Evaluation of Reduced-Graphene-Oxide Aligned with WO3-Nanorods as Support for Pt Nanoparticles during Oxygen Electroreduction in Acid Medium

Hybrid supports composed of chemically-reduced graphene-oxide-aligned with tungsten oxide nanowires are considered here as active carriers for dispersed platinum with an ultimate goal of producing improved catalysts for electroreduction of oxygen in acid medium. Here WO3 nanostructures are expected to be attached mainly to the edges of graphene thus making the hybrid structure not only highly porous but also capable of preventing graphene stacking and creating numerous sites for the deposition of Pt nanoparticles. Comparison has been made to the analogous systems utilizing neither reduced graphene oxide nor tungsten oxide component. By over-coating the reduced-graphene-oxide support with WO3 nanorods, the electrocatalytic activity of the system toward the reduction of oxygen in acid medium has been enhanced even at the low Pt loading of 30 microg cm-2. The RRDE data are consistent with decreased formation of hydrogen peroxide in the presence of WO3. Among important issues are such features of the oxide as porosity, large population of hydroxyl groups, high Broensted acidity, as well as fast electron transfers coupled to unimpeded proton displacements. The conclusions are supported with mechanistic and kinetic studies involving double-potential-step chronocoulometry as an alternative diagnostic tool to rotating ring-disk voltammetry.Comment: arXiv admin note: text overlap with arXiv:1805.0315

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

Application of a non-noble Fe-N-C catalyst for oxygen reduction reaction in an alkaline direct ethanol fuel cell

A Fe-N-C non-noble metal (NNM) catalyst for oxygen reduction reaction (ORR) was prepared via hard templating method using Fe(II)-phthalocyanine. Its electrochemical behavior towards the ORR was tested in alkaline conditions using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The kinetics of the reduction of the adsorbed oxygen, the selectivity, and the activity towards hydrogen peroxide reduction reaction (HPRR) were investigated. The ethanol tolerance and the stability in alkaline conditions were also assessed with the purpose to verify the good potentiality of this catalyst to be used in an alkaline direct ethanol fuel cell (DEFC). The results evidence that the ORR occurs mainly following the direct 4 e–reduction to OH−, and that the Fe-N-C catalyst is highly ethanol tolerant and shows a promising stability. The alkaline DEFC tests, performed after the optimization of the ionomer amount used for the preparation of the catalyst ink, show good results at low-intermediate currents, with a maximum power density of 62 mW cm−2. The initial DEFC performance can be partially recovered after a purge-drying procedureThis work was supported by the Madrid Regional Research Council (CAM) [RESTOENE-2 grant n. S2013/MAE2882], the Spanish Economy and Competitiveness Ministry [ENE2016 grant n. 77055-C3-1-R], and the Italian Ministry of Education, University and Research [PRIN NAMEDPEM grant n. 2010CYTWAW]

Metal-free carbon-based oxygen bi-functional electrocatalysts for rechargeable metal-air battery applications

With rapid advancements in mobile systems and vehicles, there is an increasing expectation of performances for energy storage/conversion devices each year. New technologies such as metal-air batteries and fuel-cells have gained much attention as better substitutes of currently widely used batteries and fossil fuels however it is crucial to improve the sluggish oxygen kinetics occurring on the surface of the electrodes of these devices. Abstaining from using scarce noble-metal containing species, this thesis outlines syntheses and electrocatalytic performances of non-metal carbon-based materials as oxygen electrocatalysts for possible cost-effective cathode candidates in metal-air batteries. To explore a different synthesis approach of graphene-like carbon material, as to a harsh acid oxidation preparation method, oxygen electrocatalysts were obtained via high temperature (>700 °C) graphitisation of glucose and dicyandiamide. Pores created during polymerisation and the nitrogen species (pyridinic, pyrrolic, and graphitic) increased catalytically active sites on as prepared graphitic carbons. Annealing temperature was varied to study the effect of the ratio of N-dopant species, and the concentration of carbon defects as a function of the annealing temperature. Increasing the number of active sites whilst preserving electronic conductivity is challenging, but crucial to enhancing electrocatalysts’ performances. Heteroatom-doped ‘carbon dots’, quasi-spherical carbon particles with size of less than 20 nm, were enriched on the surface of graphene substrates to maximise catalytically available active sites. Embedding carbon dots (with many N and S dopant species) provide many catalytically active sites for each defect site compared to direct heteroatom doping. Dual heteroatom-doped carbon dots embedded graphene catalysts, NS-CD@gf_a900 exhibited significantly high catalytic performances of 7.71 mA cm-2 at 1600 rpm and only 0.91 V overpotential for oxygen bi-functional reactions; even close to noble-metal Pt/C and Ir/C counterparts (0.77 V) as a result of the effect of many available catalytic sites and the synergistic behaviour of the dopants to the electronic structures. An alternative to the traditional method of 2D carbon preparation via exfoliation and oxidation using harsh chemicals is via a bottom-up approach of forming carbon polymers. Pyrene, a polycyclic aromatic carbon with a four fused benzene-ring structure, was rationally selected as the building block for the formation of carbon substrate via a Friedel-Craft acylation mechanism. There was a significant increase in measured pore volume in the nano-range and throughout, compared to other carbon polymers and graphene samples, which can enhance the gas/ion diffusion mechanism. Nitrogen and sulphur doping with high temperature annealing at 900 °C resulted in a highly porous carbon substrate that exhibited comparable electrocatalytic performances to the Pt/C electrocatalyst