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Abstract
Hydrogen has been the focus of research as an ideal green energy source because it has high energy density and zero consumption pollution. Today, electrocatalytic water splitting is widely regarded as the most promising technology for green hydrogen production, but the low efficiency and high cost hindered its commercial application. Today, a lot of research has been done to develop high-efficient and low-cost electrocatalysts for the two half reactions (hydrogen evolution reaction(HER)and oxygen evolution reaction (OER)) of the whole water splitting process. However, the current progress of OER is far from commercial requirements due to its sluggish kinetics and complex four electro/proton transfer process. Therefore, it is of great importance to develop high-active and low-cost electrocatalysts for OER. Among all candidates, transition metal oxides (TMOs) are widely regarded as promising OER catalysts. This thesis is about the fabrication of TMOs electrocatalysts for OER and the mechanisms behind the high OER performance.
Firstly, the results on SrCo0.85Fe0.1P0.05O3−δ nanofilm on nickel foam(SCFP-NF)fabricated by pulsed-laser deposition (PLD) were reported. The SCFP-NF exhibited excellent OER activity and durability in alkaline solutions. Experimental characterizations and theoretical calculations demonstrated that the PLD method can create a large amount of oxygen vacancies on the surface of the perovskite nanofilm, leading to a high concentration of highly oxidative oxygen O22−/O− . This O22−/O− species that is generally recognized to be active for OER. As a result, the SCFP-NF catalyst with low mass loading (≈30 µg cm-2) demonstrated a remarkable OER activity with a low overpotential of 290 mV and a superb stability up to 200h at 10mA cm−2 in alkaline media.
Secondly, in-situ Raman spectroscopy measurements were employed to observe Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) nanofilm deposited on nickel foam (BSCF-NF) for the dynamic surface change during real-time conditions for BSCF nanofilm catalysts. To improve the catalytic activity, a surface reductive strategy was applied for BSCF-NF, which was then demonstrated to facilitate the surface reconstruction of active species based on B-site cations. The reconstruction activation potentials of BSCF-NF decreased after NaBH4 treatment based on in-situ Raman results, and the correlation between surface electron structure and catalytic activity was well established and discussed.
Thirdly, the results on a nickel-cobalt-vanadium trimetallic (oxy)hydroxide nanosheets decorated with silver nanoparticles (Ag@NiVxCoy) were reported. The Ag@NiVxCoy fabricated by a simple spontaneous redox reaction showed a low overpotential of 255 mV to achieve a current density of 10 mA cm-2 and a small Tafel slope of 38.3 mV dec-1 in 1 M KOH. Its high catalytic performance is attributed to: (1) oxygen-deficient (oxy)hydroxide layer on the surface of Ag@NiVxCoy generated through the surface reconstruction, (2) the optimal local coordination induced by the interaction between silver nanoparticles (Ag NPs) and trimetallic system, (3) the extensively exposed active sites, and (4) the significantly improved charge-transfer ability due to the incorporation of Ag NPs.
In summary, this thesis reports the successful fabrication of high-efficient electrocatalysts by constructing 2D TMOs catalysts including thin film and nanosheet combined with surface reductive treatment and Ag nanoparticles decoration. It has been found that by reducing the thickness of TMOs and optimizing the electronic structure of the surface ions can directly improve their electrocatalytic activity. The results of this thesis might provide good guidance and inspiration for high-active catalysts design and their commercial applications by revealing the mechanisms behind their enhanced catalytic activity.
