Electrochemical Reduction of CO₂ on Ir<i>_x</i>Ru_{(1–<i>x</i>)}O₂(110) Surfaces

High overpotentials and low faradic efficiencies plague metal catalysts for direct conversion of CO₂ to methanol and other liquid fuels. RuO₂-based electrocatalysts have been observed to evolve methanol at low overpotentials, which has been attributed to an alternative reaction mechanism with oxygen-coordinated intermediates that can circumvent the limitations imposed by the scaling relations on metal catalysts. Here, we introduce an innovative concept of ligand effects in oxide catalysts. Both IrO₂ and RuO₂ binds OH* and other intermediates from the electrochemical reduction of CO₂ (CO2RR) strongly, but the stable and miscible system Ir_<i>x</i>Ru_(1‑x)O₂ exhibits anomalous weaker binding energy in the presence of CO* spectators, because of Ru–Ir ligand effects. The weakened adsorbate binding leads to a very low CO2RR onset potential (methanol evolution at −0.2 V RHE). An Ir atom at the bridge site with Ru neighbors binds intermediates such as OH* and OCHO* much weaker, because of synergistic ligand effects and adsorbate–adsorbate interactions. Consequently, a RuO₂ surface doped with Ir move close to the top of the predicted CO2RR volcano for oxides, which offers a significant improvement over state-of-the-art electrocatalysts for conversion of CO₂ into methanol. Analysis of electronic structure parameters with adsorbate binding energies indicates the ligand effect depletes electrons from the Ir atom and shifts the t_2g orbitals. The lack of electron donation from CO* spectators to Ir at the active site cause favorable adsorbate binding

From 3D to 2D Co and Ni Oxyhydroxide Catalysts: Elucidation of the Active Site and Influence of Doping on the Oxygen Evolution Activity

Layered oxyhydroxides (ox-hys) of Ni and Co are among the most active catalysts for oxygen evolution in alkaline media. Their activities can be further tuned by delamination into single-layer oxide sheets or by means of doping. The active site for the reaction and how doping and delamination promote the intrinsic activity, however, remain elusive. To shed light on these open questions, we have undertaken a systematic analysis of the stability, catalytic activity, and electronic conductivity of Ni and Co ox-hys ranging from bulk (3D) to single-layer (2D) catalysts. In both cases, we investigate the role of terrace and edge sites and use stability, catalytic activity, and electronic conductivity as evaluation criteria to pinpoint the best catalysts. We arrive at several important conclusions: the ox-hy surface is fully oxidized under oxygen evolution conditions, bulk terraces are ostensibly the most active sites, and Ni ox-hy sheets are more electronically conductive in comparison to their Co equivalents. Furthermore, we examine 25 different doped Co and Ni ox-hy nanosheets (V, Cr, Mn, Fe, Co/Ni, Cu, Ru, Rh, Pd, Ir, Pt, Ag, Al, Ga, In, Sn, Pb, Bi, Mg, Sc, Y, Ti, Nb, Zn, and Cd) to further tailor the catalytic performance. We establish the dependence of the electronic conductivity and activity on potential and find that it is more energetically favorable to dope Ni in comparison to Co ox-hys, with first-row transition and noble metals being the most stable dopants. Finally, we extend the analysis to include bulk terminations and reveal that most dopants, which are stable in the nanosheets, have a large propensity to segregate to the surface of bulk materials, and those that are less prone to segregation (Fe or Cr) are not electronically conductive in the bulk. Overall, we identify Rh-doped Ni ox-hy to be the best catalyst material

Functional Independent Scaling Relation for ORR/OER Catalysts

A widely used adsorption energy scaling relation between OH* and OOH* intermediates in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), has previously been determined using density functional theory and shown to dictate a minimum thermodynamic overpotential for both reactions. Here, we show that the oxygen–oxygen bond in the OOH* intermediate is, however, not well described with the previously used class of exchange-correlation functionals. By quantifying and correcting the systematic error, an improved description of gaseous peroxide species versus experimental data and a reduction in calculational uncertainty is obtained. For adsorbates, we find that the systematic error largely cancels the vdW interaction missing in the original determination of the scaling relation. An improved scaling relation, which is fully independent of the applied exchange–correlation functional, is obtained and found to differ by 0.1 eV from the original. This largely confirms that, although obtained with a method suffering from systematic errors, the previously obtained scaling relation is applicable for predictions of catalytic activity