Conventional Photoelectrochemical Cells Research Articles

Effectiveness of metal–EDTA-polluted water treatment and metal recovery at drop-casted films of TiO2 under UV-LED irradiation: A photoelectrochemical study

EDTA is an important metal sequestering agent in industrial processes and household products, but also an emerging pollutant and a challenge for wastewater treatment due to its high resistance to biodegradation. Conventional strategies for recovering metals from the metal chelates require a previous oxidation step. Herein, P25 TiO2 photocatalyst was applied to assemble nanoparticulate films on gold (model conducting substrate) with highly reproducible photocatalytic activity toward EDTA and Cu(II)-EDTA (model chelate) UVA-photooxidation. Films were obtained from P25 suspensions in acidic medium by a quick cold drop-casting method without thermal annealing. The involved TiO2 agglomerates were characterized by Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), voltammetric DC techniques and zero-current chronopotentiometry. Photocatalytic treatment of Cu(II)-EDTA solutions and electrochemical essays were accomplished in a thin layer-type cell coupled with a 365 nm UV-LED. Synthetic samples of the chelate at stoichiometric ratio or with EDTA in excess were exposed to the irradiated TiO2 film to promote the photooxidation of the ligand, and recovery of copper mainly by direct (photo)electrodeposition on gold. Three potential programs were tested during UV-irradiation: open circuit potential (OCP), +0.5 V and –0.3 V vs Ag/AgCl. Overall best results were obtained at –0.3 V, showing that it is more effective to favor in-situ metal reduction rather than collecting photogenerated electrons via +0.5 V biasing to diminish electron/hole (exciton) recombination. Nevertheless, operation without external biasing, i.e., at OCP, is nearly as effective since the substrate is spontaneously driven to circa –0.7 V by photogenerated electrons plus electrons injected into photoholes by EDTA, as it oxidizes. The Cu(II)-EDTA treatment can be abbreviated by coupling both redox processes at the same photoelectrode, thus, avoiding the transport of Cu(II) from the photoanode to the cathode, conceivably with risk of rechelation, like in conventional photoelectrochemical cells. Similar results were obtained with the TiO2 drop-casted on other conducting substrates including carbon, a relevant finding for a forthcoming scale-up of the treatment process.

Synergy of photocatalysis and fuel cells: A chronological review on efficient designs, potential materials and emerging applications.

The rising demand of energy and lack of clean water are two major concerns of modern world. Renewable energy sources are the only way out in order to provide energy in a sustainable manner for the ever-increasing demands of the society. A renewable energy source which can also provide clean water will be of immense interest and that is where Photocatalytic Fuel Cells (PFCs) exactly fit in. PFCs hold the ability to produce electric power with simultaneous photocatalytic degradation of pollutants on exposure to light. Different strategies, including conventional Photoelectrochemical cell design, have been technically upgraded to exploit the advantage of PFCs and to widen their applicability. Parallel to the research on design, researchers have put an immense effort into developing materials/composites for electrodes and their unique properties. The efficient strategies and potential materials have opened up a new horizon of applications for PFCs. Recent research reports reveal this persistently broadening arena which includes hydrogen and hydrogen peroxide generation, carbon dioxide and heavy metal reduction and even sensor applications. The review reported here consolidates all the aspects of various design strategies, materials and applications of PFCs. The review provides an overall understanding of PFC systems, which possess the potential to be a marvellous renewable source of energy with a handful of simultaneous applications. The review is a read to the scientific community and early researchers interested in working on PFC systems.

Photoelectrochemical Driving and Simultaneous Synthesis of 3-pyridinecarboxylic Acid and Hydrogen in WO3 Photoanode-Based Cell

In conventional photoelectrochemical (PEC) cell for solar water splitting, its solar-to-hydrogen efficiency is limited by the sluggish oxygen evolution on its photoanode. 3-pyridinecarboxylic acid (3-PA) is an important organic in industry, but there are several disadvantages in its conventional synthesis approaches. Here, we reported that the oxidation of 3-methylpyridine (3-MP) into 3-PA as an alternative to the oxygen evolution on WO3 photoanode for improving the hydrogen-evolution efficiency and the product added-value of PEC cell. In a WO3 film photoanode-based dual-chamber PEC cell, 3-PA and hydrogen were simultaneously synthetized. Since the oxidation of 3-MP into 3-PA is more thermodynamically and kinetically favorable than water oxidation, higher efficiency of hydrogen evolution was achieved in the PEC cell with function of 3-PA synthesis relative to the conventional PEC cell for water splitting. Our investigations indicated that the oxidation of 3-MP into 3-PA on WO3 photoanode is co-initiated by the photogenerated holes and the H2O2 intermediate that originated from two-holes pathway of solar water oxidation on WO3. Additionally, the Cr2O72−/Cr3+ redox pair could mediate and promote the oxidation of 3-MP into 3-PA on WO3 photoanode, and thus the PEC efficiency of 3-PA synthesis can be improved by adding Cr3+ into the H2SO4-3-MP electrolyte.

Porous titania photoelectrodes built on a Ti-web of microfibers for polymeric electrolyte membrane photoelectrochemical (PEM-PEC) cell applications

Conventional photoelectrochemical (PEC) cells are based on planar photoelectrodes supported on glass substrates and liquid electrolytes. Only few recent studies have examined an alternative PEC design which is robust and scalable, where the key elements are polymeric electrolyte membranes and porous photoelectrodes. This work aims to give further insights on the operation of such cells utilizing titania photoelectrodes and proton and hydroxide ion conducting membranes. Two families of photoelectrodes were developed on Ti porous substrate; TiO2 nanotubes grown by anodization and subsequent oxygen annealing, and TiO2 layers developed under oxygen annealing. Initial screening of the photoanodes for water splitting and (poly)alcohol photo-oxidation took place in conventional PEC cells. We found that the annealing temperature affects the performance of the photoanodes, evidenced by a monotonic increase in the activity for water photo-oxidation with increasing annealing temperature. Moreover it was demonstrated that anatase phase is predominantly active for the (poly)alcohol electro-oxidation, while there is a synergy between rutile and anatase which is beneficial for water splitting. In addition, the most promising photoanodes for water splitting were evaluated in our polymeric electrolyte membrane photoelectrochemical(PEM-PEC) cell during gas phase operation. It was found that PEM-PEC operation is more efficient when OH- conducting membranes are used, while the nature of the carrier gas does not significantly influence the activity. Overall, PEM-PEC operation is more promising than conventional PEC in both acidic and alkaline media, since comparable (or even at some cases higher) photocurrents were obtained while liquid pumping systems are not required for PEM-PEC devices.

Photoelectroreduction of Building-Block Chemicals.

Conventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

Polymeric Electrolyte Membrane Photoelectrochemical (PEM-PEC) Cell with a Web of Titania Nanotube Arrays as Photoanode and Gaseous Reactants

A novel photoelectrochemical (PEC) cell design is proposed and investigated for H2 production with gaseous reactants. The core of the cell is a membrane electrode assembly (MEA) that consists of a TiO2 photoanode of nanotube arrays, a Pt/C counter and reference electrodes and a polymeric electrolyte membrane (PEM) with proton conductivity, which serves both as compact reactor for water splitting and as gas separator. The design was inspired by PEM electrolysis technology and modified appropriately for allowing illumination and it is also equipped with a third compartment which enables the use of a hydrogen reference electrode.Photoanodes of titania nanotube arrays, TNTAs, were developed, for the first time, on a Ti-web of microfiber substrates, by electrochemical anodization. The performance of TNTAs/Ti-web photoanodes were evaluated in both gaseous and liquid reactants. Due to the presence of reliable reference electrode in gas phase direct comparison of the results was possible. Gas phase operation with He or Air as carrier gases and only 2.5% of water content exhibits very promising photoefficiency in comparison with conventional PEC cells.

Water Splitting Using High Temperature Solid Oxide Photoelectrochemical Cell and Visible Sunlight: Searching for the Appropriate Semiconductor Materials

In this paper, the concept of using solid oxide photoelectrochemical cells (SOPCs) to split water at high temperature using visible sunlight was experimentally investigated, in order to search for the appropriate semiconductor materials. High temperature photoelectrochemical water splitting physically broadens the selection of potential applicable semiconductor materials and enables more visible sunlight absorption. This newly conceived concept provides a unique pathway for solar hydrogen production, compared to conventional photoelectrochemical cells (PECs) that require aqueous environments, though experimental validation is needed. The main focus of this study was on the fabrication and evaluation of high temperature diodes suitable for use as the photocatalytic/oxygen electrodes of SOPCs. The rectifying characteristics of TiO2 based diodes, including Ag/TiO2, LSM/TiO2, LSC/TiO2, and LSCF/TiO2, were investigated at elevated temperatures. Among those diodes, only LSM/TiO2 demonstrated acceptable rectifying properties up to 450°C, indicating that such configuration may be applicable to the proposed SOPC. The results also excluded Schottky diodes from the candidates due to their rapidly degrading rectifying behaviors at elevated temperatures. Candidate semiconductors for the photovoltaic (PV) part of the cell and electrolytes for the electrolysis part were also identified and summarized in this paper for future investigations.

Solar fuel production in a novel polymeric electrolyte membrane photoelectrochemical (PEM-PEC) cell with a web of titania nanotube arrays as photoanode and gaseous reactants

A novel photoelectrochemical (PEC) cell design is proposed and investigated for H2 production with gaseous reactants. The core of the cell is a membrane electrode assembly (MEA) that consists of a TiO2 nanotube arrays photoanode, a Pt/C cathode, a Pt/C reference electrode and a proton conducting polymeric electrolyte membrane (PEM), which serves both as compact reactor for water splitting and as gas separator. The design is inspired by PEM electrolysis technology and modified appropriately to allow light irradiation and to host a third electrode compartment, which enables the use of a hydrogen reference electrode. The titanium dioxide nanotube arrays (TNTAs) photoanodes were synthetized, for the first time, on a Ti-web of microfiber substrates by electrochemical anodization. The performance of TNTAs/Ti-web photoanodes were evaluated in both gaseous (i.e. PEM-PEC cell) and liquid media (conventional PEC cell). Due to the presence of the PEM-PEC reference electrode comparison of the results was possible. Gas phase operation with either He or air as carrier exhibits very promising photoefficiency in comparison with conventional PEC cells. Additionally the performance of our PEM-PEC cell was also investigated for simultaneous anodic air treatment and cathodic hydrogen evolution.

Manipulation of charge transfer and transport in plasmonic-ferroelectric hybrids for photoelectrochemical applications.

Utilizing plasmonic nanostructures for efficient and flexible conversion of solar energy into electricity or fuel presents a new paradigm in photovoltaics and photoelectrochemistry research. In a conventional photoelectrochemical cell, consisting of a plasmonic structure in contact with a semiconductor, the type of photoelectrochemical reaction is determined by the band bending at the semiconductor/electrolyte interface. The nature of the reaction is thus hard to tune. Here instead of using a semiconductor, we employed a ferroelectric material, Pb(Zr,Ti)O3 (PZT). By depositing gold nanoparticle arrays and PZT films on ITO substrates, and studying the photocurrent as well as the femtosecond transient absorbance in different configurations, we demonstrate an effective charge transfer between the nanoparticle array and PZT. Most importantly, we show that the photocurrent can be tuned by nearly an order of magnitude when changing the ferroelectric polarization in PZT, demonstrating a versatile and tunable system for energy harvesting.

Enhancing the Efficiency of Electron Conduction in Spray-Coated Anode of Photoelectrochemical Cell Using Oxygenated Multi-Walled Carbon Nanotubes

A multiwalled carbon nanotube (MWNT) was physically cured with oxygen plasma treatment, and the as-prepared oxygenated MWNT (OMWNT) was incorporated into TiO2 nanopowders to prepare a spray-coatable OMWNT–TiO2 composite suspension. The composite layer was directly formed on a fluorinated tin oxide surface by spray coating and served as a photoanode of a photoelectrochemical cell (PEC). The cell performance was optimized in terms of the plasma treatment time and compared with a conventional PEC, showing 37% increased energy conversion efficiency. The efficiency improvement confirmed by the electrochemical impedance spectra was related to the reduced charge-transfer resistance and efficient electron transport through the OMWNT network.

A novel photoelectrochemical flow cell with online mass spectrometric detection: oxidation of formic acid on a nanocrystalline TiO2 electrode.

A novel thin-layer photoelectrochemical flow cell allowing the online mass spectrometric detection of volatile reaction products during photoelectrocatalytic reactions has been developed and applied for separating the contributions from photoelectrochemical water splitting and photoelectrooxidation of formic acid to the overall photocurrent in formic acid containing aqueous solution, using a nanocrystalline TiO2 (P25) thin-film electrode. The data reveal a clear suppression of the water oxidation reaction to O2 in the presence of formic acid. Advantages of this flow cell design over conventional photoelectrochemical cells with stagnant electrolyte in terms of mass transport will be demonstrated and discussed.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO<sub>2</sub>-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 µm were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and use of photocatalytically active segmented Ag|ZnO and coaxial TiO2-Ag nanowires made by templated electrodeposition.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 µm were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Energy Harvesting Capability of Lipid‐Merocyanine Macromolecules: A New Design and Performance Model Development

Light induced cis/trans isomerization in the family of merocyanine (MC) dyes offers a recyclable proton pumping ability which can potentially be used in hybrid bio-electronic devices. In this article, a hexadecyl MC dye is embedded in lipid molecules to make a macromolecular configuration of a lipid/hexadecyl MC membrane. Lipid molecules play a critical role in stabilizing the dye in a membrane structure for practical use in energy devices. In this study, we first examined the proton pumping characteristic of the lipid/hexadecyl MC membrane in a conventional photoelectrochemical cell. Next, a major modification in the cell was introduced by eliminating I2/I-electrolyte which resulted in a two-fold increase in the open circuit voltage compared with that of the conventional cell. In addition, the charging time in the new cell was reduced approximately four orders of magnitude. This research demonstrated that the newly designed lipid- MC cell can act as a promising bioelectronic device based on the green energy of photoinduced MC dye proton pumping.

Direct solar energy conversion and storage through coupling between photoelectrochemical and ferroelectric effects

Harvesting and storing solar energy has become more and more important. Current solid-state photovoltaic cells and conventional photoelectrochemical cells are not capable of directly storing the converted energy, which has to be facilitated by connecting to external storing devices. We demonstrate a device architecture that can convert and store solar energy in the electrical form within an intrinsically single structure. Mobile charge is internally stored, based on the coupling between photoelectrochemical and ferroelectric effects. The tested device architecture can be photo-charged under 1000 W/m2 of white light to an open-circuit voltage of 0.47V with a capacity of 37.62 mC/cm2. After removal of the light source, the mobile charge stored lasts more than 8 hours, and the open-circuit output voltage lasts more than 24 hours.

Effect of CdS contents on H2 production from Pt-(CdS/TiO2) film-typed photocatalysts

Pt-(CdS/TiO2) film-typed photocatalysts are prepared with a doctor-blade method followed by a chemical bath deposition (CBD) process, and the films are characterized by UV-vis spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy. The film-typed structure is composed of photocatalysts and Pt metal part on a FTO substrate without additional electric device, so it is relatively simpler than the conventional photoelectrochemical cell. CdS quantum dots are introduced as a sensitizer for visible light response. Amounts of CdS quantum dots on TiO2 surface are increased with increasing CBD cycles, but they start to aggregate after certain CdS concentration due to oversaturation phenomenon. This high CdS content induces high electron losses, and therefore it reduces amounts of hydrogen production. As a result, there is a saturation point of CdS content at Cd/Ti ratio of 0.197, and the amounts of evolved hydrogen are 5.407 μmol/cm2·h at this photocatalyst formulation.

Increased quantum efficiency in hybrid photoelectrochemical cell consisting of thionine and zinc oxide nanoparticles

Abstract In this paper we report our studies on hybrid photoelectrochemical cell made of thionine and zinc oxide nanoparticles. Nanostructured zinc oxide was synthesized in the laboratory by time and cost effective method. The photoelectrochemical cell yielded voltage of high magnitude (401.5 mV) and efficiency (1.21%) compared to other conventional photoelectrochemical cells. The morphology of the nanostructured zinc oxide was identified by X-ray diffraction and electron microscopy studies. Spectral studies indicated adsorption of the dye molecules on zinc oxide nanoparticles, the reason for efficient absorption of the incident photons. During photoexcitation the thionine molecules and the zinc oxide nanoparticles acted as donor–acceptor cluster which produced photovoltage.

On the Viability of Cyclometalated Ru(II) Complexes for Light-Harvesting Applications

The effects of replacing a single polypyridyl ligand with an analogous anionic cyclometalating ligand were investigated for a set of three structurally related series of Ru(II) compounds formulated as [Ru(bpy)(2)(L)](z), [Ru(tpy)(L)](z), and [Ru(tpy)(L)Cl](z), where z = 0, +1, or +2, and L = polypyridyl (e.g., bpy = 2,2'-bipyridine, tpy = 2,2':6',2''-terpyridine) or cyclometalating ligand (e.g., deprotonated forms of 2-phenylpyridine or 3-(2-pyridinyl)-benzoic acid). Each of the complexes were synthesized and characterized by (1)H NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and/or elemental analyses (EA). Cyclic voltammetry reveals that cyclometalation causes a shift of the first oxidation and reduction potentials by -0.5 to -0.8 V and -0.2 to -0.4 V, respectively, relative to their polypyridyl congeners. These disparate shifts have the effect of inducing a bathochromic shift of the lowest-energy absorption bands by as much as 90 nm. With the aid of time-dependent density functional theory (DFT), the lowest-energy bands (lambda(max) = 500-575 nm) were assigned as predominantly metal-to-ligand charge-transfer (MLCT) transitions from Ru to the polypyridyl ligands, while Ru-->C(wedge)NN (or C(wedge)N(wedge)N or N(wedge)C(wedge)N) transitions are found within the absorption bands centered at ca. 400 nm. The properties of a series of compounds furnished with carboxylic acid anchoring groups at various positions are also examined for applications involving the sensitization of metal-oxide semiconductors. It is determined that the thermodynamic potentials of many of these compounds are appropriate for conventional photoelectrochemical cells (e.g., dye-sensitized solar cells) that utilize a titania electrode and iodide-based electrolyte.

Photoelectrochemical cells using metal-decorated carbon nanotube electrodes

We fabricated the photoelectrochemical (PEC) cells using n-type TiO 2 and metal (Pt, Pd)-decorated carbon nanotubes (CNTs) as an anode and a cathode, respectively. Photovoltaic effects were clearly observed. Compared with conventional PEC cell with Pt cathode, a larger photo-voltage was found in the PEC cells with the metal-decorated CNT cathode due to p-type semiconducting properties of CNTs. Additionally, we connected two PEC cells in series to decompose water into oxygen and hydrogen. Indeed, the connected PEC cells yielded the photo-voltage of about 1.35 V, which is larger than 1.23 V required for water splitting. This result demonstrates the possibility of hydrogen generation using the connected PEC cells without an external bias.

Photoelectric Conversion Property of Covalent-Attached Multilayer Self-Assembled Films Fabricated from Diazoresin and Fullerol

A novel self-assembly multilayer film was fabricated from fullerenol (C60(ONa)n) and diazoresin (DR)a photosensitive polymer on indium-tin-oxide substrate. The film shows good stability after UV irradiation and has photoelectric conversion property, which can be measured in a conventional photoelectrochemical cell. The cathodal photocurrent was measured to be about 800 nA/cm2 for a nine-layer film at 100 mW/cm2 white light. The photocurrent spectroscopy responses coincide with the absorption spectrum of the self-assembly film, which means that the C60(ONa)n/DR film is responsible for the photocurrent generation.