Degradation heterogeneities induced by repetitive start/stop events in proton exchange membrane fuel cell: Inlet vs. outlet and channel vs. land

https://doi.org/10.1016/j.apcatb.2013.03.021Get rights and content

Highlights

  • The degradation rates induced by repeated start-ups are highly heterogeneous.

  • A first level of aging heterogeneity is between the H2 inlet and outlet.

  • The second level of aging heterogeneity is between the land and the channel.

Abstract

This paper investigates aging heterogeneities that set up during PEMFC operation in start-up and shut down conditions. The spatially-resolved analyses were based on in situ measurements of local current densities, electrochemical surface area of Pt at the cathode in a segmented cell and overall CO2 generation. In complement, ex situ physicochemical analyses were performed after the PEMFC testing, using scanning and transmission electron microscopy as well as focused ion beam scanning electron microscopy, to probe the micro and nano-scale of the cathode catalyst layer. In the present cell configuration (counter-flow mode, impact of the shut-down events negligible versus that of start-up), two kinds of aging heterogeneities are witnessed. Firstly, the performances loss at the air outlet/H2 inlet is less important than at the air inlet/H2 outlet; fuel starvation events are prevalent in this latter region; the resulting local loss of performances are linked to the distribution of the internal currents along the cell, and to larger physicochemical changes of the cathode catalyst layer in the air outlet/H2 inlet region. The faradic part of the internal currents (generated during the fuel starvation events) was identified to not only come from the electrooxidation of the carbon support of the cathode catalytic layer (CL): internal currents may also originate from Pt dissolution, carbon corrosion in the gas diffusion layer (GDL) and/or water oxidation. Secondly, the degradations at the air inlet/H2 outlet also depend on the position during the aging, either under a channel or a land. Fuel starvation events are more pronounced in land regions, due to slower removal of oxygen trapped under a land (the source of the fuel starvation) in the anode compartment during a start-up. Finally, it is wise pointing out that the particular degradation mechanism witnessed in this study would not have been observed if the MEA had been aged following a standardized stress test (potential cycling under N2-atmosphere); therefore, one may question whether the DOE standardized stress-test procedure is relevant to mimic real fuel cell operation.

Introduction

During the last decade, understanding the impact of environmental and operating conditions in a proton exchange membrane fuel cell (PEMFC) on the degradation rates of its platinum and carbon based electrodes has been (and is still) crucial to develop more robust catalysts, or at least to find mitigation strategies to their degradation [1], [2]. It is established that fuel cell degradation mechanisms and rates depend primarily on the PEMFC operating conditions: different phenomena leading to loss of performances occur during steady state load, potential cycling, start-up, shut-down, flooding or dry conditions [3], [4], [5], [6], [7], [8].

Degradations induced by start-up/shut-down (SU/SD) have been extensively studied recently as these events, when not well-managed, are known to be the most harmful regarding the stability of the PEMFC components [8], [9], [10], [11], [12]. Indeed, most of the damages induced by start-up or shut-down procedures come from the simultaneous presence of oxygen (air) and hydrogen in the anode compartment [13]. These conditions occur for instance during hydrogen injection at start-up or when flushing the anode compartment with air at shut-down, and are commonly referred to as “Fuel starvation events” since the pioneering work of Reiser et al. [13]. They lead to higher rates of electrochemical oxidation of the materials constituting the cathode catalytic layer than under normal fuel cell operation [2], [14]. Meanwhile, reverse currents, characteristic of these fuel starvation events, are generated and can be measured using segmented cells [15], [16]. These reverse currents occur in the passive part of the cell (i.e., the part of the anode compartment still filled with air), while the active part (i.e., the part already filed with hydrogen) operates normally, with hydrogen oxidation at the anode and oxygen reduction at the cathode (Fig. 1). It is possible to limit the intensity of the reverse current, and thus the extent of degradation of the cathode materials, by using nitrogen instead of air to purge the anode compartment, or by connecting the fuel cell to a load [17] (although SU or SD are usually performed in open circuit). These reverse currents are more or less a marker of the degradation of the cathode CL, but their exact nature still needs to be clarified: previous studies suggested [15], [16] that they could be divided into a faradic and a capacitive contribution, the order of magnitude of which being easily estimated experimentally by increasing as much as possible the hydrogen flow rate at start-up or the air/nitrogen flushing rate at shut down. However, the exact nature of the faradic contribution is not yet well understood: indeed it can include the contribution of various electrochemical oxidation reactions such as the carbon oxidation into CO2, surface oxide formation on carbon and platinum based materials, dissolution of platinum into its ionic form, or even water splitting into O2. Although extensive evidences of carbon oxidation in the cathode have been reported [18], [19], we showed recently that the corresponding charge stood for only a few tenths of the faradic contribution to the reverse currents [15].

All of these contributions have led to the concept of localized phenomena: very different operating conditions (oxygen and hydrogen concentration, gas velocity, etc.) yielding very different degradations rates of the electrodes within a distance of a few centimeters or less [18], [20]. Interestingly, the accepted description of a fuel starvation event (the main source of heterogeneous degradations) has been recently refined by Schneider et al. [21]. By using a segmented cell approach based on microstructured cathode flow fields, which enable measuring the local current flowing through channel and land areas, they emphasized the effect of the flow field design on the distribution of internal currents generated during SU [21]. Indeed, at the anode side, when the fuel is reintroduced during SU, the oxygen that is located under a channel can be more or less easily removed, while it will take more time to remove the oxygen trapped under a land. Therefore, there will be longer fuel starvation events for the portion of cathode facing anode land regions than anode channel regions. This work also introduces what should be a hot topic in a near future: more than their performances [22], the degradation rates of the electrodes might be severely impacted by the design of the other components of a PEMFC unit cell (here the flow-field design of the bipolar plates and the characteristics of the gas diffusion layers).

In the present paper, the development of aging heterogeneities (between the air inlet and the air outlet) that occur during repetitive start-up and shut down events will be surveyed in situ. In order to make the link between aging heterogeneities and degradation heterogeneities, we will first monitor in situ the modification of the local performances of each segment and correlate them with local degradation of the membrane electrode assembly; in particular the cathode catalyst layers, will be characterized ex situ at the micro- and the nano-scale. A special care will be taken to identify every kind of aging heterogeneities within a single membrane electrode assembly (MEA) that is inlet vs. outlet and channel vs. land.

Section snippets

Segmented cell and automated test bench

A 1 cm × 30 cm segmented cell, described previously in [7], [9], was used in this work. Air and hydrogen flow through five 30 cm long parallel channels (0.7 mm × 1 mm on both sides). The cathode compartment was machined in a segmented brass plate and the current was collected independently from 20 electrically insulated segments along the channel length. The anode flow-field plate was machined in a non-segmented gold-plated brass block. Hydrogen and air were fed in counter-flow in the anode and cathode

In situ evidences of the inlet vs. outlet degradation heterogeneities

The global fuel cell polarization curve, measured before and after aging, is presented in Fig. 3. The decay of performance is very significant, and in first approximation mostly due to the start-up events (the cumulated common residence time of simultaneously air and hydrogen in the anode compartment is estimated of about 800 s for a total aging time of 92,000 s including start-ups, operation at constant current density and shut-downs). The decrease of the fuel cell performance observed in Fig. 3

Conclusions

In this paper, we surveyed the development of aging heterogeneities that occur during PEMFC operation containing 136 start-up and shut down events. These aging heterogeneities, between the air outlet/H2 inlet and the air inlet/H2 outlet (where fuel starvation events will motivate the degradation rates of the MEA structure), were monitored in situ with a 30 cm2 fuel cell segmented in 20 regions of similar area/shape along the flow fields. The local loss of performances of the MEA was compared to

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