Structure of the catalytic sites in Fe/N/C-catalysts for O-2-reduction in PEM fuel cells

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH3 at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (FexN, with x ≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e− per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials

Platinum and Platinum alloy electrocatalysts for oxygen electrodes in Proton Exchange Membrane Fuel Cells : electrochemical and x-ray absorbtion spectroscopic investigation

Vita.Research on proton exchange membrane fuel cells (PEMFCs) has gained momentum due to the prospects of attaining high energy and power densities, which are essential for transportation and space applications. One of the major problems of PEMFCs is the high overpotential of the cathodic oxygen reduction reaction (ORR) on the Platinum electrocatalyst. Platinum has been the most favored electrocatalyst for all low and medium temperature fuel cells. Two possible approaches for further improving the ORR activities in PEMFCs are as follows: (i) optimization of the structure of the electrode/electrolyte interface by sputter-deposition of a thin layer of Pt onto the front surface of the electrode; and (ii) use of Pt alloy electrocatalysts. The primary objective of this dissertation is to investigate the electrocatalysis of ORR using the mentioned approaches. The first investigation consisted of electrochemical characterization to determine the electrode kinetic parameters (Tafel slopes, exchange current densities, activation energies and reaction orders) on the fuel cell electrodes with a thin layer of Pt on the front surface. The second investigation was to determine the role of alloying element on the ORR electrocatalysis using electrochemical and in-situ X-ray absorption spectroscopic (XAS) techniques. Of special significance is that the two complementary parts of XAS, the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) were used to provide information on the electronic and geometric parameters of the electrocatalysts in the in-situ electrochemical environment. The results demonstrated that sputter-deposition of Pt on the front surface of the electrode improved the ORR electrocatalysis, and is in all probability due to the non teflonized morphology of the electrode/electrolyte interface. Binary alloys of Pt with the first row transition elements (Cr, Mn, Fe, Co and Ni) show up to three fold improvement in performance for the electrocatalysis of ORR as compared to that on Pt/C electrocatalyst. Correlation of the electrocatalytic activities with electronic and geometric properties as obtained from in-situ XAS studies show volcano type relationship for both Pt and Pt alloy electrocatalysts. All the Pt alloys show higher Pt 5 d-orbital vacancies per atom as compared to the Pt/C electrocatalyst. The alloys also exhibited lattice contractions, in terms of the Pt-Pt bond distances, relative to the Pt/C electrocatalyst. There was no evidence of any redox type process involving the alloying element. The higher electrocatalysis by the Pt alloys as compared to Pt/C electrocatalyst could be accounted on the basis of the interplay of the electronic & geometric parameters and their combined effect on the chemisorption characteristics of oxygenated species

Platinum and Platinum alloy electrocatalysts for oxygen electrodes in Proton Exchange Membrane Fuel Cells : electrochemical and x-ray absorbtion spectroscopic investigation

Vita.Research on proton exchange membrane fuel cells (PEMFCs) has gained momentum due to the prospects of attaining high energy and power densities, which are essential for transportation and space applications. One of the major problems of PEMFCs is the high overpotential of the cathodic oxygen reduction reaction (ORR) on the Platinum electrocatalyst. Platinum has been the most favored electrocatalyst for all low and medium temperature fuel cells. Two possible approaches for further improving the ORR activities in PEMFCs are as follows: (i) optimization of the structure of the electrode/electrolyte interface by sputter-deposition of a thin layer of Pt onto the front surface of the electrode; and (ii) use of Pt alloy electrocatalysts. The primary objective of this dissertation is to investigate the electrocatalysis of ORR using the mentioned approaches. The first investigation consisted of electrochemical characterization to determine the electrode kinetic parameters (Tafel slopes, exchange current densities, activation energies and reaction orders) on the fuel cell electrodes with a thin layer of Pt on the front surface. The second investigation was to determine the role of alloying element on the ORR electrocatalysis using electrochemical and in-situ X-ray absorption spectroscopic (XAS) techniques. Of special significance is that the two complementary parts of XAS, the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) were used to provide information on the electronic and geometric parameters of the electrocatalysts in the in-situ electrochemical environment. The results demonstrated that sputter-deposition of Pt on the front surface of the electrode improved the ORR electrocatalysis, and is in all probability due to the non teflonized morphology of the electrode/electrolyte interface. Binary alloys of Pt with the first row transition elements (Cr, Mn, Fe, Co and Ni) show up to three fold improvement in performance for the electrocatalysis of ORR as compared to that on Pt/C electrocatalyst. Correlation of the electrocatalytic activities with electronic and geometric properties as obtained from in-situ XAS studies show volcano type relationship for both Pt and Pt alloy electrocatalysts. All the Pt alloys show higher Pt 5 d-orbital vacancies per atom as compared to the Pt/C electrocatalyst. The alloys also exhibited lattice contractions, in terms of the Pt-Pt bond distances, relative to the Pt/C electrocatalyst. There was no evidence of any redox type process involving the alloying element. The higher electrocatalysis by the Pt alloys as compared to Pt/C electrocatalyst could be accounted on the basis of the interplay of the electronic & geometric parameters and their combined effect on the chemisorption characteristics of oxygenated species

Fundamental Mechanistic Understanding of Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid versus Alkaline Media

Complex electrochemical reactions such as Oxygen Reduction Reaction (ORR) involving multi-electron transfer is an electrocatalytic inner-sphere electron transfer process that exhibit strong dependence on the nature of the electrode surface. This criterion (along with required stability in acidic electrolytes) has largely limited ORR catalysts to the platinum-based surfaces. New evidence in alkaline media, discussed here, throws light on the involvement of surface-independent outer-sphere electron transfer component in the overall electrocatalytic process. This surface non-specificity gives rise to the possibility of using a wide-range of non-noble metal surfaces as electrode materials for ORR in alkaline media. However, this outer-sphere process predominantly leads only to peroxide intermediate as the final product. The importance of promoting the electrocatalytic inner-sphere electron transfer by facilitation of direct adsorption of molecular oxygen on the active site is emphasized by using pyrolyzed metal porphyrins as electrocatalysts. A comparison of ORR reaction mechanisms between acidic and alkaline conditions is elucidated here. The primary advantage of performing ORR in alkaline media is found to be the enhanced activation of the peroxide intermediate on the active site that enables the complete four-electron transfer. ORR reaction schemes involving both outer- and inner-sphere electron transfer mechanisms are proposed

Relating Solvent Parameters to Electrochemical Properties to Predict the Electrochemical Performance of Vanadium Acetylacetonate for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries have shown promise for applications in grid energy storage. Increasing the efficiency of these batteries by developing the electrolyte chemistries is needed. Herein, we investigate the correlation between solvent properties and the electrochemical parameters of vanadium acetylacetonate V(acac)3. Using cyclic voltammetry (CV) and rotating disk electrode experiments (RDE), we show that trends in the performance of the V(acac)3 kinetics are directly related to solvent properties. We found strong relationships between the solvents polarity, viscosity, and donor number with the electrochemical behavior of V(acac)3 in terms of the electrochemical working widow, electron kinetics and stability towards cycling. Based on these finding, we also demonstrate how solvent selection can be improved with limited a priori knowledge