Search

Polyoxometalates (POMs) have emerged as potential anode materials for lithium-ion batteries (LIBs) owing to their ability to transfer multiple electrons. Although POM anode materials exhibit notable results in LIBs, their energy-storage mechanisms have not been well-investigated. Here, we utilize various in operando and ex situ techniques to verify the charge-storage mechanisms of a Keplerate-type POM Na2K23{[(MoVI)MoVI5O21(H2O)3(KSO4)]12 [(VIVO)30(H2O)20(SO4)0.5]}·ca200H2O ({Mo72V30}) anode in LIBs. The {Mo72V30} anode provides a high reversible capacity of up to ∼1300 mA h g-1 without capacity fading for up to 100 cycles. The lithium-ion storage mechanism was studied systematically through in operando synchrotron X-ray absorption near-edge structure, ex situ X-ray diffraction, ex situ extended X-ray absorption fine structure, ex situ transmission electron microscopy, in operando synchrotron transmission X-ray microscopy, and in operando Raman spectroscopy. Based on the abovementioned results, we propose that the open hollow-ball structure of the {Mo72V30} molecular cluster serves as an electron/ion sponge that can store a large number of lithium ions and electrons reversibly via multiple and reversible redox reactions (Mo6+ ↔ Mo1+ and V5+/V4+↔ V1+) with fast lithium diffusion kinetics (DLi+: 10-9-10-10 cm2 s-1). No obvious volumetric expansion of the microsized {Mo72V30} particle is observed during the lithiation/delithiation process, which leads to high cycling stability. This study provides comprehensive analytical methods for understanding the lithium-ion storage mechanism of such complicated POMs, which is important for further studies of POM electrodes in energy-storage applications.

The competition between the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) on a polycrystalline Pt (pc-Pt) electrode in weakly acidic solutions (pH ≈ 3) under the condition w...

While redox flow batteries carry a large potential for electricity storage, specifically for regenerative energies, the current technology-prone system—the all-vanadium redox flow battery—exhibits two major disadvantages: low energy and low power densities. Polyoxometalates have the potential to mitigate both effects. In this publication, the operation of a polyoxometalate redox flow battery was demonstrated for the polyoxoanions [SiW12O40]4– (SiW12) in the anolyte and [PV14O42]9– (PV14) in the catholyte. Emphasis was laid on comparing to which extent an upscale from 25 to 1400 cm2 membrane area may impede efficiency and operational parameters. Results demonstrated that the operation of the large cell for close to 3 months did not diminish operation and the stability of polyoxometalates was unaltered.

The VO2 +/VO2+ redox reaction takes place in the catholyte solution of the all-vanadium redox flow battery (VRFB), one of the few options to electrochemically store energy from intermittent renewable sources on a large scale. However, the sluggish redox kinetics of the VO2 +/VO2+ couple limit the power density of the VRFB, which increases the footprint of the power converters and increases capital costs. Therefore, catalysis of the redox reaction and a deeper understanding of its intricate reaction pathways is desirable. The kinetics of the VO2 +/VO2+ redox reaction have been investigated in 1 M sulfuric and 1 M phosphoric acid by cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy and flow battery tests. It was found that in 1 M phosphoric acid the electron transfer constant k0 is up to 67 times higher than in 1 M sulfuric acid. At higher over-potentials the determined currents match for the two electrolytes. This over-potential dependent difference in electron transfer constant is explained by variable contributions from three reaction mechanisms for the oxidation of VO2 + to VO2+, and by the presence of adsorbed intermediates for the reduction of VO2+. This study shows that the redox kinetics of the VO2 +/VO2+ can be considerably accelerated by altering the chemical environment of the vanadium ions, and that this effect can also be transferred into a flow battery.

Electrochemical energy storage is one of the few options to store the energy from intermittent renewable energy sources like wind and solar. Redox flow batteries (RFBs) are such an energy storage system, which has favorable features over other battery technologies, e.g. solid state batteries, due to their inherent safety and the independent scaling of energy and power content. However, because of their low energy-density, low power-density, and the cost of components such as redox species and membranes, commercialised RFB systems like the all-vanadium chemistry cannot make full use of the inherent advantages over other systems. In principle, there are three pathways to improve RFBs and to make them viable for large scale application: First, to employ electrolytes with higher energy density. This goal can be achieved by increasing the concentration of redox species, employing redox species that store more than one electron or by increasing the cell voltage. Second, to enhance the power output of the battery cells by using high kinetic redox species, increasing the cell voltage, implementing novel cell designs or membranes with lower resistance. The first two means reduce the electrode surface area needed to supply a certain power output, thereby bringing down costs for expensive components such as membranes. Third, to reduce the costs of single or multiple components such as redox species or membranes. To achieve these objectives it is necessary to develop new battery chemistries and cell configurations. In this review, a comparison of promising cell chemistries is focused on, be they all-liquid, slurries or hybrids combining liquid, gas and solid phases. The aim is to elucidate which redox-system is most favorable in terms of energy-density, power-density and capital cost. Besides, the choice of solvent and the selection of an inorganic or organic redox couples with the entailing consequences are discussed.

Electrochemical storage of energy is a necessary asset for the integration of intermittent renewable energy sources such as wind and solar power into a complete energy scenario. Redox flow batteries (RFBs) are the only type of battery in which the energy content and the power output can be scaled independently, offering flexibility for applications such as load levelling. However, the prevailing technology, the all Vanadium system, comprises low energy and low power densities. In this study we investigate two polyoxometalates (POMs), [SiW12O40]4− and [PV14O42]9−, as nano-sized electron shuttles. We show that these POMs exhibit fast redox kinetics (electron transfer constant k0 ≈ 10−2 cm s−1 for [SiW12O40]4−), thereby enabling high power densities; in addition, they feature multi-electron transfer, realizing a high capacity per molecule; they do not cross cation exchange membranes, eliminating self-discharge through the separator; and they are chemically and electrochemically stable as shown by in situ NMR. In flow battery studies the theoretical capacity (10.7 A h L−1) could be achieved under operating conditions. The cell was cycled for 14 days with current densities in the range of 30 to 60 mA cm−2 (155 cycles). The Coulombic efficiency was 94% during cycling. Very small losses occurred due to residual oxygen in the system. The voltage efficiency (∼65% at 30 mA cm−2) was mainly affected by ohmic rather than kinetic losses. Pathways for further improvement are discussed.

Replacing a part of energy derived from fossil fuels with bioenergy derived from solid waste streams may be a promising method to tackle the dual crisis of increasing waste pile-up and global climate change. In this study we propose a decentralised sewage sludge and woody biomass co-gasification system for Singapore. We evaluate the greenhouse gas emission of the proposed system and compare it to the existing system through life cycle assessment. The proposed system is expected to provide a net annual emission reduction of 137.0–164.1 kilotonnes of CO 2 eq. Increase in electricity recovery, carbon sequestration in the biochar produced and the avoidance of the use of supplementary fuel for sewage sludge incineration are the major contributors for the emission reduction. The proposed system is able to increase the net electricity production from sewage sludge and woody biomass by 3–24%. This could lead to an annual increase in electricity recovery of 12.1–74.8 GWh. It is estimated that the proposed system can produce 34 kilotonnes of biochar annually. It is found that decentralisation helps to reduce the annual tonne-km driven by 4.23 million tonne-km which could decrease the number of on-road vehicles required for waste handling.

Power generation from biomass residues is an attractive option for supplying the rapidly increasing power demand of the Association of South East Asian Nations (ASEAN) in a sustainable and a cost-effective manner. In this paper, we assess the total quantity and location of biomass residues from agriculture, livestock and forestry activities in ASEAN, evaluate their technical power generation potential and estimate the cost of electricity production from these residues. A cost optimization model is developed to identify cost-effective options of power generation from biomass residues using various conversion technologies. We estimate the total available thermal energy from biomass residues in ASEAN to be approximately 1076 TWh. About 86% of the total energy potential is provided by agricultural residues, with rice, sugarcane and palm oil residues being the major contributors. We find the highest energy potentials to be located in Indonesia (407 TWh), Thailand (194 TWh) and Vietnam (153 TWh). The estimated maximum technical potential for electricity generation from biomass residues in ASEAN amounts to 360 TWh. Power generation costs vary within a wide range from less than 40 USD/MWh to more than 200 USD/MWh.

Porous carbon materials are of tremendous importance for electrochemical energy storage. Their low cost, wide potential window and high surface area make them ideal electrodes for many applications. The activity of the electrode towards a certain reaction is given by both the available wetted surface area and the electron transfer constant k0. The present study investigates which electrochemical methods are suitable to determine k0 on porous carbon electrodes. For this purpose, we investigate the ferric/ferrous redox couple on a porous carbon nanotube electrode as model system. We show that results from cyclic voltammetry (CV) can yield an apparent catalytic effect and elucidate its origin. Chronoamperometry and electrochemical impedance spectroscopy are shown to produce consistent values for the exchange current density I0, which can then be normalized to k0. Limitations of both methods in terms of k0 and diffusion constants are discussed. The gathered insights in terms of validity of methods on porous electrodes are harnessed to review the recent literature on the vanadium redox reactions. Reported k0 values spread over four orders of magnitude and there is no consensus on the influence of heat- or acid-treatment on the kinetics. Taking into account the difficulties of CVs on porous electrodes we conclude that reasonable values for the vanadium reactions are k 0 1.2 10 − 4 c m s − 1 and that oxidation of the samples increases surface area, catalyzes the V2+/V3+ redox reaction but impedes the VO2+/VO2+ redox reaction.

The reversible intercalation of solvated Na-ions into graphite and the concomitant formation of ternary Na–graphite intercalation compounds (GICs) are studied using several in operando techniques, such as X-ray-diffraction (XRD), electrochemical scanning tunnelling microscopy (EC-STM) and electrochemical quartz crystal microbalance techniques (EQCM). Linear ethylene glycol dimethyl ether homologues (“glymes”) Gx with x + 1 O-atoms were used as solvents, where x is 1–4. The intercalation mechanism of Na+(Gx)y-complexes was investigated with a focus on the phase transitions and diffusion rates of the Na+(Gx)y-complexes inside the graphite lattice. For the four shortest glymes (G1 to G4), it is found using XRD that an intermediate stage 2 Na–GIC (NaC48) is formed upon the partial sodiation of the graphite electrode. At full sodiation stage 1 Na–GIC (NaC18, 112 mA h g−1) is obtained for G1, G2 and G4, while the G3 system also forms a stage 1 Na–GIC but with less Na incorporated (NaC30, 70 mA h g−1). The phase transitions of a battery electrode upon ion-intercalation are visualised using STM on the atomic scale for the first time. In addition, the local diffusion rates of the intercalated species inside the electrode were determined, a unique approach for determining kinetic effects in batteries on the atomic scale. The formation of a solid electrolyte interphase (SEI) is observed with STM as well as with an EQCM, while the latter technique is used for novel in situ hydrodynamic spectroscopy, giving further insight into the intercalation mechanism.

We demonstrate that (NH

The influence of a high defect density on an Au(111) surface on electrocatalytic properties of electrochemically deposited Pt nanoislands was evaluated. Therefore, the electrocatalytic activities of Pt nanoislands on a defect-poor and on a defect-rich Au(111) surface were compared for the hydrogen reactions (HER and HOR). These investigations were expected to yield higher activities of defect-rich surfaces since the spillover effect should be promoted. In fact, our expectations were met: The electrocatalytic activity of Pt nanoislands on defect-rich Au(111) was found to be more than twice as high as on defect-poor Au(111). This was thought to originate from a higher reactivity of the defect-rich Au surface, which plays a fundamental role for the spillover process. Another factor, which has to be taken into account, is the influence of differences in the hydrogen adsorption energies of Pt nanoislands, which are supported on Au(111) terraces and on Au(111)-defect sites. Hence, different electrocatalytic activities can also be due to different hydrogen adsorption energies.

Several alternative fuel–vehicle combinations are being considered for replacement of the internal combustion engine (ICE) vehicles to reduce greenhouse gas (GHG) emissions and the dependence on fossil fuels. The International Energy Agency has proposed the inclusion of low carbon alternatives such as electricity, hydrogen and biofuels in the transport sector for reducing the GHG emissions and providing a sustainable future. This paper compares the use of these alternative fuels, viz. electricity, hydrogen and bio-ethanol in combination with battery electric vehicle (BEV) and fuel cell electric vehicle (FCEV) technologies on the basis of their overall efficiency and GHG emissions involved in the conversion of the primary energy source to the actual energy required at wheels through a well-to-wheel analysis. The source of energy for electricity production plays a major role in determining the overall efficiency and the GHG emissions of a BEV. Hence electricity production mix of Germany (60% fossil fuel energy), France (76% nuclear energy), Sweden and Austria (60 and 76% renewable energy, respectively), the European Union mix (48% fossil fuel energy) and the United States of America (68% fossil fuel energy) are considered for the BEV analysis. In addition to the standard hydrogen based FCEVs, CNG and bio-ethanol based FCEVs are analysed. The influence of a direct ethanol fuel cell (DEFC) on GHG emissions and overall chain efficiency is discussed. In addition to the standard sources of bio-ethanol (like sugarcane, corn, etc.), sources like wood waste and wheat straw are included in the analysis. The results of this study suggest that a BEV powered by an electricity production mix dominated by renewable energy and bio-ethanol based DEFC electric vehicles offer the best solution in terms of GHG emissions, efficiency and fossil fuel dependency. Bio-ethanol as a fuel has the additional advantage to be implemented readily in ICE vehicles followed by advancements through reformer based FCEVs and DEFC electric vehicles. Although important, this analysis does not include the health effects of the alternative vehicles. Bio-ethanol used in an ICE may lead to increased emission of acetaldehydes which however might not be the case if it is used in fuel cells.

A novel nanohybrid material that combines single-walled carbon nanotubes (SWCNTs) with a polyoxometalate (TBA)5[PVV2MoVI10O40] (TBA-PV2Mo10, TBA: [(CH3(CH2)3)4N]+, tetra-n-butyl ammonium) is investigated for the first time as an electrode material for supercapacitors (SCs) in this study. The SWCNT-TBA-PV2Mo10 material has been prepared by a simple solution method which electrostatically attaches anionic [PV2Mo10O40]5− anions with organic TBA cations on the SWCNTs. The electrochemical performance of SWCNT-TBA-PV2Mo10 electrodes is studied in an acidic aqueous electrolyte (1 M H2SO4) by galvanostatic charge/discharge and cyclic voltammetry. In this SWCNT-TBA-PV2Mo10 nanohybrid material, TBA-PV2Mo10 provides redox activity while benefiting from the high electrical conductivity and high double-layer capacitance of the SWCNTs that improve both energy and power density. An assembled SWCNT-TBA-PV2Mo10 symmetric SC exhibits a 39% higher specific capacitance as compared to a symmetric SC employing only SWCNTs as electrode materials. Furthermore, the SWCNT-TBA-PV2Mo10 SC exhibits excellent cycling stability, retaining 95% of its specific capacitance after 6500 cycles.

Electrochemical properties of two tri-manganese substituted Keggin-based tungstosilicates [MnII3(OH)3(H2O)3(A-α-SiW9O34)]7− (MnII3SiW9) and[MnIII3(OH)3(H2O)3(A-α-SiW9O34)]4− (MnIII3SiW9) were investigated. The two polyanions are isostructural, the only difference being the oxidation state of the Mn-ions. Despite their structural similarity the electrochemical behaviour is not identical. While it is well established that polyoxometalate (POM) electrochemistry is influenced by interplay between the pH of the electrolyte, the present ions and the pKa values of the complex, this is the first report that the initial oxidation state of the POM has a major influence on the electrochemistry of the transition metal within the molecule. In order to understand the influence of the initial oxidation state extensive electrochemical investigations were performed and the potential dependent adsorption behavior of the molecules on graphite was observed with atomic force microscopy. The reaction mechanism of the two POMs was determined and it was asserted that the divergent redox behavior is caused by a ligand exchange which takes place during the measurement. This influences the adsorption of the molecules on graphite which can be explained by the Born solvation model. Performing controlled potential electrolysis, a stable tri-manganese substituted Keggin ion containing MnIV3 was obtained electrochemically.

The combination of surface science and electrochemistry is an effective method to approach a fundamental understanding of electrocatalytic systems, especially of the catalyst/support assemblies. Extrinsic chemical defects in the support can affect the performances and this topic is much investigated in recent electrocatalyst research. In this work, nitrogen functional groups are introduced into the outermost layers of highly oriented pyrolytic graphite (HOPG) by ion implantation with a beam energy of 100 eV. Palladium nanoparticles (Pd NPs) are then electrochemically deposited onto both pure and nitrogen doped HOPG (N-HOPG). Pd2+ species located at the interface between the NPs and the nitrogen-rich surface were observed in the latter case. The supported Pd NPs on N-HOPG show the same electrocatalytic activity for oxygen reduction reaction (ORR) as compared with those supported on pure HOPG. However, the stability of Pd NPs on N-HOPG towards potential cycling decreases strongly due to the existence of Pd2+ at the interface, which can accelerate the dissolution of Pd atoms. This result is contradictory to results on supported Pt NPs from the literature where the merit of the N-doping was outlined.

This paper demonstrates a novel polymer electrolyte membrane (PEM) based methanol electrolyzer stack with a catalyzed membrane for hydrogen production. The physical and electrochemical properties of the catalyzed membrane, single cell and stack are examined using various characterization techniques, such as X-ray diffraction, scanning electron microscopy with EDX and polarization studies. The results demonstrate that the with Pt-catalyzed membrane electrode assembly (MEA) exhibits significantly better performance than a normal MEA. The developed electrolyzer stack produces 102 L h−1 of 99% pure hydrogen without CO and CO2. The excellent stability of the PEM methanol electrolyzer system is demonstrated by running the stack for 2500 h of intermittent operation with constant current density.

Multi-walled carbon nanotubes (MWCNTs) are suitable electrode materials for the all-vanadium redox flow battery. In addition to their high specific surface area, catalytic properties for the VO 2 + / VO 2 + redox reaction have been reported in literature. Electrochemical impedance spectroscopy was employed to study the VO 2 + / VO 2 + - and the Fe2+/Fe3+-reaction on MWCNTs with varying amounts of surface functional groups. Our analysis method is based on taking the large electrochemical interface area of the MWCNTs into account to obtain a truly comparable value for the exchange current density. When evaluating the results for Fe2+/Fe3+ it was found that the exchange current density on MWCNTs decorated with a large amount of functional groups is more than 10 times larger than for thermally defunctionalized MWCNTs. For the VO 2 + / VO 2 + reaction, however, a decrease in activity for an increase in amount of functional groups was observed. A possible reaction mechanism and the influence of defects on MWCNTs are discussed. This work distinguishes itself from previous publications, by showing the absence of a catalytic effect of functional groups for the VO 2 + / VO 2 + reaction. Therefore, a new discourse in understanding the catalytic effect of MWCNTs and specifically of the surface functional groups of carbon materials in electrochemical reactions is necessary.

Direct carbon fuel cells (DCFCs) are very promising systems to provide electricity at medium term due to the more efficient utilization of carbon at high temperatures. In this work, the focus is on a DCFC operating with a SOFC system. Button cells with an anode layer for carbon conversion, based on gadolinia-doped ceria (GDC) and CuO, are used in the experimental setup. Different contents of ceria have been incorporated in the electrode composition to test its catalytic activity for the direct carbon conversion. Vulcan® XC72 is used in the fuel cell as carbon fuel, and results in the temperature range of 750–850 °C are presented. Performing measurements of YSZ-based cells with or without anode layers as well as a systematic analysis of the anodic off-gas composition, it can be demonstrated that ceria catalyzes both reactions: the direct carbon conversion and in particular the partial oxidation of carbon. Moreover, it has been found that the DCFC system is dominated by the complete oxidation of carbon.

Polyoxometalates (POMs) have been reported as promising electrode materials for energy storage applications due to their ability to undergo fast redox reactions with multiple transferred electrons per polyanion. Here we employ a polyoxovanadate salt, Na

One of the promising candidates as noble-metal-free electrode catalysts for polymer electrolyte fuel cells (PEFCs) is a carbon material with nitrogen atoms coordinating iron ions embedded on the surface (Fe N x moiety) as the active site, although the activity is insufficient compared to conventional platinum-based electrocatalysts. In order to obtain fundamental information on the activity enhancement, a simple model of the Fe N x active site was formed by adsorbing 5,10,15,20-Tetrakis(4-pyridyl)-21 H ,23 H -porphine iron(III) chloride (FeTPyPCl) on the basal plane of highly oriented pyrolytic graphite (HOPG), and cathodic oxygen reduction was investigated on the surface in 0.1 M HClO 4 . The catalytic activity for oxygen reduction was enhanced by loading transition metal ions (Co 2+ , Ni 2+ , Cu 2+ ) together with FeTPyPCl. The X-ray photoelectron spectrum of the surface suggested that the metal was coordinated by the pyridine-N. The enhancement effect of the transition metals was supported by two different measurements: oxygen reduction at HOPG in 0.1 M HClO 4 dissolving FeTPyPCl and the metal ions; oxygen reduction in 0.1 M HClO 4 at the subsequently well-rinsed and dried HOPG. The ultraviolet–visible spectrum for the solution also suggested the coordination between the pyridyl-N and the metal ions. The oxygen reduction enhancement was attributed to the electronic interaction between the additional transition metal and the Fe center of the porphyrin through the coordination bonds. These results implied that the improvement of the activity of the noble-metal-free catalyst would be possible by the proper introduction of the transition metal ions around the active site.

Gold nanoparticles were prepared by electrochemical deposition on highly oriented pyrolytic graphite (HOPG) and boron-doped, epitaxial 100-oriented diamond layers. Using a potentiostatic double pulse technique, the average particle size was varied in the range from 5 nm to 30 nm in the case of HOPG as a support and between < 1 nm and 15 nm on diamond surfaces, while keeping the particle density constant. The distribution of particle sizes was very narrow, with standard deviations of around 20% on HOPG and around 30% on diamond. The electrocatalytic activity towards hydrogen evolution and oxygen reduction of these carbon supported gold nanoparticles in dependence of the particle sizes was investigated using cyclic voltammetry. For oxygen reduction the current density normalized to the gold surface (specific current density) increased for decreasing particle size. In contrast, the specific current density of hydrogen evolution showed no dependence on particle size. For both reactions, no effect of the different carbon supports on electrocatalytic activity was observed.

Titanium dioxide (TiO2) and titanate materials such as nanotubes, nanobelts and nanorods were synthesized and their co-catalytic effects were studied by mixing them with Pt/C catalysts. After mixing with these nanomaterials, the methanol oxidation current of Pt/C was increased significantly, but ethanol oxidation current of Pt/C was not increased in acidic medium. Combining with material characterization and electrochemical measurements, the result showed that the co-catalysts with suitable porous channels and higher proton conductivity could accelerate the methanol oxidation, and the co-catalysts containing more adsorbed and structural water could better increase the oxidation of intermediates. TiO2(B) nanobelts and titanate nanotubes are newly developed nanomaterials and protons could transfer along their long-axis. They have showed good co-catalytic effect and would have more applications in direct alcohol fuel cells (DAFC).

Fuel cells offer a promising way to produce electricity efficiently. In this work, a direct carbon fuel cell (DCFC) based on a solid oxide fuel cell (SOFC) has been investigated, in which solid carbon has been used as fuel in form of a pellet. The DCFC is an interesting technology because it offers the possibility to use, as fuel source, available and abundant raw materials with only minor pretreatment. Moreover, the thermodynamic efficiency slightly exceeds 100% in a wide temperature range due to the positive near-zero value of reaction entropy change. As pure carbon dioxide is produced at the anode, it can be easily captured and sequestered. Direct carbon conversion is competed by the Boudouard reaction, which produces carbon monoxide at high operating temperatures. This reaction is endothermic and leads to a fuel loss. The present paper relates to the contribution of both reactions by a long-term run over about 12 h with a non-porous anode layer.

The influence of thermal treatment under different environments of PtRuMo/C catalyst has been investigated for CO and methanol electrooxidation in a half cell and in a DMFC single cell. The PtRuMo/C catalysts were synthesized following two step procedure while the thermal treatments consisted of heating at 300 °C in H2 or He atmosphere for 1 h. Structural characteristics of the electrocatalysts have been studied employing a wide range of instrumental methods, including physicochemical techniques like X-ray diffraction, TEM, TPR, XPS, and electrochemical techniques like single cell studies and Fourier Transform Infrared Spectroscopy adapted to the electrochemical system for in situ studies. These electrocatalysts exhibited good dispersion and small particle size, which increased upon increasing thermal treatment. Moreover, thermal treatment, mainly under H2 is responsible for the decrease of the lattice parameter and the increase of the spill over effect to Mo sites. These effects were also accompanied by increasing the proportion of the more reduced Ru species in this catalyst. The electrochemical characterization revealed that although all ternary catalysts were more active towards CO and methanol oxidation than the binary catalyst, the catalyst treated with H2 improves its performance by ca. 15% higher with respect to the ternary catalysts treated either in He treatment or with no treatment. The enhancement in activity is associated with a change in the reaction path, which promotes the direct oxidation of CHO species to CO2 without the production of the CO poisoning species. The synergistic effect of the three metals seems to be improved and the Mo–Pt and Mo–Ru interaction strengthened.

In this paper, titanate nanotubes were synthesized and their co-catalytic effects in direct alcohol fuel cells (DAFC) were studied by physically mixing them with commercial Pt/C catalyst. BET results showed the surface area increased significantly when a big amount of nanotubes were formed. Infrared (IR) spectra and thermal gravity analysis (TGA) showed that less water existed in nanoparticles and nanotubes when the raw materials were calcinated at higher temperature. Electrochemical measurements showed that titanate nanotubes which contained more structural water had a better catalytic performance. Proton conductivity of titanate nanotubes was also considered to be an important factor for enhancing oxidation reactivity and was studied by impedance tests.

A model is presented for studying the electrocatalytic activity at an array of catalyst nanoparticles that is deposited on a catalytically inactive support material. The objective is to rationalize effects on apparent reactivity of sizes and packing densities of nanoparticles. In the current version, the focus of the model is on the contribution of the spillover effect of adsorbed hydrogen to the overall rate of the hydrogen evolution reaction. For nanoparticles of fixed size, the current density per catalyst surface area exhibits a peculiar maximum as a function of the surface particle density; the optimum particle density varies with the rate of spillover of adsorbed hydrogen from particle to support and with adsorbate surface diffusion. For fixed particle density, the current density per catalyst surface area increases strongly with decreasing particle size. The model was used to fit experimental current densities for different catalyst particle coverage at varying electrode potentials. A good agreement between experimental data and calculated results was found. The fits provide key parameters of surface processes on the catalyst/support system.

A present challenge in electrocatalysis is to identify highly active catalysts. Therefore it is of fundamental importance to achieve a better understanding of the activity of catalyst materials in dependence of morphology and supporting material. Here, we used platinum nanoparticles supported on highly oriented pyrolytic graphite (HOPG) to investigate catalytic properties for hydrogen related reactions. The activity of Pt nanoparticles supported on carbon for application in fuel cells and platinum on glassy carbon has been investigated extensively for hydrogen related reactions in the past. However, there are only few experimental investigations on the activity of Pt nanoparticles supported on a HOPG basal plane, a well-defined support regarding geometric and electronic structure. Platinum was deposited electrochemically from solution containing K2PtCl6 by means of potentiostatic single pulse method to produce nanostructured HOPG surfaces with different particle sizes (in the range between

This paper reports the preparation and characterization of the solid Solution (NH(4))(2)Si(1-x)Ti(x)P(4)O(13) (0

In order to develop catalysts it is important to characterize the system in terms of particle size and size distribution. TEM or STEM are used as the state-of-the-art methods. In this study monodispersed Pt3Co alloy nanoparticles supported on carbon black (CB) prepared by the nanocapsule method are investigated and the results are compared with a commercially available Pt3Co/C catalyst system. To avoid the obstacles of conventional bright-field TEM or STEM regarding the image analysis, an advanced computerized image processing procedure is introduced. The so-called local adaptive threshold (LAT) is a method for a precise determination of particle diameter and size distribution in the subnanometer scale. The results demonstrate the advantage of a completely computerized particle evaluation as compared to a conventional particle analysis.

In this work, we prepared NH4PO3/MO2 (M Si, Ti) composite materials with various amounts of TiO2 using a sol–gel method, which are potential for application as electrolytes for intermediate temperature fuel cells (150–250 °C). The effect of the amount of TiO2 on the conductivities of the composite was investigated systematically by an impedance spectroscopy within the temperature range of 50–275 °C under different atmospheres. The composite SiTiO10APP (10 mol% TiO2) showed high proton conductivity, 0.001–0.043 S cm−1 at 150–250 °C. The maximum conductivities are 0.043 S cm−1 at 225 °C under humid H2 and 0.0085 S cm−1 at 175 °C under humid air, respectively. A thermogravimetric analysis showed the composite had high thermal stability below 300 °C. Moreover, it was found that the microstructure has significant effect on the conductivity of the composites at higher temperature.

The fuel cell differential electrochemical mass spectrometry (FC-DEMS) measurements were performed for studying the ethanol oxidation reaction (EOR), using alkaline membrane electrode assemblies (MEAs) made up of nanoparticle Pt catalyst and alkaline polymeric membranes. The obtained results indicate that in an alkaline medium, ethanol undergoes significantly more complete electro-oxidation to CO2 than in an acidic MEA using the same Pt anode. The CO2 current efficiency (CCE) can be compared for acidic and alkaline MEA with similar electrochemical active area on the anode side. The CCE estimated, in case of alkaline MEA with Pt anode, is around 55% at 0.8 V/RHE, 60 °C and 0.1 M ethanol. In comparison, under similar conditions, acidic MEAs using the same anode catalyst show only 2% CCE. This might indicate that the C–C bond scission rates are much higher in alkaline media. However, the mechanism of ethanol oxidation in alkaline media is not exactly known. CO2 produced in electrochemical reaction forms soluble carbonates in the presence of aqueous alkaline electrolyte. This makes it difficult to study ethanol oxidation in alkaline media using FTIR or model DEMS systems. The alkaline polymer electrolyte membranes as used in this study for making alkaline MEAs provide an important opportunity to observe CO2 produced during EOR using FC-DEMS system.

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.

Electrochemical reactivity regarding hydrogen reduction was studied at epitaxially grown Pd monolayers and sub-monolayers on Au(1 1 1) in 0.1 M HClO 4 solution. The rate of hydrogen evolution increases with decreasing numbers of layers, and it is considerably higher for sub-monolayers, i.e. the fewer Pd islands are on the surface the higher is the catalytic activity. No clear dependence of the reactivity on the ratio of Pd edge atoms to terrace atoms was found. Possible mechanisms explaining the experimental results are discussed.

Experimental results of tests using a micro-structured steam reformer for hydrogen generation from methane in fuel cell systems are reported. A distinctive feature of this work is the integration of a catalytic combustor into the reactor to deliver the necessary heat to the endothermic steam reforming reaction. The tests are performed under static and dynamic load conditions. The reactor is operated autonomously without external heat supply. The best results are obtained by firing the integrated combustor with a mixture of hydrogen and methane in co-flow with the reforming reaction. Providing carbon dioxide to the combustor feed in order to simulate the situation of anode off-gas firing has only minor effects on system performance. Under static load conditions up to 2 / min–1 of hydrogen are produced at high methane conversion rates of about 70%. Start-up tests show that a stable product composition is obtained in less than 20 s after admission of methane to the reformer. Furthermore, the product composition shows only minor variations with throughput and load changes if heating adjustments are done simultaneously. Overall, the reactor proves to be very suitable for operation in fuel cell systems.

Different aspects of the improvement of direct methanol fuel cell (DMFC) systems for portable power generation are investigated, in a project funded by the Bavarian state. The materials research focuses on the development of improved catalysts, in particular for the oxygen reduction reaction. Some recent results on supported ruthenium selenium catalysts are reported. In parallel, tests on other fuel cell materials are performed using MEAs made from industrial unsupported catalysts as the reference. These standard MEAs have catalyst loadings of about 11 mg cm–2 and, at high air flux, can deliver current densities of about 500 mA cm–2 and 100 mA cm–2 at 110 °C and 50 °C, respectively. At low air flux and 50 °C, current densities between 60 and 80 mA cm–2 are possible @ 500 mV. Using these MEAs, different commercial gas diffusion materials are tested as the cathode backing. Thus, it is found that the Sigracet materials by SGL Carbon are the most suitable for operation at a low air flux. Finally, a demonstration stack, comprised of up to ten cells, is developed using graphite PVDF compound bipolar plates by SGL Carbon. As will be reported, this stack shows a high homogeneity of cell voltages and stable operation under relevant conditions, using standard MEAs.

International audience; This paper provides further insights into the particle size effects in CO monolayer oxidation. Strong particle size effects are confirmed in the size range from 1.8 to 5 nm. The discrepancies in the literature concerned with the particle size effects in CO monolayer oxidation are reconciled by exploring the influence of the experimental conditions on the stripping voltammograms and chronoamperograms. Evidence supporting the contribution of slow non-electrochemical step to the overall mechanism of CO oxidation is presented. The particle size effects in CO monolayer oxidation are attributed to the size-dependent COads + OHads interaction as well as to the size-dependent COads surface diffusion coefficient.

Hydrous ruthenium oxides have been investigated as novel support materials for fuel cell electrocatalysts for use in DMFC applications. These oxides were chosen in particular due to their potential intrinsic proton conductivity. Pt nanoparticles have been deposited onto the new support, and the resulting catalysts characterized both structurally and electrochemically. The Pt nanoparticles are sized between 3–4 nm and are highly dispersed on the support. Transmission electron micrographs show that the individual Pt nanoparticles are covered by an amorphous coating layer – probably hydrous ruthenium oxide, in good agreement with the XPS data. Electrochemical measurements on model electrodes indicate that proton conductivity of the supporting material is strongly affected by interdiffusion of methanol. Nevertheless, initial tests on membrane electrode assemblies (MEAs) showed improved performance, particularly with respect to internal resistance, when compared to Pt-Ru black. The catalyst showed very high activity in CO stripping experiments performed on a full MEA, suggesting high catalyst utilization, despite the comparatively low Nafion® content used in the electrode layer.

A new proton-conductive composite of NH4PO3–(NH4)2Mn(PO3)4 was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH4)2Mn(PO3)4 was stable as a supporting matrix for NH4PO3. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH4PO3 in both dry and wet atmospheres. A conductivity of 7 mS cm−1 was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 102–105 Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH4PO3–(NH4)2Mn(PO3)4 electrolyte. Fuel cells using 2NH4PO3–(NH4)2Mn(PO3)4 as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm2 was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH4PO3–(NH4)2Mn(PO3)4 electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.

The electrode performances of the alternative oxides: La0.05Ca0.95Cr0.05Ti0.95O3-δ-8YSZ and Ce0.8TM0.2O2-δ(TM=Mn, Co) for the direct electrochemical oxidation of methane are investigated to assess their potential as anode materials for efficient methane conversion in a SOFC. The electrochemical oxidation of hydrogen was also studied, for comparison. The oxides are characterised electrochemically with impedance spectroscopy in the frequency range from 10 mHz to 1MHz, using a three-electrode geometry. They are compared to a standard Ni/8YSZ anode for the electrochemical oxidation of hydrogen. It is found that La0.05Ca0.95Cr0.05Ti0.95O3-δ-8YSZ demonstrates a poor electrochemical activity in both hydrogen and methane. However, the electrochemical activity of Ce0.8Mn0.2O2-δ is promising, but the electronic conductivity needs to be increased, e.g., by adding a conducting oxide, before it can be used as an anode material in a SOFC.

The temperature dependence of the oxygen reduction kinetics on carbon-supported RuxSey catalysts is studied using a rotating disc electrode in 0.5 M H2SO4 in the temperature interval from 25 °C to 65 °C. When the absolute value of the overpotential is below ca. 0.65 V, the reaction is limited by a one-electron charge transfer step, where the transfer coefficient is independent of the temperature and equal to 0.44. The apparent activation enthalpy at zero overpotential is 0.49 eV and the pre-exponential factor is independent of the temperature.

Solid oxide fuel cells (SOFCs) have many attractive features for widespread applications. The high operating temperature provides a valuable heat source and in contrast to low temperature fuel cells they not only tolerate substances such as CO but can even use them as fuel. Thus, reforming of hydrocarbon fuels for SOFCs can be done without additional gas purification. As both stack and hydrocarbon reformer unit have to be operated at high temperatures (700–1000 °C), thermal management plays an important role in the successful operation of SOFC systems. As the SOFC system contains ceramic components, both large thermal gradients in the system and thermal expansion coefficient (TEC) mismatch must be avoided. Matching TECs is done by selecting the suitable materials. Avoiding high temperature gradients is done by selecting the right system design and control strategies. In order to achieve both, we have built a finite element simulation for a complete SOFC systems which allows to study system parameters both during steady operation and during transients. Examples of the thermal start-up behaviour for several system configurations are given for selected components as well as internal temperatures of the SOFC-stack during start-up. The simulation model includes also the option to simulate the effects of internal methane reformation in the SOFC stack. As the minimum operation temperature is high, cooling down of the system has to be avoided if instant operation is desired. This can be achieved either passively by selecting suitable thermal insulation materials and/or actively by adopting a strategy for maintaining the temperature.

The stability of gold supported palladium clusters, generated with an electrochemical scanning tunnelling microscope, has been investigated in metal-free electrolyte. The clusters are found to be more stable, if the tip is moved further towards the electrode surface during the generation process. Only a weak dependence of the potential on the dissolution is observed, but a strong increase of the dissolution current density with decreasing particle size. Dissolution of the clusters occurs from the edges rather than layer by layer.