Microbial fuel cell

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Microbial_fuel_cell 

Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system also known as micro fuel cell that generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or electron donor) on the anode to oxidized compounds such as oxygen (also known as oxidizing agent or electron acceptor) on the cathode through an external electrical circuit. MFCs produce electricity by using the electrons derived from biochemical reactions catalyzed by bacteria. MFCs can be grouped into two general categories: mediated and unmediated. The first MFCs, demonstrated in the early 20th century, used a mediator: a chemical that transfers electrons from the bacteria in the cell to the anode. Unmediated MFCs emerged in the 1970s; in this type of MFC the bacteria typically have electrochemically active redox proteins such as cytochromes on their outer membrane that can transfer electrons directly to the anode. In the 21st century MFCs have started to find commercial use in wastewater treatment.

History

The idea of using microbes to produce electricity was conceived in the early twentieth century. Michael Cressé Potter initiated the subject in 1911. Potter managed to generate electricity from Saccharomyces cerevisiae, but the work received little coverage. In 1931, Barnett Cohen created microbial half fuel cells that, when connected in series, were capable of producing over 35 volts with only a current of 2 milliamps.

A study by DelDuca et al. used hydrogen produced by the fermentation of glucose by Clostridium butyricum as the reactant at the anode of a hydrogen and air fuel cell. Though the cell functioned, it was unreliable owing to the unstable nature of hydrogen production by the micro-organisms. This issue was resolved by Suzuki et al. in 1976, who produced a successful MFC design a year later.

In the late 1970s, little was understood about how microbial fuel cells functioned. The concept was studied by Robin M. Allen and later by H. Peter Bennetto. People saw the fuel cell as a possible method for the generation of electricity for developing countries. Bennetto's work, starting in the early 1980s, helped build an understanding of how fuel cells operate and he was seen by many as the topic's foremost authority.

In May 2007, the University of Queensland, Australia completed a prototype MFC as a cooperative effort with Foster's Brewing. The prototype, a 10 L design, converted brewery wastewater into carbon dioxide, clean water and electricity. The group had plans to create a pilot-scale model for an upcoming international bio-energy conference.

Definition

A microbial fuel cell (MFC) is a device that converts chemical energy to electrical energy by the action of microorganisms. These electrochemical cells are constructed using either a bioanode and/or a biocathode. Most MFCs contain a membrane to separate the compartments of the anode (where oxidation takes place) and the cathode (where reduction takes place). The electrons produced during oxidation are transferred directly to an electrode or to a redox mediator species. The electron flux is moved to the cathode. The charge balance of the system is maintained by ionic movement inside the cell, usually across an ionic membrane. Most MFCs use an organic electron donor that is oxidized to produce CO₂, protons, and electrons. Other electron donors have been reported, such as sulfur compounds or hydrogen. The cathode reaction uses a variety of electron acceptors, most often oxygen (O₂). Other electron acceptors studied include metal recovery by reduction, water to hydrogen, nitrate reduction, and sulfate reduction.

Applications

Power generation

MFCs are attractive for power generation applications that require only low power, but where replacing batteries may be impractical, such as wireless sensor networks. Wireless sensors powered by microbial fuel cells can then for example be used for remote monitoring (conservation).

Virtually any organic material could be used to feed the fuel cell, including coupling cells to wastewater treatment plants. Chemical process wastewater and synthetic wastewater have been used to produce bioelectricity in dual- and single-chamber mediator less MFCs (uncoated graphite electrodes).

Higher power production was observed with a biofilm-covered graphite anode. Fuel cell emissions are well under regulatory limits. MFCs convert energy more efficiently than standard internal combustion engines, which are limited by the Carnot efficiency. In theory, an MFC is capable of energy efficiency far beyond 50%. Rozendal produced hydrogen with 8 times less energy input than conventional hydrogen production technologies.

Moreover, MFCs can also work at a smaller scale. Electrodes in some cases need only be 7 μm thick by 2 cm long, such that an MFC can replace a battery. It provides a renewable form of energy and does not need to be recharged.

MFCs operate well in mild conditions, 20 °C to 40 °C and at pH of around 7 but lack the stability required for long-term medical applications such as in pacemakers.

Power stations can be based on aquatic plants such as algae. If sited adjacent to an existing power system, the MFC system can share its electricity lines.

Education

Soil-based microbial fuel cells serve as educational tools, as they encompass multiple scientific disciplines (microbiology, geochemistry, electrical engineering, etc.) and can be made using commonly available materials, such as soils and items from the refrigerator. Kits for home science projects and classrooms are available. One example of microbial fuel cells being used in the classroom is in the IBET (Integrated Biology, English, and Technology) curriculum for Thomas Jefferson High School for Science and Technology. Several educational videos and articles are also available on the International Society for Microbial Electrochemistry and Technology (ISMET Society).

Biosensor

Main article: Biosensor

The current generated from a microbial fuel cell is directly proportional to the organic-matter content of wastewater used as the fuel. MFCs can measure the solute concentration of wastewater (i.e., as a biosensor).

Wastewater is commonly assessed for its biochemical oxygen demand (BOD) values. BOD values are determined by incubating samples for 5 days with proper source of microbes, usually activated sludge collected from wastewater plants.

An MFC-type BOD sensor can provide real-time BOD values. Oxygen and nitrate are interfering preferred electron acceptors over the anode, reducing current generation from an MFC. Therefore, MFC BOD sensors underestimate BOD values in the presence of these electron acceptors. This can be avoided by inhibiting aerobic and nitrate respiration in the MFC using terminal oxidase inhibitors such as cyanide and azide. Such BOD sensors are commercially available.

The United States Navy is considering microbial fuel cells for environmental sensors. The use of microbial fuel cells to power environmental sensors could provide power for longer periods and enable the collection and retrieval of undersea data without a wired infrastructure. The energy created by these fuel cells is enough to sustain the sensors after an initial startup time. Due to undersea conditions (high salt concentrations, fluctuating temperatures and limited nutrient supply), the Navy may deploy MFCs with a mixture of salt-tolerant microorganisms that would allow for a more complete utilization of available nutrients. Shewanella oneidensis is their primary candidate, but other heat- and cold-tolerant Shewanella spp may also be included.

A first self-powered and autonomous BOD/COD biosensor has been developed and enables detection of organic contaminants in freshwater. The sensor relies only on power produced by MFCs and operates continuously without maintenance. It turns on the alarm to inform about contamination level: the increased frequency of the signal warns about a higher contamination level, while a low frequency informs about a low contamination level.

Biorecovery

In 2010, A. ter Heijne et al. constructed a device capable of producing electricity and reducing Cu²⁺ ions to copper metal.

Microbial electrolysis cells have been demonstrated to produce hydrogen.

Wastewater treatment

MFCs are used in water treatment to harvest energy utilizing anaerobic digestion. The process can also reduce pathogens. However, it requires temperatures upwards of 30 degrees C and requires an extra step in order to convert biogas to electricity. Spiral spacers may be used to increase electricity generation by creating a helical flow in the MFC. Scaling MFCs is a challenge because of the power output challenges of a larger surface area.

Types

Mediated

Most microbial cells are electrochemically inactive. Electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, pyocyanin, methyl viologen, methyl blue, humic acid, and neutral red. Most available mediators are expensive and toxic.

Mediator-free

A plant microbial fuel cell (PMFC)

Mediator-free microbial fuel cells use electrochemically active bacteria such as Shewanella putrefaciens and Aeromonas hydrophila to transfer electrons directly from the bacterial respiratory enzyme to the electrode. Some bacteria are able to transfer their electron production via the pili on their external membrane. Mediator-free MFCs are less well characterized, such as the strain of bacteria used in the system, type of ion-exchange membrane and system conditions (temperature, pH, etc.)

Mediator-free microbial fuel cells can run on wastewater and derive energy directly from certain plants and O₂. This configuration is known as a plant microbial fuel cell. Possible plants include reed sweetgrass, cordgrass, rice, tomatoes, lupines and algae. Given that the power is obtained using living plants (in situ-energy production), this variant can provide ecological advantages.

Microbial electrolysis

Main article: Microbial electrolysis cell

One variation of the mediator-less MFC is the microbial electrolysis cell (MEC). While MFCs produce electric current by the bacterial decomposition of organic compounds in water, MECs partially reverse the process to generate hydrogen or methane by applying a voltage to bacteria. This supplements the voltage generated by the microbial decomposition of organics, leading to the electrolysis of water or methane production. A complete reversal of the MFC principle is found in microbial electrosynthesis, in which carbon dioxide is reduced by bacteria using an external electric current to form multi-carbon organic compounds.

Soil-based

A soil-based MFC

Soil-based microbial fuel cells adhere to the basic MFC principles, whereby soil acts as the nutrient-rich anodic media, the inoculum and the proton exchange membrane (PEM). The anode is placed at a particular depth within the soil, while the cathode rests on top the soil and is exposed to air.

Soils naturally teem with diverse microbes, including electrogenic bacteria needed for MFCs, and are full of complex sugars and other nutrients that have accumulated from plant and animal material decay. Moreover, the aerobic (oxygen consuming) microbes present in the soil act as an oxygen filter, much like the expensive PEM materials used in laboratory MFC systems, which cause the redox potential of the soil to decrease with greater depth. Soil-based MFCs are becoming popular educational tools for science classrooms.

Sediment microbial fuel cells (SMFCs) have been applied for wastewater treatment. Simple SMFCs can generate energy while decontaminating wastewater. Most such SMFCs contain plants to mimic constructed wetlands. By 2015 SMFC tests had reached more than 150 L.

In 2015 researchers announced an SMFC application that extracts energy and charges a battery. Salts dissociate into positively and negatively charged ions in water and move and adhere to the respective negative and positive electrodes, charging the battery and making it possible to remove the salt effecting microbial capacitive desalination. The microbes produce more energy than is required for the desalination process. In 2020, a European research project achieved the treatment of seawater into fresh water for human consumption with an energy consumption around 0.5 kWh/m3, which represents an 85% reduction in current energy consumption respect state of the art desalination technologies. Furthermore, the biological process from which the energy is obtained simultaneously purifies residual water for its discharge in the environment or reuse in agricultural/industrial uses. This has been achieved in the desalination innovation center that Aqualia has opened in Denia, Spain early 2020.^[56]

Phototrophic biofilm

Phototrophic biofilm MFCs (ner) use a phototrophic biofilm anode containing photosynthetic microorganism such as chlorophyta and candyanophyta. They carry out photosynthesis and thus produce organic metabolites and donate electrons.

One study found that PBMFCs display a power density sufficient for practical applications.

The sub-category of phototrophic MFCs that use purely oxygenic photosynthetic material at the anode are sometimes called biological photovoltaic systems.

Nanoporous membrane

The United States Naval Research Laboratory developed nanoporous membrane microbial fuel cells that use a non-PEM to generate passive diffusion within the cell. The membrane is a nonporous polymer filter (nylon, cellulose, or polycarbonate). It offers comparable power densities to Nafion (a well-known PEM) with greater durability. Porous membranes allow passive diffusion thereby reducing the necessary power supplied to the MFC in order to keep the PEM active and increasing the total energy output.

MFCs that do not use a membrane can deploy anaerobic bacteria in aerobic environments. However, membrane-less MFCs experience cathode contamination by the indigenous bacteria and the power-supplying microbe. The novel passive diffusion of nanoporous membranes can achieve the benefits of a membrane-less MFC without worry of cathode contamination.Nanoporous membranes are also 11 times cheaper than Nafion (Nafion-117, $0.22/cm² vs. polycarbonate, <$0.02/cm²).

Ceramic membrane

PEM membranes can be replaced with ceramic materials. Ceramic membrane costs can be as low as $5.66/m². The macroporous structure of ceramic membranes allows for good transport of ionic species.

The materials that have been successfully employed in ceramic MFCs are earthenware, alumina, mullite, pyrophyllite, and terracotta.

Generation process

When microorganisms consume a substance such as sugar in aerobic conditions, they produce carbon dioxide and water. However, when oxygen is not present, they may produce carbon dioxide, hydrons (hydrogen ions), and electrons, as described below for sucrose:

C12H22O11 + 13H2O → 12CO2 + 48H+ + 48e−

Eqt. 1

Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and channel electrons produced. The mediator crosses the outer cell lipid membranes and bacterial outer membrane; then, it begins to liberate electrons from the electron transport chain that normally would be taken up by oxygen or other intermediates.

The now-reduced mediator exits the cell laden with electrons that it transfers to an electrode; this electrode becomes the anode. The release of the electrons recycles the mediator to its original oxidized state, ready to repeat the process. This can happen only under anaerobic conditions; if oxygen is present, it will collect the electrons, as it has more free energy to release.

Certain bacteria can circumvent the use of inorganic mediators by making use of special electron transport pathways known collectively as extracellular electron transfer (EET). EET pathways allow the microbe to directly reduce compounds outside of the cell, and can be used to enable direct electrochemical communication with the anode.

In MFC operation, the anode is the terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore, the microbial activity is strongly dependent on the anode's redox potential. A Michaelis–Menten curve was obtained between the anodic potential and the power output of an acetate-driven MFC. A critical anodic potential seems to provide maximum power output.

Potential mediators include natural red, methylene blue, thionine, and resorufin.

Organisms capable of producing an electric current are termed exoelectrogens. In order to turn this current into usable electricity, exoelectrogens have to be accommodated in a fuel cell.

The mediator and a micro-organism such as yeast, are mixed together in a solution to which is added a substrate such as glucose. This mixture is placed in a sealed chamber to prevent oxygen from entering, thus forcing the micro-organism to undertake anaerobic respiration. An electrode is placed in the solution to act as the anode.

In the second chamber of the MFC is another solution and the positively charged cathode. It is the equivalent of the oxygen sink at the end of the electron transport chain, external to the biological cell. The solution is an oxidizing agent that picks up the electrons at the cathode. As with the electron chain in the yeast cell, this could be a variety of molecules such as oxygen, although a more convenient option is a solid oxidizing agent, which requires less volume.

Connecting the two electrodes is a wire (or other electrically conductive path). Completing the circuit and connecting the two chambers is a salt bridge or ion-exchange membrane. This last feature allows the protons produced, as described in Eqt. 1, to pass from the anode chamber to the cathode chamber.

The reduced mediator carries electrons from the cell to the electrode. Here the mediator is oxidized as it deposits the electrons. These then flow across the wire to the second electrode, which acts as an electron sink. From here they pass to an oxidizing material. Also the hydrogen ions/protons are moved from the anode to the cathode via a proton exchange membrane such as Nafion. They will move across to the lower concentration gradient and be combined with the oxygen but to do this they need an electron. This generates current and the hydrogen is used sustaining the concentration gradient.

Algal biomass has been observed to give high energy when used as the substrate in microbial fuel cell.

Applications in environmental remediation

Microbial fuel cells (MFCs) have emerged as promising tools for environmental remediation due to their unique ability to utilize the metabolic activities of microorganisms for both electricity generation and pollutant degradation. MFCs find applications across diverse contexts in environmental remediation. One primary application is in bioremediation, where the electroactive microorganisms on the MFC anode actively participate in the breakdown of organic pollutants, providing a sustainable and efficient method for pollutant removal. Moreover, MFCs play a significant role in wastewater treatment by simultaneously generating electricity and enhancing water quality through the microbial degradation of contaminants. These fuel cells can be deployed in situ, allowing for continuous and autonomous remediation in contaminated sites. Furthermore, their versatility extends to sediment microbial fuel cells (SMFCs), which are capable of removing heavy metals and nutrients from sediments. By integrating MFCs with sensors, they enable remote environmental monitoring in challenging locations. The applications of microbial fuel cells in environmental remediation highlight their potential to convert pollutants into a renewable energy source while actively contributing to the restoration and preservation of ecosystems.

Challenges and advances

Microbial fuel cells (MFCs) offer significant potential as sustainable and innovative technologies, but they are not without their challenges. One major obstacle lies in the optimization of MFC performance, which remains a complex task due to various factors including microbial diversity, electrode materials, and reactor design. The development of cost-effective and long-lasting electrode materials presents another hurdle, as it directly affects the economic viability of MFCs on a larger scale. Furthermore, the scaling up of MFCs for practical applications poses engineering and logistical challenges. Nonetheless, ongoing research in microbial fuel cell technology continues to address these obstacles. Scientists are actively exploring new electrode materials, enhancing microbial communities to improve efficiency, and optimizing reactor configurations. Moreover, advancements in synthetic biology and genetic engineering have opened up possibilities for designing custom microbes with enhanced electron transfer capabilities, pushing the boundaries of MFC performance. Collaborative efforts between multidisciplinary fields are also contributing to a deeper understanding of MFC mechanisms and expanding their potential applications in areas such as wastewater treatment, environmental remediation, and sustainable energy production.

Carbon quantum dot

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Carbon_quantum_dot 

Carbon quantum dots also commonly called carbon nano dots or simply carbon dots (abbreviated as CQDs, C-dots or CDs) are carbon nanoparticles which are less than 10 nm in size and have some form of surface passivation.

History

CQDs were first discovered by Xu et al. in 2004 accidentally during the purification of single-walled carbon nanotubes. This discovery triggered extensive studies to exploit the fluorescence properties of CQDs.

As a new class of fluorescent carbon nanomaterials, CQDs possess the attractive properties of high stability, good conductivity, low toxicity, environmental friendliness, simple synthetic routes as well as comparable optical properties to quantum dots. Carbon quantum dots have been extensively investigated especially due to their strong and tunable fluorescence emission properties, which enable their applications in biomedicine, optronics, catalysis, and sensing. In most cases CQDs emits the light in a band of about several hundred nanometers in visible or near-infrared range, however it was also reported on broadband CQDs covering the spectrum from 800 to 1600 nm.

Carbon dots prepared from different precursors: urea, alanine and sucrose (made by Paliienko Konstantin)

The fundamental mechanisms responsible of the fluorescence capability of CQDs are very debated. Some authors have provided evidence of size-dependent fluorescence properties, suggesting that the emission arises from electronic transitions with the core of the dots, influenced by quantum confinement effects, whereas other works, including single particle measurements, have rather attributed the fluorescence to recombination of surface-trapped charges, or proposed a form of coupling between core and surface electronic states. The excitation-dependent fluorescence of CQDs, leading to their characteristic emission tunability, has been mostly linked to the inhomogeneous distribution of their emission characteristics, due to polydispersity, although some works have explained it as a violation of Kasha's rule arising from an unusually slow solvent relaxation.

Properties

The structures and components of CQDs determine their diverse properties. Many carboxyl moieties on the CQD surface impart excellent solubility in water and biocompatibility. Such surface moieties enable CQDs to serve as proton conducting nanoparticles. CQDs are also suitable for chemical modification and surface passivation with various organic, polymeric, inorganic or biological materials. By surface passivation, the fluorescence properties as well as physical properties of CQDs are enhanced. Recently, it has been discovered that amine and hydroxamic acid functionalized CD can produce tricolor (green, yellow and red) emission when introduced with different pH environment and this tricolor emission can be preserved in ORMOSIL film matrix.

Synthesis

Synthetic methods for CQDs are roughly divided into two categories, "top-down" and "bottom-up" routes. These can be achieved via chemical, electrochemical or physical techniques. The CQDs obtained could be optimized during preparation or post-treatment. Modification of CQDs is also very important to get good surface properties which are essential for solubility and selected applications.

Synthetic methods

"Top-down" synthetic route refers to breaking down larger carbon structures such as graphite, carbon nanotubes, and nanodiamonds into CQDs using laser ablation, arc discharge, and electrochemical techniques. For example, Zhou et al. first applied electrochemical method into synthesis of CQDs. They grew multi-walled carbon nanotubes on a carbon paper, then they inserted the carbon paper into an electrochemical cell containing supporting electrolyte including degassed acetonitrile and 0.1 M tetrabutyl ammonium perchlorate. Later, they applied this method in cutting CNTs or assembling CNTs into functional patterns which demonstrated the versatile callability of this method in carbon nanostructure manipulations.

"Bottom-up" synthetic route involves synthesizing CQDs from small precursors such as carbohydrates, citrate, and polymer-silica nanocomposites through hydrothermal/solvothermal treatment, supported synthetic, and microwave synthetic routes. For instance, Zhu et al. described a simple method of preparing CQDs by heating a solution of poly(ethylene glycol) (PEG) and saccharide in 500 W microwave oven for 2 to 10 min. By varying the molar ratio of citric acid and urea (two common precursor molecules) of the mixture that is subjected to pyrolysis, a number of distinct fluorescent materials in both liquid and solid state can be synthesised, predominantly comprising Carbon dots with embedded fluorophores. Also a laser-induced thermal shock method is exploited for synthesis ultra-broadband QCDs. Recently, green synthetic approaches have also been employed for fabrication of CQDs. Care must be taken to separate the "bottom-up" carbon dots from fluorescent byproducts such as small molecules or polyester condensates by using multiple dialysis and chromatography separation methods.

Size control

In addition to post-treatment, controlling the size of CQDs during the preparing process is also widely used. For instance, Zhu et al. reported hydrophilic CQDs through impregnation of citric acid precursor. After pyrolyzing CQDs at 300 °C for 2 hours in air, then removing silica, followed by dialysis, they prepared CQDs with a uniform size of 1.5–2.5 nm which showed low toxicity, excellent luminescence, good photostability, and up-conversion properties.

Modification

Being a new type of fluorescent nanoparticles, applications of CQD lie in the field of bioimaging and biosensing due to their biological and environmental friendly composition and excellent biocompatibility. In order to survive the competition with conventional semiconductor quantum dots, a high quantum yield should be achieved. Although a good example of CQDs with ~80% quantum yield was synthesized, most of the quantum dots synthesized have a quantum yield below 10% so far. Surface-passivation and doping methods for modifications are usually applied for improving quantum yield.

To prevent surfaces of CQDs from being polluted by their environment, surface passivation is performed to alleviate the detrimental influence of surface contamination on their optical properties. A thin insulating layer is formed to achieve surface passivation via the attachment of polymeric materials on CQDs surface treated by acid.

In addition to surface passivation, doping is also a common method used to tune the properties of CQDs. Various doping methods with elements such as N, S, P have been demonstrated for tuning the properties of CQDs, among which N doping is the most common way due to its great ability in improving the photo luminescence emissions. The mechanisms by which Nitrogen doping enhances the fluorescence quantum yield of CQDs, as well as the structure of heavily N-doped CDs, are very debated issues in the literature. Zhou et al. applied XANES and XEOL in investigating the electronic structure and luminescence mechanism in their electrochemically produced carbon QDS and found that N doping is almost certainly responsible for the blue luminescence. Synthesis of new nanocomposites based on CDs have been reported with unusual properties. For example, a nanocomposite has been designed by using of CDs and magnetic Fe₃O₄ nanoparticles as precursors with nanozyme activity.

Post synthesis electrochemical etching results in dramatic changes in GQDs size and fluorescence intensity.

Applications

CQDs with unique properties have great potential in biomedicine, optronics, catalysis and sensors

Bioimaging

CQDs can be used for bioimaging due to their fluorescence emissions and biocompatibility. By injecting solvents containing CQDs into a living body, images in vivo can be obtained for detection or diagnosis purposes. One example is that organic dye-conjugated CQDs could be used as an effective fluorescent probes for H₂S. The presence of H₂S could tune the blue emission of the organic dye-conjugated CQDs to green. So by using a fluorescence microscope, the organic dye-conjugated CQDs were able to visualize changes in physiologically relevant levels of H₂S. Another example can be dual-mode bioimaging using their highly accessible surface functional groups to conjugate them via EDC-NHS chemistry. Saladino et al. demonstrated the concept using MW-assisted synthesized nitrogen-doped excitation-independent CQDs. These were conjugated with rhodium nanoparticles – X-ray fluorescence contrast agents – leading to dual-mode nanohybrids with both optical and X-ray fluorescent properties. Moreover, the conjugation process not only accounts for dual-mode bioimaging but also passivates the rhodium nanoparticle surface, resulting in reduced cytotoxicity.

Sensing

CQDs were also applied in biosensing as biosensor carriers for their flexibility in modification, high solubility in water, nontoxicity, good photostability, and excellent biocompatibility. The biosensors based on CQD and CQs-based materials could be used for visual monitoring of cellular copper, glucose, pH, trace levels of H₂O₂ and nucleic acid. A general example is about nucleic acid lateral flow assays. The discriminating tags on the amplicons are recognized by their respective antibodies and fluorescence signals provided by the attached CQDs. More generally, the fluorescence of CQDs efficiently responds to pH, local polarity, and to the presence of metal ions in solution, which further expands their potential for nanosensing applications, for instance in the analysis of pollutants.

Drug delivery

The nontoxicity and biocompatibility of CQDs enable them with broad applications in biomedicine as drug carriers, fluorescent tracers as well as controlling drug release. This is exemplified by the use of CQDs as photosensitizers in photodynamic therapy to destroy cancer cells.

Catalysis

The flexibility of functionalization with various groups CQDs makes them possible to absorb lights of different wavelengths, which offers good opportunities for applications in photocatalysis. CQDs-modified P25 TiO₂ composites exhibited improved photocatalytic H2 evolution under irradiation with UV-Vis. The CQDs serve as a reservoir for electrons to improve the efficiency of separating of the electron-hole pairs of P25. In the recent times, metal-free CQDs have been found to improve the kinetics of hydrogen evolution reaction (HER), making CQDs a sustainable choice for catalysis.

Optronics

CQDs possess the potential in serving as materials for dye-sensitized solar cells, organic solar cells, supercapacitor, and light emitting devices. CQDs can be used as photosensitizer in dye-sensitized solar cells and the photoelectric conversion efficiency is significantly enhanced. CQD incorporated hybrid silica based sol can be used as transparent Fluorescent paint,

Rocket fuels

Recently, CQDs have been employed in hybrid rocket fuels.

Fingerprint recovery

CQDs are used for the enhancement of latent fingerprints.

Electrocatalyst

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Electrocatalyst

A platinum cathode electrocatalyst's stability being measured by chemist Xiaoping Wang

An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.

Background and theory

An electrocatalyst lowers the activation energy required for an electrochemical reaction. Some electrocatalysts change the potential at which oxidation and reduction processes occur. In other cases, an electrocatalyst can impart selectivity by favoring specific chemical interaction at an electrode surface. Given that electrochemical reactions occur when electrons are passed from one chemical species to another, favorable interactions at an electrode surface increase the likelihood of electrochemical transformations occurring, thus reducing the potential required to achieve these transformations.

Electrocatalysts can be evaluated according to activity, stability, and selectivity. The activity of electrocatalysts can be assessed quantitatively by the current density is generated, and therefore how fast a reaction is taking place, for a given applied potential. This relationship is described with the Tafel equation. In assessing the stability of electrocatalysts, the a key parameter is turnover number (TON). The selectivity of electrocatalysts refers to the product distribution. Selectivity can be quantitatively assessed through a selectivity coefficient, which compares the response of the material to the desired analyte or substrate with the response to other interferents.

In many electrochemical systems, including galvanic cells, fuel cells and various forms of electrolytic cells, a drawback is that they can suffer from high activation barriers. The energy diverted to overcome these activation barriers is transformed into heat. In most exothermic combustion reactions this heat would simply propagate the reaction catalytically. In a redox reaction, this heat is a useless byproduct lost to the system. The extra energy required to overcome kinetic barriers is usually described in terms of low faradaic efficiency and high overpotentials. In these systems, each of the two electrodes and its associated half-cell would require its own specialized electrocatalyst.

Half-reactions involving multiple steps, multiple electron transfers, and the evolution or consumption of gases in their overall chemical transformations, will often have considerable kinetic barriers. Furthermore, there is often more than one possible reaction at the surface of an electrode. For example, during the electrolysis of water, the anode can oxidize water through a two electron process to hydrogen peroxide or a four electron process to oxygen. The presence of an electrocatalyst could facilitate either of the reaction pathways.

Types of electrocatalyst materials, including homogeneous and heterogeneous electrocatalysts.

Homogeneous electrocatalysts

A homogeneous electrocatalyst is one that is present in the same phase of matter as the reactants, for example, a water-soluble coordination complex catalyzing an electrochemical conversion in solution. This technology is not practiced commercially, but is of research interest.

Synthetic coordination complexes

Many coordination complexes catalyze electrochemical reactions, although few have achieved commercial success. Well investigated processes include the hydrogen evolution reaction.

Electrification of catalytic processes

There is much interest in replacing traditional chemical catalysis with electrocatalysis. In such a scheme electrons supplied by an electrode are reagents. The topic is a theme within the area of green energy, because the electrons can be sourced from renewable resources. Several conversions that use of hydrogen gas could be transformed into electrochemical processes that use protons. This technology remains economically noncompetitive.

Another example is found in the area of nitrogen fixation. The traditional Haber-Bosch process produces ammonia by hydrogenation of nitrogen gas:

N₂ + 3 H₂ → 2 NH₃

In the electrified version, the hydrogen is provided in the form of protons and electrons:

N₂ + 6 H⁺ + 6 e⁻ → 2 NH₃

The ammonia represents an energy source since it is combustable. In this way electrification can be seen as a means for energy storage.

Another process attracting much effort is the electrochemical reduction of carbon dioxide.

Enzymes

Some enzymes can function as electrocatalysts. Nitrogenase, an enzyme that contains a MoFe cluster, can be leveraged to fix atmospheric nitrogen, i.e. convert nitrogen gas into molecules such as ammonia. Immobilizing the protein onto an electrode surface and employing an electron mediator greatly improves the efficiency of this process. The effectiveness of bioelectrocatalysts generally depends on the ease of electron transport between the active site of the enzyme and the electrode surface. Other enzymes provide insight for the development of synthetic catalysts. For example, formate dehydrogenase, a nickel-containing enzyme, has inspired the development of synthetic complexes with similar molecular structures for use in CO₂ reduction. Microbial fuel cells are another way that biological systems can be leveraged for electrocatalytic applications. Microbial-based systems leverage the metabolic pathways of an entire organism, rather than the activity of a specific enzyme, meaning that they can catalyze a broad range of chemical reactions. Microbial fuel cells can derive current from the oxidation of substrates such as glucose, and be leveraged for processes such as CO₂ reduction.

Heterogeneous electrocatalysts

A heterogeneous electrocatalyst is one that is present in a different phase of matter from the reactants, for example, a solid surface catalyzing a reaction in solution. Different types of heterogeneous electrocatalyst materials are shown above in green. Since heterogeneous electrocatalytic reactions need an electron transfer between the solid catalyst (typically a metal) and the electrolyte, which can be a liquid solution but also a polymer or a ceramic capable of ionic conduction, the reaction kinetics depend on both the catalyst and the electrolyte as well as on the interface between them. The nature of the electrocatalyst surface determines some properties of the reaction including rate and selectivity.

Bulk materials

Electrocatalysis can occur at the surface of some bulk materials, such as platinum metal. Bulk metal surfaces of gold have been employed for the decomposition methanol for hydrogen production. Water electrolysis is conventionally conducted at inert bulk metal electrodes such as platinum or iridium. The activity of an electrocatalyst can be tuned with a chemical modification, commonly obtained by alloying two or more metals. This is due to a change in the electronic structure, especially in the d band which is considered to be responsible for the catalytic properties of noble metals.

Nanomaterials

Nanoparticles

A variety of nanoparticle materials have been demonstrated to promote various electrochemical reactions, although none have been commercialized. These catalysts can be tuned with respect to their size and shape, as well as the surface strain.

Electronic density difference of a Cl atom adsorbed on a Cu(111) surface obtained with a DFT simulation.

Also, higher reaction rates can be achieved by precisely controlling the arrangement of surface atoms: indeed, in nanometric systems, the number of available reaction sites is a better parameter than the exposed surface area in order to estimate electrocatalytic activity. Sites are the positions where the reaction could take place; the likelihood of a reaction to occur in a certain site depends on the electronic structure of the catalyst, which determines the adsorption energy of the reactants together with many other variables not yet fully clarified.

According to the TSK model, the catalyst surface atoms can be classified as terrace, step or kink atoms according to their position, each characterized by a different coordination number. In principle, atoms with lower coordination number (kinks and defects) tend to be more reactive and therefore adsorb the reactants more easily: this may promote kinetics but could also depress it if the adsorbing species isn't the reactant, thus inactivating the catalyst. Advances in nanotechnology make it possible to surface engineer the catalyst so that just some desired crystal planes are exposed to reactants, maximizing the number of effective reaction sites for the desired reaction.

To date, a generalized surface dependence mechanism cannot be formulated since every surface effect is strongly reaction-specific. A few classifications of reactions based on their surface dependence have been proposed but there are still many exceptions that do not fall into them.

Particle size effect

An example of a particle-size effect: the number of reaction sites of different kinds depends on the size of the particle. In this four FCC nanoparticles model, the kink site between (111) and (100) planes (coordination number 6, represented by golden spheres) is 24 for all of the four different nanoparticles, while the number of other surface sites varies.

The interest in reducing as much as possible the costs of the catalyst for electrochemical processes led to the use of fine catalyst powders since the specific surface area increases as the average particle size decreases. For instance, most common PEM fuel cells and electrolyzers design is based on a polymeric membrane charged in platinum nanoparticles as an electrocatalyst (the so-called platinum black).

Although the surface area to volume ratio is commonly considered to be the main parameter relating electrocatalyst size with its activity, to understand the particle-size effect, several more phenomena need to be taken into account:

• Equilibrium shape: for any given size of a nanoparticle there is an equilibrium shape which exactly determines its crystal planes
• Reaction sites relative number: a given size for a nanoparticle corresponds to a certain number of surface atoms and only some of them host a reaction site
• Electronic structure: below a certain size, the work function of a nanoparticle changes and its band structure fades away
• Defects: the crystal lattice of a small nanoparticle is perfect; thus, reactions enhanced by defects as reaction sites get slowed down as the particle size decreases
• Stability: small nanoparticles have the tendency to lose mass due to the diffusion of their atoms towards bigger particles, according to the Ostwald ripening phenomenon
• Capping agents: in order to stabilize nanoparticles it is necessary a capping layer, therefore part of their surface is unavailable for reactants
• Support: nanoparticles are often fixed onto a support in order to stay in place, therefore part of their surface is unavailable for reactants

Carbon-based materials

Carbon nanotubes and graphene-based materials can be used as electrocatalysts. The carbon surfaces of graphene and carbon nanotubes are well suited to the adsorption of many chemical species, which can promote certain electrocatalytic reactions. In addition, their conductivity means they are good electrode materials. Carbon nanotubes have a very high surface area, maximizing surface sites at which electrochemical transformations can occur. Graphene can also serve as a platform for constructing composites with other kinds of nanomaterials such as single atom catalysts. Because of their conductivity, carbon-based materials can potentially replace metal electrodes to perform metal-free electrocatalysis.

Framework materials

Metal—organic frameworks (MOFs), especially conductive frameworks, can be used as electrocatalysts for processes such as CO₂ reduction and water splitting. MOFs provide potential active sites at both metal centers and organic ligand sites. They can also be functionalized, or encapsulate other materials such as nanoparticles. MOFs can also be combined with carbon-based materials to form electrocatalysts. Covalent organic frameworks (COFs), particularly those that contain metals, can also serve as electrocatalysts. COFs constructed from cobalt porphyrins demonstrated the ability to reduce carbon dioxide to carbon monoxide.

However, many MOFs are known unstable in chemical and electrochemical conditions, making it difficult to tell if MOFs are actually catalysts or precatalysts. The real active sites of MOFs during electrocatalysis need to be analyzed comprehensively.

Research on electrocatalysis

Some transition metal complexes that exhibit some activity as homogeneous electrocatalysts.

Water splitting / Hydrogen evolution

Main article: Electrolysis of water

A schematic of a hydrogen fuel cell. To supply hydrogen, electrocatalytic water splitting is commonly employed.

Hydrogen and oxygen can be combined through by the use of a fuel cell. In this process, the reaction is broken into two half reactions which occur at separate electrodes. In this situation the reactant's energy is directly converted to electricity. Useful energy can be obtained from the thermal heat of this reaction through an internal combustion engine with an upper efficiency of 60% (for compression ratio of 10 and specific heat ratio of 1.4) based on the Otto thermodynamic cycle. It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a fuel cell. In this process, the reaction is broken into two half-reactions which occur at separate electrodes. In this situation the reactant's energy is directly converted to electricity.

The standard reduction potential of hydrogen is defined as 0V, and frequently referred to as the standard hydrogen electrode (SHE).

Half Reaction	Reduction Potential Eored (V)
2H+ + 2e− → H2 (g)	≡ 0
O2(g) + 4H+ + 4e− → 2H2O(l)	+1.23

HER can be promoted by many catalysts.

Carbon dioxide reduction

Main article: Electrochemical reduction of carbon dioxide

Electrocatalysis for CO₂ reduction is not practiced commercially but remains a topic of research. The reduction of CO₂ into useable products is a potential way to combat climate change. Electrocatalysts can promote the reduction of carbon dioxide into methanol and other useful fuel and stock chemicals. The most valuable reduction products of CO₂ are those that have a higher energy content, meaning that they can be reused as fuels. Thus, catalyst development focuses on the production of products such as methane and methanol. Homogeneous catalysts, such as enzymes and synthetic coordination complexes have been employed for this purpose. A variety of nanomaterials have also been studied for CO₂ reduction, including carbon-based materials and framework materials.

Ethanol-powered fuel cells

Aqueous solutions of methanol can decompose into CO₂ hydrogen gas, and water. Although this process is thermodynamically favored, the activation barrier is extremely high, so in practice this reaction is not typically observed. However, electrocatalysts can speed up this reaction greatly, making methanol a possible route to hydrogen storage for fuel cells. Electrocatalysts such as gold, platinum, and various carbon-based materials have been shown to effectively catalyze this process. An electrocatalyst of platinum and rhodium on carbon backed tin-dioxide nanoparticles can break carbon bonds at room temperature with only carbon dioxide as a by-product, so that ethanol can be oxidized into the necessary hydrogen ions and electrons required to create electricity.

Chemical synthesis

Electrocatalysts are used to promote certain chemical reactions to obtain synthetic products. Graphene and graphene oxides have shown promise as electrocatalytic materials for synthesis. Electrocatalytic methods also have potential for polymer synthesis. Electrocatalytic synthesis reactions can be performed under a constant current, constant potential, or constant cell-voltage conditions, depending on the scale and purpose of the reaction.

Advanced oxidation processes in water treatment

Water treatment systems often require the degradation of hazardous compounds. These treatment processes are dubbed Advanced oxidation processes, and are key in destroying byproducts from disinfection, pesticides, and other hazardous compound. There is an emerging effort to enable these processes to destroy more tenacious compounds, especially PFAS

Fluid solution

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Fluid_solution
 

In general relativity, a fluid solution is an exact solution of the Einstein field equation in which the gravitational field is produced entirely by the mass, momentum, and stress density of a fluid.

In astrophysics, fluid solutions are often employed as stellar models, since a perfect gas can be thought of as a special case of a perfect fluid. In cosmology, fluid solutions are often used as cosmological models.

Mathematical definition

The stress–energy tensor of a relativistic fluid can be written in the form

${\displaystyle T^{ab}=\mu u^a{u}^b+p{h}^{ab}+\left(u^a{q}^b+q^a{u}^b\right)+{\pi }^{ab}}$

Here

• the world lines of the fluid elements are the integral curves of the velocity vector ${\displaystyle u^a}$,
• the projection tensor ${\displaystyle h_{ab}=g_{ab}+u_a{u}_b}$ projects other tensors onto hyperplane elements orthogonal to ${\displaystyle u^a}$,
• the matter density is given by the scalar function ${\displaystyle \mu }$,
• the pressure is given by the scalar function ${\displaystyle p}$,
• the heat flux vector is given by ${\displaystyle q^a}$,
• the viscous shear tensor is given by ${\displaystyle {\pi }^{ab}}$.

The heat flux vector and viscous shear tensor are transverse to the world lines, in the sense that

${\displaystyle q_a{u}^a=0,{\pi }_{ab}{u}^b=0}$

This means that they are effectively three-dimensional quantities, and since the viscous stress tensor is symmetric and traceless, they have respectively three and five linearly independent components. Together with the density and pressure, this makes a total of 10 linearly independent components, which is the number of linearly independent components in a four-dimensional symmetric rank two tensor.

Special cases

Several special cases of fluid solutions are noteworthy (here speed of light c = 1):

• A perfect fluid has vanishing viscous shear and vanishing heat flux:

${\displaystyle T^{ab}=(\mu +p)u^a{u}^b+p{g}^{ab},}$

• A dust is a pressureless perfect fluid:

${\displaystyle T^{ab}=\mu u^a{u}^b,}$

• A radiation fluid is a perfect fluid with ${\displaystyle \mu =3p}$:

${\displaystyle T^{ab}=p\left(4u^a{u}^b+g^{ab}\right).}$

The last two are often used as cosmological models for (respectively) matter-dominated and radiation-dominated epochs. Notice that while in general it requires ten functions to specify a fluid, a perfect fluid requires only two, and dusts and radiation fluids each require only one function. It is much easier to find such solutions than it is to find a general fluid solution.

Among the perfect fluids other than dusts or radiation fluids, by far the most important special case is that of the static spherically symmetric perfect fluid solutions. These can always be matched to a Schwarzschild vacuum across a spherical surface, so they can be used as interior solutions in a stellar model. In such models, the sphere ${\displaystyle r=r[0]}$ where the fluid interior is matched to the vacuum exterior is the surface of the star, and the pressure must vanish in the limit as the radius approaches ${\displaystyle r_0}$. However, the density can be nonzero in the limit from below, while of course it is zero in the limit from above. In recent years, several surprisingly simple schemes have been given for obtaining all these solutions.

Einstein tensor

The components of a tensor computed with respect to a frame field rather than the coordinate basis are often called physical components, because these are the components which can (in principle) be measured by an observer.

In the special case of a perfect fluid, an adapted frame

${\displaystyle {\overrightarrow{e}}_0,{\overrightarrow{e}}_1,{\overrightarrow{e}}_2,{\overrightarrow{e}}_3}$

(the first is a timelike unit vector field, the last three are spacelike unit vector fields) can always be found in which the Einstein tensor takes the simple form

${\displaystyle G^{\hat{a}\hat{b}}=8\pi \left[\begin{array}{cccc}\mu & 0& 0& 0\\ 0& p& 0& 0\\ 0& 0& p& 0\\ 0& 0& 0& p\end{array}\right]}$

where ${\displaystyle \mu }$ is the energy density and ${\displaystyle p}$ is the pressure of the fluid. Here, the timelike unit vector field ${\displaystyle {\overrightarrow{e}}_0}$ is everywhere tangent to the world lines of observers who are comoving with the fluid elements, so the density and pressure just mentioned are those measured by comoving observers. These are the same quantities which appear in the general coordinate basis expression given in the preceding section; to see this, just put ${\displaystyle \overrightarrow{u}={\overrightarrow{e}}_0}$. From the form of the physical components, it is easy to see that the isotropy group of any perfect fluid is isomorphic to the three dimensional Lie group SO(3), the ordinary rotation group.

The fact that these results are exactly the same for curved spacetimes as for hydrodynamics in flat Minkowski spacetime is an expression of the equivalence principle.

Eigenvalues

The characteristic polynomial of the Einstein tensor in a perfect fluid must have the form

${\displaystyle \chi (\lambda )=\left(\lambda -8\pi \mu \right){\left(\lambda -8\pi p\right)}^3}$

where ${\displaystyle \mu ,p}$ are again the density and pressure of the fluid as measured by observers comoving with the fluid elements. (Notice that these quantities can vary within the fluid.) Writing this out and applying Gröbner basis methods to simplify the resulting algebraic relations, we find that the coefficients of the characteristic must satisfy the following two algebraically independent (and invariant) conditions:

${\displaystyle 12a_4+a_2^2-3a_1{a}_3=0}$

${\displaystyle a_1{a}_2{a}_3-9a_3^2-9a_1^2{a}_4+32a_2{a}_4=0}$

But according to Newton's identities, the traces of the powers of the Einstein tensor are related to these coefficients as follows:

${\displaystyle {G^a}_a=t_1=a_1}$

${\displaystyle {G^a}_b{G^b}_a=t_2=a_1^2-2a_2}$

${\displaystyle {G^a}_b{G^b}_c{G^c}_a=t_3=a_1^3-3a_1{a}_2+3a_3}$

${\displaystyle {G^a}_b{G^b}_c{G^c}_d{G^d}_a=t_4=a_1^4-4a_1^2{a}_2+4a_1{a}_3+2a_2^2-a_4}$

so we can rewrite the above two quantities entirely in terms of the traces of the powers. These are obviously scalar invariants, and they must vanish identically in the case of a perfect fluid solution:

${\displaystyle t_2^3+4t_3^2+t_1^2{t}_4-4t_2{t}_4-2t_1{t}_2{t}_3=0}$

${\displaystyle t_1^4+7t_2^2-8t_1^2{t}_2+12t_1{t}_3-12t_4=0}$

Notice that this assumes nothing about any possible equation of state relating the pressure and density of the fluid; we assume only that we have one simple and one triple eigenvalue.

In the case of a dust solution (vanishing pressure), these conditions simplify considerably:

${\displaystyle a_2=a_3=a_4=0}$

or

${\displaystyle t_2=t_1^2,t_3=t_1^3,t_4=t_1^4}$

In tensor gymnastics notation, this can be written using the Ricci scalar as:

${\displaystyle {G^a}_a=-R}$

${\displaystyle {G^a}_b{G^b}_a=R^2}$

${\displaystyle {G^a}_b{G^b}_c{G^c}_a=-R^3}$

${\displaystyle {G^a}_b{G^b}_c{G^c}_d{G^d}_a=-R^4}$

In the case of a radiation fluid, the criteria become

${\displaystyle a_1=0,27a_3^2+8a_2^3=0,12a_4+a_2^2=0}$

or

${\displaystyle t_1=0,7t_3^2-t_2{t}_4=0,12t_4-7t_2^2=0}$

In using these criteria, one must be careful to ensure that the largest eigenvalue belongs to a timelike eigenvector, since there are Lorentzian manifolds, satisfying this eigenvalue criterion, in which the large eigenvalue belongs to a spacelike eigenvector, and these cannot represent radiation fluids.

The coefficients of the characteristic will often appear very complicated, and the traces are not much better; when looking for solutions it is almost always better to compute components of the Einstein tensor with respect to a suitably adapted frame and then to kill appropriate combinations of components directly. However, when no adapted frame is evident, these eigenvalue criteria can be sometimes be useful, especially when employed in conjunction with other considerations.

These criteria can often be useful for spot checking alleged perfect fluid solutions, in which case the coefficients of the characteristic are often much simpler than they would be for a simpler imperfect fluid.

Examples

Noteworthy individual dust solutions are listed in the article on dust solutions. Noteworthy perfect fluid solutions which feature positive pressure include various radiation fluid models from cosmology, including

• FRW radiation fluids, often referred to as the radiation-dominated FRW models.

In addition to the family of static spherically symmetric perfect fluids, noteworthy rotating fluid solutions include

• Wahlquist fluid, which has similar symmetries to the Kerr vacuum, leading to initial hopes (since dashed) that it might provide the interior solution for a simple model of a rotating star.