The use of CO stripping for in situ fuel cell catalyst characterization

doi:10.1016/j.electacta.2006.12.057

Electrochimica Acta

Volume 52, Issue 18, 10 May 2007, Pages 5606-5613

https://doi.org/10.1016/j.electacta.2006.12.057 Get rights and content

Abstract

An important parameter in normalizing activities of porous electrocatalysts is real surface area of the catalyst. In this study CO stripping voltammetry was applied for the determination of the surface area of PtRu catalysts in a membrane electrode assembly. The difficulties in applying this method are summarized and a practical solution for the “accurate” voltammetric CO charge determination is suggested. The influence of adsorption time, potential, sweeping range and sweep rate on voltammetric CO charge determination is studied. It is shown that the CO charge determined in this way is independent on the sweep rate applied.

Introduction

In electrochemistry, the term “real surface area” means the electrochemically active surface area under working conditions. For porous electrodes in fuel cells it refers to the surface area of metal particles, which are at the same time in contact with the electrolyte (in this case Nafion^®) and the current collector (usually carbon cloth or carbon paper). From the fundamental point of view, determination of the real surface area is important in order to normalize activities of different electrocatalysts to the same number of reactive surface sites. From the more practical point of view the real surface area determination is important in order to check catalyst durability by measuring the loss of catalyst active surface area during fuel cell operation.

The real surface area under fuel cell conditions can be determined by hydrogen adsorption method, but only in the case of Pt catalyst. The CO adsorption method is more promising in terms of its more general applicability, but its application is connected with many uncertainties. Some of them arise from unknown type of CO bonding on the surface [1]. The other uncertainty is connected with accurate CO stripping charge correction in respect to other contributions like double layer charging and charging due to metal oxide formation [2]. In the case of PtRu catalyst this problem is significant since oxide formation on PtRu catalyst overlaps with CO oxidation [3]. For this reason the CO voltammetric charge determination is usually used only in a qualitative way and stated that the surface area is only given (if it is given) as a relative and not absolute value.

It was shown recently that the CO charge can be accurately determined by using Infra-Red (IR) [4] or differential electrochemical mass spectroscopy (DEMS) [5]. These techniques have one major advantage compared to voltammetric CO charge determination, namely CO charge determined in this way is free from other faradaic and non-faradaic contributions. However there are some disadvantages, like applicability of these techniques in the fuel cell labs and on real catalyst and this issue is especially important in determining the loss of catalyst activity during fuel cell long term operation.

In this paper a practical solution for the “accurate” voltammetric CO charge determination is described. The influence of adsorption time, potential, sweeping range and sweep rate for real surface area determination of unsupported PtRu catalysts in a membrane electrode assembly (MEA) under fuel cell relevant conditions is studied. It is shown that the CO charge determined in this way is independent on the sweep rate applied. First, a system characterization with an unsupported Pt catalyst as a test system was carried out. Then the method is established by trying out different strategies for voltammetric CO stripping charge integration and base line subtraction. Finally the method is tested on the literature data [5] for CO stripping on PtRu catalyst in DEMS configuration and good agreement to CO charge recalculating from MS signal (which was free from other faradaic and non-faradaic contributions) is obtained.

Section snippets

Cell

All electrochemical measurements were performed in a special electrochemical cell—cyclone flow cell (Fig. 1). This cell enables the investigation of the kinetics of electrochemical reactions under fuel cell relevant conditions at well-defined potential control conditions [6], [7].

The working electrode compartment (1) was supplied with nitrogen or CO/Argon gas mixture, while the counter (2) and the reference electrode (3) compartments with 1 M sulphuric acid solution. Sulphuric acid container was

Hydrogen adsorption method

As it was mentioned before the unsupported Pt catalyst in MEA configuration was used for the system calibration. The total number of reactive surface sites on platinum was determined by the hydrogen adsorption method. As it is well known, the method is based on the determination of the charge needed to remove a monolayer of adsorbed hydrogen, which is practically done by integrating the current in the cathodic or anodic scan of the hydrogen adsorption/desorption region. The cyclic voltammogram

Conclusions

The carbon monoxide method was applied to the surface area determination of unsupported PtRu catalysts. The influence of the positive potential limit, the sweep rate, the CO adsorption potential and the time was investigated. It was shown that the positive potential limit has to be extended up to 1.0 V versus Ag/AgCl in order to capture the whole CO stripping wave. The CO charge was decaying with the sweep rate when a base line subtraction was performed as in the case of platinum, i.e. with the

Acknowledgment

We would like to express our gratitude to Dr. Peter Veit (Otto von Guericke University in Magdeburg) for performing TEM measurements.

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The use of CO stripping for in situ fuel cell catalyst characterization

Abstract

Introduction

Section snippets

Cell

Hydrogen adsorption method

Conclusions

Acknowledgment

J. Power Sources

Electrochim. Acta

J. Power Sources

Electrochim. Acta

J. Electroanal. Chem.

J. Electrochem. Chem.

J. Electroanal. Chem.

Electrochim. Acta

Electrochim. Acta

Electrochim. Acta

J. Phys. Chem.