Mesoporous TiO2 Photoanode Research Articles

Formic Acid Photo-Fuel Cells Consisting of TiO2 Photoanode and Pt Cathode.

Formic acid (HCOOH) has attracted much attention as a promising power source for portable electronic devices because of its ease of storage and transportation. Here we report that a simple HCOOH photo-fuel cell (PFC) consisting of mesoporous anatase TiO2 photoanode and Pt cathode stably delivers a short-circuit photocurrent (Jsc) of 5.94 mA cm-2 and an open-circuit voltage of 0.94 V under UV-light irradiation (light intensity, I=200 mW cm-2). The incident photon-to-current conversion efficiency and Faradaic efficiency reach ~90 % and ~100 %, respectively. The excellent performances of this HCOOH PFC, designed based on the discovery that HCOOH provides a large photocurrent by current doubling even in the presence of O2, not only solves the problem of conventional HCOOH FCs, but also achieves the performances far exceeding those of PFCs using biomass-derived organics reported so far.

Photoanode Modification Based on Lignin Extracted from Rice Husk for Dye-Sensitized Solar Cells

The photoanode optimization of dye-sensitized solar cells (DSSCs) has been developed to create renewable devices based on green chemistry. TiO2 plays a crucial role in light absorption, charge separation, and electron transport within the photoanode of DSSCs. Modifying mesoporous TiO2 photoanode using lignin-template has been considered a viable option for photocatalyst synthesis. Lignin derived from rice husk is a potential material due to its abundant sources in many countries. Therefore, in this work, rice husk lignin was employed for non-toxic and low-cost material performance improvement of TiO2. Experimental results showed that the best efficiency and current density were identified as the effect of adding a 5% concentration of lignin on the TiO2 composite, namely 4.81% PCE.

Investigating the Influence of PbS Quantum Dot-Decorated TiO2 Photoanode Thickness on Photoelectrochemical Hydrogen Production Performance.

To maximize the photoelectrochemical (PEC) hydrogen production performance of quantum dot (QD)-decorated photoelectrodes, it is crucial to prioritize the optimization of electrode's structure, including thickness and porosity. In this study, we prepare PbS QD-decorated mesoporous TiO2 photoanodes for PEC hydrogen production, and systematically investigate the influence of the photoanode thickness on optical properties and PEC performances. As the thickness of photoanodes increases from 6.4 µm to 16.3 µm, the light absorption capability is enhanced across the entire visible and near-infrared (IR) spectrum due to the improved loading of PbS QDs. However, the photocurrent density is optimized for the 11.9 µm thick photoanode (15.19 mA/cm2), compared to the 6.4 µm thick (10.80 mA/cm2) and 16.3 µm thick photoanodes (11.93 mA/cm2). This optimization is attributed to the trade-off between the light absorption capability and the efficient mass transfer of the electrolyte as the photoanode thickness increases, which is confirmed by the lowest charge transfer resistance (Rct) evaluated from the electrochemical impedance data.

(Invited) C–C Bond Cleavage of Lignin with an Organic Dye-Sensitized Photoanode

Chemoselective C–C bond cleavage remains a challenge for the degradation of polymers because of the relatively high bond dissociation energy of C–C σ-bonds at room temperature. Organic molecular-based dye-sensitized photoelectrochemical cells (DSPECs) could offer a means of using renewable solar energy to drive energetically demanding chemoselective C–C bond cleavage reaction. This study reports the solar light-driven activation of a bicyclic aminoxyl mediator to achieve C–C bond cleavage in the aryl-ether linkage of a lignin model compound (LMC) at room temperature using a donor–π-conjugated bridge–acceptor (D–π–A) organic dye-based DSPEC system. The 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid (DPTC) D–π–A organic dye was investigated along with a bicyclic aminoxyl radical mediator (9-azabicyclo[3,3,1]nonan-3-one-9-oxyl, KABNO) in solution and at the interface of a mesoporous structured TiO2 substrate in the presence of LMC. Photophysical studies of DPTC with KABNO were carried out to show intermolecular energy/electron transfer under 1 sun illumination (100 mW·cm− 2). The D–π–A type DPTC sensitized mesoporous TiO2 photoanode in the presence of KABNO can facilitate the generation of the reactive oxoammonium species KABNO+ as a strong oxidizing agent, which plays an important role in the photocatalytic oxidative C–C bond cleavage of LMC. The photoelectrochemical oxidative reaction in a complete DSPEC with KABNO afforded the chemoselective C–C bond cleavage products 2-(2-methoxyphenoxy)acrylaldehyde (94%) and 2,6-dimethoxy-1,4-benzoquinone (66%). This process provides a first report utilizing a D–π–A type organic dye in combination with a bicyclic nitroxyl radical mediator for heterogeneous photoelectrolytic oxidative cleavage of C–C σ-bonds, modeled on those found in lignin, at room temperature.

Scalable, solution-based dye-sensitized photovoltaic fibers with an electrolytic solid-state ion conductor

We have created a textile with solid-state dye-sensitized photovoltaic fibers (DSPFs) fabricated using a unique combination of inexpensive, commercial-off-the-shelf materials and solution-based processing deposition techniques. DSPFs were made possible by macroscopic support of liquid state components in both the electrolyte and photoanode of the DSPFs. A plastic crystal system of succinonitrile was doped with iodide salts to achieve a solid-state ion-conducting electrolyte. Similarly, utilization of a mesoporous TiO2 photoanode created an adsorptive scaffold for photoactive dyes in a liquid solution. An inexpensive pairing of coated carbon and stainless steel, replacing the traditional and expensive platinum-based counter electrode, was used as a catalytic counter electrode. A solid-state DSPF including both electrodes aligned in comparative orientations provided peak power of up to 0.4 µW/mm with open-circuit voltage (VOC) of 0.7 V. A module of solid-state DSPFs was then incorporated into a simple weave in a mini-loom to demonstrate textile capability. Solid-state DSPFs woven and connected in series in the loom produced peak power of over 50 µW and VOC of 4.5 V. Creation of the flexible DSPFs was achieved through a series of simple and scalable solution-based coating processes, and to the best of our knowledge, has not been previously reported in literature to produce fully encapsulated solid-state dye-sensitized photovoltaic cells with a fiber form factor. Low cost, high throughput processing of this technology can potentially support the manufactural transition for the next generation of commercial energy harvesting wearable electronics.

Enhanced photoelectrochemical performance of mesoporous TiO2 photoanode prepared by multi-step evaporation-induced self-assembly method

Mesoporous TiO2 films are promising in photocatalysis and photoelectrochemistry for energy and environment applications, because of its high specific surface area and regular mesopore structure. However, the performances of mesoporous TiO2 films are still limited by low catalyst loading due to the damage during the preparation process. In this study, a multi-step evaporation-induced self-assembly method is proposed to develop a mesoporous TiO2 photoanode with high catalyst loading. The prepared mesoporous TiO2 photoanodes possess not only high catalyst loading without damage but also large specific surface area, which provides more active sites. Moreover, regular mesopore structure is remained to benefit mass transport. Because of these merits, the mesoporous TiO2 photoanode prepared by the multi-step evaporation-induced self-assembly method exhibits superior performance than does conventional nanoparticles-packed TiO2 photoanode. The superiority of the mesoporous TiO2 photoanode prepared by this multi-step evaporation-induced self-assembly method is also demonstrated by incorporating it into a photocatalytic fuel cell. The short circuit current density and maximum power density increase by 90.9% and 25.0%, respectively. The present work provides a new way to prepare high-performance mesoporous TiO2 films for various photocatalytic and photoelectrochemical applications.

The ultrathin PEALD-GaN surface/interface layer-modulated charge dynamics in quantum dot-sensitized solar cells

In this work, gallium nitride (GaN) is employed for the first time to modulate the charge dynamics of quantum dot-sensitized solar cells (QDSCs). An ultrathin GaN layer has been coated on the surface of both mesoporous TiO2 photoanode and quantum dots (QDs) at 240 °C by plasma-enhanced atomic layer deposition (PEALD) approach. It is revealed that there exists a stepped energy level alignment among the as-prepared TiO2 film, GaN layer and QDs, which accelerates the extraction and collection of photogenerated electrons. Meanwhile, a type-II core-shell QD/GaN structure is formed benefiting from the self-limiting reactions of PEALD, resulting in an enhanced light absorption and a redshift of absorption edge. In addition, the dense GaN layer can also effectively inhibit the reverse transfer of photogenerated electrons from TiO2 to QDs or electrolyte while improving the connection between TiO2 and QDs. Ultimately, the QDSCs with a 0.68 nm-thick GaN layer achieve a 29% increase of short-circuit current density and enhanced device efficiency, even with reduced fill factor. This work has shown the multi-functions of GaN in regulating the charge dynamics of QDSCs as well as the potential advantages in replacing TiO2 as photoanode for electronic extraction and transport.

Photovoltaic Performance of TiO2 Mesoporous Films with Different Working Areas for Dye-sensitized Solar Cells

The photovoltaic performance of Dye-sensitized Solar Cells (DSSC) could be optimized by playing the working area of the mesoporous TiO2 photoanode. A small working area will enhance the photovoltage and power conversion efficiency. In comparison, a large working area will improve the recombination rate and reduce the photon particle transport rate at the photoanode. The TiO2 based photoanode layer consists of a blocking layer and mesoporous paste deposited using spin coating and screen printing technique, respectively with various working areas of 0.25, 0.30, and 0.56 cm2. The cells were characterized using XRD, SEM-EDX, UV-Vis spectroscopy, and solar simulator. XRD confirmed that the TiO2 was in the anatase phase. SEM image represented a high surface area of mesoporous TiO2 indicated by porosity level up to 70%. The elemental analysis of TiO2 using EDX observed the presence of Ti and O peaks. The TiO2 mesoporous-based photoanode immersed in 0.07 mM of N719 solution possessed an absorbance range in the ultraviolet to the visible light. The working area of 0.25 cm2 exhibited a promising power conversion efficiency of 2.02% with a circuit current density of 10.8 mA/cm2 under an illumination light of 100 mW/cm2.

Aqueous Biphasic Dye‐Sensitized Photosynthesis Cells for TEMPO‐Based Oxidation of Glycerol

AbstractThis work reports an aqueous dye‐sensitized photoelectrochemical cell (DSPEC) capable of oxidizing glycerol (an archetypical biobased compound) coupled with H2 production. We employed a mesoporous TiO2 photoanode sensitized with the high potential thienopyrroledione‐based dye AP11, encased in an acetonitrile‐based redox‐gel that protects the photoanode from degradation by aqueous electrolytes. The use of the gel creates a biphasic system with an interface at the organic (gel) electrode and aqueous anolyte. Embedded in the acetonitrile gel is 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), acting as both a redox‐mediator and a catalyst for oxidative transformations. Upon oxidation of TEMPO by the photoexcited dye, the in situ generated TEMPO+ shuttles through the gel to the acetonitrile–aqueous interface, where it acts as an oxidant for the selective conversion of glycerol to glyceraldehyde. The introduction of the redox‐gel layer affords a 10‐fold increase in the conversion of glycerol compared to the purely aqueous system. Our redox‐gel protected photoanode yielded a stable photocurrent over 48 hours of continuous operation, demonstrating that this DSPEC is compatible with alkaline aqueous reactions.

Aqueous Biphasic Dye-Sensitized Photosynthesis Cells for TEMPO-Based Oxidation of Glycerol.

This work reports an aqueous dye‐sensitized photoelectrochemical cell (DSPEC) capable of oxidizing glycerol (an archetypical biobased compound) coupled with H2 production. We employed a mesoporous TiO2 photoanode sensitized with the high potential thienopyrroledione‐based dye AP11, encased in an acetonitrile‐based redox‐gel that protects the photoanode from degradation by aqueous electrolytes. The use of the gel creates a biphasic system with an interface at the organic (gel) electrode and aqueous anolyte. Embedded in the acetonitrile gel is 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), acting as both a redox‐mediator and a catalyst for oxidative transformations. Upon oxidation of TEMPO by the photoexcited dye, the in situ generated TEMPO+ shuttles through the gel to the acetonitrile–aqueous interface, where it acts as an oxidant for the selective conversion of glycerol to glyceraldehyde. The introduction of the redox‐gel layer affords a 10‐fold increase in the conversion of glycerol compared to the purely aqueous system. Our redox‐gel protected photoanode yielded a stable photocurrent over 48 hours of continuous operation, demonstrating that this DSPEC is compatible with alkaline aqueous reactions.

Understanding the optical behaviours and the power conversion efficiency of novel organic dye and nanostructured TiO2 based integrated DSSCs

The rapid use of fossil fuels has led to quick industrialization, swift economic growth, improved living standards, a high release of greenhouse gas, and fast exhaustion of fuel resources, which is a big threat to the environment and climate changes. To keep the planet green, dye-sensitized solar cells (DSSCs) have attracted much attention as the third-generation solar cells. This article documented an extensive study on the efficient use of natural dyes (Portulaca grandiflora, pereskia bleo, and alternanthera ficoidea), mesoporous TiO2 photoanode, a carbon cathode, and conductive layers to improve efficiency of DSSCs at ambient conditions. All of the conductive/TiO2/natural dyes display enhanced structural, optical, and electrical properties. X-ray diffraction studies of conductive/TiO2 layers represent the tetragonal structure which is buttressed by the SEM imaging, whereas organic dyes are amorphous in nature. The integrated conductive/TiO2/natural dye films have good sensitization in the visible spectrum (380–750 nm) and possess a bandgap falling (1.25–1.50 eV) and high refractive index while the maximum optical conductivity is observed in pereskia bleo. On the other hand, alternanthera ficoidea and portulaca grandiflora exhibit higher electrical conductivity (7.5 × 1012 Scm−1) and optical electronegativity (1.777). The TiO2 photoanodes at a conductive layer thickness of 3886.8 nm with iodide/tri-iodide redox liquid electrolyte generated current densities of 0.071–0.106 mAcm−2 under AM 1.5 illumination. Furthermore, the short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and the corresponding photon-to-current conversion efficiency (η) estimated from the J-V characteristic curves with an effective area of 1 cm2 calculated for pereskia bleoDSSC are JSC = 0.106 mAcm−2, VOC = 0.023 V,FF = 45, andη = 0.11%. This study will open up a new door to fabricate highly efficient solar cells by modifying different parameters (photoanode, conductive layer, hybrid active layer, or different electrolyte) that can achieve meaningful success in photovoltaic industries.

Tandem DSSC fabrication by controlled infiltration of organic dyes in mesoporous electrode using electric-field assisted spray technique

Tandem absorber architectures in DSSCs for extending the light absorption range, using two dyes (N719/Red dye and N749/Black dye) appropriately infiltered in the pores of a mesoporous TiO2 photoanode, have been prepared for the first time by electric-field assisted spray technique (E-Spray). In comparison to the conventionally used dip technique, which takes several hours the E-spray technique developed in this study significantly reduces the loading time to 10–15 min by rapid pore filling of dual dyes. The shorter wavelength (450–550 nm) absorber -N719 dye and longer wavelength (550–800 nm) absorber -N749 dye have been infiltrated sequentially and in right proportion by precise control over E-Spray parameters in order to capture a wider range of the incident solar spectrum. The tandem DSSCs (T-DSSCs) thus fabricated show substantial improvement in photo-current density (Jsc), resulting in ~11% enhancement in photoconversion efficiency (PCE), as compared to a single-dye device. The best performing T-DSSC (N719:N749-75:25) yields the Jsc, Voc and FF of ~16 mA/cm2, ~0.75 V and ~0.69, respectively, resulting in PCE of 7.5%, which is ~40% higher than the PCE of 5.33% for the conventionally fabricated dip-coated DSSC. This study opens up door for tandem device fabrication using E-spray technique which is suitable for large area and roll-to-roll manufracturing aiming for high PCE solar cells.

Influence of different concentrations of 4-tert-butyl-pyridine in a gel polymer electrolyte towards improved performance of Dye-Sensitized Solar Cells (DSSC)

Developing gel polymer electrolyte (GPE) can solve the safety issues faced by liquid electrolyte and allow easy fabrication of dye-sensitized solar cell (DSSC). However, its power conversion efficiency (η) still needs improvement by bettering any other parameters such as open-circuit voltage (VOC), short–circuit current (JSC) and Fill Factor (FF). Herein, 4-tert-butyl-pyridine (TBP) is used as an additive with different concentrations, and its effects on the GPE-TBP are studied. The addition of TBP is beneficial in DSSC employing GPE-TBP as it can adsorb on the surface of the mesoporous TiO2 photoanode, which leads to the shifting of the quasi-fermi level of TiO2 photoanode to a higher potential. This phenomenon enhances open-circuit voltage (VOC) of the DSSC from 615 to 750 mV upon addition of 7 wt% of TBP content to the GPE sample. However, taking into consideration all parameters in DSSC employing GPE-TBP, GPE sample containing 3 wt% of TBP shows the highest photovoltaic conversion efficiency (ɳ) of 8.11% under light illumination of 100 mW cm−2.

Solar Energy Conversion and Storage Using a Photocatalytic Fuel Cell Combined with a Supercapacitor

This work studies the production of electricity by a photocatalytic fuel cell and its storage in a supercapacitor. We propose a simple construction, where a third electrode bearing activated carbon is added to the device to form a supercapacitor electrode in combination with the supporting electrolyte of the cell. The photocatalytic fuel cell is based on a CdS-sensitized mesoporous TiO2 photoanode and an air cathode bearing only nanoparticulate carbon as an oxygen reduction electrocatalyst.

State filling effects on photoluminescence and photovoltaic characteristic of aluminium-doped CdTe colloidal quantum dots stabilized in aqueous medium

CdTe and aluminium (Al)-doped CdTe (Al:CdTe QDs) colloidal quantum dots (QDs) were synthesized through an aqueous route. CdTe and Al:CdTe QDs colloidal quantum dots of different size were obtained by collecting the samples at different refluxing time. The size of the synthesized QDs collected after 6 h of refluxing time was measured to 3.3 and 4.8 nm for Al:CdTe QDs and CdTe QDs, respectively. The size shrinkages of Al:CdTe QDs were due to chlorine ions from AlCl3 precursor, which controls the growth kinetics of QDs and stabilizes the QDs in the aqueous phase. The size shrinkages of Al:CdTe QDs result in an increase of bandgap from 2.13 to 2.3 eV and congruently blue-shifted absorbance and emission spectra were discerned. The state filling effect, gradual filling of energy state, observed clearly in photoluminescence spectra of Al:CdTe QDs for samples collected after 30 min of refluxing time. We also fabricated Gratzel-type QDs-sensitized solar cells by loading CdTe and Al:CdTe QDs on mesoporous TiO2 photoanode. Photovoltaic efficiency is marginally increased (13%) from 0.45 to 0.51% for Al-doped CdTe QDs as compared to CdTe QDs.

A novel, PbS quantum dot-Sensitized solar cell structure with TiO2-fMWCNTS nano-composite filled meso-porous anatase TiO2 photoanode

The research deals with fabrication of a novel quantum dot sensitized solar cells (QDSSC) architecture. Three devices, device 1, 2 and 3 with TiO2-fMWCNTs nano composite filled meso porous anatase TiO2 photoanodes, sensitized with PbS by 10, 15 and 20 SILAR cycles respectively were prepared. Decrease in optical band gap has been observed by increasing the SILAR cycles for PbS sensitization. QDSSC with TiO2-fMWCNTS nano-composite filled meso-porous anatase TiO2 photoanode showed a significant energy conversion efficiency of 3.8%, 5% and 5.6% for device 1, 2 and 3 respectively under the simulated light of 100 mW cm−2 with AM 1.5 filter. EIS analysis revealed that the device with highest power conversion efficiency has highest charge combination resistance (Rcr) 2852 Ω.

Nanoscale Perovskite‐Sensitized Solar Cell Revisited: Dye‐Cell or Perovskite‐Cell?

A general and straightforward way of preparing few‐nanometer‐sized well‐separated MAPbIxBr3−x (MA=methylammonium) perovskite photosensitizers on the surface of an approximately 1 μm thick mesoporous TiO2 photoanode was suggested through a two‐step sequential deposition of low‐concentrated lead halides (0.10–0.30 m PbI2 or PbBr2) and methylammonium iodide/bromide (MAI/MABr). When those nanoscale MAPbIxBr3−x perovskites were incorporated as a photosensitizer in typical solid‐state dye‐sensitized solar cells (ss‐DSSCs), it could be verified clearly by the capacitance analysis that nano‐particulate MAPbI3 perovskites play the same role as that of a typical dye sensitizer (MK‐2 molecule) although their size, composition, and structure are different.

Ultrathin TiO2-coated SiO2 nanoparticles as light scattering centers for quantum dot-sensitized solar cells

SiO2/TiO2 core/shell nanoparticles (ST-NPs) with an ultrathin and conformal TiO2 shell were fabricated via atomic layer deposition (ALD) technique with a specially-designed rotary reactor. We successfully employed the ST-NPs as light scattering centers in a mesoporous TiO2 photoanode in quantum dot (QD)-sensitized solar cells (QDSSCs). ST-NPs enhanced the light scattering in the TiO2 photoanode without deterioration of QD loading behavior. Consequently, visible light absorbance was improved in the PbS QDs-sensitized TiO2 photoanode, and thus the photocurrent density increased from 14 to 17 mA/cm2. Detailed electron microscopy was utilized to reveal the uniqueness of the ultrathin TiO2 shell in ST-NPs, and in-depth electrochemical impedance spectroscopy divulged the increase in photogenerated carrier lifetime and improved recombination resistance for the QDSSC with ST-NPs.

Facile graft copolymer template synthesis of mesoporous polymeric metal-organic frameworks to produce mesoporous TiO2: Promising platforms for photovoltaic and photocatalytic applications

Mesoporous polymeric metal-organic frameworks (mesoporous polymeric MOFs) are prepared on fluorine-doped tin oxide (FTO) substrate using hydrophilic terephthalic acid as the ligands, titanium isopropoxide as polymeric MOF precursors, and amphiphilic graft copolymers (i.e., poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) as structure-directing agents. The hydrophilic POEM chains in amphiphilic graft copolymers interact with the hydrophilic ligands and polymeric MOF precursors. Following thermal treatment at 500°C, mesoporous polymeric MOFs are transformed to mesoporous TiO2 with high specific surface area and crystallinity, suitable for photovoltaic and photocatalytic applications. Solid-state dye-sensitized solar cells (ssDSSCs) and dye-sensitized solar cells (DSSCs) fabricated with mesoporous TiO2 photoanodes have efficiencies of 7.45 and 8.43 % at 100mW/cm2, which is much higher than that of ssDSSCs and DSSCs with photoanodes of conventional TiO2 (5.36 and 7.14 %), respectively. The enhanced efficiency is attributed to good interconnectivity, larger surface area, and high porosity of the mesoporous TiO2, which results in suppressed interfacial charge recombination loss, enhanced electron transport, increased dye loading, and facilitated penetration of the electrolytes. Mesoporous TiO2 shows excellent activity as a photocatalyst for the degradation of humic acid under UV light irradiation.

Wet chemical synthesis of lead sulfide nanoparticles and its application as light harvester in photovoltaic cell

Nanosized PbS particles are synthesized under an inert atmosphere by a low temperature method by using precursor solutions of lead nitrate (Pb(NO3)2) and sodium sulfide (Na2S). 3-Mercaptopropionic acid (MPA) as capping agent. Phase, crystal structure, morphology and optical properties of the nanoparticles are confirmed from XRD, SEM, TEM and UV–Visible-NIR analysis. Application study of the synthesized PbS nanoparticles has been done by fabricating a photovoltaic cell consisiting of a mesoporous TiO2 photoanode, electrodeposited platinum counter electrode and iodine/triiodide liquid electrolyte. I-V and impedance of the PbS sensitized cell is measured under one sun illumination and obtained an efficiency of 0.31%, open-circuit voltage of 0.61 V, short circuit current of 0.86 mA/cm2 and a fill factor of 58.2%