Investigation of the water splitting reactions on multi-heteroatom doped cobalt-based carbon catalysts

In the context of energy transition, hydrogen is addressed as a clean future energy carrier that enables the emission-free production of energy to become independent of fossil fuels. The development of hydrogen production technologies like the electrolysis is strongly driven by the two main criteria: sustainability and economy. Over the past decades, various technological achievements resulted in a reduction of costs which has dramatically improved the economic potential of hydrogen produced by electrolysis. Especially in the field of material development, great effort was devoted to replace the precious state of the art catalyst materials with abundant cost-effective catalysts accelerating sluggish water splitting reactions. This dissertation focuses on the investigation of carbon-based cobalt catalysts with multi-heteroatom doping for the oxygen evolution reaction (OER) and hydrogen evolution reactions (HER). Within this study, two major synthesis approaches, one with metal organic framework (MOF) and another with polyaniline (PANI), were investigated in terms of structural and electrochemical characterization. Moreover, the catalysts were analyzed in detail by active site identification and mechanistic understanding of the reactions within the scope of each project. Within the MOF approach, the role of the metal species on HER activity was investigated using X-ray photoelectron spectroscopy (XPS). The discussion was further supported by density functional theory (DFT) calculations resulting in structure-activity correlations with emphasis on the importance of the nature of the metal. Besides, bimetallic catalysts with optimal hydrogen binding energies were suggested as a promising active catalyst toward HER . The PANI approach was proposed to investigate multi heteroatom doping influence on the catalytic activity and material properties. Within this approach, cobalt catalysts with variation of cobalt loading and sulfur loading in the precursors were prepared. The catalysts were highly active toward both HER and OER, though the origin of activity might be different. Several physico-chemical characterization techniques combined with post mortem analysis were carried out in order to get insight into the origin of the activity. It was found that the high HER activity of PANI-based samples is attributed to MeN4 sites, and the OER activity is originating from a hybrid cobalt complex depending on the synthesis route

Investigation of the water splitting reactions on multi-heteroatom doped cobalt-based carbon catalysts

In the context of energy transition, hydrogen is addressed as a clean future energy carrier that enables the emission-free production of energy to become independent of fossil fuels. The development of hydrogen production technologies like the electrolysis is strongly driven by the two main criteria: sustainability and economy. Over the past decades, various technological achievements resulted in a reduction of costs which has dramatically improved the economic potential of hydrogen produced by electrolysis. Especially in the field of material development, great effort was devoted to replace the precious state of the art catalyst materials with abundant cost-effective catalysts accelerating sluggish water splitting reactions. This dissertation focuses on the investigation of carbon-based cobalt catalysts with multi-heteroatom doping for the oxygen evolution reaction (OER) and hydrogen evolution reactions (HER). Within this study, two major synthesis approaches, one with metal organic framework (MOF) and another with polyaniline (PANI), were investigated in terms of structural and electrochemical characterization. Moreover, the catalysts were analyzed in detail by active site identification and mechanistic understanding of the reactions within the scope of each project. Within the MOF approach, the role of the metal species on HER activity was investigated using X-ray photoelectron spectroscopy (XPS). The discussion was further supported by density functional theory (DFT) calculations resulting in structure-activity correlations with emphasis on the importance of the nature of the metal. Besides, bimetallic catalysts with optimal hydrogen binding energies were suggested as a promising active catalyst toward HER . The PANI approach was proposed to investigate multi heteroatom doping influence on the catalytic activity and material properties. Within this approach, cobalt catalysts with variation of cobalt loading and sulfur loading in the precursors were prepared. The catalysts were highly active toward both HER and OER, though the origin of activity might be different. Several physico-chemical characterization techniques combined with post mortem analysis were carried out in order to get insight into the origin of the activity. It was found that the high HER activity of PANI-based samples is attributed to MeN4 sites, and the OER activity is originating from a hybrid cobalt complex depending on the synthesis route

On the effect of sulfite ions on the structural composition and ORR activity of Fe-N-C catalysts

Fe-N-C catalysts are the most promising group of non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). This study focusses on two different porphyrin-based Fe-N-C catalysts and a Fe-N-C catalyst prepared from alternative precursors under S-addition. Catalysts are subjected to a wet-chemical poisoning treatment by sulfite ions . A mechanism for the deactivation process of the active sites is proposed. ORR activity is evaluated for the original catalysts (OC) and for the poisoned catalysts in 0.1 M H2SO4. In addition, the structural composition of the catalysts is identified by Mobauer spectroscopy. Our results show that the sulfite ions bound irreversible to the catalysts and the catalysts lose significant fractions of their ORR activity while in Mobauer spectroscopy a new doublet appears. Based on the results, possible models for the binding of the ambident sulfite ion to the FeN4 centers are discussed

Influence of sulfur in the precursor mixture on the structural composition of Fe-N-C catalysts

Fe-N-C catalysts were prepared by a new synthesis protocol at 800 ∘C with subsequent acid leaching. The effect of sulfur was investigated by a systematic study in which the molar S/Fe ratio in the precursor was varied from 0.0 to 2.45. The obtained catalysts were evaluated for their ORR activity in 0.1 M H2 SO 4. In addition, the specific BET surface area was determined from N2 sorption measurements and structural characterization was made by Mößbauer spectroscopy. Catalysts contain FeN4 moieties and inorganic iron species. Structure activity correlation indicate a dominance of the ferrous low-spin FeN4 site for the ORR activity. This is in agreement with previous findings. In addition, the optimum in terms of ORR activity is in the same S/Me range as found for porphyrin-based catalysts. However, in contrast to previous conclusions of an avoidance of iron carbide formation by sulfur addition, a very high S/Fe ratio is required to obtain a catalyst free of iron carbide. Further work is required to identify the parameter that indeed enables inhibition of iron carbide formation

Elucidating the Origin of Hydrogen Evolution Reaction Activity in Mono- and Bimetallic Metal- and Nitrogen-Doped Carbon Catalysts (Me-N-C)

In this work, we present a comprehensive study on the role of metal species in MOF-based Me-N-C (mono- and bimetallic) catalysts for the hydrogen evolution reaction (HER). The catalysts are investigated with respect to HER activity and stability in alkaline electrolyte. On the basis of the structural analysis by X-ray diffraction, X-ray-induced photoelectron spectroscopy, and transmission electron microscopy, it is concluded that MeN4 sites seem to dominate the HER activity of these catalysts. There is a strong relation between the amount of MeN4 sites that are formed and the energy of formation related to these sites integrated at the edge of a graphene layer, as obtained from density functional theory (DFT) calculations. Our results show, for the first time, that the combination of two metals (Co and Mo) in a bimetallic (Co,Mo)-N-C catalyst allows hydrogen production with a significantly improved overpotential in comparison to its monometallic counterparts and other Me-N-C catalysts. By the combination of experimental results with DFT calculations, we show that the origin of the enhanced performance of our (Co,Mo)-N-C catalyst seems to be provided by an improved hydrogen binding energy on one MeN4 site because of the presence of a second MeN4 site in its close vicinity, as investigated in detail for our most active (Co,Mo)-N-C catalyst. The outstanding stability and good activity make especially the bimetallic Me-N-C catalysts interesting candidates for solar fuel applications

Adding a New Member to the MXene Family: Synthesis, Structure and Electrocatalytic Activity for the Hydrogen Evolution Reaction of V4C3Tx

Two-dimensional transition-metal-based carbides (or nitrides), so-called MXenes, that can be derived from the three-dimensional MAX phases, have attracted considerable attention throughout the past couple of years. The particular structure together with their hydrophilic and metallic nature make them promising candidates for a plethora of applications, such as sensors, electrodes, and catalysts. Obviously, the respective chemical and physical properties are highly dependent on the chemical composition, stoichiometry, and surface structure of the MXene. Here, we introduce a new member of the MXene family, V4C3Tx (T representing the surface groups), based on the chemical exfoliation of the 413 MAX phase V4AlC3 by treatment with aqueous hydrofluoric acid. X-ray powder diffraction data together with scale-bridging electron microscopy studies prove the successful removal of aluminum from the MAX phase structure. The electrocatalytic activity for the hydrogen evolution reaction of this new MXene is tested in acidic solution over the course of 100 cycles. Interestingly, we find a significant improvement of the catalytic performance over time (i.e., the overpotential required to achieve a current density of 10 mA cm–2 decreases by almost 200 mV) that we assign to the removal of an oxide species from the surface of the MXene, as shown by XPS measurements. Our study provides crucial experimental data of the electrocatalytic activity of MXenes together with the evolution of its surface structure that is also relevant for other transition-metal-based MXenes in the context of further potential applications

Effect of metal species on the stability of Me-N-C catalysts during accelerated stress tests mimicking the start-up and shut-down conditions

Currently, Me-N-C catalysts are the most prominent alternative to Pt/C catalysts for the oxygen reduction reaction in acidic media. It is well known that the achievable activity and selectivity strongly correlates with the nature of metal species. However, so far the effect of the metal species on the stability of these catalysts was not investigated systematically. In this work, a group of 13 different Me-N-C catalysts were investigated with respect to their activity and stability in accelerated stress tests mimicking the start-up and shut-down conditions (AST_SSC). A strong correlation between the nitrogen content assigned to different MeN4 sites and the D3 band from Raman spectroscopy is found. Moreover, we were able to correlate changes in the D3 band and variations in the displacement of the metal atoms out of the N4 plane with the losses in ORR activity. Based on these findings, we propose a model for the degradation of Me-N-C catalysts during accelerated stress tests mimicking the start-up and shut-down conditions

Improved electrochemical performance of Fe-N-C catalysts through ionic liquid modification in alkaline media

It is well known that Fe-N-C catalysts reach a significantly better ORR activity in alkaline compared to acidic electrolyte. This advantage makes the material of interest for application in alkaline fuel cells. Beside this, for Pt/C catalyst it is known that the performance in acid can be significantly enhanced through ionic liquid modification following the Solid Catalysts with Ionic Liquid Layer (SCILL) concept. In our current study we combine both advantages and investigate for two Fe-N-C catalysts prepared either with or without sulfur in the precursor mixture the effect of IL modification. The unmodified catalysts are characterized using X-ray induced photoelectron spectroscopy (XPS), 57Fe Mößbauer and Raman spectroscopy as well as N2 sorption. The electrochemical behavior of the unmodified catalyst and with different pore-filling degrees of ionic liquid (IL) is analysed with respect to double layer capacitance, ORR activity and stability in accelerated stress tests mimicking the load-cycle conditions