Rational design of supported catalysts based on layered transition metal oxides towards electrochemical water splitting

Pu, Yayun (2021). Rational design of supported catalysts based on layered transition metal oxides towards electrochemical water splitting. University of Birmingham. Ph.D.

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Abstract

Under the circumstance of urgent demand for clean, renewable hydrogen fuel on a global scale, large-scale pure hydrogen production has drawn extensive attention from both industrial and science communities. To date, most commercial hydrogen is produced by steam methane reforming, which operated under harsh conditions with high energy-cost.1 Alternatively, electrolysis of water is an effective strategy that can produce high purity H2 under mild conditions. In this context, high active, low-cost electrocatalysts tend to be the specific target to realize the industrial application. To design and develop prospective catalysts, insight into the correlation between composition, coordination environment and electronic structure of electrocatalysts is undoubtedly important. With this in mind, this thesis reports various kinds of electrocatalysts that can effectively catalyze the hydrogen/oxygen evolution reaction (HER/OER), with tailored coordination structure/electronic structure. Through a variety of characterizations, the correlation between chemical environments of actives species and corresponding electrochemical activity towards HER/OER is elucidated.

Chapter 4 presents the phase transformations from lepidocrocite titania nanosheets (L-TiO2) to rutile (R-TiO2) and anatase (A-TiO2).2 We report that temperature determined the final phase-structure in the transition phase of the L-TiO2 nanosheets into TiO2 nanoparticles, while the pH determined the final morphology and particle size. Based on the experimental results, two different transition pathways of dissolution-recrystallization (pH=3) and topologically rolling transition (pH=9) have been proposed. Atomic dispersed Pt species were deposited on the pre-synthesized A-TiO2 surface via photochemical reduction reaction. Due to the strong interaction of deposited Pt species and TiO2 supports, a lower oxidation state of Pt was found on Pt@TiO2-pH9. In 0.5 M H2SO4, Pt@TiO2-pH9 delivered the best HER activity than other Pt decorated TiO2 supports. It only requires an overpotential of 30 mV to reach 10 mA cm-2 geo. In addition, with regard to the mass activity, all the Pt decorated TiO2 samples show excellent HER performance superior to commercial Pt/C due to ultralow loading of Pt on the catalyst.

Chapter 5 describes that low-loading (< 20 wt.%) Ni-confined in layered metal oxides anode catalysts have been synthesized by facile ion exchange methodology and subjected to systematic electrochemical studies.3 It was found that Ni-intercalated on K-rich birnessite (Ni-KMO) presents an onset overpotential (ηonset) as low as 100 mV and overpotential at 10 mA cm-2 (η10) of 206 mV in pH=14 electrolyte. By combining electrochemical methods XAS and XES, we demonstrate that Ni sites are the active sites for OER catalysis and that the Mn3+ sites facilitate Ni intercalation during the ion-exchange process. Different coordination structures of Ni species were confirmed on Ni-KMO and Ni-HMO, which are strongly correlated to the electrochemical behavior and corresponding OER activity. The effect of the pH and the nature of the supporting electrolyte on the electrochemical performance were also evaluated to reveal the electrochemical features of Ni intercalated KMO.

Chapter 6 shows the surface galvanic formation of Co-OH on KMO was achieved via the redox reaction of hydrated Co2+ with crystalline Mn4+.4 The synthesis method takes place at ambient temperature without using any surfactant agent or organic solvent, providing a clean, green route for the design of highly efficient catalysts. The redox reaction resulted in the formation of ultrathin Co-OH nanoflakes. XAS and XPS analysis confirmed the changes in the oxidation state of the bulk and surface species on the Co-OH nanoflakes supported on the KMO. XPS and Time of flight secondary ion mass spectrometry (ToF-SIMS) analysis demonstrated that the layer of CoOxHy deposited on the KMO and its electronic structure strongly depends on the anion of the precursor used. In particular, it was found that Cl- favors the formation of Co-OH, changing the rate-determining step of the reaction, which enhances the catalytic activity towards the OER, producing the most active OER catalyst in alkaline media.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):