Comparative assessment of synthetic strategies toward active platinum–rhodium–tin electrocatalysts for efficient ethanol electro-oxidation
Graphical abstract
Introduction
Platinum is a commonly used anodic material in acidic low temperature fuel cells. Since alcohol oxidation on pure platinum doesn't reach the desired activities, research in the field of Direct Ethanol Fuel Cells (DEFC) has focused widely on the development of binary and ternary Pt-based alloys [1], [2], [3]. The introduction of ternary electrocatalytical systems for the ethanol oxidation reaction (EOR) in recent research efforts has brought the development of DEFC as alternate power sources a big step forward [4], [5]. Ethanol is of a particular interest for mobile applications such as electric vehicles, due to high energy density 8 kWh kg−1, low toxicity, biocompatibility and abundant availability. It is, however, not easily oxidized completely to CO2 and water. This is due to difficulties in the C–C bond cleavage in ethanol and the reaction may involve several different mechanism pathways with the formation of a high number of reaction intermediates such as CHx species or acetaldehyde and, to some extent, to the formation of CO-intermediates leading to poisoning of the active sites on Pt catalysts [3], [6], [7], [8], [9], [10], [11]. Efforts to develop highly active and selective EOR electrocatalysts have therefore concentrated on the addition of co-catalysts to platinum [12], [13], [14], [15], [16], [34], [35], [36], [37], [38].
Our previous research focused on the promising family of EOR nanocatalysts based on mixtures of Pt, Rh and Sn [3], [5], [17], [18], [19], [20], [21], [22], [23]. In a recent comprehensive study on a set of Pt–Rh–SnO2 nanoparticle catalysts an optimal Pt–Rh–Sn atomic ratio of 3:1:4 has been proposed [5]. In our previous work we addressed the optimal structural arrangement of the atoms of the three components in the surface and bulk of the final active catalyst. On its surface, metallic Pt and Rh are atomically mixed with Sn, giving rise to active-surface-site ensembles. Our aim was to maximize activity and selectivity and find a single-phase Rh-doped Pt–Sn Niggliite structure as the preferred and catalytically most active nanocrystalline phase [22]. Synthesis routes to nanoparticle EOR catalysts containing Platinum, Rhodium and Sn range from impregnation-reductions methods [24] to deposition of metal atoms on oxide surfaces followed by galvanic displacement [3].We implemented in our work a modified polyol method in dioctylether solvent [25], controlling the temperature during the reaction with a heating mantel. This approach yields ternary single phased catalyst with SnOx next to metallic Pt an Rh on the surface in close proximity. Recent reports also claim improved electrocatalytic stability and elevated activities for ternary electrocatalysts by microwave-assisted selective deposition of nanoparticles onto carbon [26] and ternary PtSn@Rh/C systems by a two-step microwave-assisted polyol method as a promising catalyst preparation method for optimizing the Pt–Sn–Rh ternary system for EOR application [27].
In order to compare the two synthesis approaches and clearly establish a preferable synthetic approach towards PtRhSn catalysts, we first compared a one-pot reduction of metal precursors at ambient pressures both in the presence and absence of carbon support (referred to as “ap-PtRhSn+C” and “ap-PtRhSn/C”; ap materials). Synthesis conditions followed our previously used polyol method under a temperature control using a standard laboratory heating mantel device. Thereafter, we compared two variations of the two synthesis routes involving microwave-assisted temperature control in an autoclave associated with autogenous overpressure conditions (referred to as “op-PtRhSn+C” and “op-PtRhSn/C; op materials).
Section snippets
Catalyst preparation
All electrocatalysts (40 wt.% of metal loadings, Pt:Rh:Sn atomic ratios of 3:1:4) were prepared using Pt(acac)2, Rh2(OAc)4, and Sn(acac)2 as metal precursors, 1,2-tetradecandiol, oleic acid and oleylamine in dioctylether as reducing and capping agents, and Ketjen Black as support. All precursors were mixed together, including carbon for the direct supported electrocatalysts, heated up to 260 °C and stirred under reflux at that temperature for 30 min. For the heating mantel temperature
Structural characterization of the Pt–Rh–Sn catalysts
The Pt:Rh:Sn atomic ratios obtained by ICP and EDX were similar to the intended ratio (Table 1). The local atomic ratios from EDX measurement differ slightly from the overall ratios in the ICP results, which can be expected since they represent a more local estimation of the atomic concentration. TEM micrographs and their corresponding size distribution histograms of the electrocatalysts (Fig. 1) evidenced that the nanoparticles were well distributed across the carbon support and largely
Conclusions
Both ambient and overpressure synthesis approaches resulted in carbon supported Pt–Rh–Sn electrocatalysts that exhibited high performance for ethanol oxidation in acidic medium. The X-ray diffractogramms show fcc PtRh and hexagonal PtRhSn phases. The highest overall activity is shown by materials synthesized at ambient pressures, with very early onset potentials for an on-carbon-reduced-precursor synthesis approached catalyst ap-PtRhSn+C and the highest mass activity for ap-PtRhSn/C. This could
Acknowledgments
The project on which this Report is based was promoted with funds from the Federal Ministry of Education and Research under the promotional reference number 16N11929. Responsibility for the contents of this publication lies with the author. Partial financial support by the German Research Foundation (DFG) through grant STR 596/4-1 (“Pt stability”) is gratefully acknowledged.
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