The major bottlenecks in the development of highly productive MES systems are the low bacterial loading, low electron transfer rate and low production of relevant chemicals, which limit the future potential for scaling up this process. Graphene has excellent electrical conductivity, remarkably high carrier mobility, special intrinsic mechanical strength, chemical stability, outstanding specific surface area, and biocompatibility. Therefore, in this regard, graphene can overcome these challenges and provide new opportunities. Graphene is suited for use as a cathode for increasing the bacterial loading and boosting the performance of MES. Over the last decade, graphene has been extensively developed and explored in MES. Graphene incorporation in cathodes can augment the surface area, reduce the resistance, and increase the electron transfer rate; thus, high current density, high coulombic efficiency, and high chemical production can be achieved. To better understand and further explore the modification of graphene-based materials as cathodes in MES systems, it is quite necessary to review and summarize recent developments in this field. Therefore, in this report, we briefly survey and discuss the up-to-date research activities regarding graphene in cathode modification and fabrication, with particular emphasis on their fabrication strategies and characterization, highlighting their key roles in MES systems, as well as presenting the challenges and the future prospects.Microbial electrosynthesis (MES) is a potential energy transformation technology for the reduction of the greenhouse gas carbon oxide (CO 2) into commercial chemicals.Microbial electrosynthesis (MES) is a potential energy transformation technology for the reduction of the greenhouse gas carbon oxide (CO CO2-capturing technologies and a replacement for fossil energy sources with sustainable energy systems are necessary.2,3 The rapid development of green energy technologies such as renewable energy has become a research focus in recent years.4,5 Microbial electrochemical synthesis is a novel strategy in which electroactive microorganisms use electrons obtained from solid electrodes to convert waste organic matter or CO2 into organic compounds and store them in the form of chemical energy.6,7 Microbial electrochemical synthesis is an attractive method to generate renewable fuels, and is perhaps the most promising way to meet future human energy demand.8–10 The electrochemical synthesis of microorganisms can reduce the greenhouse effect and eliminate CO2 emissions, which can minimize the CO2 content in the atmosphere.11–13 By converting abundant and inexpensive CO2 into valuable chemicals, we can create a new sustainable energy transformation system. Microbial products synthesized with CO2 have the following advantages: they do not rely on limited arable land and precious water resources, and they are virtually non-toxic and harmless to living organisms. If the electrical energy required for microbial electrochemical synthesis comes from photovoltaic power generation systems, MES may represent an artificial form of photosynthesis with high efficiency.14 Microorganisms can use solar energy to convert CO2 and water into organic compounds with oxygen as a byproduct, and solar energy can thus be stored in the form of chemical energy. Organic matter can provide fuel for transportation or it can be extracted and used at any time when urgently needed within existing infrastructures. A diagram of CO2 capture by the microbial electrosynthesis of biochemicals from renewable energy and chemical energy utilization is shown in 15Due to the rapid development of human civilization and the economy, the increasing demand for energy has mankind facing huge challenges in the 21st century. The dependence on fossil fuels and continuous growth in annual carbon oxide  and graphene have been explored in MES systems.35–38 A schematic of the MES cathode material development history and the various related biochemicals produced is shown in In early cathode research on MES systems, a negatively charged solid-state graphite block was used as the electron donor source.2-bonded hexagonal pattern.39 It is currently known as the thinnest material in the world. Ever since it was discovered in 2004,40 studies on graphene quickly became an international hot topic.41,42 Particularly in the last decade, it has been extensively studied in the scientific and engineering fields.43,44 Owing to its distinctive properties, graphene has excellent electrical conductivity, remarkably high carrier mobility, special intrinsic mechanical strength, chemical stability, outstanding specific surface area, and biocompatibility. Due to these special characteristics, in recent years, graphene has also been extensively studied and explored in the environmental, medical and electrochemical fields, especially in bioelectrochemical systems.45 In order to better understand and outline future prospects for the modification of graphene and graphene-based composite/hybrid material cathodes in MES systems, it is quite necessary to review and summarize recent developments in this field. Therefore, in the present report, we briefly discuss and outline the up-to-date research activities on graphene in the modification and fabrication of microbial electrosynthesis cathodes, with particular emphasis on their fabrication strategies and characterization, highlighting the key roles of graphene in MES systems, and finally the challenges faced and their future prospects.Graphene is a two-dimensional atomic lattice structure in which carbon atoms are arranged in an sp2.46 Through GO reduction to reduced graphene oxide (RGO), RGO can be decorated on the cathode. GO reduction methods can be used, such as thermal annealing, chemical reducing agents, and bacterial reduction such as self-assembly .assembly . Further2.1.in situ fixed on the cathode materials, and the surface of the cathode can be enhanced. The chemical reducing agents used include sodium borohydride, ascorbic acid, and hydrazine. For example, a very simple method involves dripping GO on the carbon felt surface and then directly reducing it to RGO using the reducing agent of sodium borohydride.80 The larger specific surface area of RGO infiltrated into the carbon cloth lattice can enrich microorganism attachment and result in the high production of acetate and butyrate. Using ascorbic acid as a chemical reducing agent, a carbon felt cathode soaked with GO was directly reduced to RGO via a solvothermal reduction process.47 Using scanning electron microscopy (SEM)  , it can via the reduction of GO sheets to RGO, GO can be prepared as a paper and then directly reduced and fabricated as freestanding and flexible RGO paper, which can then be exploited as a novel bioelectrochemical cathode to enhance electron transfer between the cathode and microbe.48 As shown in In addition to its infiltration into and immobilization on the cathode of MES 2.2.49,50 In microbial electrosynthesis, electrostatic repulsion can inhibit interfacial electron transfer and reduce the bioelectrocatalytic activity between the electrode and bacteria.51 Additionally, the inherent hydrophobicity of RGO can reduce its affinity for bacterial attachment.52 Thus, the novel strategy of functionalizing RGO with positive charges can solve this problem, strengthening the interaction between bacteria and the cathode and substantially augmenting the biofilm density at the surface of the electrode. The conducting polymer of poly  (PEDOT) with a positively charged backbone covered on RGO has been constructed and used in a high-performance E. coli-driven MFC.53 The negatively charged bacteria can easily interact with the positively charged backbone of PEDOT coated on RGO and form a denser biofilm, which can augment the electron transfer. In early research on MES, chemical agents with a positive charge such as chitosan were used to modify the surface of the cathode to reinforce this interaction.33 Based on the electrostatic interaction between the bacteria and graphene, RGO was functionalized with positive charges of tetraethylenepentamine (RGO-TEPA), and a carbon cloth cathode was then modified with RGO-TEPA.54 Therefore, negatively charged S. ovata strains can be easily incorporated into the positively charged matrix of RGO-TEPA through electrostatic interaction via a simple self-assembly method  with a rough surface can provide extra room for microorganisms to colonize; therefore, the bioconversion ability was activated. Graphene modification on the cathode can also improve the available surface area and create more electroactive sites for the microbially catalyzed reaction in MES, resulting in an increased electron exchange capacity with the microbes. The high specific surface area of the graphene-based cathode can maximize the direct mass transfer from the cathode to the microbes for CO2 conversion. Taking RGO paper as an example of a cathode, its specific surface area is 5.4 times higher than that of a carbon paper cathode with a similar diameter and thickness.48 Thus, the high surface area of the electrode substantially increased the electrode surface area available for electronic interactions with the microbes. This can be confirmed by confocal laser scanning microscopy (CLSM)  . A dense2 delivery and adsorption, which can improve the bacterial growth and enhance the conversion rates in MES systems. Nanocomposites of porous copper ferrite/RGO were fabricated and applied in MES.72 The overall surface area of copper ferrite/RGO increased significantly, as verified by SEM  modified on a CF cathode showed a lower charge transfer resistance (12.1 Ω) than CF (83.3 Ω).38 The inexpensive metal oxide nickel ferrite (NiFe2O4) coated on conventional CF possessed a lower electron transfer resistance (714 Ω) than CF (898 Ω).94 In the case of the above-mentioned modified cathodes, not only was there a charge transfer resistance reduction but the current output, acetate production rate, and Faraday efficiency also increased.A low charge transfer resistance can speed up electron delivery and bring about higher electron uptake, which can boost the performance of MES. A low charge transfer resistance can result in better electrocatalytic activity due to low charge transfer between the microbes and the electrode. High-conductivity materials in cathodes can promote electron transportation and speed up the extracellular electron transfer capacity, which can achieve higher chemical production. In recent years, in order to reduce the electron transfer resistance, conductive materials or nanocomposites such as conductive polymers, metal sulfides and metal oxides have been commonly used as ideal modification materials to lower the electron transfer resistance. The charge transfer resistance of a cathode coated with the high-conductivity polymer polypyrrole (PPy) was reduced by 33–70% compared to that of the uncoated one.84–86 Due to the synergetic interaction of nanocomposites compared to their individual components, the improved electrical properties of graphene-based composites or hybrids can facilitate faster electron transfer kinetics.87–90 The low charge transfer resistance of graphene or graphene-based nanocomposites/hybrids can also boost the reduction current density and bring about excellent electrochemical properties to enable high-performance MES.91–94 In an example using only the introduction of graphene as a cathodes in MES,60 the whole-cell resistance was much smaller (12.3 Ω) than that of the control CF cathode (456.8 Ω). The introduction of graphene in the biofilm greatly reduced the cathodic charge transfer resistance and boosted the electron transfer rate. In an example with a nanocomposite of graphene and magnetite, the incorporated graphene reduced the electron transfer resistance and increased the extracellular electron transfer.73 The EIS data for an RGO with magnetite nanoparticles (RGO-MNPs) modified electrode and unmodified CF electrode with or without an attached biofilm were plotted as Nyquist curves (Rct) of the biofilm-enriched CF/RGO-MNPs cathode was low compared to that of the unmodified CF with the biofilm, which verified that it could improve the extracellular electron exchange, resulting in a higher electron uptake and high chemical production in MES. Due to the lower charge transfer resistance, the faradaic efficiency increased two times.Due to its exceptional electrical conductivity, chemical stability, and biocompatibility, graphene can serve as a desirable conductive element between microbes and bacterial electrodes for MES. When graphene is involved in microbial electrochemical systems, its superb electrical properties can speed up electron transfer and improve the overall system properties.t curves . The cha3.3.2, formate, Fe(ii) and ammonia as redox mediators or when electrophilic microorganisms release or excrete their own soluble redox mediators to carry electrons from the cathode, as shown in 2 as the sole carbon source; therefore, EET is a very important procedure for MES.95–98 Accelerating the EET rate is critical for the optimization of MES performance.99,100 A high EET rate can result in better carbon dioxide reduction, high biochemical production, high current density, and high coulombic efficiency in MES.101,102 The EET rate is associated with the cultivation medium, surface area and roughness of the cathode, interaction between the microbes and cathode, and transfer resistance.103 Increasing the interfacial area using a porous three-dimensional scaffold electrode, augmenting the catalytic activity, reducing the electron transfer resistance, and incorporating positively charged species can greatly improve the EET rate.Extracellular electron transfer (EET) is a vital pathway to transport electrons between the bacteria and electrodes in MES. According to the literature, EET can be divided into the two mechanisms of direct electron transfer and indirect electron transfer . Direct 54,73,110 The high conductivity of graphene can promote electron transfer and further accelerate the EET. The high biocompatibility and permeability of graphene together with its improved bioelectroactive sites can speed up the EET. The synergetic effects of graphene-based nanocomposites can also promote the enrichment of electroautotrophic microbes for biomass clogs, which can reduce the resistance and facilitate the EET. Particularly, the high surface area-to-volume ratio of the porous three-dimensional hierarchical structure of composites based on graphene can further increase bacterial attachment and enhance the microbial EET rate.104,105 For example, the fabrication of a novel high-surface-area three-dimensional graphene-nickel foam cathode was reported.46 The hierarchical porous graphene-nickel cathode possessed a high specific surface area, which can be beneficial to bacteria entering the inner structure of the 3D electrode to form an electrocatalytically active biofilm. The high conductivity of graphene can form multiplexed conductive pathways, which can expedite electron transfer between the microorganisms and electrode. At the same time, the graphene decorated on nickel foam can be beneficial for hydrogen generation, which can indirectly increase the indirect electron transfer rate and CO2 reduction. A volumetric acetate production rate of 3.11 mmol L−1 day−1 was achieved and 70% of the electrons consumed were recovered.The high surface area and very thin layer of graphene can favor greater bacterial adhesion and intimate contact, which can increase the direct electron exchange. The functionalization of graphene with positive charges can strength its interaction with the negatively charged bacterial cell surface, which can further increase the EET and significantly speed up acetate production. At the same time, the functionalization of graphene with positive charges can make electrophilic microorganisms release or excrete their own soluble redox mediators, which can bring about indirect electron transfer.Due to their unique physicochemical characteristics, graphene and functionalized graphene-based nanocomposites have been developed to promote the accelerated proliferation of electroactive bacteria and their strong interaction with the surface of electrodes. The enlarged surface area, low conductive resistance and high electron transfer rate can boost the capability of MES. 50,106,107 However, it needs to be clarified whether the antibacterial properties of graphene come from specific aerobic conditions. Graphene can induce oxidative stress with reactive oxygen species using molecular oxygen. The antibacterial properties of graphene have also been used in water treatment, metal recovery, medical equipment, and tissue engineering scaffolds. However, in the case of graphene in the MES of biochemicals from CO2, the growth conditions are usually maintained under strict anaerobic conditions to remove O2, with constant gas flushing. In the absence of O2, the graphene-coated bioelectrode can promote bacterial proliferation. This is why graphene can exhibit bacterial growth-promoting activity in some reports and antibacterial properties in other reports.In recent years, a vast number of studies have reported that graphene has antibacterial properties.4.2 into high-value multicarbon biochemicals using renewable sources of electricity such as solar and wind and effectively store electrical energy in the form of chemical bonds.108–111 For purpose of obtaining highly productive MES systems and speeding up their potential scaling up, the low bacterial loading, low electron transfer rate and low productivity of chemicals are great challenges that need to be overcome.112–114 High active surface area, highly hierarchical porous networks, and high electronic conductivity in materials are essential to develop highly productive MES systems. Therefore, in this regard, graphene and graphene-based composites can help overcome the challenges and provide opportunities in the future research on MES. Graphene is suited for application as a cathode to increase the bacterial loading and boost MES performance due to its high active surface area, chemical stability, high electrical conductivity, corrosion resistance and biocompatibility. Due to their additional positive synergistic effects over individual graphene, graphene-based nanocomposites have even better functional properties, which can improve the EET and increase the microorganism density due to their exceptional electrical conductivity and high surface area. Therefore, graphene and graphene-based nanocomposite bacterial electrodes possess excellent electrochemical properties for high-performance microbial electrosynthesis. Graphene-modified cathodes increase the electrosynthetic rate of chemicals from CO2 and significantly improve the biofilm density and current consumption.MES is a promising way to drive the reduction of the greenhouse gas CO115,116 Carbon nanotubes exhibit high biocompatibility, and can thus promote bacterial metabolism and growth. Therefore, carbon nanotubes have become extremely attractive for applications in MES. In the last decade, some researchers have reported carbon nanotube-based biocathodes for the EMS of chemicals from CO2.36,37,83 The main approaches for carbon nanotube-based cathode fabrication include electrophoretic deposition (EDP) and CVD. The EDP method is suitable and attractive for large-scale MES-based cathode fabrication. The CVD method is expensive, complicated, and energy and time consuming. We believe that in the near future, graphene has sufficient advantages to tackle the challenges associated with carbon nanotubes in MES. Graphene has a higher specific surface area  than carbon nanotubes. Compared with carbon nanotubes, the larger specific surface area of graphene can result in more sites available for microbial interaction and adhesion. The rough surfaces and numerous irregularities in graphene are beneficial for microbial colonization. Graphene has a higher conductivity (108 S m−1) than carbon nanotubes. The high conductivity of graphene can promote the electron transportation and speed up the extracellular electron transfer capacity. Due to its easy synthetic process, graphene has a lower fabrication cost than carbon nanotubes. Graphene-based cathode fabrication is inexpensive, simple, and can be up-scaled for industrial-scale production.Carbon nanotubes have a high aspect ratio, high surface area, good chemical stability, and excellent electrical conductivity.Over the last decade, a variety of methods have been developed for designing and fabricating graphene and graphene-based composite cathodes for MES. Although some considerable achievements have been made in this area, such as high current density, high coulombic efficiency, and high chemical production, some problems such as the high cost of graphene, the complicated process, energy/time-consuming process and the small lab-scale synthesis have been encountered, which limit their wide application. The low-cost, high efficiency, pilot-scale, high quality, and high yield synthesis of these products is still a challenge in expanding their fundamental performance and potential practical applications. The key considerations of graphene research are presented as follows:(i) The EET between microorganisms and cathodes should be further understood through theory and experimentation. Till now, a deeper understanding of the mechanism behind electron transfer and exchange is limited. When graphene is introduced in MES systems, connecting the microbes and cathode, it can accelerate electron transfer and reduce electron transfer resistance; however, there is a lack of knowledge regarding the related EET process. There are no theoretical and experimental studies in this field at present. Determining the electron transfer mechanism for graphene involved in the MES pathway can further result in a major breakthrough and boost the already high EET rate. We believe that with the development of various technical tools, such as electrostatic microscopy, electrochemical surface plasmon resonance, and the establishment of mathematical models, multidisciplinary approaches can shed fresh light on the role of graphene in the EET mechanisms.117 However, this reduces the electron mobility and conductivity of graphene. We believe that with the progress of doped and functionalized materials based on graphene, a balance between the wettability and conductivity can solve this problem.(ii) The hydrophobicity of graphene is a big challenge for the large-scale development of MES as graphene has to inevitably come in contact with the electrolyte solution. Hydrophobicity is an intrinsic property of graphene that restricts its dissolution in water, and thus it can inhibit its interaction with bacteria. In order to improve its solubility, graphene is often doped or functionalized with other functional groups such as carboxyl and hydroxy.(iii) Although graphene has an outstanding specific surface area, the sufficient utilization of its large specific surface area is another problem. As we know, due to the strong π–π stacking coupled with van der Waals forces between the layers, graphene sheets are prone to restacking and easily form irreversible agglomerates. Therefore, in graphene-based cathode fabrication, the large specific surface area of graphene can be dramatically decreased, which results in an insufficient surface for MES applications. Restraining the agglomeration of graphene sheets and making efficient use of single-layer graphene in MES cathodes should be the focus of further research. As we know, large sheets of single-layer graphene can be grown by the CVD method. Therefore, we can construct a three-dimensional single-layer graphene brush or sponge as an MES cathode. Thus, a three-dimensional graphene brush or sponge can provide a sufficient surface for MES applications.118 Its production has been substantially increased in the laboratory; however, its larger-scale production has not been exhibited at present. Its high production costs limit its commercialization. The CVD method is expensive, complicated, and energy- and time consuming. The solution-based chemical reduction method can bring about toxic chemical pollution from the GO reduction process. In contrast, mechanical exfoliation can produce graphene on a large scale at low cost and high quality. Thus, in the near future, with the advancement of graphene preparation technology, we forecast that the cost of graphene-based cathode fabrication will decrease and it will be competitive in the market.(iv) The high cost of graphene has become a significant drawback for its expansion, particularly in the competitive commercial market. Graphene and graphene-based composites have been used in MES cathode modification; however, the cost of graphene is higher than that of traditional materials such as carbon cloth and carbon felt, and its chemical production is much more expensive than other methods.55,118–120 With the development of various technologies, we expect the fabrication of large-scale graphene-based cathodes to have a bright future in CO2 microbial transformation and utilization.(v) The large-scale fabrication of graphene-based cathodes is another critical issue for its industrial-scale production. The small-scale manipulation in the lab, toxic chemicals introduced for the GO reduction process, and complex and time-consuming fabrication process have become shortcomings in their further application. Large-scale graphene-based cathode fabrication is limited by multiple factors such as its inefficient construction strategies, high cost, and low quantity of chemical production in the lab. Industrial-scale graphene-based cathode development and high-quantity chemical production should be integrated with advanced technologies such as the biologically mediated production of RGO and 3D bio-printing.5.Graphene has attracted enormous interest in the fundamental research and potential application of MES due to its suitable electrical and physical characteristics with high affinity for bacterial cells and the formation of high-density electroactive biofilms. In this report, we have briefly reviewed the recent research progress on novel cathodes of graphene and graphene-based composites/hybrids in MES systems. We have emphasized the strategies behind the design and fabrication of graphene-based cathodes with their key roles in electrosynthesis, and have included our perspective and remarks as well. This review not only provides an updated progress on the fabrication of all kinds of MES cathodes based on graphene and its composites but also demonstrates their important roles in the augmentation of the surface area, reduction in the resistance and increase in the electron transfer rate. The comprehensive discussion on the fabrication techniques and critical function of these cathodes in MES can provide fundamental insight into understanding and designing graphene-based cathodes for their further optimization and improvement. Due to the research progress on graphene and related technologies, it can be expected that graphene will play a significant role in the further development of high-efficiency MES and can speed up the commercialization process.There are no conflicts to declare.


Graphene is then produced by catalytically carbonizing this Fe-supported lignin at 1200 °C. The characteristics of both the Fe catalyst and lignin are crucial for the production of high-quality graphene. Specifically, the lignin should disperse well in water, freeze dry, and carbonize via solid-state carbonization. The obtained graphene-based material is highly resistant to electrochemical oxidation, as observed in other graphene-based materials. The direct conversion of lignin to graphene described herein is an unprecedented method for synthesizing large amounts of graphene-based material at low cost, as well as being an excellent use for lignin.Methods for effectively utilizing lignin are necessary for the realization of a sustainable society. Herein, we report a method for directly converting lignin to graphene-based materials. Fe-supported lignin is prepared by dissolving lignin in an aqueous FeCl via catalytic carbonization is expected to expand the industrial use of lignin and graphene-based materials.Directly converting lignin to graphene  Novel methods for the production of graphene using metal catalysts instead of graphene oxide have been proposed. For example, Qiu et al.15 used chemical vapor deposition (CVD) for the bottom-up deposition of graphene on the surface of a porous metal support, and examined its catalytic applicability. However, this method is not suitable for use in the industrial production of graphene-based materials because of the high price of the raw materials and the difficulty of the CVD process.In recent years, various carbon materials have been studied for use in energy storage and conversion devices.i.e., high graphene content and few defect sites).16,17 The proposed method utilizes the water solubility and ion exchange capacity of lignin to produce superior graphene directly by catalytic carbonization. The direct conversion from lignin to graphene is sustainable, inexpensive, and would provide large amounts of graphene-based material. Moreover, it is expected to greatly contribute to the industrial use of both lignin and graphene-based materials.To accelerate the industrial use of graphene-based materials, it is necessary to develop a cheap, environmentally friendly, and sustainable synthetic method that does not use graphene oxide. In this study, we report a new method for synthesizing graphene-based material directly from lignin. In recent years, several studies have been reported on methods for producing graphene-based materials using lignin as a raw material, but these methods have not yet led to the synthesis of superior graphene (2 solution (50 mL) at a predetermined concentration, and the solution was stirred for 3 h. To prevent the oxidation and precipitation of Fe2+, hydroxylammonium chloride and acetic acid were added to the FeCl2 solution at respective molar ratios of 0.36 and 0.007 (for 1 mol of Fe2+). Lignin samples supported with Fe (Fe-supported lignin) were prepared by oven drying or freeze drying the obtained aqueous lignin solutions. The Fe loading was adjusted in the range of 0.028 to 2.8 mmol g−1. The Fe-supported lignin samples were heat-treated for 1 h in an inert atmosphere at a temperature of 1200 °C to obtain carbonized samples. The sample names are distinguished by the amount of Fe (X mmol g−1) and the lignin drying method (oven dried (OD) or freeze dried (FD)), and are expressed as L-FeX(OD) and L-FeX(FD). As control samples, metal-free carbonized samples were prepared by dispersing lignin in distilled water without FeCl2 followed by drying (OD or FD) and heat treating under the conditions described above. The control samples are denoted as L(OD) and L(FD), respectively.Commercially available alkaline lignin  was used as the lignin sample. The lignin sample (1.0 g) was dispersed in FeClVmeso) were determined by Barrett–Joyner–Halenda (BJH) analysis. Raman spectra of the prepared carbon samples were obtained using Raman spectrometry  with a 532 nm Nd:YAG laser. X-ray diffraction (XRD) analysis was performed by an X-ray diffractometer  with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos AXIS NOVA instrument (Shimadzu Co.). The charge-up shift correction was conducted by setting the C 1s binding energy level of the samples to 284.5 eV.Carbonized samples were observed using transmission electron microscopy  to investigate their carbon structures. Sample lattice fringes on the microgrid were analyzed at an accelerating voltage of 200 kV. The surface areas and pore structures were evaluated using nitrogen adsorption–desorption measurements  at −196 °C. The specific surface areas were calculated based on Brunauer–Emmett–Teller (BET) theory. The volumes of mesopores  was used to measure the concentrations of the peripheral (edge) sites of the graphene layers in the carbonized samples by measuring the molecules desorbing at those sites. The average size of the graphene sheet was also calculated from the number of edge sites.−2) and Nafion®. To evaluate the electrochemical oxidation resistance and compare the electrochemical characteristics with other graphene-based materials, cyclic voltammetry (CV) was performed in the following manner. First, a CV scan (5 mV s−1) was repeated four times in the potential range of 0 to 1.0 V vs. a reversible hydrogen electrode (RHE) in 0.5 M H2SO4 aqueous solution at room temperature. Then, triangle-wave potential cycling between 1.0 and 1.5 V at 100 mV s−1 was employed as a carbon oxidation test. After 100, 200, and 500 cycles, the CV scan was repeated another four times in the potential range of 0 to 1.0 V.Electrochemical measurements were performed using a disk electrode. The working electrode was prepared by covering a glass-like carbon disk electrode with a mixture of the prepared carbonized sample ) exhibited a similar Raman spectrum to the metal-free sample, whereas the freeze-dried Fe-supported sample (L-Fe0.56(FD)) showed a strong 2D band (2700 cm−1) unique to graphene.28 Notably, the Raman spectrum of L-Fe0.56(FD) is very similar to that of commercially available graphene . Attempts to synthesize graphene from lignin have been reported so far, but the graphene obtained in those reports showed a relatively weak 2D peak.16,17 L-Fe0.56(FD) obtained by our method shows a stronger 2D band than not only the reported graphene but also the reduced graphene oxide which is often used in the production of graphene-based materials. There has never been a report of such high-quality graphene being directly synthesized from lignin, which indicates the usefulness of our approach as a method for converting lignin to graphene.3O4, as verified by XRD measurements and selected area electron diffraction (SAED) analysis  to the G band (1590 cm−1) (ID/IG) and the 2D band to the G band (I2D/IG) were calculated. The results are summarized in ID/IG was minimized, and I2D/IG was maximized, with between 0.28 and 1.4 mmol g−1 of metal loading. When the lignin contained 2.8 mmol g−1 of Fe, the ID/IG and I2D/IG values were almost the same as those of the control sample , which indicates that the beneficial effect of the Fe support on the production of graphene is lost at a high metal loading.−1 because lignin cannot maintain its dispersed state in an aqueous solution with a large amount of metal cations.29 The presence of this precipitate causes the morphology of the freeze-dried sample to differ from that of the other freeze-dried samples; the apparent density is clearly reduced, and the state appears similar to that of the oven-dried sample. This indicates that two factors are essential for graphene production from lignin, namely, solidification of the lignin molecules in a sparse state, and appropriate Fe support for the lignin molecules.30–33 but the formation of graphene observed herein has not been reported previously. Hydrophobic polymers are generally used as carbon precursors in carbon chemistry and industry. Such polymers are difficult to freeze dry, and when used as carbon precursors, a dense solid powder must be used. It is conceivable that a dense carbon precursor would yield dense carbon, that is, carbon with laminated graphene. In addition to the effect of the Fe catalyst, the use of a sparse polymer at the molecular level—such as freeze-dried lignin—is considered the most important factor in the formation of graphene in this method.Since graphene aggregates are easily laminated by van der Waals forces, the strong Raman 2D band is unique to graphene; it cannot be observed in general carbon materials. To prevent the lamination of graphene, it is considered appropriate to use a carbon precursor in which the polymer is spatially sparse, such as freeze-dried lignin. So far, we have reported on the catalytic and noncatalytic carbonization of polymers,To clarify the principle of graphene formation from metal-supported lignin, we performed a detailed structural analysis of the carbonized samples. 18 we found that the graphene sheet sizes of carbon materials had a greater dependence on the temperature of carbonization than on the carbon structure or pore structure. By comparing previous results with the current report, it can be interpreted that the spatial distribution of lignin molecules does not affect the growth rate of graphene sheets. In contrast, the sheet size of L-Fe0.56(FD), in which graphene formation was confirmed, was clearly larger than that of the other samples. This was due to the presence of the Fe support, which causes a catalytic carbonization reaction and promotes the growth of the graphene sheet. The sheet size of L-Fe2.8(FD), which had a large Fe2+ loading, was approximately the same as that of L(FD), which was formed without Fe2+. Therefore, the effect of catalytic carbonization cannot be confirmed for L-Fe2.8(FD). L-Fe0.56(FD) and L-Fe2.8(FD) differed in whether they exhibited lignin aggregation, as shown in 2+.In our previous report,31 However, the catalytic carbonization of lignin herein proceeded at an extremely high temperature of 1200 °C, which differs from previous reports on catalytic carbonization. To understand the catalytic carbonization of lignin, it is necessary to conduct more detailed investigations in the future. The high water dispersibility and solid-state carbonization characteristics of lignin play important roles in the formation of graphene from lignin. The direct conversion of lignin to graphene by catalytic carbonization in this study was achieved owing to the polymer properties of lignin.To proceed successfully with the catalytic carbonization reaction, optimal Fe loading and spatial distribution of lignin molecules are required. When the heat treatment temperature was reduced to 1100 °C or lower (data not shown), graphene formation was not confirmed for any of the lignin samples. The catalytic carbonization reaction is promoted as the temperature increases. With Fe catalysts, the catalytic carbonization reaction should start at approximately 700 °C.via catalytic carbonization promotes the reduction of these carbon edge sites, thereby improving the electrochemical oxidation resistance. Resistance to electrochemical oxidation is a common characteristic of graphene-based materials.13 Thus, this result indicates that the carbonized sample can be regarded as a graphene-based material.To compare the characteristics of the obtained graphene-based material with those of other graphene-based materials, the sample was electrochemically evaluated by CV and oxidation cycling. In this study, we show that graphene can be synthesized from lignin biomass, which is abundant in modern society. To generate graphene from lignin, two essential conditions were identified: (i) the metal-supported lignin must remain well-dispersed in an aqueous solution, and (ii) a low-density powder is required, such as that produced by freeze-drying. The water dispersibility and solid-state carbonization characteristics of lignin play important roles in conversion to graphene. Lignin-derived graphene exhibited similar electrochemical properties to other graphene-based materials. The direct conversion of lignin to graphene is an unprecedented method for synthesizing large amounts of graphene-based material at low cost and is also an excellent method for the effective use of excessive lignin produced when extracting cellulose. With further research, this method for producing graphene-based materials could revolutionize the sustainability of the energy we use.The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.There are no conflicts to declare.RA-011-D1RA02491D-s001


Capgras and Fregoli delusions are psychotic complex conditions that integrate a larger group of Delusional Misidentification Syndromes (DMS), where the patient misidentifies people, places or objects.Review of the literature and exposure of a case report of a first psychotic episode with DMS.Case report and a nonsystematic review through databases. With the keywords: Capgras; Fregoli; DMS.This case presents a caucasian male, 34-year-old, with no personal or family history of psychiatric pathology. He was taken to the emergency service, due to behavior disorder with verbal aggressiveness to his mother. During the psychiatric emergency he was restless, with accelerated speech and presented Capgras and Fregoli delusion. He believed that his mother had been replaced by an imposter and that the doctor was actually his childhood friend disguised, all with the intention of harming him. The patient was hospitalized, analytical and brain scan show no abnormally. Improvement in symptoms was been seen when a long-term injectable antipsychotic was started. The diagnosis was Bipolar type I disorder.DSM are more frequent than previously considered, they often occur in association with psychiatric or neurological disorders. Case reports like this one helps to clarify the association between DMS and psychiatric disorders. Given the high incidence of DMS, it is essential to recognize them, carry out an early treatment and be alert to other psychopathological or neurological symptoms that may coexist.No significant relationships.


Meanwhile, the extinction ratio of bistable states in the absorber reaches an ultrahigh value of 47.3 dB at 1545.3 nm. Moreover, the influence of changing the structural parameters on the bistable behaviors is discussed in detail, showing that the structure can tolerate structural parametric deviation to some extent. The proposed bistable structure with ultra-compact size, low thresholds, high extinction ratio, and ultrafast response time could be of great applications for fabricating high-performance all-optical-communication devices.A kind of graphene-based perfect absorber which can generate low-threshold and high-extinction-ratio optical bistability in the near-IR band is proposed and simulated with numerical methods. The interaction between input light and monolayer graphene in the absorber can be greatly enhanced due to the perfect absorption. The large nonlinear coefficient of graphene and the strong light-graphene interaction contribute to the nonlinear response of the structure, leading to relatively low switching thresholds of less than 2.5 MW/cm Q factor is required to generate bistable states, which limits the applications of OB. Therefore, materials with large nonlinear coefficients are needed to overcome this drawback.Optical bistability (OB) is a typical phenomenon of nonlinear optics. In some resonant structures with third-order nonlinear optical materials, one given input state has two corresponding output states. Based on this unique property, OB can be applied to the fabrication of all-optical switches, memory cells, and transistors ,3,4,5,6,−12 or even 10−11 m2/W, which is about 107 to 109 times larger than that of bulk dielectrics [Graphene, as a novel 2D material, has been studied intensively over the years due to its outstanding physical properties ,14, suchlectrics ,16,17. Slectrics ,24,25,26lectrics ,32,33,34lectrics ,37,38,39lectrics ,52,53,54lectrics ,56, and 2, and the extinction ratio is over 40 dB. The proposed structure is easy to fabricate and has a toleration to structural parametric deviation, making it practical for the real applications of high-performance graphene-based bistable devices.In this article, we demonstrate graphene-based OB with a low threshold and high extinction ratio by using a graphene-based perfect absorber. The absorber supports guided-mode resonance (GMR) in the near-IR band, which strongly enhances the electric field intensity in the graphene. With the significant light-graphene interaction as well as the large nonlinear coefficient of graphene, the switching threshold of the bistable states is cut down to a relatively low level. Moreover, by optimizing structural parameters to satisfy the critical coupling condition, the extinction ratio of bistable states reaches an ultrahigh level. As is demonstrated by the simulation, the switching threshold can be lower than 2.5 MW/cmp = 1138 nm, the grating thickness h = 67 nm, and the grating width w = p/2. The thickness of the silica layer d = 1200 nm, and the thickness of the gold substrate is 200 nm. The thickness of the graphene 16 s−1 and γ = 8.17 × 1013 s−1 [fE is the Fermi level, τ is the relaxation time and T is the temperature. In the simulation of this work, fE is set to 0.1 eV, considering that graphene might be slightly doped even if gate voltage is absent [τ and T are set to 200 fs and 300 K, respectively, which are typical values for graphene relaxation time and environmental temperature. The total surface conductivity of graphene is 2/W, which is a moderate value for Z-scan measurement in the near-IR band [−14 m2/V2. The nonlinear coefficient of graphene is several orders larger than that of bulk silica, so here silica can be treated as a linear dielectric. Besides the optical Kerr effect, we also consider the saturable absorption effect of graphene, which is necessary for intrinsic or lowly doped graphene [2 and the modulation depth of saturable absorption is k (which can be derived from the conductivity of graphene) with I is treated as time-average power flow and is calculated by I = |E|·|H|/2. Apparently, the saturable absorption effect of graphene leads to a decrease in graphene’s absorption coefficient with increasing incident intensity, which should be carefully considered when optimizing structural parameters.1013 s−1 . The int1013 s−1 :(1)σintrs absent . τ and T-IR band ,16, and graphene . The absurements . The sumxy plane is built since the structure is uniform along the z direction. Periodic boundary condition is added to the x direction of the model so that only one structural period needs to be simulated. A port is added above the grating to give a normal plane wave incidence with a certain intensity and scattering boundary condition is added below the gold layer.The model of the proposed structure is defined and simulated in the FEM software COMSOL Multiphysics. In the simulation, a 2D model of the structural cross-section in the x or y component, so only the z component of normalized electric field , the reflectance spectrum almost coincides with the linear one. When the incident intensity is strong enough (1.5 and 1.75 MW/cm2), we see a red shift of the resonant wavelength which is caused by the shift of graphene’s relative permittivity, and meanwhile, the reflectance suddenly jumps up as the incident wavelength is swept over the resonant wavelength from left to right, indicating the appearance of bistable states.It is known that the rise of incident intensity has a trend to break the spectral symmetry of a resonant structure with Kerr material, with the resonant wavelength shifting away from the initial value and the spectral curve becoming steeper and steeper on one side of the resonant wavelength. 2 (1.68 MW/cm2) for 1545.1 and 1545.6 nm, respectively. These relatively low thresholds are the main contributions of the strong GMR as well as the large nonlinear coefficient of graphene. The simulation results show that when the incident wavelength is between 1545.1 and 1545.6 nm, the extinction ratio of bistable states is greater than 30 dB, and when the wavelength is 1545.3 nm, the extinction ratio reaches the maximum value of 47.3 dB. As mentioned before, the absorption coefficient of graphene has a trend to decrease with increasing light intensity, so the critical coupling condition  within the simulation range. The peak absorption of the structure shows better stability to the deviation of d compared with the resonant wavelength. As is shown in the graph, the structure maintains a considerable peak absorption of over 98% within the simulation range, which means that the extinction ratio of bistable states at 1.75 MW/cm2 is very stable, even if d deviates 30 nm away from the optimized value. d = 1170, 1200, and 1230 nm at their corresponding working wavelengths, which are 1541.7, 1545.6, and 1549.2 nm, respectively. Bistable characteristics are clear enough for all cases. Apparently, the switching-off-to-on thresholds are maintained at nearly 1.7 MW/cm2, and the extinction ratios at 1.75 MW/cm2 are still very high, which are 18.8 and 19.7 dB for d = 1170 and 1230 nm. The switching-on-to-off threshold decreases with d increasing, which has a trend to slightly cut down the structural Q factor.h varying from 52 to 82 nm with a step of 5 nm. Other structural parameters are still the same as those in 2. The change of h has only a slight influence on the resonant wavelength, which shifts no more than 1.5 nm within the simulation range. Its influence on the peak absorption is much stronger compared with that of the silica layer thickness. The peak absorptions drop to 93.4% and 95.7% for h = 52 and 82 nm, respectively. Moreover, the jump of reflectance at the right side of the resonant wavelength is weakened with h increasing, indicating that the structural bistable behaviors will be affected. h = 52 nm, 67 and 82 nm at their corresponding working wavelengths, which are 1544.3, 1545.6, and 1546.5 nm, respectively. The extinction ratios at 1.75 MW/cm2 are 13.5 and 13.7 dB for h = 52 and 82 nm, respectively. Apparently, the hysteresis loop sharply shrinks with h increasing since h directly determines the mode leakage rate of the structure and has a tremendous influence on the structural Q factor. It is predictable that the hysteresis loop is going to vanish if h further increases unless a higher δ varying from 0.3 to 0.7. Likewise, other structural parameters are the same as those in 2. The structure shows significant stability to the change of δ since the spectrum only shifts a little with δ varying from 0.4 to 0.65. δ = 0.3, 0.5, and 0.7 at their corresponding working wavelengths, which are 1544.9, 1545.6, and 1545.9 nm, respectively. Even if δ extremely deviates away from the value of 0.5, the structure still possesses clear bistable characteristics. Similar to that in 2.Q factor of the absorber and can be calculated by Q factor is 530 at a low incident intensity and close to 1000 at extremely high incident intensity when the absorption of graphene is saturated, so Finally, we would like to discuss the response time of the OB in the proposed graphene-based absorber, which is decided by both the graphene’s Kerr effect and the photon lifetime of the absorber. The photon lifetime τR=Q/2πv . For thes 100 fs , as was The proposed perfect absorber can be easily fabricated with standard nanofabrication technology. The gold layer can be deposited onto a chromium-coated silicon substrate by magnetron sputtering. The silica layer can be deposited onto the gold layer by plasma-enhanced chemical vapor deposition (PECVD). The monolayer graphene can be obtained by mechanical exfoliation and can be transferred onto the top of the silica layer. A PMMA film can be spin coated onto the graphene, and the designed grating patterns can be formed by E-beam lithography ,30. We b2 according to the simulation. Meanwhile, the extinction ratio of bistable states exceeds 40 dB at certain incident intensities. The structure is compact and can be easily fabricated with standard nanofabrication technology, which is practical for designing and fabricating graphene-based bistable devices and all-optical switches in the near-IR band.This work demonstrates a graphene-based perfect absorber that possesses bistable characteristics in the near-IR band. By coupling monolayer graphene with dielectric grating patterns, the electric field intensity is highly enhanced within the graphene. The strong light-graphene interaction, together with the large nonlinear coefficient of graphene, highly contributes to low switching thresholds, which can be lower than 2.5 MW/cm