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14 декабря, 2021
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GENERAL OPTIMIZATION
The following section cites selected studies aimed at optimizing the performance of DMFCs through careful variation of design, materials, and operating conditions. An excellent study of a wide range of experimental conditions is presented first, then issues of cathode flooding, electrolyte/electrode contact, parasitic power loss associated with fuel pumping, electrode design, and CO2 bubble formation are considered.
A systematic study by Liu and Ge varied operational parameters such as cell temperature, methanol concentration, anode flow rate, air flow rate, and cathode humidification, and showed that changing any one of the parameters has a pronounced effect on the performance of the DMFC [30]. However, varying cathode humidification has negligible impact on DMFC performance. The range of parameters evaluated are listed in Table 9.4. In general, higher cell temperatures lead to better DMFC performance; however, other processes that diminish cell performance as temperature increases such as methanol crossover and water transfer from cathode to anode set a limit on optimum performance. The study found the optimal methanol solution to have a concentration between 1 and 2 M. This is in general agreement with other studies that found the optimum concentration to be 2.0 M [5] and 2.5 M [31] for DMFCs run under similar operating conditions. Methanol crossover from the anode to the cathode can be minimized by increasing either the cathode air flow rate or oxygen partial pressure. The work suggests that the cathode structure and operating conditions play a major role in DMFC performance. The reference by Liu and Ge contains a large amount of data, both plotted and tabulated, and is a useful resource for making comparisons of DMFC performance over a range of conditions.
For DMFCs power plants, performance degradation occurs when water builds up at the cathode. Knights et al. describe a load-cycling strategy to reduce cathode flooding [27]. By removing the load of the DMFC for 30 seconds of every 30 minutes of operation, the rate for performance degradation is shown to be 13 pV hr-1 over 2000 hr of failure-free operation, which is in the low range of the typical performance degradation rate of 10 to 25 pV hr-1.
The use of solid PEMs such as Nafion prevents electrolytes from fully enveloping the electrode as liquid electrolyte does. This limits the reaction area to points of direct contact between membrane and electrode. To increase the reactive area, Nafion suspension is often compounded directly into the catalyst layer. Sudoh et al. use the spray method to optimize the electrochemical characteristics of the catalyst layer [37]. The spray method consists of introducing a catalyst to the electrode surface and then spraying Nafion over the catalyst. Three Nafion loadings are considered: 0.5, 1.0, and 3.5 mg cm-2. The resulting electrodes are incorporated into DMFCs and the performance measured. The cell made with catalyst layers having a Nafion loading of 1.0 mg cm-2 performed the best producing 258 mA cm-2 at 0.4 V. The cell made from electrodes with 0.5 mg cm-2 Nafion loading generated roughly half the current and the cell with the highest Nafion loading was resistive and performed poorly. The performance of the spray-coated, in-house DMFC is similar to that of commercially available ELAT® electrode with similar catalyst loading (0.5 mg cm-2). ELAT is the trademarked name of gas diffusion electrode material distributed by E-Tek, Inc. It is frequently used in PEFCs and DMFCs.
Zhang and Wang present a piezoelectric micropump design for delivering fuel to a miniaturized DMFC power source [39]. For low current densities (<100 mA cm-2) methanol concentrations between 0.5 M and 2.0 M do not significantly impact the power generated. The authors suggest their DMFC running at 40°C will have a maximum current density, or Jmax of 120 mA cm 2 at 0.35 V for a 1-W system where a 25 cm2 cell will be required. The estimated power consumption of their piezoelectric pump operating at 100 Hz, pushing 1 ml min-1 over the face of the cell is on the order of 70 mW, or 7% of the power produced by the cell, which compares well with other literature examples.
Typical DMFC anode structures consist of strata of a supported/unsupported catalyst bonded with a Nafion suspension over Teflon-bonded carbon black
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![Подпись: Naf. 117 PtRu 3.0 6.0 Pt 3.0 n/a 1200 air 70 2.0 83 330 Naf. 117 PtRu 3.0 6.0 Pt 3.0 n/a 600 O2 70 2.0 132 243 Naf. 117j PtRu 4.0 0.5 Pt 4.0 0.1 20 O2 40 1.0 40 350 [39] Naf. 112 PtRu/C 3.0 9.0 Pt/C 3.0 0.3 105 O2 120 2.5 127* 550 Naf. 117 PtRu 1.0 n/a Pt 4.0 0.15 50 air 80 0.5 50* 500](/img/1174/image050.gif "image050") |
a Membrane has 1-иш sputtered Pd-Ag film b Pd impregnated Nafion; 0.0214 g Pd/cm3 of Nafion c Pd sputtered membrane d Polyaryl blend of PEK, PBI, and bPSU e Nafion has 14% mass gain from polystyrene
f Multiwall carbon nanotube support with Fe — and Ni-contaminated catalyst g Cathode impregnated with 1.0 mg cm-1 Nafion ionomer h Sulfonated poly(4-phenoxybenzoyl 1,4-phenylene)
1 Sulfonated poly[bis(3-methylphenoxy) phosphazene]
j Power calculated from model based on Nafion 117 DMFC data [40] minus power needed to drive piezoelectric pump * Not necessarily maximum power. T Reference has performance data over wide range of conditions
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k Anode made from Ti mesh with electrodeposited catalyst layer 1 Membrane is 5-^m thick copolymer of TFE and ethylene
m MEAs made of half cells placed back to back so Nafion thickness between electrodes is 7 mil and potential listed for cell is iR corrected n MEA run as component of 22-cell DMFC stack for 6 months o Same cell as listed with “n” superscript prior to use in DMFC stack p 100-^m thick PEMs made from recast Nafion and 3 wt % SiO2 — PWA filler q Single-cell DMFC after 75-hour lifetime test
r Same cell as listed with superscript “q” superscript prior to 75-hour lifetime test s 1 wt % CeO2 doped cathode t Catalyst heat treated at 500°C u No heat treatment
v Listed value is Pt loading; Pt to Ti ratio of 1:1 w Ratio Pd to Pt is 3:1
x Ratio Fe to Pt is 1:1; Pt loading 1.0 mg cm-2
Operating Conditions and Range over Which DMFC Performance is Evaluated
Parameter
Cell Temperature
Methanol Concentration
Cathode Humidification Temperature
Anode Flow Rate
Air Flow Rate
diffusion layer over carbon cloth or paper diffusion layer. This structure is an ineffectual design for the transport and release of CO2 gas produced by methanol oxidation and limits methanol transport to the anode. To remedy this problem, Scott et al. directly deposit PtRu catalyst onto a titanium mesh by electrodeposition and subsequent thermal decomposition and use the coated mesh as the anode [41]. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction analysis (XRD) are used to characterize the electrodes. Anodes are tested under galvanostatic control as well as in DMFCs. Galvanostatic testing shows the mesh and conventional anodes have similar electrochemical performance. This is somewhat unexpected given the very different morphologies of the strata and electrodeposits of electrode types. Tests using the anodes under working DMFC conditions mirror the performance of the anode tests. Catalysts loadings are in the range of 0.8-1.0 mg cm-2.
An often overlooked limitation in DMFC performance is CO2 bubble formation in the anode. Kulikovsky developed a simple DMFC model to determine how anode channel bubble formation impacts cell performance [52]. Under conditions simulating typical operating conditions, the model suggests that moderate to severe bubble formation decreases the mean methanol concentration as it passes through the anode channel, limiting the current that can be drawn for the cell. Under severe bubbling conditions, the limiting current that can be drawn from the cell is diminished by as much as a factor of four. The author speculates that faster flow rates may help offset some performance losses due to bubble formation and offers some calculations to support his speculation, but he also cautions that kinetics of bubble formation are outside the scope of the model.