This section presents a brief review of the performance degradation mechanisms of PEM fuel cells. The structure and operating principle of a PEM fuel cell, as well as the stack assembly, is illustrated in Fig. B.1 and Fig. B.2, respectively.
Degradation of the fuel cell performance can have multiple causes. Some of the major contributors to fuel cell performance degradation are corrosion, con-tamination, starvation, degradation of the membrane, poor water management, and poor thermal management, which are all briefly explained in the following sections. [6]
B.2.1 Corrosion
Corrosion is a main cause of degradation in PEM fuel cells and can occur in both the electro-catalyst layers, the gas diffusion layers (GDL), and the bipolar plates [7]. The amount of degradation caused by corrosion is linked to the time where cyclic operation of cell voltage occurs. Corrosion of the
Fuel Air
Unused Fuel
Electric Load
Anode Cathode
Membrane Catalyst
Layer GasDiffusion Layer
Catalyst Layer GasDiffusion Layer Hydrogen Ions
Electrons
Air + Water
Fig. B.1: Sketch of fuel cell components and operating principle.
Outlet
Inlet Bipolar Plate
Membrane-Electrode Assembly
Fig. B.2: Sketch of fuel cell stack assembly.
platinum in the electrodes occurs mainly at the cathode side and causes the loss of electrochemical active surface area (ECSA), increased activation loss and therefore lower cell voltage [7], [8]. This corrosion is accelerated by high relative humidity, cycling of the fuel cell voltage, and high operating temperature [7].
In the GDL, corrosion affects the carbon that supports the catalyst. Corrosion of the GDL will cause lower cell voltage and reduced performance [6] and is accelerated by voltage cycling and low relative humidity of the reactant gases [7]. Corrosion in the bipolar plates can cause the formation of a resistive surface layer, which will increase the ohmic resistance and thereby loss.
B.2.2 Contamination
Contamination is when impurities pollute or cause chemical attacks and hinder the intended reactions in the cell. Even small amounts of impurities can cause contamination that can seriously degrade the performance and lifetime of the fuel cell [8], [9]. Contamination can occur in both the anode electrode and the membrane. In the anode, carbon monoxide (CO) impurities in the hydrogen supply can lead to contamination (CO-poisoning), which leads to lower cell voltage and reduces energy conversion efficiency [6]. Membrane contamination can be caused by impurities in the hydrogen or air supply, or by impurities resulting from corrosion of fuel cell components and can cause major reduction in performance including conductivity and mass transfer [9].
B.2.3 Starvation
Starvation of the fuel cell is the lack of sufficient reactant gas and can have mul-tiple causes including: blocked pores in the GDL; poor gas feeding management;
imperfect stack and cell design; poor stack assembly; and quick load transients [8], [10]. The starvation of hydrogen will cause a high anode potential, since the current cannot be maintained. This can cause the water at the anode to split.
Hence, oxygen will be present at the anode. Similarly, oxygen starvation on the cathode side will cause a reaction where hydrogen is produced. Combined, the oxygen on the anode side and the hydrogen on the cathode side causes voltage reversal of the cell, i.e. a negative voltage between anode and cathode [11]. This voltage reversal causes acceleration of carbon corrosion and eventually damaged components [10], [12].
B.2.4 Membrane Degradation
Membrane degradation is one of the main lifetime reducing factors of PEM fuel cells and is a complicated mechanism, consisting of both mechanical and chemical degradation. The chemical degradation is caused by the formation of radicals – highly chemical reactive molecules, which cause chemical attacks in the membrane [13]. The mechanical degradation is caused by transient operating conditions such as voltage, temperature, and humidification cycling, along with the aforementioned chemical attacks [13]. These degradations can lead to severe membrane degradation such as pinhole formation.
B.2.5 Water Management
Improper water management can lead to two inappropriate scenarios: fuel cell flooding, and membrane dehydration. Flooding can occur on both the anode and cathode side of the membrane, but occurs in particular on the cathode side [8]. The excess water significantly reduces the transport rate of the reactants causing increasing mass transport loss [8]. The water blocks the reactants from passing through the GDL and thereby causes gas starvation, which leads to
an immediate drop in cell voltage. Membrane dehydration is most likely to occur on the anode side as opposed to the cathode side. A dehydrated cell experiences immediate as well as long-term degradation. The decreased water content causes a decrease in the proton conductivity, which leads to higher ionic resistance and therefore ohmic loss [8], [14]. The effect is a voltage drop and temporary power loss [14], [15]. The voltage is usually recoverable through humidification, but prolonged dry cell operation lead to irreversible damage to the membrane, such as development of crazes and cracks [16]. This can start a destructive cycle of mechanical degradation, where the cracks cause gas crossover leading to hotspots, which in turn causes pin holes, that facilitate further gas crossover [17].
B.2.6 Thermal Management
In general, the fuel cell performance is decreased at both low and high tem-peratures [8], [18]. However, sub-zero temtem-peratures in particular can cause issues in the fuel cell. Mainly due to the freezing of the water content in the cell, which can cause mechanical damage to the components, delamination, and startup issues [6]. High temperature operation has some benefits including higher tolerance to contaminants and enhanced water management and cooling.
However, the degradation rate is accelerated and the long-time performance is decreased. [19]
B.2.7 Summary
All of the presented degradation mechanisms, can affect the system performance and ultimately the produced voltage output. Although some of the mentioned mechanisms can be avoided by proper control of the fuel cell system, but many are unavoidable in long term operation in a varying environment.