Chapter 6. Summary and Future Perspectives
6.2 Future Perspectives
Further studies are required to fully understand the electrochemistry of metal-free carbon nanomaterials. First, an electronic descriptor should be developed for applying a metal-free carbon nanomaterial to an advanced electrocatalyst. Although many studies have suggested potential electronic descriptors, these are not sufficient to predict the active site or to be accessible by experiment. Thus, an electronic descriptor could be found based on our studies, in which the electronic origin of the ORR in metal-free carbon nanomaterials is elucidated. This finding can shed light on the development of carbon-based oxygen reduction electrocatalysts. When an electronic descriptor is developed, the atomic design of metal-free carbon nanomaterials is required to overcome the limitation of oxygen reduction activity in acidic solutions. To enhance the electrocatalytic activity, metal-free carbon nanomaterials should have a high concentration of active sites with high intrinsic activity. To achieve this, a number of active sites must be investigated by using a model catalyst. As in our studies on the activation of graphene basal plane by intrinsic carbon defects, edge structures have also been elucidated and designed.
Finally, the exposed surface area, defect concentration, and edge/basal plane ratio could be suggested for optimal electrocatalytic activity.
Carbon materials have been widely studied owing to their structural diversity and high utilization.
In addition, various structures could be investigated at the atomic level for catalytic application. In our preliminary study, it was thought that structural wrinkles on the graphene could enhance the adsorption energy of the reaction intermediate. This is applicable to other types of two-dimensional materials such as h-BN, MoS2, and MXene. As a support, carbon materials also enhance the surface area of the electrocatalyst. However, the degradation of these materials hampers stability during the electrocatalytic reaction, especially at high electrode potentials. Reducing this degradation is a challenge for enhancing the cyclability of carbon-based electrocatalysts. The degradation mechanism should therefore be investigated, and the contribution of this mechanism to the current density could be elucidated.
Following this concept, we can apply the reduction of CO2 or CO molecules to the formation of carbon- based electrocatalysts. The reduction of CO2 molecules is an important challenge at present. However, the efficiency of reducing CO2 to produce CO is not sufficient because of the high activation energy required and the production of undesirable by-products. Thus, if graphene-based material can be obtained from the reduction of CO2 or CO, the commercial value is expected to be high from an environmental point of view.
The system that is used to describe the electrode and electrolyte should be further studied by computational chemistry. Most studies have focused on using metal-based electrocatalysts as a basic model. However, only carbon-based electrocatalysts, including single-atom catalysts and metal-free
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catalysts, have been suggested as alternatives to precious metal-based catalysts as yet. Thus, electrolyte- graphene interface studies are required to understand the electrochemistry of carbon-based electrocatalysts. The first challenge in the development of such models is the simplification of the electrolyte. In the electrolyte-transition metal interface, a small unit cell size is sufficient to describe the interface. In contrast, the active sites are sparsely distributed in carbon-based electrocatalysts, meaning that a larger unit cell size is necessary. Solvated water cluster modelling could be applied to solve this problem. In other words, implicit and explicit models should be employed simultaneously. This could simplify the modelling of these materials; however, the cluster model, which can effectively act as the electrolyte, requires optimization. In addition, the reaction mechanism for the stepwise transfer of protons and electrons should be further studied. Within the CHE approach, most of the reactions assumed that the electrochemical reaction was occurred through the concerted proton and electron transfer step. This can be reasonable in acidic solutions. However, in alkaline solutions, there is some evidence for decoupled proton and electron transfer. If the decoupled proton and electron transfer is considered in the reaction mechanism, we can exactly identify the rate-determining step for electrochemical reactions. This could be applicable to the reaction mechanism in neutral solutions.
Additionally, the effect of counter ions on the reaction mechanism and catalytic activity of metal-free carbon nanomaterials requires further study. Thus, a design strategy for metal-free carbon nanomaterials could be suggested for the application of seawater batteries. In conclusion, it is possible to obtain fundamental knowledge that can be used to apply metal-free carbon nanomaterials in the formation of a pH-universal oxygen reduction electrocatalyst.
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List of Publications
SCI publications [†: equal contribution]
1. Jung Hwa Kim†, Se-Yang Kim†, Sung O Park†, Gwan Yeong Jung, Seunguk Song, Ahrum Sohn, Sang-Woo Kim, Sang Kyu Kwak*, Soon-Yong Kwon*, and Zhonghoon Lee*,
"Antiphase Boundaries as Faceted Metallic Wires in 2D Transition Metal Dichalcogenides"
Advanced Science, 2020, 2000788.
2. Nguyen Dien Kha Tu†, Sung O Park†, Jaehyun Park, Youngsik Kim, Sang Kyu Kwak*, and Seok Ju Kang*, "Pyridinic-Nitrogen-Containing Carbon Cathode: Efficient Electrocatalyst for Seawater Batteries"
ACS Applied Energy Materials, 2020, 3, 1602-1608.
3. Young Jin Sa†, Sung O Park†, Gwan Yeong Jung, Tae Joo Shin, Hu Young Jeong, Sang Kyu Kwak*, and Sang Hoon Joo*, "Heterogeneous Co-N/C Electrocatalysts with Controlled Cobalt Site Densities for the Hydrogen Evolution Reaction: Structure-Activity Correlations and Kinetic Insights"
ACS Catalysis, 2019, 9, 83-97.
4. Ziyauddin Khan†, Sung O Park†, Juchan Yang, Seungyoung Park, Ravi Shanker, Hyun-Kon Song, Youngsik Kim, Sang Kyu Kwak*, and Hyunhyub Ko*, "Binary N,S-doped carbon nanospheres from bio-inspired artificial melanosomes: A route to efficient air electrodes for seawater batteries"
Journal of Materials Chemistry A, 2018, 6, 24459-24467.
5. Gyutae Nam†, Yeonguk Son†, Sung O Park†, Woo Cheol Jeon, Haeseong Jang, Joohyuk Park, Sujong Chae, Younshin Yoo, Jaechan Ryu Min Gyu Kim*, Sang Kyu Kwak*, and Jaephil Cho*,
"A Ternary Ni46Co40Fe14 Nanoalloy-Based Oxygen Electrocatalyst for Highly Efficient Rechargeable Zinc-Air Batteries"
Advanced Materials, 2018, 30, 1803372.
6. Seokyoon Moon†, Sung O Park†, Yun-Ho Ahn, Heejoong Kim, Eunhye Shin, Sujin Hong, Yunseok Lee, Sang Kyu Kwak*, and Youngjune Park*, "Distinct hydrophobic-hydrophilic dual interactions occurring in the clathrate hydrates of 3,3-dimethyl-1-butanol with help gases"
Chemical Engineering Journal, 2018, 348, 583-591.
7. Ziyauddin Khan†, Baskar Senthilkumar†, Sung O Park†, Seungyoung Park, Juchan Yang, Jeong Hyeon Lee, Hyun-Kon Song, Youngsik Kim, Sang Kyu Kwak*, and Hyunhyub Ko*,
"Carambola-shaped VO2 nanostructures: a binder-free air electrode for an aqueous Na-air battery"
Journal of Materials Chemistry A, 2017, 5, 2037-2044.
8. Yuju Jeon, Sujin Kang, Se Hun Joo, Minjae Cho, Sung O Park, Nian Liu*, Sang Kyu Kwak*, Hyun-Wook Lee*, Hyun-Kon Song*, “Pyridinic-to-graphitic conformational change of nitrogen in graphitic carbon nitride by lithium coordination during lithium plating”
Energy Storage Materials, 2020, accepted.
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9. Sanghyeon Park, Changmin Kim, Sung O Park, Nam Khen Oh, Ungsoo Kim, Junghyun Lee, Jihyung Seo, Yejin Yang, Hyeong Yong Lim, Sang Kyu Kwak*, Guntae Kim*, Hyesung Park*,
“Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity, Stability, and Scalability via Molten Metal-Assisted Intercalation”
Advanced Materials, 2020, accepted.
10. Yoon Ho Lee†, Inho Song†, Su Hwang Kim, Ju Hyun Park, Sung O Park, Jeong Hun Lee, Yousang Won, Kilwon Cho, Sang Kyu Kwak*, and Joon Hak Oh*, "Perovskite Granular Wire Photodetectors with Ultrahigh Photodetectivity"
Advanced Materials, 2020, accepted.
11. Le Quan, Hanyang Zhang*, Huijie Wei, Yunqing Li, Sung O Park, Dae Yeon Hwang, Yu Tian, Ming Huang, Chunhui Wang, Sang Kyu Kwak, Faxiang Qin, Hua-Xin Peng, and Rodney S. Ruoff*, "The Electromagnetic Absorption of a Na-Ethylenediamine Graphite Intercalation Compound"
ACS Applied Materials & Interfaces, 2020, 12, 16841-16848.
12. Koeun Kim†, Daeyeon Hwang†, Saehun Kim, Sung O Park, Hyungyeon Cha, Yoon-Sung Lee, Jaephil Cho, Sang Kyu Kwak*, and Nam-Soon Choi*, "Cyclic Aminosilane-Based Additive Ensuring Stable Electrode-Electrolyte Interfaces in Li-Ion Batteries"
Advanced Energy Materials, 2020, 10, 2000012.
13. Taejung Lim†, Gwan Yeong Jung†, Jae Hyung Kim, Sung O Park, Jaehyun Park, Yong-Tae Kim, Seok Ju Kang, Hu Young Jeong, Sang Kyu Kwak*, and Sang Hoon Joo*, "Atomically dispersed Pt-N4 sites as efficient and selective electrocatalysts for the chlorine evolution reaction"
Nature Communications, 2020, 11, 412.
14. Pavel V. Bakharev*, Ming Huang, Manav Saxena, Suk Woo Lee, Se Hun Joo, Sung O Park, Jichen Dong, Dulce C. Camacho-Mojica, Sunghwan Jin, Youngwoo Kwon, Madakini Biswal, Feng Ding, Sang Kyu Kwak, Zhonghoon Lee, and Rodney S. Ruoff*, "Chemically induced transformation of chemical vapour deposition grown bilayer graphene into fluorinated single- layer diamond"
Nature Nanotechnology, 2020, 15, 59-66.
15. Xu Zhang*, Da Luo, Hanyang Zhang, Dae Yeon Hwang, Sung O Park, Bao-Wen Li, Mandakini Biswal, Yi Jiang, Yuan Huang, Sang Kyu Kwak, Christopher W. Bielawski*, and Rodney S. Ruoff*, "Effect of Copper Substrate Surface Orientation on the Reductive Functionalization of Graphene"
Chemistry of Materials, 2019, 31, 8639-8648.
16. Heming Zhang, Sung O Park, Se Hun Joo, Jin Hyun Kim, Sang Kyu Kwak*, and Jae Sung Lee*, "Precisely-controlled, a few layers of iron titanate inverse opal structure for enhanced photoelectrochemical water splitting"
Nano Energy, 2019, 62, 20-29.
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17. Joohyuk Park, Manabu Shirai, Gwan Yeong Jung, Sung O Park, Minhoon Park, Jaechan Ryu, Sang Kyu Kwak*, and Jaephil Cho*, "Correlation of Low-Index Facets to Active Sites in Micrometer-Sized Polyhedral Pyrochlore Electrocatalyst"
ACS Catalysis, 2018, 8, 9647-9655.
18. Seung-Chul Lee, Chang-Lyoul Lee*, Jeongyun Heo, Chan-Uk Jeong, Gyeong-Hui Lee, Sehoon Kim*, Woojin Yoon, Hoseop Yun, Sung O Park, Sang Kyu Kwak, Sung-Ha Park, and O-Pil Kwon*, "Molecular Viscosity Sensors with Two Rotators for Optimizing the Fluorescence Intensity-Contrast Trade-Off"
Chemistry A European Journal, 2018, 24, 2888-2897.
19. S. T. Senthilkumar, Sung O Park, Junsoo Kim, Soo Min Hwang, Sang Kyu Kwak*, and Youngsik Kim*, "Seawater battery performance enhancement enabled by a defect/edge-rich oxygen self-doped porous carbon electrocatalyst"
Journal of Materials Chemistry A, 2017, 5, 14174-14181.
20. A-Rang Jang, Seokmo Hong, Chohee Hyun, Seong In Yoon, Gwangwoo Kim, Hu Young Jeong, Tae Joo Shin, Sung O Park, Kester Wong, Sang Kyu Kwak, Noejung Park, Kwangnam Yu, Eunjip Choi, Artem Mishchenko, Freddie Withers, Kostya S. Novoselev, Hyunseob Lim*, and Hyeon Suk Shin*, "Wafer-Scale and Wrinkle-Free Epitaxial Growth of Single-Orientated Multilayer Hexagonal Boron Nitride on Sapphire"
Nano Letters, 2016, 16, 3360-3366.
21. Sung-Young Park, Prasun Ghosh, Sung O Park, Young Min Lee, Sang Kyu Kwak*, and Oh- Hoon Kwon*, "Origin of ultraweak fluorescence of 8-hydroxyquinoline in water: photoinduced ultrafast proton transfer"
RSC Advances, 2016, 6, 9812-9821.
22. Jinseon Kim, Sanghyuk Kwon, Dae-Hyun Cho, Byunggil Kang, Hyukjoon Kwon, Youngchan Kim, Sung O Park, Gwan Yeong Jung, Eunhye Shin, Wan-Gu Kim, Hyungdong Lee, Gyeong Hee Ryu, Minseok Choi, Tae Hyeong Kim, Junghoon Oh, Sungjin Park, Sang Kyu Kwak, Suk Wang Yoon, Doyoung Byun, Zhonghoon Lee, and Changgu Lee*, "Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control"
Nature Communications, 2015, 6, 8294.