Engineering inorganic nanostructured composites for boosting H2 and O2 evolution reactions

Getachew Solomon
Abstract & Cover

Hydrogen is considered a promising energy source with zero emission of CO2; it can provide higher energy density compared to other sources of energy. The amount at which H2 is produced, and the method of production need further improvement for the advancement of hydrogen energy technologies. Water electrolysis using renewable energy sources such as electrical, solar, and wind energy is one of the alternative technologies that can produce pure H2. However, water electrolysis itself is not an easy process, it requires a highly active catalyst capable of converting water into hydrogen, and oxygen.
This Ph.D. dissertation mainly focuses on developing efficient, robust, and low-cost catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and Oxygen reduction reaction (ORR). The work describes different strategies for improving the performance of the catalyst, such as creating nanocomposite, Nobel metal decoration, core-shell structures, hierarchical nanostructure, and cocatalyst and protective layers, which are vital for improving the efficiency of the catalyst. Consequently :
Nanocomposites composed of Ag2S nanoparticle, MoS2, and reduced graphene oxide (RGO) flake, with a 0D/2D/2D interface were synthesized. Ag2S nanoparticles were homogeneously distributed and embedded in a layer of semi-crystalline MoS2 nanosheets. The ternary catalyst results in a superior performance due to the intimate contact created by the 2D-2D interface (MoS2/RGO) and due to the uniformly grown Ag2S nanoparticles, which provides the ease of hydrogen adsorption by modulating the electronic properties, and exposure of highly rich active sites
Nobel metal decorated (Ag-decorated vertically aligned MoS2 nanoflakes) were developed and investigated for OER and ORR. Results of this work revealed that, due to the presence of silver, the catalyst shows more than 1.5 times an increase in the roughness-normalized rate of OER. Based on the rate constant values obtained during the ORR test, Ag-MoS2 proceeds through a mixed 4 electron and a 2 + 2 serial route reduction mechanism, suggesting that the presence of silver decreases the electron transfer number and increases the peroxide yield. 
A core-shell structure of hydrous NiMoO4 micro rods conformally covered by Co3O4 nanoparticles was developed and employed as an OER catalyst, showing a remarkable catalytic activity towards OER with a record low overpotential of 120 mV at 10 mA/cm2. Here, the strong interactions between core (hydrated NiMoO4) and shell (Co3O4) help to tune the electronic properties by modifying the active sites densities of the surface.
A hierarchical nanostructure composed of NiMoO4 nanorods and MoS2 nanosheets was synthesized on interconnected nickel foam substrates. The as-prepared hierarchical structure exhibits excellent OER performance due to its numerous exposed active sites for adsorbing oxygen intermediates which are beneficial for promoting the enhancement of the OER catalytic performance
Cu2O photocathode protected by a very thin layer of TiO2 and an amorphous Vox were synthesized and used for HER, with aim of improving the photostability of Cu2O. Photooxidation of Cu2O nanowires are minimized by growing TiO2 protective layer and an amorphous VOx cocatalyst. After optimization of the overlayer and the cocatalyst, the photoelectrode exhibits a stable photocurrent density for an extended illumination time. 
Besides, advanced characterization tools were used for tracking ORR reaction intermediates and OER active sites. RRDE, Operando Raman, and synchrotron-based photoemission spectroscopy analysis were utilized together with Post OER characterization tools to reveal the reason behind the higher catalytic activity of the catalyst. 
In summary, the presented outcomes can significantly contribute to the fundamental insight towards improving the efficiency of HER, OER, and ORR catalyst, by offering a clear and in-depth understanding of the preparation and characterization of cheap and efficient catalysts.

Luleå University of Technology
(Luleå, Sweden)
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