Dimethyl Ether Electro-oxidation Research Articles

(Invited) Development of Multifunctional Nanoscale Reactors for Electrocatalytic Oxidation of Dimethyl Ether Fuel

There has been growing interest in utilizing small (simple) organic molecules, as alternative fuels to hydrogen, for electrochemical energy conversion in fuel cells. Dimethyl ether (DME) is one of the possible choices in this respect. But electrooxidation of DME is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials.Bimetallic PtSn catalysts were found to be almost as active as PtRu for methanol oxidation, and PtSn was even more efficient toward the electrooxidation of ethanol where breaking of C-C bond is required. While PtRu forms a single-phase homogeneous (intermetallic) alloy in which Ru is largely metallic, tin does not maintain its metallic state in PtSn (heterogenous alloy) and is transformed readily to such oxygenated species as Sn oxides or hydroxides. During operation of PtSn, the Pt component retains its mostly metallic character and act independently as Pt0 catalyst. By analogy to Ru, the Sn “bifunctional agent” tends to activate the water dissociative adsorption and increases population of chemisorbed hydroxyl groups to diminish the Pt-poisoning effect through enhanced oxidation of CO-type intermediates. Nevertheless, the DME oxidation on both PtRu or PtSn is still kinetically less favorable, relative to the systems’ performance toward methanol. While Sn, and particularly Ru alloyed atoms activate readily the water dissociative adsorption on Pt and permit the oxidative stripping of the poisoning CO-type adsorbates, the feasibility of the enhancement of the of the DME adsorption and activation through alteration of the electronic structure of Pt catalytic sites has not been fully explained. By analogy to the ethanol oxidation, Pt sites in the heterogeneous PtSn alloy are active toward dissociation of the DME molecule but the Sn additive is less efficient than Ru (from intermetallic PtRu) in the oxidative removal of poisoning adsorbates. But PtRu is probably the most active during oxidation of methanol, and this small organic molecule is also the DME-oxidation-reaction intermediate. Nevertheless, in comparison to bare Pt, electrooxidation of DME starts at PtRu and PtSn at less positive potentials.We pursue a concept of utilization of multicomponent hybrid electrocatalytic systems or nanoreactors utilizing zirconia oxide matrices for supporting and activation of primarily PtSn nanoparticles supported onto Vulcan XC-72 carbon black carriers. Electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys supported onto ZrO2 matrices has been significantly enhanced toward electrooxidation of DME in acid medium (0.5 mol dm-3 H2SO4) by decorating PtSn/V with bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence (even below 0.45 V vs. RHE) of reactive ruthenium oxo/ hydroxo groups on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In this respect, the Ru-oxo species seem to support activity of Sn forming with Pt the PtSn heterogenous alloy. The PtSn/V admixed with PtRu decorated ZrO2 exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles decorated zirconia oxide seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Acknowledgement Authors acknowledge financial support of the National Science Center (Poland) under Opus Project 2018/29/B/ST5/02627.

Understanding the Electrooxidation of Dimethyl Ether on Pt3Pd3Sn2 Supported on a Mixture of Carbon Materials

Recently, we reported a rigorous study on the effect of carbon supports and their cold plasma treatment on a state-of-the-art catalyst, Pt3Pd3Sn2, for dimethyl ether (DME) electrooxidation. The catalyst supported on a mixture of 75% activated multi-walled carbon nanotube (MWCNT) (75 M) and 25% pristine black pearl 2000 (BP2000) (25B) (Pt3Pd3Sn2/75M25B) offered improved DME kinetics with respect to the single or other combinations of the same supports. In this work, the results of the electrochemical impedance spectroscopy (EIS) were coupled with physicochemical characterizations (X-ray Diffraction (XRD), Small Angle X-ray Scattering (SAXS), and Scanning Tunneling Electron Microscopy (STEM)) for a detailed understanding of the origins of the improved kinetics. With an appropriate composition of the two supports in the mixture (75M25B), a catalyst with optimized particle size, dispersion, and conductivity was obtained. A Pt3Pd3Sn2/75M25B-coated electrode exhibited a reduced charge transfer resistance of 0.63 ohms at the catalyst layer compared to BP2000 and MWCNT, which showed 1.53 and 1.31 ohms, respectively. These results provide vital insights into catalyst support design considering the use of support mixtures of optimized conductivity and surface area for enhanced power output.

Electrooxidation of Dimethyl Ether on Pt/TiO2–C Сatalysts

Electrooxidation of Dimethyl Ether on Pt/TiO2–C Сatalysts

Hybrid Electrocatalysts Composed of PtSn, Ru or PtRu Nanoparticles for Low-Temperature Oxidation of Dimethyl Ether Fuel

The science and technology of proton-exchange membrane (PEM) low-temperature fuel cells is a subject of intense during recent years, particularly with use of hydrogen gas due to its molecular simplicity and fast kinetics of its electrooxidation. However, there are still many fundamental problems to be resolved with respect to generation of pure hydrogen and related pollution side effects, the low density and safety of hydrogen storage and distribution. Thus, there are parallel studies involving such simple organic molecules as methanol, formic acid or ethanol as substitute fuels for a wide range of potential applications. Among important obstacles are slow kinetics of electrocatalytic oxidations of fuels and insufficient durability of operation of catalysts that includes poisoning of active sites by strong adsorption of the reaction intermediate products (e.g. CO-type adsorbates), in addition the fuel cross-over through the membrane, which depolarizes the cathode and decreases its activity. While methanol, which has been the most extensively studied, faces the problem of toxicity and challenges mentioned above, the low electrocatalytic efficiency of the formic acid oxidation (only 2-electron-process) is complicated by corrosion phenomena. When it comes to the electrooxidation of ethanol, extra problems concern difficulty of the strong C-C bond breaking and excessive formation of undesirable intermediate products, acetaldehyde and acetic acid. Dimethyl ether (DME), or methoxymethane having formula CH3OCH3, is the simplest ether, and it can be considered as an isomer of another small organic molecule, ethanol, but with a different functional group. DME is a colorless gas that has been demonstrated in a variety of fuel applications, e.g. as an aerosol propellant or substitute for propane in LPG. DME can be viewed as the synthetic second generation biofuel, which can be produced from lignocellulosic biomass biogas or methane from agricultural waste. With respect to potential applications in low-temperature fuel cells, DME could be an attractive simple fuel due to its high energy density, low toxicity, liquefied easy storage, as well as the molecule’s low dipole moment that should diminish crossover through Nafion membrane, when compared to methanol, ethanol and formic acid. The stoichiometric complete electrochemical oxidation of DME requires three H2O-molecules, produces two CO2-molecules, and yields as many as twelve electrons. Obviously the multi-electron and multi-proton electrode process are complex and composed of distinct and competing reaction pathways and, thus, they are subject to kinetic limitations. The main constraints related to application of common Pt-based catalysts (e.g. Pt and bimetallic PtRu) in direct DME fuel cells concern the system’s low practical electrooxidation efficiency.In the present study, electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys toward electrooxidation of dimethyl ether (DME) has been significantly enhanced in acid medium (0.5 mol dm-3 H2SO4) by intentional and controlled decoration of PtSn/V with ruthenium black or bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence of reactive ruthenium oxo/hydroxo groups (even below 0.45 V vs. RHE) on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In that respect, the Ru-oxo species originating from Ru or PtRu additives seem to support activity of Sn co-catalytic sites interacting with Pt within the PtSn heterogenous alloy. The Ru-decorated PtSn/V and, in particular, PtSn/V admixed with PtRu exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Despite possible differences in the DME-electrooxidation mechanisms at the hybrid and single component systems, the dissociation of H3C-O-CH3 molecule, which involves C-O bond breaking, formation of the methanol reaction intermediate and CO-type poisoning adsorbates should be operative.

Hybrid Electrocatalysts Composed of PtSn, Ru or PtRu Nanoparticles for Low-Temperature Oxidation of Dimethyl Ether Fuel

Electrocatalytic activity of Vulcan-supported PtSn (PtSn/C) nanostructured alloys toward electrooxidation of dimethyl ether (DME), a potential small-organic-molecule fuel, has been significantly enhanced in acid medium (0.5 mol dm-3 H2SO4) by decorating PtSn/C with bimetallic PtRu nanoparticles modified with phosphododecatungstic acid (PtRu/PW12). The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. In that respect, the Ru-oxo species originating from PtRu additives seem to support activity of Sn co-catalytic sites interacting with Pt within the PtSn heterogenous alloy. The polytungstate adsorbates on nanoparticles facilitate their distribution at the electrocatalytic interface, as well as they interact competitively with the CO adsorbates at noble metal catalytic sites. The hybrid materials composed of PtSn/C decorated with PtRu nanoparticles modified with PW12 seem to act as multifunctional nanoreactors during electrooxidation of DME.

Methyl formate and dimethyl ether electro-oxidation on Pt[sbnd]Pd[sbnd]Sn catalyst supported on carbon nanotube decorated with carbon dots

In this work, methyl formate (MF) and dimethyl ether (DME) electro-oxidation was studied on equimolar ratio PtPdSn catalyst supported on Vulcan Carbon XC-72, carbon nanotubes (CNT), and CNT–nitrogen-doped carbon dots (NCDs) composite. The ternary catalyst was synthesized by the ethylene glycol-assisted thermal reduction method and NCDs were synthesized by a hydrothermal method in the presence of CNT to form CNT-NCDs composite, in which the NCDs are incorporated onto the CNT surface. The activity of the catalyst in the oxidation of MF and DME was analyzed using cyclic voltammetry and chronoamperometry techniques. The ternary catalyst supported on CNT-NCDs composite (Pt1Pd1Sn1/CNT-NCDs) showed a peak oxidation current of 75 mA/mg and 365 mA/mg for DME and MF, respectively, highest among the studied Pt1Pd1Sn1/XC-72 and Pt1Pd1Sn1/CNT. The onset potential of DME on Pt1Pd1Sn1/CNT-NCDs catalyst shifted negatively by ∼200 mV compared to PtRu. Irrespective of the support material, the ternary catalysts showed a significant negative shift in the onset and peak potential of COad oxidation compared with commercial PtRu. The oxidation behavior of MF was studied in conjugation with its hydrolysis products methanol, formic acid, and mixed fuel (formic acid + methanol) solution, which disapproves the earlier proposed acid-catalyzed hydrolysis of MF to a significant degree during the electrochemical experiment. Hence, the CNT-NCDs supported ternary catalyst showed high catalytic activity and stability for electro-oxidation of DME and MF.

Enhancement of oxidation of dimethyl ether through formation of hybrid electrocatalysts composed of Vulcan-supported PtSn decorated with Ru-black or PtRu nanoparticles

Electrocatalytic activity of Vulcan-supported PtSn (PtSn/C) nanostructured alloys toward electrooxidation of dimethyl ether (DME), a potential small-organic-molecule fuel, has been significantly enhanced in acid medium (0.5 mol dm−3 H2SO4) by decorating PtSn/C with ruthenium black or bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence of reactive ruthenium oxo/hydroxo groups (even below 0.45 V vs. RHE) on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In that respect, the Ru-oxo species originating from Ru or PtRu additives seem to support activity of Sn cocatalytic sites interacting with Pt within the PtSn heterogenous alloy. The Ru-decorated PtSn/C and, in particular, PtSn/C admixed with PtRu exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles seem to act as multifunctional nanoreactors inducing not only oxidative stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol).

Enhancement of oxidation of dimethyl ether through application of zirconia matrix for immobilization of noble metal catalytic nanoparticles

Electrocatalytic activity of Pt and bimetallic PtRu nanoparticles (both Vulcan supported and unsupported) toward electrooxidation of dimethyl ether (DME), a potential small organic molecule fuel, in an acid medium (0.5 mol dm−3 H2SO4) has been significantly enhanced by dispersing them over a thin film of zirconia (ZrO2). The enhancement effects concern increases of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. Similar effects have been observed for the oxidation of methanol. Regarding the dissimilar DME electrooxidation mechanisms at Pt and PtRu catalytic centers, the activating capabilities of zirconia seem to originate from the high population of reactive –OH groups favoring mobility of protons and the capability of inducing the oxidative removal of poisoning (CO-type) intermediates both at platinum and ruthenium catalytic sites. In the presence of the zirconia matrix, the onset potential for the oxidation of DME (particularly at PtRu) is shifted more than 50 mV toward less positive potentials. Mutual metal-support interactions are also postulated.

Direct Electro-Oxidation of Dimethyl Ether on Pt-Cu Nanoparticles

As the use of portable devices such as mobile phones, laptops and tablets grows rapidly, efficient portable energy technologies are becoming more necessary than ever. In the past years, fuel cells made a great advancement in the field of power sources technology, and are currently referred to as the most promising alternative energy technology. The most popular type of fuel cells today is the polymer electrolyte membrane fuel cell (PEMFC), fueled with hydrogen. Although it offers very high power density, the lack of fueling infrastructure necessary for the handling of hydrogen is currently one of its biggest commercial impediments. In order to overcome this hurdle with portable devices, direct methanol fuel cells (DMFC) were proposed as an alternative. This is mainly due to the methanol’s high energy density and ease of handling. DMFC’s were studied thoroughly over the past decades, and the technology encountered some barriers such as slow anode kinetics, methanol crossover from the anode to the cathode and issues concerning methanol’s toxicity. Dimethyl ether (DME) is an ether gas under ambient conditions that could be liquefied under low pressure, traditionally used for aerosol propulsion and as an alternative fuel for diesel combustion engines. It was also recently suggested as an alternative fuel for low-temperature fuel cells. This is mainly due to its higher energy density when compared to methanol, 8.2 vs. 6.1 kWh kg-1. There are no C-C bonds to break in the reaction and its theoretical open cell voltage (OCV) is relatively close to DMFC, 1.18 V compared to 1.21 V at 25°C. In addition, DME has a lower dipole moment than methanol and therefore, the fuel crossover in a fuel cell is expected to be much lower than methanol’s, decreasing the mixed potential impact on the cathode. In this work, new catalyst for the direct electro-oxidation of dimethyl ether (DME) was synthesized and studied using an array of techniques. One of the most prominent catalysts for this reaction, platinum copper alloy (PtCu), was synthesized in an easy and low cost approach. Structural characterizations such as X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and elemental analysis revealed that the synthesized PtCu nanoparticles (3 nm on average), formed homogeneous alloy without aggregation of metallic platinum or copper. The catalyst’s activity towards electro-oxidation of DME was tested using rotating disk electrode (RDE) and in membrane-electrode assembly in a full cell. The catalyst performance was found to be promising. Direct DME fuel cell (DDMEFC) studied in this work has relatively high energy density as well as low “cross-over” tendency due to a low dipole moment and thus shows great potential as fuel for low power fuel cells. The electrocatalysis of the DME oxidation reaction (DOR) was compared between synthesized PtCu and commercial PtRu/C and exhibited an enhanced performance with the newly synthesized catalyst.

Highly active PtxPdy/SnO2/C catalyst for dimethyl ether oxidation in fuel cells

Dimethyl ether (DME) is a nontoxic gas that is considered a potential fuel for direct-feed proton exchange membrane fuel cells (PEMFCs). DME has several advantages over other fuels, including high energy density, pumpless fuel delivery, liquefied storage, low toxicity and minimal crossover through Nafion® membranes in PEMFC. However, the low activity of the state-of-the-art catalyst (Pt50Ru50) for DME oxidation is the main hurdle in the development of efficient fuel cell devices. In this work, fine layers of SnO2 on high surface-area carbon (PtxPdy/SnO2/C) catalysts were synthesized by ethylene glycol-assisted reduction and characterized by X-ray diffraction, TEM, EDX, XPS, and ICP-OES. The electro-oxidation of DME was systematically studied in a conventional three-electrode cell and a laboratory prototype direct DME fuel cell (DDMEFC) operating at 70 °C. Compared to other catalysts reported for DME oxidation, the Pt0.75Pd0.25/SnO2/C shows higher specific power density in both the conventional three-electrode cell and the fuel cell configurations. The peak power density delivered by direct gas feed DDMEFCs is 110 mW cm-2 at 0.4 V with only 1.2 mg cm−2 of platinum group metal (PGM) loading.

Direct Electro-oxidation of Dimethyl Ether on Pt-Cu Nanochains.

A new platinum-copper alloy electrocatalyst for the direct electro-oxidation of dimethyl ether (DME) has been synthesized in an easy and low-cost approach and studied by using an array of techniques, including X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and elemental analysis. Structural characterization revealed that the synthesized PtCu nanoparticles (3 nm on average) formed homogeneous nanochains without aggregation of metallic platinum or copper. The catalyst's activity towards electro-oxidation of DME was tested using cyclic voltammetry (CV) and in membrane-electrode assembly (MEA) in a full cell and was found to be promising. The direct DME fuel cell (DDMEFC) studied in this work has relatively high energy density, of 13.5 mW cm-1 and thus shows great potential as fuel for low power fuel cells. The newly synthesized PtCu catalyst exhibited almost double the performance of commercial PtRu in electrocatalytic DME oxidation.

Electrochemical Nanoreactors of Mixed Metal-Oxide-Supported Noble- Metal-Nanoparticles for Efficient Oxidation of Dimethyl Ether and Ethanol

Noble metal-based (e.g. Pt, Pd, PtRu or PtSn) nanoparticles have been recognized as the most active catalytic systems towards oxidation of small organic molecules (ethanol, dimethyl ether) at low and moderate temperatures. Although all of them can be ideally oxidized to carbon dioxide, their actual electrooxidation mechanisms can significantly differ and may result in distinct electrocatalytic efficiencies and final reaction products. There is a need to develop functionalized electrocatalytic systems exhibiting specific reactivity and capability to induce the slow reaction steps at ambient conditions. Platinum based nanoparticles are readily poisoned by the strongly adsorbed CO-type intermediate species requiring fairly high overpotentials for their removal. For example, to enhance activity of Pt-based catalysts toward the ethanol oxidation, additional metal or metal oxide nanostructures (rhodium, iridium, tin or molybdenum and tungsten oxides) will be intentionally introduced to the electrocatalytic interface. We are also going to demonstrate that catalytic activity of platinum-based nanoparticles can be significantly enhanced through their interfacial modification with ultra-thin monolayer-type films of mixed-metal-oxo-species of titanium, cerium, zirconium or molybdenum and tungsten in their simple and polyoxometallate forms Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites. It is noteworthy that certain metal and mixed-metal oxides can generate –OH groups at low potentials: they induce oxidation of passivating CO adsorbates (e.g. on Pt) or breaking C-H bonds (e.g. during oxidation of methanol or dimethyl ether). When combined with dispersed Rh or Ir, immobilized in the organized manner in the vicinity Pt sites, they tend to weaken C-C bonds during ethanol oxidation. We will also demonstrate that deposition of Pt and bimetallic PtRu nanoparticles onto nanostructured zirconia support results in the enhancement of their catalytic activities toward electrooxidation of dimethyl ether, another possible alternate fuel. Once more, it is reasonable to expect that the abilities of zirconia to facilitate proton mobility and to donate active hydroxyl groups at the electrocatalytic interface in acid media are responsible for the observed enhanced catalytic effects. Specific interactions with platinum and ruthenium components are also postulated.

Nanostructured catalysts for direct electrooxidation of dimethyl ether based on Bi- and trimetallic Pt–Ru and Pt–Ru–Pd alloys prepared from coordination compounds

Bi- and trimetallic platinum–ruthenium and platinum–ruthenium–palladium catalysts with specified atomic ratios Pt: Ru = 1: 1 and Pt: Ru: Pd = 1: 1: 0.1, respectively, were synthesized from the coordination compounds of the metals deposited on highly dispersed carbon black. The catalysts were characterized by powder X-ray diffraction, electron dispersive analysis, and transmission electron microscopy. According to voltammetry data, the highest activity in the dimethyl ether (DME) electrooxidation is exhibited by the catalyst Pt0.43Ru0.47Pd0.1/C; hence, it may be considered as a promising anode material for direct DME fuel cells.

Interactions of Metal Oxo Species with Platinum-Based Catalysts Toward More Efficient Electrooxidation of Dimethyl Ether: Comparison to Methanol

Methanol is so far the most commonly considered fuel as an alternative to hydrogen. However, the practical performance is limited by several factors, which include the slow kinetics of methanol oxidation and crossover of methanol from the anode to the cathode side of the cell. More recently, there has been growing interest in dimethyl ether (CH3OCH3), a gas commonly used in aerosol propellants, solvents and coolants, as a possible alternative fuel for low temperature fuel cells. Dimethyl ether (DME) is non-toxic, non-teratogenic, non-mutagenic and non-carcinogenic gas. When oxidized to CO2, a CH3OCH3 molecule releases 12 electrons. The fuel has several advantages over typical organic fuels. Unlike ethanol (CH3CH2OH), the CH3OCH3 ether does not require breaking of the C-C bond. A fairly low CH3OCH3 dipole moment results in the lower ether crossover (through the membrane) in comparison to methanol. Dimethyl ether is much less toxic than methanol and can be readily stored and transported using existing technologies. In addition, the CH3OCH3 ether has potentially higher energy density relative to methanol (namely, 8.2 vs. 6.1 kWh kg-1). Platinum is one of the typical catalysts for the oxidation of organic molecules. It is quite active toward the dehydrogenation step, but suffers from the CO oxidation step. The bimetalic catalysts (e.g. PtRu, PtSn) can lower the onset potential of DME oxidation peak from 0.5 V for Pt/C to 0.4 V vs. Normal Hydrogen Electrode (NHE), but the maximum current densities are obtained on Pt/C. We propose here to study the electrooxidation of DME at Pt and PtRu nanoparticles that have been deposited onto nanostructured zirconium(IV) oxide matrix (thin film on electrode surface). Zirconia is known to act as a catalytic component capable of enhancing the reactivity of the electrocatalytic system by increasing the overall acidity at the reaction interface. When combined (as a support) with Pt or PtRu centers, the existence of large population of hydroxyl groups on ZrO2 surface is believed to facilitate desorption of CO adsorbates from Pt and to induce their oxidation to CO2. In the presence of nanostructured zirconia (existing in the vicinity of the catalytic metal), the transport of protons at the electrocatalytic interface is also favored. Synergistic interactions between ruthenium and zirconia components are also feasible. In the present report, we will not limit our results to DME, but similar effects have been observed during oxidation of methanol. We are also going to consider combination of Pt or PtRu with the other metal (Pd) or metal (W, Mo) oxo nanostructures. The support from National Research Center (NCN), Poland under the MAESTRO Project No 2012/04/A/ST4/00287 is highly appreciated.

Dimethyl ether electro-oxidation on platinum surfaces

A first-principles density functional theory study was performed to elucidate the mechanism of dimethyl ether electro-oxidation on three low-index platinum surfaces (Pt(111), Pt(100), and Pt(211)). The goal of this study is to provide a fundamental explanation for the high activity observed experimentally on Pt(100) compared to Pt(111) and stepped surfaces. We determine that the enhanced activity of Pt(100) stems from more facile C–O bond breaking kinetics, as well as from easier removal of CO as a surface poison through activation of water. In general, the C–O bond (in CHxOCHy) becomes easier to break as dimethyl ether is dehydrogenated to a greater extent. In contrast, dehydrogenation becomes more difficult as more hydrogen atoms are removed. We perform two analyses of probable reaction pathways, which both identify CHOC and CO as the key reaction intermediates on these Pt surfaces. We show that the reaction mechanism on each surface is dependent on the cell operating potential, as increasing the potential facilitates C–H bond scission, in turn promoting the formation of intermediates for which C–O scission is more facile. We additionally demonstrate that CO oxidation determines the high overpotential required for electro-oxidation on Pt surfaces. At practical operating potentials (~0.60 VRHE), we determine that C–O bond breaking is most likely the most difficult step on all three Pt surfaces studied.

On the Structure Sensitivity of Dimethyl Ether Electro-oxidation on Eight FCC Metals: A First-Principles Study

The electro-oxidation of dimethyl ether (DME) was investigated using periodic, self-consistent density functional theory (DFT) calculations on the (111) and (100) facets of eight fcc metals: Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh. The goal of this study is to understand the experimentally observed structure sensitivity of this reaction on Pt, and to predict trends in structure sensitivity of this reaction across the other seven metals studied. The main conclusion is that the enhanced activity of Pt(100) originates from more facile C–O bond breaking and removal of surface poisoning species, including CO and CH. When comparing C–O bond breaking energetics, we do not find a universal trend where these elementary steps are always more exergonic on the (100) facet. However, we find that, at a given potential, DME can be dehydrogenated (prior to breaking the C–O bond) to a greater extent on the (100) facet. Additionally, we find that the reaction energy for C–O bond breaking in CHxOCHy-type species becomes increasingly exergonic as the species becomes increasingly dehydrogenated. Together, the more facile dehydrogenation on the (100) facets provides more favorable routes to C–O bond activation. Though we calculate a lower onset potential on Au(100), Ag(100), Cu(100), Pt(100), and Pd(100) than their respective (111) facets, the calculated onset potential for Ni(100), Ir(100), and Rh(100) are actually higher than for their respective (111) facets. Finally, by constructing theoretical volcano plots, we conclude that Au(100), Ag(100), Cu(100), Pt(100), and Pd(100) should be more active than their respective (111) facets, while Ni(100), Rh(100), and Ir(100) will show the opposite trend.

High-Performance Direct Dimethyl Ether Fuel Cell Operating with an Advanced Activation Process

The direct methanol fuel cell (DMFC) has been of interest due to the absence of a difficult to split carbon-carbon bond in methanol, high energy content of the liquid fuel and facile handling. Other established high energy-density liquid fuels, such as ethanol, are not yet amenable to direct electrochemical oxidation due to the lack of electrocatalysts for an efficient carbon-carbon bond scission. A related fuel to methanol, with strong direct feed fuel cell potential is dimethyl ether (DME). DME possesses many of the same physical advantages that methanol does.1-3 It is similar to methanol in terms of theoretical open cell voltage (1.18 vs. 1.21 V at 25°C) and does not require a carbon-carbon bond scission. DME is also a clean and, unlike methanol, nontoxic fuel that undergoes facile biodegradation and can be easily synthesized from recycled carbon dioxide and renewable energy.4 Due to its lower dipole moment, DME is less subject to membrane crossover than methanol; it also has a higher energy content than methanol (8.2 vs. 6.1 kWh/kg).5,6Complete electro-oxidation of DME consumes 3 equivalents of water and yields 12 equivalents of electrons: CH3 OCH3+3H2O→2CO2+12H++12e- (1) While the direct DME fuel cell (DDMEFC) performance has by now been demonstrated to match that of the state-of-the-art DMFC,7 DDMEFC still requires development to become fully viable. In addition to the needed improvements in the anode catalyst activity, the catalyst deactivation and its durability continue to pose a challenge. Several aspects of optimization have been explored of which break-in has been one area of attention.8 In this work, membrane electrode assembly (MEA) preparation and initial break-in were carried out according to standard procedure.1 Prior to operation in the DDMEFC mode, the fuel cell was operated on humidified hydrogen/air to condition the membrane. When the current at 0.7 V plateaued the break-in was stopped, fuel cell polarization recorded on H2/air, and the anode feed switched to preheated, humidified DME under a pressure of approximately 1.75 bar gauge (2.56 bar absolute).1 Performance at this point reproduced previously observed performance on DME under similar conditions. Several DDMEFC polarization plots were recorded, leading a gradual performance loss. In an attempt to recover lost performance, the cell was switched back to break-in conditions on H2/air and after break in, DDMEFC polarization plots were recorded again. With each additional iteration of this procedure, the fuel cell performance increased, up to ~0.3 A cm-2 at 0.5 V after a thirteenth iteration (Figure 1). As a result of this conditioning, the DDMEFC power density achieved reached 0.18 W cm-2 – a 50% increase over the highest power density value reported in the literature with a commercial PtRu catalyst (0.12 W cm-2).7 In this presentation, we will provide details of the procedure used and discuss the causes of such a significant performance improvement. We will also comment on the DDMEFC performance durability in the context of the conditioning approach used in this study. Acknowledgment Financial support from the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy through Fuel Cell Technologies Office is gratefully acknowledged. REFERENCES 1 Li, Q., Wu, G., Johnston, C. M., Zelenay, P. "Direct Dimethyl Ether Fuel Cell with Much Improved Performance." Electrocatalysis 5, 310-317 (2014). 2 Jensen, J. O., Vassiliev, A., Olsen, M., Li, Q., Pan, C., Cleemann, L. N., Steenberg, T., Hjuler, H. A., Bjerrum, N. "Direct dimethyl ether fueling of a high temperature polymer fuel cell." Journal of Power Sources 211, 173-176 (2012). 3 Li, Q., Wen, X., Wu, G., Chung, H. T., Zelenay, P. "High-Activity PtRuPd/C Catalyst for Direct Dimethyl Ether Fuel Cell." Angewandte Chemie International Edition(In Press). 4 Olah, G. A., Goeppert, A., Prakash, G. S. "Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons." The Journal of Organic Chemistry 74, 487-498 (2008). 5 Müller, J. T., Urban, P. M., Hölderich, W. F., Colbow, K. M., Zhang, J., Wilkinson, D. "Electro‐oxidation of dimethyl ether in a polymer‐electrolyte‐membrane fuel cell." Journal of the Electrochemical Society 147, 4058-4060 (2000). 6 Mizutani, I., Liu, Y., Mitsushima, S., Ota, K.-i., Kamiya, N. "Anode reaction mechanism and crossover in direct dimethyl ether fuel cell." Journal of Power Sources 156, 183-189 (2006). 7 Chung, H. T., Dumont, J. H., Martinez, U., Zelenay, P. Catalyst Development for Dimethyl Ether Electrooxidation. in Meeting abstracts. 929-929p (The Electrochemical Society). 8 Qi, Z., Kaufman, A. "Quick and effective activation of proton-exchange membrane fuel cells." Journal of Power Sources 114, 21-31 (2003). Figure 1

High‐Activity PtRuPd/C Catalyst for Direct Dimethyl Ether Fuel Cells

Dimethyl ether (DME) has been considered as a promising alternative fuel for direct-feed fuel cells but lack of an efficient DME oxidation electrocatalyst has remained the challenge for the commercialization of the direct DME fuel cell. The commonly studied binary PtRu catalyst shows much lower activity in DME than methanol oxidation. In this work, guided by density functional theory (DFT) calculation, a ternary carbon-supported PtRuPd catalyst was designed and synthesized for DME electrooxidation. DFT calculations indicated that Pd in the ternary PtRuPd catalyst is capable of significantly decreasing the activation energy of the CO and CH bond scission during the oxidation process. As evidenced by both electrochemical measurements in an aqueous electrolyte and polymer-electrolyte fuel cell testing, the ternary catalyst shows much higher activity (two-fold enhancement at 0.5 V in fuel cells) than the state-of-the-art binary Pt50 Ru50 /C catalyst (HiSPEC 12100).

Activation of Platinum-Based Centers through Modification with Metal Oxo Species toward Electrocatalytic Oxidation of Dimethyl Ether and Methanol

We have considered nanonstructured metal oxides as promoters for platinum, palladium or bimetallic platinum-ruthenium nanoparticles to fabricate electrocatalytic systems capable of efficient oxidation of dimethyl ether, a potentially new fuel of importance to low temperature fuel cells. The research has been performed in comparison to methanol. We explore the fact that certain inorganic oxides (e.g. WO3, TiO2, ZrO2 and CeO2) and polyoxometallates of molybdenum or tungsten tend to influence supported metal centers in ways other than simple dispersion over electrode area. Among useful characteristics of metal oxides and related systems are the following: they can generate –OH groups at low potentials that induce oxidation of passivating CO adsorbates (e.g. on Pt); they can potentially break C-H bonds (e.g. by hydrogen tungsten oxide bronzes); and they can possibly weaken C-O bonds during dimethyl ether oxidation (e.g. through changes of the electronic properties of Pt or Pd as additional component). Also nanostructured gold in combination with platinum has been demonstrated to enhance the overall electrocatalytic performance during oxidation of such small organic molecules as methanol and dimethyl ether. We pursue a concept of utilization of functionalized titanium dioxide, tungsten oxide and/or zirconium oxide matrices (by admixing them with polyoxometallate-modified gold nanoparticles) for supporting and activating noble metal nanoparticles (Pt, Pt-Ru, Pd, Pt-Pd) toward electrooxidation of dimethyl ether in comaprison to methanol. Remarkable increases of the respective electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account improvement of overall conductivity (due to the presence of nanostructured gold) at the electrocatalytic interface (formed by metal oxide support), as well as and possibility of specific Pt-metal oxide or Pt-Au electronic interactions and existence of active hydroxyl groups on transition metal oxo species.in the vicinity of catalytic Pt sites. Further, the capping layers of Keggin-type phosphomolybdates, which are known to undergo fast stepwise multi-electron redox processes as well as to activate Pt-based electrocatalysts, may also contribute to the overall enhancement effect.

Activation of Platinum-Based Centers through Modification with Metal Oxo Species toward Electrocatalytic Oxidation of Dimethyl Ether and Methanol

We studied electrooxidation of dimethyl ether on Pt and PtRu nanoparticles: bare and deposited on ZrO2 film. When compared to the behavior of bare Pt and PtRu nanoparticles (deposited on glassy carbon at the same loading of 100 μg cm-2), the enhancement effect in the presence of ZrO2 was demonstrated in terms of increases of electrocatalytic (voltammetric) currents recorded at room temperature (22oC) in 0.5 mol dm-3 H2SO4 containing 1.05 mol dm-3 DME. Introduction of the metal oxide (zirconia) nanostructures at the electrocatalytic interface should produce high population of reactive –OH groups favoring mobility of protons and inducing the oxidative removal of poisoning (CO-type) intermediates, at platinum and ruthenium sites of the deposited bimetallic PtRu nanoparticles.