摘要:

Thomas HamannThomas Hamann is an Assistant Professor of Chemistry at Michigan State University. From 2006 to mid-2008 he was a postdoctoral fellow in the Department of Chemistry at Northwestern University. He obtained a BA in chemistry from the University of Texas, an MS in chemistry from the University of Massachusetts, and a PhD in chemistry at California Institute of Technology. His research interests and expertise center on solar energy conversion with semiconductor-liquid junctions.

ISSN:0046-225X    

DOI:10.1039/b809672d

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Broader contextHeterogeneous catalysis possesses immense potential to transform lignocellulosic biomass into liquid fuels and chemicals. This article reviews the existing routes in biomass conversion that use heterogeneous catalysts and discusses the role of catalytic science. Over the last 20 years new cutting-edge experimental, synthetic and theoretical techniques have been developed that can now be applied to the conversion of biomass into fuels and chemicals. These cross-cutting tools will allow us to rapidly develop new catalysts and catalytic processes for the conversion of biomass into fuels and chemicals.

ISSN:0046-225X    

DOI:10.1039/b814955k

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Dr Asel Sartbaeva was born in Kyrgyzstan. She received her MSc degree at the Kyrgyz-Russian Slavic University in Bishkek, and MPhil and PhD degrees at Cambridge University. She has worked in the Department of Physics at ASU on glasses, zeolites and superconductors. In 2007, she was awarded a Glasstone Research Fellowship at Oxford University. Her main research interests are hydrogen storage materials and flexible frameworks.

ISSN:0046-225X    

DOI:10.1039/b810104n

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Broader contextAt present, fossil fuels (oil, gas, and coal) represent the largest share of all primary energy sources and will most likely remain dominant over the coming decades. The combustion of fossil fuels produces large amounts of CO2during the energy conversion process. With a share of over 60%, power generation represents the largest source of anthropogenic CO2. Carbon dioxide capture and storage is one option in the portfolio of mitigation methods to stabilize emissions of CO2. Oxy-combustion power plants using membrane technology have been proposed as one possible approach for CO2capture. However, the use of membranes for integrated air separation is very challenging due to several constraints that limit power plant performance and the lifetime of critical process components.

ISSN:0046-225X    

DOI:10.1039/b910124a

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Atsushi TakagakiAtsushi Takagaki received a BSc in chemistry from the Tokyo University of Science in 2001 and a MSc (2003) and PhD (2006) in chemistry from the Tokyo Institute of Technology supervised by Professor Kazunari Domen and Michikazu Hara. After graduating, Dr Takagaki had moved to The University of Tokyo as a postdoc in 2006–2008. In 2008, Dr Takagaki joined Japan Advanced Institute of Science and Technology (JAIST) as Assistant Professor. His research interests are the development of heterogeneous catalysts, especially solid acids for green and sustainable chemistry, including biomass utilization.

ISSN:0046-225X    

DOI:10.1039/b918563a

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

I.IntroductionThree important and inter-related global issues of the 21st century are: (i) atmospheric concentration of CO2at 385 ppm in 2008 (+37.5% compared with the pre-industrial level of 280 ppm) and increasing at the rate of 2 ppm y−1(0.52% y−1) with the attendant impact on the current and projected global warming, (ii) world annual energy use of 500 EJ (Exajoule = 1018Joules, 1 Quad = 1015BTU = 1.05 EJ), increasing at the rate of 2.2% y−1and projected to be 537 EJ by 2010, 590 EJ by 2015, 637 EJ by 2020, 687 EJ by 2025 and 737 EJ by 2030,1and (iii) food—insecure population of about 1 billion and increasing because of an increase in price of energy and the related input (e.g., fertilizer, irrigation), decline in per capita arable land area (caused by conversion to urban/industrial uses and increasing susceptibility to soil degradation), and reduction in per capita availability of renewable fresh water resources for agricultural use. Increasing energy demand is a major cause of CO2emission. Fossil fuel combustion for energy production emits between 0.14 to 0.28 Mg C Mwh−1of energy.2Thus, fossil fuel combustion and other anthropogenic activities have strongly impacted the atmospheric enrichment of several greenhouse gases (GHGs) with the attendant impact on climate change. Relative emission of GHGs comprises 79.9% energy-related CO2, 9.5% CH4, 5.8% N2O, 3.0% non-energy CO2and 1.8% other gases (www.climatetechnology.gov). With regards to the energy budget of the Earth, however, radiative forcing or the global warming potential of different GHGs must also be considered. Therefore, mitigating the increase in atmospheric abundance of CO2necessitates identification of options which: (i) reduce emissions by using low-carbon or no-carbon fuel sources, (ii) enhance energy use efficiency by minimizing losses, and (iii) sequester atmospheric CO2into other reservoirs with secure storage and long residence time. With increasing reliance on fossil fuel as the dominant energy source for most of the 21st century, disposal of CO2by engineering, chemical and biological techniques is important to reducing the atmospheric loading and minimizing the risks of climate change. Total emission of C, with business as usual during the 21stcentury, is estimated at 950 to 2195 Pg compared to only 300 Pg emitted between 1850 and 2000.3The rate of annual emission by 2100 is estimated at 20 to 35 Pg C y−1compared with the 1990 baseline emission rate of only 5.5 Pg y−1.3It is thus imperative that the anthropogenic emissions of CO2be sequestered in other global pools for long-term and secure storage. This article collates and synthesizes research information, and describes technological options of C sequestration to off-set anthropogenic emissions due to fossil fuel combustion, land use change and other activities to stabilize the atmospheric concentration of CO2at a desired level (e.g., 550 ppm).

ISSN:0046-225X    

DOI:10.1039/b809492f

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Xiao-Feng WangXiao-Feng Wang received his MS degree from Jilin University (Professor Xueqi Fu), received his PhD in physical chemistry in 2006 from Kwansei Gakuin University in Japan (Professor Yasushi Koyama). In 2005, he earned the Chinese Government Award for Outstanding Self-Financed Students Abroad. He conducted his postdoctoral research at Kwansei Gakuin University (2006–2008), National Institute of Advanced Industrial Science and Technology (2008–2009), and Gifu University (2009) working on biological, organic, and inorganic materials for solar energy conversion. From 2007–2010, he is the principle researcher of two research projects granted by JST and MEXT of Japan for developing chlorophyll-based solar cells.

ISSN:0046-225X    

DOI:10.1039/b918464c

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Broader contextMicrobial fuel cells (MFCs), generate electrical power through the anodic oxidation of organic substrates mediated by micro-organisms, while the reduction of the electron acceptor (oxidant) occurs at the cathode. One of the proposed applications of such bio-electrochemical devices is the bio-remediation of wastewater coupled to the production of bio-energy. In order to increase the power output of these fuel cells, work is underway to improve the fuel cell design and the electrode reaction kinetics. At the cathode, oxidants such as potassium permanganate or ferricyanide have been used to boost MFCs power output. From a sustainability point of view however, molecular oxygen is the oxidant of choice since it is freely available and can be reduced to water. In this communication we have considered the linking of an efficient O2-reducing enzymatic cathode to a microbial anode. We found that this combination gives a higher maximum power output compared to the MFC based on ferricyanide at the cathode. We advocate that enzymatic cathodes should be considered to improve the performance of MFCs, but this must be weighed against the advantages/drawbacks of chemical and microbial alternatives for the catalysis of molecular oxygen reduction at the cathode.

ISSN:0046-225X    

DOI:10.1039/b815331k

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

1.IntroductionThe rising global energy demand and the environmental impact of energy use from traditional sources pose serious challenges to human health, energy security, environmental protection and the sustainability of natural resources. It is widely understood that increased energy use presents the easiest route to achieving prosperity and enhancing quality of life for large segments of the human population.1Over the last century, this goal has been accomplished by burning fossil fuels with adverse environmental consequences. The continued growth in global population, steadily increasing per-capita energy consumption in developing countries, and dependence on fossil fuel resources located in politically volatile regions make the fossil fuel-based economy unsustainable. Recently, a hydrogen-based economy has been proposed2as an alternative to the current fossil-fuel economy, but it is far from being widely realized. Considerable progress in the fundamental science of sustainable hydrogen production, storage and efficient use in fuel cells is needed to realize the vision of a hydrogen economy.Proton exchange membrane (also known as polymer electrolyte membrane) fuel cells (PEMFCs) represent a critical step in the proposed hydrogen economy. PEMFCs convert chemical energy of the fuel (hydrogen, methanol or formic acid) into electrical energy with high efficiency (∼60%) and minimal environmental pollution. Since PEMFCs are modular and have a simple design, they can be scaled up in size to suit the demands of a variety of applications. They have the potential to revolutionize transportation, which consumes 60% of the worldwide production of petroleum,3improve stationary power generation thereby reducing dependence on the electrical grid and provide reliable portable power for our ever-increasing array of personal electronic devices. Fuel cell technology has been used to power automobiles, scooters, buses, backup power generators, boats, underwater vehicles, locomotives and laptop computers.4Widespread commercialization of PEMFC technology has been stymied by high cost and the need for significant advances in hydrogen production and storage, and the performance, durability and service life of fuel cells under operating conditions.The heart of the PEMFC is a polymer electrolyte membrane (PEM) that separates the reactant gases and conducts protons as shown inFig. 1. The hydrogen fuel (on the anode side) and oxygen from air (on the cathode side) flow separately through grooves in the bipolar plate, then through a porous gas diffusion layer and in to a catalyst layer loaded with precious metal catalysts on carbon support. At the anode catalyst layer, hydrogen dissociates into protons that are transported through the membrane and electrons that flow through an external circuit. At the cathode catalyst layer, oxygen combines with the protons and electrons to form water and heat as by-products. The desired membrane properties for PEMFC include excellent proton conductivity and poor electron conductivity, low permeability for reactant gases, minimal crossover of water and fuel especially for direct methanol and formic acid fuel cells (DMFCs and DFAFCs), compatibility with electrode materials, dimensional stability during fuel cell operation (minimal swelling and shrinking), good mechanical strength, chemical and thermal stability, durability under prolonged operation (∼5000 h for transportation applications5) at elevated temperatures and during freeze–thaw cycles, and low cost. None of the available membranes meets all of these requirements.Schematic diagram of a PEM fuel cell. BP, GD, CL and Mem represent bipolar plates, gas diffusion layers, catalyst layers and membrane, respectively.A 2005 cost analysis6of an 80 kW PEMFC system for transportation, assuming high volume production of 500 000 units, projected a cost of $108 kW−1, which is below the United States Department of Energy's (DOE) 2005 target of $125 kW−1but considerably higher than the 2015 DOE target of $30 kW−1. The cost of the stack was projected to be $68 kW−1and the rest ($40 kW−1) arises from assembly and balance-of-plant components for water, heat and fuel management. The membrane–electrode assembly (MEA) accounts for 83% of the cost of the stack ($56 kW−1). The pressing need to overcome MEA cost and membrane degradation issues has driven an exponentially growing interest in this field as shown inFig. 2by the number of journal articles found in the scientific database Scopus.7The article search was performed using all of the following keywords: polymer, electrolyte, membrane, fuel and cell.Number of journal articles on polymer electrolyte membrane fuel cell based on a search using Scopus.7The most widely used PEM, Nafion®, was developed 40 years ago by DuPont Inc.8In Nafion, a hydrophobic perfluorinated polyethylene backbone and a highly hydrophilic sulfonic acid-terminated perfluoro vinyl ether pendant are known to form nanoscale domains within which ionic transport occurs.9,10The chemical structure of Nafion is shown inFig. 3. Typical values ofxandyare 7 and 1, respectively. Nafion has endured as the prototypical membrane for PEMFCs, because it offers high proton conductivity11(0.13 S cm−1at 75 °C and 100% relative humidity [RH]), chemical stability, and longevity (> 60 000 h) in a fuel cell environment.12However, it has several deficiencies that have stimulated a vigorous search for better membrane materials. Nafion is expensive, allows methanol crossover easily in DMFCs with adverse effect on performance, cannot function well at high temperatures (above 80 °C) or low humidity (below ∼80% RH), and requires external humidification and therefore management of water.12Chemical structure of Nafion.Proton dissociation from the SO3H groups and subsequent proton transport in Nafion depends on the presence of liquid water, because water molecules facilitate proton transfer and serve as shuttles for proton transport.13The need for liquid water in the membrane constrains the operating temperature below 100 °C in principle and 80 °C in practice. Moreover, protons traversing the hydrated membrane drag water molecules along from the anode to the cathode (electro-osmotic drag) as illustrated inFig. 1. There is also a flux of water molecules (back-diffusion) from the cathode to the anode driven by the water concentration gradient. Imbalance between these two fluxes can cause severe performance degradation due to drying of the anode catalyst layer and flooding of the cathode catalyst layer, mechanical stresses in the membrane, delamination of the catalyst layer, and pinhole formation in the membrane leading to gas crossover, catalyst sintering and failure.12,14The need to externally manage water distribution in the membrane adds to the system volume, weight, complexity and cost.Operation of PEMFCs above 120 °C is desirable for a number of reasons. Elevated temperature operation enhances the kinetics of electrode reactions and improves CO tolerance. At 80 °C, CO adsorbs on the Pt catalyst and diminishes the fuel cell performance. To avoid this CO poisoning, the CO content has to be maintained as low as 20 ppm in the fuel stream at 80 °C.15CO tolerance increases to 1000 ppm at 130 °C and 30 000 ppm at 200 °C due to CO desorption.15At temperatures above 120 °C, flooding of the catalyst layers can be avoided due to the elimination of liquid water, the cooling system will be simplified as a result of a larger temperature difference with the ambient, waste heat may be recovered, and it may be feasible to use non-precious metal catalysts.16In view of these considerations, DOE has set the following 2010 targets for PEM: conductivity: 0.1 S cm−1during operation at 120 °C and 1.5 kPa inlet water vapour partial pressure; unassisted start from −40 °C; O2and H2crossover: 2 mA cm−2; elevated temperature durability: 2000 h with cycling; and cost: $20 m−2.17In order to realize the potential of fuel cells, there is a need for rational design of novel membrane materials based on a fundamental understanding of membrane chemistry, proton transfer, nanophase segregation, membrane morphology, proton and molecular transport, and mechanical properties. Current membrane research is focused on developing membranes that retain water at elevated temperatures or that provide good conductivity in the absence of water. Several articles have reviewed various aspects of membrane development for PEMFCs.9,12,15,16,18–30The strategies being explored include modifying the pendant groups of existing membranes, incorporating hygroscopic oxides and heteropoly acids in the membrane to retain water, using aromatic backbone polymers, and replacing water with a less volatile proton solvent.22The aim of this review is to highlight developments over the past five years in membranes for PEMFCs. The references cited here are not meant to be comprehensive but serve as starting points for further exploration of this fascinating subject. The review is organized as follows: section 2 reports on the current experimental and theoretical understanding of Nafion membranes; section 3 discusses other perfluorinated membranes; section 4 presents recent developments in aromatic backbone membranes; section 5 discusses anhydrous membranes; section 6 presents the summary and outlook for membrane development.

ISSN:0046-225X    

DOI:10.1039/b808149m

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Broader contextAlthough sunlight provides by far the most abundant renewable energy resource, solar energy utilization at massive scale will require integrated storage and distribution, in addition to technologies that enable solar energy capture and conversion. Due to their very high energy density, far dwarfing that of batteries, compressed air energy storage systems, or pumped hydro systems, chemical fuels present an extraordinarily attractive option for enabling persistent, long term, high deliverability and high energy density, solar energy storage on a massive scale. Photovoltaics in series with electrolysis units accomplish this functionality, but require expensive electrical interconnections between the two separate (individually and collectively expensive) components of the energy system. Photosynthesis accomplishes the conversion of sunlight into chemical fuel (initially in the form of NADH and ultimately in the form of reduction of CO2to form sugars), albeit at very low (<1%) yearly averaged real energy efficiencies. Artificial photosynthesis has been shown to produce fuel from sunlight with much higher (>10%) overall energy conversion efficiency. However, photoelectrode materials for artificial photosynthesis that exhibit long-term stability in sunlight are relatively inefficient at fuel production, and conversely photoelectrode materials that exhibit high energy conversion efficiencies are relatively unstable. New materials that simultaneously possess both of these attributes are thus desirable. One approach to this problem, as described herein, is to explore a wide set of new photoelectrode materials in a combinatorial fashion, enabling screening of a wide variety of possibilities while compiling a structure–function database to guide yet further exploration and screening of promising new photoelectrodes. If a suitable material were successfully identified and developed, in principle it would underpin the development of a complete solar energy capture, conversion, and storage system that would be capable, if suitably widely deployed, of providing hundreds of exaJ per year of energy from sunlight in a storable, transportable, readily utilizable form,i.e.as chemical fuel.

ISSN:0046-225X    

DOI:10.1039/b812177j

出版商:RSC    

年代:2008

数据来源: RSC