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Calculation Method for Hydrogen Evolution and Adsorption on Ir, Pt, and Ir-Pt Nanocatalysts Supported on Porous
Farid Taherkhani1,2, Fabian Mauss1,2
1Departments of Thermodynamics and Thermal Process Engineering, Brandenburg University of Technology, Cottbus 03046, Germany.
Abstract:
The Tafel exchange current density for the electrochemical hydrogen evolution reaction (HER) in water electrolysis was systematically evaluated on iridium (Ir) nanoelectrodes using first-principles computational methods. For the first time, the transition from crystalline (metallic) to amorphous structural regimes in Ir nanoclusters is explicitly correlated with variations in exchange current density, as determined from density functional theory (DFT) calculations. The results reveal a pronounced maximum in the exchange current density associated with the structural transformation from ordered crystalline to disordered amorphous phases under acidic conditions. This enhancement is attributed to modifications in the local electronic structure and active site distribution induced by amorphization. The theoretically predicted exchange current densities are in quantitative agreement with available experimental measurements, thereby validating the computational approach. DFT calculation predicts that for small iridium nanoparticles (NPs) (20-50 atoms), the exchange current density for the HER in acidic media is around 1.1 (mA/cm2), and it approaches 5 mA/cm2 upon undergoing amorphous phase transition. The current density has also been calculated for pure Ir, pure Pt, and Ir-Pt nanoalloy electrodes designed for the HER in alkaline media. Furthermore, molecular dynamics (MD) simulation shows that Pt doping in Ir nanoclusters, supported on single-walled carbon nanotubes (SWCNTs), enhances hydrogen adsorption and increases the current density for the HER in alkaline water electrolysis, even at low Pt concentrations. The calculated current density based on DFT computation values is consistent with the published experimental data. DFT calculation estimates that the current density for the HER in alkaline media at 50 mV is 5.5, 6, and 7 (mA/cm2) for Ir, Pt, and the Ir-Pt nanoalloy with a total atom number of 30, respectively. Furthermore, the heat capacity (C p) of Ir@SWCNT exhibits a transition from monotonic to nonmonotonic temperature dependence with increasing Ir cluster size. MD simulations further demonstrate that the hydrogen diffusion coefficient on Ir and Pt nanoparticles (NPs) supported on single-walled carbon nanotubes (Ir@SWCNT and Pt@SWCNT) decreases with decreasing cluster size. In addition, computational analysis reveals the formation of multilayer hydrogen adsorption on both monometallic Ir and bimetallic Ir-Pt nanocatalysts supported on SWCNTs (IrPt@SWCNT), suggesting strong adsorbate-adsorbate interactions under the studied conditions.
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