CVD grown graphene as catalyst for acid electrolytes
Introduction
Polymer electrolyte membrane fuel cells are potential users in need of stable and low cost catalyst. Oxygen reduction in the PEM fuel cell cathode is the largest contributor of polarization losses and efforts continuing for improved kinetics through material innovation. Platinum (Pt) is considered as the best catalyst for oxygen reduction reaction and commonly used [1], [2], [3], [4], [5]. Efforts continue to replace or reduce amount of Pt with nonmetallic catalyst or non-precious metals/alloys to enhance commercial viability of electrochemical processes [6], [7], [8], [9].
Graphite is one of the most used electrode support for its inertness as well as excellent electrical and thermal properties [10]. Graphene is the single layer of graphite providing unique features due to isotropic properties at both in-plane and through-plane structure (Fig. 1a). Each layer of spacing is 3.354 Å (A) and difficult to separate layers from each other. When several layers of graphene comes on top of each other, properties become anisotropic and 2–3 orders of magnitude of differences measured in properties between basal (in-plane) and layered (through-plane) planes. Anisotropy is resulted from multi-layer structure with graphite still carrying basic characteristics of graphene (Fig. 1b). Raman spectroscopy response with G/2D response larger than 1 indicates that multi-layer structure (Fig. 1c) leading to material called graphite.
Graphene has been researched for electro-catalytic purposes and commonly processed with chemical approaches through layer separation from graphite powder. CVD grown graphene has limited studies in acid electrolytes due to processing difficulties to modify structure. Graphene produced by CVD process requires alternative processing approaches to obtain suitable materials for heterogeneous nature of gas-liquid catalytic activities. Many metals have been tried to grow graphene in CVD process, however, copper and nickel are the most used ones [11], [12], [13], [14], [15]. Electrocatalyst that are used in heterogeneous electrochemical systems, i.e. fuel cells, require 3-phase contact where catalyst and electrolytes homogenously distributed for charge transfer and proton exchange with gas and liquid present. If graphene can be produced in 3D form, it will help achieving fast charge transfer and high gas/liquid permeability leading to high catalyst conversion. This requires 2D foils to be replaced with 3D foams to grow graphene to take the shape of base materials [16], [17], [18]. Nitrogen (N) doping on graphene can form different defect and electronic structures for electrochemical functionality. It is shown Graphitic and Pyridinic N-doping can provide additional oxygen reduction functionality [19]. N-doping was achieved by different methods and reviewed by Rao et al. [20], and Wang et al. [21]. Most of the N-doping in the literature takes place with nitrogen precursors mixed with graphene chemically, but ammonia is also used doping agents for graphene [22], [23], [24], [25], [26].
Wong et al., has reviewed status of nitrogen doped carbon structures as electrocatalyst for fuel cell applications [27]. Various carbon structures with and without metal catalyst were reviewed for their performance in both acid and alkali electrolyte. Ratso et al., studied nitrogen doped graphene and carbon nanotubes for oxygen reduction in alkali electrolyte [28]. They have shown doping graphene can significantly improve oxygen reduction kinetics without metal catalyst. Kim et al., used microwave synthesis approach to co-dope graphene with boron and nitrogen and showed acceptable oxygen reduction activity in KOH [29]. Dai et al., utilized CVD grown graphene to look at oxygen reduction reaction (ORR) and reported enhanced electrocatalytic activity for alkali electrolyte [30]. There is still lack of understanding on how CVD grown graphene behave in acid electrolyte.
Continuous-phase porous graphene produced by CVD process is not commonly utilized for heterogeneous electrocatalytic applications due to processing difficulties. Herein, In order to understand the role and contribution of CVD grown graphene in catalytic oxygen reduction process in the presence of acid electrolyte, plain, N-doped and Pt-deposited graphene structures are tested by RDE measurements by varying processing parameters.
Section snippets
Experimental study
CVD process has been utilized to grow graphene on nickel in a split furnace at 1000 °C. Nickel foam with 580 μm pore size and 1.7 mm thickness was used as substrate at varying dimensions. Nickel samples were cleaned in ethanol and acetone to remove surface impurities. Gases (Ar, N2, H2, CH4) with 99,99 and above purity were used as carrier gases at varying flow rates. The following temperature-time schedule was applied for the graphene growth (green) and N-doping (blue) (Fig. 2). After vacuum
Results and discussion
Fig. 3a shows SEM image for graphene grown on nickel foam. SEM-EDX analysis has confirmed presence of 1.6% and above carbon. After etching away nickel support in HNO3 solution, transparent multi-layer graphene network is nicely seen in Fig. 3b. Even though surface looks broken, graphene is in place as self-supporting structure. Depending on the growth time, number of graphene layer can be as little as a few layer (Fig. 3c-1 min growth) or several layers (Fig. 3d-1 h growth).
Raman Spectroscopy
Conclusions
In order to understand the role and contribution of CVD grown graphene in catalytic oxygen reduction process in the presence of acid electrolyte, plain, N-doped and Pt-deposited graphene structures have been tested by electrochemical RDE measurements.
CVD grown continuous phase graphene has shown little catalytic activity towards oxygen reduction in HClO4 acid electrolytes when doped with NH3. Sputter deposition of 1 nm and 10 nm Pt on graphene (2.1 and 21.45 μg/cm2 loading) has shown orders of
Acknowledgement
Financial support provided by the TUBITAK-ARDEB through 1003 program under contract #215M302 is greatly acknowledged.
XPS analysis carried out at Gebze Technical University, Physics Department is also acknowledged.
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