Elsevier

Applied Catalysis B: Environmental

Volume 210, 5 August 2017, Pages 121-130
Applied Catalysis B: Environmental

Research paper
Carbon nanotube supported PdAg nanoparticles for electrocatalytic oxidation of glycerol in anion exchange membrane fuel cells

https://doi.org/10.1016/j.apcatb.2017.02.082Get rights and content

Highlights

  • CNT supported PdAg nanoparticles with average particle size of <3 nm is prepared.

  • XPS shows that by alloying with Ag, more metallic state Pd is presented on the surface.

  • PdAg/CNT shows high power density and fuel utilization in direct glycerol fuel cell.

  • Electrochemical activity and product analysis of glycerol intermediates are conducted.

  • Reaction pathway for glycerol electro-oxidation on PdAg/CNT at high pH is proposed.

Abstract

Electro-oxidation of alcohol is the key reaction occurring at the anode of a direct alcohol fuel cell (DAFC), in which both reaction kinetics (rate) and selectivity (to deep oxidation products) need improvement to obtain higher power density and fuel utilization for a more efficient DAFC. We recently found that a PdAg bimetallic nanoparticle catalyst is more efficient than Pd for alcohol oxidation: Pd can facilitate deprotonation of alcohol in a base electrolyte, while Ag can promote intermediate aldehyde oxidation and cleavage of Csingle bondC bond of C3 species to C2 species. Therefore, a combination of the two active sites (Pd and Ag) with two different functions, can simultaneously improve the reaction rates and deeper oxidation products of alcohols (Applied Catalysis B, 2016, 199, 494). In this continuing work, Pd, Ag mono, and bimetallic nanoparticles supported on carbon nanotubes (Ag/CNT, Pd/CNT, Pd1Ag1/CNT, and Pd1Ag3/CNT) were prepared using an aqueous-phase reduction method; they served as working catalysts for studying electrocatalytic oxidation of glycerol in an anion-exchange membrane-based direct glycerol fuel cell. Combined XRD, TEM, and HAADF-STEM analyses performed to fully characterize as-prepared catalysts suggested that they have small particle sizes: 2.0 nm for Pd/CNT, 2.3 nm for PdAg/CNT, 2.4 nm for PdAg3/CNT, and 13.9 nm for Ag/CNT. XPS further shows that alloying with Ag results in more metal state Pd presented on the surface, and this may be related to their higher direct glycerol fuel cell (DGFC) performances. Single DGFC performance and product analysis results show that PdAg bimetallic nanoparticles can not only improve the glycerol reaction rate so that higher power output can be achieved, but also facilitate deep oxidation of glycerol so that a higher faradaic efficiency and fuel utilization can be achieved along with optimal reaction conditions (increased base-to-fuel ratio). Half-cell electrocatalytic activity measurement and single fuel cell product analysis of different glycerol oxidation intermediates, including C3: glycerate, tartronate, mesoxalate, and lactate; C2: glycolate and oxalate, over PdAg/CNT catalyst was further conducted and produced deeper insight into the synergistic effects and reaction pathways of bimetallic PdAg catalysts in glycerol electrocatalytic oxidation.

Introduction

Rapid depletion of fossil fuels makes it necessary to seek replacement of petroleum-based energy sources to lead to a sustainable future [1]. Clean and renewable energy sources are increasingly being used to replace fossil fuels, to end the progression of climate change, and to reduce pollution [2]. Prominent energy devices such as internal combustion engines have low efficiency (<13%) while emitting many harmful pollutants and greenhouse gasses [3]. Glycerol is a non-toxic, non-flammable, and non-volatile biorenewable alcohol fuel obtained as a byproduct of the transesterification reaction that occurs in the production of biodiesel [4], [5], [6]; as a result, glycerol can today be obtained at relatively lower market prices compared to other alcohol fuels (see Table 1). Glycerol can serve as a starting point for production of a series of high-value oxygenated chemicals such as glyceric acid, tartronic acid, mesoxalic acid, and glycolic acid, etc. [7], [8], [9], [10], [11]. Traditional production of these oxygenate compounds is costly, environmentally unfriendly because of stoichiometric oxidation using strong acids [12], or exhibits slow fermentation processes accompanied by low output yields [13]. A glycerol oxidation reaction (GOR) produces negative Gibbs free energy, so it can be used as a fuel fed at the anode for fuel cells to simultaneously generate electrical power and produce valuable chemicals.

Fuel cells, batteries, and electrochemical capacitors are systems considered for alternative energy/power sources. The main disadvantage of rechargeable batteries (mostly lithium-based, e.g., lithium or lithium polymer) is limited energy density [14], [15]. Fuel cell technology, a thrust research area, is an appropriate substitute to rechargeable battery technology due because fuel cells, especially direct alcohol fuel cells (DAFCs), have been recognized as green energy generators capable of converting renewable sources into electrical power [16]. To meet the world’s demand for energy, DAFCs represent a potentially promising alternative energy source to the use of fossil fuels [17], [18]. The thermodynamic efficiency of a DAFC is greater than 90% because energy from the fuel is directly transformed into electrical energy without the constraints of Carnot’s theorem [19], [20]. Anion-exchange membrane-direct alcohol fuel cells (AEM-DAFCs) have the great advantage that the kinetics of both anode and cathode reactions can be greatly enhanced by the better mass transfer and lower adsorption of spectator-charged species [17], [18], [21], [22], [23], [24]. The byproducts associated from AEM-DAFCs also appear to produce no negative environmental impact. To more completely explore such alternative fuels, numerous studies have been carried out based on AEMFC platforms using various biorenewable fuels.

The typical performances of DAFCs are shown in Table 1. Low-temperature AMFCs have exhibited significant advantages over other types of fuel cells because charge and ion transfer along with alcohol oxidation kinetics can be greatly improved in alkaline media. We have demonstrated a surprisingly high performance of 268.5 mW/cm2 (ambient O2, with a low Pt loading of 0.5 mg/cm2, 80°C) using an AEMFC directly fed with 88 wt% soybean biodiesel crude glycerol (one of the cheapest alcohols on the market), with the faradaic efficiency reaching 47% (6.5e-/14e-) [25]. In general, a PEM-direct ethanol fuel cell has a peak power density (e.g., <80 mW/cm2) and low faradaic efficiency of <30% because its dominant byproduct is acetate (4e/12e). SOFCs must operate at high temperatures (i.e. >750°C) and thus have relatively limited applications for portable electronics. Current biofuel cells employ enzymatic catalysts to achieve complete oxidation of alcohols, but their low output power density (<1.0 mW/cm2), heavy dependence on the organic-living environment, and short lifetime limit biofuel cell applications to environmental remediation rather than mobile power source application. To achieve long life-time DGFCs operation, however, more robust and cheaper catalysts must be developed.

Platinum (Pt) and Pt-based catalysts for DAFCs have been identified as the best electro-catalysts with respect to electrooxidation of alcohols at relatively low temperatures, where they exhibited high power density and fuel utilization efficiency [25], [33], [34], [35], [36], [37]. Pt can be more easily contaminated than other precious metals, limiting its stability and activity, and the high cost due to scarcity of Pt is also problematic, so extensive efforts are being carried out to rationally design new catalysts for DAFCs. Much research regarding selective oxidation of glycerol through environmentally-friendly and fast heterogeneous catalysis using monometallic/bimetallic Pd based catalysts has been conducted [7], [12], [38], [39], [40], [41]. Since Pd is much more abundant in nature and half the cost of Pt, it is a suitable replacement of Pt for oxidation of a large variety of organic molecules in alkaline environments. The addition of a second metal to create Pd-M alloy catalysts has been extensively explored [42], [43], [44], [45], [46], [47], [48], [49], [50]. For a C2+ alcohol, there is a need to rationally design Pd-M catalysts to not only improve the oxidation kinetics (activity), but also to manipulate the reaction pathway to cleave a Csingle bondC bond of alcohol. Ag has up to now been much less studied in heterogeneous oxidation catalysis, even though its addition to Pd can significantly reduce the cost of the anode catalyst and may even further improve the alcohol oxidation rate. We recently designed an efficient PdAg/CNT catalyst that demonstrated better performance than Pd/CNT for alcohol (methanol, ethanol, ethylene glycol, and glycerol) oxidation in AEM-DAFCs [51]. We found that Pd can facilitate deprotonation of alcohol in a base electrolyte, while Ag can promote intermediate aldehyde oxidation and cleavage of Csingle bondC bonds of C3 species to C2 species, so a combination of two active sites (Pd and Ag), with two different functions, can simultaneously improve both the reaction rate and the deeper oxidation products. Previous work focused on the PdAg/CNT catalyzed alcohol oxidation reaction facilitated by Ag catalyzed aldehyde oxidation, a more general catalytic mechanism.

In the present work, we focus on a more detailed analysis of glycerol oxidation over PdAg/CNT as well as a more comprehensive physical characterization of PdAg/CNT. Full characterizations such as XRD, TEM, XPS, ICP-MS, and HAADF-STEM, were used to characterize the particle size, size distribution, structure, surface chemical state and bulk metal composition of these catalysts. The catalytic activities of catalysts toward glycerol oxidation were first compared in half-cells, and then applied as electrocatalysts for oxidation of glycerol in AEM-DAFCs to determine the product distribution. Electrocatalytic activities of different reaction intermediates (C3 chemicals: glycerate, tartronate, mesoxalate, lactate; C2 chemicals: glycolate, oxalate) corresponding to these catalysts were determined to investigate the reaction pathways, with combined product distribution results obtained in single DGFCs. This study demonstrates the benefit of using an alloyed Pd-Ag bimetallic catalyst to improve peak power density and facilitate deeper oxidation products, thereby improving fuel cell performance.

Section snippets

Chemicals

Carboxyl-group functionalized short multi-wall carbon nanotubes (8–15 nm outer diameter, 0.5–2 mm length) were purchased from Cheaptubes, Inc. Palladium (II) nitrate dihydrate (40%), silver nitrate (99%), 1-propanol (99.5%), potassium hydroxide (85%), potassium sulfate (99%), sodium borohydride (99%), sodium citrate dihydrate (99%), polytetrafluoroethylene (PTFE) ionomer solution (60%), glycerol (99.5%), lactic acid (98%), d-glyceric acid calcium salt dihydrate (99%), sodium mesoxalate

Physical characterization of monometallic and bimetallic Pd and Ag electrocatalysts

Carbon nanotube (CNT) supported monometallic Pd/CNT and Ag/CNT and bimetallic PdAg/CNT and PdAg3/CNT were prepared using a modified aqueous-phase reduction method [51]. The morphology, particle size, size distribution, structure, surface chemical state, and composition of the as-prepared catalysts were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark field via aberration-corrected scanning transmission electron microscopy (HAADF-STEM), X-ray

Conclusions

Carbon nanotube (CNT) supported Pd, PdAg, PdAg3, and Ag nanoparticles with small sizes (2.0 nm for Pd/CNT, 2.3 nm for PdAg/CNT, 2.4 nm for PdAg3/CNT, and 13.9 nm for Ag/CNT) and narrow size distributions were synthesized through a modified aqueous-phase reduction method and served as working catalysts. XPS spectra show that by alloying with Ag, more metallic state Pd is presented on the surface. The higher performance of AEM-DGFC with PdAg anode catalyst compared to that with Pd/CNT anode catalyst

Acknowledgements

We acknowledge financial support from the US National Science Foundation (CBET-1501124 and 1159448), the Iowa State University startup fund, the Ames Lab startup fund and an Iowa Energy Center (IEC) Opportunity Grant. We thank Dr. Dapeng Jing of Material Analysis and Research Laboratory of ISU for XPS analysis and also Zhiyuan Qi of Chemistry Department for ICP-MS analysis. The authors are grateful to Ryan F. McSweeney and Baitong Chen of Iowa State University for assistance in fuel cell

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