Fabrication–microstructure–performance relationships of reversible solid oxide fuel cell electrodes–review

Abstract

A reversible solid oxide fuel cell system can act as an energy storage device by storing energy in the form of hydrogen and heat, buffering intermittent supplies of renewable electricity such as tidal and wave generation. The most widely used electrodes for the cell are lanthanum strontium manganate–yttria stabilised zirconia and Ni–yttria stabilised zirconia. Their microstructure depends on the fabrication techniques, and determines their performance. The concept and efficiency of reversible solid oxide fuel cells are explained, along with cell geometry and microstructure. Electrode fabrication techniques such as screen printing, dip coating and extrusion are compared according to their advantages and disadvantages, and fuel cell system commercialisation is discussed. Modern techniques used to evaluate microstructure such as three-dimensional computer reconstruction from dual beam focused ion beam–scanning electron microscopy or X-ray computed tomography, and computer modelling are compared. Reversible cell electrode performance is measured using alternating current impedance on symmetrical and three electrode cells, and current/voltage curves on whole cells. Fuel cells and electrolysis cells have been studied extensively, but more work needs to be done to achieve a high performance, durable reversible cell and commercialise a system.     

Design and testing of a 9.5 kWe proton exchange membrane fuel cell–supercapacitor passive hybrid system

The design and test of a 9.5 kWe proton exchange membrane fuel cell passively coupled with a 33 × 1500 F supercapacitor pack is presented. Experimental results showed that the system reduced dynamic loads on the fuel cell without the need for additional DC/DC converters. Fuel efficiency gains of approximately 5% were achieved by passive hybridisation in addition to addressing two main operational degradation mechanisms: no-load idling and rapid load cycling.     

Physical and sealing properties of BaO–Al2O3–SiO2–CaO–V2O5 glasses for solid oxide fuel cell applications

In this study, the properties of BaO–Al₂O₃–SiO₂ (SAB) glasses incorporated with CaO and V₂O₅ as the network modifier and additive, respectively, are evaluated. The electrical resistivities of the glasses decrease upon the addition of CaO but increase upon increasing their V₂O₅ content because the V⁵⁺ species lower the ionic mobility of the glasses. The addition of V₂O₅ improves the wettability of the glasses on the Crofer 22 APU substrate, and thus, increases the fracture strength at the glass–Crofer 22 APU couple. Among the glasses evaluated, the SAB glass with a CaO content of 20 wt% and V₂O₅ content of 2 wt% (SAB-Ca20V2) present excellent sealing properties because it adheres well to both the Zr_0·92Y_0·08O_2-δ (YSZ) and Crofer 22 APU substrates; no pores, cracks, or interfacial phases are present at the interfaces, confirming the good chemical and thermal compatibility of the glass–substrate pairs at high temperatures. After SAB-Ca20V2 is sealed on the Crofer 22 APU substrate at 850 °C, the leakage rate of the glass is low (<0.015 sccm?cm^?1 at 800 °C for 200 h), indicating negligible deterioration of its sealing efficiency and revealing its remarkable potential for use in solid oxide fuel cell applications.     

Simulation of dynamics and control of a double-effect LiBr–H2O absorption chiller

A dynamic model has been developed to simulate dynamic operation of a real double-effect absorption chiller. Dynamic behavior of working fluids in main components was modeled in first-order nonlinear differential equations based on heat and mass balances. Mass transport mechanisms among the main components were modeled by valve throttling, ‘U’ tube overflow and solution sub-cooling. The nonlinear dynamic equations coupled with the subroutines to calculate thermodynamic properties of working fluids were solved by a numerical method. The dynamic performance of the model was compared with the test data of a commercial medium chiller. The model showed a good agreement with the test data except for the first 83 min during which different flow rates of the weak solution caused some discrepancy. It was found that the chiller dynamics is governed by the inlet temperatures of the cooling water and the chilled water when the heat input to the chiller is relatively constant. For a step change of load at constant inlet temperatures of the cooling water and the chilled water, the response time of the chilled water exit temperature was about 15 min and it was due to the thermal capacities of the chiller. The dilution cycle was found to be an essential means for improvement of control performance as well as anti-crystallization.     

Synthesis and properties of a barium aluminosilicate solid oxide fuel cell glass–ceramic sealant

A series of barium aluminosilicate glasses modified with CaO and B₂O₃ were prepared and evaluated with respect to their suitability in sealing planar solid oxide fuel cells (SOFCs). At a target operating temperature of 750 °C, the long-term coefficient of thermal expansion (CTE) of one particular composition (35 mol% BaO, 15 mol% CaO, 10 mol% B₂O₃, 5 mol% Al₂O₃, and bal. SiO₂) was found to be particularly stable, due to devitrification to a mixture of glass and ceramic phases. This sealant composition exhibits minimal chemical interaction with the yttria-stabilized zirconia electrolyte, yet forms a strong bond with this material. Interactions with metal components were found to be more extensive and depended on the composition of the metal oxide scale that formed during sealing. Generally alumina-scale formers exhibited a more compact reaction zone with the glass than chromia-scale forming alloys. Mechanical measurements conducted on the bulk glass–ceramic and on seals formed using these materials indicate that the sealant is anticipated to display adequate long-term strength for most conventional stationary SOFC applications.     

Harvesting energy from real human urine in a photo-microfluidic fuel cell using TiO2–Ni anode electrode

The use of a liquid sample employed for analysis in microfluidic fuel cells has been increased because it can be used in order to obtain medical diagnostic and at the same time as fuel. This document presents the construction and evaluation of a photo-assisted microfluidic fuel cell (photo-μFC) that uses human urine as a fuel. For the construction of this photo-μFC, TiO₂ nanoparticles modified with Ni(OH)₂ were synthesized for use as a photoanode in the oxidation of the urea content in urine, finding an increase in the absorption of light in the visible spectrum with respect to TiO₂. Nanoparticles of TiO₂–Ni in a mixture of anatase (60%) and brookite (40%) phases were found with crystallite sizes of 9 and 15 nm, respectively. The photo-μFC proved with urine, showed an open-circuit potential of 0.70 V, a maximum current density of 1.7 mA cm^?2 and a maximum power density of 0.09 mW cm^?2. The photo-μFC developed was evaluated for 15 consecutive hours at room temperature to observe the lifetime and stability of the photoanode with respect to the generated current. In addition, the oxidation of the urea by the photogenerated holes (h⁺) in the TiO₂ was verified. This research shows the novelty of a promising advance in the use of a microfluidic fuel cell operated with a single-stream from human urine and using photoanodes (TiO₂–Ni) to obtain electrical power with a feasible application in low power portable medical devices.     

Cooling performance investigation of electronics cooling system using Al2O3–H2O nanofluid

In this study, the cooling performance of Al₂O₃–H₂O nanofluid was experimentally investigated as a much better developed alternative for the conventional coolant. For this purpose the nanofluid was passed through the custom-made copper minichannel heat sink which is normally attached with the electronic heat source. The thermal performance of the Al₂O₃–H₂O nanofluid was evaluated at different volume fraction of the nanoparticle as well as at different volume flow rate of the nanofluid. The volume fraction of the nanoparticle varied from 0.05 vol.% to 0.2 vol.% whereas the volume flow rate was increased from 0.50 L/min to 1.25 L/min. The experimental results showed that the nanofluid successfully has minimized the heat sink temperature compared to the conventional coolant. It was noticed also that the thermal entropy generation rate was reduced via using nanofluid instead of the normal water. Among the other functions of the nanofluid are to increase the frictional entropy generation rate and to drop the pressure which are insignificant compared to the normal coolant. Given the improved performance of the nanofluid, especially for high heat transportation capacity and low thermal entropy generation rate, it could be used as a better alternative coolant for the electronic cooling system instead of conventional pure water.     

Results of the long-term testing of the ‘Photon’ fuel cell generator

Results of the long-term testing of the Photon fuel cell generator are considered in this article. Its ability to operate with satisfactory performance for 6000–7000 h is shown. Further increase of the service life is associated with solution of the fuel cell components' corrosion problem as well as with removal of the impurities from the fuel and oxidant, which cause electrolyte carbonization.     

Modeling and optimization of a typical fuel cell–heat engine hybrid system and its parametric design criteria

A theoretical modeling approach is presented, which describes the behavior of a typical fuel cell–heat engine hybrid system in steady-state operating condition based on an existing solid oxide fuel cell model, to provide useful fundamental design characteristics as well as potential critical problems. The different sources of irreversible losses, such as the electrochemical reaction, electric resistances, finite-rate heat transfer between the fuel cell and the heat engine, and heat-leak from the fuel cell to the environment are specified and investigated. Energy and entropy analyses are used to indicate the multi-irreversible losses and to assess the work potentials of the hybrid system. Expressions for the power output and efficiency of the hybrid system are derived and the performance characteristics of the system are presented and discussed in detail. The effects of the design parameters and operating conditions on the system performance are studied numerically. It is found that there exist certain optimum criteria for some important parameters. The results obtained here may provide a theoretical basis for both the optimal design and operation of real fuel cell–heat engine hybrid systems. This new approach can be easily extended to other fuel cell hybrid systems to develop irreversible models suitable for the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Synthesis and evaluation of ATO as a support for Pt–IrO2 in a unitized regenerative fuel cell

An IrO₂ catalyst was prepared using a colloidal method followed by a thermal treatment. The catalyst was later mixed with Pt-Black and supported on the Sb-doped SnO₂ (ATO), synthesized through the same colloidal method. ATO was investigated as a possible catalyst support in an electrode of a regenerative fuel cell (URFC), where Pt–IrO₂ was used as the catalyst for the oxygen evolution and reduction reactions. The morphology and composition of the ATO support was investigated through transmission electron microscopy, X-ray diffraction (including Rietveld Refinement), BET analysis, and X-ray fluorescence. An ATO support was obtained with a highly homogeneous distribution and crystal sizes, measuring approximately 4–6 nm.     

All-solid-state supercapacitor using a Nafion® polymer membrane and its hybridization with a direct methanol fuel cell

An all-solid-state supercapacitor is fabricated and optimized using a Nafion^® membrane and an ionomer. The device shows good capacitance (ca. 200 F g⁻¹) as demonstrated by cyclic voltammograms (CVs) and charge–discharge curves. The supercapacitor exhibits a relatively stable capacitance during l0,000 cycles of operation. A hybrid system comprising a direct methanol fuel cell (DMFC) and an all-solid-state supercapacitor has been designed and tested. It is confirmed that the power discharged by the supercapacitor is transferred effectively to the DMFC. The power of the hybrid is immediately improved by 30% compared with that of a DMFC alone operating at 25 °C. The possibilities of using this system for high energy and high instantaneous power devices and integrated fabrication processes are discussed.     

Fabrication and evaluation of Nafion nanocomposite membrane based on ZrO2–TiO2 binary nanoparticles as fuel cell MEA

Synthesis and characterization of nanocomposite membranes for proton exchange membrane fuel cell (PEMFC) operating at different temperatures and humidity were investigated in this study. Recast Nafion composite membrane with ZrO₂ and TiO₂ nanoparticles with 75 nm in mean size diameter, prepared for PEM fuel cells. Nafion/TiO₂ composite membranes have been also fabricated by in-situ sol–gel method. However, fine particles of the ZrO₂ were synthesized and Nafion/ZrO₂ composite membrane were produced by blending a 5% (w/w) Nafion-water dispersion with the inorganic compound. All nanocomposite membranes demonstrated higher water retention in comparison with unmodified membranes. Proton conductivity increased with increasing ZrO₂ content while TiO₂ additive (with mean size of 25 nm) enhanced water retention. Subsequently, structures of the membranes were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) as well as X-Ray Diffraction (XRD). In addition, water uptake and proton conductivity of the modified membranes were also measured. The nanocomposite membrane was tested in a 25 cm² commercial single cell at the temperature range of 80–110 °C and in humidified H₂/O₂ under different relative humidity (RH) conditions. The membrane electrode assembly (MEA) prepared from Nafion/TiO₂, ZrO₂ presented highest PEM fuel cell performance in respect of I–V polarization under condition of 110 °C, 0.6 V and 30% RH and 1 atm.     

Power coordination in a fuel cell–battery hybrid power source using commercial power controller circuits

Hybrid power systems combine the superb energy density of a fuel cell power source with the outstanding power density of modem batteries. A hybrid power source with an integral power distribution and charge management system was designed and built using standard miniature power regulator integrated circuits with appropriate modifications to implement the necessary controls. The resulting converter not only allows the interconnection of fuel cell systems and batteries having dissimilar operating voltages, but it also imposes a power sharing strategy that elicits peak performance from each part of the device. The resulting hybrid power source can supply 70% greater peak power, with only a 6% increase in weight, and no increase in volume, compared to the as-packaged fuel cell power source on which the hybrid source was based.     

Quasi-stationary UI-characteristic model of a PEM fuel cell–Evaluating the option of self-humidifying operation

This work presents a zero-dimensional PEM fuel cell UI-characteristic model created in MATLAB Simulink® for operation with dry or humidified air supply. It is parameterised and validated based on the results of stack operation by varying stack temperature (50–80 °C), gas pressure (1.0–2.4 bar) and air humidification (0.0–1.0). The model is based on physical and electrochemical correlations and expanded by empirically assumptions concerning the influence of the humidification and limiting current density on the performance. The UI-model is intended to be integrated into a comprehensive zero-emission powertrain model. Since non-humidified operation of PEM fuel cell systems provides benefits for mobile applications by reducing space demand and system complexity, the objective of the model is to relate performance to the operating conditions and underlying physical parameters. Results confirm the feasibility of a self-humidifying stack operation at high performance by optimal parameter setting.     

Effect of orthohydrogen–parahydrogen composition on performance of a proton exchange membrane fuel cell

The effect of orthohydrogen–parahydrogen concentration on the performance of a proton exchange membrane fuel cell is calculated and experimentally investigated. Gibbs free energy and reversible cell potential calculations predict that parahydrogen at room temperature produces a voltage 20 mV/cell higher than normal hydrogen and a 1.6% increase in efficiency over normal hydrogen. Experimental data based on a 1 kW proton exchange membrane fuel cell rapidly switched between normal and parahydrogen did not show a statistically significant difference in performance. Variations due to stack humidity and anode purging are found to dominate fuel cell output. The experimental results confirm that, as anticipated, parahydrogen concentration has a negligible impact on fuel cell performance for the majority of practical applications.     

Economic evaluation and optimization of a photovoltaic–fuel cell–batteries hybrid system for use in the Brazilian Amazon

The lack of electric power in isolated communities in the Brazilian Amazon region has become one of the barriers to economic and social development. Currently, the main technologies that provide electric power to these communities are diesel generators. This non-renewable energy source, besides causing serious problems to the environment and human health, have high maintenance and operational costs. This paper presents a study on the use of photovoltaic and fuel cells for continuous supply of electric power. The paper outlines the technical and costs characteristics of a pilot project set up in an environmental protection area, located in the state of Tocantins, Brazil. The pilot project uses solar energy as the primary electric power production source. Surplus energy stored in the hydrogen produced by the electrolysis of water is later transformed into electric power by the fuel cells during periods when there is little or no sunlight. A comparative study between the technologies and potential configurations meeting the needs of isolated communities in the Amazon through simulations based on HOMER software are presented. As result, this paper outlines some policies to promote the use of renewable energy sources in isolated areas in Brazil derived from the pilot project.     

Thermodynamic models for H2O–CO2–H2 mixtures in near-critical and supercritical regions of water

The PVTx properties of the H₂O–CO₂–H₂ mixtures have significant applications in the technology of supercritical water gasification of coal. Here, we first carry out the molecular dynamics simulations of the PVTx properties of the H₂O–CO₂–H₂ mixtures in the near-critical and supercritical regions of water to generate 600 datasets at 750–1150 K and 4.0–443.5 MPa. The molar fraction of each composition in the ternary mixtures ranges from 10% to 80%. Later we investigate the applicability of a well-known thermodynamic model for the ternary mixtures, namely the Duan-Møller-Weare equation of state (DMW EOS). It is observed that the DMW EOS shows great potential in the prediction of the PVTx properties of the ternary mixtures. However, it is noted that the mixing parameters describing the binary interactions of H₂O–H₂ and CO₂–H₂ are still unknown in the DMW EOS. By determining the missing mixing parameters using the Levenberg-Marquardt algorithm, the accuracy of the original DMW EOS is improved for the ternary mixtures. Moreover, optimizing the coefficients in the DMW EOS further promotes the accuracy of the model for the H₂O–CO₂–H₂ mixtures. The results from this work may facilitate the development of supercritical water gasification of coal.     

Exploring MnCr2O4–Gd0.1Ce0.9O2-δ as a composite electrode material for solid oxide fuel cell

Symmetrical solid oxide fuel cell (SOFC) adopting the same material at both electrodes is potentially capable of promoting thermomechanical compatibility between near components and lowering stack costs. In this paper, MnCr₂O₄–Gd_0.1Ce_0.9O_2-δ (MCO-GDC) composite electrodes prepared by co-infiltration method for symmetrical electrolyte supported and anode supported solid oxide fuel cells are evaluated at a temperature range of 650–800 °C in wet (3% H₂O) hydrogen and air atmospheres. Without any alkaline earth elements and cobalt, the co-infiltrated MCO-GDC composite electrode shows excellent activity for oxygen reduction reaction but mediocre activity for hydrogen oxidation reaction. With MCO-GDC as the cathode, the Ni-YSZ (Y₂O₃ stabilized ZrO₂) anode supported asymmetrical cell demonstrates a peak power density of 665 mW cm⁻² at 800 °C. The above results suggest MCO-GDC is a promising candidate cathode material for solid oxide fuel cells.