In situ decoration of metallic catalysts in flow-through electrodes: Application of Fe/Pt/C for glycerol oxidation in a microfluidic fuel cell
Graphical abstract
Introduction
Fuel cells are clean power sources, capable of producing electric energy from two coupled electrochemical reactions. Among different cell configurations, microfluidic fuel cells (μFCs) have emerged as promising power sources for small electronic devices with minimal cost and environmental impact [[1], [2], [3]]. μFCs operate by using an anode for the fuel electrooxidation and a cathode where the oxidant is electroreduced, as it is for a conventional fuel cell; however, they dispense the use of an ionic permeable membrane [1,4]. A co-laminar flow is introduced to separate the anolyte stream from the catholyte stream whilst allowing the necessary ionic transport. Hence, the cost and crossover issues associated with the use of conventional ion exchange membranes are excluded. μFCs also offer the advantage of using mixed media electrolytes condition, enabling the control of the reversible half-cell potentials by pH modification at each stream [5,6]. Moreover, porous electrodes can be used to provide enhanced mass transport and surface area utilization in a flow-through configuration [7]. Therefore, the association of mixed media with flow-through porous electrodes configuration [3,7,8] optimizes each individual half-cell reaction leading to an improvement in the overall cell performance.
μFCs have the flexibility of using ionic or non-ionic electroactive species as fuel [3]. Small-chain organic molecules, such as formic acid [9,10], methanol [8,[11], [12], [13]], ethanol [14], ethylene glycol and glycerol [[15], [16], [17], [18]] have been considered to feed anodes of μFCs. These direct alcohol microfluidic fuel cells (μDAFCs) are environmentally friendly energy converters, since the fuel can be obtained from biomass [15], and may be interchangeably applied in fuel-flexible systems [18]. Among these alcohols, glycerol has received special attention, since it is a co-product equivalent to ∼10% of the total biodiesel fabrication, generating a massive amount of the alcohol. The surplus of glycerol decreases its market price and increases the environmental risks due to inadequate disposal. Glycerol displays three hydrated carbons, which makes it a versatile substrate for the electrosynthesis of carbonyl compounds [[19], [20], [21], [22]] and prospective fuel for fuel cells [23,24].
The performance of a microfluidic direct glycerol fuel cell (μDGFC) in terms of its output power density depends on the efficiency of the catalyzed glycerol electrooxidation reaction (GEOR) at the anode. Pt surface adsorbs the organic molecule, which needs to be further oxidized in order to extract electrons. The total glycerol electrooxidation to produce CO2 releases 14 electrons per molecule, which requires CC bond cleavage. Nevertheless, the limited ability of commercial Pt/C nanoparticles (NPs) in cleaving the CC bonds of the molecule [[25], [26], [27]] limits the anodic reaction and the overall performance of the μDGFC accordingly. In this context, much effort has been spent to synthesize multi-metallic NPs with enhanced activity for GEOR [[28], [29], [30], [31], [32], [33], [34]]. In these catalysts, Pt plays an important role in the dissociative adsorption of glycerol [35], while the additional metal may provide oxygen species to assist the surface electrooxidation reaction and/or modify the Pt d-band [31,33,34,36]. The ad-atom facilitates the adsorption of oxygenated species at low potentials, which allows for the oxidative removal of partial oxidized compounds from the catalyst surface at low potential, enhancing the alcohol electrooxidation accordingly. Since the Pt-based materials used as anode and cathode for fuel cells represent a considerable proportion of the total cost of the device, the additional metal must be of low cost in order to increase performance with negligible added cost. Among the ad-atoms, iron is a widely available metal with low cost, which has been reported as a co-catalyst for surface reactions [[37], [38], [39]]. Although Pd-based NPs associated with Fe have been reported for GEOR [40,41], the literature lacks information regarding the use of PtFe NPs for such reaction. Xu et al. reported a PtFe alloy 3D porous structure with enhanced activity towards methanol electrooxidation and CO tolerance [38]. Hu et al. also found CO tolerance and improved activity for formic acid electrooxidation by using PtFe-based materials dispersed on graphene [39]. Since CO is the main intermediate for the conversion of glycerol into CO2 [[42], [43], [44]], it is reasonable to conjecture that the association of Pt with Fe might enhance GEOR. This hypothesis is based on the fact that glycerol molecules are adsorbed on Pt, while Fe could downshift the d-band of Pt [38], or even provide oxygen species, facilitating the CO-adsorbed electrooxidation and the GEOR, accordingly.
New multi-metallic catalysts are mainly synthesized by hydrothermal polyol methods [45], but new alternative methods such as microwave-assisted chemical methods [46], potentiodynamic decoration [33,34] or decoration in wall-jet configuration [47] could be used to prepare the materials prior to their application in DAFCs [16,48]. This new catalyst is thereby immobilized by spray coating or successive immersions in the catalytic ink on the carbon paper (CP) electrodes and used to improve the respective half-cell reaction during the cell operation. However, this strategy is time-consuming and costly. The classic chemical methods consume high amounts of metallic precursors with limited efficiency, since only portions of the metallic cations are reduced to their metallic form, whereas the rest are wasted during centrifuging and/or filtration [47]. After immobilization/dispersion of the new catalysts on CP, there is no guarantee that the NPs are homogeneously distributed through the porous support or gas diffusion layer. In that case, the electroactive species (fuel or oxidant) may go intact throughout the catalyst or pass through an inefficient collision frequency. The limited encounters of reactants against the multi-metallic sites compromise the cell performance, so the output power would depend on how well the catalyst is situated in/on the electrodes. Moreover, classic methods of synthesis cannot be used post-fabrication to existing systems, without dis-assembly. In this context, Goulet et al. have partially addressed this issue [4,49]. The authors reported the use of flowing deposition of carbon nanotubes driven toward the carbon paper prior to Ref. [49] and during [4] operation of a microfluidic vanadium redox flow battery, which does not require metallic catalysts. They reported a physical deposition of carbon nanotubes throughout the porous electrodes, which increases the surface area and cell performance, accordingly.
The present research objective is to leverage the technology of in situ modification of catalysts to develop a new strategy for in situ electrochemical modification of metallic catalysts. Here, this process is accomplished based on a co-laminar flow of metallic cations through porous electrodes under applied potential to form metal-decorated catalysts. The electrodeposition of the ad-atoms is facilitated in the flow path rather than on the surface of the electrode. The highest rate of local electrodeposition likely occurs at the proper sites of high reactants collision frequency, which are ideal locations to facilitate further high current densities with porous electrodes. The proposed electrode decoration method is demonstrated in this work by application of the in situ decorated Fe/Pt/C catalyst as an anode of μDGFC showing improved performance compared to a similar cell with non-decorated Pt/C anode.
Section snippets
Experimental
The micro-channels of the microfluidic fuel cells with flow-through configuration are fabricated by ultra-violet soft-lithography of polydimethylsiloxane (PDMS) from a photoresist template, as previously reported by Goulet and Kjeang [50]. The cells are designed with two inlets, two outlets and electrical contacts, as depicted in Fig. 1A. The co-laminar micro-channel is 10 mm long, 1 mm wide and 0.15 mm thick, building a 0.015 cm2 cross-sectional electrode area normal to the flow of reactants.
In situ electrochemical catalyst decoration method
The method to electrochemically decorate catalysts in flow-through configuration is developed in a μFC, as illustrated in Fig. 1. However, it can be mimicked or adapted for other porous electrodes that contain a previously dispersed metal (i.e. Pt), which serves as active sites for the hydrodynamic electrodeposition. The architecture of the cell used here is designed to integrate two flow-through porous electrodes separated by a co-laminar micro-channel (Fig. 1A). Both electrodes are composed
Conclusions
This work demonstrated a novel in situ electrochemical decoration method as a powerful tool to modify catalysts for applications in electrochemical energy conversion with flow-through electrodes. As a proof of concept, we decorated Pt/C-modified carbon paper with Fe ad-atoms. The flow rate can be used to control the decoration process, wherein high residence times increase the ad-atom content. The concentration of the metallic precursor can also be used to control the ad-atom coverage degree;
Acknowledgements
C. A. Martins thanks Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). Funding for this research provided by Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI) and British Columbia Knowledge Development Fund (BCKDF) is appreciated. E. Kjeang acknowledges support from the Canada Research Chairs program. The work made use of the 4D LABS shared facilities at Simon Fraser University (SFU) supported by the CFI, BCKDF,
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