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@PHDTHESIS{Park:818129,
author = {Park, Heeyong},
othercontributors = {Granwehr, Josef and Klankermayer, Jürgen and Schlögl,
Robert},
title = {{NMR} studies of hydrothermal carbon materials as
electrocatalytic electrodes for water splitting},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2021-04308},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2021},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2021},
abstract = {Hydrogen has emerged as an essential part of the green
energy system needed to ensure a sustainable future. The
production of clean hydrogen using water-splitting with
renewable energy, combined with the urgency to reduce
greenhouse gas emissions, has received unprecedented
interest in politics and business. In this regard,
carbon-based materials that own unique advantages, including
structure diversity, good electrical conductivity, and
mechanical strength, have been widely investigated in
water-splitting. One of the synthetic techniques for
producing carbon materials, the hydrothermal carbonization,
allow tailoring the desired morphology, chemical
composition, and structure. The main focus of this work is
to understand the properties of Hydrothermal Carbon (HTC)
materials for the application as an electrocatalytic
electrode for water-splitting, using various NMR techniques
(i.e., Solid state NMR techniques, NMR dynamics techniques,
and Dynamic Nuclear Polarization (DNP) NMR techniques,
etc.). The fundamental understanding regarding the chemical
structure of materials and the water interaction with its
surface is essential in applying a knowledge-based approach
to the development of electrodes for water-splitting. NMR
methods to investigate the water interaction behavior of
various HTC types was developed using water dynamics, and it
was confirmed that the information regarding water
interaction was straight correlated to the electrochemical
properties in water-splitting. It was also investigated if
the introduction of nitrogen into HTC improves water
interaction. The presence of N-functional groups influences
the water interaction with (N)-HTC (HTC and
nitrogen-functionalized HTC(N-HTC)) more strongly than
surface area, pore size distribution, or oxygenated
functional groups. Furthermore, the degree of water
interaction can be tuned by adjusting the synthesis
temperature and the precursor ratio of glucose and
urotropine. Due to the high hydrophilicity, N-HTC can have
internal water in a near-surface layer inside the particles,
whereas HTC has no internal water. Surprisingly, the main
difference of HTC compared to conventional carbon materials,
such as graphene or carbon nanotubes, is the much greater
incorporation of oxygen functional groups, but the
oxygenated functionalities in HTC play a minor role in the
interaction with water. The cause of poor water interaction
behavior for HTC was revealed through NMR experiments with
non-degassed and degassed water. HTC, as revealed by
longitudinal (T1) relaxation time and diffusion NMR, can
adsorb O2 from non-degassed water, which leads to a decrease
in the degree of water interaction. On the other hand, when
HTC is present in degassed water without O2, the water
interaction with HTC increases dramatically. It suggests
that the sites related to O2 adsorption in HTC are connected
to that involved in the water interaction. Interestingly,
contact with water activates the ability of O2 adsorption in
HTC. This suggests the application of the HTC material as O2
scavenger in water, but not from air; therefore, it can
easily be handled in air. This could be another HTC
application. The use of NaOH as an activating agent for the
manufacture of activated carbon has attracted great interest
due to the valuable properties of the materials produced by
this process. NMR was applied to reveal not only why a
NaOH-activated carbon (Na-AC) exhibits a high catalytic
activity (current density of up to 30 mA/cm2) and excellent
stability (lifetime test: 48 h) compared to non-treated one
(Non-AC), but also the mechanism of chemical activation by
NaOH. The treatment with NaOH was found to induce changes on
the surface oxygen functionalities, leading to a higher
amount of carboxylic acid and lactone functional groups,
thereby improving water interaction, which plays a key role
in the catalytic process. In the non-washed Na-AC electrode,
sodium exists in the structure of sodium carbonate, whereas,
in the washed Na-AC electrode, residual Na was presented as
complex Na-containing surface groups at the edges of the
aromatic lamellae inside the pores. In particular, when the
non-washed Na-AC electrode was in water or 0.1M KOH
electrolyte, the Na-AC trapped hydrated Na ions well in the
pores, and the exchange rate between in-pore water and
ex-pore water was slower than Non-AC. Understanding
surface-activity relationships of materials is a promising
topic for revealing their working mechanisms in different
applications as well as for optimization with improved
properties. DNP-enhanced solid-state NMR was performed to
probe the surface of N-HTC selectively at atomic resolution.
In a typical magic-angle spinning (MAS) DNP experiment,
several mechanisms are simultaneously involved when
transferring much larger polarization of electron spins to
NMR active nuclei of interest. Recently, spontaneous 1H-13C
cross-relaxation induced enhancement (CRE) effect under DNP
by active motions that was not frozen out at even 100 K was
reported as one of the mechanisms. Based on this, the
spontaneous 1H-15N CRE effect under DNP was first
demonstrated using both primary ammonium and amine
structures in model compounds during 15N DPMAS DNP. The
influence on CRE efficiency caused by variation of the
radical solution composition and by temperature was also
investigated. Notably, the structural depth-profiling with
the CRE effect under DNP has been demonstrated using 13C/15N
fully labeled N-HTC having effective reorientation dynamics
of methyl and amine groups on the particle surface. CRE
contributions on the signal can be determined as a function
of polarization time, which is then associated with the spin
diffusion length inside the particle. This selectivity
showed differences in structural distributions between
surface and bulk, with carbonyl and amide groups having a
higher concentration near the surface of N-HTC. This new
approach could be beneficial for low surface area materials
that have been challenging for DNP applications, like N-HTC
(0.7 m2/g). Furthermore, this approach may also be extended
to systems where the reorientation dynamics of protons on
the surface are not available. By the intentional
introduction of isotope-labeled probe molecules with
rotatable protons on the surface of a material, surface
polarization could be enabled.},
cin = {150000 / 155520 / 154310},
ddc = {540},
cid = {$I:(DE-82)150000_20140620$ / $I:(DE-82)155520_20160614$ /
$I:(DE-82)154310_20190725$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2021-04308},
url = {https://publications.rwth-aachen.de/record/818129},
}
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