Drop-casted Platinum Nanocube Catalysts for Hydrogen Evolution Reaction with Ultrahigh Mass Activity.

Platinum hydrogen evolution reaction (HER) electrocatalysts in the form of nanocubes (NCs) were synthesized at 50 °C by aqueous-based colloidal synthesis and were applied to electrochemical (EC) and photoelectrochemical (PEC) systems by a fast and simple drop-casting method. A remarkable Pt mass activity of 1.77 A mg-1 at -100 mV was achieved in EC systems (fluorine-doped tin oxide/Pt NC cathode) with neutral electrolyte while maintaining low overpotential and Tafel slope. In the Cu(In,Ga)(S,Se)2 (CIGS)-based PEC system, a carefully chosen amount of Pt NC loading to achieve a compromise between the catalytic activity (more Pt NCs) and better light transmittance (fewer Pt NCs) led to a maximum onset potential of 0.678 V against the reference hydrogen electrode. The photoelectrodes with Pt NCs also exhibited good long-term operational stability over 9.5 h with negligible degradation of the photocurrent. This study presents an effective strategy to greatly reduce the use of expensive Pt without compromising the catalytic performance because the drop-casting of Pt NC solutions to form electrocatalysts is expected to waste less raw material than vacuum deposition.

CO2 -Reductive, Copper Oxide-Based Photobiocathode for Z-Scheme Semi-Artificial Leaf Structure.

Green plants convert sunlight into high-energy chemicals by coupling solar-driven water oxidation in the Z-scheme and CO2 fixation in the Calvin cycle. In this study, formate dehydrogenase from Clostridium ljungdahlii (ClFDH) is interfaced with a TiO2 -coated CuFeO2 and CuO mixed (ClFDH-TiO2 |CFO) electrode. In this biohybrid photocathode, the TiO2 layer enhances the photoelectrochemical (PEC) stability of the labile CFO photocathode and facilitates the transfer of photoexcited electrons from the CFO to ClFDH. Furthermore, inspired by the natural photosynthetic scheme, the photobiocathode is combined with a water-oxidizing, FeOOH-coated BiVO4 (FeOOH|BiVO4 ) photoanode to assemble a wireless Z-scheme biocatalytic PEC device as a semi-artificial leaf. The leaf-like structure effects a bias-free biocatalytic CO2 -to-formate conversion under visible light. Its rate of formate production is 2.45 times faster than that without ClFDH. This work is the first example of a wireless solar-driven semi-biological PEC system for CO2 reduction that uses water as an electron feedstock.

Tailoring Photoelectrochemical Performance and Stability of Cu(In,Ga)Se2 Photocathode via TiO2-Coupled Buffer Layers.

We report on the photoelectrochemical (PEC) performance and stability of Cu(In,Ga)Se2 (CIGS)-based photocathodes for photocatalytic hydrogen evolution from water. Various functional overlayers, such as CdS, TiO2, ZnxSnyOz, and a combination of the aforementioned, were applied on the CIGS to improve the performance and stability. We identified that the insertion of TiO2 overlayer on p-CIGS/n-buffer layers significantly improves the PEC performance. A multilayered photocathode consisting of CIGS/CdS/TiO2/Pt exhibited the best current-potential characteristics among the tested photocathodes, which demonstrates a power-saved efficiency of 2.63%. However, repeated linear sweep voltammetry resulted in degradation of performance. In this regard, we focused on the PEC durability issues through in-depth chemical characterization that revealed the degradation was attributed to atomic redistribution of elements constituting the photocathode, namely, in-diffusion of Pt catalysts, out-diffusion of elements from the CIGS, and removal of the metal-oxide layers; the best-performing CIGS/CdS/TiO2/Pt photocathode retained its initial performance until the TiO2 overlayer was removed. It was also found that the durability of CIGS photocathodes with a TiO2-coated metal-oxide buffer layer such as ZnxSnyOz was better than those with a TiO2-coated CdS, and the degradation mechanism was different, suggesting that the stability of a CIGS-based photocathode can be improved by careful design of the structure.

Design of red, green, blue transparent electrodes for flexible optical devices.

Controlling the wavelength of electrodes within a desirable region is important in most optoelectronic devices for enhancing their efficiencies. Here, we investigated a full-color flexible transparent electrode using a wavelength matching layer (WML). The WMLs were able to adjust the optical-phase thickness of the entire electrode by controlling refractive indices and were capable of producing desirable colors in the visible band from 470 to 610 nm. Electrodes with tungsten oxide (WO(3)) having a refractive index of 1.9 showed high transmittance (T = 90.5%) at 460 nm and low sheet resistance (R(s) = 11.08 Ω/sq), comparable with those of indium tin oxide (ITO, T = 86.4%, R(s) = 12 Ω/sq). The optimum structure of electrodes determined by optical simulation based on the characteristic matrix method agrees well with that based on the experimental method. Replacing the ITO electrode with the WO(3) electrode, the luminance of blue organic light-emitting diodes (λ = 460 nm) at 222 mA/cm(2) increased from 7020 to 7200 cd/m(2).

Nano-branched transparent conducting oxides: beyond the brittleness limit for flexible electrode applications.

Although the performance of transparent conducting oxides based on bixbyite In(2)O(3) (Sn doped In(2)O(3): ITO) and wurtzite ZnO (Al, In, and Ga doped ZnO) is sufficient in conventional optoelectronic devices, their flexibility remains insufficient for demands in mobile and foldable electronics generation. A lot of alternative materials such as metallic nanowires and carbon based nano-structures have been tried for transparent flexible electrodes, but poor thermal stability of metal nanowires and limits in conductivity of carbon based nano-structures are still waiting for permanent solutions. Here, we show that the cross-linked ITO nano-branches have superior mechanical flexibility compared to ITO bulk film without any cracks even with a bending radius of 0.1 cm. Moreover, for equivalent sheet resistivity, the ITO nano-branches exhibit optical transmittance comparable to that of commercial metallic nanowires (such as Ag and Cu in the visible spectrum) but show far superior thermal stability in conductivity without any degradation even at a temperature of 200 °C and a humidity of 90%.