H. Enis Karahan

All-carbon solid-state yarn supercapacitors from activated carbon and carbon fibers for smart textiles

Smart textiles are intelligent devices that can sense and respond to environmental stimuli. They require integrated energy storage to power their functions. An emerging approach is to build integratable fiber-/yarn-based energy storage devices. Here, we demonstrate all-carbon solid-state yarn supercapacitors using commercially available activated carbon and carbon fiber yarns for smart textiles. Conductive carbon fibers concurrently act as current collectors in yarn supercapacitors and as substrates for depositing large surface area activated carbon particles. Two hybrid carbon yarn electrodes were twisted together in polyvinyl alcohol/H3PO4 polymer gel, which is used as both an electrolyte and a separator. A 10 cm long yarn supercapacitor, with the optimum composition of 2.2 mg cm−1 activated carbon and 1 mg cm−1 carbon fiber, shows a specific length capacitance of 45.2 mF cm−1 at 2 mV s−1, an energy density of 6.5 μW h cm−1, and a power density of 27.5 μW cm−1. Since the yarn supercapacitor has low equivalent series resistance at 4.9 Ω cm−1, longer yarn supercapacitors up to 50 cm in length were demonstrated, yielding a high total capacitance of up to 1164 mF. The all-carbon solid-state yarn supercapacitors also exhibit excellent mechanical flexibility with minor capacitance decreases upon bending or being crumpled. Utilizing three long yarn supercapacitors, a wearable wristband was knitted; this wristband is capable of lighting up an LED indicator, demonstrating strong potential for smart textile applications.

Space-confined assembly of all-carbon hybrid fibers for capacitive energy storage: realizing a built-to-order concept for micro-supercapacitors

Miniaturized portable and wearable electronics have diverse power requirements, ranging from one microwatt to several milliwatts. Fiber-based micro-supercapacitors are promising energy storage devices that can address these manifold power requirements. Here, we demonstrate a hydrothermal assembly method using space confinement fillers to control the formation of nitrogen doped reduced graphene oxide and multi-walled carbon nanotube hybrid fibers. Consequently, the all-carbon hybrid fibers have tunable geometries, while maintaining good electrical conductivity, high ion-accessible surface area and mechanical strength; this allows us to address two important issues in micro-supercapacitor research. First, we found a clear correlation between the geometry of the hybrid fibers and their capacitive energy storage properties. Thinner fibers (30 μm in diameter) have higher specific volumetric capacitance (281 F cm−3), superior rate capability, and better length dependent performance. In contrast, larger-diameter hybrid fibers (236 μm in diameter) can achieve much higher specific length capacitance (42 mF cm−1). Second, we realized the first built-to-order concept for micro-supercapacitors by using all-carbon hybrid fibers with diversified geometry as electrodes. The device energy can cover two orders of magnitude, from <0.1 μW h to nearly 10 μW h, and the device power can be tuned in four orders of magnitude, from 0.2 μW to 2000 μW. Furthermore, multiple mechanically flexible fiber-based micro-supercapacitors can be integrated into complex energy storage units with wider operation voltage windows, demonstrating broad application potentials in flexible devices.

Amorphous Bimetallic Oxide–Graphene Hybrids as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn–Air Batteries

Metal oxides of earth-abundant elements are promising electrocatalysts to overcome the sluggish oxygen evolution and oxygen reduction reaction (OER/ORR) in many electrochemical energy-conversion devices. However, it is difficult to control their catalytic activity precisely. Here, a general three-stage synthesis strategy is described to produce a family of hybrid materials comprising amorphous bimetallic oxide nanoparticles anchored on N-doped reduced graphene oxide with simultaneous control of nanoparticle elemental composition, size, and crystallinity. Amorphous Fe0.5 Co0.5 Ox is obtained from Prussian blue analog nanocrystals, showing excellent OER activity with a Tafel slope of 30.1 mV dec-1 and an overpotential of 257 mV for 10 mA cm-2 and superior ORR activity with a large limiting current density of -5.25 mA cm-2 at 0.6 V. A fabricated Zn-air battery delivers a specific capacity of 756 mA h gZn-1 (corresponding to an energy density of 904 W h kgZn-1 ), a peak power density of 86 mW cm-2 and can be cycled over 120 h at 10 mA cm-2 . Other two amorphous bimetallic, Ni0.4 Fe0.6 Ox and Ni0.33 Co0.67 Ox , are also produced to demonstrate the general applicability of this method for synthesizing binary metal oxides with controllable structures as electrocatalysts for energy conversion.

Synergism of Water Shock and a Biocompatible Block Copolymer Potentiates the Antibacterial Activity of Graphene Oxide

Graphene oxide (GO) is promising in the fight against pathogenic bacteria. However, the antibacterial activity of pristine GO is relatively low and concern over human cytotoxicity further limits its potential. This study demonstrates a general approach to address both issues. The developed approach synergistically combines the water shock treatment (i.e., a sudden decrease in environmental salinity) and the use of a biocompatible block copolymer (Pluronic F-127) as a synergist co-agent. Hypoosmotic stress induced by water shock makes gram-negative pathogens more susceptible to GO. Pluronic forms highly stable nanoassemblies with GO (Pluronic-GO) that can populate around bacterial envelopes favoring the interactions between GO and bacteria. The antibacterial activity of GO at a low concentration (50 μg mL(-1) ) increases from <30% to virtually complete killing (>99%) when complemented with water shock and Pluronic (5 mg mL(-1) ) at ≈2-2.5 h of exposure. Results suggest that the enhanced dispersion of GO and the osmotic pressure generated on bacterial envelopes by polymers together potentiate GO. Pluronic also significantly suppresses the toxicity of GO toward human fibroblast cells. Fundamentally, the results highlight the crucial role of physicochemical milieu in the antibacterial activity of GO. The demonstrated strategy has potentials for daily-life bacterial disinfection applications, as hypotonic Pluronic-GO mixture is both safe and effective.

Bacterial physiology is a key modulator of the antibacterial activity of graphene oxide

Carbon-based nanomaterials have a great potential as novel antibacterial agents; however, their interactions with bacteria are not fully understood. This study demonstrates that the antibacterial activity of graphene oxide (GO) depends on the physiological state of cells for both Gram-negative and -positive bacteria. GO susceptibility of bacteria is the highest in the exponential growth phase, which are in growing physiology, and stationary-phase (non-growing) cells are quite resistant against GO. Importantly, the order of GO susceptibility of E. coli with respect to the growth phases (exponential ≫ decline > stationary) correlates well with the changes in the envelope ultrastructures of the cells. Our findings are not only fundamentally important but also particularly critical for practical antimicrobial applications of carbon-based nanomaterials.

Hydrogen evolution reaction activity of nickel phosphide is highly sensitive to electrolyte pH

The nickel phosphide (Ni2P) family of materials have become a hot subject in hydrogen evolution reaction (HER) electrocatalyst research. Various studies have reported their high activity, high stability, and high faradaic efficiency. To date, there have been no systematic studies regarding the influence of pH on the HER performance of Ni2P. Here we show that the pH of electrolytes can strongly influence the HER activity of Ni2P electrocatalysts. Tests in 19 electrolytes with pH ranging from 0.52 to 13.53 show that Ni2P is much more active in strongly acidic and basic electrolytes. With the increase of pH, the lower H+ concentration reduces the formation of adsorbed H atoms in the Volmer reaction, resulting in poorer activities. However, the high activity observed in the strongly basic electrolytes is not the intrinsic property of Ni2P. We found that Ni oxides/hydroxides are formed in strongly basic electrolytes under applied potentials, resulting in improved activities. Furthermore, the specific activity based on the electrochemically active surface area of recently reported Ni2P catalysts is not high and requires significant improvements for practical applications.

Metal-free bifunctional carbon electrocatalysts derived from zeolitic imidazolate frameworks for efficient water splitting

Metal-free carbon catalysts have attracted great interest because of their high electrical conductivity, tailorable porosity and surface area, affordability, and sustainability. In particular, their bifunctional activity for hydrogen and oxygen evolution reactions (HER and OER) is attractive for electrochemical splitting of water. However, pristine carbon materials have low activities for HER/OER. Here, a high-performance carbon electrocatalyst is demonstrated by first pyrolyzing a metal–organic framework (MOF), i.e., zeolitic imidazolate framework-8 (ZIF-8), followed by optimized cathodic polarization treatment (CPT). Pyrolyzing ZIF-8 produces a highly N-doped (8.4 at%) carbon material having a large specific surface area of 1017 m2 g−1 with micro and mesopores. CPT in 0.5 M H2SO4 for up to 8 hours modulates the composition of N- and O-containing surface functional groups of the pyrolyzed ZIF-8 without sacrificing its large surface area and pore size distribution. After the 6-hour CPT, this material shows an excellent HER activity in 0.5 M H2SO4 electrolyte with an overpotential of 155 mV, a Tafel slope of 54.7 mV dec−1, and an exchange current density of 0.063 mA cm−2. And the 4-hour CPT results in excellent OER activity in 0.1 M KOH electrolyte with an overpotential of 476 mV and a Tafel slope of 78.5 mV dec−1. In a demonstration, these two carbon electrocatalysts steadily run a two-electrode water electrolyzer at a current density of 10 mA cm−2 over 8 hours under a potential of 1.82 V with a Faradaic efficiency of 98.0–99.1% in 0.1 M KOH electrolyte. The superior activity of the designed carbon electrocatalysts can be attributed to the functional group composition modulation achieved by CPT. High-performance metal-free carbon electrocatalysts derived from MOFs show excellent potentials for energy and environmental applications.

Simultaneous DLS–SLS study of titanium and titanium/silicon oxide sol growth

AbstractA commercial DLS setup was used for simultaneous DLS/SLS analysis of sol growth of titanium and titanium/silicon oxides. The scattering data were analyzed in dynamic and static modes which allowed evaluating particle size and concentration simultaneously. A binary solvent (acetone/ethanol mixture) was introduced which effectively controls monodisperse growth behavior by simply adjusting its ratio. Fixing the solvent composition to the ratio which delayed gelation the most, the effect of the amount of catalyst (acetic acid), hydrolyzing agent (water) and titanium oxide precursor (titanium tetraisopropoxide) on growth kinetics were studied. Taking the advantage of extra functionalities of the catalyst used, acetic acid, i.e., decreasing the reactivity of titanium tetraisopropoxide and increasing the reactivity of tetraethyl orthosilicate, hybrid titanium/silicon oxide growth was also studied. Here, we step-by-step showed that particle size, particle concentration and sol-to-gel transition time of titanium and titanium/silicon oxide systems can be well controlled by adjusting the composition of formulations in ambient conditions. We also showed how practical the laser light scattering is to evaluate even the early onsets of growth profiles long before visual identification of clouding. The findings reported here are particularly important for practical applications of sol–gel technology where the control of particle size/concentration and gelation time is advantageous. Graphical Abstract

Assembly of pi-functionalized quaternary ammonium compounds with graphene hydrogel for efficient water disinfection

Graphene hydrogels hold great potential for the disinfection of bacteria-contaminated water. However, the intrinsic antibacterial activity of graphene hydrogels is not satisfactory, and the incorporation of other antibacterial agents often results in their unwanted releases. Here, we present a new strategy to improve the antibacterial activities of graphene hydrogels. We first synthesized a new pi-conjugated molecule containing five aromatic rings and two side-linked quaternary ammonium (QA) groups, denoted as piQA. Next, we fabricated composite gravity filters by assembling piQA with reduced graphene oxide (rGO) hydrogel. The rGO hydrogel helps to form a sponge-like physical sieve, contributes to the overall antibacterial activity, and provides abundant pi-rich surfaces. The large aromatic cores of piQA allow the formation of collectively strong pi-pi interactions with rGO, resulting in a high piQA mass loading of ∼31 wt%. Due to the sieving effect of rGO hydrogel and the synergistic antibacterial activity of rGO and piQA, the filters prepared based on piQA-rGO assemblies can remove over 99.5% of Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) cells with a high-water treatment capacity of 10 L g-1. Furthermore, the piQA-rGO assemblies show low toxicity towards two different mammalian cell lines (L929 and macrophages), and the release of piQA is also negligible. Overall, the new piQA-rGO assembly demonstrates high potential for water disinfection applications.

Membrane-based technologies for post-treatment of anaerobic effluents

Anaerobic digestion-based processes for converting wastewater into clean water and energy are attracting ever-growing industrial interest. However, apart from the microbial digestion step, current technologies require further progress from an integrated process point of view, including post-treatment steps. Anaerobic effluents normally undergo extensive post-treatment steps to meet stringent discharge standards, while valuable nutrients are rarely recovered. Additionally, a significant portion of the produced methane remains inevitably dissolved in the effluent, which is eventually released into the environment, causing economic loss and global warming concerns. To address these issues, several membrane-based technologies show significant promise. Here, we review current progress in membrane-based recovery of dissolved methane and nutrients, highlighting opportunities where membrane-based technologies can improve the post-treatment of anaerobic effluents. Lastly, we also share our perspectives for promising research directions and how to secure the competitiveness of membrane-based technologies for anaerobic wastewater treatment processes, focusing on current challenges for membrane development, biofouling mitigation strategies, and small-scale to large-scale implementation.