Synthesis and Applications of Cobalt Sulfide Nanostructures for Energy Materials

Abstract

저비용의 고효율 에너지 변환 시스템의 개발은 미래의 에너지 문제에 대한 해결책을 제공 할 수 있는 중요한 영역이며, 이러한 에너지 변환 장치의 핵심 부분은 전기화학적 촉매이다. 현재, 가장 일반적으로 사용되고 있는 것은 고가의 귀금속 촉매이다. 이러한 귀금속 촉매를 대체하기 위해 수많은 유형의 촉매성 물질이 연구되고 있다. 이 중에서 코발트 황화물은 저렴한 비용, 높은 매장량, 화학적 안정성 및 전기화학적 촉매로서의 효율성 등으로 주목받고 있다. 또한 추가적인 나노구조화를 통해 보다 강력하면서도 비용대비 효율적인 전기화학적 촉매로 만들 수 있다. 지금까지 많은 연구에서 코발트 황화물의 나노 구조화를 다뤄 왔고 그 대부분은 상향식 (Bottom-up) 합성법을 사용하였다. 이러한 합성법은 코발트 황화물의 전기화학적 촉매로서의 성능을 상당히 증가시켰다. 하지만 일반적으로 상향식 합성법은 물질의 표면 에너지를 제어하는데 많은 수고가 들고, 대규모 합성에 어려움을 겪는 경우가 많다. 이러한 문제를 해결하기 위해서 본 연구에서는, 간단한 세 가지 하향식 (Top-down) 방법을 통해 나노 구조화시켜 보다 높은 전기화학적 촉매 효율을 갖도록 만들었다.

첫 번째는 두 단계에 걸친 간단한 합성법으로 나노구조화시키는 방법이다. 암모니아 (NH3) 분위기 하에서 열처리를 한 후, 황화수소 (H2S) 분위기에서 후속 열처리를 사용하여 벌크 Co3O4에서 코어/쉘 CoO/CoSx를 합성하였다. 이 합성법은 각 반응 가스에서 열처리를 통한 고유 특징이 활용되었다. 암모니아 열처리는 Co3O4를 CoO로 환원시키며, 이 환원 반응 동안 Co3O4와 CoO 사이의 격자 구조 차이는 결정 구조를 파괴하고 산소 결함을 만들어 다공성 구조를 형성한다. 이후 황화수소 열처리를 통해 CoO 표면에 전기화학적 촉매 활성 물질인 CoSx를 형성한다. 귀금속 촉매에 대한 대신 합성한 나노 구조화된 CoO/CoSx을 triiodide 환원 및 산소 발생 반응에 적용하였고, 그 촉매 활성이 백금, 이리듐산화물과 비견할 만한 효과를 나타냈다.

두 번째 실험에서, 앞선 연구보다 반응 단계를 한 단계 더 줄여 벌크 Co3O4을 황화수소 열처리만으로 코어/쉘 Co3O4/CoO@CoSx 합성물을 합성하는 하향식 접근법을 연구했다. 황화수소 분위기 하에서 열처리하면, 황화수소는 초기에 표면에 합성된 CoSx에 의해 수소와 황으로 열분해 되며, 각각의 성분이 Co3O4를 CoO와 CoSx로 변형시키게 된다. 황화수소와의 반응 시간에 따라 Co3O4/CoO@CoSx 코어/쉘 화합물이 서로 다른 표면 상태와 조성을 가졌다. 특히 다양한 열처리 시간 조건 중 10분간 열처리한 Co3O4/CoO@CoSx 코어/쉘은 산소 발생 반응(OER)에서 이리듐산화물 (0.347V)보다 낮은 과전위 (0.320V)를 나타내어 우수한 전기화학적 효과를 보였다.

세 번째로, 산화 및 황화 반응을 반복적으로 시행하여 벌크 CoSx에서 거대다공성을 갖는 구 모양의 CoSx 합성법을 제시하였다. 산소 분위기 하에서 벌크 CoSx를 열처리하면, 산소는 황과 반응하여 이산화황 가스 형태로 빠져나가고 반대로 CoSx에 다공성 구조를 형성했다. 이후 합성된 매크로 포러스 Co3O4를 다시금 황화수소 가스 하에서 열처리하면, 매크로 포러스 CoSx가 생성된다. 매크로 포러스 CoSx에 대해 산소 발생 반응을 적용해본 결과, 하향식 접근법에 의해 제조된 다공성 CoSx가 시작 물질인 벌크 CoSx나 기준 촉매인 이리듐산화물에 비해 높은 산소발생반응 성능을 나타내었다.

이 연구에서 제공한 세 가지 하향식 합성법은 기존의 나노 구조체 합성법에 비해 간단하며 다양한 금속으로 응용 가능성이 많기 때문에, 높은 활성도의 전기화학적 촉매 합성에 새로운 길을 열 것으로 기대한다.

|The development of low-cost, highly efficient energy conversion systems is an important area that can offer a solution to future energy challenges. A key part of the energy conversion device is a catalyst, by which electrochemical reactions are promoted significantly by lowering activation energy battier. Currently, the most commonly used are expensive noble metal catalysts. Numerous types of catalytic materials have been tested to replace these noble metal catalysts. Among them, cobalt sulfide is attracting attention due to its advantages such as low cost, high abundance, chemical stability and high electrocatalytic activity. In the case of these electrocatalyst materials, nanostructuring method make more powerful and cost-effective. Therefore, nanostructuring has been an important research topic. To date, many studies have dealt with nanostructuring of catalytic materials and have shown results using bottom-up synthesis. The bottom-up nanostructured approach has benefited electrocatalyst research by significantly increasing the catalytic activity of electrochemical catalysts. In general, however, they require tedious wet chemistry steps and delicate control while managing considerable surface energies For this reason, it is often difficult in large scale synthesis. To solve this problem, I have proposed three simple methods for producing nanostructured electrocatalysts with high catalytic activity from bulk form.

The first method is nanostructuring by simple two-step thermal synthesis. In this study, synthesizing method of core/shell CoO/CoSx from bulk Co3O4 is proposed. My novel approach uses a low-temperature thermal treatment under NH3 and a subsequent treatment in H2S atmosphere, judiciously exploiting the unique features of each reaction atmosphere. Hot NH3 treatment reduces Co3O4 to CoO, during which a significant lattice mismatch between the Co3O4 and CoO fractures crystallites, and the coalescence of oxygen vacancies introduces porosity. Subsequent H2S treatment specifically forms electrocatalytically active CoSx species on the surface of CoO. I explore the use of the resulting nanostructured CoO/CoSx as an alternative to state-of-the-art noble metal catalysts in the triiodide reduction and oxygen evolution reactions, finding that its activity is comparable to those of standard catalysts.

In the second experiment, I present a top-down approach to synthesize core/shell Co3O4/CoO@CoSx composites, one step simpler than previous studies, through H2S heat treatment of bulk Co3O4. During the heat treatment with H2S, H2S is thermally reduced to H2 and S by CoSx surface, and each component transforms Co3O4 into CoO and CoSx. Co3O4/CoO@CoSx core/shell composites were found to have different surface states and compositions depending on the reaction time with H2S. In particular, the Co3O4/CoO@CoSx core/shells after heat treatment for 10 minutes showed lower overpotential (0.320V) at 10mAcm-2 current density than IrO2 (0.347V) in the oxygen evolution reaction (OER), showing excellent electrochemical effects.

Third, I show the preparation of porous spherical CoSx in bulk CoSx through repeated thermal oxidation and sulfidation reactions. When the bulk CoSx is heat treated under oxygen atmosphere, oxygen reacts with sulfur to escape in the form of SO2 gas and conversely forms a porous structure in CoSx. After the heat treatment of the synthesized porous Co3O4 under H2S gas, the porous CoSx form. Application of oxygen evolution reaction (OER) to porous CoSx showed that porous CoSx prepared by top-down approach showed better OER performance compared to bulk CoSx and IrO2 catalysts.

The three top-down synthesis methods presented in this study are simpler and more versatile than conventional nanostructure synthesis methods, and are expected to open up new avenues for the synthesis of highly active electrophilic catalysts.; The development of low-cost, highly efficient energy conversion systems is an important area that can offer a solution to future energy challenges. A key part of the energy conversion device is a catalyst, by which electrochemical reactions are promoted significantly by lowering activation energy battier. Currently, the most commonly used are expensive noble metal catalysts. Numerous types of catalytic materials have been tested to replace these noble metal catalysts. Among them, cobalt sulfide is attracting attention due to its advantages such as low cost, high abundance, chemical stability and high electrocatalytic activity. In the case of these electrocatalyst materials, nanostructuring method make more powerful and cost-effective. Therefore, nanostructuring has been an important research topic. To date, many studies have dealt with nanostructuring of catalytic materials and have shown results using bottom-up synthesis. The bottom-up nanostructured approach has benefited electrocatalyst research by significantly increasing the catalytic activity of electrochemical catalysts. In general, however, they require tedious wet chemistry steps and delicate control while managing considerable surface energies For this reason, it is often difficult in large scale synthesis. To solve this problem, I have proposed three simple methods for producing nanostructured electrocatalysts with high catalytic activity from bulk form.

The first method is nanostructuring by simple two-step thermal synthesis. In this study, synthesizing method of core/shell CoO/CoSx from bulk Co3O4 is proposed. My novel approach uses a low-temperature thermal treatment under NH3 and a subsequent treatment in H2S atmosphere, judiciously exploiting the unique features of each reaction atmosphere. Hot NH3 treatment reduces Co3O4 to CoO, during which a significant lattice mismatch between the Co3O4 and CoO fractures crystallites, and the coalescence of oxygen vacancies introduces porosity. Subsequent H2S treatment specifically forms electrocatalytically active CoSx species on the surface of CoO. I explore the use of the resulting nanostructured CoO/CoSx as an alternative to state-of-the-art noble metal catalysts in the triiodide reduction and oxygen evolution reactions, finding that its activity is comparable to those of standard catalysts.

In the second experiment, I present a top-down approach to synthesize core/shell Co3O4/CoO@CoSx composites, one step simpler than previous studies, through H2S heat treatment of bulk Co3O4. During the heat treatment with H2S, H2S is thermally reduced to H2 and S by CoSx surface, and each component transforms Co3O4 into CoO and CoSx. Co3O4/CoO@CoSx core/shell composites were found to have different surface states and compositions depending on the reaction time with H2S. In particular, the Co3O4/CoO@CoSx core/shells after heat treatment for 10 minutes showed lower overpotential (0.320V) at 10mAcm-2 current density than IrO2 (0.347V) in the oxygen evolution reaction (OER), showing excellent electrochemical effects.

Third, I show the preparation of porous spherical CoSx in bulk CoSx through repeated thermal oxidation and sulfidation reactions. When the bulk CoSx is heat treated under oxygen atmosphere, oxygen reacts with sulfur to escape in the form of SO2 gas and conversely forms a porous structure in CoSx. After the heat treatment of the synthesized porous Co3O4 under H2S gas, the porous CoSx form. Application of oxygen evolution reaction (OER) to porous CoSx showed that porous CoSx prepared by top-down approach showed better OER performance compared to bulk CoSx and IrO2 catalysts.

The three top-down synthesis methods presented in this study are simpler and more versatile than conventional nanostructure synthesis methods, and are expected to open up new avenues for the synthesis of highly active electrophilic catalysts.