Photoelectrochemical Energy Conversion Research Articles

Nanoscale Photoelectrochemical Mapping of Two-Dimensional Electrodes

Electron transfer plays a significant role in many chemical processes, especially in the catalytic reactions involved in photoelectrochemical energy conversion and storage. Revealing the electron transfer behavior at the interface of electrode-electrolyte is thus of great importance. Here we fabricate atomically thin semiconductor MX2 (M = Mo, W; X = S, Se) by chemical vapor deposition method and directly map the photoelectrochemical behavior using Atomic Force Microscopy-combined with light-induced Scanning Electrochemical Microscopy (AFM-SECM). AFM-SECM feedback mapping shows a layer-dependent photoelectrocatalytic ability of MX2 to oxidize the redox species Ru2+ back to Ru3+. Moreover, AFM-SECM generation and collection mode shows the potential-dependent edge and basal plane activity for the hydrogen evolution reaction of WSe2. Compared with monolayer and bilayer, few-layer WSe2 shows better stability in an electrochemical environment, faster electron transfer, and higher hydrogen production. Furthermore, finite element method based numerical simulations using MATLAB® and COMSOL Multiphysics® is performed to calculate the electron transfer rate constants k0 and simulate the steady-state concentration gradient. Finally, the photoelectrochemistry at MX2 electrode-electrolyte interfaces are spatially resolved and understanding this behavior will be useful for the future photoelectrochemical devices.

Rational design of nanostructured BiVO4/FeOx photoanode coupling with 2D Co(OH)2 cocatalyst for enhanced photoelectrochemical water splitting

Efficient charge transport and separation is of prime significance to achieve high efficiency of photoelectrochemical (PEC) solar energy conversion. In this study, we designed a novel ternary nanostructured BiVO4/FeOx/Co(OH)2 composite as photoelectrode for enhanced PEC water splitting. Compared with pristine BiVO4, the fabricated BiVO4/FeOx/Co(OH)2 photoelectrode produced a doubled increase in photocurrent (from 0.36 to 0.72 mA cm−2 at 1.23 V versus RHE) with a negative shift of 390 mV in the initial potential (from 0.72 to 0.33 V). In addition, the conversion efficiency and photostability for PEC water oxidation were also improved. The elevated PEC properties of ternary composite photoelectrode were originated from the synergy of heterojunction construction and 2D cocatalyst loading that provides effective pathway for promoting charge separation as well as sufficient sites for photoelectrochemical reaction. These findings could provide an exemplary PEC model for enhanced solar hydrogen evolution of BiVO4 photoanode.

Resonance Raman spectra and excited state properties of methyl viologen and its radical cation from time-dependent density functional theory.

Time-dependent density functional theory (TDDFT) was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+ ). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, environmental remediation, and chemical sensing and electrosynthesis. MV2+ is easily reduced by a single electron transfer to form a radical cation species (MV•+ ), which has an intense UV-visible absorption near 600 nm. The redox properties of the MV2+ /MV•+ couple and light-sensitivity of MV•+ have made the system appealing for photo-electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo-induced charge transfer processes through electronic absorption and resonance Raman spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+ . Using a conventional hybrid exchange functional (B3-LYP) and a long-range corrected hybrid exchange functional (ωB97X-D3), including with a conductor-like polarizable continuum model to account for solvation, the electronic absorption and resonance Raman spectra predicted are in good agreement with experiment. Also analyzed are the charge transfer character and natural transition orbitals derived from the TDDFT vertical excitations calculated. The findings and models developed further the understanding of the electronic properties of viologens and related organic redox mediators important in renewable energy applications and serve as a reference for guiding the interpretation of electronic absorption and Raman spectra of the ions.

Atomistic insight into the defect-induced tunable plasticity and electronic properties of tetragonal zirconia

Tetragonal zirconia (t-ZrO2) with exceptional properties is crucial in catalytic applications. Defects effectively enhance its electronic properties and plasticity, making t-ZrO2 a valuable material for high-efficiency industrial catalysts, and flexible electronic devices. In this study, we utilize first-principles calculations to compare the impact of vacancy defects and nitrogen doping on the structural and electronic properties of wide bandgap semiconductor t-ZrO2. Additionally, we analyze the tuned plasticity of t-ZrO2 by varying N-dopant and vacancy concentrations using molecular dynamics simulations and cyclic-nanoindentation tests. The estimation of energy release associated with plastic deformation is conducted using the Griffith energy balance model. Our study reveals significantly distinct electronic properties and plastic deformation maps in oxygen-deficient and N-doped t-ZrO2 compared to the perfect counterpart, where the defect nature dictates the band gap energy and plastic zone size. t-ZrO2−x exhibits a greater bandgap narrowing than t-ZrO2−xNx, resulting from increased atomic displacement, decreased free energy for plastic deformation, and enhanced plastic dissipated energy. Furthermore, we demonstrate that electronic properties and the plasticity of t-ZrO2−xNx, including bandgap energy and energy release rate during cyclic-nanoindentation, are unable to compete with those of the small-bandgap counterpart t-ZrO2−x. Herein, t-ZrO2−x, x = 0.2 exhibits the highest plasticity and the smallest bandgap of 1.28 eV, contrasting with the 5.6 eV bandgap of perfect t-ZrO2. Thereby, t-ZrO2−x displays pronounced band gap tightening and enriched flexibility, providing it a favorable semiconductor for photoelectrochemical energy conversion (PEC) applications.

Microalgae-material hybrid for enhanced photosynthetic energy conversion: a promising path towards carbon neutrality.

Photosynthetic energy conversion for high-energy chemicals generation is one of the most viable solutions in the quest for sustainable energy towards carbon neutrality. Microalgae are fascinating photosynthetic organisms, which can directly convert solar energy into chemical energy and electrical energy. However, microalgal photosynthetic energy has not yet been applied on a large scale due to the limitation of their own characteristics. Researchers have been inspired to couple microalgae with synthetic materials via biomimetic assembly and the resulting microalgae-material hybrids have become more robust and even perform new functions. In the past decade, great progress has been made in microalgae-material hybrids, such as photosynthetic carbon dioxide fixation, photosynthetic hydrogen production, photoelectrochemical energy conversion and even biochemical energy conversion for biomedical therapy. The microalgae-material hybrid offers opportunities to promote artificially enhanced photosynthesis research and synchronously inspires investigation of biotic-abiotic interface manipulation. This review summarizes current construction methods of microalgae-material hybrids and highlights their implication in energy and health. Moreover, we discuss the current problems and future challenges for microalgae-material hybrids and the outlook for their development and applications. This review will provide inspiration for the rational design of the microalgae-based semi-natural biohybrid and further promote the disciplinary fusion of material science and biological science.

Highly conducting flake-like CuS nanostructured counter electrode for photoelectrochemical solar energy conversion

Fabrication of platinum (Pt) free, low cost, environmentally friendly counter electrode is highly attractive for electrochemical solar energy conversion. In this report, highly conducting flake-like CuS nanostructured counter electrode has been prepared for photosensing study and semiconductor sensitized solar cell application. Flake-like CuS nanostructures were synthesized by sulfuration of CuO nanorods grown by hydrothermal process. Scanning electron microscopy and atomic force microscopy study confirmed the growth of flake-like CuS nanostructures on glass substrates. CuS nanoflakes were highly crystalline with hexagonal phase. The various electronic states of Cu and S in CuS nanoflakes were studied using X-ray photoelectron spectroscopy. Chemically grown CuS films were highly conducting and highest conducting film was obtained at 85°C growth temperature with sheet resistance ∼20.02±0.02Ω/sq. Photoelectrochemical solar cell (PEC) was fabricated using CdS decorated TiO2 nanorods as working electrode and CuS film as counter electrode. The short circuit current density, open circuit voltage and power conversion efficiency of the PEC device were found to be ∼4.8 A/m2, ∼0.38 V and ∼0.17%, respectively.

Internal magnetic-field-enhanced photogenerated charge separation in ferromagnetic TiO2 surface heterojunctions

The use of the internal magnetic field of ferromagnets can effectively promote charge separation and transfer (CST) in photoelectrochemical energy conversion. However, photoelectrochemical materials with a ferromagnetic field are scarce, and the internal magnetic field is negligible in nonferromagnetic materials. To address this issue, we propose a rational method for preparing ferromagnetic TiO2 powder using controllable oxygen vacancies in anatase TiO2 with co-exposed {001} and {101} facets. Accordingly, an excellent saturation magnetisation of 0.0014 emu/g in TiO2 is achieved owing to an asymmetric and uneven charge distribution. Compared with that of nonferromagnetic TiO2, the efficiency of photocatalytic hydrogen generation of ferromagnetic TiO2 is improved by 0.64 times. The enhancement of photocatalytic hydrogen generation is due to the different forces exerted on the electrons and holes in the magnetic field, which significantly improve the photogenerated CST efficiency of ferromagnetic TiO2. This result highlights the significant role of the synergistic regulation of the crystal structure and defects in regulating the ferromagnetic characteristics of materials. The findings of this study provide guidance for leveraging point defects to promote CST for high-efficiency solar-energy conversion systems.

Photoelectrocatalytic Dioxygen Reduction Based on a Novel Thiophene-Functionalized Tricarbonylchloro(1,10-phenanthroline)rhenium(I)

A novel Re (I) complex of [Re(CO)3Cl(L)], {L = 2-([2,2'-bithiophen]-5-yl)-1-phenyl-1H-imidazo [4,5-f][1,10]phenanthroline}, was synthesized, and its optical (UV-Visible absorption and emission spectroscopy), cyclovoltammetric and photoelectrochemical oxygen reduction properties were studied. The geometric and electronic properties were also investigated by density functional theory calculations. It was found that the ITO electrode coated with drop-casted [Re(CO)3Cl(L)] film exhibited cathodic photocurrent generation characteristics. The illuminated film exhibited a maximum cathodic photocurrent up to 30.4 μA/cm2 with an illumination intensity of 100 mW/cm2 white light at a bias potential of -0.4 V vs. SCE in O2-saturated electrolyte solution, which was reduced by 5.1-fold when thoroughly deoxygenated electrolyte solution was used, signaling that the electrode performed well on the photoelectrochemical oxygen reduction. The photo-electrocatalytic hydrogen peroxide production was proved with a maximum H2O2 concentration of 6.39 μM during 5 h of the photoelectrocatalytic process. This work would guide the construction of more efficient rhenium-based photo(electro)catalytic molecular systems for O2 sensing, hydrogen peroxide production and other types of photoelectrochemical energy conversion and storage.

Investigation of non-stoichiometric delafossite photoelectrodes composed of cuprous oxide and iron oxide for enhanced water-splitting

Photoelectrochemical energy conversion is the ultimate approach for converting sunlight energy into value-added chemical fuels. The path of achieving economically feasible, highly efficient, and earth-abundant material-based photoelectrodes can be a driving force for realizing PEC on a global scale. Copper-based delafossite photoelectrodes are attractive candidates because they exhibit appropriate absorption properties and structural stability in an aqueous system. In this study, delafossite CuxFe2−xO2 films of varying compositions ranging from 0 ≤ x ≤ 2 are prepared by adjusting the Cu/Fe composition via radio frequency plasma co-sputtering to evaluate the fundamental optoelectronic properties. The optical bandgap of the CuFeO2 films tuned from 1.4 to 1.65 eV, with the maximum carrier concentration of 5.9 × 1017 cm−3 noticed in Cu1.5Fe0.5O2 film. The X-ray photoelectron spectroscopy measurements confirm the existence of a high level of oxygen-deficient sites in CuFeO2 film photoelectrodes that are suitable for PEC performance. The fabricated non-stoichiometric photoelectrode exhibits an excellent hydrogen evolution reaction of 4.3 μmol/hcm2, nearly 1.8 times improvement over Cu2O photoelectrode with a long-term photostability even in an aqueous system. The ability to fabricate tunable electronic structures photoelectrodes along with oxygen-deficient surface defects facilitates to localize band edge closer to the HER of water, and the Fermi level is slightly away from the reduction potential of Cu2O, which supports to improve PEC performance by suppressing photo-induced corrosion to retain greater photostability.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties.

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2 O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Triclinic-Phase Bismuth Chromate: A Promising Candidate for Photocatalytic Water Splitting with Broad Spectrum Ranges.

Photocatalytic water splitting for solar energy conversion remains challenged by the lack of novel semiconductor photocatalysts with paramount parameters including wide light-harvesting ranges and suitable band structures. Here, a novel triclinic-phase bismuth chromate (Bi2 CrO6 ) acting as a semiconductor photocatalyst candidate is reported. Triclinic Bi2 CrO6 exhibits a broad absorption range of ≈650nm with a direct bandgap of 1.86eV and shows a suitable band structure for water splitting. Theoretical simulations of triclinic Bi2 CrO6 reveal a high charge mobility,possibly owing to the strong hybridized covalent bonds, large elastic modulus, and small carrier effective mass. The triclinic Bi2 CrO6 is demonstrated to work well toward photocatalytic water oxidation and hydrogen production reactions under visible light and match well with its absorption ranges. In particular, it exhibits decent photocatalytic water oxidation performance in the presence of various electron scavengers. Furthermore, the visible-light-driven Z-scheme overall water splitting system is fabricated by coupling triclinic Bi2 CrO6 as the oxygen evolution photocatalyst with SrTiO3 :Rh as the hydrogen evolution photocatalyst, giving a stable overall water splitting with stoichiometric evolution of H2 and O2 . This work presents a promising semiconductor material enabling wide-range light harvesting for photocatalytic and photo-electrochemical solar energy conversion.

Porous Single Crystals at the Macroscale: From Growth to Application.

ConspectusPorous materials have wide applications in the fields of catalysis, separation, and energy conversion and storage. Porous materials contain pores that are specifically designed to achieve expectant performance. The solid phases in porous materials are normally completely continuous to form the basic porous frame while the pores are fluid phase within the solid phase. Single crystals are macroscopic materials in three spatial dimensions with the constituent atoms, ions, molecules, or molecular assemblies arranged in an orderly repeating pattern with the ordered structures. The growth of single crystals is indeed a process to arrange these constituents in three dimensions into a repeating pattern within the materials. Today the applications of single crystals are exponentially growing in wide fields, and single crystals are therefore unacknowledged as the pillars of our modern technology. Introducing porosity into single crystals would be expected to create a new kind of porous material in which the basic porous frames are single-crystalline and free of grain boundaries. The structural symmetry is completely maintained within the basic porous frames which are a continuous solid phase, but it is completely lost inside the pores. The porous architecture is free of grain boundaries, and the fully interconnected skeletons are in single-crystalline states within the basic porous frames. Single crystals with porosities can therefore be considered to be a new kind of porous material, but they are single-crystal-like because the structural symmetry is maintained only in the skeletons and completely lost within the pores. We therefore call them porous single crystals or consider them in porous single-crystalline states to stand out with their structural features. Porous single crystals at the macroscale combine the advantages of porous materials and single crystals to incorporate both porosity and structural coherence in a porous architecture, leading to invaluable opportunities to alter the material's properties by controlling the unique structural features to enhance its performance. However, the growth of single crystals in three dimensions reduces the formation of porosities, leading to a fundamental challenge for introducing porosity into single crystals in a traditional process of crystal growth. In this Account, we report the rational design, growth methodology, and microstructural engineering of porous single crystals in a solid-solid transformation. We rationally design a high-density mother phase in a single-crystalline state and transform it into a low-density new phase in a single-crystalline state to introduce porosities into single crystals even incorporating the removal of specific compositions from the mother phase during the growth of porous single crystals. The porosity can be tailored by controlling the change in relative densities from the mother phase to the porous single crystals while the pore size can be engineered by controlling the fabrication conditions. Considering the unique structural features, we explore their functionalities and applications in photoelectrochemical energy conversion, electrochemical alkane conversion, and electrochemical energy storage. We believe that the materials, if tailored into porous single-crystalline states, would not only find a broad range of applications in other fields but also enable a new path for material innovations.

Quantitative framework development for understanding the relationship between doping and photoelectrochemical energy conversion of TiO2

This review suggests an alternative quantitative framework for developing and establishing the relationship between doping and photoelectrochemical energy conversion in TiO2 based devices.

Photoelectrochemical aptasensor based on nanocomposite of CdSe@SnS2 for ultrasensitive and selective detection of sulfamethazine.

A photoelectrochemical (PEC) aptasensor based on CdSe@SnS2 nanocomposite has been developed to detect sulfamethazine (SMZ). The introduction of CdSe into SnS2 displayed an amplified PEC signal, which was higher than that ofpure CdSe and SnS2, attributable to its enhanced light harvesting capacity and promoted PEC energy conversion efficiency. Due to the formation of specific non-covalent bonds, theSMZ-binding aptamer (SBA) has significant specificity and sensitivity. When SMZ was incubated on a CdSe@SnS2 modified electrode fixed with aminated SBA, the formation of the SMZ/SBA complex increased the space resistance of electron transfer and hindered the electronic migration between the electrodes, resulting in a decrease in photocurrent. The greater the adsorbed amount on the SBA, the lower the photocurrent produced. Under optimized conditions the photocurrent response of MCH/SBA/CdSe@SnS2/FTO was inversely proportional to the SMZ concentration in the range 0.1 to 100pM, with a detection limit (3S/N) of 0.025pM (at 0V vs. Hg/HgCl). The recoveries ranged from 95.8 to 104% with relative standard deviations (RSDs) < 6.3% (n = 3) in actual water sample. This PEC aptasensor which shows considerable potential in SMZ detection applications has high selectivity, reproducibility, and good stability.

Toward Ideal Metal–Organic Framework Thin-Film Growth via Automated Layer-by-Layer Deposition: Examples Based on Perylene Diimide Linkers

Photoactive metal–organic framework (MOF) thin films offer an opportunity for translating the advantages of periodic, crystalline, and tailorable light-harvesting materials directly into devices such as those for photoelectrochemical solar energy conversion. In this study, we report the fabrication of light-absorbing perylene-diimide-containing pillared-paddlewheel MOF thin films using an automated layer-by-layer (LbL) deposition technique. Our focus here is on optimizing the growth of representative chromophoric MOFs as oriented films of uniform and predefined thickness. Growth was examined as a function of metal identity, pillaring ligand composition, and supporting-surface chemical functionality. Application of atomic force microscopy (AFM) and complementary techniques revealed that the surface-supported MOFs initially display island-type film growth (Volmer–Weber growth), resulting in comparatively rough films. Further growth is accompanied by the merging of islands, resulting in films that, depending on experimental details, can be remarkably smooth (i.e., roughness on the order of ±1 nm (one structural repeat unit in the pillaring direction)). These details include the use of 1,4-diazabicyclo[2.2.2]octane (DABCO) as a MOF pillar and ALD-grown zinc oxide as a film support (ALD = atomic layer deposition). Also helpful for mitigating island-type growth, at least in part, is the replacement of Zn2+ by Cu2+ as the metal component of the MOF. Notably, each of these adjustments entails replacing weaker chemical bonds with stronger ones.

Indium Selenide/Indium Tin Oxide Hybrid Films for Solution‐Processed Photoelectrochemical‐Type Photodetectors in Aqueous Media

Abstract2D metal monochalcogenides have recently attracted interest for photoelectrochemical (PEC) applications in aqueous electrolytes. Their optical bandgap in the visible and near‐infrared spectral region is adequate for energy conversion and photodetection/sensing. Their large surface‐to‐volume ratio guarantees that the charge carriers are photogenerated at the material/electrolyte interface, where redox reactions occur, minimizing recombination processes. However, solution‐processed photoelectrodes based on these materials exhibit energy conversion efficiencies that are far from the current state of the art expressed by established technologies. This work reports a systematic morphological, spectroscopic, and PEC characterization of solution‐processed films of photoactive InSe flakes for PEC‐type photodetectors. By optimizing the thickness and hybridizing InSe flakes with electrically conductive Sn:In2O3 (ITO) nanocrystals, photoanodes with a significant photoanodic response in both acidic and alkaline media are designed, reaching responsivity up to 60.0 mA W−1 (external quantum efficiency = 16.4%) at +0.4 V versus RHE under visible illumination. In addition, a strategy based on the use of sacrificial agents (i.e., 2‐propanol and Na2SO3) is proposed to improve the stability of the InSe and ITO/InSe photodetectors. Our data confirm the potential of 2D InSe for PEC energy conversion and sensing applications, remarking the challenges related to InSe stability during anodic operation.

(Invited) Efficient and Selective Electrocatalytic and Photoelectrochemical Conversion of Energy and Chemicals

Due to the intermittent nature of most renewable energy sources (such as solar and wind), practical large scale renewable energy utilization demands both efficient energy conversion and large scale energy storage or alternative usage. Earth-abundant but highly active and selective electrocatalysts are needed to enable efficient and sustainable production of fuels and chemicals using electrocatalytic and photoelectrochemical (PEC) energy conversion. We developed earth-abundant electrocatalysts for highly efficient hydrogen evolution reaction (HER) and oxygen evolution (OER). We recently combined computations and experiments to develop metal compounds as selective catalysts for two-electron oxygen reduction reaction (2e- ORR) to produce hydrogen peroxide (H2O2) and the subsequent electro-Fenton process for upgrading biomass molecules. We have integrated these earth-abundant electrocatalysts with efficient semiconductor materials to demonstrate efficient photoelectrochemical hydrogen generation systems. We also developed high performance hybrid solar-charged storage devices that integrate photoelectrochemical solar cells and redox flow batteries (RFBs). In these integrated solar flow batteries (SFBs), solar energy is absorbed by semiconductor electrodes to directly charge up the redox couples without external electric bias. Such charged redox molecules can then be either discharged to generate the electricity when needed or used to produce fuels and chemicals.

Nanospace-confined worm-like BiVO4 in TiO2 space nanotubes (SPNTs) for photoelectrochemical hydrogen production

Titanium dioxide (TiO2) space nanotubes (SPNTs) have attracted considerable attention in the fields of photocatalysis and photoelectrochemical energy conversion because of their large surface area and good physical and chemical stability. However, their large bandgap (3.0–3.2 eV) and poor charge-carrier separation and transport properties lead to unsatisfactory photo-conversion efficiency. To overcome these drawbacks, we performed in vivo and in vitro deposition of BiVO4 (BVO) on SPNTs, obtaining a sophisticated BVO/SPNTs heterojunction via electrochemical anodization and microwave-assisted synthesis. Unlike conventional TiO2 nanotube structures, TiO2 SPNTs possess empty spaces between the tubes (tube-to-tube interface), which can effectively admit BVO particles and generate worm-like BVO, yielding an intimate heterojunction interface and enlarged surface area. The prepared BVO/SPNTs exhibited a narrow bandgap (2.32 eV), small charge-transfer resistance, and accelerated reaction kinetics, as confirmed by incident-photon-to-current conversion efficiency, ultraviolet diffuse reflectance spectroscopy, and electrochemical impedance spectroscopy. These salient features, together with a strong response to visible light, resulted in a photocurrent density of 0.68 mA/cm2 without any co-catalyst at 1.23 V vs. a reversible hydrogen electrode in 0.5 M Na2SO4, which is 14 times higher than that of the pure SPNT electrode film. Thus, this study highlights that the integration of the materials with different dimensions by tailoring the surface properties may yield efficient and stable catalysts for multiple applications, particularly for hydrogen production at a significantly improved rate.

Photoelectrochemical Behavior of Phthalocyanine-Sensitized TiO2 in the Presence of Electron-Shuttling Mediators.

Dye-sensitized TiO2 has found many applications for dye-sensitized solar cells (DSSC), solar-to-chemical energy conversion, water/air purification systems, and (electro)chemical sensors. We report an electrochemical system for testing dye-sensitized materials that can be utilized in photoelectrochemical (PEC) sensors and energy conversion. Unlike related systems, the reported system does not require a direct electron transfer from semiconductors to electrodes. Rather, it relies on electron shuttling by redox mediators. A range of model photocatalytic materials were prepared using three different TiO2 materials (P25, P90, and PC500) and three sterically hindered phthalocyanines (Pcs) with electron-rich tert-butyl substituents (t-Bu4PcZn, t-Bu4PcAlCl, and t-Bu4PcH2). The materials were compared with previously developed TiO2 modified by electron-deficient, also sterically hindered fluorinated phthalocyanine F64PcZn, a singlet oxygen (1O2) producer, as well as its metal-free derivative, F64PcH2. The PEC activity depended on the redox mediator, as well as the type of TiO2 and Pc. By comparing the responses of one-electron shuttles, such as K4Fe(CN)4, and 1O2-reactive electron shuttles, such as phenol, it is possible to reveal the action mechanism of the supported photosensitizers, while the overall activity can be assessed using hydroquinone. t-Bu4PcAlCl showed significantly lower blank responses and higher specific responses toward chlorophenols compared to t-Bu4PcZn due to the electron-withdrawing effect of the Al3+ metal center. The combination of reactivity insights and the need for only microgram amounts of sensing materials renders the reported system advantageous for practical applications.

A New Figure of Merit for Solar Charging Systems: Case Study for Monolithically Integrated Photosupercapacitors Composed of a Large‐Area Organic Solar Cell and a Carbon Double‐Layer Capacitor

The global trend toward automatization and miniaturization of smart devices has triggered the development of reliable off‐grid power sources with low economic and environmental impact. Such autonomy can be provided when a photovoltaic cell is integrated with an electrochemical double‐layer capacitor in one monolithic power pack. This work demonstrates a reliable and straightforward approach to monolithically integrate high‐performance organic solar cells with mesoporous nitrogen‐doped carbon nanosphere‐based supercapacitors in a single device with a three‐electrode configuration. To assess the efficiency of the device, a novel approach is presented that relies on the direct monitoring of both integrating parts during illuminated and dark phases and accounts for possible losses. The evaluation with the standard literature approach shows an outstanding performance of the integrated photosupercapacitor with a peak photoelectrochemical energy conversion efficiency of 17%. However, this type of efficiency does not properly represent the real overall efficiency of the device. Based on the newly developed efficiency calculation, a more modest overall cycle efficiency of 2% is obtained, which represents the overall performance of the integrated device in a better way. This versatile evaluation approach is applicable for all kinds of integrated multifunctional photoconversion–storage systems.