Synthesis, characterization and electrochemical activity studies of metal-modified zirconium phosphate electrocatalysts for the oxygen evolution reaction

Abstract

The preparation of several metal-modified zirconium phosphate (ZrP) nanomaterials was performed for their used as electrocatalysts for the oxygen evolution reaction (OER). ZrP nanoparticles were intercalated or their surface modified with first-row transition metals. All metal-modified nanomaterials were thoroughly characterized and their electrochemical OER performance assessed.<br /> <br /> In Chapter 3, two metal-modified ZrP catalyst systems were synthesized: metal- intercalated ZrP and metal-adsorbed ZrP, each involving Fe(II), Fe(III), Co(II), and Ni(II) cations. Characterization through X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirm the incorporation of the metal cations either between the layers or on the surface of ZrP. Catalytic activity studies of both systems for the OER were performed using Rotating Disk Electrode (RDE) voltammetry using 0.1 M KOH as the electrolyte. All metal-modified systems are catalytically active, requiring between 0.5-0.7 V of overpotential to reach 10 mA/cm², depending on the choice of metal cation, the M:ZrP molar ratio used during synthesis, and whether the metal was intercalated into or adsorbed onto ZrP. These findings indicate that the metal species that were supported within ZrP required similar or higher overpotentials to drive the OER when compared to their metal-adsorbed counterparts. This may result from differences in mass transport and/or electronic conductivity in the two systems.<br /> <br /> In Chapter 4, zirconium phosphate nanosheets (exfoliated ZrP) were modified with Co(II) and Ni(II) cations and studied as OER electrocatalysts. Their OER activity showed an improved performance compared to that of surface modified ZrP nanoparticles. Inductively coupled plasma- mass spectrometry (ICP-MS) measurements show that there is a large difference in the metal content of two different metal-modified ZrP systems. The intrinsic activity of the electrocatalysts was probe through mass activity and turnover frequency (TOF) calculations. In these calculations, it was assumed that all metal content quantified by ICP-MS in the materials were active and accessible to perform the OER. These results show that the increased activity of the exfoliated samples is not due to higher intrinsic activity of the catalytic sites. Instead, the enhanced OER performance can be attributed to a larger number of active sites as observed by the higher metal content. For this reason, it is believed that the enhancement in catalytic activity is due to the fact that the inner layer surfaces are now more electrochemically accessible stemming from being more exposed to the electrolyte in the exfoliated samples.<br /> <br /> In Chapter 5, the synthesis and characterization of metal-modified zirconium phosphate support structures with controlled morphology was performed and their electrochemical OER performance assessed and correlated to electrical conductivity (four-point probe measurements). Results elucidate the possible nature of the geometric and mass normalized activities of Co- and Ni-modified ZrP OER electrocatalysts in unique morphologies. It is found that the loading and the coverage of cobalt and nickel species is important for optimal electrocatalysis. By modifying the morphology of the ZrP support, the maximum ion exchange capacities and coverage of nickel and cobalt cations is altered which directly influences the observed geometric and mass activity trends. Thus, the OER activity can be tuned by selection of the ZrP support and the metal used for its modification. Furthermore, a spherical ZrP morphology with low-loading nickel specie showcases a mass activity that is 1-2 orders of magnitude higher over nickel species at higher loadings on hexagonal, rod, and cube-like ZrP structures. This work paves the way for future investigations for improving the activities of metal-modified ZrP by targeting improved conductivities of the as-synthesized composite materials.