Access Type
Open Access Dissertation
Date of Award
January 2022
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Chemistry
First Advisor
Stephanie Brock
Abstract
Nanoscale transition metal phosphides are emerging as efficient catalysts for hydrogen production via electrochemical water splitting. Although monometallic phases have been extensively studied, enhanced catalytic activities and stabilities can be gained by the synergism of two metals in bimetallic ternary transition metal phosphides. In this dissertation, we report (1) the effect of the counter electrode and electrolyte on stabilities of Co2−xRhxP nanoparticles for water reduction (hydrogen evolution reaction, HER), (2) novel synthetic protocol development and electrocatalytic water splitting activity and stability study on Ni2-xRhxP nanoparticles, and (3) a preliminary investigation employing cation exchange methods to synthesize Cu-M-P (M= Fe, Co, and Ni) nanoparticles.Co2−xRhxP nanoparticles show promising electrocatalytic activity toward HER in acidic and basic media. However, Co is not stable in acidic media under electrocatalytic conditions due to Co dissolution, and the activity was found to deplete 50% with Co loss. Studies on the use of Pt counter electrode and its interference on the working electrode at lower potentials (<0.6 V vs.RHE) revealed that the deactivation of Co2−xRhxP/C due to Co dissolution was masked by Pt dissolution and redeposition on the working electrode. However, in basic media, no Co dissolution, activity deactivation, or Pt redeposition was observed. At high pH, the increased activity and stability are attributed to in-situ formed metal oxide/hydroxides on the catalytic surface. In order to create a more stable catalyst, we replaced Co in Co2−xRhxP nanoparticles with Ni to produce Ni2−xRhxP (x=0.25, 0.5, and 1.75) nanoparticles with low polydispersity. Intermediate compositions were not phase pure. Ni-rich compositions (x=0.25 and 0.5) were hollow spherical nanoparticles with hexagonal structures, while the Rh-rich composition (x=1.75) adopted a quasi-spherical shape with a cubic antifluorite structure. X=0.25 showed the highest OER intrinsic and geometric catalytic activities in basic media with overpotentials of 261.8 mV and 273.8 mV to achieve 2.0 μA/cm2ECSA and 10 mA/cm2Geo, respectively. X=1.75 showed the highest HER intrinsic catalytic activity in basic media with an overpotential of 44.5 mV to get producing -2.0 μA/cm2ECSA. The x=1.75 composition also had competitive geometric HER activity with an overpotential of 82.1 mV to obtain 10 mA/cm2Geo. The XPS and EDS mapping data revealed that for both HER and OER, in-situ formed oxide/hydroxide species play a role in the catalytic activity, and Rh phase segregation is found post-OER. The use of Ni0.25Rh1.75P as a cathode and Ni1.75Rh0.25P as an anodic in an overall water splitting electrolyzer resulted in an overpotential of 554 mV to obtain a current density of 20 mA/cm2. In acidic media, the x=1.75 phase showed comparable HER activity and stability relative to Rh2P. For OER, the x=1.75 phase is unique in terms of its stability. While it is initially less active than Rh2P, the latter phase rapidly deactivates, whereas Ni1.75Rh0.25P has a stability profile similar to the state-of-the-art RuO2 catalyst. The enhanced stability is attributed to a nickel-rich surface with a Rh underlayer, in which the active nickel catalyst is moderated by the Rh, enabling turnover. Finally, the synthesis of bimetallic phosphides of Cu-Fe, Cu-Ni and Cu-Co was explored by cation-exchange reactions of Cu3-xP. Preliminary data suggest that treatment of Cu3-xP with equimolar Fe3+ at 200 °C produces a crystalline product with the Cu3-xP structure but a slightly compressed lattice. STEM/EDS mapping suggests the hexagonal plates of the starting Cu3-xP are retained, with Cu and Fe forming separate domains. We hypothesize a redox-mediated uptake in which Fe3+ oxidizes Cu1+ departing the lattice, with Fe2+ then incorporated. Attempted exchanges with Ni2+ and Co2+ were not successful. Optimization of protocols is expected to lead to make new bimetallic catalytic materials that could be utilized in electrochemical water splitting applications.
Recommended Citation
Batugedara, Tharanga Nisansala, "Synthesis And Investigation Of Properties In Ternary Transition Metal Phosphide Nanomaterials For Electrochemical Water Splitting" (2022). Wayne State University Dissertations. 3659.
https://digitalcommons.wayne.edu/oa_dissertations/3659