We report annealing-free compact TiOx layer by atomic layer deposition for high efficiency flexible perovskite solar cells, and maintained 95% of the initial PCE after 1000 bending cycles with 10 mm bending radius.
The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.
For the efficient electroconversion of CO2 to formate, CO and H2 evolution must be suppressed. Herein, carbon-supported BiO x nanoparticles (BiO x /C) were investigated as a potential candidate for CO2 reduction. In bicarbonate solutions, the BiO x /C catalysts exhibited a high Faradaic efficiency of 93.4% for formate from −1.37 to −1.70 V versus Ag/AgCl with a negligible amount of CO and H2. Stable partial current densities and high Faradaic efficiencies were also achieved in 0.5 M NaCl (12.5 mA cm–2 and 96.0%, respectively). The possible reaction pathways and kinetic parameters of formate formation were examined using systematic electrochemical methods, including Tafel, pH dependence, and in situ X-ray absorption near-edge structure analyses. From the results of these mechanistic studies, we propose that dual mechanisms are functional on the BiO x /C catalysts. Specifically, a two-electron and one-proton transfer reaction to adsorbed CO2 or a chemical proton transfer reaction to CO2 – anion are the possible rate-determining steps (RDSs) at low potentials, whereas a one-electron transfer reaction to CO2 is the RDS at high potentials.
supply of fossil fuels. [1][2][3][4][5][6][7][8][9][10] Thus far, recycling CO 2 into renewable carbon-based fuels has been solely dependent on biomass, which has inherent problems of low productivity and limited product. [ 3 ] To produce various chemicals with high yield, the direct conversion of CO 2 by simply applying electric power is more feasible. However, achieving high selectivity with low overpotential remains challenging in CO 2 reduction due to the closely positioned thermodynamic energies required for each reaction and the kinetic barrier to activation of the CO 2 molecule. [ 11,12 ] Although rare-earth metals such as Au, Re, and Rh, as well as their complexes, have been intensively studied to achieve those requirements, [13][14][15][16][17] their application on the industrial scale is diffi cult. [ 18,19 ] With respect to cost and environmental ramifi cations, developing an effi cient and inexpensive CO 2 -reducing catalyst has a great impact on CO 2 utilization. Here, for the fi rst time, we demonstrate a p-type Si-based solar-driven CO 2 conversion on a metal-free catalyst in a heterogeneous system. Nitrogen-doped graphene quantum sheets (N-GQSs) enhance the performance of the silicon photocathode by signifi cantly decreasing the overpotential and by providing high selectivity for the conversion of CO 2 to CO.CO 2 can be reduced to form carbon monoxide (CO), formate (HCOOH), methane (CH 4 ), or other hydrocarbons at moderate potentials of around −0.2 to −0.6 V versus the normal hydrogen electrode (NHE). Those reactions are listed as follows with their thermodynamic redox potentials (V vs. NHE) CO 2H 2e CO + H O 0.53 versus NHE 2 2The reduction of carbon dioxide (CO 2 ) into chemical feedstock is drawing increasing attention as a prominent method of recycling atmospheric CO 2 . Although many studies have been devoted in designing an effi cient catalyst for CO 2 conversion with noble metals, low selectivity and high energy input still remain major hurdles. One possible solution is to use the combination of an earth-abundant electrocatalyst with a photoelectrode powered by solar energy. Herein, for the fi rst time, a p-type silicon nanowire with nitrogen-doped graphene quantum sheets (N-GQSs) as heterogeneous electrocatalyst for selective CO production is demonstrated. The photoreduction of CO 2 into CO is achieved at a potential of −1.53 V versus Ag/Ag + , providing 0.15 mA cm −2 of current density, which is 130 mV higher than that of a p-type Si nanowire decorated with well-known Cu catalyst. The faradaic effi ciency for CO is 95%, demonstrating signifi cantly improved selectivity compared with that of bare planar Si. The density functional theory (DFT) calculations are performed, which suggest that pyridinic N acts as the active site and band alignment can be achieved for N-GQSs larger than 3 nm. The demonstrated high effi ciency of the catalytic system provides new insights for the development of nonprecious, environmentally benign CO 2 utilization.
The effects of exchange current density, Tafel slope, system resistance, electrode area, light intensity, and solar cell efficiency were systematically decoupled at the converter-assisted photovoltaic–water electrolysis system. This allows key determinants of overall efficiency to be identified. On the basis of this model, 26.5% single-junction GaAs solar cell was combined with a membrane-electrode-assembled electrolysis cell (EC) using the dc/dc converting technology. As a result, we have achieved a solar-to-hydrogen conversion efficiency of 20.6% on a prototype scale and demonstrated light intensity tracking optimization to maintain high efficiency. We believe that this study will provide design principles for combining solar cells, ECs, and new catalysts and can be generalized to other solar conversion chemical devices while minimizing their power loss during the conversion of electrical energy into fuel.
As the demand for energy has dramatically increased in the past decade, electrochemical water splitting has been regarded as an attractive approach to produce renewable hydrogen energy. However, large overpotentials of oxygen‐evolving reaction (OER) is a key bottleneck for practical application. Thus, water‐oxidizing electrocatalysts with low cost and high efficiency should be developed. Here, 5 nm‐sized Mn3O4 nanoparticles (NPs) are synthesized by a hydrothermal method, which is appropriate for large‐scale production. To further improve their performance, various 3d transition metal elements are successfully doped in Mn3O4 NPs. Ni‐doped Mn3O4 NPs exhibit the highest efficiency among the Mn3O4 NPs doped with various elements. Based on structural analysis, the Ni‐doping process leads to the lattice distortion of their tetragonal spinel structure and it strongly correlates with the enhancement of OER activity. The overpotential at the current density of 10 mA cm–2 is 524 and 458 mV for pristine and 5 at% doped Mn3O4 NPs under neutral condition. The heteroatom‐doping process in sub‐10 nm‐sized nanocatalysts is expected to be a promising methodology to induce distorted structure related to active species. Thus, it can be effective to improve catalytic performance of various heterogeneous nano‐catalysts.
The electrochemical reduction of carbon dioxide (CO2) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO2 to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO2 to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO2 in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.
The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand‐engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO4) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm−2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm−2) of BiVO4. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems.
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