Bio-mimic energy storage system with solar light conversion to hydrogen by combination of photovoltaic devices and electrochemical cells inspired by the antenna-associated photosystem II.

Global warming caused by anthropogenic activity is one of the serious problems today. In order to suppress the global warming, the shift from fossil fuel-based energy source to the nature-oriented sustainable energy is encouraged. In this concept paper, possible biomimetic engineering approach inspired by the efficient and sustainable natural energy utilization in living plants is demonstrated. The focal features in plants include (1) the light-harvesting and energy condensing apparatus, (2) water splitting O2 evolving apparatus, (3) storage of energy-related chemicals, and (4) reversal conversion of storage into the "energy in use" by meeting the demands. Demonstration of solar-driven chemical energy conversion was performed using a system consisted of (i) photovoltaic power-generating device, (ii) an electrochemical unit converting electric power into chemical energy, (iii) storage of H2, and (iv) polymer electrolyte cells converting H2 back to electricity by meeting the demands on site. The present concept paper presenting a technical perspective based on the plant-inspired knowledge (conceptual similarity between natural photosynthesis and solar-to-H2 conversion) is a fruit of interdisciplinary collaboration between the team of chemical energy conversion renown for the world highest record of solar-to-hydrogen conversion efficiency (24.4%, as of 2015) and a group of plant biologists.

First principles calculations of surface dependent electronic structures: a study on ß-FeOOH and γ-FeOOH.

We report a theoretical study on iron oxyhydroxide (FeOOH). The FeOOH surface is expected to act as an efficient electrochemical catalyst for the oxygen evolution reaction (OER), because it is based on iron, an element of the fourth highest Clarke number. Experimentally, the OER activity of ß-FeOOH is known to be higher than that of γ-FeOOH. However, the details of the OER mechanism and the surface reactivities of the FeOOH polymorphs have not yet been fully understood. We performed first-principles calculations of bulk and surfaces of ß-FeOOH and γ-FeOOH using density functional theory, to investigate their electronic structures and catalytic activities. The calculations suggest that depending on the surface indices, several surfaces may be favored for catalytic activities.

First-principles modeling of GaN(0001)/water interface: Effect of surface charging.

The accumulation properties of photogenerated carriers at the semiconductor surface determine the performance of photoelectrodes. However, to the best of our knowledge, there are no computational studies that methodically examine the effect of "surface charging" on photocatalytic activities. In this work, the effect of excess carriers at the semiconductor surface on the geometric and electronic structures of the semiconductor/electrolyte interface is studied systematically with the aid of first-principles calculations. We found that the number of water molecules that can be dissociated follows the "extended" electron counting rule; the dissociation limit is smaller than that predicted by the standard electron counting rule (0.375 ML) by the number of excess holes at the interface. When the geometric structure of the GaN/water interface obeys the extended electron counting rule, the Ga-originated surface states are removed from the bandgap due to the excess holes and adsorbates, and correspondingly, the Fermi level becomes free from pinning. Clearly, the excess charge has a great impact on the interface structure and most likely on the chemical reactions. This study serves as a basis for further studies on the semiconductor/electrolyte interface under working conditions.

Structural changes in the S3 state of the oxygen evolving complex in photosystem II.

The S3 state of the Mn4 CaO5 -cluster in photosystem II was investigated by DFT calculations and compared with EXAFS data. Considering previously proposed mechanism; a water molecule is inserted into an open coordination site of Mn upon S2 to S3 transition that becomes a substrate water, we examined if the water insertion is essential for the S3 formation, or if one cannot eliminate other possible routes that do not require a water insertion at the S3 stage. The novel S3 state structure consisting of only short 2.7-2.8 Å Mn-Mn distances was discussed.

Analysis of the Gas Phase Kinetics Active during GaN Deposition from NH3 and Ga(CH3)3.

The results of a systematic investigation aimed at determining the dominant gas phase chemistry active during GaN MOVPE are reported and discussed in this work. This study was performed developing a thermodynamic database including the most stable GaN gas phase species and a gas phase mechanism that could efficiently describe their interconversion kinetics. The thermodynamic data and the kinetic mechanism were calculated combining density functional theory and ab initio simulations. Structures and vibrational frequencies of reactants and transition states were determined at the M062X/6-311+G(d,p) level, while energies were computed at the ROCBS-QB3 level. Rate constants were calculated using transition state theory using the rigid rotor - harmonic oscillator approximation and considering the possible degeneration of internal motions in torsional rotations. The thermodynamic analysis indicated that the Ga gas phase species formed in the highest concentration at the standard GaN deposition temperature (1300 K) is GaNH2, followed by GaH and Ga. The diatomic GaN gas phase species, often considered to be the main precursor to the film growth, is predicted to be unstable with respect to GaNH2. Among the gas phase species containing two Ga atoms, the most stable are GaNHGaH(NH2)3, GaNHGaH2(NH2)2, and GaNHGa(NH2)4, thus indicating that the substitution of the methyl groups of the precursor with H or amino groups is thermodynamically favored. Several kinetic routes leading to the formation of these species were examined. It was found that the condensation of Ga(R1)x(R2)3-x species, with R1 and R2 being either CH3, NH2, or H, is a fast process, characterized by the formation of a precursor state whose decomposition to products requires overcoming submerged energy barriers. It is suggested that these species play a key role in the formation of the first GaN nuclei, whose successive growth leads to the formation of GaN powders. A kinetic analysis performed using a fluid dynamic model allowed us to identify the main reactive routes of this complex system.

New insights into highly efficient reduction of CO2 to formic acid by using zinc under mild hydrothermal conditions: a joint experimental and theoretical study.

We report here a theoretical study with quantum chemical calculations based on experimental results to understand highly efficient reduction of CO2 to formic acid by using zinc under hydrothermal conditions. Results showed that zinc hydride (Zn-H) is a key intermediate species in the reduction of CO2 to formic acid, which demonstrates that the formation of formic acid is through an SN2-like mechanism.

A nitride based polarization-engineered photocathode for water splitting without a p-type semiconductor.

Photoelectrochemical water splitting is a promising way for hydrogen production with low environmental burden. Although III-nitride semiconductors have potentially favorable properties as water splitting photoelectrodes, they have several limitations for practical use currently. In this study, the concept of a polarization-engineered nitride photocathode for water splitting is proposed to overcome this problem. We observed that the proposed GaN/AlN/GaN structure worked as a photocathode even though it consisted of only n-type III-nitride semiconductors. This polarization-engineered photocathode showed a remarkably stable and relatively high photocurrent since it can avoid the causes of problems from which both n-type and p-type conventional GaN photoelectrodes suffer.

Photoelectrochemical reaction and H2 generation at zero bias optimized by carrier concentration of n-type GaN.

The authors studied the photoelectrochemical properties dependent on carrier concentration of n-type GaN. The photocurrent at zero bias became the maximum value at the carrier concentration of 1.7x10(17) cm-3. Using the sample optimized carrier concentration, the authors achieved H2 gas generation at a Pt counterelectrode without extra bias for the first time. The authors also discussed the mechanism of the dependence of photocurrent on the carrier concentration of GaN.