Relationships between Atomic Level Surface Structure and Stability/Activity of Platinum Surface Atoms in Aqueous Environments

Abstract

The development of alternative energy systems for the clean production, storage, and conversion of energy is strongly dependent on our ability to understand, at atomic molecular levels, the functional links between the activity and stability of electrochemical interfaces. Whereas structure− activity relationships are rapidly evolving, the corresponding structure− stability relationships are still missing. This is primarily because there is no adequate experimental approach capable of monitoring the stability of welldefined single crystals in situ. Here, by utilizing the power of inductively coupled plasma mass spectrometry (ICP-MS) connected to a stationary probe and coupling this technique to the rotating disk electrode method, it was possible to simultaneously measure the dissolution rates of surface atoms (as low as 0.4 pg cm −2 s −1 ) and correlate them with the kinetic rates of electrochemical reactions in real time. Making use of this unique probe, it was possible to establish almost "atom by atom" structure−stability−activity relationships for platinum single crystals in both acidic and alkaline environments. We found that the degree of stability is strongly dependent on the coordination of surface atoms (less coordinated yields less stable), the nature of covalent and noncovalent interactions (i.e., adsorption of hydroxyl groups, oxygen atoms, and halide species vs interactions between hydrated Li cations and surface oxide), the thermodynamic driving force for Pt complexation (Pt ion speciation in solution), and the nature of the electrochemical reaction (the oxygen reduction/evolution and CO oxidation reactions). These findings open new opportunities for elucidating key fundamental descriptors that govern both activity and stability trends and will ultimately assist in the development of real energy conversion and storage systems.

Potential Dependence of Pt and Co Dissolution from Platinum-Cobalt Alloy PEFC Catalysts Using Time-Resolved Measurements

“…For comparison, Jovanovic et al 18 reported 0.0045% or 150 μML for 3 nm Pt/C cycled between 0.05 V and 1.0 V at 5 mV/s and Lopes et al 19 reported 103±10 μML for 3 nm Pt/C cycled between 0.05 V and 1.05 V at 10 mV/s. The percentage reported here equates to a Pt dissolution rate of 3.8 × 10 −13 g-cm −2 s −1 .…”

“…19 The silver electrode was made by coating a thin layer of micron-size silver particles on the glassy carbon working electrode of the BASi cell. The silver particles were firmly bound to the electrode using a thin film of perfluorosulfonic acid (PFSA) ionomer (5 wt% in lower aliphatic alcohols, Sigma-Aldrich).…”

“…[17][18][19][20][21][22][23] In a series of papers, Mayrhofer and co-workers showed that during potential cycling with UPL higher than 1.1 V, greater Pt dissolution occurs during the cathodic scan than during the anodic scan. 17,21,22 The amount of Pt dissolved during the cathodic scan increased with increasing UPL as well as with decreasing scan rate and occurred at potentials below 1 V in the cathodic scan.…”

“…18 Jovanovic et al also detected Pt at 1.0 V. The Mayrhofer et al 21,22 and Jovanovic et al 18 results are less conclusive for UPL lower than 1.1 V. Lopes et al developed a stationary ICP-MS probe coupled with a rotating disk electrode (RDE) which allows measurement of electrocatalytic activity simultaneously with catalyst dissolution. 19 They were able to detect Pt dissolution from Pt(111), Pt(100), polycrystalline Pt, and Pt nanoparticles on high surface area carbon during both anodic and cathodic sweeps with UPL as low as 1.05 V. 19 The purpose of this paper is to investigate the aqueous stability of commercially-available state-of-the-art PtCo alloy catalysts supported on high surface area carbon (PtCo/C). These alloy catalysts have much higher mass activity for ORR than Pt/C catalysts, and, due to their larger particle size (4-5 nm), can be more stable than the 2-3 nm Pt/C traditionally used to obtain high ORR mass activities.…”

Standards and Protocols for Data Acquisition and Reporting for Studies of the Electrochemical Reduction of Carbon Dioxide

“…[43][44][45][46][47][48] The transition metal cations evolved from the anode can reach the cathode even when an anion exchange membrane is utilized to separate the electrode chambers. 49 Whether this crossover occurs during operation or during the storage and cleaning of the electrochemical cell has yet to be resolved conclusively.…”