Electrochemical Pt surface area

Pt Catalyst-Coated Electrode Surface to Obtain the Electrochemical Pt-Surface Area (EPSA) 189... 

In fuel cell testing and diagnosis, CV is used to determine the electrochemical Pt surface area (EPSA) of a catalyst layer, and LSV is used to evaluate and monitor fuel crossover. For these methods, pure H2 and pure inert N2 or He are passed over the anode and the cathode, respectively. The anode is used as both the reference and counter electrode, whereas the cathode is used as the working electrode. 

RDF and RRDE techniques have been widely used for studying the kinetics of the ORR [1-10] and HOR [11-17] in PEM fuel cells. Particularly in ex situ evaluations of catalysts and catalyst layers, the conventional half-cell is used to measure the electrochemical Pt surface area (EPSA), ORR mass activity, catalyst stability, as well as non-noble metal catalyst activity and stability, as described in later sections of this chapter. 

The stability during potential cycling and ORR activity of Pt (20 wt°/o) supported on MWCNTs and carbon black was also investigated [136]. Two different potential cycling conditions were used, namely lifetime (0.5 to 1.0 V vs. RHE) and start-up (0.5 to 1.5 V vs. RHE). Pt supported on MWCNTs catalyst exhibited a significantly lower drop in normalized electrochemically active surface area (ECA) values compared to Pt supported on Vulcan (Fig. 14.10), showing that MWCNTs possess superior stability to commercial carbon black under normal and severe potential cycling conditions [137]. 

Figure 6.33. Trends in Pt surface-area normalized electrochemical activity of various ternary alloy compositions with respect to the electrooxidation of methanol. Activity gains are seen in the order Pt, PtRu, PtRuNi, and PtRuCo (adapted from [69]).

Fig. 11.12 Electrochemical activity (area-specific activity) normalized by the surface area of Pt for the most active alloys. The Pt surface area was determined electrochemically using the hydrogen adsorption integral of each catalyst.

Note that the farther away the electric potential of the carbon surface is from the potential of zero charge point ( pzc) l e higher the disjoining pressure is. In principle, this may result in a systematic variation of the support pore size in Me/C catalysts with potential (similar to the electrocapillary curve [96,97]) and consequently the efficiency of metal particle blocking by the pore walls. Such behavior of porous carbons obviously can influence the measurements of the electrochemically active surface area and might be one of the reasons for the observed correlation between the apparent dispersion of Pt/C catalysts, measured by cyclic voltammetry, and pHpzc of the supports [95], whereas no noticeable difference in the particle size has been observed with HRTEM. Undoubtedly, this problem needs further investigation. 

Figure 6 Comparison of the mass activity for ORR at 0.9 V as a function of electrochemically active surface area (m /gm Pt) for supported Pt and Pt alloy electrocatalysts. (From Ref. 18.)...

In all three MEAs the rate of methanol oxidation was facilitated by the platinum-ruthenium unsupported catalyst, which in the presence of CO as a byproduct of the reaction, exhibit an electrochemical activity higher than pure Pt. However, compared to Pt supported and unsupported catalysts, the electrochemically active surface area of PtRu alloys cannot be determined by hydrogen adsorption using cyclic voltammetiy due to the overlap of hydrogen and oxygen adsorption potentials, and the tendency for hydrogen to absorb in the ruthenium lattice [xvi]. However, under the same operation conditions, cyclic voltammetry can be used for qualitative estimation of the similarity in the PtRu anode layer properties. 

The electrochemical activity of the catalyst is normally measured in the electrolyte solution saturated with inert gas such as pure N2 or Ar and the electrode is at static state (not rotated). The purpose is to obtain information about the redox activity of the catalyst itself or some information about the catalyst surface behavior, from which the catalyst electrochemical active surface area (for Pt-based catalysts) or the concentration of catalyst active center (for non-noble metal catalysts) an be obtained. To give some basic sense about these measurements, we will give two examples as follows. 

A small amount of Pt was loaded on WC, and the Pt/WC electrocatalyst was tested for hydrogen oxidation. Electrocataljdic activities can be evaluated by measuring electrochemical active surface area (EAS, m /gpt) from cyclovoltammetry. 10 wt% Pt/WC prepared via TPRe of various tungsten oxide based precursors showed 5.9-11.4 m /gpt in 0.5 Af H2SO4 electrolyte at room temperature (28). 7.5 wt% Pt/W2C and Pt/WC fabricated by polymer-induced carburization led to high EAS value of 327 m /gpt (33) and 316 m /gpt (34) in 1 Af H2SO4 electrolyte. 

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