precursors. Different synthesis approaches were used in order to obtain an improvement of the activity, and to understand the influence of the synthesis process variables. In particular, the influence of carbon supports (commercial and synthesized in the lab), nitrogen and transition metals precursors, templating agents, number and temperature of pyrolysis were examined. The catalysts produced were characterized by means of several instrumental techniques such as N2 physisorption, XRD, XPS, EDX, SEM, FESEM, TEM, Raman and FTIR. The effect of the presence of different transition metals on the pyrolysis process was investigated by TGA coupled with a mass spectroscopy analysis, in order to have an insight on their influence in the formation of ORR active sites. The activity toward ORR was assessed by RDE-RRDE (rotating disk electrode - rotating ring disk electrode) analysis and by gas-diffusion electrode in a 3-electrodes electrochemical cell configuration. The electrochemical techniques used were cyclic voltammetry (CV), linear sweep voltammetry (LSV), staircase voltammetry (SV), chronoamperometry and electrochemical impedance spectroscopy (EIS). These electrochemical tests were performed in both acid and alkaline conditions, with reference to the potential applications in both H+ and OH– conducing polymer electrolyte membrane fuel cells. This first part of research was carried out in the laboratories of the Gre.En2 (Green Energy and Engineering) Group in the Department of Applied Science and Technology (DISAT) at Politecnico di Torino. Then, in the second part, some of the most promising electrocatalysts in terms of ORR activity were in different types of single PEMFC. In particular, using acidic electrolyte membrane, the tests were performed using H2 or methanol as fuels. In the case of direct methanol fuel cell (DMFC) tests, short-term durability tests were done in order to compare the durability performance of our catalysts with a standard Pt-based catalysts. The tests with alkaline electrolyte membrane were performed using ethanol as fuel. This second part of research was carried out at the Universidad Autonoma de Madrid in the laboratories of the Department of Applied Physical-Chemistry. Here the structure of the thesis: Chapter 1 is a general introduction about the PEMFC fuel cell technology, particularly focusing on the non-noble metal catalysts for ORR as potential alternative to Pt. Chapter 2 is focused on the use of different types of reduced graphene oxide as support for the synthesis of Fe-N/C catalysts. In Chapter 3, a complex between Co ions and a N-containing ligand molecule is impregnated on multi walled carbon nanotubes and pyrolyzed one or two times for producing a Co-N-C catalyst, and the influence of the second pyrolysis on the activity improvement was investigated. Chapter 4 deals the optimization of the synthesis process of a Fe-N-C catalyst using polypyrrole as N source and mesoporous carbon a C-support. In Chapter 5 the study of the influence of different silica templates on the morphology on the ORR activity of a Fe-N-C catalyst synthesized using Fe-phthalocyanine as precursor is presented. In Chapter 6, different Me-phthalocyanines (Me = Fe, Co, Cu, Zn) were used as precursor for the synthesis of Me-N-C catalysts using SBA-15 silica as hard template. The influence of the different transition metals on the pyrolysis process and on the ORR activity and selectivity toward a complete 4 e- oxygen reduction was investigated in both acid and alkaline conditions. A detailed kinetic analysis in acid conditions is also presented. The most active catalyst was tested in different types of PEMFCs. Finally, in Chapter 7, the influence of four different carbon supports on the ORR activity of Fe-N/C catalysts in acid and alkaline conditions as well as the performance in single PEMFC is examined. The general conclusions of the thesis are presented in Chapter 8.