Impact of Hydrogen Concentration for CO2 reduction on PdHx

Abstract

Pd hydride has shown much better electrochemical CO2 reduction reaction (CO2RR) performance compared to metal Pd implying that H in the PdHx surface plays a vital role in affecting the performance. Some experiments show phase transformation from metal Pd to PdHx via in-situ X-ray diffraction and electrochemical method. However, studying the stable atomic structures and reaction mechanisms at different H concentrations is experimentally very difficult. Most previous experimental and com- putational studies only focus on the comparison of CO2RR performance of pure metal Pd (0% H concentration) and Pure PdH (100% H concentration). However, there is also no systematical theoretical study about the H concentration effect of PdHx (from 0% to 100%) on CO2RR and the hydrogen evolution reaction (HER), and the atomic structure of the active substoichiometric PdHx catalyst have not been identi- fied. Based on density functional theory (DFT) calculations, active learning cluster expansion equipped with Monte Carlo simulated annealing is carried out to search for stable PdHx(111) surface configurations in this work. We identify 12 stable PdHx(111) configurations from the converged DFT convex hull and theoretically investigate the evolution of the H concentration of the stable structures as a function of applied potential. After automatically finding unique adsorption sites and calculating the binding energies of adsorbates, we analyze their free energy diagram, activity volcano and selectivity toward CO and H2. Finally, we identify the atomic structure of the PdHx phase most likely to produce syngas. It’s activity can be attributed to the fact that H segregation breaks the linear relation between COOH and CO adsorbates.

High-throughput compositional screening of PdxTi1–xHy and PdxNb1–xHy hydrides for CO2 reduction

“…We notice that most candidates have stronger binding of H* than Pd 64 H 64 and the bindings are not strong, which leads to their high HER activity. However, the candidates with high concentrations of H and Pd have weaker binding of H* than Pd 64 H 64 and thus lower HER activity, which is consistent with our previous work [21]. In addition, there are no obvious scaling relations between the binding energies of different adsorbates for both Pd x Ti 1x H y and Pd x Nb 1x H y as shown in Figure…”

“…The workflow for the screening of stable and active candidates of Pd method is implemented to find unique sites. [21] After finding the unique adsorption sites, CO* adsorbate is added to the bare slabs, and its binding energies are calculated using DFT. During The small grey spheres in adsorbates are C atoms, and the red spheres are O atoms.…”

“…Monte Carlo Simulated Annealing (MCSA) is carried out to find the ground-state structure at each During the MCSA, the acceptance probability P acc can be expressed as: [21,29] P acc = min 1, exp…”

“…Due to the vast search space of composition of Pd x Ti 1x H y and Pd x Nb 1x H y , their stabilities are first explored by an active learning cluster expansion model equipped with Monte Carlo simulated annealing of each concentration. [21] The advantage of this method is to search the ground-state structure at each concentration efficiently.…”