摘要:

Broader contextPhotovoltaics (PVs) harvest electrical energy directly from sunlight and are potentially a major component of the solution to the growing energy challenge. Current PV technology is dominated by silicon (Si) and the performance has improved steadily as the technology has matured. However, the cost remains uncompetitive with traditional sources of power and there is a clear need for new inexpensive PV technologies. Devices based on organic semiconductors (OPVs) are therefore attracting a great deal of attention. They can be fabricated on large area, lightweight, flexible substrates and they offer a path to cost competitiveness with fossil fuel power generation. However, the performance of OPVs remains poor when compared to Si-based devices and it is widely recognised that significant improvements will be required for the technology to develop. In this communication, we focus on one of the most important issues in OPV device development, namely the interface between the transparent conducting electrode (indium tin oxide, ITO) and the organic donor layer (chloroaluminium phthalocyanine, ClAlPc). Specifically, we show how insertion of an ultra-thin molybdenum oxide layer at the ITO/ClAlPc interface greatly improves both the efficiency and stability of ClAlPc/C60devices. Our results highlight the importance of understanding and controlling interface properties.

ISSN:0046-225X    

DOI:10.1039/b915764f

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Broader contextThe direct conversion of organic wastes and biomass to electricity with microbial fuel cells offers the potential for producing high-value, carbon-neutral energy, from inexpensive source materials. However, at present, the power output of microbial fuel cells is too low for most envisioned applications. Further optimization has been stymied by a lack of understanding of the factors controlling the activity of the microorganisms that colonize the anode of microbial fuel cells and are responsible for producing the current. Here we report on a novel approach which makes it feasible to image actively metabolizing and growing cells within the anode biofilm in real time with a confocal scanning laser microscope.G. sulfurreducens, a well studied current-producing microorganism, was engineered to express the red fluorescent protein, mcherry. The growth of the fluorescent cells on the anode was monitored over time. When a fluorescent dye that is sensitive to pH was introduced it was possible to measure pH throughout different layers of the biofilm. During active current production, the pH deep within the biofilm dropped to levels shown to inhibit the activity ofG. sulfurreducens. These results suggest that strategies to facilitate proton flux out of the biofilm may increase power output.

ISSN:0046-225X    

DOI:10.1039/b816445b

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Broader contextDirect methanol fuel cells (DMFCs) are being considered as a possible solution to replace the current battery as the dominant power provider for portable electronic application, but they currently experience significant power density and efficiency losses due to high methanol crossover through proton exchange membranes (PEMs). Currently, Nafion® perfluorosulfonic acid (PFSA) membrane made by DuPont is the most frequently used PEM in DMFC. However, it is a poor barrier to methanol crossover, which limits its wide application in DMFC. In this communication, our synthesized PFSA polymers with a similar chemical structure to Nafion® were used as membrane materials, and PFSA membranes with various ion exchange capacities (IECs) from 0.36 mmol g−1to 1.17 mmol g−1were prepared. The relationship between the IECs of PFSA membranes and their transport properties (including water uptake, proton conductivity and methanol permeability) for DMFC applications was investigated.

ISSN:0046-225X    

DOI:10.1039/b917352h

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

1.IntroductionAs a renewable alternative energy source, solar energy is a potential choice for solving the shortage of fossil energy. In this regard solar cells have drawn much attention towards the realization of efficient conversion of solar energy into electric power. However, the widespread use of solar cells has been seriously hampered by the high production cost of electricity generated from the present silicon-based solar cells. Therefore, it is highly desirable to develop low-cost solar cells exhibiting high cell performance. The prospect of utilizing inexpensive materials that can be mass-produced makes organic solar cells fascinating alternatives for future energy sources.1–4It should be pointed out that they have several unique advantages over inorganic solar cells, other than low-cost, for example, light weight, flexibility and colorfulness.The production of electrical power from sunlight in organic solar cells involves the following processes: (i) sunlight photons are absorbed within a photoactive layer, leading to the formation of locally confined excitons, (ii) the excitons migrate to the interface of a donor–acceptor heterojunction and subsequently they dissociate to form free charges consisting of electrons and holes, and (iii) the charges are moved towards respective electrodes to eventually yield a photocurrent in an external circuit. To develop highly efficient organic solar cells, it is essential to elucidate the controlling factors in the processes and optimize each process in the photovoltaic event based on the fundamental information. In this context, extensive efforts have been made in recent years to select suitable donor and acceptor molecules and organize them on an electrode surface at the nanometer scale towards the realization of cell optimization.1–4It has been well established that fullerenes have small reorganization energies of electron transfer (ET), which leads to remarkable acceleration of photoinduced charge separation (CS) and of charge shift as well as deceleration of charge recombination (CR).5The excellent ET properties of fullerenes as acceptors have prompted many researchers to construct fullerene-based photoelectrochemical devices and photovoltaic cells.5–7For instance, fullerenes and their derivatives have always been employed for bulk heterojunction solar cells exhibiting a high cell performance, together with small donor molecules and p-type conjugated polymers, respectively.1,8,9On the other hand, we have successfully combined fullerenes with porphyrins that are an electron donor with excellent light-harvesting propeties, to develop a novel organic solar cell possessing both characters of dye-sensitized and bulk heterojunction solar cells (i.e., a dye-sensitized bulk heterojunction solar cell).10–13In either case the construction of nanohighways for efficient electron and hole transport in donor–acceptor multilayers on electrodes is highly crucial to attain efficient photocurrent generation. In fact many researchers have suggested the importance of an interpenetrating, bicontinuous electron- and hole-transporting network in the blend films of bulk heterojunction solar cells,1,8,9whereas we have exemplified the importance of such nanostructured electron- and hole-transporting highways using alternate porphyrin–fullerene multilayer structures on semiconducting electrodes.13Recently, the integration of a new class of carbon allotropes (i.e., carbon nanotubes (CNTs)) into organic solar cells has attracted much attention in connection with similar carbon-based structures with fullerenes.14Compared with the spherical shape of fullerenes, however, CNTs reveal unique one-dimensional (1-D), nanowire-like structures. The 1-D structures associate closely with the ideal electron- or hole-transporting highway in the active layer of organic solar cells (see Section 3 in detail), as in the case of one-dimensional materials comprising of semiconductors.15Therefore, CNTs are highly promising for transporting electrons or holes efficiently in the blend films of CNTs with donor or acceptor molecules on electrodes. Although CNTs are still expensive relative to other carbon-based materials such as graphite and fullerenes, low-cost production may be possible considering that CNTs also consist of naturally abundant carbon atoms. Intensive efforts have been devoted to reduce the production cost and the price is now continuously reducing as the demand is increasing.16In this review article we will mainly focus on the application of CNTs, especially single-walled carbon nanotubes (SWNTs), to a photoactive layer of photoelectrochemical devices and photovoltaic cells using liquid electrolytes. The applications of CNTs to polymer-based organic solar cells without liquid electrolytes are also described briefly. Representative examples are highlighted with the results of our SWNT-based photoelectrochemical devices.

ISSN:0046-225X    

DOI:10.1039/b805419n

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Broader contextAlthough public support for the expansion of nuclear power is increasing, significant growth is liable to be hindered or even halted by the seemingly intractable nuclear waste problem. A particularly difficult component of the waste problem is that any solution must be highly predictable at time scales not conducive to direct experimental verification. However, we have recently discovered a phenomenon that may permit improved predictability of long-term waste form performance. Specifically, from first principles theoretical methods, we have found that unconventional compounds and crystal structures may formviathe chemical transmutation that occurs during radioactive decay (e.g.rock salt137BaCl formation from the β- decay of137CsCl) (C. Jiang, C.R. Stanek, N. A. Marks, K. E. Sickafus and B. P. Uberuaga,Phys. Rev. B: Condens. Matter Mater. Phys., 2009,79, 132110). We refer to this phenomenon as “radioparagenesis.” For crystalline nuclear waste forms, understanding this phenomenon and its consequences may allow us to examine stability far beyond what is accessible by experiments. More importantly, we may also be able to use insight gained from radioparagenesis to revise how waste forms are designed.

ISSN:0046-225X    

DOI:10.1039/b915493k

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Robert H. CrabtreeEducated at Oxford and Sussex Universities and CNRS Natural Products Institute in Paris. Now, a Professor of Chemistry at Yale, he works extensively on catalysis, both organometallic and bioinorganic. Appointed Dow lecturer at Berkeley, Sabatier Lecturer at Toulouse, and will be Osborn Lecturer at Strasbourg and Mond Lecturer in the UK. He has been awarded ACS and RSC prizes for organometallic chemistry.

ISSN:0046-225X    

DOI:10.1039/b805644g

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Although the mechanisms of bioelectrocatalytic substrate oxidation processes in microbial fuel cells and, especially, of anodic electron transfer are of utmost importance for the performance of microbial fuel cells,1little is known, so far, of the nature of the underlying mechanisms. For this reason, research activities in this field have considerably intensified over the past years. Different concepts and mechanisms for electron transfer from the biocatalyst to the fuel cell anode have been proposed.2Thus, it can be distinguished between direct electron transfer (DET) and mediated electron transfer (MET) mechanisms. Examples of DET are, the electron transferviamembrane bound cytochromes (e.g., fromGeobacter sp.,3Rhodoferax ferrireducens)4orviaconductive bacterial pili (“nanowires”), as recently proposed forShewanella oneidensis MR-1.5MET, on the other hand, has been reported to occur either by primary metabolites (e.g., hydrogen, formate)6–8or by secondary metabolites, such as phenazine derivates9or quinones10or flavines, as has just been discovered forShewanella oneidensis MR-1.11Many of these proposed transfer mechanisms are of a putative nature and are controversially discussed. Often, the involved redox species are barely identified or understood. Some of the greatest challenges in their study are (i) the complexity of the microbial metabolism, (ii) the often extremely low concentrations of the involved redox species and (iii) the complex (electro)chemical nature of bacterial cultures and even of microbial cell membranes, which may contain several redox active species that do not necessarily contribute to the bioelectrocatalytic current flow.Cyclic voltammetry is a standard tool in electrochemistry12and has regularly been exploited to study and to characterize the electron transfer interactions between microorganisms or microbial biofilms and microbial fuel cell anodes.13–15In most of these publications the microorganisms have been studied either in their bioelectrocatalytically active state or in the inactive state. In this communication, we demonstrate that by applying cyclic voltammetry at different stages of microbial growth and metabolic activity, valuable information on the anodic electron transfer processes in microbial fuel cells can be gained. We also show that due to the complexity of the underlying processes, the simplified assumptions and models of anodic electron transfer are difficult to prove.The experiments described in this communication are based onGeobacter sulfurreducensbiofilm modified graphite electrodes, obtained when graphite electrodes are immersed in the inoculated medium and are poised at a constant anodic potential (here +0.3 V,vs.Ag/AgCl, sat. KCl), in order to form an electrochemically active bacterial biofilm.16Fig. 1illustrates such biofilm formation and bioelectrocatalytic current generation by means of a chronoamperometric experiment. The figure shows that 168 h after inoculation withG. sulfurreducens, the electrode reached a first activity maximum, followed by a decrease in the current, caused by substrate exhaustion. After replenishment of the bacterial medium at 210 h the current generation commenced again, reaching a maximum current density of about 75 μA cm−2, 265 h after inoculation.Chronoamperometric plot of the formation and the bioelectrocatalytic activity of aG. sulfurreducensbiofilm at a graphite electrode; semi-batch experiment, substrate: 10 mM L−1acetate.By recording cyclic voltammograms at different stages of biofilm formation and substrate availability (and thus different stages of current generation), valuable information on the electron transfer mechanism can be gained.Fig. 2Ashows a typical cyclic voltammogram of aG. sulfurreducensbiofilm modified electrode recorded at the first maximum of the bioelectrocatalytic activity (seeFig. 1). The figure shows a typical sigmoidal shape based on, at first glance, one single underlying redox centre. Yet, the first derivative of this voltammetric curve (Fig. 2B) reveals that the oxidative as well as reductive potential sweep possess two inflection points, reflected by two maxima in the derivative curve. This behaviour has also recently been reported for artificially immobilizedG. sulfurreducenscells.17(A) Cyclic voltammogram of a metabolizingG. sulfurreducensbiofilm. The voltammogram was recorded at maximum biofilm activity 168 h after the start of the chronoamperometric experiment (seeFig. 1). The scan rate was 5 mV s−1. (B) First derivatives of the voltammetric curve over the potential.A more detailed view on the electrochemical features of the biofilm are obtained at substrate depletion (in this experiment 190 h after inoculation; seeFig. 1). Under these non-catalytic conditions, the voltammetric behaviour strongly depends on the scan rate of the voltammetric experiment. As illustrated inFig. 3A, for scan rates above 20 mV s−1the voltammogram shows one major redox system, at a formal potential of −0.331 V. The shape of the voltammogram appears typical for bacteria, whose electrochemical activity is ascribed to outer membrane cytochromes.18,19At lower scan rates (seeFig. 3B), the simplicity of the voltammogram gives way to more complex behaviour. Now, four major redox systems can be distinguished, indicated as systems 1–4. Besides the two less evolved redox systems, at −0.515 V (Ef,1) and +0.059 V (Ef,4), the major redox system inFig. 3Ais split into two systems, with formal potentials of −0.376 V and −0.295 V (Ef,2andEf,3). Both systems possess a fine structure,i.e., they may consist of further redox processes. None of the redox peaks are obtained in the sterile culture medium or at a blank (biofilm-less) graphite electrode in the inoculated medium. Additionally, all peaks remain present when the medium is exchanged to a fresh, sterile medium, which demonstrates that all redox signals are caused by biofilm based redox compounds. A comparison ofFig. 2Band3Breveals that both redox processes associated with the formal potentialsEf,2andEf,3contribute to the bioelectrocatalytic anodic electron transfer (seeFig. 2), whereas, systems 1 and 4 appear electrocatalytically inactive. From the literature, it can be derived that electron transfer fromG. sulfurreducensto a solid electron acceptor is accomplished by outer membrane cytochromes, like OmcB, OmcE and OmcS.20–22For OmcB, a formal potential of −190 mVvs.SHE (corresponding to −387 mVvs.Ag/AgCl) has been reported.23This potential most likely corresponds to redox system 2 (−376 mV,vs.Ag/AgCl), found in our experiments. The presence of a second involved species at more positive potential now indicates a parallel, competing path, possiblyviaa similar membrane associated species.Cyclic voltammograms of aG. sulfurreducensbiofilm. The voltammograms were recorded in a substrate depleted culture medium (seeFig. 1). Scan rate: (A) 50 mV s−1, (B) 1 mV s−1.The anodic electron transfer ofG. sulfurreducensis considered to be an archetype for direct electron transfer.24From an electrochemical point of view, one would (for redox systems 2 and 3) expect typical thin film behaviour, manifested ase.g., a linear relationship of the peak current with the scan rate (i∼v) in cyclic voltammetry. Yet, how much the electrochemistry of living bacterial cells (and bacterial biofilms) may differ from that of isolated and immobilized redox enzymes or other “simple” redox compounds is illustrated inFig. 4. These experimental results show a scan rate dependence of the peak currents ofi∼v0.50(reduction peak)i∼v0.70(oxidation peak) as derived from the slope of the double logarithmic plot of the peak currentsversusscan rate (not shown). This dependence strongly indicates diffusion control, a result that cannot be explained by the current knowledge of DET. It can be speculated that the electron hopping between the heme centres of the bacterial outer membrane cytochromes, or electron transfer between electrochemically linked redox enzymes show diffusion characteristics in cyclic voltammetry.Absolute values of the peak currents of aG. sulfurreducensbiofilm electrode as a function of the scan rate. The voltammograms were recorded in a substrate depleted culture. (A) Oxidation peak, (B) reduction peak.At increasing bioelectrocatalytic activity, as is found after replenishment of the medium (seeFig. 1), the electrocatalytic curve (Fig. 5) did not reveal any fine structure. Here, the electrocatalytic wave possessed only a single inflexion point (and thus, only one single maximum in first the derivative ΔI/ΔE), at a potential of −0.335 V, corresponding to the arithmetic mean of systems 2 and 3 (Fig. 2andFig. 3). Yet, after subsequent substrate exhaustion, again a cyclic voltammogram equivalent to that shown inFig. 3Bwas obtained, which showed that both redox systems, 2 and 3, were still present as separate features. It can be assumed, that at high catalytic activity these features may be resolved only at extremely small scan rates.Cyclic voltammogram of a metabolizingG. sulfurreducensbiofilm. The voltammogram was recorded at a scan rate of 5 mV s−1268 h after the start of the chronoamperometric experiment (seeFig. 1). Inset: first derivatives of the voltammetric curve over the potential.The height of the bioelectrocatalytic current strongly correlates with the peak currents measured under substrate depletion. For example, the increase of the bioelectrocatalytic performance from 30 μA cm−2at 168 h by a factor of 2.5 to 76 μA cm−2at 265 h (see chronoamperometric curve,Fig. 1) was accompanied by a proportional increase (factor 2.5) of the peak currents measured by cyclic voltammetry at the subsequent periods of substrate depletion (at 190 h and 320 h, respectively). This finding indicates that the increase of the electrocatalytic biofilm activity is either caused by a growing cell density at the electrode surface, or by an increase in the number of membrane bound electron transfer proteins in the individual cells.In summary, we demonstrate the principle suitability of cyclic voltammetry to study biofilm based anodic electron transfer processes in microbial fuel cells. The experiments underline the advantages of performing voltammetric experiments at different stages of biofilm growth and activity. We also demonstrate that the complexity of microbial and bioelectrochemical processes make evaluation and interpretation of the voltammetric results a challenging endeavour. The results show that data gained by a purely electrochemical study needs to be combined with data from,e.g., molecular biology and metabolomics, in order to gain a better understanding of the mechanisms of cell–electrode interactions at the molecular level.

ISSN:0046-225X    

DOI:10.1039/b802363h

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

1.IntroductionFuel cell technology offers many advantages based on its high energy efficiency and scalability. One of the main difficulties for current fuel cell systems, especially those based on hydrogen, is low energy density. The use of solid carbon as a fuel in direct carbon fuel cells (DCFCs) would certainly remove this problem as the energy density of carbon is higher than alternative sources available both on a volumetric1and a mass density2basis. Carbon fuels can be obtained from coal, cracking of hydrocarbons, or pyrolysis of biomass. Biomass is a very attractive energy source in terms of renewable energy.The DCFC itself has a long history.3The first report relating to DCFC was in 1896 by Jacques.4This fuel cell used molten hydroxide as the electrolyte, operating in the temperature range of 400–500 °C. It, however, suffered from poisoning due to the build up of carbonate in the electrolyte. More recently, DCFCs using molten carbonate electrolyte have been reported,2,5–7as the stability, ionic conductivity and thermal properties of carbonates for molten carbonate fuel cell (MCFC) applications have been widely investigated since the 1960s.8The DCFCs with molten carbonate, however, require complex CO2management and cathode materials that are tolerant to molten carbonate at high temperature. The DCFCs based on solid oxide fuel cell (SOFC) technology have also been reported.9–12It is, however, difficult to get sufficient interaction between solid carbon fuel and solid electrode/electrolyte.The hybrid direct carbon fuel cell (HDCFC) with a binary electrolyte merges SOFC and MCFC technologies.13–15A solid oxide electrolyte is employed to separate the cathode and anode compartments while a molten carbonate electrolyte is utilised in the anode compartment. Oxygen is reduced to O2−at the cathode and transported across the solid electrolyte membrane to the carbon/carbonate slurry, where carbon is oxidised. The ideal overall reaction is the oxidation of carbon to carbon dioxide:1C + O2→ CO2This concept has some advantages in comparison to normal MCFC and SOFC. CO2circulation, which is required in normal MCFC, is not necessary. The cathode is not exposed to carbonate; therefore, the cathode materials already developed for SOFC applications are available. An enhancement of carbon oxidation by the carbonate slurry can be expected.We have already demonstrated this concept using tubular HDCFCs,14,15which are suitable for practical demonstrations, with some model fuels including biomass based fuel. It is, however, difficult to determine the exact nature of the anode reaction with those tubular cells because the active area of the electrolyte/electrode and the state of active sites are difficult to be certain of in a closed cell above temperatures where the carbonate has melted. The purpose of this study is to investigate, in detail, the electrochemistry of the oxidation of solid carbon in the carbon/carbonate slurry using a test cell with planar geometry. Investigating the HDCFC reaction over a specific anode with small area should contribute to better understanding of the anode reaction. A test cell was fabricated employing standard materials for SOFC: a yttria-stablilized zirconia (YSZ) electrolyte, Ni/YSZ cermet anode, and (La0.8Sr0.2)0.95MnO3(LSM) cathode. A eutectic carbonate mixture of lithium carbonate and potassium carbonate was utilised for the molten carbonate.

ISSN:0046-225X    

DOI:10.1039/b804785e

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

1.IntroductionThe worldwide growing demand for electrical energy cannot be compensated by renewable energies in the medium term. Thus, new and improved concepts are required in order to use the limited fossil energy carriers in a more efficient way. A promising development in this context could be direct carbon fuel cell systems (DCFC) that use cheap and abundant coal as fuel and additionally benefit from its high thermodynamic conversion efficiency. In a DCFC in contrast to all other gas-fed fuel cell systems the stored chemical energy of a solid fuel is directly converted into electricity. The overall cell reaction (eqn (1)) is based on the complete electrochemical oxidation of carbon to carbon dioxide (CO2) in a four-electron process.1C + O2→ CO2The thermodynamic efficiency slightly exceeds 100%—almost independent of conversion temperature (Fig. 1). This is due to a positive near-zero entropy change of the cell reaction (ΔS° = 2.9 J K−1mol−1)1. Furthermore, the anodic exhaust gas of a DCFC consists of almost pure CO2that can be captured and sequestered with less difficulty than fore.g.conventional thermal power plants. The latter is due to the fact that either the oxy-fuel process needs to be used or additional energy is required in order to separate nitrogen (N2) from the off-gas-stream. Another advantage of a DCFC is that the fuel utilisation can reach up to 100%, since a solid fuel is used. This is due to the fact that the reaction product, CO2, exists in a separate gas phase and thus does not influence activity of the solid carbon.Thermodynamic efficiency as function of temperature for different reactions; triangles: carbon oxidation; squares: hydrogen oxidation; circles: carbon monoxide oxidation.Recently, three different concepts of a DCFC based on different electrolytes have been discussed: molten carbonate, molten hydroxide or solid ceramic material YSZ (yttria-stabilised zirconia). It should be mentioned that performance data of different concepts, presented in the following, cannot be compared directly, because they strongly depend one.g.used carbon fuel material, operating temperature and the concept itself (anode/electrolyte material).The most developed DCFC-systems are those based on molten carbonate electrolyte.1–3Cooper and coworkers1from Lawrence Livermore National Laboratory (LLNL) achieved power densities in the range of 40 to 100 mW cm−2(0.8 V cell voltage, 800 °C) for different carbon materials. They operated a cell for 30 h at a power output of 27 mW cm−2(∼1 V cell voltage, 800 °C). Nevertheless, carbonate-based cells suffer from corrosion problems which limits the choice of materials.1,4–6Zecevicet al.7at Scientific Applications and Research Associates (SARA) developed a DCFC using a molten hydroxide electrolyte. Peak power densities up to 120–180 mW cm−2were observed and an average power output of 40 mW cm−2(0.3 V cell voltage, 630 °C) was achieved over 540 h. The most important problems with hydroxide cells are (i) corrosion of materials and (ii) degradation of the electrolyte due to formation of carbonates during carbon electrooxidation.4,5,7Another concept of a DCFC is based on the combination of solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) technology. Balachov and coworkers at SRI International demonstrated peak power densities of 10 to 110 mW cm−2(0.7 V cell voltage) in a temperature range of 700 to 950 °C, using different carbon containing materialse.g.plastic.4Irvine and coworkers5,8,9kept a cell running over ten hours at a power output of 10 mW cm−2(0.5 V cell voltage, 700 °C).Duskin and Gür at Direct Carbon Technologies (DCT) operate a DCFC combining SOFC and fluidized-bed technologies.10,11This concept is based on the gasification of carbon to carbon monoxide (CO). The CO isin situformedviaBoudouard reaction (CO2+ C → 2CO) and then electrochemically oxidized to CO2(CO + 0.5O2→ CO2) in a two electron process. For this cell reaction the thermodynamic efficiency is smaller as compared to thermodynamic efficiency of the direct carbon electrooxidation (Fig. 1). Peak power densities up to 140 mW cm−2(0.5 V cell voltage, 900 °C) have been achieved using this concept.10,11It has been reported6,7that the direct conversion of a solid fuel on a solid electrolyte (e.g.YSZ) suffers from low performance because of poor point-like contact between two solid materials. Additionally, carbon fuel needs to be in close contact with the anode material and the active area is limited to the geometric surface area. But as particle density in solid carbon is about 4 orders of magnitude higher than particle density of a gaseous fuel, a smaller active contact area can even be overcompensated, although for gaseous fuels a porous electrode shows typically an at least 3 orders of magnitude larger surface area12compared to the flat electrode.Solid oxide fuel cells (SOFC) show high fuel flexibility. As oxygen ions are transported through the electrolyte a SOFC can theoretically be operated on any combustible fuel, thus also on carbon. In principle, a DCFC with a solid electrolyte may be advantageous for fundamental investigations because it does not suffer from corrosion or degradation of the electrolyte. Compared to other DCFCs, a SOFC based system is rather simple, as no recycle loops for CO2or water management solutions are needed. Furthermore, SOFC systems allow implementation of catalytically active materials within modified anodes in order to enhance carbon electrooxidation.

ISSN:0046-225X    

DOI:10.1039/b916995d

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Broader contextDye-sensitized solar cells transforming solar light into electricity can serve to alleviate the shortage of fossil fuels, can contribute to the reduction of CO2emissions and in the long term can form part of the pool of renewable, sustainable energy resources. However, there are still important problems with this type of cell that need to be solved in order to facilitate the widespread application of this technology. In the current state of the art, the maximum overall efficiency for the conversion of solar energy into electrical power is about 10% and there is still much room for improvement. In this context, most of the dye-sensitized solar cells are based on conventional titania nanoparticles as the active semiconductor. The efficiency of solar cells based on titania nanoparticles has been continuously improved in a large number of contributions, but, in spite of the intensive research in this area, the increment in the efficiency is very minor. The approach described in our contribution is to develop novel semiconductor materials that derive from layered hydrotalcites as semiconductors. The synthesis of these materials is simple and advantageous because they can be reliably prepared in large quantities by precipitation from aqueous solutions. In this paper we show that the efficiency of solar cells derived from hydrotalcites is similar to that of titania. Considering the wide range of hydrotalcites that can be prepared with various di- and trivalent metals, our report can serve to develop new semiconductors that eventually can overcome the efficiency of titania nanoparticles.

ISSN:0046-225X    

DOI:10.1039/b916515k

出版商:RSC    

年代:2009

数据来源: RSC