Modeling and Investigation of Carbon Loss on the Cathode Electrode during PEMFC Operation

Abstract

Severe carbon loss phenomena, such as partial hydrogen starvation and start-up, have been investigated in previous works. Even for short times, these acute situations can be fatal, and some methods have been suggested to mitigate carbon oxidation and improve proton exchange membrane fuel cell ͑PEMFC͒ durability. By comparison, the rate of carbon loss is low during an ordinary PEMFC operation. Although less severe than for hydrogen starvation and start-up, the impact is still large enough so that the cell performance degrades markedly after a long-term operation. In this work, carbon loss during a PEMFC operation is investigated with a physical model. These results quantify the effects that cell voltage, activity for oxygen reduction, humidity, and temperature have on the rate of carbon loss. More rapid kinetics for oxygen reduction promotes carbon loss by lowering the solution potential at the fuel cell cathode.Carbon is used extensively as the support for catalysts in proton exchange membrane fuel cells ͑PEMFCs͒. Loss of carbon is a wellknown degradation phenomenon in PEMFC electrodes, and this electrochemical oxidation of carbon causes cell performance to decay. The relationship between carbon loss and cell performance was confirmed by Yu et al. 1 A reduction in carbon of only 5 wt % results in a large drop in cell voltage under a fuel cell operation at a constant current. Because Reiser et al. reported a "reverse current mechanism," 2 carbon losses under partial hydrogen starvation 3 and start-up 4 conditions have been discussed collectively as carbondegradation phenomena in PEMFCs, and the potentially fatal consequences have been noted. 5-12 Makharia et al. suggested that only 1 h under conditions of partial hydrogen starvation caused a loss of more than 5 wt % carbon. 13 Takeuchi and Fuller reported that under severe conditions, a large carbon loss ͑Ͼ1 wt %͒ was expected during the start-up of the fuel cell when air and hydrogen gases were exchanged in the fuel cell anode. 14 As the details of the reverse current mechanism have been elucidated, some methods to mitigate these effects for partial hydrogen starvation and start-up conditions have been proposed. Reducing the loading of a Pt catalyst on the fuel cell anode is an effective mitigation for both cases of carbon loss. 9,10,14,15 At start-up, careful control of the cell voltage also reduces carbon loss, and keeping the cell voltage low can prevent fatal damage for the fuel cell cathode. 14,16 These approaches help to improve the durability of PEMFC membrane electrode assemblies ͑MEAs͒ by eliminating the most severe conditions that cause fatal damage to fuel cell cathodes.Even if we can avoid these catastrophic situations mentioned above, low rates of carbon oxidation are still expected under an ordinary PEMFC operation, especially under high temperature conditions. Whereas a minimum operational life of 5000 h is required for transportation applications, 17 the rate of carbon loss may be 1-10 wt % per 1000 h, large enough to cause the decay of PEMFC perf...

Toyota MIRAI Fuel Cell Vehicle and Progress Toward a Future Hydrogen Society

“…8,9 Modeling and simulation are expected to improve the associated issues. [10][11][12] At the same time, non-noble metal catalysts, oxide catalysts, and carbon alloys are being actively researched. 13,14 The Pt loading in the anode catalyst layer is also expected to be reduced after Pt reduction is realized at the cathode.…”

A review on performance degradation of proton exchange membrane fuel cells during startup and shutdown processes: Causes, consequences, and mitigation strategies

“…In addition to measuring or mathematically predicting the high cathode potential experienced during the startup and shutdown processes, researchers have also conducted other related tests, such as measuring the current distribution in segmented cells [60][61][62], monitoring the concentration of CO or CO 2 during startup and shutdown [59,63], evaluating the effect of cell design (flow field structure and GDL thickness) on start-stop phenomena [64,65], and conducting modeling studies [58,[66][67][68][69][70][71][72][73] on water management and carbon-corrosion during startup and shutdown. All the test results have confirmed performance degradation during the startup and shutdown processes of PEMFCs.…”

Synthesis and Electrochemical Evaluation of Carbon Supported Pt-Co Bimetallic Catalysts Prepared by Electroless Deposition and Modified Charge Enhanced Dry Impregnation

“…Proton exchange fuel cells (PEMFCs) are highly considered for wide scale use in small form factor applications such as vehicles and mobile technology owing to their high performance and compact design. Improving the cost competitiveness of PEMFCs can be done by enhancing the activity and durability of the catalyst employed [1,2]. One of the most widely used catalysts for electrochemical (i.e., oxygen reduction reaction, ORR) applications is a carbon-supported platinum catalyst (Pt/C) which has well-recognized issues with activity degradation from particle instability and corrosion of the metal and support.…”

“…Increasing catalyst activity can be done by synthesizing smaller particle sizes to achieve greater metal dispersion, where dispersion is defined as the fraction of the metal atoms exposed on the surface. Various methods have been developed to increase metal dispersion specially for Pt/C catalysts, however there are also arguments against smaller Pt particles in ORR catalysts as some studies have pointed to lower specific activity for highly dispersed (50%-100%) platinum for He of the impregnated carbon support, expectations were that exposure to elevated temperatures in flowing inert gas should cause decomposition of the Co(NO 3 ) 2 component to form Co 3 O 4 and N 2 , while the citric acid additive should decompose to form CO 2 and H 2 O. The inverse peaks of the TPD profile in Figure 1 show thermal conductivity changes of the flowing He where the primary and most intense peak at approximately 60˝C is attributed to water evolution while the second peak is associated with evolution of CO 2 and N 2 as decomposition products.…”

“…Various methods have been developed to increase metal dispersion specially for Pt/C catalysts, however there are also arguments against smaller Pt particles in ORR catalysts as some studies have pointed to lower specific activity for highly dispersed (50%-100%) platinum for He of the impregnated carbon support, expectations were that exposure to elevated temperatures in flowing inert gas should cause decomposition of the Co(NO 3 ) 2 component to form Co 3 O 4 and N 2 , while the citric acid additive should decompose to form CO 2 and H 2 O. The inverse peaks of the TPD profile in Figure 1 show thermal conductivity changes of the flowing He where the primary and most intense peak at approximately 60˝C is attributed to water evolution while the second peak is associated with evolution of CO 2 and N 2 as decomposition products. The second and third peaks at 220-250˝C and 420-440˝C, respectively, may be due to decomposition products such as N 2 , CO 2 , and H 2 O; the actual compositions could not be identified because of the non-selective nature of the thermal conductivity detector.…”