NMR studies of hydrothermal carbon materials as electrocatalytic electrodes for water splitting
Park, Heeyong; Granwehr, Josef (Thesis advisor); Klankermayer, Jürgen (Thesis advisor); Schlögl, Robert (Thesis advisor)
Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2021
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.