Coffee Waste-Derived Hierarchical Porous Carbon as a Highly Active and Durable Electrocatalyst for Electrochemical Energy Applications

Oxygen-enriched carbon quantum dots from coffee waste: Extremely active organic photocatalyst for sustainable solar-to-H2O2 conversion

Solar-driven artificial photosynthesis offers a promising avenue for hydrogen peroxide (H2O2) generation, an efficient and economical replacement for current methods. The efficiency and selectivity hurdles of the two-electron oxygen reduction reaction (ORR) in solar-to- H2O2 conversion are substantial barriers to large scale production. In this manuscript, a simple biomass-assisted synthesis was performed to produce oxygen-enriched carbon quantum dots (OE-CQDs) from spent coffee waste, acting as an efficient photocatalyst for solar-powered H2O2 production. OE-CQDs can stabilize and store light-generated electrons effectively, boosting charge separation and enhancing photocatalytic performance with longevity. The maximal photocatalytic H2O2 production was achieved viz the utilization of OE-CQDs with generation rate of 356.86 μmol g-1 h-1 by retaining 80% activity without any external sacrificial donors. The outstanding performance of synthesized OE-CQDs under light exposure at wavelength (λ) of 280 nm has been ensured by the quantum yield value of 9.4% upon H2O2 generation. The combinatorial benefits of OE-CQDs with their authentic crystalline structure and oxygen enrichment, is expected to be enhancing the ORR activity through accelerating charge transfer, and optimizing oxygen diffusion. Consequently, our eco-friendly method holds considerable promise for creating highly efficient, metal-free photocatalysts for artificial H2O2 production.

Catalytically Active Carbon for Oxygen Reduction Reaction in Energy Conversion: Recent Advances and Future Perspectives

The shortage and unevenness of fossil energy sources are affecting the development and progress of human civilization. The technology of efficiently converting material resources into energy for utilization and storage is attracting the attention of researchers. Environmentally friendly biomass materials are a treasure to drive the development of new-generation energy sources. Electrochemical theory is used to efficiently convert the chemical energy of chemical substances into electrical energy. In recent years, significant progress has been made in the development of green and economical electrocatalysts for oxygen reduction reaction (ORR). Although many reviews have been reported around the application of biomass-derived catalytically active carbon (CAC) catalysts in ORR, these reviews have only selected a single/partial topic (including synthesis and preparation of catalysts from different sources, structural optimization, or performance enhancement methods based on CAC catalysts, and application of biomass-derived CACs) for discussion. There is no review that systematically addresses the latest progress in the synthesis, performance enhancement, and applications related to biomass-derived CAC-based oxygen reduction electrocatalysts synchronously. This review fills the gap by providing a timely and comprehensive review and summary from the following sections: the exposition of the basic catalytic principles of ORR, the summary of the chemical composition and structural properties of various types of biomass, the analysis of traditional and the latest popular biomass-derived CAC synthesis methods and optimization strategies, and the summary of the practical applications of biomass-derived CAC-based oxidative reduction electrocatalysts. This review provides a comprehensive summary of the latest advances to provide research directions and design ideas for the development of catalyst synthesis/optimization and contributes to the industrialization of biomass-derived CAC electrocatalysis and electric energy storage.

Waste-derived carbon nanostructures (WD-CNs): An innovative step toward waste to treasury

With the growing population, the accumulation of waste materials (WMs) (industrial/household waste) in the environment incessantly increases, affecting human health. Additionally, it affects the climate and ecosystem of terrestrial and water habitats, thereby needing effective management technology to control environmental pollution. In this aspect, managing these WMs to develop products that mitigate the associated issues is necessary. Researchers continue to focus on WMs management by adopting a circular economy. These WMs convert into useful/value-added products such as polymers and nanomaterials (NMs), especially carbon nanomaterials (CNs). The conversion/transformation of waste material into useful products is one of the best solutions for managing waste. Waste-derived CNs (WD-CNs) have established boundless promises for numerous applications like environmental remediation, energy, catalysts, sensors, and biomedical applications. This review paper discusses the several sources of waste material (agricultural, plastic, industrial, biomass, and other) transforming into WD-CNs, such as carbon nanotubes (CNTs), biochar, graphene, carbon nanofibers (CNFs), carbon dots, etc., are extensively elaborated and their application. The impact of metal doping within the WD-CNs is briefly discussed, along with their applicability to end applications.

Waste Coffee Ground-Derived Porous Carbon for Neurochemical Detection

We present an optimized synthetic method for repurposing coffee waste to create controllable, uniform porous carbon frameworks for biosensor applications to enhance neurotransmitter detection with fast-scan cyclic voltammetry. Harnessing porous carbon structures from biowastes is a common practice for low-cost energy storage applications; however, repurposing biowastes for biosensing applications has not been explored. Waste coffee ground-derived porous carbon was synthesized by chemical activation to form multivoid, hierarchical porous carbon, and this synthesis was specifically optimized for porous uniformity and electrochemical detection. These materials, when modified on carbon-fiber microelectrodes, exhibited high surface roughness and pore distribution, which contributed to significant improvements in electrochemical reversibility and oxidative current for dopamine (3.5 ± 0.4-fold) and other neurochemicals. Capacitive current increases were small, showing evidence of small increases in electroactive surface area. Local trapping of dopamine within the pores led to improved electrochemical reversibility and frequency-independent behavior. Overall, we demonstrate an optimized biowaste-derived porous carbon synthesis for neurotransmitter detection for the first time and show material utility for viable neurotransmitter detection within a tissue matrix. This work supports the notion that controlled surface nanogeometries play a key role in electrochemical detection.

Xerogel-Derived Manganese Oxide/N-Doped Carbon as a Non-Precious Metal-Based Oxygen Reduction Reaction Catalyst in Microbial Fuel Cells for Energy Conversion Applications

Current study provides a novel strategy to synthesize the nano-sized MnO nanoparticles from the quick, ascendable, sol-gel synthesis strategy. The MnO nanoparticles are supported on nitrogen-doped carbon derived from the cheap sustainable source. The resulting MnO/N-doped carbon catalysts developed in this study are systematically evaluated via several physicochemical and electrochemical characterizations. The physicochemical characterizations confirms that the crystalline MnO nanoparticles are successfully synthesized and are supported on N-doped carbons, ascertained from the X-ray diffraction and transmission electron microscopic studies. In addition, the developed MnO/N-doped carbon catalyst was also found to have adequate surface area and porosity, similar to the traditional Pt/C catalyst. Detailed investigations on the effect of the nitrogen precursor, heat treatment temperature, and N-doped carbon support on the ORR activity is established in 0.1 M of HClO4. It was found that the MnO/N-doped carbon catalysts showed enhanced ORR activity with a half-wave potential of 0.69 V vs. RHE, with nearly four electron transfers and excellent stability with just a loss of 10 mV after 20,000 potential cycles. When analyzed as an ORR catalyst in dual-chamber microbial fuel cells (DCMFC) with Nafion 117 membrane as the electrolyte, the MnO/N-doped carbon catalyst exhibited a volumetric power density of ~45 mW m2 and a 60% degradation of organic matter in 30 days of continuous operation.

A portable electrochemical immunosensor for ovarian cancer uses hierarchical microporous carbon material from waste coffee grounds

A simple label-free electrochemical immunosensor for ovarian cancer (OC) detection was developed using a hierarchical microporous carbon material fabricated from waste coffee grounds (WCG). The analysis method exploited near-field communication (NFC) and a smartphone-based potentiostat. Waste coffee grounds were pyrolyzed with potassium hydroxide and used to modify a screen-printed electrode. The modified screen-printed electrode was decorated with gold nanoparticles (AuNPs) to capture a specific antibody. The modification and immobilization processes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The sensor had an effective dynamic range of 0.5 to 50.0 U mL^-1 of cancer antigen 125 (CA125) tumor marker with a correlation coefficient of 0.9995. The limit of detection (LOD) was 0.4 U mL^-1. A comparison of the results obtained from human serum analysis with the proposed immunosensor and the results obtained from the clinical method confirmed the accuracy and precision of the proposed immunosensor.

Determination of Heavy Metal Ions in Infant Milk Powder Using a Nanoporous Carbon Modified Disposable Sensor

Due to the risk of heavy metal pollution in infant milk powder, it is significant to establish effective detection methods. Here, a screen-printed electrode (SPE) was modified with nanoporous carbon (NPC) to detect Pb(II) and Cd(II) in infant milk powder using an electrochemical method. Using NPC as a functional nanolayer facilitated the electrochemical detection of Pb(II) and Cd(II) due to its efficient mass transport and large adsorption capacity. Linear responses were obtained for Pb (II) and Cd(II) in the range from 1 to 60 µg L^-1 and 5 to 70 µg L^-1, respectively. The limit of detection was 0.1 µg L^-1 for Pb(II) and 1.67 µg L^-1 for Cd(II). The reproducibility, stability, and anti-interference performance of the prepared sensor were tested as well. The heavy metal ion detection performance in the extracted infant milk powder shows that the developed SPE/NPC possesses the ability to detect Pb(II) and Cd(II) in milk powder.

Natural resources for dye-sensitized solar cells

While the development of dye-sensitized solar cells (DSSCs) has been ongoing for more than 30 years, the currently obtained efficiency is unsatisfactory. However, the study of DSSC development has produced a fundamental understanding of cell performance and inspired other devices, such as perovskite cell solar cells. DSSCs consist of a dye-sensitized photoanode, a counter electrode, and a redox couple in the electrolyte system. Each of the components has an important role and cofunctions with each other to obtain a high power conversion efficiency. Various modifications to each DSSC component have been applied to improve their performance. Additionally, to generate improvements, the effort to reduce production costs has been crucial. The utilization of natural sources for DSSC components is a possible solution to this issue. The utilization of natural resources also aims to increase the value of the natural resource itself. In this review, the applications of various natural sources for DSSC components are described, as well as the modification efforts that have been made to enhance their performance. The discussion covers the utilization of natural dye for sensitizer dyes in liquid DSSC applications: (1) utilization of biopolymers for quasi-solid DSSC electrolytes, (2) green synthesis methods for photoanode semiconductors, and (3) development of natural carbon counter electrodes. The detailed factors that influence improvements in cell performance are also addressed.

Synthesis of unused-wood-derived C-Fe-N catalysts for oxygen reduction reaction by heteroatom doping during hydrothermal carbonization and subsequent carbonization in nitrogen atmosphere

There is an urgent need to develop renewable sources of energy and use existing resources in an efficient manner. In this study, in order to improve the utilization of unused biomass and develop green processes and sustainable technologies for energy production and storage, unused Douglas fir sawdust (SD) was transformed into catalysts for the oxygen reduction reaction. Fe and N were doped into SD during hydrothermal carbonization, and the N- and Fe-doped wood-derived carbon (Fe/N/SD) was carbonized in a nitrogen atmosphere. After the catalyst had been calcined at 800°C, its showed the highest current density (-5.86 mAcm^-2 at 0.5 V versus reversible hydrogen electrode or RHE) and E_onset value (0.913 V versus RHE). Furthermore, its current density was higher than that of Pt/C (20 wt% Pt) (-5.66 mA cm^-2 @0.5 V versus RHE). Finally, after 50 000 s, the current density of sample Fe/N/SD (2 : 10 : 10) remained at 79.3% of the initial value. Thus, the synthesized catalysts, which can be produced readily at a low cost, are suitable for use in various types of energy generation and storage devices, such as fuel cells and air batteries. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Acid-directed preparation of micro/mesoporous heteroatom doped defective graphitic carbon as bifunctional electroactive material: Evaluation of trace metal impurity

Extensive research to explore cost-effective carbon materials as electrocatalysts and electrode materials for energy conversion and storage has been conducted in the recent literature. This raised a crucial question regarding the origin of this electrocatalytic activity from the heteroatom doping/ hierarchical porous defect-rich architecture and/ or the trace metal impurities introduced during synthesis/ inherent to the precursor. In this work, an insight into this issue is provided by considering a lignocellulosic biowaste, Euryale Ferox (foxnut) shells as a precursor to derive micro/ mesoporous defective graphitic carbon sheets by the phosphoric acid (H₃PO₄) dictated in-situ carbonization for oxygen reduction reaction (ORR) and supercapacitor applications. The sample synthesized at 900 °C (FP900) shows an onset potential of 0.98 V vs. reversible hydrogen electrode (RHE), ORR current density of 5.5 mA cm^-2, and current stability of 93% (in 10 h measurement) in 0.1 M KOH. In addition, a symmetric supercapacitor device is assembled using the prepared material and the specific capacitance of 207.5 F g^-1 at 1 A g^-1 is obtained. An attempt to explain the origin of the electrochemical performance is made by establishing parallels with the physicochemical characterizations. The inherently doped heteroatoms give rise to electroactive functionalities and the wide enough pore size distribution improves the active sites utilization efficiency by enhancing the accessibility to electrolytic ions resulting in better electrochemical performance. Furthermore, the contribution from the intrinsic trace metal impurities is evaluated using X-ray fluorescence (XRF) spectroscopy. The presented research clarifies the non-contributing nature of trace metal species owing to the inaccessibility of active sites and lower abundance in F900 and FP900, respectively.

Biomass-derived carbon boosted catalytic properties of tungsten-based nanohybrids for accelerating the triiodide reduction in dye-sensitized solar cells

Manganese tungstate (MnWO₄), zinc tungstate (ZnWO₄), and copper tungstate (CuWO₄) embedded biomass-derived carbon (MWO-C, ZWO-C, CWO-C) was synthesized by hydrothermal treatment and investigated as counter electrode (CE) catalysts to test electrochemical activity. Biomass-derived carbon was used as the shape controlling agent, which changed the morphology of MWO from spherical to spindle-like. Owing to the synergistic effect between tungsten-based bimetal oxides and biomass-derived carbon, the MWO-C, ZWO-C, and CWO-C catalysts exhibited enhanced electrochemical performance in dye-sensitized solar cells (DSSCs) system. The MWO-C, ZWO-C and CWO-C catalysts in DSSCs showed outstanding power conversion efficiency (PCE) of 7.33%, 7.61%, and 6.52%, respectively, as compared with 7.04% for Pt based devices. Biomass-derived carbon improves the catalytic properties of tungsten-based nanohybrids. The results showed that biomass-derived carbon-enhanced inorganic compound as CE catalysts are promising alternatives to Pt-based CE catalysts for energy conversion devices.

Recent progress on nanostructured carbon-based counter/back electrodes for high-performance dye-sensitized and perovskite solar cells

Dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) favor minimal environmental impact and low processing costs, factors that have prompted intensive research and development. In both cases, rare, expensive, and less stable metals (Pt and Au) are used as counter/back electrodes; this design increases the overall fabrication cost of commercial DSSC and PSC devices. Therefore, significant attempts have been made to identify possible substitutes. Carbon-based materials seem to be a favorable candidate for DSSCs and PSCs due to their excellent catalytic ability, easy scalability, low cost, and long-term stability. However, different carbon materials, including carbon black, graphene, and carbon nanotubes, among others, have distinct properties, which have a significant role in device efficiency. Herein, we summarize the recent advancement of carbon-based materials and review their synthetic approaches, structure-function relationship, surface modification, heteroatoms/metal/metal oxide incorporation, fabrication process of counter/back electrodes, and their effects on photovoltaic efficiency, based on previous studies. Finally, we highlight the advantages, disadvantages, and design criteria of carbon materials and fabrication challenges that inspire researchers to find low cost, efficient and stable counter/back electrodes for DSSCs and PSCs.

Biomass Juncus Derived Nitrogen-Doped Porous Carbon Materials for Supercapacitor and Oxygen Reduction Reaction

Juncus is a perennial herb aquatic plant found worldwide, with high reproductive ability in warm regions. It has three-dimensional hierarchical porous triangular networks structures composited of tubular fibers. Here, juncus derived nitrogen-doped porous carbon (NDPC) was prepared by mixing juncus and ZnCl₂ through one-step pyrolysis and activation which is a low-cost, simple, and environmentally friendly method. The NDPC had hierarchical porous structures and a high specific surface area and was applied for supercapacitor and oxygen reduction reaction (ORR). The resulted NDPC-3-800 was prepared by mixing juncus with ZnCl₂ at a mass ratio of 1:3 and then carbonized at 800°C, it was used as electrode material of a supercapacitor. The supercapacitor exhibited excellent specific capacitance of 290.5 F g⁻¹ and 175.0 F g⁻¹ in alkaline electrolyte at the current densities of 0.5 A g⁻¹ and 50 A g⁻¹, respectively. The supercapacitor showed good cycle stability, and the capacitance was maintained at 94.5% after 10,000 cycles. The NDPC-5-800 was prepared by mixing juncus with ZnCl₂ at a mass ratio of 1:5 and then carbonized at 800°C. It exhibited outstanding ORR catalytic activity and stability attributing to their high specific surface area and abundant actives sites. The juncus can derive various materials for application in different fields.

Soybean straw biomass-derived Fe-N co-doped porous carbon as an efficient electrocatalyst for oxygen reduction in both alkaline and acidic media

The development of highly efficient oxygen reduction reaction (ORR) catalysts is of great significance for the large-scale commercialization of fuel cells. In this work, honeycomb-like Fe-N co-doped porous carbon materials (Fe-N-PC) were prepared through a facile one-step pyrolysis strategy using soybean straw biomass as the precursor. The obtained Fe-N-PC catalyst exhibits excellent ORR performance with an onset potential of 0.989 V and a half-wave potential of 0.854 V in alkaline conditions, which positively shift only by 5 mV and 27 mV, respectively than those of the commercial Pt/C catalyst. Furthermore, the onset potential and the half-wave potential of the Fe-N-PC catalysts are up to 0.886 V and 0.754 V, respectively, under acidic conditions, which are superior to those of many other Fe, N-doped electrocatalysts. The ORR process can be regarded as a four-electron transfer process based on RRDE measurements. Moreover, the Fe-N-PC catalyst also shows greater stability and satisfactory methanol tolerance than the Pt/C catalyst. The superior electrocatalytic performance of Fe-N-PC may be attributed to the abundant nanoporous structure, large BET surface area, and Fe-N co-doping, which provide abundant and highly efficient active sites.

Dendritic Nanostructured Waste Copper Wires for High-Energy Alkaline Battery

Rechargeable alkaline batteries (RABs) have received remarkable attention in the past decade for their high energy, low cost, safe operation, facile manufacture, and eco-friendly nature. To date, expensive electrode materials and current collectors were predominantly applied for RABs, which have limited their real-world efficacy. In the present work, we propose a scalable process to utilize electronic waste (e-waste) Cu wires as a cost-effective current collector for high-energy wire-type RABs. Initially, the vertically aligned CuO nanowires were prepared over the waste Cu wires via in situ alkaline corrosion. Then, both atomic-layer-deposited NiO and NiCo-hydroxide were applied to the CuO nanowires to form a uniform dendritic-structured NiCo-hydroxide/NiO/CuO/Cu electrode. When the prepared dendritic-structured electrode was applied to the RAB, it showed excellent electrochemical features, namely high-energy-density (82.42 Wh kg-1), excellent specific capacity (219 mAh g-1), and long-term cycling stability (94% capacity retention over 5000 cycles). The presented approach and material meet the requirements of a cost-effective, abundant, and highly efficient electrode for advanced eco-friendly RABs. More importantly, the present method provides an efficient path to recycle e-waste for value-added energy storage applications.

Facile One-Pot Synthesis of Bimetallic Co/Mn-MOFs@Rice Husks, and its Carbonization for Supercapacitor Electrodes

Novel hybrid nanomaterials comprising metal-organic framework compounds carbonised in the presence of biomass material derived from rice husk have been investigated as a new class of sustainable supercapacitor materials for electrochemical energy storage. Specifically, two synthetic routes were employed to grow Co/Mn metal-organic framework compounds in the channels of rice husks, which had been activated previously by heat treatment in air at 400 °C to produce a highly porous network. Pyrolysis of these hybrid materials under nitrogen at 700 °C for 6 h produced metal-containing phases within the nanocarbon, comprising intimate mixtures of Co, MnO and CoMn2O4. The materials thus produced are characterized in detail using a range of physical methods including XRD, electron microscopy and X-ray photoelectron spectroscopy. The synthetic pathway to the metal-organic framework compound is shown to influence significantly the physical properties of the resulting material. Electrochemical evaluation of the materials fabricated revealed that higher specific capacitances were obtained when smaller crystallite sized bimetallic Co/Mn-MOFs were grown inside the rice husks channels compared to larger crystallite sizes. This was in-part due to increased metal oxide loading into the rice husk owing to the smaller crystallite size as well as the increased pseudocapacitance exhibited by the smaller crystallite sizes and increased porosity.

Sustainable Carbonaceous Materials Derived from Biomass as Metal-Free Electrocatalysts

Although carbon is the second most abundant element in the biosphere, a large proportion of the available carbon resources in biomass from agriculture, stock farming, ocean fisheries, and other human activities is currently wasted. The use of sustainable carbonaceous materials as an alternative to precious metals in electrocatalysis is a promising pathway for transforming sustainable biomass resources into sustainable energy-conversion systems. The development of rational syntheses of metal-free carbonaceous catalysts derived from sustainable biomass has therefore become a topic of significant interest in materials chemistry. However, great efforts are still required to develop methods that are low cost, scalable, and environmentally friendly and which afford carbonaceous materials having an electrocatalytic performance comparable to, or even better than, existing precious metal catalysts. Herein, recent achievements in developing metal-free carbonaceous catalysts based on biomass are reviewed and discussed and the critical issues which still need to be addressed are highlighted. The focus is on representative synthesis and optimization strategies applicable to different kinds of biomass, as well as studies of the physicochemical structure and electrochemical performance of the resulting metal-free carbonaceous catalysts. Finally, some guidelines for the future development of this important area are provided.