Design of Oxygen Reduction Bimetallic Catalysts:  Ab-Initio-Derived Thermodynamic Guidelines

Abstract

The Gibbs free energies of key elementary steps for the electrocatalytic oxygen reduction reaction (ORR) are calculated with B3LYP type of density functional theory: O2 + M + H+ + e- (0 eV) --> HOO-M (deltaG1), HOO-M + M --> HO-M + O-M (deltaG2), O2 + 2M + H+ + e- (0 eV) --> O-M + HO-M (deltaG3), and HO-M + O-M + 3H+ + 3e- (0 eV) --> 2H2O + 2M (deltaG4), where H+ is modeled as H3(+)O(H2O)3 and M stands for the adsorption site of a metal catalyst modeled by a single metal atom as well as by an M3 cluster. Taking Pt as a reference, deltaG4 is plotted against deltaG1 for 17 metals from groups V to XII. It is found that no single metal has both deltaG1 and deltaG4 more negative than Pt, although some of them have either more negative deltaG1 or more negative deltaG4. This enables us to explain thermodynamically why no other single metal catalyzes the ORR as effectively as Pt does. Moreover, a thermodynamic analysis reveals that the signs of delta deltaG (the difference between deltaG of other metals and deltaG of Pt) strongly correlate with the valence electronic structure of metals, i.e., delta deltaG1 < 0 and delta deltaG4 > 0 for metals M with vacant valence d orbitals, whereas delta deltaG1 > 0 and delta deltaG4 < 0 for metals M' with fully occupied valence d orbitals. Thus, a simple thermodynamic rule for the design of bimetallic catalysts for the ORR is proposed: couple a metal M (delta deltaG1 < 0) with a second metal M' (delta deltaG4 < 0) to form an alloy catalyst MM'3. The rationale behind this selection is based on M being more efficient for the rate-determining step, i.e., for the formation of the adsorbed species M-OOH, while M' can enhance the reductions of O and OH in the last three electron-transfer steps.

Cobalt Monolayer Islands on Ag(111) for ORR Catalysis

“…The catalytic activity of such samples towards ORR was evaluated and the best improvement in activity was found to be that of the sample with a cobalt percentage of approximately 30 % with respect to the bare silver, which is in good agreement with theoretical hypotheses. This paper studies the catalytic effect of silver substrates, which are electrochemically modified on a nanometer scale through electrodeposition of monolayer islands of Co. Based on literature data, [11][12][13] Co should support the initial dissociative chemisorption (eventually coupled with the first electroreduction step) [12] of oxygen, whereas silver should promote the following charge transfer steps. On silver, well-ordered layers of many metals can be obtained by exploiting surface phenomena such as underpotential deposition (UPD).…”

Preparation, characterization and electrocatalytic activity for oxygen reduction reaction in PEMFCs of bimetallic PdNi nanoalloy

“…The catalyst consists of two metals, one with a low occupancy of d-orbitals (such Fig. 2 TEM micrographs of the carbon-supported PdNi alloy calcined for 3 h a at 300°C, b at 500°C, c at 700°C and d at 300°C for 5 h, respectively, all of these images with 100 nm magnification as Ni) and the other with fully occupied d-orbitals (such as Pd), the d-orbital coupling effect between them can significantly decrease the Gibbs free energy of the electron transfer, resulting in an improvement in ORR kinetics [19]. The incorporation of Ni with Pd could bring favourable change of Pd electron structure to modify the reaction kinetics for ORR on PdNi alloy, thus resulting in a higher efficiency of the reduction process.…”

“…The interest in Pd is not only for the purpose to lower the cost of catalysts but pursuits an improved catalytic activity [13][14][15]. The catalytic activity of Pd can be modified by alloying with metals having smaller atomic size such as V, Cr, Fe, Co and Ni is particularly effective in enhancing the catalytic activity [16][17][18][19][20][21][22]. Due to the higher electronic density of nickel compared with the other metals in the first transition series, the metallic bond with palladium is stronger which results in increasing and improving the process of catalytic activity [23][24][25][26][27][28][29][30][31].…”

Carbon-Supported Palladium-Cobalt-Noble Metal (Au, Ag, Pt) Nanocatalysts as Methanol Tolerant Oxygen-Reduction Cathode Materials in DMFCs

“…Pd-Co, being very catalytically active towards ORR, this small extent of PtOH formation will not affect the reaction. Using theoretical calculations, Wang and Balbuena 45 showed that this bimetallic system thermodynamically has performance similar to Pt to catalyze ORR. However, in the presence of methanol, a cathodic shift of ϳ110 mV in the half-wave potential of ORR is observed ͑Fig.…”