The lack of a polymer electrolyte membrane (PEM, e.g. Nafion) in membraneless, laminar flow-based micro fuel cells (LF-FCs) eliminates several PEM-related issues such as fuel crossover, cathode flooding, and anode dry-out, as we reported previously. This paper explores the media flexibility of LF-FCs by working in acidic and alkaline media, as well under “mixed-media” conditions in which the anode is in acidic media while the cathode is in alkali, or vice versa. Operating a fuel cell under alkaline conditions has positive effects on the reaction kinetics, both at the anode and cathode, while the cell performance under “mixed-media” conditions offers an opportunity to increase the maximum achievable open cell potential (OCP). The lack of media-related constraints and the simplicity of the LF-FC design allow for these experiments to be performed consecutively in a single LF-FC without changing the system, except for altering the composition/pH of the fuel and oxidant stream. The performance of LF-FCs operated with different media is described and compared.

This communication reports the design and characterization of an air-breathing laminar flow-based microfluidic fuel cell (LFFC). The performance of previous LFFC designs was cathode-limited due to the poor solubility and slow transport of oxygen in aqueous media. Introduction of an air-breathing gas diffusion electrode as the cathode addresses these mass transfer issues. With this design change, the cathode is exposed to a higher oxygen concentration, and more importantly, the rate of oxygen replenishment in the depletion boundary layer on the cathode is greatly enhanced as a result of the 4 orders of magnitude higher diffusion coefficient of oxygen in air as opposed to that in aqueous media. The power densities of the present air-breathing LFFCs are 5 times higher (26 mW/cm2) than those for LFFCs operated using formic acid solutions as the fuel stream and an oxygen-saturated aqueous stream at the cathode ( approximately 5 mW/cm2). With the performance-limiting issues at the cathode mitigated, these air-breathing LFFCs can now be further developed to fully exploit their advantages of direct control over fuel crossover and the ability to individually tailor the chemical composition of the cathode and anode media to enhance electrode performance and fuel utilization, thus increasing the potential of laminar flow-based fuel cells.

We report the first direct measurement of CO diffusion on nanoparticle Pt electrocatalysts at the solid/liquid interface, carried out using 13C nuclear magnetic resonance (NMR) with a spin-labeling pulse sequence. Diffusion parameters were measured in the temperature range of 253-293 K for CO adsorbed on commercial Pt-black under saturation coverage. 2H NMR of the same system indicates that the electrolyte remains in the liquid state at temperatures where the CO diffusion experiments were performed. The CO diffusion parameters follow typical Arrhenius behavior with an activation energy of 6.0 +/- 0.4 kcal/mol and a pre-exponential factor of (1.1 +/- 0.6) x 10-8 cm2/s. Exchange between different CO populations, driven by a chemical potential gradient, is suggested to be the main mechanism for CO diffusion. The presence of the electrolyte medium considerably slows down the diffusion of CO as compared to that seen on surfaces of bulk metals under UHV conditions. This work opens up a new approach to the study of surface diffusion of adsorbed molecules on nanoparticle electrode catalysts, including the possibility of correlating diffusion parameters to catalytic activity in real world applications of broad general interest.

In this paper, we report some new findings obtained by in situ radiotracer and voltammetric studies of the simultaneous adsorption of HSO4 −/SO4 2− ions labelled with 35S, and Cr-containing species labelled with 51Cr on a gold electrode in the course of dichromate reduction in 1 mol dm−3 ClO4 − supporting electrolyte at various pH values (pH=0 and pH=2.0). Special attention is paid to show a modified version of the in situ radiotracer “foil” method, as well as to present a detection and colculation procedure elaborated for the quantitative evaluation of the surface excess of radiolabelled Cr species via measurement of the intensity of low energy X-rays (E=4.90 keV) emitted by 51Cr. From our experimental results it can be stated that: (i) the electroreduction of Cr(VI) particles presumably proceeds via a ce (chemical-electron-transfer) mechanism to yield a gold surface covered with intermediate surface adlayer containing Cr(VI) species and added anions (HSO4 −/SO4 2−, ClO4 −); (ii) the extent and mechanism of adlayer formation are affected by the solution pH values; (iii) the maximum surface excess of Cr-containing species (Γ pH=0=1.2×-9 mol cm-2; Γ pH=2.0= 1.6×10-9 mol cm-2), as well as the molar ratio between the Cr species and HSO4 −/SO4 2− ions (m pH=0≈6 at E=0.8 V; m pH=2.0≈2 at E=0.05 V) attest that the coverage of the gold surface with intermediate complexes does not exceed one monolayer.

The influence of solution pH on chromium(VI) deposition from 0.1 M NaCl + 0.1 mM Na2CrO4 solution onto Al 1100 alloy was studied under open circuit potential (OCP) condition. To monitor the deposition of chromium(VI) on the aluminum surface we applied an experimental methodology based on the energy selective measurement of the characteristic Kalpha, beta X-radiation emitted by the Cr-51-labeled chromate species. Data obtained from this in situ radiochemical analysis were complemented by X-ray photoelectron spectroscopy (XPS) and electrochemistry. The results of these analyses indicated that both the kinetics and the extent of deposition are strongly pH dependent. Specifically, after 2 h of solution exposure, chromate deposition exhibited a maximum surface coverage at pH 2. Chromate, even in 0.1 mM concentration, was found to effectively reduce the chloride content of the oxide film. In addition, the presence of chromate in the solution phase was found to alter the hydration and thickness of the passive film. The XPS analyses revealed that chromate not only adsorbed but also electrochemically interacted with aluminum, creating a Cr(III,VI)-rich layer on the surface. The Cr(III,VI) species is bound to the passive film in a largely irreversible manner. (C) 2002 The Electrochemical Society.

The specific adsorption of sulfate ions on Cr2O3 was studied by a radiotracer technique using 35S-labelled sulfuric acid in low concentrations (c<10–3 mol dm–3) in the presence of a large excess of perchlorate supporting electrolyte. The pH and concentration dependence were determined. It was found that the extent of adsorption is determined by the protonation of the surface sites, similar to other oxides studied previously. A comparison of Cr2O3 and Al2O3 in this respect shows that the protonation of the former takes place at significantly lower pH values than that of the latter. The indirect radiotracer study of the adsorption of chromate on Al2O3 was carried out using labelled sulfate ions as indicator species. The results obtained show that the adsorption strength of chromate species is very low in comparison to sulfate ions and a regular Langmuir-like adsorption behaviour can be observed. It is believed that the observations presented may contribute to a better understanding of the behaviour of surface layers with a mixed oxide content.

The specific adsorption of sulfate ions on Cr2O3 was studied by a radiotracer technique using (35)Slabelled sulfuric acid in low concentrations (c < 10(-3) mol dm(-3)) in the presence of a large excess of perchlorate supporting electrolyte. The pH and concentration dependence were determined. It was found that the extent of adsorption is determined by the protonation of the surface sites, similar to other oxides studied previously. A comparison of Cr2O3 and Al2O3 in this respect shows that the protonation of the former takes place at significantly lower pH values than that of the latter. The indirect radiotracer study of the adsorption of chromate on Al2O3 was carried out using labelled sulfate ions as indicator species. The results obtained show that the adsorption strength of chromate species is very low in comparison to sulfate ions and a regular Langmuir-like adsorption behaviour can be observed. It is believed that the observations presented may contribute to a better understanding of the behaviour of surface layers with a mixed oxide content.

A suppressing/enhancing effect of chloride, nitrate, and perchlorate on phosphate deposition was studied on aluminum in dilute sodium phosphate solutions under open-circuit conditions. Despite weak adsorption of these anions on the oxidized aluminum surfaces, the deposition of phosphate in chloride-containing and chloride-free solutions is markedly different. While in the chloride-containing solution the deposition extended over several days and resulted in a large phosphate uptake, the deposition of phosphate in chloride-free solutions occurred within 1 h (or shorter) and produced only a thin oxide film. That is, some anions can "turn on" and some can "turn off" the multilayer phosphate deposition process. We believe that such a difference is due to the effect of the anions on aluminum dissolution. Chloride, by accelerating the release of aluminum ions from the metal through pitting corrosion, enhances the deposition of the phosphate-rich aluminum oxide. Perchlorate neither promotes nor inhibits the phosphate deposition, while nitrate, by mitigating the pitting corrosion, strongly retards the phosphate-rich film formation even in the presence of chloride ions. In this case, phosphate deposition proceeds via sorption rather than by a precipitation mechanism. The presented data can be read as evidence for a strong correlation between aluminum corrosion/corrosion inhibition and phosphate deposition processes. (C) 2000 The Electrochemical Society. S1099-0062(00)01-076-2. All rights reserved.

The sorption of phosphate ions on aluminum was studied by an in situ radiotracer technique. The kinetics and reversibility of sorption and the influence of the solution pH on phosphate coverage were followed in 0.1 mol dm−3 NaCl solution. It was found that phosphate adsorption on aluminum is determined by the surface charge and the protonation state of phosphate in the bulk solution. Despite the high chloride concentration, significant phosphate sorption was identified, indicating strong phosphate binding to the surface. Phosphate sorption increased with pH up to pH=4.7. As the pH of the solution increased further, phosphate desorbed from aluminum. The exchange experiments indicated that the phosphate sorption on the passive aluminum surface is a largely irreversible process. It is assumed that, in near neutral solution, there is an ongoing surface transformation, i.e., oxide film growth, that entraps phosphate ions from the solution.