Ju Hun Kim

Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf

A stand-alone, wireless solar water splitting device without external energy supply has been realized by combining in tandem a CH3NH3PbI3 perovskite single junction solar cell with a cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped). The photoanode recorded one of the highest photoelectrochemical water oxidation activity (4.8 mA/cm(2) at 1.23 VRHE) under simulated 1 sun illumination. The oxygen evolution Co-Ci co-catalyst showed similar performance to best known cobalt phosphate (Co-Pi) (5.0 mA/cm(2) at 1.23 VRHE) on the same dual-doped BiVO4 photoanode, but with significantly better stability. A tandem artificial-leaf-type device produced stoichiometric hydrogen and oxygen with an average solar-to-hydrogen efficiency of 4.3% (wired), 3.0% (wireless) under simulated 1 sun illumination. Hence, our device based on a D4 tandem photoelectrochemical cell represents a meaningful advancement in performance and cost over the device based on a triple-junction solar cell-electrocatalyst combination.

Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system

A gold-coupled ZnTe/ZnO-nanowire array is a new photocathode for selective CO2 reduction to CO. At −0.7 VRHE under simulated 1 sun illumination, its photocurrent (−16.0 mA cm−2) and incident photon-to-current conversion efficiency (97%) represent the highest among reported ZnTe photocathodes for CO2 reduction and dramatic enhancement from those of a bare electrode (−7.9 mA cm−2, 68%). In addition, the Au nanoparticles convert mainly-hydrogen-producing bare ZnTe/ZnO-nanowires into mainly-CO-producing photocathodes in photoelectrochemical CO2 reduction. The remarkable effects of the Au co-catalyst originate from the formation of a Schottky junction with ZnTe to improve charge separation and to provide reaction centers for CO2 reduction suppressing competing water reduction.

Nanostructure-Preserved Hematite Thin Film for Efficient Solar Water Splitting.

High-temperature annealing above 700 °C improves the activity of photoelectrochemical water oxidation by hematite photoanodes by increasing its crystallinity. Yet, it brings severe agglomeration of nanostructured hematite thin films and deteriorates electrical conductivity of the transparent conducting oxide (TCO) substrate. We report here that the nanostructure of the hematite and the conductivity of TCO could be preserved, while the high crystallinity is attained, by hybrid microwave annealing (HMA) utilizing a graphite susceptor for efficient microwave absorption. Thus, the hematite thin-film photoanodes treated by HMA record 2 times higher water oxidation photocurrents compared to a conventional thermal-annealed photoanode. The enhanced performance can be attributed to the synergistic effect of a smaller feature size of nanostructure-preserved hematite and a good electrical conductivity of TCO. The method could be generally applied to the fabrication of efficient photoelectrodes with small feature sizes and high crystallinity, which have been mutually conflicting requirements with conventional thermal annealing processes.

Ultrafast fabrication of highly active BiVO4 photoanodes by hybrid microwave annealing for unbiased solar water splitting

Hybrid microwave annealing (HMA) with a silicon susceptor in a household microwave oven produces BiVO4-based photoanodes of much improved performance in photoelectrochemical water oxidation in only 6 min relative to conventional thermal annealing in a traditional muffle furnace (FA) that needs a much longer time, 300 min. This technique can apply equally effectively to bare as well as modified BiVO4 by Mo-doping, heterojunction formation with WO3, and an oxygen evolution co-catalyst. Relative to FA, HMA forms BiVO4 films of smaller feature sizes, higher porosity, and increased three dimensional roughness, which decrease the diffusion distance of holes to the surface and thereby increase mainly the bulk charge separation efficiency (ηbulk) of the photoanodes. Thus, the HMA-treated BiVO4/WO3 film achieves the state-of-the art ηbulk of ∼90% for water oxidation. Combination of a photoanode of NiOOH/FeOOH/BiVO4/WO3 (HMA, 6 min) with a 2p c-Si solar cell allows a solar to hydrogen conversion efficiency of ∼5.0% in unbiased overall water splitting, which is also comparable to the state-of-the-art for a similar material combination.

Fabrication of periodically polarity-inverted ZnO films

One-dimensional periodically polarity-inverted (PPI) structures of ZnO for nonlinear optical devices are fabricated on c-plane Al2O3 substrates. To do so, corrugated MgO buffer layers are fabricated by etching after patterning, which is followed by the growth of ZnO layers by plasma-assisted molecular beam epitaxy. The polarity-inverted structures are confirmed by scanning piezoresponse microscopy and atomic-force microscopy. PPI structures with submicron periodicity are fabricated to satisfy the quasiphase matching condition for second harmonic generation of light.

Correction: Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system

Correction for ‘Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system’ by Youn Jeong Jang et al., Energy Environ. Sci., 2015, 8, 3597–3604.