Piotr Waszczuk

A NANOPARTICLE CATALYST WITH SUPERIOR ACTIVITY FOR ELECTROOXIDATION OF FORMIC ACID

This paper reports the synthesis and characterization of a series of Pt-based nanoparticle catalysts with high activity for formic acid electrooxidation. The catalysts were prepared using spontaneous deposition to decorate platinum nanoparticles with controlled amounts of palladium and palladium/ruthenium. Among all the catalysts investigated, the Pt/Pd catalyst shows the best performance; the steady-state formic acid oxidation current is ca. at 0.27 V vs. RHE. This current is two orders of magnitude higher than that obtained from pure platinum, and the catalyst is ideally suited to be an anode in the direct oxidation formic acid fuel cell. The enhancement in formic acid oxidation by the admetal addition does not correlate with the threshold for CO oxidative stripping (the CO tolerance). The Pt/Pd catalyst requires the highest potential to remove the CO, yet it is the most active. We suggest, therefore, that within the dual path mechanism of formic acid oxidation, the direct CO2 formation channel on Pt/Pd is much less affected by the CO chemisorption than on Pt, or on the Pt/Pd/Ru catalyst, also studied in this report.

UHV and electrochemical studies of CO and methanol adsorbed at platinum/ruthenium surfaces, and reference to fuel cell catalysis

This paper reviews surface chemistry of carbon monoxide and methanol in ultra high vacuum (UHV) and in the electrochemical environment on clean and Ru modified Pt single crystal surfaces, and on Pt and Pt/Ru nanoparticles. The results show that CO behaves very similarly in UHV and in the electrochemical environment. Cyclic voltammetry (CV), temperature programmed desorption (TPD) and radioactive labeling all show similar behavior in terms of numbers of peaks, peak splitting etc. Both UHV and CV measurements show that there is about a 200-meV change in the potential for CO removal in the presence of ruthenium. Earlier 13C EC-NMR data indicated a 30% reduction in the Ef-LDOS of CO bound to Ru islands deposited on platinum, and 15% of CO bound to Pt sites, and TPD and CV also show that the binding of CO is modified. The present data confirm that Pt atoms away from Ru are only weakly affected, and the overall CO binding energy modification is quite moderate. We conclude that the changes in the CO binding energy only play a small role in enhancing methanol electrooxidation rates. Instead, the main effect of the ruthenium is to activate water to form OH. Quantitative estimates of the reduction in CO desorption barrier indicate that the effect of bifunctional mechanism is about four times larger than that of ligand effect. In contrast to the results for CO, methanol behaves quite differently in UHV and in an electrochemical environment. Pt(111) is unreactive at room temperature in UHV, while Pt(110) is quite reactive. Initially, clean Pt(111) is less reactive than clean Pt(110) even in the electrochemical environment. However, Pt(110) is quickly poisoned in the electrochemical environment, so at steady state, Pt(111) is more reactive than Pt(110). Another issue is that the mechanism of methanol decomposition is quite different in UHV and in the electrochemical environment. There are three pathways in UHV, a simple decomposition via a methoxonium (CH3O(ad)) intermediate, an SN1 pathway via a methoxonium cation ([CH3OH2]+), and an SN2 pathway via a methoxonium intermediate. So far, none of these pathways have been observed in an electrochemical environment. Instead, the decomposition goes mainly through a hydroxymethyl (CH2OH(ad)) intermediate. These results show that there are both similarities and differences in the behavior of simple molecules in UHV and in the electrochemical environment.

Adsorption of CO poison on fuel cell nanoparticle electrodes from methanol solutions: a radioactive labeling study

Abstract Combined radioactive labeling and electrochemical measurements were conducted to study adsorption and desorption of methanol-derived surface CO on fuel-cell grade platinum and platinum–ruthenium alloy nanoparticle catalysts. The adsorption results were obtained under constant potential and voltammetric conditions, and the experiments were carried out in sulfuric acid solutions containing methanol (at room temperature). The electrode potential effect on the CO coverage, the rates of methanol adsorption and CO desorption, as well as the susceptibility of the CO adsorbate to exchange with bulk methanol (surface/bulk exchange) were investigated. We found that CO adsorption at low electrode potentials was slower than at higher potentials, but higher coverages were obtained at the low potentials. Adsorption, desorption and surface/bulk exchange processes were significantly different on Pt than on Pt/Ru, confirming some of the previous results published in the electrochemical literature. The higher rate of methanol adsorption on Pt/Ru is explained on the basis of electronic modification of platinum by ruthenium using state-of-the-art concepts from the theory of electrocatalytic reactivity. We also report that combined bifunctional and ligand effects account for the difference in desorption kinetics. Finally, the relevance of our observations to the processes occurring on the anode of the direct methanol oxidation fuel cell is highlighted.

Characterization of limiting factors in laminar flow-based membraneless microfuel cells

This paper characterizes the performance-limiting factors of a membraneless microfuel cell in which two aqueous streams flow laminarly in parallel in the absence of a physical membrane without turbulent mixing. The all-liquid configuration allows for easy external addition of a reference electrode, enabling the determination of the type of performance limiting factors such as kinetics and mass transfer limitations, including the source ~anode or cathode!, and the cell resistance. In addition, options to address the present dominating mass transfer limitations at the cathode are discussed.

Oxidation of CO adsorbed from CO saturated solutions on the Pt(111)/Ru electrode

Abstract Using Pt(111) surfaces covered by ruthenium by a spontaneous deposition method we conducted voltammetric oxidation of chemisorbed CO in clean sulfuric acid electrolyte at 50 mV s−1 and under variable sweep rate conditions. The deposition yields Pt(111) surfaces decorated by two-dimensional nanosized ruthenium islands previously reported. At 50 mV s−1, the CO stripping produces two well-resolved current peaks, at 0.55 and 0.67 V versus RHE. The first peak originates from CO chemisorbed on (and strictly around) ruthenium islands deposited on Pt(111), while another comes from CO chemisorbed on ‘pure’ Pt(111) phases of the Pt(111)/Ru electrode, away from the Ru islands. Using double-potential step chronoamperometry we investigated the CO oxidation on these two surface phases, and found that the low potential oxidation of chemisorbed CO occurred via the Langmuir–Hinshelwood mechanism. This shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across an island. In contrast, CO chemisorbed on Pt sites not occupied by Ru is oxidized at the Pt(111)/Ru edge, and there may be a surface diffusion contribution to a complete removal of CO from the surface. However, a part of the CO adlayer oxidation in the 0.67 V peak area occurs with no participation of surface diffusion, according to a pseudo-first order surface reaction kinetics, and the reaction may involve a ‘reactant pair’ (PtOH⋯OCPt) breakdown on the electrode surface.

Kinetic Study of Electro-oxidation of Formic Acid on Spontaneously-Deposited Pt/Pd Nanoparticles CO Tolerant Fuel Cell Chemistry

Pt/Pd catalysts have recently been found to have exceptional properties in formic acid fuel cells. In this paper, the kinetics of formic acid electro-oxidation on spontaneously-deposited Pt/Pd nanoparticles were measured to provide a basis for fuel cell design. The results confirmed previous findings that palladium covered platinum catalysts showed exceptional activity for formic acid electro-oxidation. The Pt/Pd catalysts were a factor of thirty more active than Pt under the conditions examined. The catalysts also continued to function as CO built up on the catalyst surface and showed reasonable activity. Thus, the catalysts were CO tolerant. In addition, the results showed that the Tafel slope of formic acid electro-oxidation on Pt/Pd was time dependent. Generally, the Tafel slope decreased as COad was deposited on the surface, suggesting a change in rate determining step with CO coverage. The formic acid concentration effect was also studied in detail to give guidance to the practical fuel cell design. Similar studies on Pt nanoparticles were performed for comparison

Membraneless Fuel Cell Based on Laminar Flow

An increasing societal demand for a wide range of small, often portable devices that can operate for an extended period of time without recharging has resulted in a surge of research in micropower sources. Most efforts in this area focus on downscaling of existing fuel cell technology such as the well-known proton exchange membrane (PEM) fuel cells. Here we study a novel concept for fuel cells: the use of laminar flow instead of a physical barrier such as a PEM to separate the fuel and oxidant streams. Laminar flow, i.e. low Reynolds number flow, is a property of fluid flow at the microscale: one or more liquid streams that are brought together under low Reynolds number conditions flow in parallel and contact with each other without turbulent mixing. Mass transport transverse to the direction of flow takes place by diffusion only. In our laminar flow-based fuel cell a fuel-containing stream and an oxidant-containing stream are brought together in laminar flow conditions with the electrodes placed on opposite walls within the channel. In un-optimized fuel cell configurations, current densities as high as 10 mA/cm2 are obtained at room temperature using different fuels such as methanol or formic acid vs. oxygen saturated solvents or other oxidants.© 2003 ASME

A nanoparticle catalyst with superior activity for electrooxidation of formic acid [Electrochem. Commun. 4 (2002) 599-603]

This paper reports the synthesis and characterization of a series of Pt-based nanoparticle catalysts with high activity for formic acid electrooxidation. The catalysts were prepared using spontaneous deposition to decorate platinum nanoparticles with controlled amounts of palladium and palladium/ruthenium. Among all the catalysts investigated, the Pt/Pd catalyst shows the best performance; the steady-state formic acid oxidation current is ca. 1μAcm−2 at 0.27 V vs. RHE. This current is two orders of magnitude higher than that obtained from pure platinum, and the catalyst is ideally suited to be an anode in the direct oxidation formic acid fuel cell. The enhancement in formic acid oxidation by the admetal addition does not correlate with the threshold for CO oxidative stripping (the CO tolerance). The Pt/Pd catalyst requires the highest potential to remove the CO, yet it is most active. We suggest, therefore, that within the dual path mechanism of formic acid oxidation, the direct CO2 formation channel on Pt/Pd is much less affected by the CO chemisorption than on Pt, or on the Pt/Pd/Ru catalyst, also studied in this report.