Tanja Kallio

Single‐Shell Carbon‐Encapsulated Iron Nanoparticles: Synthesis and High Electrocatalytic Activity for Hydrogen Evolution Reaction

Efficient hydrogen evolution reaction (HER) through effective and inexpensive electrocatalysts is a valuable approach for clean and renewable energy systems. Here, single-shell carbon-encapsulated iron nanoparticles (SCEINs) decorated on single-walled carbon nanotubes (SWNTs) are introduced as a novel highly active and durable non-noble-metal catalyst for the HER. This catalyst exhibits catalytic properties superior to previously studied nonprecious materials and comparable to those of platinum. The SCEIN/SWNT is synthesized by a novel fast and low-cost aerosol chemical vapor deposition method in a one-step synthesis. In SCEINs the single carbon layer does not prevent desired access of the reactants to the vicinity of the iron nanoparticles but protects the active metallic core from oxidation. This finding opens new avenues for utilizing active transition metals such as iron in a wide range of applications.

Electrochemical characterization of radiation-grafted ion-exchange membranes based on different matrix polymers

Seven proton conducting membranes based on different commercial fluoropolymer films were prepared by radiation grafting with styrene followed by sulfonation. These membranes were studied as candidates for fuel cell electrolyte membranes and compared to Nafion® 105 and 117 with respect to conductivity, oxygen and hydrogen permeability, kinetics of the oxygen reduction reaction (ORR) and performance in a fuel cell. The dependence of the conductivity of the membranes on the relative humidity (RH) and temperature was also determined. The conductivity was observed to depend on the membrane thickness and the water uptake. The dependence of the conductivity on the temperature and the RH was the same for all of the experimental membranes. Reactant gas permeabilities appeared to depend only slightly on the matrix material and no major differences in the Tafel slopes and exchange current densities of the ORR were observed. Membranes with high water uptakes appeared to be less durable in the fuel cell than membranes with low water uptakes. Thus to prepare a membrane that is durable under the fuel cell conditions, the water uptake must remain low even at the expense of the conductivity.

Effects of a fuel cell test on the structure of irradiation grafted ion exchange membranes based on different fluoropolymers

The role of the fluoropolymer matrix in the stability of irradiation grafted proton conducting membranes under fuel cell conditions is investigated. The structure of a series of membranes with poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene), poly(tetrafluoroethylene-co-hexafluoropropylene), and poly(ethylene-alt-tetrafluoroethylene) matrices with poly(styrene sulfonic acid) side chains is studied before and after a fuel cell test using X-ray scattering techniques and confocal micro-Raman spectroscopy. All tested membranes suffer from a loss of poly(styrene sulfonic acid) leading to a decrease in conductivity. Changes in crystallinity, lamellar period, orientation and thickness of the membranes are reported and compared to corresponding properties of the initial polymer films and the pristine membranes. The membranes where most severe changes in the structure of the matrix polymer can be observed have the shortest lifetimes in the fuel cell.

Transparent and flexible high-performance supercapacitors based on single-walled carbon nanotube films

Transparent and flexible energy storage devices have garnered great interest due to their suitability for display, sensor and photovoltaic applications. In this paper, we report the application of aerosol synthesized and dry deposited single-walled carbon nanotube (SWCNT) thin films as electrodes for an electrochemical double-layer capacitor (EDLC). SWCNT films exhibit extremely large specific capacitance (178 F g(-1) or 552 μF cm(-2)), high optical transparency (92%) and stability for 10 000 charge/discharge cycles. A transparent and flexible EDLC prototype is constructed with a polyethylene casing and a gel electrolyte.

Maghemite nanoparticles decorated on carbon nanotubes as efficient electrocatalysts for the oxygen evolution reaction

The oxygen evolution reaction (OER) is a critical reaction in electrochemical water splitting and rechargeable metal–air batteries to generate and store clean energy. Therefore, the development of efficient and low cost electrocatalysts for the OER with high activity and stability is of great technological and scientific interest. We demonstrate here for the first time that maghemite (γ-Fe2O3) nanoparticles decorated on carbon nanotubes (CNTs) function as low cost, highly active and durable OER electrocatalysts. The material generates a current density of 10 mA cm−2 at overpotentials of 0.38 and 0.34 V in 0.1 and 1 M NaOH, respectively. These values are comparable to those of the best OER electrocatalysts reported so far. Moreover, γ-Fe2O3/CNTs show a stable performance at a potential of ∼1.64 V vs. RHE during 25 h stability tests. The γ-Fe2O3 nanoparticles are formed from carbon encapsulated iron nanoparticles (CEINs) during the first OER measurements of the CEIN/CNT electrode. The CEIN/CNT material itself is synthesized by a fast and low cost floating catalyst chemical vapor deposition method in a one-step synthesis with a similar growth process to that of CNTs.

Proton transport in radiation-grafted membranes for fuel cells as detected by SECM

Scanning electrochemical microscopy (SECM) has been applied to investigate counter ion transport through four different proton conducting membranes with poly(styrene sulfonic acid) side chains. These membranes, intended for the polymer electrolyte fuel cell, are based on PVDF and PVDF-co-HFP matrix materials and have been prepared by an irradiation grafting method. SECM is found to be suitable for mapping variations in proton diffusion coefficient and concentration in these inhomogeneous membranes. It was found that the variations in these parameters are most considerable in a membrane with a high degree of grafting. Ionic conductivities measured with impedance spectroscopy were in agreement with calculated values obtained on the basis of the SECM measurements.

Performance of Liquid Fuels in a Platinum-Ruthenium-Catalysed Polymer Electrolyte Fuel Cell

Introduction Polymer electrolyte fuel cells (PEFCs) are promising electrochemical power generators, especially for transport and portable applications. Pure hydrogen or a hydrogen-rich gas is often used as the fuel in order to obtain high electrical efficiency at ambient temperature. However, storage, transport and refuelling are more problematic for a gaseous fuel than for a liquid. If fuel reforming is used, this renders the system even more complex. Therefore, research has recently been focused on the use of hydrogen carriers such as aqueous solutions of small organic solvents, which offer easier storage, improved safety and high energy densities (between 1750 and 7080 Wh l depending on the component) compared to hydrogen gas (180 Wh l at 1000 psi and 25oC) (1). At present, of the liquid systems, only the direct methanol fuel cell (DMFC) has been widely studied (2–7). Disadvantages of methanol include its toxicity, high flammability and tendency to permeate through the widely-used Nafion membrane – a phenomenon known as ‘crossover’. However, several other organic fuels that possess higher boiling points and lower crossover rates than methanol have shown promising preliminary results (8–13). At low temperatures, Pt nanoparticles are widely used as catalyst materials for organic fuels in the fuel cell anode. However, pure Pt catalysts only extract the hydrogen atoms from the carbon chain. They cannot cleave the carbon–carbon bond or oxidise the carbon–oxygen bond at the range of potentials normally encountered in a liquid-fuelled fuel cell. Therefore, Ru is added to offer a possible catalyst site for water to decompose to hydrogen and hydroxyl (OH) groups at lower potentials (6, 14), enabling the organic fuel to be oxidised completely to carbon dioxide.

Integration of carbon felt gas diffusion layers in silicon micro fuel cells

We have integrated carbon felt, a traditional fuel cell gas diffusion layer, with silicon micro fuel cells. To this end we used two silicon microfabrication procedures using reactive ion etching: formation of black silicon and sinking of flowfield. The former decreases electrical contact resistance to the diffusion layer, the latter serves to contain the reactant gases. The micro fuel cells, where the flowfield was covered by black silicon nano-needles, showed better performance (127 mW cm−2) compared to the same cells without black silicon (114 mW cm−2). The black silicon fuel cells were also more stable during an overnight chronoamperometric measurement.

Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis

Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys.

Methanol, Ethanol and Iso-Propanol Performance in Alkaline Direct Alcohol Fuel Cell (ADAFC)

The performance of an alkaline direct alcohol fuel cell (DAFC) supplied with a FAA-2 membrane as an electrolyte and utilizing methanol, ethanol and iso-propanol solutions as a fuel was studied. In addition, the permeability of these fuels through the membrane is investigated at different concentration to facilitate the interpretation of the fuel cell results. The permeability decreased with the increasing size of the alcohol molecule and it was some ten times lower through FAA-2 compared to Nafion® 115 for all the studied fuels. In the alkaline DAFC, the highest power density was obtained with 1 mol dm-3 iso-propanol (0.75 mW cm-2). However, the cell showed good performance with more concentrated fuels, even with 15 mol dm-3 methanol solution. Solution resistances of these MEA materials at different fuels and concentrations were obtained with electrochemical impedance spectroscopy.