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Synthesis And Characterization Of The Ternary Phase Co2-Xrhxp Nanoparticles And Their Composition-Dependent Electrocatalytic Activity Towards The Oxygen Evolution And Hydrogen Evolution Reactions, And Their Performance In Overall Water Splitting
Date of Award
Stephanie . Brock
The present dissertation work reports on the synthesis and characterization of ternary phase Co2-xRhxP nanoparticles for electrolytic water oxidation, using the solution-phase arrested precipitation technique. The relatively low cost and earth abundance of cobalt, coupled with the ability of rhodium to promote the activity of Co2P, are some of the major motivators for this work. The bulk phase diagram of Co2-xRhxP is largely unknown, with only one composition reported, namely Co1Rh1P. Thus, one of the most significant contributions of this work is perhaps the fact that we were able to form a wide range of Co2-xRhxP solid solutions at the nanoscale for the first time, by solubilizing crystal systems that clearly differ extensively. Co2P and Rh2P systems crystallize in the orthorhombic (Co2P-type) and cubic (antifluorite-type) structures respectively and exhibit major structural disparities that may favor phase separation. In addition to developing a synthetic protocol for the above-mentioned nanoparticles, our work also presents results on their electrolytic activity towards both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).
With respect to OER catalysis, the incorporation of small amounts of Rh (x = 0.25) is found to have a profound effect on the activity of Co2P towards the oxygen evolution reaction in basic media as proved by the overpotential needed to drive a current density of 10 mA/cm2 and Tafel slopes, while in acidic media, the Co2-xRhxP electrocatalysts undergo abrupt deactivation during anodic polarization. On the other hand, the same ternary phase nanoparticles are active towards the hydrogen evolution reaction in both acidic and basic media, with rhodium-rich compositions being more active than their cobalt-rich counterparts in both cases. In addition, DFT calculations are used to further elucidate the reactivity of different compositions of the above-mentioned electrocatalysts in acidic media, where it will be shown that the reactivity is composition-dependent among samples that crystallize in the cubic antifluorite phase (x ≥ 1), since the hydrogen adsorption energies and Gibb’s free energies of hydrogen adsorption among different sample compositions decrease almost linearly as one moves from pure Rh2P end to the mid-range (Co1Rh1P) composition. The DFT calculations were computed based on the (100) Rh2P surfaces.
The HER reactivity of the electrocatalysts in both acidic and basic media was further elucidated using double-layer capacitance (Cdl) and electrochemical surface area (ECSA) measurements, in order to establish whether there was a correlation between the observed activities and the surface active sites, which can be quantified as a function of the calculated surface areas. Surprisingly, these studies established that while activities were found to be positively correlated to the calculated surface areas, there exists considerably composition-dependent variation in the quality of active sites and their ability to function at low potentials.
In addition, since it was established that Co2-xRhxP electrocatalyst nanoparticles can be utilized as both OER and HER electrocatalysts in basic media, this dissertation will demonstrate that these nanoparticles can be used for overall water splitting in basic media, where the overall bifunctionality is harnessed by utilizing cobalt-rich (Co1.75Rh0.25P) and rhodium-rich (Co0.25Rh1.75P) electrocatalysts as anodic and cathodic electrode materials, respectively where a current density of 10 mA/cm2 is achieved at a potential of 1.62 V. This represents an overpotential of 0.39 V above to the thermodynamic minimum energy (1.23 V) that is needed to achieve overall water splitting.
Mutinda, Samuel Itumo, "Synthesis And Characterization Of The Ternary Phase Co2-Xrhxp Nanoparticles And Their Composition-Dependent Electrocatalytic Activity Towards The Oxygen Evolution And Hydrogen Evolution Reactions, And Their Performance In Overall Water Splitting" (2019). Wayne State University Dissertations. 2304.