Synthesis of Graphene Oxide by Atmospheric plasma for biological applications

알람쿠시드 전남대학교 대학원 2020 국내석사

그래핀은 나노카본의 새로운 응용 분야로, 특이한 응용법으로 인해 많은 관심을 끌었으며 나노 기술 분야를 개편했다. 지르코니아, 스테인리스 스틸 및 티타늄과 같은 다른 유형의 기판을 그래 핀 증착에 사용할 수 있다. 본 연구는 티타늄에 본질적인 생체 적합성이 있기 때문에 티타늄을 기판으로써 선택했지만, 세포 상호 작용을 향상시키기 위해서는 추가적인 표면 처리가 필요하기 때문에 환원된 산화 그래핀 및 산화 그래 핀의 증착을 통해 표면 개질을 수행하였다. 본 연구는 Phoenix 300, SEO Korea에 의해 접촉각이 변하여 다양한 표면 특성을 갖는 환원된 그래핀 옥사이드 (rGO) 및 그래핀 옥사이드 (GO)의 증착에 관한 것이다. 메탄 (10 %)과 아르곤의 혼합 가스를 탄소 공급원으로 사용하고 60 SCCM (분당 표준 센티미터 입방)의 속도로 플라즈마에 도입하여 6 분 동안 증착 공정을 진행시켰고, 아르곤 가스만을 사용한 추가 플라즈마 처리에 의해서 산화 그래핀 (GO)을 얻었다. 이때 증착 된 환원 그래 핀 옥사이드 층은 소수성을 나타내었지만, 아르곤 가스만으로 플라즈마를 추가로 처리하여 이를 친수성으로 만들었으며, 초 소수성 코팅은 또한 200 ℃에서 1 시간 30 분 동안 열처리하여 수득되었다. XPS를 사용하여 대기 플라즈마 처리를 통한 탄소 코팅 합성을 평가하고 라만 분광법, TEM 및 접촉각 (Phoenix 300, SEO Korea) 및 세포 생존력은 MTT 분석에 의해 측정되었다. 소수성 표면을 초 소수성 및 친수성으로 변화시키는 다양한 산소 함유 작용기를 드러냄으로써 그래핀 옥사이드와 환원 된그래 핀 옥사이드를 구별하기 위해 X 선 광전자 분광법이 수행되었다. 표면 특성이 다른 rGO 및 GO의 생체 적합성, 즉 티타늄 기판에 증착 된 초 소수성, 소수성 및 친수성은 MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) 분석법으로 표면 특성이 다른 카본 코팅과 베어 티타늄 기판의 다양한 비교를 보여주었다. 티타늄이 생체 적합성 물질이라는 것이 보편적으로 인정되고 있지만, 본 연구에서 소수성 및 친수성 특성을 갖는 탄소 코팅은 베어 티타늄보다 섬유 모세포에 대해 훨씬 더 나은 생체 적합성 표면임을 입증했다. 또한, 소수성 표면과 친수성 표면 사이의 비교에서, 친수성을 갖는 탄소 코팅은 세포 생존력에 대해 소수성보다 훨씬 더 우수한 기질 인 것으로 관찰되었다. Graphene is a new class of nanocarbon that has attracted much attention due to its unusual applications and has reshaped the field of nanotechnology. Different types of substrates like zirconia, stainless steel and titanium can be used for graphene deposition. We chose titanium as a substrate because titanium has intrinsic biocompatibility. However, it requires further surface treatment to enhance cells interaction. Surface modifications were performed through deposition of reduced graphene oxide and graphene oxide. This study comprised on deposition of reduced graphene oxide (rGO) and graphene oxide (GO) with diverse surface properties differentiating contact angle by Phoenix 300, SEO Korea. A mixture gas of methane (10%) and argon was used as a source of carbon and introduced to the plasma at a rate of 60 SCCM (standard centimeter cubic per minute), running the deposition processes for six minutes. Graphene oxide (GO) was obtained by the additional plasma treatment using argon gas alone. As deposited reduced graphene oxide layer showed hydrophobic properties, but the additional treatment of plasma with argon gas alone turned it into hydrophilic one. A superhydrophobic coating was also obtained by heat treatment at 200 0C for one hour and 30 minutes. Carbon coating synthesis through atmospheric plasma treatment is evaluated by using XPS; Raman spectroscopy; TEM and contact angle (Phoenix 300, SEO Korea) and cells viability is measured by MTT assay. X-ray photoelectron spectroscopy is performed for differentiating graphene oxide and reduced graphene oxide by revealing various oxygen containing functional groups turning hydrophobic surface into super-hydrophobic and hydrophilic one. Biocompatibility of rGO and GO with different surface properties i.e. super hydrophobic, hydrophobic and hydrophilic deposited on titanium substrate is evaluated by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay shows diverse comparison of carbon coating with different surface properties and bare titanium substrate. Though it is universally accepted that titanium is a biocompatible material but here in this report we have proved that carbon coating with hydrophobic and hydrophilic characters are far better biocompatible surfaces for fibroblast cells than bare titanium. In comparison between hydrophobic and hydrophilic surfaces, it is observed that carbon coating with hydrophilic property is even better substrate than hydrophobic for cells viability.

Study of Reduced Graphene Oxide Property and Application : Reduced Graphene Oxide 속성과 응용에 관한 연구

우성용 DGIST 2016 국내석사

Graphene Oxide(GO)는 일반적인 실험실 혹은 실내 환경에서 작업이 가능한 물질이다. 사용방법도 간단하고 물질 접촉에 따른 독성도 없어서 작업에 아주 용이하다. 자체 특성상 비전도성 특성을 가지지만 열이나 레이저로 가공을 하면 에너지가 가해진 부분에서 환원과정이 발생하여 “Reduced Graphene Oxide(rGO)”가 되면서 전기 전도성을 가진다. 당연히 가공이 안된 면은 비전도성 성질을 그대로 가지면서 두 가지 특성을 동시에 가진 형태로 제작이 가능하게 된다. 입력된 에너지의 양에 따라서 전기적 특성이 달라지는데, 즉 전기적 저항에 차이가 발생한다. 레이저로 같은 위치를 반복하여 가공을 하면 저항 값이 감소하는 특성을 내장 하고 있다. 이러한 특성으로 인하여 반도체 및 메모리에 활용이 되고 있으며, 향후 자체 복합 회로도 제작이 가능할 것으로 보인다. 본 연구에서는 rGO 의 전기적 특성을 재 조명하면서 레이저 에너지의 양과 그에 따른 변화를 보고 활용분야의 확인과 적용을 하려고 한다. rGO 프로세서 과정에서 형태상 부풀어 오르는 성질을 이용하여 분자간의 접촉 면적이 커지는 점을 착안하여 다양한 이온을 혼합하여 특정 센서로 활용을 모색하고 있으며, 특히 의료 분야에서 혈액을 대상으로 하는 센서의 개발을 주로 이루어 지고 있다. 이러한 방법은 기존의 단백질과 각종 효소에 의지하여 제작되고 있는 고가의 반응 물질을 이용한 센서를 물리적인 성질을 이용한 센서로 대처하며, 효소나 단백질 센서의 고질적인 보관과 유통기한의 문제에서 해결의 돌파구를 제시할 것으로 보여진다. 의료분야에서 효소를 사용한 측정분야에서 가장 크게 사용되고 있는 분야는 혈당의 측정 즉 포도당을 측정하는 것으로 당뇨병 환자를 주 타깃으로 하는 분야이다. 본 분야의 센서부분은 위에서 언급한 효소를 사용한 센서로 인하여 생산단가가 높고 유통기한으로 이한 보관의 주의가 필요한 문제가 있다. 때문에 포도당 측정에 있어서 rGO 를 활용한 센서를 제작할 경우 낮은 가격에 대량의 생산의 길이 열릴 것으로 예상이 된다. 이러한 대량 생산을 위해서는 쉽고 빠르게 제작이 가능한 기기가 있어야 하며 본 연구에서는 직접 레이저 가공기를 활용한 방법을 제안한다. 레이저 가공기는 산업용 로봇이나 기타 범용 모터를 활용한 구성으로 제작과 구성이 용이하다. 또한 소형화를 통해 센서를 개인적으로 프린터에서 인쇄를 하듯 센서도 필요할 때 가정이나 장소에 구애가 없이 제작이 가능하며, 재료의 독성이 없으므로 폐기나 보관시 제약이 없어야 한다. 이러한 보관과 폐기의 용이성에 쉬운 제작과 대량 생산 및 자가 생산으로 접근성을 위한 여러 실험과 환경조건을 제시를 해야 한다. 본 연구는 독성이 없는 GO 를 사용하여 rGO 를 생성하는 방법을 쉽고 빠르게 제작이 가능한 레이저 스크라이버의 사용과 함께 제작된 rGO 의 성능 분석과 포도당의 직접적인 측정을 통하여 포도당측정의 가능여부 및 향후 개선안과 방향을 소개할 것이다. Graphene Oxide is a substance capable of working in a typical laboratory or indoor environment. Usage is also simple and also because of the toxic materials in contact is very easy to work. If the non-conductive nature of the process itself have only column characteristics or by laser energy reduction processes occurring in the applied part "Reduced Graphene Oxide" has electrical conductivity. While as the non-processed surface of non-conductive properties is manufactured in a form to enable this with the two characteristics simultaneously. The electrical resistance value varies according to the amount of energy input. That is the difference in electrical resistance occurs. When the process is repeated for the same position as the laser has a built-in property of the resistance value decreases. Because of this characteristic it has been utilized in the semiconductor and memory, which appears to be the next circuit making the composite itself. In this study, the trying to make the application of the application fields to see a change with the amount of laser energy thereto and refocus the electrical characteristics of rGO. rGO is utilized to expect sensor which using swell property in reduction processor and mixing nano particle. Particularly, recent rGO research is bio and blood base clinic (medical field) to target. This method is coping with the sensor using an expensive reactive substances that are produced by the will of the existing proteins and various enzymes to the sensor using the physical properties, the enzyme or the protein sensor a breakthrough resolved in a matter of persistent storage and expiration date. In fabricating the sensor, so that utilized rGO in glucose measurement is expected to be opened, the length of the large quantity of production at a lower price. These should be capable of quick and easy to manufacture device to mass production. In this study, we propose a method using direct laser scribe. Laser equipment is easy to manufacture and consists of a configuration using the industrial robot or other general-purpose motor. In addition, easily and expeditiously making processor can manufacture sensor with a laser scribe. There shall be no restrictions on the disposal or storage of materials. The storage and disposal easier and self-produced and mass production of the sensor should be presented several experimental and environmental conditions for access to the produce. This study presents a laser scriber capable of easily and quickly making how to create the rGO with the non-toxic GO. rGO sensor performance analysis that manufactured through laser fabrication process. These will be proposed the availability, future development direction and improvement of the glucose measurement.

Study on graphene composite anode materials prepared by spray pyrolysis for lithium ion battery : 분무열분해 공정에 의해 합성된 리튬이차전지용 그래핀 복합체 음극 소재 연구

이수민 건국대학교 대학원 2015 국내석사

Graphene-MnO composite and hollow-structured MnO powders are prepared by a simple one-pot spray pyrolysis process. Based on the results of thermogravimetric analysis, the graphene content in the graphene-MnO composite powder is estimated to be 10 wt.%. Furthermore, morphological analysis of the graphene-MnO composite powder indicate that the fine MnO crystals of size several tens of nanometers are uniformly distributed all over the graphene. The BET specific surface areas of the graphene-MnO composite and hollow-structured MnO powders are found to be 20 and 5 m2 g-1, respectively. The graphene-MnO composite powders have high initial discharge and charge capacities of 1207 and 849 mA h g-1, respectively, at a current density of 500 mA g-1. The initial discharge and charge capacities of the hollow-structured MnO powders are 1004 and 673 mA h g-1, respectively. The discharge capacities of the graphene-MnO composite and hollow-structured MnO powders for the 130th cycle at a current density of 500 mA g-1 are 1313 and 701 mA h g-1, respectively. In the measurement of the rate performances, the gap between the discharge capacities of both the graphene-MnO composite and hollow-structured MnO powders increases with increase in the current densities. Hierarchically structured tin oxide-reduced graphene oxide (RGO)-carbon composite powders are prepared using a one-pot spray pyrolysis process. SnO nanoflakes several hundred nanometers in diameter and a few nanometers thick are uniformly distributed over the micron-sized spherical powder particles, as are ultrafine nanometer-scale SnO2 particles. The initial discharge and charge capacities of the tin oxide-RGO-carbon composite powders at a current density of 1000 mA g-1 are 1543 and 1060 mA h g-1, respectively. The discharge capacity of the tin oxide-RGO-carbon composite powders after 175 cycles is 844 mA h g-1 and the capacity retention measured from the second cycle is 80%. The transformation during cycling of SnO nanoflakes, uniformly dispersed in the tin oxide-RGO-carbon composite powder, into ultrafine nanocrystals, results in hollow nanovoids that act as buffers for the large volume changes that occur during cycling, and thereby improve the cycling and rate performance of the tin oxide-RGO-carbon composite powders. Nickel sulfide-reduced graphene oxide (RGO) composite powders with spherical shapes were prepared by a one-pot spray pyrolysis process. The optimum mole ratio of nickel nitrate and thiourea to obtain nickel sulfide–RGO composite powders with high initial capacities and good cycling performance is 1:8. The bare nickel sulfide and nickel sulfide–RGO composite powders prepared directly by spray pyrolysis from spray solutions with Ni nitrate and thiourea in a mole ratio of 1:8 had mixed crystal structures of hexagonal -NiS and cubic Ni3S4 phases. The bare nickel sulfide powders were prepared from the spray solution without graphene oxide sheets. The nickel sulfide–RGO composite powders had sharp mesopores approximately 3.5 nm in size. The discharge capacities of the nickel sulfide–RGO composite powders for the 1st and 200th cycles at a current density of 1000 mA g-1 were 1046 and 614 mA h g-1, respectively, and the corresponding capacity retention measured from the second cycle was 89%. However, the discharge capacities of the bare nickel sulfide powders for the 1st and 200th cycles at a current density of 1000 mA g-1 were 832 and 16 mA h g-1, respectively, and the corresponding capacity retention measured from the second cycle was 2%. The electrochemical impedance spectroscopy (EIS) measurements revealed the high structural stability of the nickel sulfide–RGO composite powders during cycling. 이차전지는 현재까지 상용화되어 산업 분야 및 일상생활 등의 여러 분야에 사용되고 있으나 새로운 기술 기반의 충족하는 고안정성, 장수명 이차전지의 필요성이 대두되고 있다. 최근에 이차전지의 고안정성 및 장수명 전지개발을 위하여 금속 산화물계 및 금속 황화물계 전극 소재에 대한 전기화학적 반응 기구에 대한 연구가 가장 큰 핵심 이슈가 되고 있다. 현재까지 이차전지 전극재료로 기존에 사용하고 있는 흑연은 낮은 용량의 한계를 가지고 있어서 흑연을 대체할 많은 연구들이 되고 있으며 고용량의 주석(Sn), 실리콘(Si) 계열의 물질들이 많은 관심을 가지고 연구가 되었다. 그러나 이러한 전극재료들은 부피팽창 및 분쇄로 인한 전지의 성능감소를 가져오는 등의 여러 가지 개선해야 할 문제점을 가지고 있다. 이런 문제점을 극복하고 고안정성, 장수명의 이차전지 개발을 위한 여러 가지 방법들이 모색되고 있으나 아직까지 극복해야 할 많은 문제점을 가지고 있다. 본 연구는 이러한 문제점을 극복하기 위하여 이차전지 음극 재료로 사용되는 금속 산화물과 금속황화물을 분무열분해 공정으로 합성하고 그래핀 첨가로 인한 전기화학적 특성을 증가시키고자 한다. 그래핀은 음극활물질의 전기화학적 특성을 향상시키는데 중요한 역할을 한다. 충방전 동안의 음극활물질의 뭉침 현상을 방지해주어 그 구조를 안정하게 해준다. 또한 리튬의 삽입과 탈리 과정 중에 일어나는 부피팽차을 보호해주는 역할을 한다. 분무열분해 공정에 의해 MnO-graphene 복합체 분말을 합성하였고 비교를 위해 순수한 MnO 분말을 같은 공정을 이용하여 제조하였다. 그래핀 시트에 수십 나노의 MnO 입자들이 고루 퍼져 있는 형태를 갖는 MnO-graphene 분말은 Graphene 함량은 10%이고, BET 표면적은 20 m2 g 1로, 순수한 MnO 분말에 4배정도이다. 전류밀도가 500mA g-1일 때, 방전용량은 130 cycle에서 1313mA h g-1으로 우수한 특성을 나타내었다. Tin oxide-graphene-carbon 복합체는 nanoflake 형태의 SnO와 nanoparticle인 SnO2의 혼합된 상으로 존재한다. 충전, 방전을 진행 시 SnO nanoflake가 산화되어 SnO2로 변형되며 그 빈 공간이 완충역할을 수행함으로써 리튬이온의 삽입과 탈리로 발생하는 부피팽창으로 인한 내부 스트레스를 줄여준다. 175번의 충방전 후 방전용량은 844 mA h g-1로 80%의 보존율을 기록했다. Nickel sulfide-graphene 복합체는 분무열분해 공정에 의해 니켈과 황의 여러 비율의 복합체 분말을 합성하였다. 최적의 조건인 니켈과 황의 몰비는 1:8로 높은 용량과 고안정성을 나타내었다. 200번의 충방전 후 방전용량은 614 mA h g-1로 89%의 보존율을 기록했다.

Study on high efficiency fabrication and quality improvement of graphene oxide via Couette-Taylor flow

박원규 성균관대학교 일반대학원 2016 국내박사

In summary, a practical approach to bulk-scale graphene-based materials is critically important for their use in the industrial applications. Here, we describe a facile method to prepare graphite oxide using a Couette–Taylor flow reactor for the oxidation of bulk graphite flakes. We found that the turbulent Couette–Taylor flow in the reactor could be engineered to result in the efficient mixing and mass transfer of graphite and oxidizing agents (KMnO4 and H2SO4), thereby improving the efficiency of graphite into graphene oxide. As compared to the standard Hummers’ method, higher fraction of a single- and few-layer graphene oxide can be yielded in a dramatically shortened reaction time, by optimizing the processing parameters, we have shown that ~93% of graphene oxide yield could be achieved within 60 min of reaction time. This method also allowed for the in-situ functionalization of graphene oxide with metal oxide nanoparticles to give a nanoparticle-decorated graphene oxide hybrid material. Furthermore, we describe a green method to prepare graphene oxide using a recycled sulfuric acid through filter process after oxidation of natural graphite in a Couette-Taylor flow reactor. The volume of water for the washing of graphite oxide was dramatically decreased. As compared to the conventional Hummers’ method, viscosity of the mixture containing graphite oxide and sulfuric acid after oxidation reaction using the Couette-Taylor flow was very low as 200 cP (25ºC), and it enabled the filtering process. In conclusion, although the recycled sulfuric acid was used for the fabrication of graphene oxide, high quality, a single- or few-layer, graphene oxide could be obtained at the same time while saving the process costs via reducing the washing water and reuse the sulfuric acid. To utility in a range of applications of graphene oxide, high efficiency exfoliation of graphite oxide with controlled area is required. Here, we describe a facile method to prepare large-area and single layer graphene oxide using a Couette-Taylor flow reactor, a novel exfoliation method. The Couette-Taylor reactor consists of two concentric cylinders and the inner cylinder rotates at a controlled speed while the outer cylinder is kept stationary. We found that the formation of Taylor vortex flow with shearing stress in the reactor is effective for exfoliation of graphite oxide, which allows for the production of a more than 40μm in lateral size single or few-layer graphene oxide platelets at a high yield of 90% or above within 60 min of exfoliation reaction time. Moreover, we could control the lateral size of graphene oxide sheets through different rotation speed of the inner cylinder and reaction time. Our method for facile and eco-friendly fabrication method of graphite oxide and their high efficiency exfoliation with controlled area may find utility in a range of applications including energy storage, conducting composite, electronic device and supporting frameworks of catalyst.

Graphene oxide를 기반으로 한 PEBAX 혼합막의 CO2/N2 기체 투과 특성

이은선 상명대학교 일반대학원 2023 국내석사

In this study, graphene oxide (GO), polyethylenimine-graphene oxide (PEI-GO), and polyethylenimine-graphene oxide@zeolite imidazolate framework-8 (PEI-GO@ZIF-8) were added to poly (ether-block-imide) 2533 (PEBAX 2533) by content and mixed to be used as mixed matrix membranes (MMMs). The MMMs penetrated single CO2 and N2 gas, to investigate the performance of each filler and content. Through Fourier-transform infrared spectrometer (FT-IR) and powder X-ray diffractometer (XRD), it confirmed GO, PEI-GO, and PEI-GO@ZIF-8 were synthesized well, and changes in crystallinity of the materials were confirmed due to modification. When MMMs, FT-IR peak change was not observed, because the small amount of the filler. As a result of analysis through Thermogravimetric Analyzer (TGA), it was confirmed that the thermal stability of the filler increased as GO was modified to PEI and ZIF-8. The thermal stability of the MMMs was also increased with the content of the filler, and it was confirmed that the PEBAX/PEI-GO@ZIF-8 MMMs had the best thermal stability. In Gas permeation, case of the PEBAX/GO MMMs, the permeability of both N2 and CO2 gas tended to decrease as the GO content increased. On the other hand, the CO2/N2 selectivity steadily increased to 58.89 which was the highest at 0.3 wt%, decreased at 0.5 wt% to 55.34 which was similar to 0.1 wt%, and the overall selectivity was improved overall compared to the pure PEBAX 2533 membrane. In the PEBAX/PEI-GO MMMs, the N2 permeability decreased steadily. The CO2 permeability decreases from 0.1wt% and then increases from 0.3wt%, and decreases slightly from 0.5wt%, but shows a value similar to 0.3wt%. The CO2/N2 selectivity steadily increased to 73.5, the highest at 0.3wt%, and slightly decreased at 0.5wt%. The N2 permeability of the PEBAX/PEI-GO@ZIF-8 MMMs shows a decreasing trend. The CO2 permeability was from 0.1wt% to 221.88 Barrer, showing the highest permeability in all MMMs. It was confirmed that the CO2 permeability gradually decreased at the above content. Unlike the PEBAX/GO and PEBAX/PEI-GO MMMs, the CO2/N2 selectivity recorded the highest 60 at a content of 0.1wt%, and then decreased. When the MMMs subjected to gas permeation were illustrated in upper bound (2008), it was confirmed that all membranes were close to the upper bound compared to the pure PEBAX 2533 membrane. Among them, PEBAX/PEI-GO of 0.3wt% and 0.5wt% showed excellent performance beyond the upper bound. 본 연구는 합성한 graphene oxide (GO), polyethylenimine-graphene oxide (PEI-GO), polyethylenimine-graphene oxide@zeolite imidazolate framework-8 (PEI-GO@ZIF-8)을 poly(ether-b-amide) 2533 (PEBAX 2533)에 함량별로 첨가, 혼합하여 mixed matrix membranes (MMMs)로 사용하였다. 제조된 혼합막은 단일 기체인 CO2, N2 기체에 투과하여 충진재별, 함량별 성능을 알아보았다. PEBAX 2533에 혼입되는 GO는 기존 발표된 논문을 기초로 합성하였고, PEI-GO는 GO 표면에 PEI 사슬을 접합하는 형태로 합성되었다. PEI-GO@ZIF-8의 경우 PEI-GO 내 GO 표면에 ZIF-8 입자가 형성된 구조를 이루도록 합성하였다. Fourier-transform infrared spectrometer (FT-IR)를 통해 GO, PEI-GO, PEI-GO@ZIF-8의 합성이 잘 되었음을 판단하였다. 각 충진재의 powder X-ray diffractometer (XRD)를 통해 PEI와 ZIF-8을 통한 개질이 GO sheets 간 거리에 영향을 미친 것으로 확인하였다. Thermogravimetric analyzer (TGA)를 통한 분석 결과 GO를 PEI와 ZIF-8으로 개질할수록 충진재의 열적 안정성이 증가하는 것을 확인할 수 있었다. 또한 GO, PEI-GO, PEI-GO@ZIF-8을 첨가한 혼합막의 열적 안정성은 충진재의 함량에 따라 증가하였으며, PEBAX/PEI-GO@ZIF-8 혼합막이 가장 우수한 열적 안정성을 가진다는 것을 확인하였다. 기체 투과는 3atm, 25˚C 조건에서 이루어졌다. PEBAX/GO 혼합막의 경우 GO의 함량 증가에 따라 N2 기체와 CO2 기체 모두에서 투과도가 감소하는 경향을 보였다. 반면 CO2/N2 선택도는 꾸준히 증가하여 0.3wt%에서 가장 높은 58.89를 보였으며, 0.5wt%에서는 감소하여 0.1wt%와 유사한 55.34를 기록하였고 전체적인 선택도는 순수한 PEBAX 2533 막에 비해 전반적으로 개선된 값을 보였다. PEBAX/PEI-GO 혼합막에서는 N2 투과도가 꾸준히 감소하는데, 그 감소 폭은 PEBAX/GO 막에서보다 큰 것으로 나타났다. CO2 투과도는 0.1 wt%에서 일시적으로 감소하였다가 0.3wt%에서 증가하고, 0.5wt%에서 약간 감소하나 0.3wt%와 유사한 값을 보인다. CO2/N2 선택도는 PEBAX/GO 혼합막과 유사하게 꾸준히 증가하여 0.3wt%에서 가장 높은 73.5를 기록하였으며, 0.5wt%에서 약간 감소한다. PEBAX/PEI-GO@ZIF-8 혼합막의 N2 투과도는 감소하는 추세를 보이나 PEBAX/PEI-GO 혼합막보다 완만하게 감소한다. CO2 투과도는 0.1wt%에서 221.88 Barrer로 모든 혼합막에서 가장 높은 투과도를 보였으며 그 이상의 함량에서는 서서히 투과도가 감소하는 것을 확인하였다. CO2/N2 선택도는 PEBAX/GO, PEBAX/PEI-GO 혼합막과 달리 0.1wt% 함량에서 가장 높은 60을 기록하였으며 그 후 감소하였다. 혼합막의 기체 투과 결과를 upper bound (2008)에 도시하였을 때 순수한 PEBAX 2533 막에 비해 모든 혼합막이 upper bound에 근접한 것을 확인할 수 있었다. 그 중 PEBAX/PEI-GO 0.3wt%, 0.5wt% 혼합막은 upper bound를 넘는 우수한 성능을 보였다.

Controlling molecule and ion permeation properties of multilayer graphenes assisted by pore generation

김정필 Graduate School, Yonsei University 2024 국내박사

Graphene possesses excellent properties such as mechanical strength, electrical and thermal conductivities, making it highly valuable in various fields such as electronics and energy storage. However, it is extremely challenging to produce high-quality graphene in large quantities, while reducing production costs. This has inspired lots of active research to replace graphene in various applications by controlling the structure of graphene derivatives like graphene oxide, which can be mass-produced inexpensively. The control of graphene oxide's structure can be generally categorized into two aspects: (1) adjusting the interlayer spacing of graphene and (2) tuning the pore size within graphene sheets. In this study, we employed various methods to simultaneously control the interlayer spacing and pore size of multilayer graphene oxide. We further investigated the effects of structural changes on versatile applications of synthesized multilayer graphene oxide. The work of this thesis is organized into 3 sections. In the first section (Chapter 2), we carried out research about the influence of the porous structure of reduced graphene oxide on supercapacitor performance. Reduced graphene oxide with a hierarchical structure and high surface area was synthesized through a two-step thermal treatment process. Initially, rapid high-temperature treatment in air leads to the formation of a hierarchical structure due to the decomposition of oxygen functional groups into CO/CO2 gases. Subsequently, thermal treatment in a nitrogen atmosphere allowed us to maintain the developed hierarchical structure while modifying the surface area of micropores. Consequently, we optimized the process conditions of both thermal treatments for the reduced graphene oxide to be utilized as a supercapacitor electrode material with high capacitance and long-term stability, which was confirmed through performance evaluations. In the second part (Chapters 3 and 4), reduced graphene oxide was prepared by the hot-press method and we investigated the effect of structural changes in reduced graphene oxide, on gas and ion permeation phenomena. We observed that the reduced graphene oxide membrane fabricated via hot-press exhibited a more efficient reduction in interlayer spacing compared to thermally treated graphene oxide without external pressure. Moreover, as the duration of the hot-press increased, the interlayer spacing did not change significantly, while the nanopore size within the graphene sheet increased. In Chapter 3, we investigated the gas separation membranes with rapid hydrogen permeation and optimized the hot-press time for better control of the interlayer spacing and nanopore size. In Chapter 4, we explored the permeation behavior of monovalent and divalent ions through reduced graphene oxide sheets and investigated the influence of oxygen functional groups and interlayer spacing on the permeation behavior. In the third part (Chapter 5), Li ion-selective membranes were manufactured using zwitterionic monomer, and their performance was evaluated. Zwitterionic monomers intercalated between graphene oxide sheets, increased its interlayer spacing, resulting in higher ion permeability compared to hot-pressed graphene membranes. Furthermore, the positively charged part of the zwitterionic monomer inhibited the permeation of divalent cations, enhancing the selectivity for monovalent lithium ions. The manufactured graphene oxide composite membrane exhibited higher lithium ion permeability and selectivity. We also confirmed that high-purity lithium solution was obtained through a series of ion separation processes. This study provides a deep understanding of the structural changes of reduced graphene oxide using various advanced methods. This work also presents strategies to enhance the properties of graphene oxide, overcoming its limitations such as low electrical conductivity and surface area. This was achieved by precisely controlling the interlayer spacing and pore structure of multilayer graphene oxide. The high-performance graphene-based materials of this study facilitate the development of impactful industrial applications in energy storage and separation membranes. 그래핀은 우수한 전기 및 열 전도성, 기계적 강도 등 다양한 특성을 가지고 있어 전자공학, 에너지 저장 등 다양한 분야에서 유용하게 활용되고 있다. 하지만 고품질의 그래핀의 생산 비용을 절감하면서 대량 생산하는데 여전히 어려움을 겪고 있다. 저렴한 가격으로 대량 생산이 가능한 산화 그래핀과 같은 그래핀 유도체의 구조를 제어하여 다양한 응용 분야에서 그래핀을 대체하려는 활발한 연구가 진행되고 있다. 산화 그래핀의 구조 제어는 일반적으로 두 가지 측면으로 나뉘며, (1) 그래핀의 층간 간격을 조절하는 것이며, (2) 그래핀 내의 기공 크기를 조절하는 것이다. 본 논문에서는 다층 산화 그래핀의 층간 간격과 기공 크기를 동시에 조절하기 위해 다양한 방법을 활용하였으며, 산화 그래핀의 구조적 변화가 산화 그래핀의 다양한 응용 분야에 미치는 영향을 이해하기 위한 목적으로 이를 수행하였다. 이 학위논문은 3 개 부분으로 구성되어 있다. 첫 번째 부분(제 2 장)에서는 환원된 산화 그래핀의 기공 구조가 슈퍼캐패시터 성능에 미치는 영향 연구를 진행하였다. 계층 구조와 높은 표면적을 가진 환원된 그래핀 산화물은 2 단계 열처리 공정을 통해 합성되었다. 먼저, 공기 중에서의 빠른 고온 열처리 과정에서 발생하는 CO/CO2 가스에 의해 계층적 구조가 형성됨을 관찰하였다. 이어서 질소 분위기에서의 열처리를 통해 앞서 형성된 계층 구조를 유지하면서 마이크로 기공의 표면적이 변화됨을 확인하였다. 결과적으로, 환원된 산화 그래핀을 슈퍼캐패시터의 전극 재료로 사용하기 위한 높은 용량과 장시간 안정성을 갖도록 두 단계 열처리 방식의 조건을 최적화하였으며, 이를 통해 성능 평가를 통해 확인하였다. 두 번째 부분 (제 3 장과 4 장)에서는 열압축 방식을 통해 환원된 산화 그래핀 분리막을 제조하였고, 산화 그래핀의 구조 변화가 가스 및 이온 투과 현상에 미치는 영향을 연구하였다. 열압축 방식을 통해 제조된 환원된 산화 그래핀 분리막은 압력 없이 열처리 공정한 산화 그래핀보다 층간 간격이 효율적으로 감소함을 관측하였다. 또한, 열압축 시간에 증가함에 따라 층간 간격은 크게 변화하지 않고, 그래핀 시트 내의 나노기공 크기가 커짐을 확인하였다. 이에 3 장에서는 빠른 수소 투과도를 가진 가스 분리막에 대한 연구를 하였고, 산화 그래핀의 층간 간격과 나노기공 크기를 제어하기 위한 최적화된 열압축 시간을 제시하였다. 4 장에서는 환원된 산화 그래핀 막을 통한 1 가 및 2 가 이온의 투과 현상을 분석하였으며, 산화 그래핀의 산소작용기와 층간 간격에 의해 이온들의 투과 현상에 미치는 연구를 진행하였다. 세 번째 부분(제 5 장)에서는 양쪽성 단량체를 이용하여 이온 선택성 분리막을 제조하고 성능을 평가하였다. 양쪽성 단량체는 산화 그래핀 시트 사이에 삽입되어 층간 간격을 증가시킴으로써 열처리만 환원된 분리막보다 높은 이온 투과도를 가짐을 확인하였다. 또한, 양쪽성 단량체의 양이온성 성질에 의해 2 가 양이온의 투과를 억제하여 1 가 이온인 Li 의 선택도를 높여주었다. 제작된 산화 그래핀 복합체 이온 분리막은 산화 그래핀 이온 분리막보다 높은 Li 이온 투과도와 선택도를 나타냈으며, 여러 이온 분리막 공정을 통해 고순도 Li 용액을 얻을 수 있음을 확인했습니다. 본 연구는 여러 가지 방법을 사용하여 환원된 그래핀 산화물의 구조 변화에 대한 심층적인 이해를 제공하였다. 또한, 낮은 전기전도도와 낮은 표면적과 같은 산화 그래핀의 한계를 극복하고 향상시키는 전략을 제시한다. 이러한 성능 향상은 다층 산화 그래핀의 층간 간격과 기공 구조를 정밀하게 조절하는 것을 통해 달성하였다, 이를 통해 에너지 저장, 분리막 등 다양한 분야에 활용 가능한 고성능 그래핀 기반 소재 개발에 활용될 수 있을 것이다.

Electrical field- and flow-induced alignment of graphene oxide dispersion

심전자 성균관대학교 일반대학원 2016 국내박사

In this dissertation, the graphene oxide as lyotropic liquid crystals for electro-optic response convention, have been emphasized for inverstigation.Graphene oxide liquid phase was used as the main material for testing electro-optic response and also calculated large level Kerr coefficient. In chapter 1 the technical back ground of liquid crystal and graphene oxide liquid crystal phase have been briefly reviewed. In chapter 2, the high-frequency electric fields induced liquid crystallinity in GO dispersions at bi-phase state (~ 0.2wt %) was first reported, when fields off the GO dispersion to be isotropic state. The Kerr coefficient, was measured to be 1.8 × 10−5 mV−2, it is the highest value ever report for a molecular liquid crystal. In chapter 3, we investigated the relationship between the quality of the GO dispersion and the electro-optic response of a cell containing the GO dispersion. Here, the quality of the GO dispersion was controlled by the number of centrifugal cleaning cycles in the sample preparation. It was apparent that excellent GO dispersion is required for fabricating a quality electro-optic device using the GO dispersion. The residual salts of oxidizing reagents paralyze the electro-optical response of GO dispersions, which can be explained by the increasing solvent conductivity. At the same time, the small aspect ratio due to stacked GO particles contributes partially to the desensitization of the electro-optic response as well. In chapter 4 we do research into ion effect on electro-optic response of GO–LC. We measured the optical Kerr coefficients of GO dispersions with varying ion types and concentrations. We also determined the zeta potential, conductivity, and pH of these GO dispersions, as they are important parameters in the electro-optic effect of GO dispersions. The pH of the pure GO dispersion was 3.7, indicating a H+ ion concentration of 2 × 10−3 M in the solvent. The solvent conductivity was slightly decreased by addition of NaOH, which may result from the replacement of H+ ions in the solvent with Na+ ions due to the recombination of H+ with OH−. This result is directly reflected in the Kerr coefficient. Although we added 10−3 M NaOH, the electro-optic sensitivity increased rather than decreased. The calculated surface charge density and mobility indicate that H+ ions are more attracted to the surface than are Na+ ions, and that H+ ions persist in the electrical double layer instead of being replaced by Na+ ions. In chapter 5, the velocity profile and the order parameters of GO particles in a flowing GO dispersion within a cylindrical tube was obtained by measuring the optical birefringence and dichroic. We found that the tube flow transits from a parabolic profile to one with a nearly uniform velocity near the centre and a large velocity gradient near the tube edge, which is Newtonian to non-Newtonian transition owing to shear thinning effect. The S- and P-order parameters of GO particles exhibit concentric alignment with a largely biased fluctuation and a disordered state in the very centre. The order parameters increase concurrently with the increase of m, indicating that the ordering of GO particles and shear thinning effect correlate to each other. Interestingly, the P-order parameter exhibits more correlation with m than the S order parameter does, which implies that the interparticle interaction influences the shear thinning effect as well. Thus, the ordering of the individual GO particles and the interparticle interaction causes the shear thinning of an aqueous GO dispersion. At the last chapter, chapter 6, We demonstrated that the structural color in a cell containing an aqueous GO dispersion can be electrically erased and rewritten through the introduction of a appropriate electrode structure and driving mechanism. Since periodic GO alignment parallel to the substrate is responsible for color reflection, the application of a vertical field can destroy the structural color by rearranging the GO alignment along the vertical direction. In the presence of a vertical field, the reflection color becomes black. The operation voltage of the cell increases with increasing GO concentration due to an increase in inter-particle friction. When the field is removed, the initial structural color can be partially recovered via spontaneous diffusive particle motion, although this process is extremely slow. This process can be accelerated by applying a horizontal field to cause the GO particles to align parallel to the surface. By applying vertical and horizontal fields repeatedly, the structural color can be written and erased accordingly.

(An) empirical approach for industry-compatible graphene with superior electronic properties

신소명 세종대학교 대학원 2021 국내박사

In order to apply the superior properties of graphene to industrial applications, large-scale growth and mass production techniques are required. Many research groups and companies have achieved to grow a thousand-centimeter scale graphene film by chemical vapor deposition (CVD) growth method and the earlier studies on graphite oxide exfoliated in water on a scale of tons led to breakthroughs in its low-cost and massive productivity for the real-world applications. However, the high sheet resistance of the CVD graphene of a single atomic layer is still difficult to replace ITO used as a conventional transparent electrode, and rGO has a lower conductivity than the natural graphene due to the defects caused by the oxidation process. Therefore, we investigate methods to improve the conductivity of CVD graphene and rGO and evaluate their electrical and optical characteristics for the industry-compatible graphene. First, metal substrates are commonly used as a catalyst for CVD graphene growth, thus, a transfer process from the metal substrate to a target substrate, mainly oxide substrates, is required for devices applications. A variety of transfer methods have developed for the high-quality graphene sheet, however, the residues of the sacrificial polymer layers and wrinkles and cracks occurred during the transfer process severely deteriorate the conductivity of graphene. Here, polymers with aromatic structure are used as an alternative to PMMA, resulting in the lower density of polymer residues, the better roughness, and enhancement in the conductivity of graphene by hole-doping. In addition, we lowered the sheet resistance by the deposition of molybdenum oxide layer on graphene with a high work function on the graphene surface. The lower sheet resistance can be ascribed to the hole-doping through the charge transfer from graphene to molybdenum oxide. Another main factor to decrease the conductivity of CVD graphene is the grain boundaries formed during the growth process. We selectively deposited Al-doped ZnO at the grain boundaries using Atomic layer deposition technique and achieved the low resistivity while maintaining the high transmittance. In the case of rGO, since defects such as oxygen functional groups residues and structural defects caused during the oxidation and reduction process, the rGO materials has inherently a lower conductivity than graphene. Most of studies to acquire the high-quality rGO have focused on the reduction process or the healing methods after reduction. In this study, we investigate the effect of the oxidation process by adjusting the pH of the oxidizing agent. We confirmed that the oxidizing agent of the low oxidation number facilitates to produce the high quality rGO, representing the high conductivity close to the one of CVD Graphene. In addition, the essential parameters for the material properties evaluation, such as the localization length and bandgap are extracted through the careful analysis of the temperature-dependent measurement from 50 K to 300 K and the related theoretical transport models. It is confirmed that the oxygen double bond formed in the oxidation process can generate defects such as vacancies and Stone-wales, and the structural defects are found to be decisive factors to degrade the properties of the resultant reduced graphene. Our studies for improving graphene conductivity provide a practical approach for the real-world application in graphene electrodes or graphene-based nanodevices.

Theoretical and Empirical Approaches to Easy Band Gap Engineering of Graphene for a Variety of Applications

박지수 서울대학교 대학원 2020 국내박사

그래핀은 그 자체가 가지고 있는 기존 재료 대비 뛰어난 특성 때문에 그 동안 큰 주목을 받아왔다. 이에 따라, 다양한 응용 분야에서 그래핀을 사용하여 고성능 장치를 구현하고자 수 많은 연구들이 진행되어 왔다. 그러나, 그래핀 제조 공정에 의해 변경 될 수 있는 그래핀의 그 자체의 재료 특성에 대한 인지 부족으로 인해 실제 디바이스에 대한 적용 시 요구되는 다양한 범위의 그래핀을 만족시키지 못하여 그래핀이 가지고 있는 뛰어난 물성을 완전하게 구현하는데 실패하였다. 특히, 전자소자로의 응용의 경우, 전기적 특성을 결정하는 중요한 특성인 밴드갭은 그래핀의 경우 존재하지 않으며, 물리적 혹은 화학적 구조 변화에 의해서만 확보 될 수 있으므로, 그래핀 기반 장치의 성능의 비약적인 향상을 위해 그래핀의 밴드갭 확보 과정에 대한 분석이 필수적이라고 할 수 있다. 이를 위해 일부 연구자들은 밴드갭을 결정하는 그래핀 자체의 물질적 파라미터를 찾아내고자 하였으나, 그 밴드갭과 물질적 파라미터 간의 관계를 실질적으로 명확하게 밝혀내는 데에는 실패하였다. 이러한 그래핀 연구의 장애물을 극복하기 위해서는 기존 연구진이 시도해왔던 것과는 차별화하여, 그래핀이 가지고 있는 탄소원자를 기반으로 그래핀의 밴드갭을 제어하는 숨겨진 파라미터를 밝혀내는 것이 중요하다. 따라서, 이 연구는 기존의 관점에서 탈피하여 공액구조를 가지고 있는 탄소원자의 비율과 경계탄소의 비율, 그리고 그래핀의 밴드갭 사이의 관계 확립에 중점을 두어 명확한 선형관계가 성립함을 입증하였다. 또한, 대표적인 응용 분야들을 위해 실제 미세 조정된 밴드갭을 갖는 그래핀을 실험적으로 적용하여 그 효과를 실질적으로 증명하고자 하였다. Part I 에서는 그래핀의 밴드갭 미세 조정에 대한 배경과 필요성을 정리하였다. 그래핀의 밴드갭 엔지니어링에 대한 현재 연구 동향을 면밀하게 검토하였으며, 이를 기반으로 그래핀의 이론적으로 예측된 성능과 실제 그래핀 기반 장치 사이의 격차를 극복하기 위한 이 연구의 목적을 확립하고자 하였다. Part II에서는 이론적으로 그래핀의 밴드갭과 표면 파라미터 간의 관계를 예측하고, 이를 검증하기 위해 기본적인 이론을 확립하는데 중점을 두었으며, 공액구조를 가지는 탄소원자의 비율과 그 중 경계탄소의 비율을 기반으로 표면 특성에 따라 실제 밴드갭을 결정하는 주요 파라미터를 도출하고자 하였다. 주요 재료적 파라미터들은 기본 전자 공학 이론을 기반으로 검토 후 도출하였다. 이를 바탕으로 화학적인 방법으로 합성된 그래핀 옥사이드 기반 그래핀의 밴드갭을 예측하기 위해 공액구조를 가지는 탄소원자 및 공액구조 경계 탄소원자의 비율을 도입하였으며, 이들 파라미터들과 실제 그래핀 옥사이드 기반 그래핀의 관계에 대한 실험적인 검증을 통해 확립된 이론의 신뢰성을 확보하고자 하였다. Part III은 Part II에서 제시된 그래핀 밴드갭과 그 자체의 물질적 파라미터 간의 관계를 따라 맞춤형 밴드갭을 갖는 그래핀의 설계 및 합성에 중점을 둔다. Chapter 3에서는 그래핀의 가장자리 선택적 산화에 의해 환원 후 그래핀 옥사이드 기반 그래핀의 밴드갭을 낮추는 것을 시도한다. 그 결과 그래핀 산화물의 환원공정 동안 발생하는 결함을이 기존 그래핀 옥사이드에 비해 크게 감소하었으며 그 결과 환원된 그래핀의 전기전도도를 크게 향상시키는데 성공하였다. Chapter 4 에서는 그래핀 옥사이드의 표면의 에폭사이드 작용기를 선택적으로 감소시킴으로써, 표적 가스 분자의 흡착 및 전하 운반체의 이동을 향상 시켜 타겟 물질에 대한 감도를 증가시키는데 성공하였다. Chapter 5에서는 인듐-갈륨-아연-산화물 (IGZO) 박막 트랜지스터의 채널 물질의 전기적 수송을 개선하기 위해 그래핀의 양에 따른 전체 계의 밴드갭을 미세하게 조정하였고, 채널 물질의 열처리 동안 기공 발생을 최소화하기 위해 에폭사이드 작용기가 감소된 그래핀 옥사이드를 사용하여 밴드갭이 조정된 그래핀 / IGZO 복합체의 전하 수송을 효과적으로 증가시켰다. 이 연구는 그래핀 기반 장치의 성능과 관련된 변수에 대한 이론적 고려를 통해 그래핀의 밴드갭에 영향을 미치는 주요 재료 매개 변수를 도출하고 이를 실질적으로 검증한다는 점에서 그 의의를 찾을 수 있으며, 실제적으로 대표적인 적용 분야에서 성능 향상을 보인 만큼, 이 연구에서 제시된 그래핀 설계 지침은 이론적으로 예측된 그래핀의 성능과 실제 장치 성능 사이의 격차를 극복하고 차세대 그래핀 기반 재료로 연구를 발전시키는 것에 대한 통찰력을 제공 할 수 있을 것이라 사려된다. Graphene has been taken attention because of its astounding properties than other transitional materials. Therefore, there have been tremendous studies on developing high performance devices using graphene in a variety of applications. However, many researchers failed to achieve devices with high performance because of negligence about the material properties of graphene that can be altered by the preparation process. Especially in the case of electronic applications, band gap, which considered as important property that decides electrical properties, can be altered by the physical and chemical characteristics of the graphene itself. Therefore, for the improved performance of graphene based devices, fine tuning of band gap is essential. For the fine tuning of graphene’s band gap, some of the researchers tried to find the materials parameters of graphene which decide the band gap itself, but failed to find the clear relationship. In order to overcoming this huddle, unveiling of hidden parameters which control the band gap of graphene based on carbon atom is important. Therefore this research is focused on the establishment of the relationship between sp2 carbon and graphene’s band gap. In addition, through the empirical application of graphene which has fine-tuned band gap for representative applications. Part I summarizes the background and necessity for fine tuning of graphene’s band gap. The current research trends in the band gap engineering of graphene are reviewed. In order to overcome the gap between the theoretically predicted performance of graphene and the practical graphene based devices, the objectives and scopes of this study were established. In Part II, basic fundamentals are studied to examine the theoretically predicted band gap of graphene. Based on the sp2 carbon, key parameters that determine practical band gap according to the surface characteristics are derived. Key material parameters are derived by examining the theory of basic electronics, while the area of sp2 carbon and boundary sp2 carbon is introduced to predict the band gap of graphene from chemical route. And this relationship between sp2 carbon and band gap is experimentally reviewed and extended to graphene embedded materials. Part III focuses on the design and synthesis of the graphene with tailor fitted band gap according to the design guidelines presented in Part II. Chapter 4 attempt to overcome the lower the band gap of graphene by induced defect. In Chapter 4, edge selectively oxidation of graphene are used to overcome the induced defect on basal plane during reduction process of graphene oxide, resulting in improved electrical conductivity. In Chapter 5, the surface of the graphene oxide is modified to improve the absorption of target gas molecule and transfer of charge carrier by selectively reduction of epoxide group, thereby increasing the sensitivity of the sensing material. In Chapter 6, graphene is embedded in indium-gallium-zinc-oxide (IGZO) for the improvement of electrical transport of channel materials of thin film transistor. By using lower functionalized graphene oxide to minimize the pore generation during heat treatment of channel materials to effectively increase the charge transport of band gap tuned graphene / IGZO composite. As a result, the mobility of graphene embedded thin film transistor is maximized resulting in much lowered roughness. This study derives key material parameters that affect the band gap of graphene through theoretical considerations for variables related to the performance of graphene based devices. The graphene design guidelines presented in this study can provide insight to overcoming the gap between the theoretically predicted performance of graphene and practical device performance in addition to advancing the research into next generation graphene based materials.

Graphene-based electrode materials for supercapacitor applications

Khoh, Wai Hwa Incheon National University 2016 국내박사

Extensive research efforts have been made on development of high performance Li-ion batteries and fuel cells in the past. However, low power density, low charge/discharge rate and environmental unfriendliness have kept them away from many applications. Recently, supercapacitors have drawn great attention because of their high charge/discharge rate, long life cycle, high power density and environmental friendliness. However, supercapacitors generally have low energy density. Electrode material with desirable properties is the key for improving performance supercapacitors. Graphene has been considered a promising electrode material for supercapacitor applications due to its unique lattice structure, excellent electrical conductivity, chemical stability and high surface area (2600 m2/g). However, graphene nanosheets tend to aggregate and restack to multilayer structure, thus, leading to a dramatic decrease in the surface area and hinder ion-diffusion from the electrolyte to the electrode, resulting in low electrochemical performance. Therefore, our research have mainly focused on development graphene-based supercapacitor by introducing either metal oxide nanoparticles or conducting polymer into the interlayer spacing of graphene nanosheets to prevent them from aggregating and also to enhance the energy density of the supercapacitor. These materials were designed to enlarge the interplanar spacing with aiming at maintaining the high surface area of graphene nanosheet, and making both sides of the nanosheets accessible. Moreover, these metal oxides and conducting polymer were able to contribute an additional electrochemical performance in the supercapacitor. This dissertation is divided into several chapters, each discussing a specific topic. Chapter 2 describes the application of layer-by-layer (LBL) self-assembled multilayer film composed of Fe3O4/reduced graphene oxide in supercapacitor applications. Chapter 3 and 4 described the assembly of asymmetric supercapacitor using ITO substrate and flexible PET substrate, respectively. Chapter 2 illustrated the electrochemical properties of LBL self-assembled multilayer films composed of magnetite (Fe3O4) nanoparticle and chemically reduced graphene oxide (RGO) for supercapacitor application. The thickness of the Fe3O4/GO bilayer was determined by using optical ellipsometry to be 6.53±0.17 nm, which agreed well with the sum of the independently measured thickness; 5.61±0.14 nm for a Fe3O4 layer and 0.91±0.1 nm for a GO layer component, respectively. Each layer was found to be deposited uniformly and regularly. Fe3O4/RGO multilayer film was obtained by reducing a Fe3O4/GO film using hydrazine. The multilayer film yielded a minimum resistance of 1.0 × 104 Ω/sq (2.8 S/cm) for a film composed of 50 Fe3O4/GO bilayers. The electrodes fabricated from 30 Fe3O4/RGO bilayers exhibited excellent capacitive performances with a maximum specific gravimetric capacitance (151 F/g) at a current density of 0.9 A/g. In Chapter 3, manganese dioxide/reduced graphene oxide/indium tin oxide (MRI) and polypyrrole/reduced graphene oxide/indium tin oxide (PRI) electrodes were prepared via the chronopotentiometric deposition of either manganese oxide or polypyrrole, respectively, onto a RGO/ITO film at a constant current density. Solid-state asymmetric supercapacitors (ASC) were assembled with MRI as the positive electrode and PRI as the negative electrode in a PVA/LiCl gel electrolyte. These devices displayed a power density of 7.4 kW/kg (for an energy density of 13 Wh/kg), an energy density of 16 Wh/Kg (for a power density of 0.3 kW/kg), and a capacitance retention of 75% over 2000 cycles. The MRI//PRI ASC exhibited a much improved capacitive performance compared to the symmetric PRI//PRI (3 Wh/kg at 0.47 kW/kg) and MRI//MRI (9 Wh/kg at 0.12 kW/kg) supercapacitors. The superior capacitive performance of the MRI//PRI ASC was ascribed to the improved conductivities and mechanical stabilities of MRI and PRI electrodes, which were obtained by fabricating either the polypyrrole or manganese oxide films on a graphene-coated electrode. In Chapter 4, flexible solid-state asymmetric supercapacitor based on multilayer of [Polyaniline(PANI)/PEDOT/PANI/reduced ultra-large GO]n (PPPrG) and PEDOT/MoS2 (PMo) multilayer film were successfully fabricated on flexible PET substrate using a facile coating method like either LBL self-assembly or drop-coating. The PPPrG and PMo electrode were integrated into a flexible supercapacitor cell with including PVA/H2SO4 gel electrolyte. The structure of flexible supercapcitor was simplified with no need of expensive current collector and binders, because the multilayer film work well as the active electrode and current collector owing to the high conductivity. This ASC showed the maximum energy density of 5.4 mWh/cm3 at a power density of 110 mW/cm3, and still maintained 4.0 mWh/cm3 at the power density of 265 mW/cm3 in optimized cell voltage of 0.8 V. Moreover, the ASC exhibited excellent mechanical flexibility without sacrificing electrochemical performances. The superior capacitive performance of the PPPrG//PMo ASC was ascribed to the synergistic combination of good electrical conductivity in the rULGO and a high pseudocapacitance of the eletroactive materials including PANI and PEDOT. Despite the intensive research activities for the enhancement electrochemical performance of supercapacitor, scientists are still looking for the advanced novel materials, which would satisfy the complex requirement for high performance of the supercapacitor including energy density, power density, long-term cycle stability, and low cost production.