A chemically regenerative redox fuel cell. II

Our previous paper [1] has described an unconventional chemically regenerative redox fuel cell, including a modest amount of performance data, and has pointed out a number of problem areas. This paper describes additional work and some attempts that have been made to improve the cell performance in several of the problem areas described in the first paper. Two different redox fuel cell systems employing the V²⁺/V³⁺ and Mo³⁺Mo⁴⁺ redox couples as the anolyte, and a new NO ₃ ^–/NO catalyst system for VO²⁺ oxidation were explored and described. The reduction of Na₂MoO₄ to Mo⁴⁺ and Mo³⁺ by H₂ in concentrated H₂SO₄ solution was studied in small scale laboratory experiments and discussed. The performance of a redox cell in which H₂ was obtained from the Pd-catalysed decomposition of formic acid, HCOOH, is described and the results of experiments with different membranes are reported. The results of experiments using WC as a hydrogenation catalyst at a temperature of 70–90° C with mixed molybdo-tungsto-silicic acids are reported and discussed.     

Chemically regenerative redox fuel cells

Exploratory experiments with three types of redox fuel cells utilizing the VO ₂ ⁺ /VO²⁺-Sn²⁺/Sn⁴⁺, VO ₂ ⁺ /VO²⁺-Fe²⁺/Fe³⁺ and VO ₂ ⁺ /VO²⁺-Cu/Cu²⁺ redox couples are reported. The results show the major features and problems, and suggest possible solutions to some of the problems associated with operating redox fuel cells. In this phase of experimentation the best individual cell performances that were achieved showed that the VO ₂ ⁺ /VO²⁺-Sn²⁺/Sn²⁺ redox cell had a power density of 0.049 W cm^–2, the VO ₂ ⁺ /VO²⁺-Fe²⁺/Fe³⁺ redox cell had a power density equal to 0.049 W cm^–2 and the V ₂ ⁺ /VO²⁺-Cu/Cu²⁺ redox fuel cell had a power density of 0.093 W cm^–2.     

Chemically regenerative redox fuel cells. I Membrane studies

Three different types of membrane have been tested in a chemically regenerative redox fuel cell. It was found that a Nafion membrane gave the best polarization curves, but also that a much cheaper silica-filled polyethylene membrane could be used. A polysulphone membrane ranked number three     

Chemically regenerative redox fuel cells II. Regeneration reaction studies

The kinetics of the reduction of TiO²⁺ with H₂ has been studied using a platinum catalyst in hydrochloric acid medium. The kinetic results were used to design the performance of a chemically regenerative redox fuel cell with the Ti³⁺/TiO²⁺ redox couple as anode system. Also the results from another chemically regenerative redox fuel cell with the Fe(EDTA)^2–/Fe(EDTA)^– redox couple as anode system are presented. In both redox cells the VO ₂ ⁺ /VO²⁺ redox couple was used as cathode system.     

Performance of vanadium-oxygen redox fuel cell

A promising approach to improving the energy density of the all-vanadium redox flow battery while also saving on raw materials costs, is to eliminate the positive half-cell electrolyte and replace it with an air electrode to produce a hybrid vanadium–oxygen redox fuel cell (VOFC). This concept was initially proposed by Kaneko et al. in 1992 and first evaluated at the University of New South Wales by Menictas and Skyllas-Kazacos in 1997. In this project the performance of the VOFC over a range of temperatures and using different types of membranes and air electrode assemblies was evaluated. Despite early problems with the membrane electrode assemblies that saw separation of the membrane due to swelling and expansion during hydration, with improved fabrication techniques, this problem was minimized and it was possible to operate a 5-cell VOFC system for a total of over 100 h without any deterioration in its performance.     

High-performance microfluidic vanadium redox fuel cell

We demonstrate a new microfluidic fuel cell design with high-surface area porous carbon electrodes and high aspect ratio channel, using soluble vanadium redox species as fuel and oxidant. The device exhibits a peak power density of 70 mW cm⁻² at room temperature. In addition, low flow rate operation is demonstrated and single pass fuel utilization levels up to 55% are achieved. The proposed design facilitates cost-effective and rapid fabrication, and would be applicable to most microfluidic fuel cell architectures.     

Improved performance of the direct methanol redox fuel cell

Advancements in the performance of the direct methanol redox fuel cell (DMRFC) were made through anolyte/catholyte composition and cell temperature studies. Catholytes prepared with different iron salts were considered for use in the DMRFC in order to improve the catholyte charge density (i.e., iron salt solubility) and fuel cell performance. Following an initial screening of different iron salts, catholytes prepared with FeNH₄(SO₄)₂, Fe(ClO₄)₃ or Fe(NO₃)₃ were selected and evaluated using electrolyte conductivity measurements, cyclic voltammetry and fuel cell testing. Solubility limits at 25 °C were observed to be much higher for the Fe(ClO₄)₃ (>2.5 M) and Fe(NO₃)₃ (>3 M) salts than FeNH₄(SO₄)₂ (~1 M). The Fe(ClO₄)₃ catholyte was identified as a suitable candidate due to its high electrochemical activity, electrochemical reversibility, observed half-cell potential (0.83 V vs. SHE at 90 °C) and solubility. DMRFC testing at 90 °C demonstrated a substantial improvement in the non-optimized power density for the perchlorate system (79 mW cm⁻²) relative to that obtained for the sulfate system (25 mW cm⁻²). Separate fuel cell tests showed that increasing the cell temperature to 90 °C and increasing the methanol concentration in the anolyte to 16.7 M (i.e., equimolar H₂O/CH₃OH) yield significant DMRFC performance improvements. Stable DMRFC performance was demonstrated in short-term durability tests.     

Gas diffusion layer with titanium carbide for a unitized regenerative fuel cell

In this work, a gas diffusion layer (GDL) prepared with metallic ceramics TiC for a unitized regenerative fuel cell (URFC) was first investigated. By the measurements of morphological characteristic, electrical conductivity, absolute through-plane permeability and electrochemical stability, the characteristics of the novel GDLs and the conventional one were compared. A high corrosion-resistive and low-cost GDL with 80 wt.% TiC and 20 wt.% IrTiO_x was expected to enhance the cycle performance of URFC. And the total loading of Ir in the novel URFC was only 1.3 mg cm⁻². The URFC with the novel GDL exhibited the similar initial performance under both fuel cell and electrolysis modes as that using the conventional GDL. However, the life cycle testing over 60 h showed that the URFC with the novel GDL was more stable than the URFC with the traditional GDL, indicating that the GDL with TiC and IrTiO_x was beneficial to improve the cycle life of the URFC.     

A unitized approach to regenerative solid polymer electrolyte fuel cells

In this work on regenerative fuel cells, the initial part deals with water electrolysis using a cell design that closely resembled that of a solid polymer fuel cell. The electrolytes were Nafion® 117 and the Dow experimental membrane. The electrodes were Pt-on-C and Pt/Ir-on-C gas diffusion electrodes on the oxygen side and Pt-on-C on the hydrogen side. Fuel cells were built with the above mentioned electrodes and membranes. These cells were run to obtain fuel cell and electrolysis data. Data for a maximum of five regenerative cycles were obtained. The current-potential data in the regenerative electrolysis were characterized by a gradual decay with time. The fuel cell data were very stable. The membrane-electrode assemblies were found in very good condition, and no visible corrosion of electrodes was evident.     

Bifunctional electrodes for unitised regenerative fuel cells

The effects of different configurations and compositions of platinum and iridium oxide electrodes for the oxygen reaction of unitised regenerative fuel cells (URFC) are reported. Bifunctional oxygen electrodes are important for URFC development because favourable properties for the fuel cell and the electrolysis modes must be combined into a single electrode. The bifunctional electrodes were studied under different combinations of catalyst mixtures, multilayer arrangements and segmented configurations with single catalyst areas. Distinct electrochemical behaviour was observed for both modes and can be explained on the basis of impedance spectroscopy. The mixture of both catalysts performs best for the present stage of electrode development. Also, the multilayer electrodes yielded good results with the potential for optimisation. The influence of ionic and electronic resistances on the relative performance is demonstrated. However, penalties due to cross currents in the heterogeneous electrodes were identified and explained by comparing the performance curves with electrodes composed of a single catalyst. Potential improvements for the different compositions are discussed.     

Effect of fabrication methods of bifunctional catalyst layers on unitized regenerative fuel cell performance

In order to understand the origins of performance variations in unitized regenerative fuel cells (URFCs), bifunctional catalyst layers (BCLs) fabricated with two different methods, i.e., ink deposition on membrane or GDL, were designed in this paper. The performances of the two different methods were evaluated, and their reaction dynamics were measured by electrochemical impedance spectra. The different BCLs, caused by the different preparation processes, were found to influence the fuel cell performance. The cell potentials of the URFCs using platinum sprayed onto the gas diffusion layer (GDL) are above 0.100 V higher than those with platinum sprayed onto the membrane at 800 mA cm⁻² in fuel cell (FC) mode. The mass transport resistances of the URFCs at different operation modes were also compared. It was proved that the platinum layer formed by applying platinum onto the GDL could prevent the cell from water flooding in FC mode. However, it was found that the cell performance changed slightly in water electrolysis mode with different BCLs. The electron conduction path was also found to be hindered by an IrO₂ agglomerate, which led to a decrease in cell performance. The highest and lowest round-trip efficiencies of the URFC with different BCLs were 42.1% and 22.3%, respectively, at 800 mA cm⁻².     

A tubular microbial fuel cell

Cell potential and power performance for tubular microbial fuel cells utilising manure as fuel are reported. The microbial fuel cells do not use a mediator, catalysts or a proton exchange membrane. The cell design has been scaled up to a size of 1.8 m in length using electrodes of 0.4 m² in area. The cell does not require a strictly controlled anaerobic environment and has potential practical applications when adapted into the form of a helix allowing fuel to flow through it. The cell could be used for power generation in remote applications. The peak power density of the cell is over 3 μW cm ⁻² (30 mW m⁻²). The performance can be improved by a more effective design of the interface between the anode and cathode chambers.     

Nafion®-bonded porous titanium oxide electrodes for oxygen evolution: towards a regenerative fuel cell

Oxygen evolution on PTFE/Nafion^®-bound porous activated XC-72 carbon and on PTFE- and PTFE/Nafion^®-bound Ebonex^® has been studied, using RuO₂/IrO₂ catalysts. High activity was found for Ebonex^®, but carbon, even when coated with Nafion, proved qquite unstable.     

A multi-function compact fuel reforming reactor for fuel cell applications

A multi-function compact chemical reactor designed for hydrocarbon steam reforming was evaluated. The reactor design is based on diffusion bonded laminate micro-channel heat exchanger technology. The reactor consists of a combustor layer, which is sandwiched between two steam reforming layers. Between the two function layers, a temperature monitor and control layer is placed, which is designed to locate the temperature sensors. The combustor layer has four individually controlled combustion zones each connected to a separate fuel supply. The reactor design offers the potential to accurately control the temperature distribution along the length of the reactor using closed loop temperature control. The experimental results show that the variance of temperature along the reactor is negligible. The conversion efficiency of the combustor layer is approximately 90%. The heat transfer efficiency from combustion layer to reforming layers is 65-85% at 873 K and 673 K, respectively. The heat transfer rate to the reforming layers is sufficient to support a steam reformation of propane at a rate of 0.7 dm³/min (STP) with a steam to carbon ratio of 2 at 873 K.     

Integrated fuel processors for fuel cell application: A review

This report documents the key technological progress made over last two decades in the field of development of integrated fuel processor for hydrogen generation. Studies on process optimization based on numerical simulation/calculation, mass and energy management, parametric adjustment have been reported. A number of these studies discuss the application of reforming process assisted by other technologies such as pressure swing adsorption and membrane separation to enhance the hydrogen productivity and/or purity. However, for such systems the extent of integration among and between components remains limited. Accordingly, the net efficiency is compromised due to the mass/heat transfer rate and reaction dynamics either in the individual units or the complete system. Process intensification technologies such as engineered catalysts, on-site heat production/removal and product purification can not only allow precise control of reaction and heat/mass transfer rates, but also help optimize the operation conditions, and, consequently, improve overall efficiency and mitigate the requirement for materials and capital investment. It seems that micro-scale technologies, possessing the typical characteristics of process intensification technologies, have potential for making the integrated fuel processor into practice.     

可再生燃料电池及其在风/光电储能调峰中的应用研究进展

介绍了可再生燃料电池的工作原理、结构组成、分类和国内外可再生燃料电池技术的研究状况及现阶段存在的主要技术问题,对可再生燃料电池在风/光电储能调峰中的应用进行了分析总结,并对其发展趋势进行了展望。     

微生物燃料电池的基础研究

构建了一个无中间体、无膜的微生物燃料电池,电池以生活污水为初始接种体,以醋酸钠水溶液为原料,考察了溶液浓度和电子受体等对电池性能的影响,分析了电池中微生物的反应动力学特征,并对电池中微生物的种属进行了鉴定.结果显示,微生物燃料电池中电流的产生主要是依靠电极表面吸附微生物直接将代谢产生的电子传递给电极表面所致;溶液COD的降解符合一级反应动力学特征;当溶液中存在Q2和NO3-时溶液输出电压或电量均会降低,前者主要是发生好氧呼吸,后者主要是发生了反硝化作用.通过16 s rDNA的基因分析,确定了电池中的优势微生物为假单胞菌属.     

A high performance molten carbonate fuel cell cathode

A highly-conducting ceramic material, identified as stable in molten carbonate electrolyte, has been fabricated by partial sintering of the powder into porous gas-diffusion electrodes. These have been tested with commercial molten carbonate electrolyte tiles, to compare performance with that of state-of-the-art NiO cathodes. Although the fabrication technique was quite simple, the resultant La_0.8Sr_0.2CoO₃ electrodes showed kinetic performance equivalent to the NiO. This, or other perovskite or spinel-structure ceramic shown to be totally system compatible in long-term tests, can be formed into effective electrodes to replace NiO.