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.
Study of the diffusion of small molecules on catalyst surfaces is of broad general interest, and there have been numerous investigations of surface CO diffusion on Pt under ultrahigh vacuum (UHV) or gas phase conditions. 1-7 Both diffusion coefficients (D CO ) as well as activation energies (E d ) for diffusion have been measured and are of importance in the context of, among other topics, CO hydrogenation in fuel synthesis 8 and CO oxidation in heterogeneous catalysis. 9 The latter topic is also of interest in the context of fuel cell catalysis, but there has been no direct experimental determination of D CO in an electrochemical environment due to problems associated with the presence of the electrolyte. 10 Fortunately, however, NMR methods are not plagued by these problems, [11][12][13] and in this paper, we report the first direct determination of the diffusion constants of CO on Pt in a liquid electrochemical environment, together with the activation energy for diffusion, using the techniques of electrochemical NMR (EC-NMR) 11-14 and selective spin inversion NMR. 7 To determine diffusion constants, we used the "S-shape" pulse sequence developed by Becerra et al. 7 The S-shape pulse sequence (Figure 1) exploits the fact that CO molecules adsorbed on a Pt nanoparticle can have different 13 C resonance frequencies, depending on the angle of CO's unique molecular axis with respect to the external magnetic field. A part of the magnetization is selectively inverted by the first two pulses, and the 13 C spins are then allowed to diffuse to different regions of the nanoparticle during the evolution period T ev . Motion of a CO molecule due to surface diffusion alters the 13 C spin's Larmor frequency (ω), and experimentally, the amplitude M + (T ev ) of the noninverted part of the spectrum is measured for various values of T ev . If only T 1 processes are involved, M + (T ev ) grows back to its equilibrium value independent of T ev , but when CO molecules diffuse, a mixing of inverted and noninverted parts of the nuclear magnetization occurs, leading to an initial decrease in M + (T ev ), which then grows back to its equilibrium value with increasing T ev .To calculate the diffusion rate, we follow the time evolution of a normalized signal amplitude, A + , defined by the following equation, at various T ev :where ∆ is the line width and ω 0 is the frequency where M + (T ev ) has its maximum; λ n and A n are the coefficients from a Fourier series solution that are determined by boundary and initial conditions. 7 D ω (the diffusion coefficient in the frequency domain) is obtained as the sole fitting parameter to eq 2, and D ω can then be converted to D CO , the diffusion constant, using the relation, D CO ) (π 2 d 2 /32Ω 2 )D ω , where d is the average particle diameter, and Ω is the upper bound for diffusion in the frequency domain, as reported elsewhere. 7 NMR measurements were carried out on 13 CO adsorbed onto electrochemically cleaned platinum black in 0.5 M D 2 SO 4 using a home-built 8.47 T NMR spectrometer. 14...
This paper reports a study of thin films of Pt and Pt-alloys deposited on oriented crystalline organic whiskers using high resolution transmission electron microscopy imaging. It is our opinion that these nanostructured thin film (NSTF) electrocatalysts are the leading electrode technology for polymer electrolyte fuel cells because of their significant advantages in durability and specific activity. The sputterdeposited thin catalyst films on the whiskers grow as polycrystalline layers that expose highly oriented fcc crystallites. Within the films, truncated pyramid-shaped crystallites grow into acicular whiskerettes on the whisker sides, and the whiskerette shapes display a growth mechanism governed by a strong support-metal interaction and surface energy minimization. The surface structure of the crystalline whiskers facilitates metal nucleation sites that are spaced evenly along the whisker length by ∼6-8 nm. The whiskerettes are the dominant structural form of the NSTF catalyst coatings at the loading levels investigated. Along just a few nanometers of a whisker's long axis, the whiskerettes can exhibit changes in aspect ratios up to a factor of 5 and thus may increase the surface roughness by a factor of 6. Diffractogram indexing suggests that the truncated pyramids expose four (111) and one (100) planes, which grow further into pillars by elongating their base along two of the (111) facets. When the atomic scale roughness is disregarded, the supported NSTF Pt-based electrocatalysts expose predominantly (111) facets.
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