Prof. Zhiqun Lin, Advisor, MSE
Prof. Meilin Liu, Co-advisor, MSE
Prof. Seung Soon Jang, MSE
Prof. Vladimir Tsukruk, MSE
Prof. Angus Wilkinson, CHEM/MSE
Investigation into Strategies for Enhanced Electrocatalytic Activities of Spinel-Based Transition Metal Oxide Nanoparticles
As the severity of global climate issues continues to build, the need for clean energy storage and conversion devices has become increasingly pressing. The production of green hydrogen through water electrolysis is a promising route to alleviating these challenges. However, the high cost and scarcity of the state-of-the-art noble metal-based electrocatalysts utilized in such processes represents one of the critical hurdles to be overcome prior to their practical implementation. A promising direction is to utilize transition metal-based nanoparticles (NPs), which offer superior electrocatalytic performance over their bulk counterparts. The work in this dissertation systematically investigates strategies to improve the electrocatalytic performance of transition metal-oxide NPs.
Capping the surface of NPs with polymers is widely recognized as an effective means towards their dispersion and stabilization. However, it is often circumvented due to its tendency to lower the electrocatalytic activity of the ligated NPs. In this context, the first systematic investigation into the impact of the chain density and hydrophilicity of the surface-capping polymers, which can be judiciously regulated, on the oxygen evolution reaction (OER) activity is performed. By capitalizing on star-like diblock copolymers as nanoreactors, spinel CoFe2O4 (CFO) NPs permanently ligated with polymers of interest (i.e., varied chain density and characteristic) are crafted. The correlation between the chain density and hydrophilicity of surface-capping polymers and the OER activity of CFO NPs are scrutinized. Intriguingly, decreasing the number of surface-capping chains and increasing the chain hydrophilicity result in significantly decreased overpotential, caused by an increased exposure of the active material (CFO) to the electrolyte and reduced diffusion resistance. This study provides insight into the strategies for mitigating the activity-limiting properties of surface polymers and tailoring the electrocatalytic properties of polymer-ligated NPs.
Recently, the use of externally applied magnetic fields has garnered significant attention as a promising strategy to enhance OER electrocatalytic performance. OER exhibits spin-dependent kinetics, producing triplet O2 from singlet reactants (OH-, H2O). Notably, magnetization can reduce this kinetic barrier by aligning the spin ordering of ferromagnetic (FM) electrocatalysts. Unfortunately, some of the most active OER catalysts, namely transition metal oxyhydroxides, are paramagnetic (PM). This can be circumvented by utilizing a spin pinning effect in FM/PM core/shell materials, which has already been successfully demonstrated in a bulk CFO/CoFeOxHy system. In this work, previous research is built upon by examining a similar system at the nanoscale. Star-like block copolymers prepared via sequential atom transfer radical polymerization were successfully utilized as nanoreactors to synthesize CFO nanoparticles. The surfaces of CFO nanoparticles were successfully doped with sulfur under mild conditions, enabling the successful surface reconstruction of S-doped CFO into a more active oxyhydroxide phase. Successful spin-pinning was verified by an observed increase in OER activity following the application and removal of a magnetic field; thus, confirming that spin-pinning remains a viable OER-enhancement technique even at the nanoscale. This study lays the groundwork for future systematic studies on the effects of NP size and core-to-shell ratio on the magnetic field-rendered OER enhancement.
In addition to externally applied magnetic fields, other effects can be introduced during electrocatalysis to improve performance. Previous research has found that some spinel NPs, NiFe2O4 (NFO) for example, experience a photothermal effect upon near-infrared light irradiation which promotes the dynamic generation of active OER sites. Thus, in this dissertation, both the magnetic field-based enhancement and photothermal effect are collectively exploited to further improve the OER electrocatalytic ability of NFO NPs. Concurrent application of magnetic field and photothermal effect is demonstrated to further enhance the OER activity of NFO NPs. Interestingly, the significant increase in activity observed was primarily attributed to a greatly promoted surface reconstruction. It is determined that the application of a magnetic field during chronopotentiometry can promote surface reconstruction to a similar degree of inducing the photothermal effect. This work documents a new strategy to induce surface reconstruction in NiFe2O4, opening up the door for future studies employing different electrocatalytic materials and investigating the mechanisms of magnetic-field enhanced surface reconstruction.
The findings in this dissertation serve as an important step towards the practical implementation of OER-limited devices, such as water electrolyzers. Various strategies to enhance the OER activity of metal-oxide nanoparticles have been presented. Excitingly, future work can build upon the investigated methods to enable low-cost, low-complexity electrocatalysts to serve as competitive alternatives to state-of-the-art noble-metal based catalysts.