عنوان

Design and Development of Low Cost Materials for Water Electrolyzers

Number

TLpq2475572096

.Language of Text, Soundtrack etc

انگلیسی

Title Proper

Design and Development of Low Cost Materials for Water Electrolyzers

General Material Designation

[Thesis]

First Statement of Responsibility

Munonde, Tshimangadzo Saddam

Subsequent Statement of Responsibility

Nomngongo, P. N.

Name of Publisher, Distributor, etc.

University of Johannesburg (South Africa)

Date of Publication, Distribution, etc.

2020

Specific Material Designation and Extent of Item

202

Dissertation or thesis details and type of degree

Ph.D.

Body granting the degree

University of Johannesburg (South Africa)

Text preceding or following the note

2020

Text of Note

The construction and modifications of highly active, stable and low cost electrocatalysts promoting the oxygen and hydrogen evolution reactions (OER and HER) in water splitting are the key areas of research in energy generation and conversion. Given the significance of the HER and OER electrocatalysts, the objective of this study was to fabricate low-cost materials that are capable of being used as cathode and anode electrocatalysts in water splitting. Thus, nanomaterials and nanostructures provide unique properties that have allowed then to play an essential role in recent developments in energy-related applications. As known, the structural and electronic properties of nanomaterials/nanostructures stimulate their physical and chemical properties, and therefore their catalytic activity. Thus, tailoring the morphology of nanomaterials/nanostructures either during the synthesis or post the synthesis process, have been known to promote their structural and electronic properties, thus leading to enhanced catalytic activity. Inspired by these factors, NiFe2O4 nanospheres were synthesized and exfoliated using the developed sonication-assisted liquid exfoliation strategy in water dispersions, with the aim of improving their catalytic activity for HER. Impressively, after exfoliation, NiFe2O4 nanospheres displayed improved HER activity in acidic media, affording the current density of 10 mA cm-2 at an overpotential of 186 mV, as opposed to 274 mV of the unexfoliated NiFe2O4 nanospheres. Additionally, the exfoliated NiFe2O4 nanospheres demonstrated superior stability, with the overpotential improvement of ~100 mV at 50 mA cm-2 after 6000 CV cycles. It was concluded that the powerful ultrasonic exfoliation process exposes more catalytically active sites by deaggregating and deagglomerating the agglomerated and aggregated particles.  Following the observed catalytic activity enhancements after exfoliation, the adopted sonication-assisted liquid exfoliation strategy was optimized and the effects of different sonicating conditions on the OER performance of the hydrothermally synthesized NiFe LDH electrocatalysts were studied. The concentration, sonication time and ultrasonic amplitude were parameters evaluated in the exfoliation process. Noticeably, after optimization, the exfoliated NiFe LDH nanosheets required only 250 mV overpotentials to reach the current density of 10 mA cm-2 in alkaline media, which was 100 mV better than the stacked NiFe LDH. The exfoliated NiFe LDH nanosheets also displayed an excellent stability with a negligible degradation. The improved charge-transfer process at the electrode/electrolyte interface validated the improved OER activity after exfoliation. Then, using the optimum conditions, the sonication-assisted liquid exfoliation strategy was used to exfoliate the stacked NiFe LDH/CB to boost its OER activity in alkaline media. Impressively, after exfoliation, the exfoliated NiFe LDH/CB nanosheets displayed a superior OER activity in alkaline media with an overpotential of 220 mV to reach the current density of 10 mA cm-2 compared to 280 mV of the bulk NiFe LDH. Coupled with the improved conductivity from carbon black, the improved OER performance was ascribed to the synergistic effects of the Ni2+ and Fe3+ enriched surface and the exposed active sites after exfoliation. In an effort to tailor the morphology and improve the electronic structure, the catalytically active CoFe2O4 nanoparticles were hybridized with conductive nanocarbons (carbon black, carbon nanofibers and reduced graphene oxide) and the OER performance were studied in alkaline media. The CoFe2O4/CNF catalyst showed superior OER performance, affording the current density of 10 mA cm-2 at 260 mV overpotentials, which was 60 mV and 110 mV better than CoFe2O4/CB and CoFe2O4/RGO, respectively. The high OER performance and stability of the CoFe2O4/CNF catalyst was attributed to the improved charge/mass transport emanating from the successful integration of CNF on CoFe2O4 nanoparticles.  Lastly, MoS2@NiFe2O4/CB hybrid prepared by integrating MoS2 with the previously synthesized NiFe2O4/CB nanoparticles was used to improve the HER and OER catalytic performance in acidic and alkaline media, respectively. The MoS2@NiFe2O4/CB hybrid required lower overpotentials of 260 mV to reach a current density of 10 mA cm-2 for OER in alkaline media, compared to 400 and 450 mV overpotentials of NiFe2O4/CB nanoparticles and MoS2 nanosheets, respectively. Contrary to this, the HER performance of MoS2@NiFe2O4/CB hybrid (423 mV at j = 10 mA cm-2 ) in acidic media was lower than those of NiFe2O4/CB nanoparticles (274 mV at j = 10 mA cm-2 ) and MoS2 nanosheets (395 mV at j = 10 mA cm-2 ). The results indicate that the MoS2@NiFe2O4/CB hybrid favour the adsorption of hydroxyl ions (OH- ) in alkaline media than the adsorption of hydrogen ions (H+ ) in acidic media, thus accelerating the OER kinetics than HER kinetics, leading to higher OER performance than the HER performance.

Atomic physics

Energy

Environmental engineering

Munonde, Tshimangadzo Saddam

Nomngongo, P. N.

Electronic name

p

[Thesis]

276903

a

Y