CLOSING THOUGHTS

A general observation made while surveying the DMFC literature is the unsettling lack of replicates in many studies. Frequently, one test cell is compared to one control cell and no statistics are provided to gauge cell performance. The incom­plete reporting of test condtions further confounds comparison. While impractical for some studies, an analysis of cell performance over extended periods of time and with intermittent startup and shutdowns should be the standard. As a result of the limited data, broad conclusions drawn from these studies must be assessed with caution.

As can be seen from the breadth of topics covered in this chapter, many avenues are being pursued to make DMFC technology a practical reality. Effort is expended in the engineering of hardware and fuel delivery systems and in developing PEMs impermeable to methanol. That being said, the outlook for DMFC technology appears mixed. At this point, the development of alternative (e. g., less expensive, more effective, more robust) electrocatalysts appears to be the foremost obstacle in making DMFCs a practicable power generation alternative.

An important study by Zelenay et al. shows that use of the highly active PtRu electrocatalyst present in the great majority of DMFC anodes (see Tables 9.2 and 9.3) will contaminate the cathode with Ru [43]. The migration of Ru occurs under nearly all operating conditions, including conditions where the cell is only humid­ified with inert gases and no current is drawn from the cell. Cathode contamination with Ru inhibits oxygen reduction kinetics and reduces cathode electrocatalyst tolerance to methanol crossover. The degree to which Ru contamination occurs depends on such factors as anode potential and operating lifetime of the cell. Depending upon the degree of Ru contamination at the cathode, the associated loss of cell performance is estimated to be from as little as ~40 mV to as much as 200 mV Figure 9.6 shows data for a Pt-only cathode subject to Ru contamination.

The top plots of Figure 9.6, marked “a” and “b,” include CO stripping scans (a) and cyclic voltammograms recorded in the absence of CO (b) for a DMFC cathode made with Pt only as the electrocatalyst. The DMFC cathode was part

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of a 22-cell stack run intermittently for 6 months that experienced voltage reversal. The plots include data for control electrodes, Pt only (short dashed lines) and PtRu (long dashed lines). As can be seen in (a), the Pt-only DMFC cathode behaves in a near identical fashion to the PtRu control, consistent with PtRu in the electrocatalyst layer of the cathode. The CVs of plot (b) shows a similar trend with the traces for the DMFC cathode morphologically similar to the PtRu control.

The bottom plot of Figure 9.6 shows how cell performance can diminish with Ru migration. The plot is the iR corrected voltage-current plot for the DMFC cathode described above. The cell was fed a 0.3-M methanol solution, cell temperature is 70°C, dry air is the oxidant, and no backpressure is applied at the cathode. The solid line plots the initial performance of the cell and the dashed line plots the performance of the cell after inclusion in the DMFC stack for 6 months.

It should be noted the fate of the cathode shown in Figure 9.6 is a rather extreme example of Ru migration and the conditions the cell experienced are not ideal, however, as the researchers found, Ru migration occurs even under the most benign conditions. Similar electrocatalyst migration in the form of agglom­eration was found by Yi et al. following a 75-hour DMFC life test [29]. The PtRu electrocatalyst used by Yi differed from the Zelenay study in that the anode catalyst is carbon supported PtRu rather than PtRu black and suggests that all forms of PtRu electrocatalysts are likely susceptible to migration in DMFC.

In summary, the principle challenges to commercializing DMFC technology are effective and stable electrocatalysts tolerant to methanol contamination of the cathode and membranes significantly less permeable to methanol. The most commonly used separator, Nafion, is not sufficient for the task. Nor are Pt and PtRu electrocatalysts. The outlook for membrane development for the near future seems to be somewhat static. However, promising electrocatalyst developments such as relatively inexpensive Pd-based catalysts that are stable and reaction specific may be the breakthrough that allows for the use of a less than ideal separator such as Nafion.

ABBREVIATIONS

AFC

Alkaline Fuel Cell

BET

Brunauer Emmett and Teller

BOL

Beginning of Life

CS

Chitosan

DARPA

Defense Advanced Research Projects Agency

PPQ

Poly(Phenyl Quinoxaline)

DMFC

Direct Methanol Fuel Cell

EDX

Energy Dispersive X-ray

ELAT

Commercial Gas Diusion Electrode (E-Tek)

PVA

Poly(Vinyl Alcohol)

EOL

End of Life

IEC

Ion Exhcange Capacity

Jmax

Maximum Current Density

MCFC

Molten Carbonate Fuel Cell

MOR

Methanol Oxidation Reaction

MWNT

Multiwall Carbon Nanotubes

NP-PCM

Nanoporous Proton-Conducting Membrane

SSA

Sulfosuccinic Acid

ORR

Oxygen Reduction Reaction

OTC

Operational Test Command

PAFC

Phosphoric Acid Fuel Cell

PBI

Polybenzimidazole

PEFC

Polymer Electrolyte Fuel Cell

PEK

Poly(Ether Ketone)

PEM

Polymer Electrolyte Membrane

PES

Poly(Ether Sulfone)

PFA

Polymerized Furfuryl Alcohol

PSEPVE

Perfluorovinyl Ether

PSU

Polysulfone

SA

Sodium Alginate

SCE

Saturated Calomel Electrode

SEM

Scanning Electron Microscopy

SiO2-PWA

SiO2-Phosphotungstic Acid

SOFC

Solid Oxide Fuel Cell

sPEEK

Sulfonated Poly(Ether Ether Ketone)

SWNT

Single-Wall Carbon Nanotubes

TEM

Transmission Electron Microscopy

TFE

Tetrafluoroethylene

XPS

X-ray Photoelectron Spectroscopy

XRD

X-ray Diffraction Analysis

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