Catalyst degradation electrochemical surface area

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

Much of the knowledge in Pt/C durability derives from the experience with phosphoric acid fuel cells (PAFCs) at operating temperatures of about 200°C. Catalyst degradation is witnessed as an apparent loss of platinum electrochemical surface area over time, " associated with platinum crystal growth. These changes are ascribed to different processes which include... 

One critical issue facing the commercialization of low-temperature fuel cells is the gradual decline in performance during operation, mainly caused by the loss of the electrochemical surface area (EGA) of carbon-supported platinum nanoparticles at the cathode. The major reasons for the degradation of the cathodic catalyst layer are the dissolution of platinum and the corrosion of carbon under certain operating conditions, especially those of potential cycling. Cycling places various loads on... 

From the above experimental results, it can be seen that the both PtSn catalysts have a similar particle size leading to the same physical surface area. However, the ESAs of these catalysts are significantly different, as indicated by the CV curves. The large difference between ESA values for the two catalysts could only be explained by differences in detailed nanostructure as a consequence of differences in the preparation of the respective catalyst. On the basis of the preparation process and the CV measurement results, a model has been developed for the structures of these PtSn catalysts as shown in Fig. 15.10. The PtSn-1 catalyst is believed to have a Sn core/Pt shell nanostructure while PtSn-2 is believed to have a Pt core/Sn shell structure. Both electrochemical results and fuel cell performance indicate that PtSn-1 catalyst significantly enhances ethanol electrooxidation. Our previous research found that an important difference between PtRu and PtSn catalysts is that the addition of Ru reduces the lattice parameter of Pt, while Sn dilates the lattice parameter. The reduced Pt lattice parameter resulting from Ru addition seems to be unfavorable for ethanol adsorption and degrades the DEFC performance. In this new work on PtSn catalysts with more... 

DMFC performance loss due to catalyst degradation has been attributed to several factors a decrement of the electrochemically active surface area (ECSA) of the platinum electrocatalyst supported on a high-surface-area carbon, a loss of cathode activity towards the ORR by surface oxide formation, and ruthenium crossover [83, 85, 116, 117]. 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

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. 

In fact, this potential driven corrosion of carbon can be quite severe, causing substantial loss of the electrochemically active surface area as the electrode degrades with the loss of the catalyst support. Enhanced fuel cell degradation can occur under the additional stress conditions associated with cold start and hot stopping [62]. 

The durability of the catalysts and their supports has been investigated in real or simulated low-temperature PEMFC condition [18, 39, 45]. It has been found that the electrochemical active surface area of the electrodes will be decreased during PEMFC running [45, 46]. The decrease in the electrochemical active surface area contributes to the main performance degradation of PEMFC [47]. Pt or Pt-alloy... 

Ferreira et al. (2005) used a similar accelerated catalyst degradation condition to investigate platinum surface area loss during fuel cell operation. MEAs with 0.4 mg Pt cm" loading in the anode and cathode were subjected to 10 000 cycles (0.6-1 V vs RHE 20 mV s sweep rate) at 80 °C cell temperature and fully humidified (100% RH) reactants (H2 and N2 in the anode and cathode respectively). Areduction of approximately 64% in electrochemically active surface area was observed at the end of 10 000 cycles. X-ray difiraction (XRD) and transmission electron microscopy studies (TEM) revealed a significant increase of platinum particle size (-2 tun initial... 

When residual water produced during fuel cell operation remains in the electrodes after the stack is shut down, problems can arise, particularly when the environmental temperature is <0 °C. When the stack is exposed to subzero conditions, the residual water will freeze, so the volume of the electrodes (in particular, the catalysts layers) will expand due to ice formation, which will lead to structural damage and decreased electrochemical active surface area. This has been reported as an additional degradation mechanism in PEM fuel cells [21]. However, if the PEM fuel cell is operated at high temperatures, less liquid water will remain in the electrode and thus decrease the impact of fuel cell structure failure caused by frozen water. 

Electrochemical Active Surface Area The characterization of the ECS A is a measure of the efficiency of the catalyst loading. The ECS A is an extremely useful tool to compare the efficiency of one electrode to another and also to examine the degradation of the electrode over time resulting from a variety of physical and chemical mechanisms. Eor the ECSA measurement on a platinum electrode, the experimental configuration shown in Eigure 9.7 is used and two assumptions are made [13] ... 

How to reduce the degradation of the components in a CL. As described above, the catalyst particles, binder and carbon support degrade during the operation due to chemical and electrochemical corrosion. Graphitization is one way to increase the stability and conductivity of the support. Another possible way is to make uniform defects on the outer walls of the carbon nanotubes to anchor the metal particles. Part of the metal particle embeds into the etched pore on the surface, and these embedded particles can increase the contact area and consequently increase the stability. 

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