Ammonia, being a good source of hydrogen, has the potential to play a significant role in a future hydrogen economy. The hydrogen content in liquid ammonia is 17.6 wt% compared with 12.5 wt% in ...methanol. Although a large percentage of ammonia, produced globally, is currently used in fertiliser production, it has been used as a fuel for transport vehicles and for space heating. Ammonia is an excellent energy storage media with infrastructure for its transportation and distribution already in place in many countries. Ammonia is produced at present through the well known Haber–Bosch process which is known to be very energy and capital intensive. In search for more efficient and economical process and in view of the potential ammonia production growth forecast, a number of new processes are under development. Amongst these, the electrochemical routes have the potential to substantially reduce the energy input (by more than 20%), simplify the reactor design and reduce the complexity and cost of balance of plant when compared to the conventional ammonia production route. Several electrochemical routes based on liquid, molten salt, solid or composite electrolytes consisting of a molten salt and a solid phase are currently under investigation. In this paper these electrochemical methods of ammonia synthesis have been reviewed with a discussion on materials of construction, operating temperature and pressure regimes, major technical challenges and materials issues.
•Electrochemical methods of ammonia synthesis are reviewed.•Construction materials, operating regimes and technology status have been discussed.•Proton conduction mechanism in potential solid electrolytes has been discussed.•The ammonia production rates achieved are in the 10−13–10−8 mol cm−2 s−1 range.•Further advances are required on electrolytes, catalysts and technology up-scaling.
Ammonia synthesized using hydrogen from renewable sources offers a vast potential for the storage as well as transportation of renewable energy from regions with high intensity to regions lean in ...renewable sources. Ammonia can be used as an energy vector for an emissionless energy cycle in a variety of ways. Ammonia at the point of end use can be converted to hydrogen for fuel cell vehicles or alternatively utilized directly in solid oxide fuel cells, in an internal combustion engine or a gas turbine. One ton of ammonia production requires 9–15 MWh of energy. However, its conversion back to useful form or direct utilization can lead to substantial energy losses. In this paper, we present an overview of the current processes and technologies for ammonia synthesis and its utilization as an energy carrier. We have performed an estimation of the round-trip efficiency of different routes for ammonia utilization at the point of end use along with some sensitivity analysis, and we discuss the outcomes resulting from the best and worst case scenarios.
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•A comprehensive review of various direct ethanol fuel cells has been provided.•Bio-ethanol sources and production processes have been discussed in detail.•Fuel cells operating on ...bio-ethanol fuel offer economic and environmental advantages.•Materials, operating regimes, performance and life time issues are discussed.•The technology status and market applications have been detailed.
Fuel cells are one of the most efficient means of converting chemical energy into electrical energy. The major deterrents to the commercialisation of fuel cell technologies, especially for the transport sector, are the hydrogen storage and almost non-existence of hydrogen transportation and distribution infrastructure. The utilisation of bio-fuels such as methanol and ethanol instead of hydrogen as a fuel in fuel cells, not only reduces issues with fuel transportation and storage, but can also provide a CO2 neutral power generation technology and lead to a reduction in CO2 and other pollutants. In particular bioethanol is attractive as it is non-toxic, inexpensive, renewable and readily available. Currently around 90billionlitres per annum of ethanol is produced globally. It can be produced from a range of feedstock which includes sugar-cane, wheat, corn and low grade biomass such as woodchips, bagasse, waste from agro-industries, organic fractions from municipal waste or forestry residue. These factors make ethanol, especially when used with a low emission technology such as fuel cells, attractive from both an economic and environmental perspective. This has lead to a considerable interest in developing fuel cell systems operating directly on bioethanol. In this paper various types of direct ethanol fuel cells currently under development have been reviewed with emphasis on ethanol sources and production methods, cell construction materials, operating regime, cell and stack fabrication, performance and life time issues, technology status and market applications.
Fuel cells are under development for a range of applications for transport, stationary and portable power appliances. Fuel cell technology has advanced to the stage where commercial field trials for ...both transport and stationary applications are in progress. The electric efficiency typically varies between 40 and 60% for gaseous or liquid fuels. About 30–40% of the energy of the fuel is available as heat, the quality of which varies based on the operating temperature of the fuel cell. The utilisation of this heat component to further boost system efficiency is dictated by the application and end-use requirements. Fuel cells utilise either a gaseous or liquid fuel with most using hydrogen or synthetic gas produced by a variety of different means (reforming of natural gas or liquefied petroleum gas, reforming of liquid fuels such as diesel and kerosene, coal or biomass gasification, or hydrogen produced via water splitting/electrolysis). Direct Carbon Fuel Cells (DCFC) utilise solid carbon as the fuel and have historically attracted less investment than other types of gas or liquid fed fuel cells. However, volatility in gas and oil commodity prices and the increasing concern about the environmental impact of burning heavy fossil fuels for power generation has led to DCFCs gaining more attention within the global research community. A DCFC converts the chemical energy in solid carbon directly into electricity through its direct electrochemical oxidation. The fuel utilisation can be almost 100% as the fuel feed and product gases are distinct phases and thus can be easily separated. This is not the case with other fuel cell types for which the fuel utilisation within the cell is typically limited to below 85%. The theoretical efficiency is also high, around 100%. The combination of these two factors, lead to the projected electric efficiency of DCFC approaching 80% - approximately twice the efficiency of current generation coal fired power plants, thus leading to a 50% reduction in greenhouse gas emissions. The amount of CO2 for storage/sequestration is also halved. Moreover, the exit gas is an almost pure CO2 stream, requiring little or no gas separation before compression for sequestration. Therefore, the energy and cost penalties to capture the CO2 will also be significantly less than for other technologies. Furthermore, a variety of abundant fuels such as coal, coke, tar, biomass and organic waste can be used. Despite these advantages, the technology is at an early stage of development requiring solutions to many complex challenges related to materials degradation, fuel delivery, reaction kinetics, stack fabrication and system design, before it can be considered for commercialisation. This paper, following a brief introduction to other fuel cells, reviews in detail the current status of the direct carbon fuel cell technology, recent progress, technical challenges and discusses the future of the technology.
Molten carbonate fuel cell is one of the most promising high efficiency and sustainable power generation technologies, as demonstrated by the availability of several commercial units nowadays. ...Despite the significant progress made over the past few decades, the issues like component stability in carbonate melts and lower power density as compared to other high-temperature fuel cell systems need to be overcome to meet cost and lifetime targets. An improvement in the catalysts and system design for in situ reforming of fuel is critical to make molten carbonate fuel cells (MCFCs) compatible with real world fuels with minimal preprocessing requirements. Thus a significant opportunity exists for materials R&D in the MCFC area. In the present review, the key issues with MCFC component materials: cathode, anode, matrix, current collectors and bipolar plates, are discussed. The alternative materials and strategies adapted by the MCFC R&D community to mitigate these issues are discussed with emphasis on research trends and developments over the past decade.
Electrochemical cells and systems play a key role in a wide range of industry sectors. These devices are critical enabling technologies for renewable energy; energy management, conservation, and ...storage; pollution control/monitoring; and greenhouse gas reduction. A large number of electrochemical energy technologies have been developed in the past. These systems continue to be optimized in terms of cost, life time, and performance, leading to their continued expansion into existing and emerging market sectors. The more established technologies such as deep-cycle batteries and sensors are being joined by emerging technologies such as fuel cells, large format lithium-ion batteries, electrochemical reactors; ion transport membranes and supercapacitors. This growing demand (multi billion dollars) for electrochemical energy systems along with the increasing maturity of a number of technologies is having a significant effect on the global research and development effort which is increasing in both in size and depth. A number of new technologies, which will have substantial impact on the environment and the way we produce and utilize energy, are under development. This paper presents an overview of several emerging electrochemical energy technologies along with a discussion some of the key technical challenges.
Direct carbon fuel cells (DCFC) offer clear advantages over conventional power generation systems including higher conversion efficiency, low emissions and production of a near pure CO2 exit stream ...which can be easily captured for storage. When operated on biomass-derived fuels and combined with carbon capture and storage they have the potential to be a carbon negative technology. Currently most studies relating to DCFC's focus on the use of synthetic high purity fuels. Although of significant academic interest, the high energy requirements for the production of such fuels and high cost would negate the advantages offered by DCFCs over conventional combustion technologies that can produce power from lower-grade fuels. A number of industrial processes (such as pyrolysis or gasification) can produce high carbon containing and low cost chars from biomass sources. This paper describes the operation of a novel solid state direct carbon fuel cell operated on two such commercially available bio-mass derived chars, an agricultural waste derived bio-char used for soil enrichment and coconut char used for the processing of ceramics. Chemical analysis (ICP, XRF), X-ray diffraction and thermo-gravimetric analysis have been used to characterise the fuels. Testing on small button cells showed that it is possible to operate fuel cells directly on low grade unprocessed chars. Although initial power densities were low, significant improvements to cell materials and designs can lead to practical devices. Overall the stability of the fuel cell materials in contact with bio-chars appeared to be good with no phase decomposition of any material observed.
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•A solid state direct carbon fuel cell was operated on biomass derived chars.•The ash species in biomass derived fuels did not degrade fuel cell materials used.•The biomass with high ash content, volatiles and moisture resulted in poor performance.
Hydrogen as an energy currency, carrier and storage medium may be a key component of the solution to problems of global warming, poor air quality and dwindling reserves of liquid hydrocarbon fuels. ...Hydrogen is a flexible storage medium and can be generated by the electrolysis of water. It is particularly advantageous if an electrolyser may be simply and efficiently coupled to a source of renewable electrical energy. This paper examines direct coupling of a polymer electrolyte membrane (PEM) electrolyser to a matched solar photovoltaic (PV) source for hydrogen generation and storage. Such direct coupling with minimum interfacing electronics would lead to substantial cost reduction and thereby enhance the economic viability of solar-hydrogen systems. The electrolyser is designed for fail-safe operation with multiple levels of safety and operational redundancy. A control system in the electrolyser unit provides for disconnection when required and for auto-start in the morning and auto shut-down at night, simultaneously addressing the goals of minimum energy loss and maximum safety. The PV system is a 2.4
kW array (20.4
m
2 total area) comprising 30, 12
V, 80
W, Solarex polycrystalline modules in a series–parallel configuration. The integrated system has been operated for approximately 60 days over a 4-month period from September 2007 to January 2008 with many periods of unattended operation for multiple days, experiencing weather ranging from hot and sunny (above 40
°C) to cool and cloudy. The principle and practicality of direct coupling of a suitably matched PV array and PEM electrolyser have been successfully demonstrated. Details of electrolyser operation coupled to a PV array along with modelling work to match current–voltage characteristics of the electrolyser and PV system are described.
A direct carbon fuel cell offers a high efficiency alternative to traditional coal fired electrical power plants. In this paper, the electrochemical performance of electrolyte supported button cells ...with Gd
2O
3-doped CeO
2 (CGO) electrolyte is reported over the temperature range 600 to 800
°C with solid carbon as a fuel and He/CO
2 as the purge gases in the fuel chamber. The electrochemical characterisation of the cells was carried out by the Galvanostatic Current Interruption (GCI) technique and measuring V-I and P-I curves. Power densities over 50 mWcm
-2 have been demonstrated using carbon black as the fuel. Results indicate that at low temperatures around 600
°C, the direct electrochemical oxidation of carbon takes place. However, at higher temperatures (800
°C) both direct electrochemical oxidation and the reverse Boudouard reaction take place leading to some loss in fuel cell thermodynamic efficiency and reduced fuel utilisation due to the in-situ production of CO. In order to avoid reverse Boudouard reaction whilst maximising performance, an operating temperature of around 700
°C appears optimal. Further, the electrochemical performance of fuel cells has been compared for graphite and carbon black fuels. It was found that graphitic carbon fuel is electrochemically less reactive than relatively amorphous carbon black fuel in the DCFC when tested under similar conditions.
► The performance of CGO electrolyte based direct carbon fuel cell with carbon fuel was evaluated successfully over the temperature range from 600
°C to 800
°C in He and CO
2 atmospheres in the fuel chamber. ► The power densities up to 52
mW/cm
2 were achieved at 800
°C in electrolyte supported cells with amorphous carbon black fuel. ► Results obtained indicate that the direct electrochemical oxidation of carbon occurs at lower temperature (<
700
°C) whereas mixed mechanism consisting of both indirect electrochemical oxidation (via CO formation by the reverse Boudouard reaction) and direct oxidation of carbon is likely at higher temperature (>
700
°C). ► The amorphous (less-crystalline) carbon fuel is found to be more reactive than crystalline micronized graphite fuel in DCFCs discussed in this work.
The conversion of carbonaceous materials to electricity in a Direct Carbon Fuel Cell (DCFC) offers the most efficient process with theoretical electric efficiency close to 100%. One of the key issues ...for fuel cells is the continuous availability of the fuel at the triple phase boundaries between fuel, electrode and electrolyte. While this can be easily achieved with the use of a porous fuel electrode (anode) in the case of gaseous fuels, there are serious challenges for the delivery of solid fuels to the triple junctions. In this paper, a novel concept of using mixed ionic electronic conductors (MIEC) as anode materials for DCFCs has been discussed. The lanthanum strontium cobalt ferrite, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) was chosen as the first generation anode material due to its well known high mixed ionic and electronic conductivities in air. This material has been investigated in detail with respect to its conductivity, phase and microstructural stability in DCFC operating environments. When used both as the anode and cathode in a DCFC, power densities in excess of 50 mW/cm2 were obtained at 804 °C in electrolyte supported small button cells with solid carbon as the fuel. The concept of using the same anode and cathode material has also been evaluated in electrolyte supported thick wall tubular cells where power densities around 25 mW/cm2 were obtained with carbon fuel at 820 °C in the presence of helium as the purging gas. The concept of using a mixed ionic electronic conducting anode for a solid fuel, to extend the reaction zone for carbon oxidation from anode/electrolyte interface to anode/solid fuel interface, has been demonstrated.
The article professes the use of mixed ionic/electronic conducting (MIEC anode to extend the reaction zone for carbon oxidation from anode/electrolyte interface to anode/solid fuel interface for continuous operation of the direct carbon fuel cell. This type of fuel cell and operation mode offer the highest efficiency for direct conversion of carbon from coal or biomass to electricity. Display omitted
► A new concept of using mixed ionic electronic conducting (MIEC) anode in DCFC. ► MIEC extends reaction zone for solid carbon fuel to external surface of anode. ► The concept of MIEC anode demonstrated in button and tubular cells with good performance. ► Synchrotron high resolution XRD identified phases not shown by normal X-ray diffractometer. ► LSCF fully characterised for phase assemblage and electrical conductivity.
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