Polarization and power density curves

The OCV measured for an MFC is the maximum voltage that can be obtained with the system, with the limitations imposed by the specific bacterial community and the obtained OCR of the cathode. For an MFC, as with any power source, the objective is to maximize power output and therefore to obtain the highest current density under conditions of the maximum potential. The OCV is only achieved under a condition where there is infinite resistance. As we reduce that resistance, we lower the voltage. Thus, we look to have the smallest possible drop in voltage as the current is increased in order to maximize the power production over the current range of interest. 

The power density curve is calculated from the measured voltage as P = E /R, or alternatively as P = l R. Be careful when calculating power not to use a surface normalized current (i.e., I divided by electrode surface area) as the power will then be calculated incorrectly when the term is squared. 

MFC researchers typically use the top of the power curve to report the maximum power , which for this case shown in Fig. 4.2B would be 700 mW/m. When reporting polarization and power densities it is important to include the OCV and show a complete curve up to the maximum power, and then include a few points to the right of the maximum power to fully establish the peak in the power density curve. 

Activation losses are due to energy lost (as heat) for initiating the oxidation or reduction reactions, and the energy lost through the transfer of an electron from the cell terminal protein or enzyme to the anode surface i.e., the nanowire, mediator, or terminal cytochrome at the cell surface). These losses are especially apparent at low current densities i.e., the first region in Fig. 4.3). They can be reduced using improved catalysts at the cathode, different bacteria on the anode, or by improving electron transfer between bacteria and the anode. 

Concentration, or mass transfer losses, arise when the flux of reactants to the electrode or the flux of products from the electrode are insufficient and therefore limit the rate of reaction. At the anode, the substrate flux to the anode has yet to be an obvious problem in MFC operation (see Chapter 6) as there is little evidence that we have achieved maximum possible power densities based on substrate flux to a surface. However, proton flux from the anode can be a problem as proton accumulation will lower the local pH, adversely affecting bacterial kinetics. Increases in pH have been observed in the bulk fluid near the anode Kim et al. 2007b), and pH within the biofilm could be even lower. Mass transfer limited proton transfer to the cathode can also limit power generation, and result in elevated pH at the cathode Kim et al. 2007b). It is important 

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Fig. 8. Comparison of the polarization and power density curves with air and oxygen for the MEA equipped with the PWA-modified membrane at 145°C. Operating conditions oxygen feed, 2M methanol air feed, IM methanol.

Fig. 8.22 Cell polarization and power density curves for the borohydride fuel cell at 50°C, 70°C and 85°C. Anode 0.2g alloycm , 10wt% NaBH4 in 20wt% NaOH at a flow rate of0.2Lmin h Cathode 2 mg Ptcm , humidified O2 at 0.21 min (1 atm).

The polarization and power density curves obtained with methanol, ethanol and glycerol in 2 M KOH solutions at room temperature (20-22 °C) are reported in Fig. 24 for DAFCs where the cathode is exposed to either air or 1 bar oxygen and the fuel solution (13-15 mL) is introduced into a static compartment facing the anode. 

Figure 24. Polarization and power density curves provided by oxygen-breathing DAFCs fuelled with 2 M KOH solutions of methanol (10 wt%), ethanol (10 wt%) and glycerol (5 wt%) at 20-22 °C containing MEAs with P MWCNT anodes, Hypermec K-14 Fe-Co cathodes and Tokuyama A006 membrane. Reprinted from Ref. 29, Copyright (2009) with permission from Elsevier.

For comparative purposes. Fig. 25 reports also the polarization and power density curves exhibited by a DEFC containing a Pd/C anode catalyst obtained by reduction with ethylene glycol of Vulcan XC-72-adsoibed PdCl2. " Although providing good results, there is little doubt that the Pd/C catalyst is much less efficient, especially in terms of potential output and electrochemical stabihty, than the Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C catalysts, obtained by the spontaneous deposition procedure. 

The polarization and power density curves of the DAFCs with Pd-(Ni-Zn)/C anodes are shown in Fig. 32. Only slightly inferior results have been obtained for DAFCs containing Pd-(Ni-Zn-P)/C anodes." ... 

Figure 32. Polarization and power density curves at dilferent temperatures of active DAFCs with Pd-(Ni-Zn)/C anodes fuelled with aqueous 2 M KOH solutions of ethanol (10 vrt%) (a), methanol (10 vrt%) (b) and glycerol (5 wt%) (c) Pd loading 1 mg cm. The inset reports the temperatures of fuel (left), cell (central), oxygen gas (right). ...

Fig. 15.16 Polarization and power density curves of AEMFCs using XC-72, MnPc/C, FePc/C, CoPc/C, and NiPc/C as cathode catalysts, respectively. Anode 50 wt.% Pl/C, flow rate 200 seem H2,100 % humidity. Cathode 2.0 mg catalyst cm , flow rate 200 seem O2,100 % humidity. Test temperature 50 °C back pressure 20 psi [92]...

Figure 20.6. Performance of a spray-manufactured electrocatalyst polarization and power density curves for a single MEA PEM fuel cell with a 20 wt% Pt/Shawinigan black electrocatalyst employed as cathode catalyst at 0.2 mg Pt/cm loading. Performance measured at 50 °C at atmospheric pressure in a 50 cm cell at constant flow corresponding to stoichiometry of 1.2 for hydrogen and 2.2 for air at 1 A/cm, and with 100% humidification of the gases, Nation 112 membrane. (Reproduced with permission of Cabot CorporatioiL)...

Fig. 2.11 Polarization and power density curves for a cell equipped with sulfonated polysulfone, SPSf-60, at 90 and 100 °C in 2 M MeOH/02- Reproduced from [16] with permission of Elsevier...

Fig. 2.15 Polarization and power density curves for a direct ethanol fuel cell equipped with a composite Nafion-Si02 in the temperature range from 90 to 145 °C (CNR-ITAE internal report)...

Figure 2.15 shows the polarization and power density curves obtained for this cell from 90 to... 

FIGURE 1.5 Polarization and power density curves of a typical PEFC. 

Figure 2.14 Polar and power density curves showing the effect of CO2 on the performance of the membranes. The performance in CO2-free air (gray) is good, but the carbonate effect has a drastic effect on performance in...

Fig. 4.2 Polarization and power density curves. (A) By switching out the circuit load (external resistance), we obtain a data set on the cell voltage as a function of resistance. Using these data, we calculate (B) the current and plot voltage versus current or current density to obtain the polarization curve and the power, to obtain the power density curve.

Compare the internal resistance for the polarization and power density curves shown in Fig. 4,2 using (a) the peak power, and (b) the polarization curve. Assume the anode surface area is 7.1 cml... 

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