Reactant starvation

Colbow K etal., 2001, Method and Apparatus for Operating an Electrochemical Euel Cell with Periodic Reactant Starvation. WO 01/01508. 

Wilkinson DP (2002) Improving PEM fuel cells robutness and lifetime with respect to reactant starvation. 202nd meeting of electrochemical society. Salt Lake City. Abstract 113... 

Reactant depletion is a function of reactant conversion and exercises rate control when the reduction of reactant concentration becomes significant. A significant reduction could occur for conversions of 20-KX) %, depending on the process. Reactant starvation occurs for conversions near 100 %, causing loss of reaction current and possibly the onset of undesirable reactions. In fuel cells, for example, fuel or oxygen starvation can lead to cell reversal and onset of corrosion reactions that may be irreversible. 

Uneven distribution of reaetant among eells is often caused by the aeeumulation of water droplets within the flow-field channels. If water aeeumulates within the ehannels of a unit cell, it will pose a higher resistance to the flow of the reaetant. This eauses the cell to reeeive a smaller amount of the reactant, which in turn carries a smaller amount of water out of the channels. These two events reinforce each other, and eventually could result in the flow of the reactant into this cell being less flian the stoichiometric amount, and reactant starvation occurs. Starvation of the luel can quickly damage the cell. Once a single cell is damaged, flie entire stack may have to be replaced. 

The performance of each cell in a stack is typically monitored individually. The 1-V curves of all the cells are collected at the same time. Since the cells are connected in series, each cell is generating exactly the same amount of current at any given moment even if some cells are under reactant starvation. The difference in performance among cells is shown by the cell voltage. Uniform performance among cells is preferred. 

Key words PEM fuel cell durability, reactant starvation, duty cycle, temperature, humidity, contamination. 

Electrode corrosion may occirr due to partial or complete reactant starvation in some cells due to chaimel or port ice blockages. 

The transition region between the reactant manifold ports and the reactant channels is particularly prone to water accirmirlation. Design of this region to either prevent water accumulation during shut-down, or to ensure sufficient channel depth and/or provide alternate paths for gas flow in the event of ice formation, is important to prevent reactant starvation during freeze-start. 

Colbow, K. M., Van Der Geest, M., Longley, C. J. et al. 2001. Method and apparatus for operating an electrochemical fuel cell with periodic reactant starvation. WO Patent 2001001508. 

Excess water is a common issue in PEMFC operation. As reactant gas humidity increases, Pt dissolution-precipitation and carbon support corrosion accelerate. Too much water can block the flow channels and pores of the GDL and lead instantly to reactant starvation, which can induce catalyst support degradation (see Chapter 3). 

As discussed above, interfacial delay and delamination between the CL and PEM, resulting from variations in operating conditions such as load cycling, freeze/thaw cycling, or reactant starvation (mostly fuel starvation), can severely... 

Reactant Starvation If a location in the catalyst layer of the anode or cathode is blocked with liquid or the flow rate to a stack cell is reduced due to maldistribution, poor performance from fuel or oxidizer starvation can result. Prolonged fuel starvation may result in voltage reversal and in some cases carbon corrosion, which is irreversible. 

In region 111, for convenience, both dry anode and cathode cases are shown to peak at the same location, although this depends on the individual conditions and is not necessarily the case. Following the maximum local current, there is a downward trend resulting from local flooding or gas-phase mass transport losses at the electrode(s). This peak and downward trend will only occur if a reactant starvation condition (via flooding or high utilization) is reached. 

A word of caution must be interjected here at low gas velocities there may be an apparent dependence on the gas velocity because of reactant starvation. Starvation occurs when a sufficient portion of the reactant is consumed by the solid, so that in actual fact, the particle is not contacted with a gas of bulk composition Cao but the gaseous reactant concentration is less than this value. When operating in this region the effective driving force will depend on the gas velocity irrespective of whether the process is mass transfer controlled. In planning experiments care must be taken to avoid this starvation phenomenon whether this criterion is met can be checked by a simple mass balance. All the above considerations were essentially qualitative. The real quantitative verification that a process is mass transfer controlled is by the fact the the actual extent of reaction as a function of time is then predictable from Eq. (2.2.29), where the mass transfer coefficient h y is calculated from the appropriate correlation of the pellet geometry and Reynolds number. (A selection of such correlations was presented in Chapter 2.) Alternatively, mass transfer control may be proved conclusively, if the experimentally measured mass transfer coefficients are found to agree with those predicted on the basis of the appropriate mass transfer correlations. 

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