Platinum dissolution degradation

It is perhaps not immediately obvious that the precious-metal catalysts that are employed for use in PEM fuel cells will be subject to degradation, agglomeration, and even dissolution. Most of us are familiar with platinum as an example of a noble metal, which, according to its definition, means that it resists chemical action and does not corrode. Yet there is compelling evidence that platinum can degrade under conditions experienced in the fuel cell operating environment. Within the catalyst and separator of the fuel cell, the conditions are quite acidic, and the presence of oxygen results in an environment that is extremely oxidative. 

The reasons for the deterioration of ceU performance can be distinguished in reversible and irreversible power loss. Inevitable irreversible performance loss is caused by carbon oxidation, platinum dissolution, and chemical attack of the membrane by radicals [7]. Reversible power loss can be caused by flooding of the cell, dehydration of the membrane electrode assembly (MEA), or change of the catalyst surface oxidation state [8]. If corrective actions are not started immediately, reversible effects lead to irreversible power loss that we define as degradation. In this chapter, we focus on the degradation of the catalyst layer due to undesired side reactions. 

In addition to carbon corrosion and platinum dissolution, water electrolysis is a further undesired side reaction in PEMFCs. It is the back reaction of the oxygen reduction that is given in Eq. (20.1). Water electrolysis does not cause ceU degradation directly but results in dehydration of the MEA, temporarily reducing the electrolyte conductivity. According to Eq. (20.2), it occurs at electrode potentials (p>1.23 V. 

In summary, it can be concluded that platinum dissolution and catalyst particle growth are particularly fast during potential transients because of the delayed PtO formation. Furthermore, accelerated oxidation of the carbon support is observed under cycling conditions. These mechanisms result in cell degradation lowering the durability of the fuel cell significantly. 

In summary, we conclude that oxygen starvation results in a slightly negative cell voltage and hydrogen production at the cathode side. The simultaneous power loss is reversible and the mechanism does not lead to cell degradation since high electrode potentials required for carbon corrosion or platinum dissolution do not... 

Higher operational temperatures of HT-PEMFCs as compared to LT-PEMFCs impose more challenges with catalyst degradation. This issue coupled with the presence of free acid in the membrane obviously aggravates both carbon corrosion and platinum dissolution, which in turn triggers significant agglomeration of the... 

Guilminot E, Corcella A, Chatenet M, Maillard F, Chariot F etal. (2007), Membrane and active layer degradation upon PEMFC steady-state operation I. Platinum dissolution and redistribution within the MEA , J. Electrochem. Soc., 154, B1106-B1114. DOI 10.1149/1.2775218. 

Platinum dissolution from the cathode and particle deposition inside the bulk membrane, called the platinum band, is another serious degradation phenomenon... 

Summarizing Table 1, the potentials typically experienced on PEFC electrodes range from approximately 0 to 1.5 V or more. Since higher potentials promote degradation modes such as carbon corrosion and platinum dissolution, one should strive to minimize exposure to high potentials. Before discussing ways of accomplishing this, we briefly review how potential affects platinum and carbon. 

Typical examples of degradation phenomena include (1) carbon support oxidation/corrosion, (2) platinum dissolution/agglomeration, (3) chemical degradation of the electrolyte membrane, and (4) three-dimensional structural changes in the electrocatalyst layers, among others. These phenomena do not occur individually, but rather simultaneously and in a compound manner. 

Durable and reliable operation for several thousand hours are considered crucial for the successfully commercialization of DAFC. The factors that determine a PEM fuel cell s lifetime (as platinum-particle dissolution and sintering, carbon-support corrosion, and membrane thinning) is currently studied by many researchers in order of increase the lifetime without increasing cost or losing performance [83-85]. The relative contribution of each component s degradation to the degradation of the entire fuel cell is not completely understood yet. 

Ettingshausen, E, Kleemann, J., Marcu, A., Toth, G., Fuess, H., and Roth, C. (2011) Dissolution and migration of platinum in PEMFCs investigated for start/stop cycling and high potential degradation. Fuel Cells, 11, 238- 245. 

The results for constant potential holds over 400 h show that the loss of electrochemically active platinum surface area is neghgible at 0.87 and 1.2 V, respectively, whereas it is significant at 1.05 V [72]. The reason is that at 0.87 V the reaction kinetics are slow whereas at 1.2 V a PtO monolayer is formed, blocking any dissolution or precipitation. When the electrode is held at intermediate potentials, significant catalyst degradation occurs. 

A significant part of the performance degradation of fuel cells with current platinum-based catalysts derives from the dissolution and sintering of platinum. This is due to the relatively high solubility of platinum in the strongly acidic... 

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