Environmental Engineering Reference
In-Depth Information
Despite the above limitations, electrocatalytic activities are often evaluated from
polarization measurements in a single-cell configuration (for references, see the
review [Gasteiger et al., 2005]). Various methods are applicable for the investigation
of electrocatalytic materials, including conventional electrochemical [Gasteiger et al.,
2005; Dinh et al., 2000], impedance spectroscopy [Diard, 1998; Mazurek et al., 2006],
and recently also in situ Fourier transform infrared (FTIR) spectroscopy [Tkach et al.,
2004] and X-ray absorption (XAS) spectroscopy (see the review [Russell and Rose,
2004] and references therein), provided that the characteristic time of the measure-
ments exceeds the time constant of the cell. The high selectivity of XAS allows detec-
tion of particular elements in multicomponent fuel cell systems. XAS measurements in
transmission form, however, require either removal of the catalyst from the side of the
MEA not under investigation [Roth et al., 2002] or exclusion of the absorbing element
from this electrode [Viswanathan et al., 2002]. In situ XAS allows the monitoring of
transformations of particle size, morphology, and oxidation state of individual com-
ponents, and, for particles with sufficiently high dispersion, also detection of adsor-
bate molecules [Russell and Rose, 2004]. Nevertheless, studies of size effects in
fuel cell configurations are rare, because of the complexity of the MEA and its nonuni-
form structure and composition.
Another approach to measuring the activity of fuel cell catalysts is based on gas dif-
fusion electrodes (GDEs) in the so-called “half-cell” configuration (Fig. 15.2b). The
catalytic layer is prepared as outlined above and sandwiched between a gas diffusion
medium and a proton-conducting membrane, the other side of the latter being in con-
tact with an aqueous electrolyte of a conventional electrochemical cell. The working
electrode potential is measured against a reference electrode within the accuracy of the
ohmic drop in the PEM, and processes at the counter-electrode do not affect the
catalyst under study [Antoine et al., 1998]. Hence, the first of the above-mentioned
limitations is lifted, while the second and third are partially relaxed, allowing the appli-
cation of a wider range of experimental techniques with somewhat shorter time
constants. Compared with the single-cell approach, this allows more detailed studies
of electrochemical kinetics and size effects. Antoine and co-workers applied the GDE
approach to the investigation of size effects in the HOR and ORR [Antoine et al.,
1998]. While the results of this study will be discussed in more detail in Section
15.5, here we would like to mention that Antoine et al. concluded that ORR kinetic
currents could be measured quite accurately, whereas those for the HOR were notice-
ably influenced by hydrogen mass transport. The latter currents were therefore sub-
jected to correction using a mathematical model taking into account mass transport
losses in the GDE and the discrete character of the nanoparticle distribution, resulting
in the overlap of diffusion zones at adjacent catalytic particles.
At the next level of abstraction are measurements performed at a thin film of fuel
cell catalyst immobilized on the surface of an inert substrate, such as glassy carbon
(GC) or gold (Fig. 15.2c). Essentially, three versions of this approach have been
described in the literature. In the first case (a “porous electrode”), an ink containing
catalyst and Nafion
w
ionomer is spread onto an inert nonporous substrate
[Gloaguen et al., 1994; Gamez et al., 1996; Kabbabi et al., 1994]. In the second
case (a “thin-film electrode”), the ink does not contain Nafion
w
, but the latter is
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