Semiconductor-based photocatalysts and photoelectrochemical cells for solar fuel generation: a review

Hybrid metal-ion capacitors (MICs) (M stands for Li or Na) are designed to deliver high energy density, rapid energy delivery, and long lifespan. The devices are composed of a battery anode and a supercapacitor cathode, and thus become a tradeoff between batteries and supercapacitors. In the past two decades, tremendous efforts have been put into the search for suitable electrode materials to overcome the kinetic imbalance between the battery-type anode and the capacitor-type cathode. Recently, some transition-metal compounds have been found to show pseudocapacitive characteristics in a nonaqueous electrolyte, which makes them interesting high-rate candidates for hybrid MIC anodes. Here, the material design strategies in Li-ion and Na-ion capacitors are summarized, with a focus on pseudocapacitive oxide anodes (Nb2 O5 , MoO3 , etc.), which provide a new opportunity to obtain a higher power density of the hybrid devices. The application of Mxene as an anode material of MICs is also discussed. A perspective to the future research of MICs toward practical applications is proposed to close.

Nonaqueous Hybrid Lithium-Ion and Sodium-Ion Capacitors

Vanchiappan Aravindan,*,† Joe Gnanaraj,*,‡ Yun-Sung Lee,* and Srinivasan Madhavi*,†,∥ †Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Research Techno Plaza, 50 Nanyang Drive, Singapore 637553, Singapore ‡Yardney Technical Products, Inc., 2000 South County Trail, East Greenwich, Rhode Island 02818, United States Faculty of Applied Chemical Engineering, Chonnam National University, Gwang-ju 500-757, Korea School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

Insertion-Type Electrodes for Nonaqueous Li-Ion Capacitors

Photoelectrochemical (PEC) water splitting is a promising technology for solar hydrogen production to build a sustainable, renewable and clean energy economy. Hematite (α-Fe2O3) based photoanodes offer promise for such applications, due to their high chemical stability, great abundance and low cost. Despite these promising properties, progress towards the manufacture of practical water splitting devices has been limited. This review is intended to highlight recent advancements and the limitations that still hamper the full utilization of hematite electrodes in PEC water splitting systems. We review recent progress in manipulating hematite for PEC water splitting through various approaches, focused on e.g. enhancing light absorption, water oxidation kinetics, and charge carrier collection efficiency. As the morphology affects various properties, progress in morphological characterization from thicker planar films to recent ultrathin nanophotonic morphologies is also examined. Special emphasis has been given to various ultrathin films and nanophotonic structures which have not been given much attention in previous review articles.

Using hematite for photoelectrochemical water splitting: a review of current progress and challenges.

Hydrogen is readily obtained from renewable and non-renewable resources via water splitting by using thermal, electrical, photonic and biochemical energy. The major hydrogen production is generated from thermal energy through steam reforming/gasification of fossil fuel. As the commonly used non-renewable resources will be depleted in the long run, there is great demand to utilize renewable energy resources for hydrogen production. Most of the renewable resources may be used to produce electricity for driving water splitting while challenges remain to improve cost-effectiveness. As the most abundant energy resource, the direct conversion of solar energy to hydrogen is considered the most sustainable energy production method without causing pollutions to the environment. In overall, this review briefly summarizes thermolytic, electrolytic, photolytic and biolytic water splitting. It highlights photonic and electrical driven water splitting together with photovoltaic-integrated solar-driven water electrolysis.

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Recent Progress in Energy-Driven Water Splitting

In this Article, we report a strategy to perform in situ incorporation of oxygen evolution catalyst, Co3O4, during hydrothermal growth of Fe2O3 nanorod arrays. It was found that the highest photocurrent increase and onset potential shift was observed with 5% Co2+. The photocurrent density increases from 0.72 for the pristine Fe2O3 nanorod to 1.20 mA/cm2 at 1.23 V versus RHE (i.e., 67% improvement) with 5% Co2+ added. Concomitant with this improvement was a shift in the onset potential by ∼40 mV and improvements in incident-photon-to-current efficiencies and oxygen evolution. Hematite photoanodes with in situ deposition of Co3O4 nanoparticles showed better performance than those prepared by ex situ procedures because of high surface roughness, larger Co3O4/hematite interfacial area, and smaller Co3O4 particle size.

https://dr.ntu.edu.sg/bitstream/10356/94973/1/Co3O4%20decorated%20hematite%20nanorods-6th%20Jun.pdf

Co3O4-decorated hematite nanorods as an effective photoanode for solar water oxidation

In this paper, we report a novel strategy for surface treatment of hematite nanorods for efficient photo-driven water oxidation. This is the first report describing the growth of Sn treated hematite from α-FeOOH nanorod arrays in one step without substantially altering morphologies. With this treatment the photocurrent density increased from 1.24 for pristine hematite nanorods to 2.25 mA cm−2 at 1.23 V vs. RHE (i.e. 81% improvement). The increase in photocurrent density was also accompanied by improved incident-photon-to-current efficiencies and oxygen evolution. The photocurrent improvement is mainly attributed to a reduced electron–hole recombination at the hematite–electrolyte interface through the formation of FexSn1−xO4 layer at the hematite nanorod surface as shown by XPS, HRTEM, EDAX line scan analyses and PEC measurements.

A novel strategy for surface treatment on hematite photoanode for efficient water oxidation

A simple electrospinning technique is employed for the preparation of high-performance V2O5 nanofibers. The fibers thus prepared are subjected to heat treatment under the optimized conditions at 400 °C in air to achieve a single phase. The powder X-ray diffraction pattern confirms the formation of an orthorhombic structure with Pmmn space group. Morphological studies conducted by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), clearly reveal the presence of a highly interconnected network of fibers with the diameter ranging from approximately 500–800 nm. After the heat treatment, translation of smooth fibrous morphology into porous fibers with embedded nanocrystals of V2O5 is noticed from the SEM measurements. The sintered V2O5 nanofibers are used to fabricate a hybrid electrochemical capacitor (HEC) and it is coupled with a substrate-free single-walled carbon nanotube (SWCNT) network (called “Bucky paper”) in a conventional organic electrolyte solution. Supercapacitive behavior of HEC is studied in both potentiostatic and galvanostatic modes at room temperature. The HEC demonstrated very stable and excellent cycling behavior during 3500 cycles of galvanostatic charge and discharge tests. This hybrid system is also well established during the rate capability studies from the applied current density of 30 to 210 mA g−1 and delivered maximum energy and power densities of 18 Wh kg−1 and 315 W kg−1, respectively.

Fabrication of High Energy‐Density Hybrid Supercapacitors Using Electrospun V2O5 Nanofibers with a Self‐Supported Carbon Nanotube Network

A simple and inexpensive method to form a hematite photoanode for efficient water oxidation is reported. A very thin ZnO overlayer was deposited on top of a thin film of hematite and found, compared with non-treated hematite, to increase the photocurrent and reduce the onset potential for generating oxygen from water. After 3 cycles of ZnAc treatment, the photocurrent increased more than 40% to 1.08 mA cm−2 at 0.23 V vs. Ag/AgCl and onset potential for water oxidation shifted by −170 mV. It is proposed that the ZnO overlayer changes the flat band potential of hematite and reduces the surface defects.

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Surface treatment of hematite photoanodes with zinc acetate for water oxidation

/pdf/high-energy-density-asymmetric-supercapacitor-based-on-3b6bjtkngu.pdf

Wai Fatt Mak

Papers

Co3O4-decorated hematite nanorods as an effective photoanode for solar water oxidation

A novel strategy for surface treatment on hematite photoanode for efficient water oxidation

Fabrication of High Energy‐Density Hybrid Supercapacitors Using Electrospun V2O5 Nanofibers with a Self‐Supported Carbon Nanotube Network

Surface treatment of hematite photoanodes with zinc acetate for water oxidation

High-Energy Density Asymmetric Supercapacitor Based on Electrospun Vanadium Pentoxide and Polyaniline Nanofibers in Aqueous Electrolyte