High-yield production of graphene sheets by graphite electro-exfoliation for application in electrochemical power sources

This thesis first aims at developing an electrochemical approach for low temperature, simple, and cost-effective synthesis of graphene microsheets (GNs) using graphitic electrodes in ionic liquid (IL) medium. The second major focus involves the products application as cathode-modifying microporous layers (MPLs) in proton exchange membrane fuel cells (PEMFCs) as well as anode-modifying materials in microbial fuel cells (MFCs). For the electrochemical exfoliation, a novel IL/acetonitrile electrolyte is introduced, and investigated with low concentration of ionic liquids. Using iso-molded graphite rod as the anode, up to 86% of exfoliation was achieved with the majority of the products as graphene flakes in addition to smaller quantities of carbonaceous particles and rolled sheets. Moreover, the simultaneous anodic and cathodic GN production was developed here with a synergistic exfoliation effect. When graphitic anode and cathode were subjected to a constant cell potential, up to 3 times higher exfoliation yields were generated compared to single-electrode studies on each side (~6-fold improvement in total). Thorough materials characterization confirmed the production of ultrathin GNs (< 5 layers) on both electrodes, with cathodic sheets being relatively larger and less functionalized. On the application side, the successful integration of GNs in MPLs resulted in enhanced PEMFC performance over a wide range of operating conditions. GN-based MPLs improved performance in the kinetic and ohmic regions of the polarization curve, while the addition of carbon black (CB), particularly Vulcan XC72, to form a composite GN+CB MPL, further extended the improvement to the mass transport limiting region. This was reflected by an approximate 30% and 70% increase in peak power densities compared to CB and GN MPLs, respectively, at the relative humidity (RH) of 100%. Despite the presence of CB, GN+CB MPLs also retained their superior performance at a much lower RH of 20%, thereby widening the peak power gap with CB MPLs to 80%. On the other side, the functionalized GN-modified carbon cloth anodes integrated within single-chamber MFCs generated an over four-fold improvement in peak power density compared to the plain carbon cloth (2.85 W m-² vs 0.66 W m-², respectively), exceeding the previously reported values with graphene anodes.Applied Science, Faculty ofGraduat

Fundamental studies on solar-activated zeolite-supported photocatalysts for water splitting application

Robust calculations show that the incidence of solar energy on the earth’s surface by far exceeds all human energy needs. Undoubtedly, the most trusted way of utilizing solar energy is to convert and store it in the form of an energy carrier such as hydrogen. Semiconductors capable of absorbing light energy so-called photocatalysts can potentially drive water splitting reaction for hydrogen generation. In this research, fundamental studies on a new class of solar-activated supported photocatalysts for water splitting application are presented. This resulted in significantly higher rates of H₂ production in comparison to the existing supported TiO₂ series under visible light. The composition comprises silico-aluminates (zeolite) as the support, titanium dioxides (TiO₂) as the semiconductor, cobalt compounds as hydrogen evolution sites and heteropolyacids (HPAs) as multifunctional solid acids with excitability under visible light. Using this composition, I ended up with at least 2.6 times higher hydrogen evolution rates under visible light in comparison to Degussa P25, the best commercially available titania product. The chemical point of view of this successful combination was investigated, attributing the higher photocatalytic activity of the synthesized chemical compositions to the basicity of the matrix. The more basicity properties besides HPA presence can overcome the negative impacts of titania interactions with the zeolite which are band gap widening and anodic shift of the TiO₂ band edges. Furthermore, the effect of cobalt precursors (nitrates and chlorides) on the photocatalytic activity of the prepared photocatalysts was also investigated. Although nitrate-based photocatalysts exhibited an improvement in the UV-VIS absorbance spectra toward visible light, they caused an almost 30% lower H₂ production rate in comparison to the chloride salts. Overshadowing the poisoning and parasitic effects of Cl⁻ anions on the photooxidation sites in the zeolite-supported composition was another notable outcome of this study. This suggests emulation of the core-shell photocatalysis concept insofar with providing a reasonable distance between redox sites. The results indicate the importance of zeolite’s structural and chemical properties as the photocatalyst support. This can be addressed through the selection of suitable zeolite types, taking an important step in the development of visible-light-activated photocatalysts based on earth-abundant materials.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

CO2chemical conversion to useful products: an engineering insight to the latest advances toward sustainability

In the fossil‐fuel‐based economies, current remedies for the CO2 reduction from large‐scale energy consumers (e.g. power stations and cement works) mainly rely on carbon capture and storage, having three proposed generic solutions: post‐combustion capture, pre‐combustion capture, and oxy fuel combustion. All the aforementioned approaches are based on various physical and chemical phenomena including absorption, adsorption, and cryogenic capture of CO2. The purified carbon dioxide is sent for the physical storage options afterwards, using the earth as a gigantic reservoir with unknown long‐term environmental impacts as well as possible hazards associated with that. Consequently, the ultimate solution for the CO2 sequestration is the chemical transformation of this stable molecule to useful products such as fuels (through, for example, Fischer–Tropsch chemistry) or polymers (through successive copolymerization and chain growth). This sustainably reduces carbon emissions, taking full advantage of CO2‐derived chemical commodities, so‐called carbon capture and conversion. Nevertheless, the surface chemistry of CO2 reduction is a challenge due to the presence of large energy barriers, requiring noticeable catalysis. This work aims to review the most recent advances in this concept selectively (CO2 conversion to fuels and CO2 copolymerization) with chemical engineering approach in terms of both materials and process design. Some of the most promising studies are expanded in detail, concluding with the necessity of subsidizing more research on CO2 conversion technologies considering the growing global concerns on carbon management

High-yield graphene production by electrochemical exfoliation of graphite: novel ionic liquid (IL)–acetonitrile electrolyte with low IL content

Electrochemical exfoliation of graphite assisted by ionic liquids (ILs) has been proposed as a high-throughput, green and scalable graphene production technique. Previous research has focused on IL/water electrolytes with high IL content (from 1:0.1 to 1:1 IL/water volume ratios). Here, we introduce and investigate a novel IL/acetonitrile electrolyte with dramatically lower loads of ionic liquids (∼1:50 IL/acetonitrile vol. ratio). Our approach provides three main advantages: cost efficiency due to low IL content, extended electrochemical stability in a non-aqueous electrolyte, and high exfoliation yield by effective anionic intercalation within the graphitic layers. Using iso-molded graphite rod as the anode, we achieved up to 86% of exfoliation with the majority of the products as graphene flakes in addition to smaller quantities of carbonaceous particles and rolled sheets. We also demonstrate by Raman spectroscopy the beneficial sonication effect on improving the quality of the graphene-based products. Moreover, in contrast with previous literature, we prove that the electrolyte coloration during electro-exfoliation in the IL media is related to the occurrence of diverse reactions involving the IL moieties and cannot be associated with different stages of graphene formation. The cathodically generated species can also interfere with the anionic intercalation in the graphite anode

Physicochemical impact of zeolites as the support for photocatalytic hydrogen production using solar-activated TiO2-based nanoparticles

Silico-aluminates (zeolites) have been recently utilized promisingly as the support for photocatalytic hydrogen production using solar-activated TiO2-based nanoparticles. Aside from conventional advantages offered by the supports in photocatalysis, we demonstrate the unique physicochemical impact of zeolites on photocatalytic hydrogen production. Beside zeolites, our synthesized materials comprise titanium dioxide (TiO2) as the semiconductor, cobalt ions as the hydrogen evolution sites, and heteropolyacids (HPAs) as the multifunctional solid acids with significant excitability under visible light. Four classes of zeolites (Na-Y, Na-mordenite, H-Y, and H-beta) with different Si/Al ratios and sodium contents were evaluated. Among the studied photocatalysts, Na-Y and Na-mordenite containing 10 wt% titania emerged as the potential candidates for the hydrogen evolution reaction, with corresponding rates of 250.8 and 187.2 μmol/g h, in comparison to 84.2 μmol/g h for Degussa P25; while these values for H-Y and H-beta were 96.8 and 100.1 μmol/g h, respectively. The higher photocatalytic activity of the first two classes is attributed to the basicity of the zeolite matrix, which is possibly due to the pH dependency of the TiO2 band edges. The results indicate the importance of controlling the chemical properties of the zeolite as a photocatalyst support through the selection of suitable types. Furthermore, our analyses show that the precise pore size distribution of the zeolite framework rules over accommodating the impregnated species whether in the pores or on the surface. This ultimately enables a vast array of synthesis opportunities for development of the-state-of-the-art solar-activated photocatalysts based on earth-abundant materials

Cobalt precursor role in the photocatalytic activity of the zeolite-supported TiO2-based photocatalysts under visible light: A promising tool toward zeolite-based core–shell photocatalysis

A new class of supported photocatalysts is introduced recently with high activity under visible light for water splitting purposes. The composition comprises silicoaluminates (zeolite) as the support, titanium dioxide (TiO2) as the semiconductor, cobalt ions (Co2+) as the hydrogen evolution sites and heteropolyacid (HPA) as the multifunctional solid acid with visible light activity. From photocatalyst preparation point of view, the synthesis consists of three impregnation steps of TiO2, Co2+, and HPA, respectively, followed by a specific thermal treatment for each stage. The focus of this study is on the cobalt impregnation step. The objective is to investigate the impact of employing two commercially available cobalt precursors on the photocatalytic activity of the synthesized photocatalysts, especially hydrogen production rates. Nitrate and chloride compounds of cobalt were examined on two classes of the zeolites namely Na–Y and Na–Mordenite which have emerged as suitable supports for hydrogen evolution application. Although nitrate-based photocatalysts exhibited an improvement in the UV–VIS absorbance spectra toward visible light, they caused an almost 30% lower H2 production rate in comparison to the chloride salts. The favorable shift toward visible light is possibly due to the incorporation of nitrogen (N) anions in the photocatalyst structure. However, their lower hydrogen production rate is mainly attributed to the competitive photo-reduction reactions of remnant nitrate anions, suggesting chloride (Cl−) species as a remedy of this so-called parasitic phenomenon. In addition, overshadowing the poisoning and parasitic effects of Cl− ions on the photo-oxidation sites of the zeolite-supported composition was another notable outcome of this study. This emulates core–shell photocatalysis concept insofar with providing a reasonable distance between redox sites

Electrochemically exfoliated graphene anodes with enhanced biocurrent production in single-chamber air-breathing microbial fuel cells

Microbial fuel cells (MFCs) present promising options for environmentally sustainable power generation especially in conjunction with waste water treatment. However, major challenges remain including low power density, difficult scale-up, and durability of the cell components. This study reports enhanced biocurrent production in a membrane-free MFC, using graphene microsheets (GNs) as anode and MnOx catalyzed air cathode. The GNs are produced by ionic liquid assisted simultaneous anodic and cathodic electrochemical exfoliation of iso-molded graphite electrodes. The GNs produced by anodic exfoliation increase the MFC peak power density by over 300% compared to plain carbon cloth (i.e., 2.85 W m−2 vs 0.66 W m−2, respectively), and by 90% compared to conventional carbon black (i.e., Vulcan XC-72) anode. These results exceed previously reported power densities for graphene-containing MFC anodes. The fuel cell polarization results are corroborated by electrochemical impedance spectroscopy indicating three times lower charge transfer resistance for the GN anode. Material characterizations suggest that the best performing GN samples were of relatively smaller size (~500 nm), with higher levels of ionic liquid induced surface functionalization during the electrochemical exfoliation process

Synergistic production of graphene microsheets by simultaneous anodic and cathodic electro-exfoliation of graphitic electrodes in aprotic ionic liquids

Electrochemically mediated exfoliation of graphite is a promising green and high throughput approach for production of graphene sheets (GNs). Previous research focused mostly on either anode or cathode exfoliation due to restrictions imposed by the investigated intercalating ions and insufficient consideration given to the design of the electrochemical cell. Consequently, in single graphite electrode studies, at the non-graphitic counter-electrode (e.g. Pt), unwanted electrode reactions such as gas evolution and electrolyte decomposition take place, leading to significant energy and chemical losses. Here, we report the simultaneous anodic and cathodic GN production in two types of electrochemical cells (undivided and divided) using aprotic electrolytes containing ionic liquids (ILs). We demonstrate a synergistic exfoliation effect when the iso-molded graphite anode and cathode are subjected to a constant cell potential, generating up to 3 times higher exfoliation yields compared to single-electrode studies on each side (∼6-fold improvement in total). Thorough characterization of the products collected from both electrode compartments confirmed the production of ultrathin GNs (<5 layers). The cathodic exfoliates were almost exclusively composed of GNs; whereas among the anodic products, in addition to the majority GNs, we detected traces of other morphologies such as nanoparticles, nanotubes, and larger rolled sheets

Research data supporting Graphene-Black Phosphorus Printed Photodetectors

Data are generated based on experimental results such as Raman, Scanning tunneling electron microscopy, atomic force microscopy, Absorption spectroscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, inkjet printing, chemical vapor deposition, electrical characterizations, bending tests, and theoretical calculations for theoretical absorption of the photodetector.EU grants Graphene Flagship, HiGraphInk, Neurofibres, ERC Grants Hetero2D, EPSRC Grants EP/K01711X/1, EP/K017144/1, EP/N010345/1, EP/L016087/1, EP/V000055/1, EP/P00119X/1, DSTL, ISF grant 1732/18

Emerging applications of graphene and its derivatives in carbon capture and conversion: current status and future prospects

Alarming carbon dioxide emissions and its detrimental environmental impacts (e.g. climate change and global warming) are the major consequences of the undue reliance of the modern civilization on fossil fuels. Long-term solutions to address these issues are based on developing sustainable alternatives for the human energy thirst. However, the versatilities offered by the carbonaceous fuels have still preserved their popularity as the main source of energy for a wide variety of applications. After decades of practicing conventional carbon capture and storage, researchers believe the ultimate solution of realistically facing with CO2 sequestration problem is the chemical conversion of carbon dioxide to valuable products. However, substantial development of state-of-the-art materials remains the major bottleneck of such technologies. Graphene, as the rising star of the materials world in 21st century, offers game-changing prospects towards a more sustainable future for fossil-fuel-based economies. This two-dimensional planar sheet of sp2-bonded carbon atoms is the most widely studied nanomaterial since its discovery in 2004. Here we aim to highlight various aspects of graphene research in carbon dioxide capture and conversion from materials viewpoint. After presenting an overview of the most common and effective synthesis and doping/functionalization methods, the application of graphene and its derivatives in CO2 capture and conversion is discussed in detail. Catalytic, electrocatalytic and photoelectrocatalytic use of graphene-based compounds could potentially revolutionize some of the current techniques for CO2 transformation to valuable chemical commodities. CO2 to graphene conversion pathways are also covered extensively in this review paper as another intriguing relation of graphene with CO2