Parametric and transient analysis of non-isothermal, planar solid oxide fuel cells

A multidimensional, model of non-isothermal planar solid oxide fuel cells (SOFCs) including detailed coupled mass and charge transport phenomena, has been developed. The dusty-gas model has been used, in this a comprehensive SOFC model, and has been explicitly written/constructed, for the first time in the COMSOL multiphysics modelling framework to describe mass transport in the porous electrode and detailed charge conservation equations have been taken into account. As we have shown in a recent publication [9] the incorporation of the dusty-gas model results in more accurate predictions of the SOFC behaviour compared to mass transport models based on Fick’s law or Stefan-Maxwell multi-component diffusion. Our model allows prediction of the species composition profiles, temperature profiles, electronic and ionic voltage and current density distributions, and polarisation curves in a single cell. SOFC dynamics have also been considered including responses to step changes in the operating conditions. The model is implemented in two-spatial dimensions, however, the underlying theory is independent of the geometry used. Extensive parametric analysis has been performed and the corresponding SOFC behaviour has been analysed through the resulting polarisation curves. It is shown that SOFCs exhibit higher power outputs at increased operating temperatures and pressures. It was also found that the electrodes’ porosity and tortuosity have a smaller effect on power output. Furthermore, step changes in the inlet temperatures were found to induce slower dynamic behaviours than step changes in the operating voltage.     

Brownian dynamics simulations of electrons and ions in mesoporous films

This paper presents a simulation model to study charge transport processes in mesoporous films for dye-sensitized solar cells. By simulating electron and ion transport by Brownian dynamics in these films, we achieve a direct relation between the grain connectivity and the effective diffusion coefficients. By comparing the macroscopic properties of a simple cubic and a diamond structured unit cell, we conclude that the latter better resembles the properties of the mesoporous oxide films in comparison with experimental results. The model has been used to optimize the size of the contact area between the interconnected particles in the mesoporous film with respect to the photocurrent.     

Photoelectrochemical studies of the junction between poly[3-(4-octylphenyl)thiophene] and a redox polymer electrolyte

The photoelectrochemical properties of all-solid-state photoelectrochemical cell constructed from a conjugated polymer poly[3-(4-octylphenyl)thiophene] and an amorphous poly(ethylene oxide) complexed with iodide/triiodide redox couple were studied. In order to develop flexible photoelectrochemical cells, we have used a transparent polymeric metal, doped poly(3,4-ethylenedioxythiophene), as a counter electrode. It was shown that poly(3,4-ethylenedioxythiophene) improved the charge transfer between indium tin-oxide and iodide/triiodide redox couple. The spectral response, photocurrent time, and open-circuit voltage and short-circuit current dependence on light intensity have been studied. The photon to electron conversion efficiency obtained was low. The photocurrent and photovoltage dependence studies on light intensity indicate exciton recombination and/or traps as limiting factors.     

Generation of molybdenum hydride species via addition of molecular hydrogen across metal-oxygen bond at monolayer oxide/metal composite interface

Generation of molybdenum hydride species on monolayer oxide/metal composite via addition of molecular hydrogen across metal-oxygen bond is investigated for the first time utilizing periodic Van der Waals density-functional calculations. Lewis acid-base pair constructed by the interfacially defected oxide film and the metal support provides novel active sites for activating H₂. The produced heterolytic dissociative state exhibits negative dissociative adsorption energy of −0.315 eV which thermodynamically facilitate the dissociation process of H₂ on insulating oxide films. The penitential energy pathways are calculated to reveal the dynamics and reaction processes for H₂ splitting at the oxide-metal interface. The differential charge density contour, electronic density plots, particular occupied orbitals, work function and electron localization function of H₂ dissociation are interpreted to better understand the electronic properties of the unique dissociation behavior of H₂ at interfacially defected magnesia. It is anticipated that the results here could help understand the mechanism of hydrogenation reactions on nanostructured oxide film and provide useful clue for enhancing the reactivity of insulating oxide toward activating H₂.     

Nanostructured Ti-doped hematite (α-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation

We present a form of hematite (α-Fe₂O₃) nanostructured architecture suitable for photoelectrochemical water oxidation that is easily synthesized by a pulsed laser deposition (PLD) method. The architecture is a column-like porous nanostructure consisting of nanoparticles 30–50 nm in size with open channels of pores between the columns. This nanostructured film is generated by controlling the kinetic energy of the ablated species during the pulsed laser deposition process. In a comparison with the nanostructured film, hematite thin film was also synthesized by PLD. All of the developed films were successfully doped with 1.0 at% of titanium. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy were used to characterize the films. To fabricate the photoelectrochemical (PEC) cell, Ti-doped hematite films were used as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and an aqueous solution of 1 M NaOH as the electrolyte. The photovoltaic characteristics of all cells were investigated under AM 1.5G sunlight illumination of 100 mW/cm². The photocurrent density was enhanced by approximately 220% using nanostructured film at 0.7 V versus Ag/AgCl compared to hematite thin film, and the highest photocurrent density of 2.1 mA/cm² at 0.7 V/Ag/AgCl was obtained from the 1.0 at% Ti-doped hematite nanostructured film. The enhanced photocurrent density is attributed to its effective charge collection due to its unique column-like architecture with a large surface area.     

Photocatalytic water oxidation at bismuth vanadate thin film electrodes grown by direct liquid injection chemical vapor deposition method

Photoelectrochemical cells (PECs) are devices that can harvest and convert solar energy to produce consumable fuel, e.g. by splitting water into oxygen and hydrogen. Photocatalytic semiconductor materials play a major role in PECs, and their overall efficiency is usually limited by short carrier diffusion length because of structural defects, poor light absorptivity, and sluggish kinetics of photoelectrochemical reactions at the semiconductor electrode. Synthesis of high quality defect-free semiconductor materials using high temperature deposition techniques generally yield films with good adhesion to substrates while improving charge carrier transport and hence the overall efficiency of a PEC. A direct liquid injection chemical vapor deposition (DLI-CVD) technique has been utilized to synthesize monoclinic clinobisvanite phase bismuth vanadate (BiVO₄) films for photocatalytic water oxidation. The technique yields dense high quality epitaxial and polycrystalline BiVO₄ films on Yttria stabilized zirconia (YSZ) and Fluorine doped tin oxide (FTO) substrates, respectively, at growth temperature in the range of 500–550 °C. The photoelectrochemical characteristics of the films grown on FTO have been studied and a photocurrent value of 2.1 mA/cm² at 1.23 V vs Normal hydrogen electrode (NHE) (0.5 V vs. Ag/AgCl), with onset potential values as low as 0.23 V vs. NHE (?0.5 V vs. Ag/AgCl), are obtained despite the low porosity of the films. The PEC performance is further improved by synthesizing BiVO₄ directly on top of a tungsten oxide interlayer and modifying its surface with FeOOH co-catalyst.     

Nanostructured hematite photoelectrochemical electrodes prepared by the low temperature thermal oxidation of iron

We report on a facile low temperature method for the preparation of high surface area, nanostructured α-Fe₂O₃ (hematite) thin films and their application as photoelectrochemical (PEC) water splitting electrodes. The hematite films are fabricated by thermal oxidation in air of DC sputter deposited iron films at temperatures as low as 255 °C. This method results in films with a higher surface area than typically obtained by directly sputtering α-Fe₂O₃. It is shown that beyond a minimum iron thickness, α-Fe₂O₃ nanowires result upon thermal treatment in atmospheric conditions. Structural and optical characteristics of the resulting films are analyzed. The oxidation process is studied in detail and correlated to the photoelectrical properties. The Fe films oxidize in stages via Fe-oxide layers of increasing oxidation states. Resulting photoelectrochemical performance of fully oxidized films is a balance between optical absorption and charge collection, which varies with film thickness. The optimum film achieved a net photocurrent density of 0.18 mA/cm² in 1 M NaOH at 1.23 V vs. RHE under simulated AM1.5 sunlight, amongst the highest values reported for undoped hematite films produced at low temperature.     

Thin inclusion approach for modelling of heterogeneous conducting materials

Experimental data show that heterogeneous nanostructure of solid oxide and polymer electrolyte fuel cells could be approximated as an infinite set of fiber-like or penny-shaped inclusions in a continuous medium. Inclusions can be arranged in a cluster mode and regular or random order. In the newly proposed theoretical model of nanostructured material, the most attention is paid to the small aspect ratio of structural elements as well as to some model problems of electrostatics. The proposed integral equation for electric potential caused by the charge distributed over the single circular or elliptic cylindrical conductor of finite length, as a single unit of a nanostructured material, has been asymptotically simplified for the small aspect ratio and solved numerically. The result demonstrates that surface density changes slightly in the middle part of the thin domain and has boundary layers localized near the edges. It is anticipated, that contribution of boundary layer solution to the surface density is significant and cannot be governed by classic equation for smooth linear charge. The role of the cross-section shape is also investigated. Proposed approach is sufficiently simple, robust and allows extension to either regular or irregular system of various inclusions. This approach can be used for the development of the system of conducting inclusions, which are commonly present in nanostructured materials used for solid oxide and polymer electrolyte fuel cell (PEMFC) materials.     

Hydrogen transport and embrittlement in structural metals

The close relationship between hydrogen transport and embrittlement is indicated by evidence of hydrogen absorption preceding degradation of mechanical properties. Concentration of hydrogen at a crack or flaw by diffusion or by transport with moving dislocations is probably necessary also. Experimental studies show that hydrogen permeation is significantly influenced by surface conditions, particularly oxide films, and internal defects and impurities that trap diffusing hydrogen. The usual thermodynamic and diffusion relations, therefore, do not predict accurately the final distribution of hydrogen and the kinetics of the processes.Investigation of the effects of hydrogen on the mechanical properties of approximately fifty structural alloys at ambient temperature and pressures up to 69 MPa indicates that all alloys show evidence of susceptibility to hydrogen embrittlement. The degree of hydrogen embrittlement appears to be related to the amount of hydrogen absorbed and its distribution within the metal lattice. Surface condition, defect structure, hydrogen purity, and hydrogen pressure influence embrittlement. Of major importance is the transport of hydrogen with moving dislocations, a mechanism for concentration and redistribution of hydrogen that is operative at temperatures lower than for diffusion.     

The kinetic behaviour of ion injection in WO3 based films produced by sputter and sol–gel deposition: Part II. Diffusion coefficients

The use of AC impedance spectroscopy for kinetic study of the ion intercalation into WO₃ films is reviewed, and methods for extracting the diffusion coefficient of the ion diffusion process from AC impedance spectroscopy data are described. These are applied to several different electrochromic thin films, all based on tungsten oxide, and the electrochromic performance is correlated with the diffusion coefficient. The results are also compared with results of a previous paper which concentrated on modelling the voltage response of films coloured and bleached using constant current charge injection techniques. Several examples of non-ideal behaviour of the impedance spectra are observed, including depressed semicircles and evidence of two semicircles. A full discussion of these effects is given in a following paper.     

Photoelectrochemical performance of anodic n-TiO2 films submitted to hydrogen reduction

The photoelectrochemical behaviour of semiconducting n-TiO₂ films prepared by anodic oxidation of titanium plates in concentrated alkaline solutions at very high current densities, and subsequently cathodically reduced or thermally reduced in a hydrogen atmosphere, was investigated. The original and reduced films were examined by scanning electron microscopy and X-ray diffraction analysis. Hydrogen reduction improved the photoelectrochemical performance of the oxide giving the best results when reducing the films at 600°C for 2 h. Preliminary water photoelectrolysis experiments showed that hydrogen could be produced at the counter electrode even without applying an external bias voltage, in agreement with the proposed energy diagram for the TiO₂ films.     

氢气非预混燃烧的流场输运特性分析

采用高精度直接数值模拟的方法对氢气非预混燃烧流场进行了精细的预测.模拟所求解的控制方程为三维可压缩的无量纲形式的Navier-Stokes方程,采用六阶精度紧致差分格式,结合基于详细化学反应和输运过程的FGM化学反应机制,利用768个处理器核、共近4.53亿网格点进行了基于CPU的大规模高效并行计算,分析氢气非预混燃烧特性,并进一步探讨了浮力对氢气燃烧流场输运特性的影响.研究发现,由于氢气燃烧过程中产生不同扩散性质的化学组分,使燃烧过程中遵循优势扩散的行为.这将影响流场的输运特性和火焰不稳定性的形成.在浮力驱动的氢气优势扩散燃烧流场中,对流是质量、动量及热量输运行为的主要影响因素,而无浮力火焰中优势扩散主导着流场的输运特性.平均统计结果表明,有浮力和无浮力的燃烧流场中都可以捕捉到逆梯度输运现象,且浮力会促进逆梯度输运行为的发生.     

The effects of turbulence on molten pool transport during melting and solidification processes in continuous conduction mode laser welding of copper–nickel dissimilar couple

The melting and solidification stages of a continuous copper–nickel dissimilar metal conduction mode laser welding have been simulated numerically in this study. The heat, mass and momentum transports in molten metal pool have been analysed using both laminar and turbulent flow models separately for the same process parameters. The phase change aspects related to solidification and melting are accounted for by a modified enthalpy–porosity technique while the turbulent transport is modelled by a high Reynolds number k–ε model. It has been observed that temperature fields obtained from both laminar and turbulent transport simulations are qualitatively similar to each other. The molecular thermal diffusivity of the molten metal mixture is found to be in the same order of magnitude as eddy thermal diffusivity, as a result of which the thermal field gets marginally affected by fluid turbulence. By contrast, eddy viscosity remains much greater than molecular viscosity, which leads to greater amount of momentum diffusion in the case of a turbulent molten metal pool, in comparison to that obtained from the corresponding laminar simulation. This is reflected in the reduction in maximum velocity magnitude in the turbulent simulation in comparison to the maximum velocity obtained from laminar simulation. In the case of species transport, the turbulent mass diffusivity is found to be about 10⁷–10⁸ times greater than molecular mass diffusivity. As a result, the species field in turbulent simulation shows characteristics of better mixing between two dissimilar molten metals than the species field obtained using the laminar transport model. The species distribution obtained from turbulent transport is shown to be in better agreement with experimental data reported in literature than the corresponding mass fraction distribution obtained from laminar simulation. It is also found that species distribution in the molten pool is principally determined by advective and diffusive transport during the melting stage and species transport by advection and eddy diffusion in turbulent pool increasingly weakens with decreasing temperature during the cooling following the laser melting stage.     

A new method for manufacturing nanostructured electrodes on glass substrates

The present paper describes a new method for manufacturing a nanostructured porous layer of TiO₂ on a conducting glass substrate for use in a dye-sensitized photoelectrochemical cell. The method involves deposition of a layer of semiconductor particles onto a conducting substrate and compression of the particle layer to form a mechanically stable, electrically conducting, and porous nanostructured film at room temperature. Photoelectrochemical characteristics and morphology of the resulting nanostructured films are presented. The potential use of the new manufacturing method in the future applications of nanostructured systems is discussed.     

Band gap optimization of tin tungstate thin films for solar water oxidation

Semiconducting ternary metal oxide thin films exhibit a promising application for solar energy conversion. However, the efficiency of the conversion is still limited by a band gap of a semiconductor, which determines an obtainable internal photovoltage for solar water splitting. In this report the tunability of the tin tungstate band gap by O₂ partial pressure control in the magnetron co-sputtering process is shown. A deficiency in the Sn concentration increases the optical band gap of tin tungstate thin films. The optimum band gap of 1.7 eV for tin tungstate films is achieved for a Sn to W ratio at unity, which establishes the highest photoelectrochemical activity. In particular, a maximum photocurrent density of 0.375 mA cm⁻² at 1.23 V_RHE and the lowest reported onset potential of −0.24 V_RHE for SnWO₄ thin films without any co-catalyst are achieved. Finally, we demonstrate that a Ni protection layer on the SnWO₄ thin film enhances the photoelectrochemical stability, which is of paramount importance for application.     

Charge transfer in iron oxide photoanode modified with carbon nanotubes for photoelectrochemical water oxidation: An electrochemical impedance study

Iron oxide photoanode was modified with multi-wall carbon nanotubes (MWCNTs) to improve the charge transport property of iron oxide in the photoelectrochemical water oxidation under solar light. The MWCNT-modified Fe₂O₃ electrode exhibited markedly increased photocurrent generation (by 66%) relative to unmodified Fe₂O₃ electrode. Electrochemical impedance spectroscopy demonstrated that MWCNT modification dramatically decreased resistance over the entire electrode and increased capacitance at the interface between carbon nanotubes and conducting substrate. The Mott-Schottky analysis showed that the flat band potential of the Fe₂O₃ electrode shifted to a more positive potential in the MWCNT-modified anode, indicating the charge migration from Fe₂O₃ to MWCNT. Thus the role of the MWCNT as an expressway for electron transport has been clearly demonstrated, which would help charge separation and improve photoelectrochemical water oxidation efficiency of the poorly conducting Fe₂O₃ electrode.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Electrical conductivity as a function of temperature in amorphous lithium tungsten oxide

Tungsten oxide is a widely used electrochromic material for smart windows. In order to study the charge carriers involved in the electrochromic process, it is important to characterize the electrical transport in tungsten oxide. Substoichiometric amorphous tungsten oxide films were prepared by DC-magnetron sputtering. The films were electrochemically intercalated with lithium. The Li/W intercalation ratios for the tungsten oxide films were in the range 0.15–0.53. Temperature dependent resistivity measurements were performed in the temperature range 77–300 K for samples at different lithium intercalation levels. It was found that the data are consistent with the variable range hopping model.     

Photoresponse and stability improvement of ZnO nanorod array thin film as a single layer of photoelectrode for photoelectrochemical water splitting

ZnO nanorod array thin film with Al-doping and hydrogen treatment was developed as a photoelectrode combining the functions of transparent conducting oxide thin film and photoactive 1-dimensional nanostructured semiconductor into a single layer for photoelectrochemical water splitting. It was demonstrated that hydrogen treatment and Al-doping enhanced the dark currents, photocurrents, and hydrogen generation efficiencies largely and the enhancement by hydrogen treatment was more significant. The maximum photoinduced hydrogen generation efficiency was about 0.020%. Furthermore, hydrogen treatment also improved the photosensitivity and the stability under illumination significantly. The minimum decay time constant and rise time constant were 1.71 and 1.22 s, respectively. And after current-voltage scanning upon illumination for 50 cycles, the 1-dimensional morphology still remained unchanged but those without Al-doping and/or hydrogen treatment were altered seriously. The good photoresponse and stability made the Al-doped ZnO nanorod array thin film with hydrogen treatment have wide applications in the photoelectrochemical field.     

Comparison of charge accumulation and transport in nanostructured dye-sensitized solar cells with electrolyte or CuSCN as hole conductor

The charge transport properties of the dye-sensitized solar cells consisting of Ru(dcbpyH₂)₂(NCS)₂-sensitized nanostructured TiO₂ with either redox electrolyte or CuSCN as hole conductor have been compared. The electron transport time and the electron charge in the TiO₂ varies in a similar way with the incident light intensity for both hole conductors: electron transport becomes faster and electron accumulation increases with increasing light intensity. Electron transport in the CuSCN-based cells is significantly faster than in electrolyte cells under conditions where the accumulated charge is equal. An ultra-thin aluminum oxide layer on the nanocrystalline titanium oxide has a beneficial effect as it reduces the recombination and increases the open-circuit potential.