Edge-Functionalized Graphene Nanoplatelets as Metal-Free Electrocatalysts for Dye-Sensitized Solar Cells

Recent Advances on Carbon-Based Metal-Free Electrocatalysts for Energy and Chemical Conversions

Over the last decade, carbon-based metal-free electrocatalysts (C-MFECs) have become important in electrocatalysis. This field is started thanks to the initial discovery that nitrogen atom doped carbon can function as a metal-free electrode in alkaline fuel cells. A wide variety of metal-free carbon nanomaterials, including 0D carbon dots, 1D carbon nanotubes, 2D graphene, and 3D porous carbons, has demonstrated high electrocatalytic performance across a variety of applications. These include clean energy generation and storage, green chemistry, and environmental remediation. The wide applicability of C-MFECs is facilitated by effective synthetic approaches, e.g., heteroatom doping, and physical/chemical modification. These methods enable the creation of catalysts with electrocatalytic properties useful for sustainable energy transformation and storage (e.g., fuel cells, Zn-air batteries, Li-O2 batteries, dye-sensitized solar cells), green chemical production (e.g., H2O2, NH3, and urea), and environmental remediation (e.g., wastewater treatment, and CO2 conversion). Furthermore, significant advances in the theoretical study of C-MFECs via advanced computational modeling and machine learning techniques have been achieved, revealing the charge transfer mechanism for rational design and development of highly efficient catalysts. This review offers a timely overview of recent progress in the development of C-MFECs, addressing material syntheses, theoretical advances, potential applications, challenges and future directions.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

3D graphene: synthesis, properties, and solar cell applications

Three-dimensional (3D) graphene is one of the most important nanomaterials. This feature article highlights the advancements, with an emphasis on contributions from our group, in the synthesis of 3D graphene-based materials and their utilization in solar cells. Chemistries of graphene oxides, hydrocarbons, and alkali metals are discussed for the synthesis of 3D graphene materials. Their performances in dye-sensitized solar cells and perovskite solar cells (as counter electrodes, photoelectrodes, and electron extracting layers) were correlatively analyzed with their properties/structures (accessible surface area, electrical conductivity, defects, and functional groups). The challenges and prospects for their applications in photovoltaic solar cells are outlined.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO2 reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Cement Composites with Carbon-based Nanomaterials for 3D Concrete Printing Applications - A Review

3D concrete printing (3DCP) is an emerging additive manufacturing technology in the construction industry. Its challenges lie in the development of high-performance printable materials and printing processes. Recently developed carbon-based nanomaterials (CBNs) such as graphene, graphene oxide, graphene nanoplatelets, and carbon nanotubes, have various applications due to their exceptional mechanical, chemical, thermal, and electrical characteristics. CBNs also have found potential applications as a concrete ingredient as they enhance the microstructure and modify concrete properties at the molecular level. This paper focuses on state-of-the-art studies on CBNs, 3DCP technology, and CBNs in conventional and 3D printable cement-based composites including CBN dispersion techniques, concrete mixing methods, and fresh and hardened properties of concrete. Furthermore, the current limitations and future perspectives of 3DCP using CBNs to produce high-quality composite mixtures are discussed.

A Molecularly Tailored Photosensitizer with an Efficiency of 13.2% for Dye-Sensitized Solar Cells

Photosensitizers yielding superior photocurrents are crucial for copper-electrolyte-based highly efficient dye-sensitized solar cells (DSCs). Herein, two molecularly tailored organic sensitizers are presented, coded ZS4 and ZS5, through judiciously employing dithieno[3,2-b:2″,3″-d]pyrrole (DTP) as the π-linker and hexyloxy-substituted diphenylquinoxaline (HPQ) or naphthalene-fused-quinoxaline (NFQ) as the auxiliary electron-accepting unit, respectively. Endowed with the HPQ acceptor, ZS4 shows more efficient electron injection and charge collection based on substantially reduced interfacial charge recombination as compared to ZS5. As a result, ZS4-based DSCs achieve a power conversion efficiency (PCE) of 13.2% under standard AM1.5G sunlight, with a high short-circuit photocurrent density (J_sc ) of 16.3 mA cm^-2 , an open-circuit voltage (V_oc ) of 1.05 V and a fill factor (FF) of 77.1%. Remarkably, DSCs sensitized with ZS4 exhibit an outstanding stability, retaining 95% of their initial PCE under continuous light soaking for 1000 h. It is believed that this is a new record efficiency reported so far for copper-electrolyte-based DSCs using a single sensitizer. The work highlights the importance of developing molecularly tailored photosensitizers for highly efficient DSCs with copper electrolyte.

Development of strong, tough and flame-retardant phenolic resins by using Acacia mangium tannin-functionalized graphene nanoplatelets

Constructing an eco-friendly phenolic resin with high toughness, strength, and flame retardancy is of great significance and challengeable in the wood-based panels industry. Acacia mangium tannin (AMT), as a biological macromolecule, was decorated onto graphene through ball milling. The formed AMT-functionalized graphene nanoplatelets (AMT@GnPs) were used to replace 40 % phenol to greenly modify and reinforce phenolic resins. The fabricated phenolic resin (BGTPF) exhibited high wet bonding strength of 1.58 MPa, high tensile strength of 24.4 MPa, and large toughness of 0.35 MJ m^-3, which were 38.6 %, 27.7 %, and 75.0 % increments compared with the 1.14 MPa, 19.1 MPa and 0.20 MJ m^-3 of the neat AMT-modified phenolic resin (TPF). These improvements were attributable to the good compatibility and strong interfacial interactions between AMT@GnPs and the resin matrix, which promoted the transfer and dissipation of load energy. The prepared BGTPF resin showed good flame retardancy and high thermal stability. The peak HRR decreased from 15.5 for TPF to 6.9 W/g for BGTPF. This work presents a new, low-cost, and sustainable strategy to construct mechanically strong, tough, and flame-retardant tannin-based phenolic resins for many promising applications such as engineered wood-based products.

Graphene oxide for photonics, electronics and optoelectronics

Graphene oxide (GO) was initially developed to emulate graphene, but it was soon recognized as a functional material in its own right, addressing an application space that is not accessible to graphene and other carbon materials. Over the past decade, research on GO has made tremendous advances in material synthesis and property tailoring. These, in turn, have led to rapid progress in GO-based photonics, electronics and optoelectronics, paving the way for technological breakthroughs with exceptional performance. In this Review, we provide an overview of the optical, electrical and optoelectronic properties of GO and reduced GO on the basis of their chemical structures and fabrication approaches, together with their applications in key technologies such as solar energy harvesting, energy storage, medical diagnosis, image display and optical communications. We also discuss the challenges of this field, together with exciting opportunities for future technological advances.

α-Fe2O3/Reduced Graphene Oxide Composites as Cost-Effective Counter Electrode for Dye-Sensitized Solar Cells

The counter electrode (CE) is an important and vital part of dye-sensitized solar cells (DSSCs). Pt CEs show high-performance in DSSCs using iodide-based electrolytes. However, the high cost of Pt CEs restricts their large-scale application in DSSCs and the development of Pt-free CE is expected. Here, α-Fe2O3/reduced graphene oxide (α-Fe2O3/RGO) composites are prepared as the Pt-free CE materials for DSSCs. A simple hydrothermal technique was used to disseminate the α-Fe2O3 solid nanoparticles uniformly throughout the RGO surface. The presence of the α-Fe2O3 nanoparticles increases the specific surface area of RGO and allows the composites to be porous, which improves the diffusion of liquid electrolyte into the CE material. Then, the electrocatalytic properties of CEs with α-Fe2O3/RGO, α-Fe2O3, RGO, and Pt materials are compared. The α-Fe2O3/RGO CE has a similar electrocatalytic performance to Pt CE, which is superior to those of the pure α-Fe2O3 and RGO CEs. After being fabricated as DSSCs, the current–voltage measurements reveal that the DSSC based on α-Fe2O3/RGO CE has a power conversion efficiency (PCE) of 6.12%, which is 88% that of Pt CE and much higher than that of pure α-Fe2O3 and pure RGO CEs. All the results show that this work describes a promising material for cost-effective, Pt-free CEs for DSSCs.

Date-Leaf Carbon Particles for Green Enhanced Oil Recovery

Green enhanced oil recovery (GEOR) is an environmentally friendly enhanced oil recovery (EOR) process involving the injection of green fluids to improve macroscopic and microscopic sweep efficiencies while boosting tertiary oil production. Carbon nanomaterials such as graphene, carbon nanotube (CNT), and carbon dots have gained interest for their superior ability to increase oil recovery. These particles have been successfully tested in EOR, although they are expensive and do not extend to GEOR. In addition, the application of carbon particles in the GEOR method is not well understood yet, requiring thorough documentation. The goals of this work are to develop carbon nanoparticles from biomass and explore their role in GEOR. The carbon nanoparticles were prepared from date leaves, which are inexpensive biomass, through pyrolysis and ball-milling methods. The synthesized carbon nanomaterials were characterized using the standard process. Three formulations of functionalized and non-functionalized date-leaf carbon nanoparticle (DLCNP) solutions were chosen for core floods based on phase behavior and interfacial tension (IFT) properties to examine their potential for smart water and green chemical flooding. The carboxylated DLCNP was mixed with distilled water in the first formulation to be tested for smart water flood in the sandstone core. After water flooding, this formulation recovered 9% incremental oil of the oil initially in place. In contrast, non-functionalized DLCNP formulated with (the biodegradable) surfactant alkyl polyglycoside and NaCl produced 18% more tertiary oil than the CNT. This work thus provides new green chemical agents and formulations for EOR applications so that oil can be produced more economically and sustainably.

Identifying the impact of the covalent-bonded carbon matrix to FeN4 sites for acidic oxygen reduction

The atomic configurations of FeN_x moieties are the key to affect the activity of oxygen rection reaction (ORR). However, the traditional synthesis relying on high-temperature pyrolysis towards combining sources of Fe, N, and C often results in the plurality of local environments for the FeN_x sites. Unveiling the effect of carbon matrix adjacent to FeN_x sites towards ORR activity is important but still is a great challenge due to inevitable connection of diverse N as well as random defects. Here, we report a proof-of-concept study on the evaluation of covalent-bonded carbon environment connected to FeN₄ sites on their catalytic activity via pyrolysis-free approach. Basing on the closed π conjugated phthalocyanine-based intrinsic covalent organic polymers (COPs) with well-designed structures, we directly synthesized a series of atomically dispersed Fe-N-C catalysts with various pure carbon environments connected to the same FeN₄ sites. Experiments combined with density functional theory demonstrates that the catalytic activities of these COPs materials appear a volcano plot with the increasement of delocalized π electrons in their carbon matrix. The delocalized π electrons changed anti-bonding d-state energy level of the single FeN₄ moieties, hence tailored the adsorption between active centers and oxygen intermediates and altered the rate-determining step.

Unveiling the effect of carbon matrix adjacent to Fe-N towards oxygen reduction reaction is important yet challenging. Here the authors investigate the carbon environment covalent-connected to FeN₄ sites on their catalytic activity using models prepared by pyrolysis-free approach.

Synthesis, Applications, and Prospects of Graphene Quantum Dots: A Comprehensive Review

Graphene quantum dot (GQD) is one of the youngest superstars of the carbon family. Since its emergence in 2008, GQD has attracted a great deal of attention due to its unique optoelectrical properties. Non-zero bandgap, the ability to accommodate functional groups and dopants, excellent dispersibility, highly tunable properties, and biocompatibility are among the most important characteristics of GQDs. To date, GQDs have displayed significant momentum in numerous fields such as energy devices, catalysis, sensing, photodynamic and photothermal therapy, drug delivery, and bioimaging. As this field is rapidly evolving, there is a strong need to identify the emerging challenges of GQDs in recent advances, mainly because some novel applications and numerous innovations on the ease of synthesis of GQDs are not systematically reviewed in earlier studies. This feature article provides a comparative and balanced discussion of recent advances in synthesis, properties, and applications of GQDs. Besides, current challenges and future prospects of these emerging carbon-based nanomaterials are also highlighted. The outlook provided in this review points out that the future of GQD research is boundless, particularly if upcoming studies focus on the ease of purification and eco-friendly synthesis along with improving the photoluminescence quantum yield and production yield of GQDs.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Highly efficient and stable ionic liquid-based gel electrolytes

Gel electrolytes are promising candidates for dye-sensitized solar cells (DSSCs) and other devices, but the ways to obtain stable gels always result in sacrifice of their ionic conductivity. This contradiction seriously limits the practical application of gel electrolytes. Herein, a new design strategy using rich carboxylic group-modified silica nanoparticles (COOH-SiO₂) with a branched, well-organized framework to develop ionic liquid-based gel electrolytes possessing high conductivity is demonstrated. The branched network of COOH-SiO₂ and the strong interaction in electrolytes result in the effective solidification of ionic liquids. Moreover, adding COOH-SiO₂ to ionic liquid electrolytes contributes to salt dissociation, decreases the activation energy, and improves the charge transport and recombination characteristics at the electrolyte/electrode interface. DSSCs fabricated with COOH-SiO₂ nanoparticles deliver a higher short-circuit photocurrent density (J_sc) than the reference cell. A maximum efficiency of 8.02% with the highest J_sc value of 16.60 mA cm^-2 is obtained for solar cells containing 6 wt% COOH-SiO₂.

Self-assembled one-dimensional Co coated with N-doped carbon nanotubes for dye-sensitized solar cells with high activity and remarkable durability

An important challenge for commercializing dye-sensitized solar cells (DSSCs) is the development of efficient non-precious metal counter electrode (CE) materials, which must be solved using non-precious metal catalysts because of their poor stability and low efficiency.

Well-dispersed Te-doped mesoporous carbons as Pt-free counter electrodes for high-performance dye-sensitized solar cells

A tellurium-doped carbon nanomaterial (Te-MC(P)) was newly developed by the soft-templated carbonization of the PAN-b-PBA copolymer with poly(3-hexyltellurophene). Te-MC(P) was characterized with various characterization methods, including the nitrogen sorption isotherm measurement (BET), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS), which reveal that the Te atoms are homogeneously dispersed in the three-dimensional hierarchical, graphite-like mesoporous carbon matrix with a Te doping level of 0.27 atom %. Based on the characterization results, the electrocatalytic ability of Te-MC(P) was evaluated by using a symmetrical dummy cell test with both Co(bpy)32+/3+ (bpy = 2,2'-bipyridine) and I-/I3- redox electrolytes as counter electrodes (CEs). The Te-MC(P) CEs showed remarkably lower charge-transfer resistance (Rct) values by approximately 10 times in the electrochemical impedance spectroscopy (EIS) measurement, compared to the counterpart platinum (Pt) and the tellurium-based material (Te-MC(A)), prepared with a telluric acid precursor that has a lower Te doping level of 0.15 at%. As a result, the excellent electrocatalytic ability of Te-MC(P) resulted in the improvement of photovoltaic performance. The power conversion efficiencies (PCEs) of Te-MC(P)-based dye-sensitized solar cells (DSSCs) were 12.69% for the Co(bpy)32+/3+ redox electrolyte with the SGT-021 porphyrin dye and 9.73% for the I-/I3- redox electrolyte with the N719 ruthenium dye. Furthermore, Te- MC(P) CEs exhibited remarkable electrochemical stability in the two redox electrolytes. These results could suggest that the Te-MC(P) CE is one of the best promising alternatives to Pt CEs as a low-cost, highly stable and efficient electrocatalytic CE for practical applications.

A versatile route to edge-specific modifications to pristine graphene by electrophilic aromatic substitution

Electrophilic aromatic substitution produces edge-specific modifications to CVD graphene and graphene nanoplatelets that are suitable for specific attachment of biomolecules.

Sb2S3 Thickness-Related Photocurrent and Optoelectronic Processes in TiO2/Sb2S3/P3HT Planar Hybrid Solar Cells

In this work, a comprehensive understanding of the relationship of photon absorption, internal electrical field, transport path, and relative kinetics on Sb₂S₃ photovoltaic performance has been investigated. The n-i-p planar structure for TiO₂/Sb₂S₃/P3HT heterojunction hybrid solar cells was conducted, and the photon-to-electron processes including illumination depth, internal electric field, drift velocity and kinetic energy of charges, photo-generated electrons and hole concentration-related surface potential in Sb₂S₃, charge transport time, and interfacial charge recombination lifetime were studied to reveal the key factors that governed the device photocurrent. Dark J-V curves, Kelvin probe force microscope, and intensity-modulated photocurrent/photovoltage dynamics indicate that internal electric field is the main factors that affect the photocurrent when the Sb₂S₃ thickness is less than the hole diffusion length. However, when the Sb₂S₃ thickness is larger than the hole diffusion length, the inferior area in Sb₂S₃ for holes that cannot be diffused to P3HT would become a dominant factor affecting the photocurrent. The inferior area in Sb₂S₃ layer for hole collection could also affect the V_oc of the device. The reduced collection of holes in P3HT, when the Sb₂S₃ thickness is larger than the hole diffusion length, would increase the difference between the quasi-Fermi levels of electrons and holes for a lower V_oc.

Prussian blue nanoparticle-labeled aptasensing platform on graphene oxide for voltammetric detection of α-fetoprotein in hepatocellular carcinoma with target recycling

An enzyme-free electrochemical aptasensing platform based on a graphene oxide nanosheet-modified gold-disk electrode was developed for the voltammetric detection of alpha-fetoprotein (AFP) in hepatocellular carcinoma by using a Prussian blue nanoparticle (PBNP)-labeled aptamer. The electroactive PBNP, a typical signal-generation tag, was utilized for the labeling of the aminated AFP aptamer by using covalent conjugation. The electrochemical sensing platform was prepared in a simple manner on the basis of a π-π stacking reaction between the immobilized graphene oxide and the PBNP-labeled AFP aptamer. Upon target AFP introduction, the analyte reacted with the aptamer, thus resulting in the dissociation of the PBNP from the nanosheets. In the presence of DNase I, the newly formed AFP/aptamer-PBNP complex was cleaved to release target AFP, which could react again with the aptamer on the nanosheets, thereby causing target recycling. During this process, the cleaved PBNP-aptamer was far away from the electrode to decrease the voltammetric signal. Under optimum conditions, the voltammetric peak current of the modified electrode decreased with the increment of the target AFP concentration within the linear range of 0.01-300 ng mL^-1 at a low detection limit of 6.3 pg mL^-1. The precision and reproducibility of the aptasensing protocol were acceptable (CV: <15% for intra-assay and inter-assay). Other possible nontarget biomarkers did not interfere significantly with the voltammetric signal of this system. Human serum samples containing target AFP were assayed with electrochemical aptasensing and a commercial human AFP ELISA kit, and gave well-matched results from these two methods. Importantly, our strategy provides a new horizon for the determination of disease-related proteins.