Photoelectrochemical Reactor Research Articles

Photoelectrochemical Reactors for CO2 Utilization

The photoelectrochemical reduction of CO2 to renewable fuels and valuable chemicals using solar energy is a research topic that has attracted great attention recently due to its potential to provide value-added products under the Sun, solving the issues related to global warming at the same time. This review covers the main research efforts made on the photoelectrochemical reduction of CO2. Particularly, the study focuses in the configuration of the applied reactor, which is a topic scarcely explored in the literature. This includes the main materials used as photoelectrodes and their configuration in the photoelectrochemical reactor, which are discussed for technical uses. The review provides an overview of the state-of-the-art processes and aims to help in the development of enhanced photoelectroreactors for an efficient CO2 utilization.

Degradation of EDTA in H2O2 -containing wastewater by photo-electrochemical peroxidation

Semiconductor wastewater currently contains H2O2 which is an important reagent in wafers cleaning. Recalcitrant organic pollutant such as EDTA are always present in this type of wastewater and may represent a threat for the environment. In this work, a new photoelectrochemical reactor is proposed to remove EDTA from H2O2 contaning wastewater. First, photolysis, electrochemical peroxidation and photo-electrochemical peroxidation were compared. The results showed that the removal efficiency decreases in the sequence: UV/H2O2 «EC/H2O2 <UV/EC/H2O2. The main parameters affecting the photo-electrochemical peroxidation process were studied. The study revealed that the optimal current was 50 mA, while the optimum initial pH was found in the acidic range from 2.25 to 3.0, with a peak at 2.25. Increasing H2O2 concentration results in increasing in EDTA removal up to 92.1%, while decreasing initial EDTA concentration up to 22.5 mg L−1 gives a complete removal in only 90 min. It was also found that the degradation of EDTA greatly depends on the nature of the present cation.

Enhanced visible light catalytic activity of MoS2/TiO2/Ti photocathode by hybrid-junction

In photoelectrochemical (PEC) water splitting systems, crucial obstacles limiting their performance are poor charge carrier dynamics and high recombination rate of photoexcited electron-hole pairs. Here, we report that this issue can be alleviated by engineering a hybrid-junction that is composed of homo- and hetero- junctions. This strategy is performed by facile hand-spraying MoS2 over the surface of anatase/rutile homo-junction TiO2 film on the Ti substrate to further form a hybrid-junction photocathode. By applying this photocathode into PEC reactor, enhanced catalytic activity is achieved under visible light (AM1.5 illumination of 300 W/m2) with hydrogen evolution reaction (HER) potential of −114 mV versus reversible hydrogen electrode (RHE) at 10 mA/cm2 and long-term stability of more than 10 times improvement comparing to ordinary electrode without the introduction of hybrid-junction. The hybrid-junction that effectively regulates charge separation and transfer pathways is proven to be responsible for the enhanced activity. As a novel exploration, this hybrid-junction system comprising of low-cost, efficient charge separation and transfer, and visible light responsivity offers a new path for relative materials to boost their PEC performance.

Experimental investigation and analysis of a new photoelectrochemical reactor for hydrogen production

In this research paper, an experimental investigation of photoactive material titanium dioxide (TiO2) coated on 180 cm2 316 stainless steel anode is undertaken to study the photoresponse on photoelectrochemical (PEC) hydrogen production. The TiO2 nanoparticles are first prepared via sol-gel method. A large surface 316 stainless steel anode is coated with TiO2 nanoparticles by a dip coating apparatus at a withdraw rate of 2.5 mm/s. The nanoparticles are carried on the stainless steel substrate by two-step annealing procedure. The potentiostatic studies confirm the photoactivity of TiO2 nanoparticles in a photoelectrochemical reactor when exposed to solar ultraviolet (UV) light. The photon to current efficiency measurements carried out on the PEC reactor with TiO2 coated large surface stainless steel as photoanode demonstrate a significant increase of photoresponse in UV light compared to the uncoated stainless steel prepared under similar conditions. Upon illumination at a power density of 600 W/m2, the hydrogen production is observed in TiO2 coated stainless steel substrate at a measured rate of 51 ml/h while no illumination conditions show a production rate of 42 ml/h. In comparative assessments, the TiO2 coated substrate shows an increase in photocurrent of 10 mA with an energy efficiency of 1.32% and exergy efficiency of 3.42% at an applied potential of 1.6 V. The present results show a great potential for titanium nanoparticles semiconductor metal oxide in photoelectrochemical hydrogen production application.

Solar carbon fuel via photoelectrochemistry

A promising strategy to mitigate both energy shortage and global warming is the conversion of CO2 into chemicals that can be used as fuels (chemical fuels) by utilizing renewable energy sources. Up to date, solar-driven CO2 reduction has been achieved with photochemical (PC) and photoelectrochemical (PEC) systems or electrochemical cells combined with a photovoltaic system (PV-EC). This study is intended to compare and highlight the state-of-the-art PEC systems for CO2 reduction and show the limitation factors that still hinder their widespread utilization. The review starts with a description of semiconducting photocatalyst properties and fundamental understanding of PEC CO2 reduction process. Then, the most significant performance metrics used for evaluation of PEC systems are explained in details. In addition, recent progress in PEC CO2 reduction systems is summarized and classified in different categories according to the chemical product. Different strategies such as doping, combination of two or more semiconductors, synthesis of nanostructured materials, passivation layers and co-catalysts that enhance light absorption, chemical stability, charge transfer and reduce ohmic losses and overpotentials of photoactive materials are reviewed. Besides the improvement of photocatalysts, research progress on the front of PEC reactor design, combined with the development of advanced modelling tools and characterization techniques are expected to bring PEC CO2 reduction a step closer to commercialization.

Experimental study of a solar-driven photo-electrochemical hybrid system for the decolorization of Acid Red 26

This study presents a new solar-driven hybrid system that integrated a photo-electrochemical reactor with a photovoltaics (PV) panel for azo dyes’ decolorization and electricity generation. Full spectrum of sunlight is utilized to optimize the color removal of Acid Red 26 (AR26) in this hybrid system. Persulfate (PS, S2O42−) was selected as the photochemical oxidant and Ti/IrO2-Ta2O5 electrode was used as the anode. Experiments were made to evaluate the efficiency of decolorization and the performance of PV panels in different reaction conditions outdoors. The results showed that the synergistic effect of two processes was observed for the AR26 decolorization. Comparing with the solar/persulfate process or the electrochemical process alone, the complete color removal time by the hybrid system decreased up to 50% and 44.4% respectively. In this system, the water layer in the flow channel cooled PV panels by absorbing the far infrared spectrum of sunlight, and the increased temperature of wastewater from 7°C to 16°C enhanced the decolorization efficiency of AR26. Moreover, the generated electricity by PV panels could satisfy the energy demand of electrochemical oxidation.

Performance evaluation of a photoelectrochemical hydrogen production reactor under concentrated and non-concentrated sunlight conditions

In this paper, we present the experimental performance evaluations of a newly developed photoelectrochemical (PEC) reactor for the production of hydrogen under no-light and concentrated solar radiation conditions. With a newly developed experimental setup, the solar light is concentrated about ten times, and the spectrum is divided using cold mirrors for better sunlight utilization. The photoelectrochemical reactor is examined at different applied potentials and the hydrogen production quantities are measured. Copper oxide, which is used as a light-sensitive material, is electrochemically coated on the cathode metal plate to increase the rate of hydrogen evolution under illumination. The present experiments are conducted to investigate the variation of reactor performance with intensified light conditions and the obtained results are compared with the dark conditions. The results of this study reveal that the hydrogen evolution rate was 41.34 mg/h for concentrated light measurement and 34.73 mg/h for no-light measurements at 2.5 V applied potential. The corresponding photocurrent generated under concentrated light at 2.5 V is found to be 0.63 mA/cm2. Under the concentrated sunlight, the hydrogen production rates increase considerably which is led by the positive effect of the photocurrent contribution.

Assessment of a Sustainable Electrochemical Ammonia Production System Using Photoelectrochemically Produced Hydrogen under Concentrated Sunlight

Intensive fossil fuel usage in ammonia production is considered nonsustainable; hence, alternative ammonia synthesis options are under investigation. In this study, a comprehensive study on environmental impact assessment is performed to investigate the electrochemical synthesis of ammonia at ambient pressure using photoelectrochemically produced hydrogen under concentrated solar light. The photoelectrochemical reactor consists of a membrane electrode assembly with a copper oxide semiconductor on a stainless steel cathode plate. The electrolyte for ammonia synthesis is molten salt containing a eutectic mixture of NaOH and KOH. The electrodes and wires are made of nickel. The life cycle assessment of the concentrated light photoelectrochemical hydrogen production is initially performed and integrated to molten-salt-based electrochemical ammonia synthesis. The material and energy requirements of the life cycle assessment are taken from the experimental data. The results imply that electrochemical ammonia synthesis driven by solar energy can significantly reduce the total environmental impact, corresponding to about 50% of the current steam methane reforming based ammonia production.

Advanced Photoelectrocatalysts for TEMPO-Mediated Alcohol Oxidations

The transformation of primary alcohols to carbonyls or acids is a fundamental chemical process of interest from both academic and industrial viewpoints, but typically requires stoichiometric oxidants which are costly and generate potentially toxic waste. As an alternative approach, conversion of biomass-derived alcohols in a photoelectrochemical (PEC) cell is a promising environmentally-friendly route, which takes advantage of renewable carbon feedstocks, renewable solar energy, and renewable electricity from wind, geothermal, or photovoltaic sources. In this talk, we will share our recent progress in designing semiconductor / electrocatalyst composite photoanodes made of earth-abundant elements for photoelectrochemical alcohol oxidation. Previously, we have studied selective electrocatalytic oxidation of biomass-derived alcohols and 5-hydroxymethylfurfural (HMF) on noble metal catalysts.1-2 Electroactive homogeneous catalysts such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) represent an alternative to expensive metal catalysts.3 Furthermore, it was recently reported that the electrode potential required to drive TEMPO-mediated alcohol oxidation can be significantly reduced in a PEC reactor, where solar energy is harvested with a bismuth vanadate (BiVO4) semiconductor electrode.4 However, significant electron-hole recombination and efficiency losses due to the competing oxygen evolution reaction (OER) make BiVO4 a poor electrode material for TEMPO oxidation. While it is common to modify semiconductor electrodes with electrocatalysts to enhance OER, such strategies to selectively promote TEMPO oxidation have not been explored. We present the first composite photoanode for enhanced TEMPO-mediated alcohol oxidation. BiVO4 modified with a cobalt-based electrocatalyst layer facilitated TEMPO oxidation at potentials >400 mV lower than unmodified BiVO4 by enabling facile charge transfer across solid–solid and solid–electrolyte interfaces. The competing OER was completely suppressed, allowing nearly 100% faradaic efficiency for TEMPO oxidation. In this talk, factors that tune the competition between TEMPO oxidation and OER, including photoanode structure, applied potential, and other reaction conditions will be discussed.

From millimetres to metres: the critical role of current density distributions in photo-electrochemical reactor design

0.1 × 0.1 m2 tin-doped hematite photo-anodes were fabricated on titanium substrates by spray pyrolysis and deployed in a photo-electrochemical reactor for photo-assisted splitting of water into hydrogen and oxygen.

Membrane-less photoelectrochemical cells: product separation by hydrodynamic control

This novel photoelectrochemical reactor design, utilising hydrodynamic product separation, removes the conventionally required membrane and relaxes classical material constraints.

En route to a unified model for photo-electrochemical reactor optimisation. I - Photocurrent and H2 yield predictions

A photo-electrochemical cell model was developed accounting for photon flux, electron–hole recombination rates, gas desorption, bubble formation and cross-over losses.

High performance of N-doped TiO2-magnetic activated carbon composites under visible light illumination: Synthesis and application in three-dimensional photoelectrochemical process

Visible light-sensitized, magnetically separable photocatalysts have been prepared and investigated in this study. Magnetic activated carbon (MAC) prepared via the co-precipitation method is chosen as the carrier with the surface cross-linked by chitosan-glutaraldehyde (Cts-GLA). Cts-GLA is designed as an encapsulated layer with the nitrogen-doped TiO2 deposited through a sol-gel method. Considerations have been given to the influences of different calcination temperatures and dosages of Cts. Results of this study indicated that lower ratio of Cts (MAC: Cts=10:1) and moderate calcination temperature at 550°C are beneficial to the formation of anatase TiO2. Carbonized Cts not only serve as the encapsulated layer, but also induce surface decoration of TiO2. The resultant composites demonstrate a good photo-absorption in the visible light region and magnetic property. When used as the particle electrode in a three-dimensional photoelectrochemical reactor, the pseudo-first-order rate constant of the optimized catalyst reacting with salicylic acid (SA, initial concentration=50mg/L) reached 0.0093min−1, which was much higher than that (0.0011min−1) with the individual use of the composite in photocatalysis. An increase in the initial concentration of SA did not lead to significant deterioration of the process performance. The high degradation efficiency of the novel system should be ascribed to the synergy effect of activated carbon adsorption, photocatalytic and electrochemical oxidation.

(Keynote) Interfacial Layers and Catalyst Approaches for Stabilizing a p-GaInP2 Photoelectrode for Hydrogen Production

The use of sunlight to drive the conversion of water into H2 and O2 via a photoelectrochemical (PEC) reactor provides a strategy for storing solar energy, providing an energy carrier for transportation and a feedstock for ammonia production. To collect enough sunlight to produce the amounts of hydrogen necessary will require covering large land areas. Utilizing precious metal catalysts over such large areas could prove to be difficult, thus alternative catalyst systems need to be considered. Alternative catalyst schemes include non-precious metal hydrogen evolution catalysts based on materials such as MoS2 and homogeneous catalysts. Homogeneous catalysts provide a rich area for both hydrogen and oxygen catalysts with a variety of redox potentials and catalytic activity. Combining the high photoconversion efficiency of a III-V semiconductor electrode with these catalysts could prove to be a winning combination making PEC hydrogen production viable.Our work has shown that by employing a hybrid molecular/semiconductor interface with atomic layer deposited (ALD) TiO2 as an intermediate layer, a robust and corrosion resistant GaInP2-TiO2-cobaltoxime-TiO2 photocathode can be operated in alkaline media (pH =13). An earth-abundant first row transition Cobalt catalyst Co(oxm)(pyCOOH) [oxm=bis-glyoxime; pyCOOH=4-carboxyl pyridine] as shown in figure 1 was attached onto a TiO2 modified GaInP2 surface through a carboxylic linkage group. A 35 nm amorphous TiO2 layer was deposited onto bare GaInP2 as protection and linkage layer. To further protect the linkage stability, an additional 10 cycle (∿0.4 nm) TiO2 ALD layer was deposited on top of the catalyst layer. The photoelectrochemical photocurrent density (J) versus potential (V) plots of the cobaltoxime modified electrode showed that its activity and stability were comparable to the similarly modified Pt electrodes.Note that for homogeneous catalysts used in a PEC system, the stability is less important, due to the fact that the system will be inoperative every night allowing for the possibility of rejuvenating the catalyst system by some means.In an alternative scheme, we developed a robust and catalytically active interfacial layer for stabilizing a p-GaInP2 photocathode based on MoSx/MoOx/TiO2. We find that annealing a bilayer of amorphous titanium dioxide (TiOx) and amorphous molybdenum sulfide (MoSx) deposited onto GaInP2 results in the MoSx/MoOx/TiO2 coating that provides the GaInP2 photocathode with good efficiency (current density of 11 mA/cm2 at a potential of 0 V vs. RHE under 1 sun illumination) and stability (retains 80% of its initial photocurrent density over a 20 h period) for the hydrogen evolution reaction in strong acid solution compared with traditional surface-deposited platinum-ruthenium cocatalysts on a GaInP2 photocathode. We explore the composition and phase of the interfacial layer using transmission electron microscopy , scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy before and after annealing. These studies reveal that annealing results in a graded MoSx/MoOx/TiO2 layer that retains much of the high catalytic activity of amorphous MoSx but with stability similar to crystalline MoS2.

Simulation of transport phenomena in a photo-electrochemical reactor for solar hydrogen production

A numerical simulation of transport phenomena in the photo-electrochemical (PEC) reactor is performed. The transport phenomena equations include the Navier–Stokes, the respective energy equation for electrolyte, and the radiative transfer equation (RTE). Two different designs, design A, and design B of photoelectrochemical reactors are suggested. The hydrogen production rate and solar-to-hydrogen efficiency are estimated for each design at different solar incident flux ranged from 500 to 2000 W/m2 in increments of 500 W/m2. Results have shown that the solar-to-hydrogen efficiency increases as solar flux increases for both designs. Its predicted values could reach 12.8% for design A, and 13.1% for design B. Moreover, by increasing the solar incident flux, the hydrogen volume production rate is increased as well. It is found to be 79 L/m2·h for design A, and 85.4 L/m2·h for design B. Comparison between currently predicted results and previous data indicates an enhancement of solar-to-hydrogen efficiency and hydrogen production that can be achieved with the suggested design.

Experimental investigation of a PV-Coupled photoelectrochemical hydrogen production system

In this study, an experimental setup is designed and constructed to investigate the effects of varying solar light intensity on the PV (photovoltaic) cell efficiency and the performance of photo-electrochemical (PEC) based hydrogen production. In the experimental setup, the solar light is split to be utilized by PV module for electricity production and by photoelectrochemical reactor for hydrogen production, and the sunlight is concentrated using Fresnel lens. Furthermore, the losses caused by transmittance and reflectance of the materials within the system are determined. The efficiencies for each light conversion process are calculated together with the overall efficiency and comparatively assessed for concentrated and non-concentrated light conditions. The characterization results of the solar PV module show an increase in total power output and efficiency from non-concentrated to concentrated conditions, corresponding to 12% to 16%, respectively, while only a higher wavelength spectrum is exposed to PV surface. The energy efficiency of light conversion from concentrated light to PEC reactor is obtained to be 49.3%.

Experimental investigation of hydrogen production in a photo-electrochemical chloralkali processes reactor

This paper develops and analyzes a new photo electrochemical reactor that uses zinc sulfide as a photo catalyst to produce hydrogen, chlorine and sodium hydroxide. The effects of different parameters on the rate of hydrogen, chlorine and sodium hydroxide production are experimentally studied. The parameters include the applied voltage, varied from 4 V to 5 V, amount of catalyst, varied from 1 g/425 mL to 5 g/425 mL and light intensity, varied from 20 W/m2 to 55 W/m2. A factorial design of experiments is applied and an analysis of variance (ANOVA) is used to examine the experimental results. Energy and exergy efficiencies are also calculated. An optimization is performed to find the optimal catalyst concentration. At the optimized catalyst concentration, salt water is used to check its effect on the rate of hydrogen production.

CFD simulation and development of an improved photoelectrochemical reactor for H2 production

In this paper an improved dual chamber photoelectrochemical (PEC) reactor for water splitting is developed. COMSOL Multiphysics (4.2b) software is utilized to simulate two and three-dimensional PEC reactor. Different geometries are studied to have a uniform flow with minimum recirculation zones inside the photoreactor chamber. Furthermore, the gas evolution rate in the photoreactor at different current densities is studied. As expected, the simulation results showed that by increasing the current density, the gas production rate increases. Integration of a thin layer of golden grid on the transparent conductive films (TCFs) is proposed to reduce potential drop across the sheet resistance of TCFs in photoelectrods which is one of the major problems in construction of larger photoelectrods. The simulation results showed that the potential drop is decreased significantly (from about 25% drop to roughly 6% drop) after applying the golden grid on the fluorine doped tin oxide (F-TO) film. Finally, some practical considerations and data are provided for fabrication of the PEC reactor.

RuO2–NaTaO3 heterostructure for its application in photoelectrochemical water splitting under simulated sunlight illumination

In this paper, we report the synthesis of RuO2–NaTaO3 films for their application as photocathodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. RuO2 was electrodeposited on NaTaO3 films at constant current varying the deposition time with the finality to obtain 1–3wt.% RuO2–NaTaO3. The characterization of the photoelectrodes RuO2–NaTaO3 was carried out by linear voltammetry (VL), chronoamperometry (CA), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), ultraviolet–visible absorption spectroscopy (UV–vis) and X-ray photoelectron spectroscopy (XPS). The hydrogen production was conducted in a photoelectrochemical reactor coupled with a gas chromatograph. One of the main contributions of this work is the incorporation of RuO2 on NaTaO3 films in order to enhance its activity in the visible region, achieving a maximum photoelectrochemical hydrogen production of 15.7mmolh−1g−1 for the film with 2wt.% RuO2 with a solar-to-hydrogen (STH) conversion efficiency of 4.29%.

Continuous Flow Photoelectrochemical Reactor for Solar Conversion of Carbon Dioxide to Alcohols

A newly-designed continuous flow photoelectrochemical reactor (CFPR) was applied to the solar photoelectrosynthesis of alcohols from carbon dioxide (CO2) dissolved in aqueous media. Hybrid p-type CuO/Cu2O semiconductor nanorod arrays, prepared by a three-step synthesis method (sol-gel synthesis, thermal anneal, and cathodic electrodeposition) were used as photocathodes. Scanning electron microscopy and powder X-ray diffraction were used to characterize these hybrid materials at various stages of the preparation protocol as well as before and after being subjected to photoelectrolysis for selected periods of time. The performance of the CFPR with the hybrid photocathode was analyzed in CO2-saturated aqueous bicarbonate solution at −0.3 V vs Ag/AgCl under simulated AM 1.5 solar irradiation for periods up to 5 h. The hybrid photocathode formulation facilitated efficient photoelectron injection to CO2 while its robustness and high photoelectrochemically active surface area enhanced the formation of alcohols. Gas chromatographic analyses of the photoelectrolysis solutions under a flow rate of 5 ml/h revealed ethanol to be the main product followed by isopropanol and methanol. The amounts of alcohol assayed indicated a faradaic efficiency ranging from 75 to 96%. The formation of C-C bonds in the products is a significant finding in this study and a mechanistic hypothesis is presented to account for products such as ethanol instead of methanol as is usually seen. Finally, the CFPR performance was numerically simulated to unravel the influence of flow rate and channel geometry on CO2 conversion and consumption rate.