Woo-Sung Choi

Progress in flexible energy storage and conversion systems, with a focus on cable-type lithium-ion batteries

The unending demand for portable, flexible, and even wearable electronic devices that have an aesthetic appeal and unique functionality stimulates the development of advanced power sources that have excellent electrochemical performance and, more importantly, shape versatility. The challenges in the fabrication of next-generation flexible power sources mainly arise from their limited form factors, which prevent their facile integration into differently shaped electronic devices, and from the lack of reliable electrochemical materials that exhibit optimized attributes and suitable processability. This review describes the technological innovations and challenges associated with flexible energy storage and conversion systems such as lithium-ion batteries and supercapacitors, along with an overview of the progress in flexible proton exchange membrane fuel cells (PEMFCs) and solar cells. In particular, recently highlighted cable-type flexible batteries with extreme omni-directional flexibility are comprehensively discussed.

Nanostructured metallic foam electrodeposits on a nonconductive substrate

A way to prepare the nanostructured metallic foam (NMF) electrodeposits on nonconductive materials was suggested for the first time. This innovative process involves the electrochemical preparation of NMFs on a metal-coated nonconductive substrate and subsequent removal of the deliberately heat-treated dense layer around the NMF trunks, leaving self-supported NMFs on the nonconductive substrate. The creation of nanostructured (or nanoramified) nickel foams on alumina was demonstrated as an exemplary application of the fabrication process.

Degradation of Co 3 O 4 anode in rechargeable lithium-ion battery: a semi-empirical approach to the effect of conducting material content

A large amount of conducting materials has typically been blended with transition metal oxides (MxOy, M = Fe, Co, Ni, Cu), and their electrochemical properties as the anode in lithium-ion batteries have been studied. Here, we report that a higher content of the conducting material results in poorer cycling stability of Co3O4. From the analysis of the cumulative irreversible capacity loss, a high content of conducting material is proven to promote irreversible electron consumption for the growth of a polymeric surface layer which is the origin of degradation. Furthermore, its formation is mathematically modeled on the basis of the Butler–Volmer relation. From the physical parameters of the polymeric surface layer determined by fitting the model to the experimental data, the degradation mechanism of Co3O4 is discussed.

Preparation of Electrolytic Tungsten Oxide Thin Films as the Anode in Rechargeable Lithium Battery

Tungsten oxide films were prepared by an electrochemical deposition method for use as the anode in rechargeable lithium batteries. Continuous potentiostatic deposition of the film led to numerous cracks of the deposits while pulsed deposition significantly suppressed crack generation and film delamination. In particular, a crack-free dense tungsten oxide film with a thickness of ca. 210 nm was successfully created by pulsed deposition. The thickness of tungsten oxide was linearly proportional to deposition time. Compositional and structural analyses revealed that the as-prepared deposit was amorphous tungsten oxide and the heat treatment transformed it into crystalline triclinic tungsten oxide. Both the as-prepared and heat-treated samples reacted reversibly with lithium as the anode for rechargeable lithium batteries. Typical peaks for the conversion processes of tungsten oxides were observed in cyclic voltammograms, and the reversibility of the heat-treated sample exceeded that of the as-prepared one. Consistently, the cycling stability of the heat-treated sample proved to be much better than that of the as-prepared one in a galvanostatic charge/discharge experiment. These results demonstrate the feasibility of using electrolytic tungsten oxide films as the anode in rechargeable lithium batteries. However, further works are still needed to make a dense film with higher thickness and improved cycling stability for its practical use.

Pore Gradient Nickel-Copper Nanostructured Foam Electrode

기공 경사화된 마이크론 단위의 구조 틀 및 나노 수지상 구조 벽을 가지는 니켈-구리 거품 전극을전기화학적인 방법으로 합성하였다. 전해 도금 시 순수한 니켈은 치밀한 층으로 성장하는 양상을보였으나, 구리와 함께 도금시키는 경우 그 성장 양상이 순수한 니켈과는 매우 다르게 관찰되었다.특히, 첨가제로써 염소 이온의 농도가 증가함에 따라 니켈-구리 도금 층의 수지상 성장이 뚜렷해지는 모습을 보였다. 또한, 기재와 먼 부분일수록 도금 층 내 구리 대비 니켈의 상대적인 양이감소하였으며, 염소 이온 농도가 높아짐에 따라 전 도금 층에 걸쳐 니켈의 양이 증가하였다. 수지상 구조 벽의 가지 내부 조성을 분석한 결과, 중심부로 갈수록 구리 함량이 점차 높아지는 조성 구배를 확인하였으며, 적절한 열처리를 통해 상호 확산을 유도하여 균일한 조성의 니켈-구리 합금을얻어낼 수 있었다. 본 연구를 통해 제작된 재료는 기능성 전기 화학 장치용 고성능 전극에 활용될수 있을 것으로 기대된다.Abstract: Nickel-copper foam electrodes with pore gradient micro framework and nano-ramifiedwall have been prepared by using an electrochemical deposition process. Growth habit ofnickel-copper co-deposits was quite different from that of pure nickel deposit. In particular, theramified structure of the individual particles was getting clear with chloride ion content in theelectrolyte. The ratio of nickel to copper in the deposits decreased with the distance away fromthe substrate and the more chloride ions in the electrolyte led to the more nickel contentthroughout the deposits. Compositional analysis for the cross section of a ramified branch,together with tactical selective copper etching, proved that the copper content increased withapproaching central region of the cross section. Such a composition gradient actually disap-peared after heat treatment. It is anticipated that the pore gradient nickel-copper nanostructuredfoams presented in this work might be a promising option for the high-performance electrodein functional electrochemical devices.Keywords : Co-deposition, Metallic foam, Porous structure, Ni-Cu alloy, Hydrogen evolution