Alessandro Lavacchi

Improvement in the efficiency of an OrganoMetallic Fuel Cell by tuning the molecular architecture of the anode electrocatalyst and the nature of the carbon support

The electrooxidation of ethanol to acetate is achieved with Rh(I) diolefin amine complexes of the general formula [Rh(Y)(trop2NH)(L)] (L = PPh3, P(4-n-BuPh)3; Y = triflate, acetate; Bu = butyl) in direct alcohol fuel cells that have the peculiarity of containing a molecular anode electrocatalyst and, hence, are denoted as OrganoMetallic Fuel Cells (OMFCs). Changing the carbon black support from Vulcan XC-72 (Cv) to Ketjenblack EC 600JD (Ck) and/or the axial phosphane to produce non crystalline complexes has been found to remarkably change the electrochemical properties of the organorhodium catalysts, especially in terms of specific activity and durability. An in-depth study has shown that either Ck or P(4-n-butylPh)3 favour the formation of an amorphous Rh-acetato phase on the electrode, leading to a much more efficient and recyclable catalyst as compared to a crystalline Rh-acetate complex which is formed on Cv with PPh3 as the ligand. The ameliorating effect of the amorphous phase has been ascribed to its higher number of surface complex molecules as compared to the crystalline phase. A specific activity as high as 10 000 A gRh−1 has been found in a half cell, which is the highest value ever reported for ethanol electrooxidation.

Nanostructured Fe–Ag electrocatalysts for the oxygen reduction reaction in alkaline media

The impregnation of Ketjen Black (C) with iron(II) and silver(II) phthalocyanines (MPc) individually or as a 1 : 1 stoichiometric mixture, followed by heat treatment at 600 °C under inert atmosphere, gave a series of novel nanostructured electrocatalysts AgPc/C(600), FePc/C(600) and FeAgPc/C(600) (ca. 3 wt% metal loadings) for the oxygen reduction reaction (ORR) in alkaline media. The catalysts were structurally characterized by XRPD, XPS, HR-TEM/STEM and chemisorption measurements. During the synthetic heat treatment of AgPc/C(600) at temperatures above 250 °C, the AgPc decomposed to form small finely dispersed carbon supported Ag nanoparticles (mean diameter 8.5 nm). This process was delayed for FeAgPc/C(600) to above 300 °C and the resulting Ag nanoparticles were much smaller (mean diameter 2.3 nm). As expected, at 600 °C the FePc/C(600) forms highly dispersed arrays of single Fe ions coordinated by four nitrogen atoms (Fe–N4 units). Electrodes coated with AgPc/C(600), FePc/C(600) and FeAgPc/C(600) were investigated for ORR in alkaline media by linear sweep voltammetry and the RRDE system was used to probe the production of HO2−. The electrochemical activity of all materials was analyzed by Tafel and Koutecky–Levich plots and the stability of each catalyst was followed using chronopotentiometry. Both Fe-containing electrocatalysts, FeAgPc/C(600) and FePc/C(600), were highly active for the ORR promoting exclusively the four electron pathway also during chronopotentiometry, while AgPc/C(600) was found to produce up to 35 mol% HO2−. Compared to FePc/C(600), the binary FeAgPc/C(600) catalyst displayed remarkably higher activity and stability. This experimental evidence could be explained in terms of a synergistic Ag–Fe interaction which results from the unique nanostructure that forms during heat treatment which consists of very finely dispersed Ag nanoparticles and Fe–N4 moieties.

Ionic liquids: Electrochemical investigation on corrosion activity of ethyl-dimethyl-propylammonium bis(trifluoromethylsulfonyl)imide at high temperature

In the present work we report on our investigation on the corrosion properties of the ethyl-dimethyl-propylammonium bis(trifluoromethylsulphonyl)imide at temperatures up to 473 K. The tests were performed both for commercially pure iron alloys and for pure copper. The electrochemical measurements showed that the metals corrosion rates can be dramatically reduced by purging the ionic liquid with inert gases to remove the dissolved oxygen.

Direct Alcohol Fuel Cells: Nanostructured Materials for the Electrooxidation of Alcohols in Alkaline Media

The exploitation of biomass derived alcohols in direct fuel cells is an attractive and simple way to transform their chemical energy into electrical power. Renewable alcohols such as ethanol, ethylene glycol and glycerol could replace methanol in traditional direct methanol fuel cells (DMFCs) due to their high energy densities, low vapor pressure, low toxicity and well established distribution infrastructure. For these devices to be practical alternatives, fast anode electrode kinetics of alcohol electrooxidation are required together with complete oxidation to CO2. To date the most successful exploitation of such alcohols in direct fuel cells have been under alkaline conditions. This is because alcohol electrooxidation kinetics under acidic conditions such as those of DMFCs are very sluggish. This combined with the highly corrosive conditions of the acidic electrolyte limits the choice of electrocatalyst materials to high concentrations of platinum alloys. In alkaline media, platinum can be replaced by more abundant transition metal based materials, either nanoparticles or molecular complexes, that show significantly higher activity and selectivity. This chapter provides an overview of recent developments in the preparation of electrocatalytic materials for alcohol electrooxidation in alkaline media. In addition to maximizing electrical power output these materials can be tuned to optimize the selectivity of oxidation thus leading to the co-production of partially oxidized industrially relevant intermediates. A discussion of recent work on exploiting SMSIs (Strong Metal Support Interactions) to improve activity, fuel efficiency and stability of electrocatalyst materials is also included.

Carbon-Based Nanomaterials

In fuel cells both the anode and cathode electrocatalysts are generally composed of nanosized metal particles which are supported on high surface area materials. The electrochemically active surface area (EASA), of catalysts used in low temperature fuel cells, such as polymer electrolyte membrane fuel cells (PEMFC, fed with hydrogen), direct alcohol fuel cells (DAFCs, alcohols: ethanol, methanol, polyalcohols), and direct formic acid fuel cells (DFAFC), has been found to be greatly enhanced when high surface area carbon-based materials are used as the support. As described in detail in Chap. 3, at the anode of a PEMFC dihydrogen is oxidized yielding protons and electrons.

A Bird’s Eye View of Energy-Related Electrochemistry

An electrochemical cell is a device capable of either obtaining electrical energy directly from a chemical reaction or of converting electrical energy into chemical transformations. Electrochemical devices where the conversion of the chemical energy (the free energy of a spontaneous chemical reaction) into electrical energy (e.g., combination of molecular hydrogen and oxygen to form water) occurs in fuel cells and batteries.

An increase in hydrogen production from light and ethanol using a dual scale porosity photocatalyst

The stable photocatalytic production of hydrogen is demonstrated under simulated solar irradiation from the aqueous solutions of ethanol over a dual porosity 3D TiO2 nanotube array (TNTA). The photocatalytic material consists of a uniform layer of TNTAs grown on each titanium fiber of a commercial sintered titanium web (TNTA-web). Under simulated solar irradiation, a stable H2 production rate of 40 mmol h−1 m−2 is observed. In comparison, TNTAs grown on a flat titanium foil (TNTA-foil) do not produce H2 under the same conditions. The addition of small (4–5 nm) and well distributed Pd nanoparticles to the TNTA-web increases the production of hydrogen to 130 mmol h−1 m−2 with a solar-to-fuel efficiency of 0.45%. The same Pd loading on the TNTA-foil support resulted in a H2 production rate of 10 mmol h−1 m−2. Each catalytic material is characterized by a combination of SEM, HR-TEM, XRD, XPS and Raman spectroscopy. The enhancement in H2 production is attributed to the increased light absorption properties of the TNTA-web material enabled by its unique dual porosity. The analysis of the reaction by-products shows that ethanol is transformed into acetaldehyde as a single oxidation product. Additionally, it is shown that an optimal Pd loading maximizes the H2 production rate, since agglomeration of the metal nanoparticles takes place at high loading, decreasing the Pd–TiO2 interface where the photoreforming reactions take place.

Molecular Complexes in Electrocatalysis for Energy Production and Storage

The employment of metal complexes as electrocatalysts represents a potentially very important development in the field of energy production and storage. From a practical perspective, a molecular metal complex, soluble in different solvents or easily dispersible on very small surfaces as well as bound to electrodes, but capable of promoting an electrochemical reaction, has the potential advantage to overcome the drawbacks that present the technologies based on metal nanoparticles.

Other Support Nanomaterials

Corrosion of carbon supports has been identified as one of the major factors hampering the durability of fuel cell electrocatalysts. In Chap. 5 we have seen that carbon support corrosion mainly occurs at the fuel cell cathode and is accelerated by the presence, even in traces, of hydrogen peroxide.

Supported Metal Nanoparticles

The purpose of this chapter is to summarize the main synthetic techniques used to prepare supported metal nanoparticles for use in electrolytic cells (primarily fuel cells and electrolyzers). Each method provides a strategy which has the main goal of controlling particle size, alloy composition and catalyst distribution over the support material. As a consequence the reader is presented here with a general overview of the most common synthetic methods. Some of the more established procedures are now used industrially to produce large quantities of materials. This does not mean they are superior to the others, depending on the end application of the catalyst and the instrumentation available, one method may be advantageous over another. Therefore, the reader is advised to consider the intrinsic advantages and disadvantages of each method when selecting which to use. More recently methods have been developed to control structure on an atomic scale by the formation of surface defects such as twins or stacking faults that can lead to dramatic increases in activity. This will be covered in detail elsewhere.