Photo Electrode Research Articles

(Invited) Supported Design and Synthesis of Energy Materials Towards More Effective and Selective Approaches to Solar Fuels Production

Appropriate engineering of photoelectrodes is still emerging as a challenge in the search for efficient and stable materials for energy purposes. Hybrid CuO:Zn that owns small bandgap energy similar to that of pristine CuO and relatively high theoretical photocurrent still suffers from photo corrosion. Therefore, we tuned the electro-deposition of CuO:Zn nanostructures in such a way that the twisting of the coordination sphere or the Fermi level pinning of Zn ions could govern the photoelectrochemical properties, which allowed to double its apparent photocurrent compared to that of CuO. Implementing Zn ions enlarged the space-charge region, thus triggering the electron availability for the photolysis of water. Another cooper oxide, Cu2O is considered a suitable candidate for photoelectrochemical CO2 reduction to produce a variety of hydrocarbons, but its low selectivity, current density, and stability restrict further development. Herein, we also demonstrate a planar heterojunction of Cu2O@Zn5(OH)8Cl2 photocathode architecture for photo-electrochemical CO2 reduction. It provides C1, C3, C4 products (methane, propylene and butadiene) at 0.4 V vs RHE with current density of -1.2 mA/cm2 and shows excellent stability for 20 hours. The high selectivity is attributed to the synergetic effects between the zinc intermediate and Cu2O in sustaining the adsorption of CO* intermediates, enhancing the selectivity. These results provide new insights into the design of highly efficient Cu-based photo electrodes for enhancing the selectivity of photo-electrochemical CO2 reduction to produce high-value C3, and C4 products.

Electrochemically Determined and Structurally Justified Thermochemistry of H atom Transfer on Ti-Oxo Nodes of the Colloidal Metal-Organic Framework Ti-MIL-125.

Titanium dioxide (TiO2) has long been employed as a (photo)electrode for reactions relevant to energy storage and renewable energy synthesis. Proton-coupled electron transfer (PCET) reactions with equimolar amounts of protons and electrons at the TiO2 surface or within the bulk structure lie at the center of these reactions. Because a proton and an electron are thermochemically equivalent to an H atom, these reactions are essentially H atom transfer reactions. Thermodynamics of H atom transfer has a complex dependence on the synthetic protocol and chemical history of the electrode, the reaction medium, and many others; together, these complications preclude the understanding of the H atom transfer thermochemistry with atomic-level structural knowledge. Herein, we report our success in employing open-circuit potential (EOCP) measurements to quantitatively determine the H atom transfer thermochemistry at structurally well-defined Ti-oxo clusters within a colloidally stabilized metal-organic framework (MOF), Ti-MIL-125. The free energy to transfer H atom, Ti3+O-H bond dissociation free energy (BDFE), was measured to be 68(2) kcal mol-1. To the best of our understanding, this is the first report on using EOCP measurements to quantify thermochemistry on any MOFs. The proton topology, the structural change upon the redox reaction, and BDFE values were further quantitatively corroborated using computational simulations. Furthermore, comparisons of the EOCP-derived BDFEs of Ti-MIL-125 to similar parameters in the literature suggest that EOCP should be the preferred method for quantitatively accurate BDFE calculations. The reported success in employing EOCP for nanosized Ti-MIL-125 should lay the ground for thermochemical measurements of other colloidal systems, which are otherwise challenging. Implications of these measurements on Ti-MIL-125 as an H atom acceptor in chemical reactions and comparisons with other MOFs/metal oxides are discussed.

The influence of dissolved gas supersaturation on bubble detachment from planar (photo)electrodes

The influence of dissolved gas supersaturation on bubble detachment from planar (photo)electrodes

Recent advances in copper kesterite thin film photocathodes for hydrogen production via water splitting: A review

The increasing scarcity of fossil fuels has made it crucial to investigate alternative, clean, and renewable power sources. Hydrogen, the simplest element on the planet can act as an energy carrier and can be a promising solution for this quest of carbon-free energy alternates. The practical implementation of cost-effective, efficient, and stable production of hydrogen fuel from water presents a significant challenge. Researchers around the globe have been actively exploring photoelectrochemical (PEC) water-splitting methods for large-scale commercialization of hydrogen production through solar-driven processes. Among these methods, the use of semiconductor photoelectrodes (PEs) is considered highly efficient. In this review, of the effectiveness and stability of the PEs, we describe the recent progress of copper-based chalcogenides for hydrogen generation by water splitting using solar energy. It is anticipated that this report will provide insightful guidance to study the low-cost materials for solar to chemical fuel energy conversion.

Performance test of Zn-astaxanthin complex-sensitized solar cell: effect of light intensity on open-circuit voltage and short-circuit current values.

The sensitizer is one of the most essential dye-sensitized solar cell (DSSC) components. In the present research, a Zn-astaxanthin complex was investigated as a sensitizer, compared to pure astaxanthin. The complex with a 1:1 mole ratio between astaxanthin and Zn2+ was synthesized in a reflux reactor at 37-60 °C. The product was analyzed using Proton Nuclear Resonance (1H-NMR), which indicates the presence of chelate formation between Zn2+ with two atoms of oxygen on the terminal cyclohexane ring of astaxanthin. The interaction of sensitizers (astaxanthin and Zn-astaxanthin) on the photoelectrode surface in this study was analyzed using a Fourier Transform Infra-Red (FTIR) and Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS). The FTIR spectra of photoelectrode immersed in Zn-astaxanthin show peaks of C=O stretching and vibration -OH group at 1730 and 1273 cm-1, respectively, and H-C-H stretching vibration with high intensity in 2939, 2923, and 2853 cm-1. The UV-Vis DRS analysis shows the band gap of photoelectrode (PE), photoelectrode immersed in astaxanthin (PE/astaxanthin), and Zn-astaxanthin (PE/Zn-astaxanthin) are 3.19, 1.65, and 1.59 eV, respectively. Under illumination intensity of 300 W/m2, the maximum energy conversion efficiency of DSSC with Zn-astaxanthin as sensitizer is (0.03 ± 0.0022)%, higher than DSSC with astaxanthin as sensitizer ((0.12 ± 0.0052)%). Up to 70 h of illumination, DSSC with Zn-astaxanthin as a sensitizer also has better stability than astaxanthin-based DSSC.

Electrode Surface Heating with Organic Films Improves CO2 Reduction Kinetics on Copper.

Management of the electrode surface temperature is an understudied aspect of (photo)electrode reactor design for complex reactions, such as CO2 reduction. In this work, we study the impact of local electrode heating on electrochemical reduction of CO2 reduction. Using the ferri/ferrocyanide open circuit voltage as a reporter of the effective reaction temperature, we reveal how the interplay of surface heating and convective cooling presents an opportunity for cooptimizing mass transport and thermal assistance of electrochemical reactions, where we focus on reduction of CO2 to carbon-coupled (C2+) products. The introduction of an organic coating on the electrode surface facilitates well-behaved electrode kinetics with near-ambient bulk electrolyte temperature. This approach helps to probe the fundamentals of thermal effects in electrochemical reactions, as demonstrated through Bayesian inference of Tafel kinetic parameters from a suite of high throughput experiments, which reveal a decrease in overpotential for C2+ products by 0.1 V on polycrystalline copper via 60 °C surface heating.

Accelerated Characterization of Electrode‐Electrolyte Equilibration

AbstractOperational durability is poorly characterized by traditional (photo)electrocatalyst discovery workflows, creating a barrier to scale‐up and deployment. Corrosion is a prominent degradation mechanism whose thermodynamics depend on the concentration of corrosion products in electrolyte. We present an automated system for characterizing the equilibration of (photo)electrodes with dissolved metals in electrolyte for a given electrode, pH, and electrochemical potential. Automation of electrode selection, electrolyte preparation, and electrolyte aliquoting enables rapid identification of self‐passivating electrodes and estimation of the equilibrium dissolved metals concentrations. The technique is demonstrated for metal oxide photoanodes in alkaline electrolyte, where BiVO4 is found to continually corrode, in agreement the literature. An amorphous Ni−Sb−O photoanode is found to passivate with a Ni‐rich coating on the order of 1 monolayer with less than 1 μM total dissolved metals in electrolyte, demonstrating its suitability for durable photoelectrochemical operation. The automation and throughput of the instrument are designed for incorporation in accelerated electrocatalyst discovery workflows so that durability can be considered on equal footing with activity.

MoS2 modified NiFe LDH hierarchical structure as efficient photoanode towards photoelectrochemical water splitting

Designing effective photo electrode for photo-electrochemical (PEC) water splitting is highly demanded yet challengeable because of the limited sunlight utilization and retarded redox reaction kinetics. This work introduces a feasible one-pot hydrothermal strategy of fabricating MoS2/NiFe LDH p-n heterojunctions on conductive porous nickel foam as an efficient photoanode for PEC water splitting. A great enhancement for PEC water oxidation is achieved because of the improved sunlight absorption and the increased photo-excited charge separation. The optimal 10% MoS2/Ni0.9Fe0.1 LDH/nickel foam photoanode demonstrates 3.8 times enhanced PEC activity than the Ni0.9Fe0.1 LDH/NF counterpart and a low Tafel plot of 54 mV dec−1. The bandgap modulation and the Z-scheme charge transfer dynamics due to the formation of MoS2/NiFe LDH p-n heterostructure, along with the high conductivity of porous nickel foam account for the increased photo-current conversion efficiency and the boosted charge transfer.

Aminosilanized Interface Promotes Electrochemically Stable Carbon Nitride Films with Fewer Trap States on FTO for (Photo)electrochemical Systems.

We have demonstrated the direct growth of a CNx layer on a plasma-cleaned and aminosilanized F-doped SnO2 (FTO) electrode to study the CNx|FTO interface that is critical for (photo)electrocatalytic systems. The (3-aminopropyl)triethoxysilane (APTES) was chosen as a bifunctional organosilane, with the amino end incorporating into CNx and the silane end connecting to the hydroxylated FTO surface. Plasma cleaning and aminosilanization resulted in a highly hydrophilic surface, which leads to better contact of melted thiourea to the aminosilanized FTO (p-FTONH2) during CNx polymer condensation, thus generating a thinner and more compact CNx layer. The modification at the interface was shown to influence the CNx growth on length scales of tens of micrometers. We grew CNx thin films on p-FTONH2 (CNx/p-FTONH2) and nonaminosilanized p-FTO (CNx/p-FTO). CNx/p-FTONH2 had a smaller density of trap states and passed 2.4 times the charges before failure compared to CNx/p-FTO. Additionally, a 40% decrease in interfacial charge transfer resistance at the CNx|electrolyte interface was measured for CNx/p-FTONH2 compared to CNx/p-FTO under -0.5 V vs RHE in 0.1 M Na2SO4. Furthermore, with the CNx surface coated with a Pt cocatalyst, Pt/CNx/p-FTONH2 exhibited faster hydrogen evolution rates and larger current densities than Pt/CNx/p-FTO. The highest Faraday efficiency toward electrochemical hydrogen evolution (FEH2) in 0.1 M Na2SO4 (pH = 7) was 46.1%, 37.3%, 57.7%, and 70.5% for Pt/CNx/p-FTONH2, Pt/CNx/p-FTO, CNx/p-FTONH2, and CNx/p-FTO, respectively. The increase in hydrogen evolution rate did not follow the magnitude of the current density change, indicating electrochemical processes other than proton reduction. Overall, we have carefully investigated the CNx|FTO interface and suggested potential solutions to make CNx films better (photo)electrodes for (photo)electrochemical systems.

(Invited) Interfacial Potential Distribution at Semiconductor/Liquid Interfaces: A Framework for the Quantitative Understanding of Voltammetric Data with Semiconductor (Photo)Electrodes

This presentation will introduce a quantitative framework for interpreting and analyzing voltammetric data from semiconductor (photo)electrodes immersed in liquid electrolytes. The basis of the approach is to use the dielectric properties of the semiconductor, electrolyte, and interfacial layer(s) to calculate explicitly the fractional potential drops across the system that occur when a bias is applied relative to an external reference. These fractional potential drops are then used to determine the potential dependences of the forward and back charge-transfer rates between the semiconductor and redox acceptors/donors of interest. These rates, in conjunction with mass transport considerations, are used to predict voltammetric shapes from a variety of useful experimental configurations including diffusing and adsorbed redox species at either semiconductor macro- or ultramicroelectrodes. The goal of the work is to understand semiconductor voltammetric data directly rather than through the (limiting) lenses of either metal electrode responses or solid-state diodes.The presented framework assumes charge transfer occurs predominantly through the semiconductor band edges but a description of how contributions from surface states can affect the data will be discussed. In addition, this talk will also provide examples of how this framework improves the electroanalytical utility of semiconductor voltammetry to measure semiconductor surface reactions, the flat band potential, and interfacial dielectric properties.

Low Catalyst Loading Enhances Charge Accumulation for Photoelectrochemical Water Splitting

AbstractSolar water oxidation is a critical step in artificial photosynthesis. Successful completion of the process requires four holes and releases four protons. It depends on the consecutive accumulation of charges at the active site. While recent research has shown an obvious dependence of the reaction kinetics on the hole concentrations on the surface of heterogeneous (photo)electrodes, little is known about how the catalyst density impacts the reaction rate. Using atomically dispersed Ir catalysts on hematite, we report a study on how the interplay between the catalyst density and the surface hole concentration influences the reaction kinetics. At low photon flux, where surface hole concentrations are low, faster charge transfer was observed on photoelectrodes with low catalyst density compared to high catalyst density; at high photon flux and high applied potentials, where surface hole concentrations are moderate or high, slower surface charge recombination was afforded by low‐density catalysts. The results support that charge transfer between the light absorber and the catalyst is reversible; they reveal the unexpected benefits of low‐density catalyst loading in facilitating forward charge transfer for desired chemical reactions. It is implied that for practical solar water splitting devices, a suitable catalyst loading is important for maximized performance.

Low Catalyst Loading Enhances Charge Accumulation for Photoelectrochemical Water Splitting.

Solar water oxidation is a critical step in artificial photosynthesis. Successful completion of the process requires four holes and releases four protons. It depends on the consecutive accumulation of charges at the active site. While recent research has shown an obvious dependence of the reaction kinetics on the hole concentrations on the surface of heterogeneous (photo)electrodes, little is known about how the catalyst density impacts the reaction rate. Using atomically dispersed Ir catalysts on hematite, we report a study on how the interplay between the catalyst density and the surface hole concentration influences the reaction kinetics. At low photon flux, where surface hole concentrations are low, faster charge transfer was observed on photoelectrodes with low catalyst density compared to high catalyst density; at high photon flux and high applied potentials, where surface hole concentrations are moderate or high, slower surface charge recombination was afforded by low-density catalysts. The results support that charge transfer between the light absorber and the catalyst is reversible; they reveal the unexpected benefits of low-density catalyst loading in facilitating forward charge transfer for desired chemical reactions. It is implied that for practical solar water splitting devices, a suitable catalyst loading is important for maximized performance.

Assessment of the effective parameters for the enhancement of light-harvesting power in the photoelectrochemical microbial fuel cell

ABSTRACT Photo-assisted microbial fuel cells (PMFCs) are novel bioelectrochemical systems that employ light to harvest bioelectricity and efficient contaminant reduction. In this study, the impact of different operational conditions on the electricity generation outputs in a photoelectrochemical double chamber configuration Microbial fuel cell using a highly useful photocathode are evaluated and their trends are compared with the photoreduction efficiency trends. As a photocathode, a binder-free photo electrode decorated with dispersed polyaniline nanofiber (PANI)−cadmium sulphide Quantum Dots (QDs) is prepared here to catalyse the chromium (VI) reduction reaction in a cathode chamber with an improvement in power generation performance. Bioelectricity generation is examined in various process conditions like photocathode materials, pH, initial concentration of catholyte, illumination intensity and time of illumination. Results show that, despite the harmful effect of the initial contaminant concentration on the reduction efficiency of the contaminant, this parameter exhibits a superior ability for improving the power generation efficiency in a Photo-MFC. Furthermore, the calculated power density under higher light irradiation intensity has experienced a significant increase, which is due to an increment in the number of photons produced and an increase in their chance of reaching the electrodes surface. On the other hand, additional results indicate that the power generation decreases with the rise of pH and has witnessed the same trend as the photoreduction efficiency.

Air Cathode Design for Light-Assisted Charging of Metal–Air Batteries: Recent Advances and Perspectives

Metal–air batteries are a type of electrochemical cell that generates electrical energy by combining metal and oxygen from the air. They are a promising technology for energy storage and portable devices because of their high energy density, low cost, and environmental friendliness. However, the discharge products of these batteries are notoriously difficult to decompose, resulting in a high overpotential. Recent works have shown that semiconductors can capture solar energy and store it in batteries. When exposed to light, photocatalysts generate carriers (strong redox pairs), thereby increasing the electron migration rate and significantly reducing the overpotential of the metal–air battery reaction. Nevertheless, single photocatalysts often exhibit poor cycle performance due to serious carrier recombination. This review paper begins by briefly describing the fundamentals of photoelectrochemistry and then focuses on commonly used photocathode design principles and various reported photocatalysts. It also discusses the main challenges and issues caused by light, such as photocorrosion and electrolyte decomposition. Finally, the critical issues associated with photo electrodes, electrolytes, and electrode/electrolyte interfaces are emphasized, and perspectives on the development of high-performance light-assisted metal–air batteries are presented.

OH−/H+ dual-ion energy assisted electricity effective photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is an emerging technology to store the solar energy in the chemical bonds of molecular hydrogen. Among several photo electrodes used for PEC water splitting, α-Fe2O3 is a promising material due to its suitable bandgap, chemical stability, and abundance. Despite these, the position of its conduction band does not allow spontaneous movement of photo-generated electrons to cause the water reduction. This demands the application of a minimum electrical bias of ∼1.5 V vs. SHE to increase the energy of the conduction band such that it will be energetically above the H2O/H2 redox level. We show that by utilizing the energy of neutralization, the minimum electrical voltage required for PEC water splitting can be brought down to ∼0.8 V. By employing an OH−/H+dual-ion configuration. OH−/H+dual-ion assisted PEC water splitting required only 0.95 V to produce a current density of 10 mA/cm2, and for achieving the same rate in a conventional symmetric ion configuration, at least a doubling of the applied electrical bias (∼1.8 V) is required.

Electrochemical and Photoelectrochemical Water Splitting: Operando Raman and Fourier Transform Infrared Spectroscopy as Useful Probing Techniques

The urgent need for the clean energy transition of our society is the incentive for the scientific research groups to further advance the development of green hydrogen production. The field of (photo)electrochemical water splitting is currently experiencing a significant expansion of scientific investigations. Despite the technological and material development brought by this technique, the complex reaction mechanisms on the surface of (photo)electrodes during water splitting have led to considerable scientific questions and many contradictions to theoretical expectations. Therefore, a deeper tracking of the surface‐active centers, the experimental reaction mechanisms, and the nature of intermediates requires the use of in situ spectroscopic techniques. In this review, operando Raman and Fourier transform infrared (FTIR) spectroscopies in the field of (photo)electrochemical water splitting are presented. In the first section, the environmental challenges and possible solutions are discussed, and then the principle of vibrational spectroscopy and its applications in the field of (photo)catalysis are presented in detail. In the second part, the recent results of operando Raman and FTIR techniques for (photo)electrochemical water splitting, including challenges and reaction mechanisms, are reviewed. To simplify the presentation, the results are subdivided according to the materials used as (photo)electrodes.

Facile substrate-agnostic approach in preparing high-performance water splitting (photo)electrodes

Facile substrate-agnostic approach in preparing high-performance water splitting (photo)electrodes

Electrospun TiO2/carbon composite nanofibers as effective (photo)electrodes for removal and transformation of recalcitrant water contaminants

Electrochemical (EC) and photoelectrochemical (PEC) water treatment systems are gaining popularity, necessitating new electrode materials that offer reliable performance across diverse application platforms.

UV–Vis operando spectroelectrochemistry for (photo)electrocatalysis: Principles and guidelines

Electrocatalytic and photoelectrocatalytic technologies are key players in the transition to a circular economy. In order to rationally design the new generation of efficient (photo)electrocatalysts it is critical to establish which performance bottlenecks must be overcome during the complex catalytic transformations. Obtaining such detailed mechanistic and kinetic knowledge requires the use of operando spectroscopic techniques that are capable of probing the system under operating conditions. Of all the methods available UV–Vis spectroelectrochemistry stands out for its versatility and experimental simplicity which enables the systematic study of samples under a wide range of reaction conditions. From a mechanistic viewpoint, this technique allows us to quantify the accumulation of reactive species at the catalyst–electrolyte interface and, through a kinetic population model, opens the door to characterising the kinetics of the rate determining step of catalysis. Here, we review the different information that can be extracted from UV–Vis spectroelectrochemistry and how it can be used to understand catalytic mechanisms in solid (photo)electrodes.

Rhenium Carbonyl Molecular Catalysts for CO2 Electroreduction: Effects on Catalysis of Bipyridine Substituents Mimicking Anchorage Functions to Modify Electrodes.

Heterogenization of molecular catalysts on (photo)electrode surfaces is required to design devices performing processes enabling to store renewable energy in chemical bonds. Among the various strategies to immobilize molecular catalysts, direct chemical bonding to conductive surfaces presents some advantages because of the robustness of the linkage. When the catalyst is, as it is often the case, a transition metal complex, the anchoring group has to be connected to the complex through the ligands, and an important question is thus raised on the influence of this function on the redox and on the catalytic properties of the complex. Herein, we analyze the effect of conjugated and non conjugated substituents, structurally close to anchoring functions previously used to immobilize a rhenium carbonyl bipyridyl molecular catalyst for supported CO2 electroreduction. We show that carboxylic ester groups, mimicking anchoring the catalyst via carboxylate binding to the surface, have a drastic effect on the catalytic activity of the complex toward CO2 electroreduction. The reasons for such an effect are revealed via a combined spectro-electrochemical analysis showing that the reducing equivalents are mainly accumulated on the electron-withdrawing ester on the bipyridine ligand preventing the formation of the rhenium(0) center and its interaction with CO2. Alternatively, alkyl-phosphonic ester substituents, not conjugated with the bpy ligand, mimicking anchoring the catalyst via phosphonate binding to the surface, allow preserving the catalytic activity of the complex.