Elsevier

Journal of Power Sources

Volume 248, 15 February 2014, Pages 474-482
Journal of Power Sources

Experimental study of hydrogen purge effects on performance and efficiency of an open-cathode Proton Exchange Membrane fuel cell system

https://doi.org/10.1016/j.jpowsour.2013.09.122Get rights and content

Highlights

  • Optimal purge strategy strongly depends on proper heat and water management.

  • Anode flooding and cathode drying may occur simultaneously during purged operation.

  • The major DEA operation performance limitation is related to water instead of N2.

  • O2 detection in the anode after long purge intervals indicates cathode corrosion.

Abstract

The performance and efficiency of an open-cathode PEM fuel cell system in dead-ended anode (DEA) configuration and hydrogen purges is analyzed in this work. Excess water and crossed-over nitrogen in the anode decrease the hydrogen concentration at the catalyst surface, which in turn causes performance losses. Purging the anode with hydrogen removes water and nitrogen and thus recovers the performance. However, this means wasting hydrogen and decreasing overall system efficiency. Gas chromatography was used to detect and quantify the accumulated nitrogen in the anode during DEA operation. The experiments show that the major performance limitation in the studied system is related to water instead of nitrogen. Moreover, oxygen was detected in the anode exhaust gas after long purge intervals, which is an indicator for corrosion of the cathode carbon support structure. Experimental observations revealed that the need for a hydrogen purge strongly depends on the operating conditions and the state-of-health of the fuel cell. It is shown that flooding on the anode and drying of the cathode catalyst layer may occur simultaneously during purged operation. Therefore, purge decisions must be evaluated online, depending on the operating conditions.

Introduction

Water management in Proton Exchange Membrane (PEM) Fuel Cells is a crucial issue. On the one hand, water is needed to maintain good proton conductivity and therefore has to be kept in the membrane. On the other hand, too much liquid water in the Catalyst Layers (CL) reduces the electrochemically active surface area and if present in the pores of the Gas Diffusion Layers (GDL) it hinders the reactant gases to diffuse to the catalyst surface. Thus, both effects reduce the performance of the system. The goal is to maintain an almost uniform distribution in the Membrane Electrode Assembly (MEA) by keeping a balance between the two conflicting requirements. To control water transport within a fuel cell system and thereby optimize the MEA hydration at any point of operation, proper dynamic water management strategies have to be developed. This has been analyzed recently by Hussaini and Wang [1].

Today most PEM fuel cell systems work either with a Dead-Ended Anode (DEA), which is fed by dry hydrogen at a regulated inlet pressure or a flow-through (FT) configuration. A DEA system provides savings in costs, volume and weight compared to a FT system, as it does not require a hydrogen recirculation loop with the specific auxiliary components, such as a pump, water separator and humidifier. The water back diffusion through the membrane in the anode during DEA operation leads to a certain degree of self-humidification. Thus, the anode may be fed by dry hydrogen.

However, there are some major problems involved in the DEA operation. Water accumulation in the anode CL and GDL can significantly reduce the performance, as stated above. Nitrogen permeation from the cathode through the membrane and accumulation in the anode leads to a similar decrease in performance, as recently investigated by Siegel et al. [2]. Thus, the anode has to be purged regularly in order to remove accumulated water and nitrogen. Improper control of the purge can lead to hydrogen starvation along the channel due to the accumulation of water and nitrogen, which in turn could increase the carbon corrosion rate significantly [3].

However, purging with hydrogen means wasting energy and thus decreasing the overall system efficiency. In order to minimize the waste of hydrogen, the need for a purge has to be evaluated online, depending on the actual operating conditions. Feeding the purged hydrogen back into the fuel supply line does not solve the efficiency problem due to the extra power consumption of a necessary hydrogen pump. Moreover, purging is still required in order to prevent nitrogen accumulation in the recirculation loop.

Recently several articles were published that describe experimental and modeling work on DEA single cells [2], [3], [4]. In the work of Siegel et al. [2] the effect of nitrogen blanketing is studied in detail, which was extended later by Chen et al. [4] to account for liquid water transport and accumulation. In the work of Chen et al. [3] DEA operation effects on carbon corrosion due to hydrogen depletion along the channel are highlighted.

This work contributes to the literature with a thorough experimental analysis of performance losses during DEA operation of a 20-cell, open-cathode PEM fuel cell stack for the purpose to optimize purge strategies. Possible degradation effects due to improper purge intervals are proved experimentally. Moreover, a comparison to a FT system in terms of system efficiency is provided.

Section snippets

Open-cathode PEM fuel cell system

The system under observation in this work is the commercially available 100 W, 20 cell PEM fuel cell stack H-100 from Horizon Fuel Cells Technologies. This open-cathode system with an active area of 22.5 cm2 is self-humidified and air-cooled. It includes a cooling fan directly attached to the fuel cell housing, which removes heat from the stack by forced convection and at the same time provides oxygen to the cathode. The anode inlet is supplied with dry hydrogen and the outlet features a

Effects of humidification and DEA operation on stack impedance spectra

As explained in Section 2.2, the stack was initially dry and had to be humidified. The impedance spectra before and after humidification are shown in Fig. 1. The high frequency resistance (RHF) represents the sum of the membrane resistances of the 20 cells in the stack [8]. Fig. 1 shows a decrease of RHF from 0.3 to 0.2 Ω within the first 10 min of running on humidified gas due to the very dry initial membrane states. Moreover, the low frequency resistance (RLF) decreases as well from 2.5 to

Conclusions

The presented experimental work on DEA fuel cell systems and hydrogen purges shows that this kind of system still has a great potential for optimization based on water and thermal management. Electrical and chemical experiments, such as EIS and gas chromatography highlight the water-related performance challenges. On the one hand, water is building up in the anode during DEA operation, which causes the blockage of active sites and thus a voltage drop due to local hydrogen starvation. On the

Acknowledgments

The electrical analysis was performed at the Fuel Cells Laboratory of the Institut de Robótica i Informática Industrial (CSIC-UPC, Barcelona). The gas chromatography was carried out in the hydrogen laboratory of the Institute of Energy Technologies (UPC, Barcelona). Special thanks go to Prof. Jordi Llorca and Noemí Gasamans for enabling and supporting the GC experiments. All experiments were only possible due to the laboratories' advanced equipment and proficient technical staff. This work is

References (15)

  • I.S. Hussaini et al.

    J. Power Sources

    (2010)
  • J. Chen et al.
  • S. Strahl et al.

    J. Power Sources

    (2011)
  • A. Husar et al.

    Int. J. Hydrogen Energy

    (2012)
  • S. Maass et al.

    J. Power Sources

    (2008)
  • H. Tang et al.

    J. Power Sources

    (2006)
  • J.B. Siegel et al.

    J. Electrochem. Soc.

    (2010)
There are more references available in the full text version of this article.

Cited by (78)

View all citing articles on Scopus
View full text