Electrical properties of SDC–BCY composite electrolytes for intermediate temperature solid oxide fuel cell

The electrolyte material Ce_0.85Sm_0.15O_1.92 (SDC) powders are synthesized by glycine–nitrate processes and BaCe_0.83Y_0.17O_3−δ (BCY) powders are synthesized by sol–gel processes, respectively. Then SDC–BCY composite electrolytes are prepared by mixing SDC and BCY. The SDC and BCY powders are mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as SB95, SB90 and SB85, respectively. The electrical properties of SDC and SDC–BCY composites are investigated. The experimental results show that SDC–BCY composites exhibit the excellent conductivity and could significantly enhance the fuel cell performances. The behavior that SDC–BCY composites display hybrid proton and oxygen ion conduction is substantiated. Among these electrolytes, the maximum power density reaches as high as 159 mW cm⁻² for the fuel cell based on SB90 composite electrolyte at 600 °C.     

Novel doped barium cerate–carbonate composite electrolyte material for low temperature solid oxide fuel cells

A composite of a perovskite oxide proton conductor (BaCe_0.7Zr_0.1Y_0.2O_3−δ, BCZ10Y20) and alkali carbonates (2Li₂CO₃:1Na₂CO₃, LNC) is investigated with respect to its morphology, conductivity and fuel cell performance. The morphology shows that the presence of carbonate phase improves the densification of oxide matrix. The conductivity is measured by AC impedance in air, nitrogen, wet nitrogen, hydrogen, and wet hydrogen, respectively. A sharp increase of the conductivity at certain temperature is seen, which relates to the superionic phase transition at the interface phases between oxide and carbonates. Single cell with the composite electrolyte is fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. The cell shows a maximum power density of 957 mW cm⁻² at 600 °C with hydrogen as the fuel and oxygen as the oxidant. The remarkable proton conductivity and excellent cell performance make this kind of composite material a good candidate electrolyte for low temperature solid oxide fuel cells (SOFCs).     

Intermediate temperature fuel cells based on doped ceria–LiCl–SrCl2 composite electrolyte

A new type of oxide-salt composite electrolyte, gadolinium-doped ceria (GDC)–LiCl–SrCl₂, was developed and demonstrated its promising use for intermediate temperature (400–700 °C) fuel cells (ITFCs). The dc electrical conductivity of this composite electrolyte (0.09–0.13 S cm⁻¹ at 500–650 °C) was 3–10 times higher than that of the pure GDC electrolyte, indicating remarkable proton or oxygen ion conduction existing in the LiCl–SrCl₂ chloride salts or at the interface between GDC and the chloride salts. Using this composite electrolyte, peak power densities of 260 and 510 mW cm⁻², with current densities of 650 and 1250 mA cm⁻² were achieved at 550 and 625 °C, respectively. This makes the new material a good candidate electrolyte for future low-cost ITFCs.     

Investigation of La2NiO4+δ-based cathodes for SDC–carbonate composite electrolyte intermediate temperature fuel cells

Lanthanum nickelate based oxides, including La₂NiO_4+δ (LN), La₂Ni_0.8Co_0.2O_4+δ (LNC82) and La₂Ni_0.8Fe_0.2O_4+δ (LNF82), were investigated as cathodes for intermediate temperature fuel cells with samaria doped ceria (SDC)–carbonate composite electrolytes. These oxides were synthesized by glycine–nitrate process and characterized by XRD and SEM, showing that all samples annealed at 800 °C for 2 h exhibit a K₂NiF₄ phase and a foam-like structure. The electrochemical properties of these cathodes were evaluated by fabricating and testing fuel cells with two kinds of composite electrolytes, SDC-20 wt.% (0.53Li/0.47Na)₂CO₃ and SDC-30 wt.% (0.67Li/0.33Na)₂CO₃, referred to as SDC(53L47N)20 and SDC(67L33N)30, respectively. Among these three cathodes, LNC82 shows the best cell performances at 500–600 °C. Moreover, fuel cells with SDC(67L33N)30 composite electrolyte present much higher power output than those with SDC(53L47N)20 composite electrolyte. It reveals that cobalt doping greatly enhances the electrochemical property of lanthanum nickelate, and such cathodes are more compatible with the SDC(67L33N)30 composite electrolyte.     

Endurance study of a solid polymer electrolyte direct ethanol fuel cell based on a Pt–Sn anode catalyst

The performance decay of a solid polymer electrolyte direct ethanol fuel cell (DEFC) based on a Pt₃Sn₁/C anode catalyst during an endurance test has been investigated. The effect of different cell shut-down procedures on the cycled behaviour of the DEFC has been studied. To get specific insights into the degradation mechanism, polarization and ac-impedance spectroscopy studies have been carried out. These analyses have been complemented by post-operation transmission electron microscopy and X-ray diffraction studies. The combination of these techniques has allowed to get information on recoverable and unrecoverable losses. This provides a basis for further improvement of DEFC components.     

Electrochemical properties of micro-tubular intermediate temperature solid oxide fuel cell with novel asymmetric structure based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ proton conducting electrolyte

This study employed a simple phase-inversion method to achieve anode-supported micro-tubular solid oxide fuel cells on the basis of the BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ proton conducting electrolyte. The typical cell with configuration of Ni–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ|BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ|La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-Sm_0.2Ce_0.8O_2-δ. The novel “sponge-like micro-pores electrode | homogeneous porous functional layer” asymmetric pore structure is obtained. Achieved results include: i) the electrodes revealed the single phase collected by the powder X-Ray Diffractometer analysis; ii) observed by Scanning Electron Microscope, the single cell presenting uniform distribution of micro sponge-like pores electrode was well-adhered to the dense and crack-free 12 μm thick electrolyte layer; iii) the cells showed excellent electrochemical performance with the maximum power densities of 1.070, 0.976, 0.815, and 0.700 W cm⁻² at 750, 700, 650 and 600 °C, respectively, characterized by Electrochemical Impedance Spectroscopy; iv) the designed cell clearly indicated a very low concentration polarization value (0.01 and 0.02 Ω cm² at 750 and 700 °C). Our findings provide a promising approach to improve intermediate temperature solid oxide fuel cells performance by optimizing the electrode-electrolyte interface microstructure, based on proton and oxide ion mixed conductor electrolytes.     

Modeling and simulation of solid oxide fuel cell based on the volume–resistance characteristic modeling technique

Solid oxide fuel cell (SOFC) is a complicated system with heat and mass transfer as well as electrochemical reactions. The real-time dynamic simulation of SOFC is still a challenge up to now. This paper develops a one-dimensional mathematical model for direct internal reforming solid oxide fuel cell (DIR-SOFC). The volume–resistance (V–R) characteristic modeling technique is introduced into the modeling of the SOFC system. Based on the V–R modeling technique and the modular modeling idea, ordinary differential equations meeting the quick simulation are obtained from partial differential equations. This model takes into account the variation of local gas properties. It can not only reflect the distributed parameter characteristics of SOFC, but also meet the requirement of the real-time dynamic simulation. The results indicate that the V–R characteristic modeling technique is valuable and viable in the SOFC system, and the model can be used in the quick dynamic and real-time simulation.     

Advanced fuel cell based on semiconductor perovskite La–BaZrYO3-δ as an electrolyte material operating at low temperature 550 °C

As an electrolyte, enough ionic conductivity, either proton (H⁺) or oxide (O²⁻) conduction, has demanded the better performance of low-temperature (especially below 550 °C) solid oxide fuel cell (LT-SOFCs). Notably, that either conductivity, higher performance, reliability, or higher cost is hampering the LT-SOFC marketing. In our current subject, we report the La-doped BZY proton conductor as an electrolyte has exhibited high ionic conductivity of 0.15 S/cm with a higher performance of 0.78 W/cm² at 550 °C. Also, the performance of LBZY is superior to the un-doped BZY electrolyte. Such high performance mainly ascribed due to the doping of La into BZY. Besides, the mechanism for high ion conductivity is explained. This work manifests that using the LBZY semiconductor perovskite as an electrolyte is more suitable for fuel cell technology.     

Pr2NiO4–Ag composite cathode for low temperature solid oxide fuel cells with ceria-carbonate composite electrolyte

Pr₂NiO₄–Ag composite was synthesized and evaluated as cathode component for low temperature solid oxide fuel cells based on ceria-carbonate composite electrolyte. X-ray diffraction analysis reveals that the formation of a single phase K₂NiF₄–type structure occurs at 1000 °C and Pr₂NiO₄–Ag composite shows chemically compatible with the composite electrolyte. Symmetrical cells impedance measurements prove that Ag displays acceptable electrocatalytic activity toward oxygen reduction reaction at the temperature range of 500–600 °C. Single cells with Ag active component electrodes present better electrochemical performances than those of Ag-free cells. An improved maximum power density of 695 mW cm⁻² was achieved at 600 °C using Pr₂NiO₄–Ag composite cathode, with humidified hydrogen as fuel and air as the oxidant. Preliminary results suggest that Pr₂NiO₄–Ag composite could be adopted as an alternative cathode for low temperature solid oxide fuel cells.     

YSZ electrolyte coating on NiO–YSZ composite by electrophoretic deposition for solid oxide fuel cells (SOFCs)

A thick dense film of YSZ has been fabricated on a porous NiO–YSZ substrate from the YSZ powders in the mixtures of absolute acetyl acetone–ethanol suspensions by electrophoretic deposition (EPD) method. Parameters affected on substrate porosity like pre-sintering temperature and percentage of starch and parameters affected on EPD process like applied voltage and time of deposition have been investigated. Linear dependence between weights of deposition, deposition time and applied voltage were observed. A crack-free dense thick film of YSZ was obtained on porous NiO–YSZ substrate. Adhesion between the two layers was observed by SEM. The ability of ionic transfer and permeability of the YSZ electrolyte were investigated by EIS, as well.     

A novel system for the production of pure hydrogen from natural gas based on solid oxide fuel cell–solid oxide electrolyzer

In this paper, a novel process for the production of pure hydrogen from natural gas based on the integration of solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs) is presented. In this configuration, the SOFC is fed by natural gas and provides electricity and heat to the SOEC, which carries out the separation of steam into hydrogen and oxygen. Depending on the system layout considered, the oxygen available at the SOEC anode outlet can be either mixed with the SOFC cathode stream in order to improve the SOFC performance or regarded as a co-product. Two configurations of the cell stack are studied. The first consists of a stack with the same number of SOFCs and SOECs working at the same current density. In this case, since in typical operating conditions the voltage delivered by the SOFC is lower than the one required by the SOEC, the required additional power is supplied by means of an electric grid connection. In the second case, the electricity balance is compensated by providing additional SOFCs to the stack, which are fed by a supplementary natural gas feed. Simulations carried out with Aspen Plus show that pure hydrogen can be produced with a natural gas to hydrogen LHV-efficiency that is about twice the value of a typical water electrolyzer and comparable to that of medium-scale reformers.     

Lifetime prediction of a polymer electrolyte membrane fuel cell via an accelerated startup–shutdown cycle test

To expand commercial applications of polymer electrolyte membrane fuel cells (PEMFCs), the evaluation time for their durability must be shortened. This article provides a straightforward accelerated degradation testing (ADT) procedure for PEMFC for easy and quick implementation of the procedure. The ADT procedure includes statistical modeling of degradation patterns of membrane electrode assemblies (MEAs) in PEMFCs under startup–shutdown cycling conditions. For this purpose, we propose a nonparametric degradation model to describe the nonlinear performance degradation paths of PEMFC MEAs. The analysis results indicate that the nonparametric approach provides more accurate estimates of the observed degradation data than other parametric approaches. Based on the nonparametric degradation model, we suggest a method to predict failure-times under normal operating conditions by estimating the time-scale factor under accelerated operating conditions.     

Development of Pr2-xSrxCuO4±δ mixed ion-electron conducting system as cathode for intermediate temperature solid oxide fuel cell

Mixed ionic-electronic conducting oxides Pr_2-xSr_xCuO_4±δ (x = 0, 0.2, 0.4, 0.5 and 0.6) are synthesized by combusting mixed precursors (metal acetates) under microwave heating followed by microwave sintering at 1000 °C for 4 h. Each composition of Pr_2-xSr_xCuO_4±δ solid solutions crystalizes into tetragonal structure within one single phase across the compositional range 0 ≤ x ≤ 0.5. The T′-phase Pr₂CuO_4±δ transforms to the T*-phase at x = 0.4. Crystallite size reduces from 801(9) to 728(6) nm with an increase in the Sr content. The maximum dc-conductivity (σ_dc = 63.1(2) S cm⁻¹ at 700 °C) and minimum activation energy (E_a = 0.118(3) eV, below pseudo-transition temperature 620 °C) in Pr_1.6Sr_0.4CuO_4±δ is ascribed to optimum concentration of extrinsic dissociated defects. The performance of electrochemical symmetrical cells (Pr_2-xSr_xCuO_4±δ/ Ce_0.9Gd_0.1O_1.95/ Pr_2-xSr_xCuO_4±δ) prepared using inkjet printing is found to be superior than spin coated (cathode) symmetrical cells. Increase in oxygen ion conductivity and surface oxygen exchange reaction rate with increasing Sr dopant concentration improve overall electrochemical performance of cathode. Minimum electrode polarization resistances 0.30(7) Ω cm² and 0.21(3) Ω cm² (at 700 °C) are obtained for symmetrical cells of Pr_1.6Sr_0.4CuO_4±δ cathode using spin-coating and inkjet printing methods, respectively. Electrochemical impedance spectroscopy studies suggest that the charge transfer step is the rate-limiting step during oxygen reduction reaction.     

An energetic–exergetic analysis of a residential CHP system based on PEM fuel cell

The use of fuel cell systems for distributed residential power generation represents an interesting alternative to traditional thermoelectric plants due to their high efficiency and the potential recovering of the heat generated by the internal electrochemical reactions. In this paper the study of a micro cogenerative (CHP) energy system based on a Proton Exchange Membrane fuel cell (PEMFC) is reported.     

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.     

Direct reforming of Methane–Ammonia mixed fuel on Ni–YSZ anode of solid oxide fuel cells

To control the temperature distribution in the Ni–YSZ (yttria-stabilized zirconia) anode of solid oxide fuel cells (SOFCs) by efficiently utilizing the heat generated by electrochemical reactions, the supply of methane–ammonia mixed fuel is proposed. The reaction characteristics of reforming/decomposition of the mixed fuel on a Ni–YSZ catalyst are experimentally investigated. A mixture gas of methane, steam, ammonia, and balance argon is supplied to a packed bed catalyst placed in a quartz tube in an electric furnace. The crushed Ni–YSZ anode of SOFCs is used as the catalyst. The exhaust gas composition is analyzed by gas chromatography and the streamwise temperature distribution of the catalyst bed is measured by an infrared camera. It is found that ammonia decomposition preferentially proceeds and steam methane reforming becomes active after sufficient ammonia has been consumed. A low-temperature region is formed by steam methane reforming owing to its strongly endothermic nature. Its position moves downstream while its magnitude decreases as the ammonia concentration in the fuel increases. This shows that the local temperature distribution can be controlled by tuning the ratio of methane to ammonia in the mixed fuel. It is also found that, at a certain mixture ratio, the mixed fuel realizes a hydrogen production rate higher than that for only methane or ammonia.     

Electrochemical properties and catalyst functions of natural CuFe oxide mineral–LZSDC composite electrolyte

In this study, a new functional composite based on CuFe-oxide mineral (CF) was prepared. This material was first investigated as a novel electrolyte material for low-temperature solid oxide fuel cells (LTSOFCs). The CF and an oxygen ion conducting Li_xZnO–Sm_0.2Ce_0.8O_2?δ (LZSDC) composite were prepared via a solid-phase blending method. The fuel cell device was fabricated by using the CF–LZSDC composite as an electrolyte layer sandwiched between symmetric electrodes of Ni_0.8Co_0.2Al_0.5Li (NCAL) coated Ni foam. The results showed that device performance increased with increasing compaction pressure. When the compaction pressure was 450 MPa, the maximum output power was 637 mW cm^?2, and the lowest ohmic resistance was 0.58 Ω. The electrochemical catalytic activity and the optimal design of LTSOFCs based on the mineral composite materials were investigated.     

Integrated carbon composite bipolar plate for polymer–electrolyte membrane fuel cells

The electrical resistance of bipolar plates for polymer–electrolyte membrane fuel cells (PEMFCs) should be very low to conduct the electricity generated with minimum electrical loss. The resistance of a bipolar plate consists of the bulk material resistance and the interfacial contact resistance when two such plates are contacted to provide channels for fuel and air (oxygen) supplies.     

Ag–polytetrafluoroethylene composite coating on stainless steel as bipolar plate of proton exchange membrane fuel cell

Forming a coating on metals by surface treatment is a good way to get high performance bipolar plate of proton exchange membrane fuel cell (PEMFC). In our research, Ag–polytetrafluoroethylene (PTFE) composite film was electrodeposited with silver-gilt solution of nicotinic acid by a bi-pulse electroplating power supply on 316 L stainless steel bipolar plate of PEMFC. Surface topography, contact angle, interfacial conductivity and corrosion resistance of the bipolar plate samples were investigated. Results showed that the defects on the Ag–PTFE composite coating are greatly reduced compared with those on the pure Ag coating fabricated under the same condition; and the contact angle of the Ag–PTFE composite coating with water is 114°, which is much bigger than that of the pure Ag coating (73°). In addition, the interfacial contact resistance of the composite coating stays as low as the pure Ag coating; and the bipolar plate sample with composite coating shows a close corrosion resistance to the pure Ag coating sample in potentiodynamic and potentiostatic tests. Coated 316 L stainless steel plate with Ag–PTFE composite coating exhibits well hydrophobic characteristic, less defects, high interfacial conductivity and good corrosion resistance, which shows a great potential of the application in PEMFC.