Heterostructured Bimetallic MOF-on-MOF Architectures for Efficient Oxygen Evolution Reaction.

Electron modulation presents a captivating approach to fabricate efficient electrocatalysts for the oxygen evolution reaction (OER), yet it remains a challenging undertaking. In this study, an effective strategy is proposed to regulate the electronic structure of metal-organic frameworks (MOFs) by the construction of MOF-on-MOF heterogeneous architectures. As a representative heterogeneous architectures, MOF-74 on MOF-274 hybrids are in situ prepared on 3D metal substrates (NiFe alloy foam (NFF)) via a two-step self-assembly method, resulting in MOF-(74 + 274)@NFF. Through a combination of spectroscopic and theory calculation, the successful modulation of the electronic property of MOF-(74 + 274)@NFF is unveiled. This modulation arises from the phase conjugation of the two MOFs and the synergistic effect of the multimetallic centers (Ni and Fe). Consequently, MOF-(74 + 274)@NFF exhibits excellent OER activity, displaying ultralow overpotentials of 198 and 223 mV at a current density of 10 mA cm-2 in the 1.0 and 0.1 M KOH solutions, respectively. This work paves the way for manipulating the electronic structure of electrocatalysts to enhance their catalytic activity.

Heterostructured Ultrathin Two-Dimensional Co-FeOOH Nanosheets@1D Ir-Co(OH)F Nanorods for Efficient Electrocatalytic Water Splitting.

It is highly desirable to develop high-performance and robust electrocatalysts for overall water splitting, as the existing electrocatalysts exhibit poor catalytic performance toward hydrogen and oxygen evolution reactions (HER and OER) in the same electrolytes, resulting in high cost, low energy conversion efficiency, and complicated operating procedures. Herein, a heterostructured electrocatalyst is realized by growing Co-ZIF-67-derived 2D Co-doped FeOOH on 1D Ir-doped Co(OH)F nanorods, denoted as Co-FeOOH@Ir-Co(OH)F. The Ir-doping couples with the synergy between Co-FeOOH and Ir-Co(OH)F effectively modulate the electronic structures and induce defect-enriched interfaces. This bestows Co-FeOOH@Ir-Co(OH)F with abundant exposed active sites, accelerated reaction kinetics, improved charge transfer abilities, and optimized adsorption energies of reaction intermediates, which ultimately boost the bifunctional catalytic activity. Consequently, Co-FeOOH@Ir-Co(OH)F exhibits low overpotentials of 192/231/251 and 38/83/111 mV at current densities of 10/100/250 mA cm-2 toward the OER and HER in a 1.0 M KOH electrolyte, respectively. When Co-FeOOH@Ir-Co(OH)F is used for overall water splitting, cell voltages of 1.48/1.60/1.67 V are required at current densities of 10/100/250 mA cm-2. Furthermore, it possesses outstanding long-term stability for OER, HER, and overall water splitting. Our study provides a promising way to prepare advanced heterostructured bifunctional electrocatalysts for overall alkaline water splitting.

Metal-organic framework-derived 2D layered double hydroxide ultrathin nanosheets for efficient electrocatalytic hydrogen evolution reaction.

A facile approach to synthesize a Pt-doped metal-organic framework (MOF)-derived layered double hydroxide (LDH), denoted as Pt@CuFe-LDHm, is developed. It consists of highly dispersed 2D nanosheets, and the excellent properties (high specific surface area and large porosity) inherited from CuFe(dobpdc) bestow it with superior catalytic properties. Pt@CuFe-LDHm shows low overpotentials of 33, 47 and 120 mV at 10 mA cm-2 with small Tafel slopes of 34.0, 50.2 and 85.6 mV dec-1 in 1.0 M KOH, 0.1 M KOH and 1.0 M PBS. This work paves the way for high-performance and durable HER electrocatalysts in alkaline and neutral electrolytes.

Orientation-Adjustable Metal-Organic Framework Nanorods for Efficient Oxygen Evolution Reaction.

A series of orientation-adjustable metal-organic framework (MOF) nanorods, CoFe(dobpdc)-I to CoFe(dobpdc)-III (dobpdc = 4,4'-dihydroxybiphenyl-3,3'-dicarboxylate), is developed on a 3D nickel foam (NF) template. By modulating the solvent composition for synthesis, the feature of MOF nanorods on the template can be varied from disorganized to a unidirectional orientation perpendicular to the NF. Well-aligned, vertically oriented CoFe(dobpdc)-III nanorods are hydrophilic and have more exposed active sites and interfacial charge transfer ability. Consequently, they exhibit a superior activity for oxygen evolution reaction (OER) with ultralow overpotentials of 176 and 240 mV at 10 and 300 mA cm-2 in 1.0 M KOH (aq), respectively. CoFe(dobpdc)-III also shows a record low overpotential of 204 mV at J10 mA cm-2 among the electrocatalysts based on CoFe MOF and an excellent overpotential at a high current density (100 mA cm-2) of 312 mV in 0.1 M KOH (aq). This is the first report of a convenient method to straighten up MOF nanorods on a template for highly efficient OER.

Sensitizers for Aqueous-Based Solar Cells.

Aqueous dye-sensitized solar cells (DSSCs) are attractive due to their sustainability, the use of water as a safe solvent for the redox mediators, and their possible applications in photoelectrochemical water splitting. However, the higher tendency of dye leaching by water and the lower wettability of dye molecules are two major obstacles that need to be tackled for future applications of aqueous DSSCs. Sensitizers designed for aqueous DSSCs are discussed based on their functions, such as modification of the molecular skeleton and the anchoring group for better stability against dye leaching by water, and the incorporation of hydrophilic entities into the dye molecule or the addition of a surfactant to the system to increase the wettability of the dye for more facile dye regeneration. Surface treatment of the photoanode to deter dye leaching or improve the wettability of the dye molecule is also discussed. Redox mediators designed for aqueous DSSCs are also discussed. The review also includes quantum-dot-sensitized solar cells, with a focus on improvements in QD loading and suppression of interfacial charge recombination at the photoanode.

High-performance aqueous/organic dye-sensitized solar cells based on sensitizers containing triethylene oxide methyl ether.

Metal-free dyes (EO1 to EO4) containing the hydrophilic triethylene oxide methyl ether (TEOME) unit in the spacer have been synthesized and used in dye-sensitized solar cells (DSSCs). Efficient lithium-ion trapping by TEOME results in improved open-circuit voltage (VOC ), leading to excellent conversion efficiency of the cells, ranging from 9.02 to 9.98 % with I(-) /I3 (-) electrolyte in acetonitrile under AM 1.5 illumination. The TEOME unit also enhances the wettability of the dye molecules for application in aqueous-based DSSCs. Aqueous-based DSSCs with a dual TEMPO/iodide electrolyte exhibit high VOC values (0.80-0.88 V) and very promising cell performances of up to 5.97 %.

Anthracene/phenothiazine π-conjugated sensitizers for dye-sensitized solar cells using redox mediator in organic and water-based solvents.

Metal-free dyes (MD1 to MD5) containing an anthracene/phenothiazine unit in the spacer have been synthesized. The conversion efficiency (7.13 %) of the dye-sensitized solar cell using MD3 as the sensitizer reached approximately 85 % of the N719-based standard cell (8.47 %). The cell efficiency (8.42 %) of MD3-based dye-sensitized solar cells (DSSCs) with addition of chenodeoxycholic acid is comparable with that of N719-based standard cell. The MD3 water-based DSSCs using a dual-TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl)/iodide electrolyte exhibited very promising cell performance of 4.96 % with an excellent Voc of 0.77 V.

Phenothiazinedioxide-conjugated sensitizers and a dual-TEMPO/iodide redox mediator for dye-sensitized solar cells.

Metal-free dyes containing a phenothiazinedioxide entity in the spacer were synthesized. The best conversion efficiency (7.47%) of the dye-sensitized solar cell (DSSC) by using new sensitizers with chenodeoxycholic acid as a co-adsorbent and the I(-) /I3 (-) electrolyte reached over 90% of that of the standard N719-based cell (8.10%). A new type of ionic liquid containing the nitroxide radical (N-O(.) ) and iodide was successfully synthesized and applied to the DSSCs. If the I(-) /I3 (-) electrolyte was replaced with a dual redox electrolyte, that is, a TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) derivative with a dangling imidazolium iodide entity, the cell exhibited a high open-circuit voltage of 0.85 V and a cell efficiency of 8.36%.

Ionic liquid with a dual-redox couple for efficient dye-sensitized solar cells.

A new type of ionic liquid that contains a nitroxide radical (N-O(.)) and iodide as two redox couples, JC-IL, has been successfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I2 in the wavelength region of 350-500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electrolyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electrolyte show promising cell efficiencies if coupled with dyes CR147 (8.12%) or D149 (6.76%). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electrolyte alone.

Materials for the active layer of organic photovoltaics: ternary solar cell approach.

Power conversion efficiencies in excess of 7% have been achieved with bulk heterojunction (BHJ)-type organic solar cells using two components: p- and n-doped materials. The energy level and absorption profile of the active layer can be tuned by introduction of an additional component. Careful design of the additional component is required to achieve optimal panchromatic absorption, suitable energy-level offset, balanced electron and hole mobility, and good light-harvesting efficiency. This article reviews the recent progress on ternary organic photovoltaic systems, including polymer/small molecule/functional fullerene, polymer/polymer/functional fullerene, small molecule/small molecule/functional fullerene, polymer/functional fullerene I/functional fullerene II, and polymer/quantum dot or metal/functional fullerene systems.

Benzothiadiazole-containing donor-acceptor-acceptor type organic sensitizers for solar cells with ZnO photoanodes.

Dye-sensitized solar cells using nanocrystalline ZnO as the photoanode and a metal-free sensitizer with a benzothiadiazole entity directly connected to the 2-cyanoacrylic acid acceptor exhibited an efficiency (5.18%) higher than those using the TiO(2) photoanode. Use of a hierarchical ZnO back scattering layer further improved the efficiency to 5.82%.

Dihydrophenanthrene-based metal-free dyes for highly efficient cosensitized solar cells.

Metal-free dyes (BP-1 to BP-3) containing a 9,10-dihydrophenanthrene unit in the spacer have been synthesized. The dye with the highest cell efficiency, BP-2, was used in combination with SQ2 for cosensitized DSSCs. The cosensitized DSSC in which the ratio of BP-2 and SQ2 is 8:2 (v/v) has a record high efficiency of 8.14% among cosensitized systems using all metal-free sensitizers. Dye distribution along the TiO2 film depth was analyzed by an Auger electron spectroscopy technique.

Solid-state dye-sensitized solar cells based on spirofluorene (spiro-OMeTAD) and arylamines as hole transporting materials.

Dye-sensitized solar cells are a promising solar technology because of their low cost, reliability, and high efficiency, compared with silicon-based solar cells. Efforts over the last two decades have increased solar cell efficiency to 12% based on liquid electrolytes, and more research on solid-state devices is necessary to determine their practical usage and long-term stability. The development of solid-state devices has achieved an overall efficiency over 7% using hole transporting materials. This study reviews current progress on hole transporting materials, sensitizers, and mesoporous TiO(2) in solid-state dye-sensitized solar cells using small organic molecules as the hole transporting material. This study also discusses the key factors, such as molecular structure design and interfacial problems, affecting device performance.