Young Jin Sa

G ENERAL I NTRODUCTION

F UEL C ELL

Introduction to Fuel Cell
Electrochemistry of PEMFC Electrode Reactions
Electrocatalysts for PEMFCs

Therefore, the kinetic loss is largely influenced by the exchange current (the activation energy) of the reaction. The final factor for the loss of fuel cell performance is diffusion loss caused by the limited mass transport of reactants and products.

W ATER E LECTROLYZER

Introduction to Water Electrolyzer
Electrochemistry of Water Electrolysis
Electrocatalysts for Water Electrolysis

Although Ru is a much cheaper element and more active for the OER than Ir, Ir-based OER catalysts are more desirable in terms of stability.64 However, such precious metals are expensive and very scarce in nature, making device operation uneconomical. . Volcano plot of (a) the exchange current density for the HER and the Gibbs free energy of hydrogen bonding (obtained from theoretical calculations) for some metals, and (b) the overpotential for the OER and the binding energy difference between O and OH absorbed species of some metal oxides.

C ARBON N ANOTUBES

Finally, the OER and HER catalysts operate at the same pH to implement the water electrolyzer. The development of active OER and HER catalysts in acidic and alkaline electrolytes, respectively, will be necessary for cheaper H2 production from water electrolysis.

A SSESSMENT OF E LECTROCATALYSIS

General Methodology
Selection of Reference Electrode
Selection of Counter Electrode
Working Electrode: Rotating (Ring) Disk Electrode
Electrolyte
Kinetics Analysis by Tafel Plot
Measurement of Benchmark Catalysts
Figure of Merit of the Activity

The measurement method is only systematically established for evaluating the Pt/C catalyst's ORR activity, supported by the USA. In the case of OER, no standard measurement protocol has been established either.

O UTLINE OF T HIS D ISSERTATION

In all cases, oxidation and capacitive currents in the OER region may overestimate the activity. ORR in single cell application: Current and power densities are typically compared to the 0.6 V cell voltage at which fuel cell systems typically operate.

R EFERENCES

We found that the ORR activity of CNT/PC is one of the highest compared to the reported M–N/C catalysts. The pH-dependent HER activity of the CNT/Co-PcC catalyst is shown in Figure 5.10a.

C ARBON N ANOTUBES /H ETEROATOM -D OPED C ARBON C ORE –S HEATH

E XPERIMENTAL M ETHODS

Synthesis of CNT/HDC Catalysts
Characterization Methods
Electrochemical Characterizations
RHE Calibration
Analysis of ORR Kinetics
AEMFC Performance Tests

200 mg of treated CNTs were suspended in 2 mL of IL (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; BMITFSI) and the mixture was ground in an agate mill for 15 minutes. 1 mL of formic acid was added to initiate gel formation on the silica gel.

R ESULTS AND D ISCUSSION

Synthesis of CNT/HDC Catalysts
Physicochemical Characterizations
ORR Activity in Half-Cell Configurations
AEMFC Performance

The formation of CNT/HDC nanostructures was observed using atomic resolution transmission electron microscopy (AR-TEM) (Figure 2.3). Summary of surface elemental composition of CNT/HDC nanostructures of HDC analyzed by XPS. Summary of textural properties of CNT/HDC, HDC, and acid-treated CNT nanostructures.

In particular, the exchange current density of the CNT/HDC-1000 is of the same order of magnitude as that of Pt/C. Finally, we tested the applicability of the CNT/HDC-1000 as a cathode catalyst for alkaline AEMFC.

C ONCLUSION

R EFERENCES

Ex situ Fe K-edge XANES spectra of the CNT/PC catalyst, comparative samples, and reference materials. ORR polarization curves of CNT/PC catalyst and control samples measured in (a) 0.1 M KOH and (b) 0.1 M HClO4. Significantly, the extrapolated volume current density at 0.8 V of the CNT/PC-based MEA in the PEMFC is 320.

The reaction kinetics of the CNT/Co-PcC catalyst with ∼100% Co-Nx sites was investigated by pH-dependent HER activity measurements. This is consistent with the appearance of the leading edge peak A in the XANES spectrum of CNT/Co-PcC-1 (Figure 5.3a).

S TRATEGY FOR P REFERENTIAL G ENERATION OF A CTIVE Fe–N X S ITES FOR

E XPERIMENTAL M ETHODS

Synthesis of CNT/PC Catalysts
XAS Experiments
RHE Calibration
Four-Electron Selectivity Evaluation
AEMFC Performance Tests
PEMFC Performance Tests

To evaluate the four-electron selectivity, the Pt ring potential was fixed at 1.30 V (vs. RHE) during the LSV scan for ORR. During these procedures, care must be taken not to scratch the surface of the polymer membrane. During sonication, the catalyst slurry was stirred with a stiff rod every 1 hour to minimize agglomerate attached to the side wall of the vial.

All single-cell experiments were performed at 80 °C, and the temperature of the gas lines to the anode and the cathode was always set at 10 °C above the temperature of the humidifier to avoid condensation of water vapor. The temperature of the gas lines to the anode and the cathode was always set 10 °C above the temperature of the humidifier to avoid condensation of water vapor.

R ESULTS AND D ISCUSSION

Synthesis Optimization of CNT/PC Catalysts
Physicochemical Characterizations
XAS Analysis
ORR Activity in Half-Cell Configurations
Effect of Catalyst Surface Area
Electrochemical In Situ XAS
TOF Calculations
AEMFC and PEMFC Performances
Role of the Silica Coating
Generalization of the Silica Coating to Other Carbon Supports
Catalytic Role of Fe–N x Sites and Fe/Fe 3 C NPs

To examine the role of silica layers, CNT/PC was prepared without the silica coating step (CNT/PC_w/o SiO2). The TEM image of CNT/PC_w/o SiO2 (Figure 3.7a) shows that the porphyrinic carbon layers were not uniformly formed on the CNTs. The formation of Fe-based NPs is not evident as in the case of CNT/PC_w/o SiO2.

Relative amount of Fe-Nx sites to Fe-based particles in CNT/PC and CNT/PC_w/o SiO2. ORR polarization curves of CNT/PC and CNT/PC_w/o SiO2 catalysts pyrolyzed at different temperatures measured in (a) 0.1 M KOH and (b) 0.1 M HClO4.

C ONCLUSION

Electron transfer numbers of carbon/PC and carbon/PC_w/o SiO2 catalysts measured in (a) 0.1 M KOH and (b) 0.1 M HClO4.

R EFERENCES

The coexistence of Co–Nx and Co@C sites in Co–N/C catalysts hinders the identification of the active sites for the HER. To measure the activation energy for the HER, the HER activity of the CNT/Co-PcC-1 catalyst was tested in temperature-controlled electrolytes and 55 °C). Therefore, Co–Nx sites contribute to the high HER activity while the presence of the Co@C species accounts for the lower bulk activity.

HER polarization curves of CNT/Co-PcC-1 catalyst at different reaction temperatures measured in (a) 0.5 M H2SO4 and (b) 1 M KOH. Pyrolysis of a mixture of CoIIPc and CNTs under the protection of a silica layer gave a CNT/Co-PcC catalyst with the exclusive presence of Co–Nx sites.

I N S ITU X-R AY A BSORPTION S PECTROSCOPY S TUDY ON O XYGEN R EDUCTION AND

E XPERIMENTAL M ETHODS

Synthesis of Size-Controlled CoO x NPs
Preparation of CoO x /CNTs
Synthesis of Bulk-CoOOH
XAS Experiments

The suspension was filtered, washed with copious amounts of DI water until the pH of the filtrate reached ∼7, and dried at 60 °C. First, 350 mg of acid-treated CNTs were dispersed in 50 mL of chloroform in a 100 mL Erlenmeyer flask. To estimate the kinetics for the ORR, the kinetic flux was extracted from the following equation.

A logarithmic plot of kinetic current density (measured current density in the case of OER) versus overvoltage gives a linear Tafel plot. The four-electron selectivity of CoOx/CNT was analyzed by the RRDE technique and calculated using the given equation.

R ESULTS AND D ISCUSSION

Synthesis and Characterization of CoO x /CNTs
In Situ XAS Study
Electrochemical Redox Behavior by CV
Size-Dependent ORR and OER Activities
OER Stability Test and Post Mortem XPS Analysis

Then, the change in the local environment was further observed by in situ EXAFS of the CoOx(4.3)/CNTs (Figure 4.7b). From these observations, we concluded that the phase of the CoOx NPs changed to CoOOH under OER conditions. Therefore, the experimental conditions for in situ XAS could not reflect the initial Co oxidation state of the CoOx/CNTs.

Electrocatalytic OER and ORR activities of the CoOx/CNTs were measured using RRDE in 0.1 M KOH (Figure 4.11). For the CoOx(4.3)/CNTs, the Co 2p XPS spectra before and after the OER almost overlapped.

C ONCLUSION

R EFERENCES

We found that the silica coating before the pyrolysis is critical to produce CNT/Co-PcC catalyst consisting mainly of Co-Nx sites without Co@C sites. The EXAFS analysis confirms that CNT/Co-PcC-1 catalyst mainly consists of Co-Nx sites. CNT/Co-PcC-1 catalyst is mainly composed of Co-Nx sites, while the 1_w/o SiO2 catalyst contains significant amounts of Co@C species.

Besides Co-Nx and Co@C sites, CNT/Co-PcC-1 and 1_w/o SiO2 also contain N-doped carbon species (C-N). We performed the HER measurements with the CNT/Co-PcC-1 catalyst with ∼100% Co-Nx sites in acidic and alkaline electrolytes with different pH values.

S TRUCTURE –A CTIVITY C ORRELATION AND K INETIC I NSIGHTS FOR H YDROGEN

E XPERIMENTAL M ETHODS

Synthesis of CNT/Co-PcC Catalysts
XAS Experiments

The suspension was filtered, washed with copious amounts of DI water until the pH of the filtrate reached 7, and dried at 60 °C. The incident photon energy was then calibrated using a standard Co foil where the maximum of the first derivative of absorption of the Co foil is located at 7709 eV. 8 μL of the catalyst ink was deposited onto glassy carbon (GC) disc (5.61 mm in diameter) using a micropipette and dried at RT.

Before each experiment, the temperature of the electrochemical cell was immersed in a water bath for at least 15 min to reach a temperature equilibrium. The linear region of the Tafel plot was extrapolated to the zero overpotential point to obtain the exchange current, according to the following Tafel equation.

R ESULTS AND D ISCUSSION

Synthesis and Characterization of CNT/Co-PcC Catalysts
HER Activity of CNT/Co-PcC Catalysts
Control of Active Site Density
Structure–Activity Correlation
Reaction Kinetics Study
Durability and Stability Tests

First, we increased a mass ratio of CNT to CoIIPc in the precursor mixture to 3.0 to yield CNT/Co-PcC-3 catalyst. RDFs of k3-weighted EXAFS spectra and EXAFS fit of (a) CNT/Co-PcC-1 series and (b) CNT/Co-PcC-3 series. Therefore, pH dependence experiments combined with the Tafel slope analyzes indicate that the RDS for the HER on our CNT/Co-PcC catalyst is the first hydrogen adsorption (Volmer step).

We then investigated the temperature-dependent HER activity to access the activation energy of the CNT/Co-PcC-1 catalyst. The activation energies of CNT/Co-PcC-1 were compared with those of representative catalysts (Table 5.5).

C ONCLUSION

R EFERENCES

26] Jongsik Park†, Young Jin Sa†, Hionsuck Baik, Taehyun Kwon, Sang Hoon Joo*, and Kwangyeol Lee* (†equal contribution). 23] Taehyun Kwon†, Hyeyoun Hwang†, Young Jin Sa†, Jongsik Park, Hionsuck Baik, Sang Hoon Joo* and Kwangyeol Lee* (†equal contribution). 16] Jisun Yoon†, Jongsik Park†, Young Jin Sa†, Yoojin Yang, Hionsuck Baik, Sang Hoon Joo* and Kwangyeol Lee* (†equal contribution).

12] Jongwoo Han, Young Jin Sa, Yeonjun Shim, Min Choi, Noejung Park, Sang Hoon Joo* og Sungjin Park*. 4] Young Jin Sa†, Kyungjung Kwon†, Jae Yeong Cheon, Freddy Kleitz og Sang Hoon Joo* ( †lige bidrag).

S UMMARY AND S UGGESTIONS FOR F UTURE W ORKS

S UGGESTIONS FOR F UTURE W ORKS

Although we have demonstrated spectroscopic evidence of the formation of the axial bond of silica to Fe–. N4 site in Fe porphyrin (Chapter 3), detailed molecular structure after the silica coating is still unknown, which may provide important insight into the role of silica. We hypothesize that the silica coating strongly affects the thermal decomposition behavior of the Fe-porphyrin precursor during the pyrolysis.

From the perspective of practical application in fuel cells, Fe-N/C catalysts are the most promising candidates as replacements for Pt/C. The rapid deactivation of the Fe-N/C-based single cell is known as membrane (Nafion) deterioration by hydro(pero)xyl free radicals.6 The radicals are formed by the Fenton reaction of peroxide intermediates, which are formed by the less efficient 2-electron ORR, with Fe2+ and Fe3+.

R EFERENCES

27] Yeonjun Shim, Young Jin Sa, Yunseok Shin, Junghoon Oh, Hyunchul Ju, Sang Hoon Joo en Sungjin Park*. 19] Aram Oh†, Young Jin Sa†, Hyeyoun Hwang†, Hionsuck Baik, Jun Kim, Byeongyoon Kim, Sang Hoon Joo* en Kwangyeol Lee*. 17] Bora Seo †, Young Jin Sa†, Jinwoo Woo, Kyungjung Kwon, Jongnam Park, Tae Joo Shin, Hu Young Jeong*, Sang Hoon Joo* (†gelijke bijdrage).

15] Nam-In Kim, Young Jin Sa, Sung-Hwa Cho, Insub So, Kyungjung Kwon, Sang Hoon Joo and Jun-Young Park (permanent contributions). 7] Young Jin Sa†, Chiyoung Park†, Hu Young Jeong, Seok-Hee Park, Zonghoon Lee, Kyuoung Taek Kim, Gu-Gon Park and Sang Hoon Joo* (†permanent contributions).