Characteristics of energy storage and conversion of surface-modified nanostructure: ab-initio study

Abstract

Recently, the severities of climate change caused by the global warming has emerged all over the world. It is well known that the CO2 emissions from fossil fuels mainly cause the global warming. Therefore, in order to prevent further global warming, the dependence on fossil fuels must be reduced. As an alternative to fossil fuels, renewable energy sources such as solar, wind and geothermal energy have been developed for protection of the environment. However, because energy production from renewable energy sources has the characteristics dependent on the external environment, many scientists pay attention to store and utilize the surplus energy from renewable energy.

In this thesis, strategies to improve the storage efficiency of surplus energy in renewable energy using the nanostructures are studies by the means of the density functional theory (DFT). This thesis explores two strategies to investigate the efficient storage of surplus energy for renewable energy: (i) electrochemical energy storage using Lithium – air batteries, and (ii) conversion to hydrogen energy using water electrolysis.

Due to its extremely high specific energy density, the post Lithium batteries, Lithium-air batteries (LABs) has attracted worldwide as an alternative of conventional Li-ion batteries with low energy storage efficiency. However, there are many obstacle to prevent the commercialization of LABs, such as poor efficiency induced by high overpotential and decomposing of conventional carbon-based cathode. The first part of this thesis investigates the discharge/charge mechanism of two-dimensional materials based transition metal disulfides as a cathode candidate for LABs. To optimize overpotential properties of vanadium disulfide (VS2), one of the transition metal disulfides, high efficiently catalyst design was conducted through substitutional anion doping on VS2. Among target anions, F and N doping shows outstanding overpotential properties. Moreover, thermodynamic modeling displays that the process condition of N doping on VS2 will be feasible compared to F doping. Therefore, it is revealed that the process of N-doping on VS2 can occur easily in experimental condition and shows better performance than conventional carbon-based cathodes.

In addition, it is found that the phase transition of T-phase WS2 is induced by intermediates of LABs and T-WS2 shows comparable performance with carbon-based cathode. The charge transfer of intermediates formation induces the clustering of W atoms and eventually results in new T-WS2 phases, DT-WS2 and ZT-WS2. It is revealed that the DT-WS2 and ZT-WS2 shows low discharge overpotential. Moreover, the electron transfer of intermediates formation has the effect of increasing the electrical conductivity of DT-WS2 and ZT-WS2. A series of research about the catalyst performance for LABs can provide the practical guideline for electrochemical application of transition metal disulfide and the insight into the two-dimensional based materials design.

The second part of this thesis investigated high efficiently hydrogen production catalyst using nanostructures. Hydrogen energy, one of renewable energies, has received attention as an alternative to fossil fuels. However, as recent hydrogen production which is generally the reforming natural gas, CO2 emission is inevitable. Therefore, it is essential to improve the efficiency of hydrogen production by using water electrolysis for literally renewable energy. Water electrolysis is con of two half reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) which acts as a bottleneck in hydrogen production due to the complexity and high overpotential of OER. Therefore, this study investigates the mechanism of OER for quaternary metal boride hollow nanoprism (V-doped Co-Ni boride, VNCB) through collaboration with the experimental group. DFT calculations reveals that the impressive OER properties of VCNB originate from the synergetic physicochemical effects of the different elements, Co and B as active sites, Ni as a surface electronic structure modifier, and V as a charge carrier transporter and supplier. In other words, each atoms of VCNB have a synergy effect and shows superior performance than the conventional noble metal catalyst, finally, it is revealed that VCNB is high efficiently OER catalyst for hydrogen production.

|최근, 전 세계에 지구 온난화로 인한 기후변화의 심각성에 대한 많은 논의가 되고 있다. 이는 화석연료 사용에 의한 이산화탄소 배출이 가장 큰 원인이 되는데, 더 이상의 기후 변화를 막기 위해서 화석연료 의존도를 감소하고자 하는 움직임이 일고 있다. 화석연료의 대안으로써 태양광, 풍력, 지열 등의 친환경에너지를 이용해 실생활에서 필요한 에너지를 생산하는 발전방식이 환경보호를 위해 대두 되고 있다. 하지만 친환경에너지는 에너지생산량이 외부환경에 의존하는 특성이 있기 때문에 친환경에너지의 잉여에너지를 저장하여 활용하는 데에 많은 연구가 진행 중이다.

본 학위 논문에서는 밀도범함수 이론을 통해 나노구조체를 이용한 친환경에너지의 잉여에너지 저장 효율을 높일 수 있는 방안에 대한 연구를 진행하였다. 잉여에너지 저장을 위한 연구는 다음과 같은 두 개의 핵심 전략을 통해 이루어 졌다.

1) 리튬공기전지를 이용한 전기화학적 에너지로 저장

2) 물 전기분해를 통한 수소생산반응을 이용한 수소에너지로 변환

에너지 저장효율이 떨어지는 기존의 리튬이온배터리와 다르게 리튬공기전지가 극히 높은 에너지 밀도 때문에 차세대 리튬배터리로써 많은 주목을 받고 있다.. 그러나 실용화 단계에 접어 들지 못하는 여러 문제들이 있는데 그 중 너무 높은 과전압에 의해 발생하는 에너지손실과, 탄소기반인 양극재가 분해되면서 생기는 배터리 수명 감소 등이 있다.

이 논문의 첫 번째 부분에서는 리튬공기전지의 양극 물질로써 이차원 물질 기반의 이황화전이금속의 리튬공기 전지 양극반응에 대한 충 방전 메커니즘에 대해 연구를 하였다. 이황화전이금속 중 하나인 VS2의 과전압 성능의 최적화를 위해 음이온의 치환성 도핑을 통해 고효율 촉매 설계를 하였다. 여러 가지 음이온 중 불소와 질소 도핑이 뛰어난 과전압 성능을 보여주는 것을 밝혀 내었다. 또한 열역학적 모델링을 통해 불소 도핑에 비해 질소도핑의 공정조건이 실현 가능하다는 것을 밝혀 내었다. 따라서 VS2의 질소도핑이 현실적인 실험조건에서 구현이 가능하고, 기존의 탄소 기반의 전극보다 더 좋은 과전압 성능을 보여준다는 것을 밝혀냈다.

또한, T phase의 WS2의 경우 리튬공기전지의 중간생성물에 의해 상전이가 발생하며, 기존의 탄소 기반의 전극과 비견될만한 좋은 과전압 특성을 보인다는 것을 밝혀내었다. 중간생성물의 전자이동 때문에 텅스텐의 금속 원자간의 군집화가 유도되고 결국 새로운 WS2 상인 DT-WS2와 ZT-WS2로의 전이가 발생하게 된다. DT-WS2와 ZT-WS2는 낮은 방전 과전압 특성을 보이게 됨을 밝혀 냈다. 또한 중간생성물의 전자이동은 DT-WS2와 ZT-WS2의 전기전도도를 상승시켜주는 효과가 있다. 이런 일련의 리튬공기전지의 촉매 특성에 대한 연구는 이황화전이금속 물질의 응용에 가이드를 제시할 뿐 아니라 이차원 기반의 새로운 물질을 설계할 수 있는 통찰력을 제공할 것으로 기대된다.

이 논문의 두 번째 부분에서는 나노구조체를 이용한 고효율 수소발생 촉매에 대한 연구를 하였다. 수소에너지는 친환경 에너지로써 기존의 화석연료를 대체할 에너지로 많은 주목을 받고 있다. 하지만 현재의 수소생산은 천연가스를 이용한 개질이 대부분이라 CO2 발생이 필연적이기 때문에, 물 전기분해를 이용한 수소발생의 효율을 높이기 위한 연구에 초점을 맞추고 있다. 물 전기분해는 수소생성반응과 산소생성반응으로 나뉘게 되는데, 산소생성반응은 반응의 복잡성과 높은 과전압으로 수소생산에 있어 병목으로 작용한다. 따라서 본 연구에서는 산소발생반응 촉매로써 나노구조체인 바나듐이 도핑 된 코발트-니켈 붕화물의 사성분계의 성능을 실험과의 협업을 통해 산소발생반응의 메커니즘에 대해 연구하였다. 밀도범함수 계산을 통해, 코발트와 붕소는 촉매반응의 활성부위의 역할을 하며, 니켈은 표면의 전자구조 개선을 통해 흡착 정도를 조절하고, 바나듐은 전기전도도 향상에 기여하게 되는 사성분계의 각 원소들의 역할을 규명하였다. 다시 말해서, 각 원자들이 시너지효과를 내어 기존의 귀금속 촉매보다 더 뛰어난 성능을 보여 바나듐이 도핑된 코발트-니켈 붕화물은 고효율 수소생산을 위한 산소발생반응 촉매임을 이 연구를 통해 밝혀냈다.; Recently, the severities of climate change caused by the global warming has emerged all over the world. It is well known that the CO2 emissions from fossil fuels mainly cause the global warming. Therefore, in order to prevent further global warming, the dependence on fossil fuels must be reduced. As an alternative to fossil fuels, renewable energy sources such as solar, wind and geothermal energy have been developed for protection of the environment. However, because energy production from renewable energy sources has the characteristics dependent on the external environment, many scientists pay attention to store and utilize the surplus energy from renewable energy.

In this thesis, strategies to improve the storage efficiency of surplus energy in renewable energy using the nanostructures are studies by the means of the density functional theory (DFT). This thesis explores two strategies to investigate the efficient storage of surplus energy for renewable energy: (i) electrochemical energy storage using Lithium – air batteries, and (ii) conversion to hydrogen energy using water electrolysis.

Due to its extremely high specific energy density, the post Lithium batteries, Lithium-air batteries (LABs) has attracted worldwide as an alternative of conventional Li-ion batteries with low energy storage efficiency. However, there are many obstacle to prevent the commercialization of LABs, such as poor efficiency induced by high overpotential and decomposing of conventional carbon-based cathode. The first part of this thesis investigates the discharge/charge mechanism of two-dimensional materials based transition metal disulfides as a cathode candidate for LABs. To optimize overpotential properties of vanadium disulfide (VS2), one of the transition metal disulfides, high efficiently catalyst design was conducted through substitutional anion doping on VS2. Among target anions, F and N doping shows outstanding overpotential properties. Moreover, thermodynamic modeling displays that the process condition of N doping on VS2 will be feasible compared to F doping. Therefore, it is revealed that the process of N-doping on VS2 can occur easily in experimental condition and shows better performance than conventional carbon-based cathodes.

In addition, it is found that the phase transition of T-phase WS2 is induced by intermediates of LABs and T-WS2 shows comparable performance with carbon-based cathode. The charge transfer of intermediates formation induces the clustering of W atoms and eventually results in new T-WS2 phases, DT-WS2 and ZT-WS2. It is revealed that the DT-WS2 and ZT-WS2 shows low discharge overpotential. Moreover, the electron transfer of intermediates formation has the effect of increasing the electrical conductivity of DT-WS2 and ZT-WS2. A series of research about the catalyst performance for LABs can provide the practical guideline for electrochemical application of transition metal disulfide and the insight into the two-dimensional based materials design.

The second part of this thesis investigated high efficiently hydrogen production catalyst using nanostructures. Hydrogen energy, one of renewable energies, has received attention as an alternative to fossil fuels. However, as recent hydrogen production which is generally the reforming natural gas, CO2 emission is inevitable. Therefore, it is essential to improve the efficiency of hydrogen production by using water electrolysis for literally renewable energy. Water electrolysis is con of two half reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) which acts as a bottleneck in hydrogen production due to the complexity and high overpotential of OER. Therefore, this study investigates the mechanism of OER for quaternary metal boride hollow nanoprism (V-doped Co-Ni boride, VNCB) through collaboration with the experimental group. DFT calculations reveals that the impressive OER properties of VCNB originate from the synergetic physicochemical effects of the different elements, Co and B as active sites, Ni as a surface electronic structure modifier, and V as a charge carrier transporter and supplier. In other words, each atoms of VCNB have a synergy effect and shows superior performance than the conventional noble metal catalyst, finally, it is revealed that VCNB is high efficiently OER catalyst for hydrogen production.