Therefore, improving the ORR performance of metal-free carbon nanomaterials in acidic solutions is a problem that needs to be solved before these materials can be used in a proton exchange membrane fuel cell. Designing metal-free carbon nanomaterials with adequate ORR performance requires high intrinsic activity and active site density, which means that the active sites where ORR takes place in metal-free carbon nanomaterials must be understood. However, there is room to improve the ORR activity of metal-free carbon nanomaterials by activating the basal plane as a potential ORR active site, as it is found in many carbon frameworks.
In this thesis, the ORR active sites of metal-free carbon nanomaterials are discussed with the aim of revealing the active sites at an atomic level, and design strategies are proposed to achieve high ORR performance. Next, the relationship between the ORR performance and the electronic structure was investigated to determine the origin of the catalytic activity in metal-free carbon nanomaterials.
Introduction
General Introduction
Oxygen Reduction Reaction
- Introduction
- Oxygen Reduction Electrocatalyst
- Reaction Mechanism
Theoretical Study on Electrochemistry
- Introduction
- Computational Hydrogen Electrode
- Theoretical Descriptors
- Reaction Kinetics
Outline of Dissertation
Oxygen Reduction Active Sites of Heteroatom-Doped Graphene as a Cathode
- Introduction
- Simulation Details
- Model Systems
- Density Functional Theory Calculations
- Results and Discussion
- O-Doped Graphene
- N-Doped Graphene
- N,S-Doped Graphene
- Conclusion
- References
- Introduction
- Simulation Details
- Model Systems
- Density Functional Theory Calculations
- Results and Discussion
- Intrinsic Carbon Defects
- Scaling Relationship
- Free Energy Diagram
- Conclusion
- References
Dark grey, white, blue, red colored spheres represent carbon, hydrogen, nitrogen and oxygen atoms respectively. Note that among enolate and epoxide configurations for O* intermediate, thermodynamically stable structure is selectively chosen in each free energy diagram. Dark grey, white, blue, red and yellow colored spheres represent carbon, hydrogen, nitrogen, oxygen and sulfur atoms respectively.
Dark gray, white, blue, red, and yellow spheres represent carbon, hydrogen, nitrogen, oxygen, and sulfur atoms, respectively. The corresponding defect concentration is estimated to be ~1% in terms of a number of atoms in the system.
Electronic Origin of Oxygen Reduction Activity in Metal-Free Carbon
- Introduction
- Simulation Details
- Results and Discussion
- Conclusion
- References
The blue surface corresponds to an electrostatic potential of zero, while the red surface indicates the region of positive electrostatic potential.
Kinetic Insights for Oxygen Reduction Activity in Metal-Free Carbon
Introduction
Using the computational hydrogen electrode (CHE)1 approach, the thermodynamic evaluation of electrocatalytic activity can reduce the computational costs arising from the complexity of considering the activation energy for the reaction mechanism2,3. However, the kinetics of the reaction mechanism must be investigated to understand the selectivity phenomenon of the oxygen reduction reaction (ORR). A two-electron pathway is likely at the transition metal surface where the reaction intermediates are weakly adsorbed.
For such types of electrocatalysts, the energetics of the two reaction pathways must be compared to determine the ORR reaction mechanism. However, the reaction rates between two different reactions are difficult to compare with a thermodynamic study. In addition, a kinetic study could provide insight into the overall rate of reactions and the experimental Tafel slope.
To calculate the reaction rate, we estimated the activation energies with density functional theory (DFT) calculations. Since the electron transfer occurring in the reaction path causes a change in the work function of the catalyst surface, it is difficult to measure the activation energy at a constant potential. A useful approach is to use the charge extrapolation scheme that considers changes in energy, charges, and work functions before and after an electrochemical reaction.
This approach has been applied to interpret the reaction kinetics of the complex electrochemical reactions that occur in transition metals. Herein, the reaction kinetics of ORR on metal-free carbon nanomaterials was investigated using a charge extrapolation scheme. The selectivity, rate-determining step, and structure of the electrolyte-electrode interface are discussed in this chapter.
Simulation Details
This approach can be applied to other types of electrochemical reactions (eg, CO2 reduction reaction and N2 reduction reaction). For electrochemical reactions, it is still challenging to calculate the potential-dependent activation energy involving electron transfer during the ORR. Mulliken population analysis was used to evaluate the charge states of the catalyst surface.
The hydronium ion and its first solvation shell were included in the cluster model and the ClO4- counterion was explicitly considered in the calculations. In the cluster model, the ClO4- ion was too far from the active site to avoid hindering the reaction. During the discharge process, ORR can occur in acidic solution by the following reaction mechanisms.
The room temperature (T) was applied to the system, and ∆GU represents the contributions of electrode potential to the energetics of elementary reaction, calculated by ∆GU = -eU.
Results and Discussion
An activation energy of 0.22 eV was required for this transition of the spin state during adsorption. The work function of the surface was 6.04 eV before the transfer of the proton occurred, which means that the reaction of O2* + H+ + e- → OOH* is expected to be spontaneous below the electrode potential of 1.64 V. Note that for to evaluate the work function of the reactant, the O-H distances of hydronium were fixed during the geometry optimization, limiting the proton transfer.
After the formation of the OOH* reaction intermediate, the reaction proceeds via a two- or four-electron pathway. The activation energy for the two-electron pathway was always higher than that of the four-electron pathway. The reduction of O* that followed the reduction of the reaction intermediate of OOH* to O* was also fast with a low activation energy (< 0.2 eV under ORR operating conditions).
Since the transition state required for the reduction of O* was close to the reactant, where the symmetry factor was almost 0 (Table 5.1), the activation energy was not significantly changed by changing the electrode potential. The water reorganization energy was therefore added to the activation energy for all the steps in the electrochemical reaction. In particular, the competing reaction step of the formation of OOH* required an activation energy of 0.35 eV.
The activation energy of the corresponding step (i.e., 0.64 eV) was comparable to that of Pt at an electrode potential of 0.9 V. This was caused by the difference in the symmetry factors of the two reaction paths. Thus, the change in the activation energy of the two-electron path was relatively larger as a function of electrode potential than the four-electron path.
Conclusion
COSMO: A new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient. Surface Pourbaix diagrams and oxygen reduction activity of Pt, Ag and Ni(111) surfaces studied by DFT. Active sites and mechanisms for oxygen reduction reaction on nitrogen-doped carbon alloy catalysts: Stone-Wales defect and curvature effect.
Single Mo1(Cr1) atom on nitrogen-doped graphene enables highly selective electroreduction of nitrogen to ammonia.
Summary and Future Perspectives
Summary
For the advanced electrocatalyst, more active sites were needed to investigate in the metal-free carbon nanomaterial. Calculation of the formation energy showed that the stability of carbon defects can be controlled by changing the external environment (i.e., the chemical potential of carbon), This may suggest a reasonable synthetic condition for obtaining a specific carbon defect. The electrocatalytic activity of internal carbon defects was evaluated thermodynamically using the computational hydrogen electrode (CHE) method.
In other words, the adsorption energies of the reaction intermediates were too weak to proceed with the reaction. However, it is also important to clarify the origin of the increased adsorption energy in the ISW defect. We performed the crystal orbital Hamiltonian population (COHP) analysis of the covalent bond between the intermediate and active site oxygens.
Next, we clarify that the s and pz orbitals of the active site strongly contribute to the interaction between the reaction intermediates and the catalyst surface. Finally, a kinetic study of the ORR on ISW defects was performed to gain deeper insight into the reaction mechanism, rate-determining step, and selectivity. After the adsorption of O2 molecules, the activation energies of the electrochemical reactions were calculated with a charge-extrapolation scheme.
The reaction rates for the formation of OOH*, O* and OH* were much faster than those of H2O2 and H2O. Among them, the production of H2O was evaluated as the rate-determining step for the ORR occurring on ISW defects. For the application in a proton exchange membrane fuel cell, the generation of H2O2 during ORR must be suppressed.
Future Perspectives
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*. Nguyen Dien Kha Tu†, Sung O Park†, Jaehyun Park, Youngsik Kim, Sang Kyu Kwak* and Seok Ju Kang*, “Pyridine nitrogen-containing carbon cathode: efficient electrocatalyst for seawater batteries”. 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 Hydrogen Evolution Reaction: Structure- activity correlations and kinetic insights”.
जियाउद्दीन खान†, सुंग ओ पार्क†, जुचन यांग, सेउंगयॉन्ग पार्क, रवि शंकर, ह्यून-कोन सॉन्ग, यंगसिक किम, सांग क्यू क्वाक*, और ह्यूनह्युब को*, "बायो-प्रेरित कृत्रिम से बाइनरी एन, एस-डोप्ड कार्बन नैनोस्फेयर "मेलानोसोम्स: समुद्री जल बैटरियों के लिए कुशल वायु इलेक्ट्रोड का मार्ग"। ग्युटे नाम†, येओंगुक सोन†, सुंग ओ पार्क†, वू चेओल जियोन, हासेओंग जांग, जूह्युक पार्क, सुजोंग चाए, युनशिन यू, जाचेन रयु मिन ग्यु किम*, संग क्यू क्वाक * और जैफिल चो*, जियाउद्दीन खान†, बास्कर सेंथिलकुमार†, सुंग ओ पार्क†, सेउंगयॉन्ग पार्क, जुचन यांग, जियोंग ह्योन ली, ह्यून-कोन सॉन्ग, यंगसिक किम, सांग क्यू क्वाक* और ह्यूनहुब को*, .
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*,. 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*, “Atomic Scattered Pt-N4 Sites as efficient and selective electrocatalysts for the chlorine evolution reaction". Heming Zhang, Sung O Park, Se Hun Joo, Jin Hyun Kim, Sang Kyu Kwak* and Jae Sung Lee*, "Precisely controlled, few-layer iron titanate inverse opal structure for enhanced photoelectrochemical water splitting".
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 with Active Sites in Micrometer-Size Polyhedral Pyrochlore Electrocatalyst". Senthilkumar, Sung O Park, Junsoo Kim, Soo Min Hwang, Sang Kyu Kwak*, and Youngsik Kim*, "Improvement of seawater battery performance enabled by an oxygen-deficient/edge-rich self-doped porous carbon electrocatalyst". 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".
