Surface Pourbaix diagrams and oxygen reduction activity of Pt, Ag and Ni(111) surfaces studied by DFT

Abstract

Based on density functional theory calculations we investigate the electrochemically most stable surface structures as a function of pH and electrostatic potential for Pt(111), Ag(111) and Ni(111), and we construct surface Pourbaix diagrams. We study the oxygen reduction reaction (ORR) on the different surface structures and calculate the free energy of the intermediates. We estimate their catalytic activity for ORR by determining the highest potential at which all ORR reaction steps reduce the free energy. We obtain self-consistency in the sense that the surface is stable under the potential at which that particular surface can perform ORR. Using the self consistent surfaces, the activity of the very reactive Ni surface changes dramatically, whereas the activity of the more noble catalysts Pt and Ag remains unchanged. The reason for this difference is the oxidation of the reactive surface. Oxygen absorbed on the surface shifts the reactivity towards the weak binding region, which in turn increases the activity. The oxidation state of the surface and the ORR potential are constant versus the reversible hydrogen electrode (RHE). The dissolution potential in acidic solution, on the other hand, is constant vs. the standard hydrogen electrode (SHE). For Ag, this means that where the potential for dissolution and ORR are about the same at pH = 0, Ag becomes more stable relative to RHE as pH is increased. Hence the pH dependent stability offers an explanation for the possible use of Ag in alkaline fuel cell cathodes.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…In these approaches, termed ab initio thermodynamics in the context of heterogeneous catalysis23, 99 and computational hydrogen electrode in the context of electrocatalysis,100, 101 the surrounding gas or liquid phase is represented by reservoirs, which then allows to compare structures of differing composition within a grand‐canonical framework. This leads to the prediction of thermodynamically stable (surface) phases either in form of phase diagrams as a function of the chemical potentials of the reactants or in form of Pourbaix diagrams as a function of pH and applied potential.…”

Ab initio GGA+U study of oxygen evolution and oxygen reduction electrocatalysis on the (001) surfaces of lanthanum transition metal perovskites LaBO3(B = Cr, Mn, Fe, Co and Ni)

“…We note that our thermodynamic analysis is within the scheme of the concerted proton-electron transfer 29 , where transfer of single (or multiple) proton and electron occurs in a concerted elementary step, instead of stepwise mechanisms in which the electron and proton are transferred sequentially. Consequently, all equilibrium potentials of (electrochemical) reaction steps contain the same G(H + )/e 0 term (G(H + ) is the free energy of proton and e 0 is the magnitude of its charge) and the activities/overpotentials of catalysts exhibit no pH dependences 23 . The stability of solvated metal ionic species, which requires decoupling of protons and electrons in reaction free energies (relative to the SHE reference) and incorporation of available experimental data for free energies of solvated metal ionic species vs. solid states 41 , are not included in this work.…”

“…Although these recent studies have demonstrated that improved exchangecorrelation functionals yield significantly different results than traditional LDA/GGA approaches, these more advanced models have not yet been applied to develop a realistic model of the stable surfaces and their coverage under catalytic conditions. Such effects have been shown to cause significant changes in the predicted ORR activity of Ni metal 23 . Furthermore, detailed experimental characterizations 24 has also observed a correlation between the surface HO* coverage and the ORR activities of LaBO 3 (001) films (B=Cr, Mn, Fe, Co, and Ni).…”

Theoretical Limiting Potentials in Mg/O2 Batteries

“…Several studies have noted the presence or importance of intermediate species in reactions occurring in metal-oxygen batteries, 51,67 or in other contexts. 59,62,64,65 Finally, it is instructive to compare the present results for an Mg/O2 cell to studies on the Li/O2 system. [35][36][37]66,73 A recent report by Viswanathan et al 37 Table 2.…”

“…Limiting potentials were evaluated for two reaction pathways (described below) involving discharge/charge reactions on MgO (100), following the approach of Norskov et al 30,32,35,36,[59][60][61][62] This treatment models the discharge process as a series of adsorption events onto the surface of an existing particle of the discharge product. For illustrative purposes, Figure 6 presents a generic discharge process.…”