Photoenhanced Electrochemical Interaction between <i>Shewanella</i> and a Hematite Nanowire Photoanode

Here we report the investigation of interplay between light, a hematite nanowire-arrayed photoelectrode, and <i>Shewanella oneidensis</i> MR-1 in a solar-assisted microbial photoelectrochemical system (solar MPS). Whole cell electrochemistry and microbial fuel cell (MFC) characterization of <i>Shewanella oneidensis</i> strain MR-1 showed that these cells cultured under (semi)­anaerobic conditions expressed substantial <i>c</i>-type cytochrome outer membrane proteins, exhibited well-defined redox peaks, and generated bioelectricity in a MFC device. Cyclic voltammogram studies of hematite nanowire electrodes revealed active electron transfer at the hematite/cell interface. Notably, under a positive bias and light illumination, the hematite electrode immersed in a live cell culture was able to produce 150% more photocurrent than that in the abiotic control of medium or dead culture, suggesting a photoenhanced electrochemical interaction between hematite and <i>Shewanella</i>. The enhanced photocurrent was attributed to the additional redox species associated with MR-1 cells that are more thermodynamically favorable to be oxidized than water. Long-term operation of the hematite solar MPS with light on/off cycles showed stable current generation up to 2 weeks. Fluorescent optical microscope and scanning electron microscope imaging revealed that the top of the hematite nanowire arrays were covered by a biofilm, and iron determination colorimetric assay revealed 11% iron loss after a 10-day operation. To our knowledge, this is the first report on interfacing a photoanode directly with electricigens in a MFC system. Such a system could open up new possibilities in solar-microbial device that can harvest solar energy and recycle biomass simultaneously to treat wastewater, produce electricity, and chemical fuels in a self-sustained manner

Multiscale Pore Network Boosts Capacitance of Carbon Electrodes for Ultrafast Charging

Increasing charge storage capability during fast charging (at ultrahigh current densities) has been a long-standing challenge for supercapacitors. In this work, a novel porous carbon foam electrode with multiscale pore network is reported that achieves a remarkable gravimetric capacitance of 374.7 ± 7.7 F g^–1 at a current density of 1 A g^–1. More importantly, it retains 235.9 ± 7.5 F g^–1 (60% of its capacitance at 1 A g^–1) at an ultrahigh current density of 500 A g^–1. Electron microscopy studies reveal that this carbon structure contains multiscale pores assembled in a hierarchical pattern. The outstanding capacitive performance benefits from its extremely large surface area of 2905 m² g^–1, as around 88% of the electric charges are stored via electrical double layer. Significantly, electrochemical analyses show that the hierarchical porous structure containing macro-, meso-, and micropores allows efficient ion diffusion and charge transfer, resulting in the excellent rate capability. The findings pave the way for improving rate capability of supercapacitors and enhancing their capacitances at ultrahigh current densities

Hydrogenated TiO₂ Nanotube Arrays for Supercapacitors

We report a new and general strategy for improving the capacitive properties of TiO₂ materials for supercapacitors, involving the synthesis of hydrogenated TiO₂ nanotube arrays (NTAs). The hydrogenated TiO₂ (denoted as H–TiO₂) were obtained by calcination of anodized TiO₂ NTAs in hydrogen atmosphere in a range of temperatures between 300 to 600 °C. The H–TiO₂ NTAs prepared at 400 °C yields the largest specific capacitance of 3.24 mF cm^–2 at a scan rate of 100 mV s^–1, which is 40 times higher than the capacitance obtained from air-annealed TiO₂ NTAs at the same conditions. Importantly, H–TiO₂ NTAs also show remarkable rate capability with 68% areal capacitance retained when the scan rate increase from 10 to 1000 mV s^–1, as well as outstanding long-term cycling stability with only 3.1% reduction of initial specific capacitance after 10 000 cycles. The prominent electrochemical capacitive properties of H–TiO₂ are attributed to the enhanced carrier density and increased density of hydroxyl group on TiO₂ surface, as a result of hydrogenation. Furthermore, we demonstrate that H–TiO₂ NTAs is a good scaffold to support MnO₂ nanoparticles. The capacitor electrodes made by electrochemical deposition of MnO₂ nanoparticles on H–TiO₂ NTAs achieve a remarkable specific capacitance of 912 F g^–1 at a scan rate of 10 mV s^–1 (based on the mass of MnO₂). The ability to improve the capacitive properties of TiO₂ electrode materials should open up new opportunities for high-performance supercapacitors

LiCl/PVA Gel Electrolyte Stabilizes Vanadium Oxide Nanowire Electrodes for Pseudocapacitors

Here we report a new strategy to improve the electrochemical stability of vanadium oxide electrodes for pseudocapacitors. Vanadium oxides are known to suffer from severe capacitance loss during charging/discharging cycling, due to chemical dissolution and ion intercalation/deintercalation-induced material pulverization. We demonstrate that these two issues can be addressed by using a neutral pH LiCl/PVA gel electrolyte. The function of the gel electrolyte is twofold: (i) it reduces the chemical dissolution of amphoteric vanadium oxides by minimizing water content and providing a neutral pH medium and (ii) it serves as a matrix to maintain the vanadium oxide nanowire network structure. Vanadium oxide nanowire pseudocapacitors with gel electrolyte exhibit excellent capacitance retention rates of more than 85% after cycling for 5000 cycles, without sacrificing the electrochemical performance of vanadium oxides

Dependence of Interfacial Charge Transfer on Bifunctional Aromatic Molecular Linkers in CdSe Quantum Dot Sensitized TiO₂ Photoelectrodes

Quantum dot (QD) sensitization of TiO₂ is a powerful method to improve its performance as a photoanode material in solar energy conversion. The efficiency of sensitization depends strongly on the rate of interfacial electron transfer (ET) from the QDs to TiO₂. To understand the key factors affecting the ET, arene-substituted (ortho, meta, and para) bifunctional linkers with single or double aromatic rings were employed to link CdSe QDs to TiO₂ and control the strength of their interaction as well as the rate of interfacial ET. Interestingly, the para-substituted aromatic linker, 4-mercaptobenzoic acid (4MBA) with the longest distance between the carboxyl and thiol groups, shows the best photoelectrochemical (PEC) performance, when compared to those of ortho-subtituted (2-mercaptobenzoic acid, 2MBA) and meta-substituted (3-mercaptobenzoic acid, 3MBA) aromatic linkers. Two other bifunctional linkers with double aromatic rings, 4′-mercapto-[1,1′-biphenyl]-4-carboxylic acid (4M1B4A) and 6-mercapto-2-naphthioc acid (6M2NA), were also studied for comparison. Ultrafast transient absorption (TA) spectroscopy was used to study the exciton dynamics in CdSe QDs and determine the interfacial ET rate constant (<i>k</i>_ET). The <i>k</i>_ET results are consistent with the trend of PEC measurements in that 4MBA shows the highest <i>k</i>_ET. To gain further insight into the ET mechanism, we performed density functional theory (DFT) calculations to examine the intrinsic properties of the linkers. The results revealed that the favorable wave function distribution of the molecular orbitals of 4MBA and 4M1B4A are responsible for the higher interfacial ET rate and PEC performance due to better interfacial coupling, a factor that dominates over distance. The present study provides important new insight into the mechanism of interfacial ET using aromatic bifunctional linkers, which is useful in designing QD sensitized semiconductor metal oxide nanostructures for applications including photovoltaics and solar fuel generation

Photohole Induced Corrosion of Titanium Dioxide: Mechanism and Solutions

Titanium dioxide (TiO₂) has been extensively investigated as photoanode for water oxidation, as it is believed to be one of the most stable photoanode materials. Yet, we surprisingly found that TiO₂ photoanodes (rutile nanowire, anatase nanotube, and P25 nanoparticle film) suffered from substantial photocurrent decay in neutral (Na₂SO₄) as well as basic (KOH) electrolyte solution. Photoelectrochemical measurements togehter with electron microscopy studies performed on rutile TiO₂ nanowire photoanode show that the photocurrent decay is due to photohole induced corrosion, which competes with water oxidation reaction. Further studies reveal that photocurrent decay profile in neutral and basic solutions are fundamentally different. Notably, the structural reconstruction of nanowire surface occurs simultaneously with the corrosion of TiO₂ in KOH solution resulting in the formation of an amorphous layer of titanium hydroxide, which slows down the photocorrosion. Based on this discovery, we demonstrate that the photoelectrochemical stability of TiO₂ photoanode can be significantly improved by intentionally coating an amorphous layer of titanium hydroxide on the nanowire surface. The pretreated TiO₂ photaonode exhibits an excellent photocurrent retention rate of 97% after testing in KOH solution for 72 h, while in comparison the untreated sample lost 10−20% of photocurrent in 12 h under the same measurement conditions. This work provides new insights in understanding of the photoelectrochemical stability of bare TiO₂ photoanodes

An Electrochemical Capacitor with Applicable Energy Density of 7.4 Wh/kg at Average Power Density of 3000 W/kg

Electrochemical capacitors represent a new class of charge storage devices that can simultaneously achieve high energy density and high power density. Previous reports have been primarily focused on the development of high performance capacitor electrodes. Although these electrodes have achieved excellent specific capacitance based on per unit mass of active materials, the gravimetric energy densities calculated based on the weight of entire capacitor device were fairly small. This is mainly due to the large mass ratio between current collector and active material. We aimed to address this issue by a 2-fold approach of minimizing the mass of current collector and increasing the electrode performance. Here we report an electrochemical capacitor using 3D graphene hollow structure as current collector, vanadium sulfide and manganese oxide as anode and cathode materials, respectively. 3D graphene hollow structure provides a lightweight and highly conductive scaffold for deposition of pseudocapacitive materials. The device achieves an excellent active material ratio of 24%. Significantly, it delivers a remarkable energy density of 7.4 Wh/kg (based on the weight of entire device) at the average power density of 3000 W/kg. This is the highest gravimetric energy density reported for asymmetric electrochemical capacitors at such a high power density

Polyaniline and Polypyrrole Pseudocapacitor Electrodes with Excellent Cycling Stability

Conducting polymers such as polyaniline and polypyrrole have been widely used as pseudocapacitive electrode materials for supercapacitors. However, their structural instability resulting from repeated volumetric swelling and shrinking during charge/discharge process has been a major hurdle for their practical applications. This work demonstrates a simple and general strategy to substantially enhance the cycling stability of conductive polymer electrodes by deposition of a thin carbonaceous shell onto their surface. Significantly, carbonaceous shell-coated polyaniline and polypyrrole electrodes achieved remarkable capacitance retentions of ∼95 and ∼85% after 10 000 cycles. Electron microscopy studies revealed that the presence of ∼5 nm thick carbonaceous shell can effective prevent the structural breakdown of polymer electrodes during charge/discharge process. Importantly, the polymer electrodes with a ∼5 nm thick carbonaceous shell exhibited comparable specific capacitance and pseudocapacitive behavior as the bare polymer electrodes. We anticipate that the same strategy can be applied for stabilizing other polymer electrode materials. The capability of fabricating stable polymer electrodes could open up new opportunities for pseudocapacitive devices

High Energy Density Asymmetric Quasi-Solid-State Supercapacitor Based on Porous Vanadium Nitride Nanowire Anode

To push the energy density limit of asymmetric supercapacitors (ASCs), a new class of anode materials is needed. Vanadium nitride (VN) holds great promise as anode material for ASCs due to its large specific capacitance, high electrical conductivity, and wide operation windows in negative potential. However, its poor electrochemical stability severely limits its application in SCs. In this work, we demonstrated high energy density, stable, quasi-solid-state ASC device based on porous VN nanowire anode and VO_<i>x</i> nanowire cathode for the first time. The VO_<i>x</i>//VN-ASC device exhibited a stable electrochemical window of 1.8 V and excellent cycling stability with only 12.5% decrease of capacitance after 10 000 cycles. More importantly, the VO_<i>x</i>//VN-ASC device achieved a high energy density of 0.61 mWh cm^–3 at current density of 0.5 mA cm^–2 and a high power density of 0.85 W cm^–3 at current density of 5 mA cm^–2. These values are substantially enhanced compared to most of the reported quasi/all-solid-state SC devices. This work constitutes the first demonstration of using VN nanowires as high energy anode, which could potentially improve the performance of energy storage devices

Synthesis, Optical Properties, and Exciton Dynamics of Organolead Bromide Perovskite Nanocrystals

Organolead bromide CH₃NH₃PbBr₃ perovskite nanocrystals (PNCs) with green photoluminescence (PL) have been synthesized using two different aliphatic ammonium capping ligands, octylammonium bromide (OABr) and octadecylammonium bromide (ODABr), resulting in PNC–OABr and PNC–ODABr, respectively. Structural studies by X-ray diffraction (XRD) and transmission electron microscopy (TEM) determined that the PNCs exhibit cubic phase crystal structure with average particle size dependent on capping ligand (3.9 ± 1.0 nm for PNC–OABr and 6.5 ± 1.4 nm for PNC–ODABr). The exciton dynamics of PNCs were investigated using femtosecond transient absorption (TA) techniques and singular value decomposition global fitting (SVD-GF), which revealed nonradiative recombination on the picosecond time scale mediated by surface trap states for both types of PNCs. The PL lifetime of the PNCs was measured by time-resolved photoluminescence (TRPL) spectroscopy and fit with integrated SVD-GF to determine the radiative as well as nonradiative lifetimes on the nanosecond time scale. Finally, a simple model is proposed to explain the optical and dynamic properties of the PNCs with emphasis on major exciton relaxation or electron–hole recombination processes. The results indicate that the use of capping ligand OABr resulted in PNCs with a high PL quantum yield (QY) of ∼20% (vs fluorescein, 95%), which have interesting optical properties and are promising for potential applications including photovoltaics, detectors, and light-emitting diodes (LEDs)