Carbon corrosion currents

Equation (8) states the fact that the sum of carbon corrosion current (f co, = /cor,call,) and oxygen evolution current (/o2 = /oER.cath) on the cathode must be equal to the oxygen reduction current on the anode determined by the 02 crossover rate ... 

Carbon corrosion current density can be further reduced by controlling the cell voltage at a low level during the start-stop process.25,27 Neglecting /qer, Eq. (12) is simplified to... 

Figure 7. Evolution of cathode potential (defined as rj>s 4>e at cathode electrode) and carbon corrosion current distributions corresponding to H2 depletion and N2 bubble buildup as shown in Fig. 6.

Figure 9. Top HOR (less O2 crossover) current density distribution with respect to length scales of a local H2-starved region determines where H2 depletes. Bottom carbon corrosion current distribution with respect to length scales of a local H2-starved region shows how a fully starved region develops. The cell operates on neat H2/air at s 1.5 A/cm2 (80 °C, 150 kPaabs, 100% RHin).

Figure 10. The scale of H2 depletion (top) and corresponding carbon corrosion current distribution (bottom) depends on the apphed current density. The cell operates on neat H2/air (80 °C, 150 kPaa ,s, 100% RIIjn) with a local -starved region of 20 mm in radius.

Figure 12 shows the time scales at which the carbon corrosion current reaches 50% of its maximum value at the center of the H2-starved region. Local H2 starvation takes place within 2—40 s... 

Figure 11. Calculated length scales with respect to applied current density for a cell operating on neat H2/air (80 °C, 150 kPaabs, 100% RHin). The solid line represents the length scale beyond which Fl2 depletes. The long and short dashed lines denote the length scales beyond which the maximum carbon corrosion current density would exceed 10% and 50% of O2 crossover current density, respectively.

The two-dimensional, coupled kinetic and transport model can also be used to simulate start-stop processes. Figure 14 plots cathode potential and carbon corrosion current distribution at three instants when the H2/02 front passes through 10, 50, and 90% of anode flow path during the start process. As H2 displaces air in the anode flow-field, the size of the power source increases and the load size decreases accordingly. The balanced current density becomes larger, causing higher carbon corrosion current density. 

Figure 14. Calculated cathode potential and carbon corrosion current distributions when H2/02 front passes through 10%, 50%, and 90% of anode flow-field during a cell start from air/air state (80 0 C, 101 kPaabs, 66% RHin). The cell has a catalyst loading of 0.4 mgpt/cm2 using a 50%wt Pt/Vulcan catalyst in both anode and cathode electrodes.

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

Severe carbon corrosion produces carbon dioxide and results in the loss of the carbon material as shown by Eq. 11. For a fuel cell cathode containing 0.6 mg cm of carbon, a simple calculation according to the Faraday s law shows how many hours the carbon can last before it is completely corroded. The results are shown in Table 1. It is striking to see that the carbon corrosion current density needs to be less than 0.15 pA cm in order for the carbon to last for 40,000 hours. If we assume that the electrode wlU not function properly when 20% of carbon is corroded, then a corrosion current density should be lower than 0.03 pA cm. ... 

Figure 5.31b shows that the carbon corrosion current density in the R-domain is equal to the oxygen-limiting current density (2 A cm in this simulation. Table 5.10). Thus, during the start-stop cycle, carbon corrosion runs very fast and even short transients can severely damage the catalyst. The solution to this problem is in lowering the cell potential during the transient (Takeuchi and Fuller, 2(X)8). 

On the cathode side, the ORR and carbon corrosion currents are well separated (Figure 5.31b). The feature of the problem is the formation of the HOR peak on the anode side of the cell (Figure 5.31c). This peak manifests the following effect. 

The negative effect of cell reversal under local hydrogen starvation can be mitigated by using a thicker membrane. The equivalent current density of oxygen crossover, through the membrane, is inversely proportional to the membrane thickness. Thus, larger Ipem lowers the carbon corrosion current density and increases the cell lifetime. 

The carbon corrosion current density on the anode side is given by... 

FIGURE 5.36 (a) The normalized methanol concentration and the shape of the membrane phase potential along the chaimel. (b) The ORR current density in linear and logarithmic scales, (c) The MOR and carbon corrosion current densities. For aU the curves, the mean current density is 100 mA cm, s — 0.01, and the cell potential = 0.5168 V. 

Carbon corrosion superficial exchange current density (A cm" ) Carbon corrosion current density on the anode (A cm ). Equation 5.223... 

Detailed kinetic studies of a commercial conventional-carbon-supported MEA were conducted to predict its lifetime. The carbon corrosion rates of conventional-carbon-support MEAs as a function of time at 80°C with potential hold at 1.1,1.2, and 1.3 V versus the RHE, respectively, are shown in Eig. 2. As might be expected, the carbon corrosion current increases as the potential increases. The response of the CO current versus corrosion time under these experimental conditions follows a linear log-log relation, which is consistent with the description of the corrosion current of carbon blacks in HjPO by Kinoshita (Kinoshita 1988 Kinoshita and Bett 1973). Detailed smdies of corrosion currents of conventional-carbon-support... 

Fig. 15 Carbon corrosion currents of approximately 50% Pt/C catalysts normalized by carbon mass versus time at 95°C, 1.2 V (vs. RHE) and 80% RH. i. The working electrode was fed with Nj, while the counter/reference electrode was pure at the same temperature. BP Black Pearls, KB Ketjenblack, and Vulcan Vulcan XC-72C...

In an ideal cyclic voltammetric (CV) set-up, one would choose the appropriate scan rate and a potential window for scanning. In an aqueous electrolyte, one can scan the 1.5 V potential window to get mechanistic information, but in a real fuel cell, potentials above 0.75 V (vs. SHE) is not advised if the electro-active Pt is supported on carbon. At these elevated potentials, the carbon corrosion current density increases significantly and such a useful technique could impact the cell performance post diagnostics. Scan rates are typically... 

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