Carbon-based electrode materials for application in the electrochemical water splitting process

The synthesis and characterization of N-HTC materials for the application in the water electrolysis are the main focus of this work. N-incorporation is of fundamental importance throughout this work since, first of all, N is an n-type dopant by acting as electron-donor. The technique of N-doping tunes the physicochemical material properties. It is cheap, non-toxic and has the advantage to tailor the desired electrical, mechanical, optical, magnetic, structural, morphological or chemical properties. N-doping may improve conductivity, surface wettability, catalytic or storage characteristics. Besides, N-doping creates active sites and thus might enhance electrocatalytic activity as well as long-term electrochemical stability. The hydrothermal synthetic route has been applied based on the precursors glucose and urotropine. During hydrothermal synthesis, urotropine together with glucose undergoes a plethora of complex reaction pathways. Ammonia, in particular, the decomposition product of urotropine, contributes to a wide range of reaction mechanisms. The molar ratio of glucose and urotropine has been modified in order to achieve a steady increase of the N-content. Urotropine was proved to be a highly effective N-precursor. With raising the mass fraction of urotropine towards glucose, maximal N-proportion of 19 wt% can be achieved. The pH plays a major role during synthesis and has a high impact in the reaction mechanisms of the corresponding reaction steps. Based on HTC chars, to date, no systematic pH investigations exist by modification of the molar ratios of the applied precursors. In this work, systematic studies on the pH have been performed. By variation of the molar ratio of urotropine to glucose the pH can be tuned due to the decomposition of the N-precursor urotropine into ammonia and formaldehyde. The pH changes to a more alkaline behavior if the molar ratio of urotropine to glucose is increased. Reversely, rising the molar ratio of glucose to urotropine or if only glucose is used as starting material, the pH tend to be more acidic. In both cases ≤ C6 products are obtained. At pH 7 the degradation via HMF is depressed and reducing effects yields deprotonations or benzylic acid-type rearrangements. Decarbonylations and decarboxylations arise only to a lesser extent. Besides, fragmentations are the predominant reactions enclosing β-elimination, retro-aldol fragmentation or hydrolytic β-dicarbonyl fragmentation. Throughout this work, both reaction pathways have been selected in order to analyze the degree of internal condensation of the polymeric carbon network, the preferential structural moieties or functional groups that have been formed. These special features are the main reason for the high diversity of organic compounds, functional groups and numerous structural motifs in the hydrochar. 
 A large variety of characterization techniques have been employed. It is the first comprehensive analytical study of a series of as-synthesized N-containing hydrothermal carbon (N-HTCs), consisting of analytical data of UV/VIS, HPLC, optical microscopy, SEM/(HR)TEM, BET, elemental analysis, FTIR, electronic structure calculations, TG-MS, zeta potential, acid-base titration, Raman, XRD, EELS and solid-state NMR. Due to the combination of these characterization techniques, scientific findings on the complex molecular structure could be gained. In N-free HTCs, solely O-functional groups exist, for the most part furan-based functions with aliphatic compounds acting as linkers. In contrast to it, N-HTCs consist of a large number of both, O- and N-functional groups. Only by adding higher amount urotropine, more temperature stable N-based structural elements such as pyrrole and pyridine are formed. 
 The N-HTC disc electrodes are subjected to electrocatalytic investigations with regard to water splitting process. Electrochemical characterization studies include cyclic voltammetry (CV), modular potentiometry (MP, stationary polarization), chronopotentiometry (CP) and chronoamperometry (CA) to investigate electron transfer processes, electrochemical activity as well as stability. The pellets exhibit a remarkable mechanical stability. It is the very first approach in which no binder needed to be used to stabilize the material for electrochemical applications. Disc electrodes at high anodic-potentials are tested under oxygen evolution reaction (OER) conditions in alkaline media according to a standardization protocol. N-free disc electrodes are most susceptible to carbon corrosion whereas the incorporation of N improves the material properties. The higher the amount of N is, the higher the electrochemical activity and stability. Besides, the electrolyte undergoes a color change turning from colorless into dark brown. The brown substance was isolated and is presumed to be a polymeric humic acid-like organic compound.