ABSTRACT
Perovskite solar cells (PSCs) have attracted great research interest in the scientific community due to their extraordinary optoelectronic properties and the fact that their power conversion efficiency (PCE) has increased rapidly in recent years, surpassing other 3rd generation photovoltaic (PV) technologies. Graphitic carbon nitride (g-C3N4) presents exceptional optical and electronic properties and its use was recently expanded in the field of PSCs. The addition of g-C3N4 in the perovskite absorber and/or the electron transport layer (ETL) resulted in PCEs exceeding 22%, mainly due to defects passivation, improved conductivity and crystallinity as well as low charge carriers' recombination rate within the device. Significant performance increase, including stability enhancement, was also achieved when g-C3N4 was applied at the PSC interfaces and the observed improvement was attributed to its wetting (hydrophobic/hydrophilic) nature and the fine tuning of the corresponding interface energetics. The current review summarizes the main innovations for the incorporation of graphitic carbon nitride in PSCs and highlights the significance and perspectives of the g-C3N4 approach for emerging highly efficient and robust PV devices.
ABSTRACT
Fe-N-C catalysts are very promising materials for fuel cells and metal-air batteries. This work gives fundamental insights into the structural composition of an Fe-N-C catalyst and highlights the importance of an in-depth characterization. By nuclear- and electron-resonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of α-iron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in Fe-N-C catalysts prepared by pyrolysis at 900 °C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with end-on bonded oxygen as one of the axial ligands.
ABSTRACT
Today, metal-N- as well as metal-S-doped carbon materials are known to catalyze the hydrogen evolution reaction (HER). However, especially N- and S-co-doped catalysts reach highest activity, but it remains unclear if the activity is related to MNx or MSy (M=metal) sites. In this work we apply a simple method for multi-heteroatom doping and investigate the effect of cobalt content on the HER in acidic medium. The CoNx and CoSy sites were evidenced on the basis of structural characterization by Raman, X-ray induced photoelectron spectroscopy, and TEM. The presence of sulfur enables the formation of a larger number of CoNx sites. Structure-performance relationship proves that the HER activity is dominated by CoNx rather than CoSy sites. The most active catalysts also exhibit an excellent stability under galvanostatic conditions making them of interest for electrolyser application.
ABSTRACT
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.
