Production of hydrogen with low COx-content for PEM fuel cells by cyclic water gas shift reactor

Hydrogen gas with low CO content was produced by cyclic water gas shift (CWGS) reactor based on the periodic reduction and re-oxidation of Fe₂O₃–CeO₂–ZrO₂. The process was operated with CO/H₂ mixtures produced by e.g. auto-thermal reforming of hydrocarbons. During the reduction phase of the cyclic process, the incoming CO/H₂ mixture converted Fe₂O₃–CeO₂–ZrO₂ into a reduced form. Subsequently, steam was fed into the reactor for re-oxidation of the reduced material. Thereby, H₂ was released which can be used for a proton exchange membrane fuel cell (PEMFC) without any further purification. As side product, some coke can be formed on the solid surface by Bouduard reaction. This coke is removed in the re-oxidation step with steam leading to the formation of carbon monoxide. The extent of coke formation is controllable by keeping the oxygen conversion of the material below a certain degree. The feasibility of the novel process was demonstrated by combining the CWGS reactor with a 5-cell PEMFC stack.     

A dynamic two-dimensional heterogeneous model for water gas shift reactors

A dynamic, heterogeneous, two-dimensional model for packed-bed water gas shift reactors is presented. It can be applied to both high and low temperature shifts, and at scales ranging from industrial (for power plant applications) to small (such as automotive fuel cell applications). The model is suitable for any catalyst for which kinetic data are available, and shows excellent agreement with available experimental data for non-equilibrium conditions. The model is applied to an IGCC-TIGAS polygeneration plant to examine the dynamic behavior of the WGS units. The development of catalyst hot-spots is predicted during start-up or transition between steady states under certain conditions.     

微型燃机与燃料电池复合装置的应用

对微型燃机发电装置及与燃料电池复合装置作了简介,并比较了采用顶层循环的固体氧化物燃料电池-微型燃机复合发电装置与单独微型燃机发电装置各自的循环特点,以燃机功率为50kW的微型燃机及其复合发电装置为例,进行了两者的性能分析比较：在复合发电装置中,分析了余热利用的优越性,并对余热供热进行了计算分析.     

Multi-scale model based design of membrane reactor/separator processes for intensified hydrogen production through the water gas shift reaction

This work aims to first quantify the impact of various diffusion models (Maxwell-Stefan, Wilke, Dusty-Gas) on the predictions of a multi-scale membrane reactor/separator mathematical model, and to then demonstrate this model's use for the design and process intensification of membrane reactor/separator systems for hydrogen production. This multi-scale model captures velocity, temperature and species' concentration profiles along the catalyst pellet's radial direction, and along the reactor's axial direction, by solving the momentum, energy, and species transport equations, accounting for convection, conduction, reaction, and diffusion mechanisms. In the first part of work, the effect of pellet-scale design parameters (mean pore diameter, volumetric porosity, tortuosity factor, etc.) and various species' flux models on the model predictions is studied. In the second part, the study focuses on the comparison, in terms of their process intensification characteristics, of various hydrogen production processes. These include a conventional high-temperature shift reactor (HTSR)/low-temperature shift reactor (LTSR) sequence, a novel HTSR/membrane separator (MS)/LTSR/MS sequence, and a process that involves low-temperature shift membrane reactors-LTSMR in a series.     

Evaluation of a low temperature fuel cell system for residential CHP

CHP (combined heat and power) is a technology that allows to provide electrical and thermal energy. CHP is normally used in systems that produce wasted heat at high temperature to recover energy and increase overall system efficiency. The aim of this work is to investigate the possibility to recover heat produced by a 5 kW PEFC system for residential applications (hot water and building heating). As known, PEFCs work at low temperature (60-90 °C) and the experiments have been carried out in order to improve the overall system efficiency by reusing heat that is normally wasted.The work was developed during an Italian National project PNR-FISR “Polymeric and Ceramic Fuel Cell” coordinated by CNR-ITAE. A 5 kW PEFC system, developed with NUVERA Fuel Cells in the framework of the project, was tested in cogeneration configuration recovering wasted heat with a heat exchanger directly connected to cathode out.Tests on PEFC system were carried out in the range 2.5-5 kW, maintaining the working stack temperature at 71 °C. Heat, produced at different power levels, was removed from the system by using a regulated water flow in the heat exchanger. A peculiar feature of the system is the so-called “direct water injection” at the cathode, that allows simultaneous cooling and humidification of the stack. This characteristic permitted the recovery of most of the waste heat produced by the fuel cell.The performance of the PEFC unit was analyzed in terms of electrical, thermal and total efficiency. Tests showed that it is possible to obtain water at about 68 °C under different power levels. Moreover, experimental data showed that heat recovered was maximum when heat exchanger worked at nominal power and, under these conditions, the overall system efficiency increased up to 85%.     

高温燃料电池/燃气轮机混合循环发电技术

高温燃料电池／燃气轮机混合循环系统以其效率高、排放低的特点,在未来分布式发电和集中式大规模发电中占有重要地位。本文首先简介了高温燃料电池和先进燃气轮机的结构特点及其分类,在此基础上阐述了高温燃料电池与先进燃气轮机混合系统的基本模式,然后对适用于分布式发电和集中式发电的几种典型混合循环系统的结构和相应的流程及特点进行了详细的描述,最后给出了高温燃料电池和燃气轮机混合循环发电系统中的一些主要代表性技术以及目前研究的进展、挑战和目标。     

A hybrid model of cathode of PEM fuel cell using the interdigitated gas distributor

A two-dimensional (2D), single- and two-phase, hybrid multi-component transport model is developed for the cathode of PEM fuel cell using interdigitated gas distributor. The continuity equation and Darcy's law are used to describe the flow of the reactant gas and production water. The production water is treated as vapor when the current density is small, and as two-phase while the current density is greater than the critical current density. The advection–diffusion equations are utilized to study species transport of multi-component mixture gas. The Butler–Volmer equation is prescribed for the domain in the catalyst layer. The predicted results of the hybrid model agree well with the available experimental data. The model is used to investigate the effects of operating conditions and the cathode structure parameters on the performance of the PEM fuel cell. It is observed that liquid water appears originally in the cathodic catalyst layer over outlet channel under intermediate current and tends to be distributed uniformly by the capillary force with the increase of the current. It is found that reduction of the width of outlet channel can enhance the performance of PEM fuel cell via the increase of the current density over this region, which has, seemingly, not been discussed in previous literatures.     

A simple model of a high temperature PEM fuel cell

We develop a simple analytical model of a high temperature hydrogen fuel cell with proton exchange membrane. The model is validated against experimental results obtained in our group. The model is pseudo two dimensional, steady-state and isothermal, it accounts for the crossover of reactant gases through the membrane and it can be solved analytically. The role of the crossover is considered in detail.     

Proton exchange membrane fuel cell from low temperature to high temperature: Material challenges

Proton exchange membrane fuel cell (PEMFC) is considered as one promising clean and highly efficient power generation technology in 21st century. Current PEMFC operating at low temperatures (<80 °C) encounters several difficulties, such as CO tolerance, heat rejection, which can be, to a great extent, surmounted at higher temperatures (120–150 °C). However, the higher temperature conditions are much more challenging to implement, particularly with regards to the durability of the cell component materials. This paper overviews the drivers behind the interest in high-temperature PEMFC, and the challenges in developing novel materials to enable high-temperature PEMFC, including cell component durability (catalysts, polymer, bipolar plates, etc.), candidate polyelectrolytes for the electrode catalyst layer, and material compatibility in novel membrane electrode assembly (MEA), and provides an insight into the material research and development for PEMFC.     

H2 production via water gas shift in a composite Pd membrane reactor prepared by the pore-plating method

In this work, H₂ production via catalytic water gas shift reaction in a composite Pd membrane reactor prepared by the ELP “pore-plating” method has been carried out. A completely dense membrane with a Pd thickness of about 10.2 μm over oxidized porous stainless steel support has been prepared. Firstly, permeation measurements with pure gases (H₂ and N₂) and mixtures (H₂ with N_2, CO or CO₂) at four different temperatures (ranging from 350 to 450 °C) and trans-membrane pressure differences up to 2.5 bar have been carried out. The hydrogen permeance when feeding pure hydrogen is within the range 2.68–3.96·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5, while it decreases until 0.66–1.35·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 for gas mixtures. Furthermore, the membrane has been also tested in a WGS membrane reactor packed with a commercial oxide Fe–Cr catalyst by using a typical methane reformer outlet (dry basis: 70%H₂–18%CO–12%CO₂) and a stoichiometric H₂O/CO ratio. The performance of the reactor was evaluated in terms of CO conversion at different temperatures (ranging from 350 °C to 400 °C) and trans-membrane pressures (from 2.0 to 3.0 bar), at fixed gas hourly space velocity (GHSV) of 5000 h⁻¹. At these conditions, the membrane maintained its integrity and the membrane reactor was able to achieve up to the 59% of CO conversion as compared with 32% of CO conversion reached with conventional packed-bed reactor at the same operating conditions.     

Optimization of a water–gas shift reactor over a Pt/ceria/alumina monolith

The water–gas shift (WGS) reaction is an important step in the purification of hydrogen for fuel cells. It lowers the carbon monoxide content and produces extra hydrogen. The constraints of automotive applications render the commercial WGS catalysts unsuitable. Pt/ceria catalysts are cited as promising catalysts for onboard applications as they are highly active and non-pyrophoric. This paper reports on a power law rate expression for a Pt/CeO₂/Al₂O₃ catalyst. This rate equation is used to compare different reactor configurations for an onboard water–gas shift reactor. A one-dimensional heterogeneous model that accounts for the interfacial and intraparticle gradients has been used to optimize a dual stage adiabatic monolith reactor.     

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

A solid oxide fuel cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm⁻² at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm⁻² or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.     

Visualization of unstable water flow in a fuel cell gas diffusion layer

Modeling two-phase flow in proton exchange membrane (PEM) fuel cells is hampered by a lack of conceptual understanding of flow patterns in the gas diffusion layer (GDL). In this paper, pore-scale visualizations of water in different types of GDLs were used to improve current understanding of flow and transport phenomena in PEM fuel cells. Confocal microscopy was used to capture the real-time transport of water, and pressure micro-transducers were installed to measure water breakthrough pressures. Three types of fuel cell GDLs were examined: TO series (Toray Corp., Tokyo, Japan), SGL series (SGL Carbon Group, Wiesbaden, Germany), and MRC series (Mitsubishi Rayon Corp., Otake City, Japan). The visualizations and pressure measurements revealed that despite difference in “pore” structures in the three types of GDLs, water followed distinct flow paths spanning several pores with characteristics similar to the “column flow” phenomena observed previously in hydrophobic or coarse-grained hydrophilic soils. The results obtained from this study can aid in the construction of theories and models for optimizing water management in fuel cells.     

Effect of the precursor on the activity of high temperature water gas shift catalysts

The conventional catalysts used for high temperature water gas shift reaction are composed of Fe₃O₄–Cr₂O₃–CuO. The synthesis method to prepare them affects to their properties and catalytic activity. Moreover, the raw material used during the synthesis has also influence on their performance. In this work, the effect of the precursor on the activity of magnetite-based catalysts prepared by oxidation–precipitation method was studied. Ferrous sulfate and chloride were the precursors selected and the addition of Cu and Co as activity promoters was also studied. The materials prepared with sulfate showed larger crystallite sizes and lower specific surface areas but they were more active and selective than those obtained from chloride. This could be due to a decrease of the surface basicity that improves the reducibility and affects to the hydrogen adsorption and CO chemisorption. Addition of Cu or Co increased the CO conversion for the materials prepared with chloride but not for those prepared with sulfate. In the former case, Cu improved the reducibility of the samples. Cobalt was incorporated into magnetite lattice improving the covalency of Fe²⁺ ↔ Fe³⁺ redox couple of magnetite.     

Coupling of a continuum fuel cell model with a discrete liquid water percolation model

An iterative algorithm is developed to directly integrate a discrete liquid water percolation model into a 3D continuum fuel cell model. In the continuum model the thermodynamic processes, most relevant for the water management and fuel cell performance, are calculated. For the discrete liquid water distribution in the porous transport layer (PTL), a water path network model is used, calculating the discrete, injection pressure and condensation scenario dependent saturation distribution.     

Simulation on water transportation in gas diffusion layer of a PEM fuel cell: Influence of non-uniform PTFE distribution

Proton exchange membrane fuel cells (PEMFCs) are promising clean power sources with high energy conversion efficiency, fast startup, and no pollutant emission. The generated water in the cathode can cause water flooding of the catalyst layer (CL), which in turn can significantly decrease the fuel cell performance. To address this significant issue of PEMFC, a new gas diffusion layer (GDL) with non-uniform distribution of PTFE is proposed for water removal from the CL. The feasibility of this new GDL design is numerically evaluated by a Lattice-Boltzmann Method (LBM)-based two-phase flow model. The porous structure of the new GDL design is numerically reconstructed, followed by LBM simulations of the water transport in GDL. Three types of different wetting conditions are considered. It is found that liquid water transported 7.87% more with a single row of wetted solids and 13.36% more with two rows of wetted solids. The results clearly demonstrate that the liquid water can be effectively removed from the GDL by proper arrangement of hydrophilic solids in the GDL.     

Experimental study on mitigating the cathode flooding at low temperature by adding hydrogen to the cathode reactant gas in PEM fuel cell

Severe flooding can be critical in a fuel cell vehicle operating at a high current density, and in a fuel cell vehicle at the initial stage of start up. It is often difficult to remove the condensed water from the cathode gas diffusion layer (GDL) of the fuel cell because of the surface tension between the water and the GDL. In this research, in order to remove the condensed water from the cathode GDL, a small amount of hydrogen was injected into the cathode reactant gases. The results showed that the hydrogen addition method successfully removed the liquid water from the cathode GDL. Water removal was verified for various hydrogen flow rates and hydrogen addition durations. Furthermore, the dew point temperature of the outlet gas at the cathode was observed to determine the amount of water removed from the cathode GDL. In addition, the water droplet in the cathode gas flow channel was visualized by using a transparent cell. Furthermore, degradation tests are also performed. Considering the degradation test, the hydrogen addition method is expected to be effective in mitigating cathode flooding.