Graphene as catalyst support: The influences of carbon additives and catalyst preparation methods on the performance of PEM fuel cells
Introduction
Graphene is known as monolayers of sp2 bonded carbon atoms into a two-dimensional structure and has become of rapidly expanding interest due to its extraordinary features such as high electronic and thermal conductivity [1], [2], [4], an ambipolar field-effect [3], intriguing (magneto-)transport characteristics [5], [7], and mechanical properties [6]. Graphene exhibits also a very high surface area (∼2630 m2 g−1) [8], much higher than graphite (∼10 m2 g−1) or carbon nanotubes (1300 m2 g−1). All these remarkable properties make graphene promising for applications like polymer-composite materials [9], photo-electronics [10], field-effect transistors [11], electromechanical systems [12], sensors [13], [15], [16], hydrogen storage [14], energy conversion and storage, batteries [17] and drug delivery systems [18].
PEM fuel cells are clean and environmentally-friendly power sources, which are essential for future energy solutions [19]. They exhibit good energy efficiency and high power density per volume, but the high costs of the noble metal catalysts, electrolyte membranes, bipolar plates and the control system constrain a widespread commercialization. The MEA-fabrication costs (membrane electrode assemblies) can be reduced through several approaches such as (i) reducing the platinum loading (ii) the use of high temperature-tolerant membranes and (iii) new alternative electrocatalyst materials [20], [21]. In the last 3 years some very promising efforts were made in electrode development and in membrane technology to reduce the price of fuel cell systems. Zirfon™ was sucsessfully tested to replace the expensive Nafion™-membranes [22] and activated carbon air-cathodes have a great potential as inexpensive alternative to Pt-catalysed electrodes in microbial fuel cells [23], [24]. For electrochemical cogeneration of chemicals and also for NO/hydrogen fuel cells some effective concepts for the preparation of gas diffusion electrodes in a cold rolling or casting process with carbonaceous compounds and binders were reported recently [25].
Our strategy to lower the MEA-costs is the improvement of the cell performance by developing more efficient catalyst support systems which has the following properties:
- (i)
high specific surface area, which improves the dispersion of the catalytic metals,
- (ii)
improved chemical and electrochemical stability at operation temperatures between 60 and 100 °C,
- (iii)
enhanced electronic conductivity due to strong percolation, and
- (iv)
high catalytic activity for hydrogen oxidation of the graphene itself [26], [27].
Currently, the most common electrocatalysts are platinum and platinum-based alloys supported on carbon black. A major problem of the catalysts is aggregation on the surface of the carbon black support [28]. A further problem is the formation of inactive catalysts during the manufacturing process that means about 30% of catalysts are not located at the triple-phase boundaries (TPBs) where the fuel cell reactions take place.
To reduce the amount of inactive catalysts, catalyst supports and also catalyst particles with a strong reduced size were developed [26] but these systems are very hard to handle in coating processes and additionally the very small catalyst particles are thermally instable. A third problem is the electronic conductivity because in such small carbon particles the graphite content is very low. Graphitized carbon black particles with good electronic conductivity have a larger specific surface of about 65 m2 g−1 (e.g. TIMREX™ HSAG400, Ensaco™ 350G). An alternative to conventional carbon black materials could be graphene flakes. The theoretical specific surface area of single-layer graphene is about 2600 m2 g−1, but the specific surface area of multilayer graphene is much smaller (70–130 m2 g−1). An advantage of these materials is the high graphite content. Only a few fundamental research results for the preparation of isolated metal particles on the surface of single-, double-, and few-layer graphene sheets [29], [30] are known.
In addition to the active surface area of the catalyst/carbon black system, the stability is very important. There are two mechanisms of degradation: coalescence sintering and Ostwald ripening [27]. Coalescence sintering is enhanced by carbon corrosion which occurs under a high potential in an acidic environment. During the oxidizing process surface-oxygen containing species are formed and the catalyst–carbon interactions are weakened. Graphene may improve catalyst stability by providing a more stable support material and by strengthening the interaction of the catalyst with the support [26]. These enhanced interactions between the metal particles and the graphene surface are described in [31].
Finally, in a high-performance PEM fuel cell electrode the supported catalysts should be located at the TPBs. The flake-like structure of the graphene should lead to a very porous and percolated system which is essential for the mass transport and the electronic conductivity [32]. This work shows two routes of platinum catalyst immobilization on the surface of multilayer graphene flakes: (i) thermally-induced chemical reduction of a platinum precursor and (ii) electrochemical reduction of a platinum precursor by potentiostatic electrodeposition using a hydrogen depolarized anode (HDA).
The advantage of the first method is the decrease of the degree of agglomeration of platinum precursors during the reduction process because of the high interaction between adsorbed precursor ions and the graphene surface. The catalyst loading can be controlled by the amount of hexachloroplatinic acid. The HDA-method [33] has the advantage that it enables a very precise control of the catalyst loading and leads to a very homogeneous platinum particle distribution. This method assures the formation of nearly 100% of active catalyst particles which are located at the TPBs. The samples were characterized by ICP-OES, SEM, TEM, XRD and Raman spectroscopy. The electrodes were used to prepare MEAs which were characterized in a modular fuel cell test station.
Section snippets
Materials
Many preparation methods for graphene and graphene oxide exist in the literature but most of them work in small laboratory scale [34], [35], [36], [37]. The evaluation of graphene as catalyst support requires relatively large sample amounts in the range of 10–20 g, each. This amount can only be prepared in several batches in a time consuming and extensive way. For reproducibility reasons commercial graphene samples which are available in the required amounts were used (Table 1).
Microscopic techniques
TEM pictures
Macrostructures of different graphene types
Fig. 2A and B show electron microscopic images of AO-2 graphene. The graphene flakes are well separated and consist of 44-52 monolayers, which can be clearly seen in the TEM image (Fig. 2B). The thickness of the multilayer flakes is between 11 and 13 nm.
Fig. 2C shows also well-dispersed graphene flakes of sample C1, which tend to exhibit a very rough surface structure. The average multilayer flake thickness is 3.7–4.0 nm (Fig. 2D), corresponding to 15–32 monolayers. As can be seen in the SEM
Conclusion
Different graphene types were tested for electrochemical energy conversion especially for fuel cell applications. It could be shown that pure graphene forms a very dense and ordered structure during the coating process of gas diffusion layers. For this reason the produced MEAs suffered from a very low performance. Carbon black and MWCNT were used to act as spacers between the graphene flakes, in order to increase the exposed surface area of the graphene, the electronic conductivity and the mass
Acknowledgments
This work was financially supported by the Bundesministerium für Bildung und Forschung within the framework of the WING programme (Einsatz von Graphenen in der Energietechnik – Lithiumbatterien und Brennstoffzellen – LiBZ). We wish to thank Sylvia Kuhn, Tanja Müller, Elfi Jungblut, Dieter Münch, Dr. Haibin Gao, Christoph Thome, Sabrina Fricke, Franco Cappuccio and Andreas Witzmann. We also thank Prof. Dr. Rolf Hempelmann, Bernd Oberschachtsiek and Prof. Dr. Axel Lorke for stimulating and
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