Abstract
Platinum is one of the most important electrode materials for continuous electrochemical energy conversion due to its high activity and stability. The resistance of this scarce material towards dissolution is however limited under the harsh operational conditions that can occur in fuel cells or other energy conversion devices. In order to improve the understanding of dissolution of platinum, we therefore investigate this issue with an electrochemical flow cell system connected to an inductively coupled plasma mass spectrometer (ICP-MS) capable of online quantification of even small traces of dissolved elements in solution. The electrochemical data combined with the downstream analytics are used to evaluate the influence of various operational parameters on the dissolution processes in acidic electrolytes at room temperature. Platinum dissolution is a transient process, occurring during both positive- and negative-going sweeps over potentials of ca. 1.1 VRHE and depending strongly on the structure and chemistry of the formed oxide. The amount of anodically dissolved platinum is thereby strongly related to the number of low-coordinated surface sites, whereas cathodic dissolution depends on the amount of oxide formed and the timescale. Thus, a tentative mechanism for Pt dissolution is suggested based on a place exchange of oxygen atoms from surface to sub-surface positions.
Highlights
Nowadays fuel cells, electrolyzers and metal–air batteries are considered as key elements in the development of sustainable energy architecture for our future societies
In order to investigate dissolution processes at positive potentials with and without oxide reduction, we carried out a sequence of potential cycling experiments with a constant upper potential limit of 1.55 VRHE and varying lower potential limits, complementary to the experiment described in our last report.[21]
Scheme 1 Alternation of the platinum surface state during: (a) anodic polarization, above ca. 1.1 VRHE dissolution and passivation of the surface are in competition; and (b) cathodic polarization, during surface reduction below ca. 1.0 VRHEdeposition and dissolution are in competition
Summary
Electrolyzers and metal–air batteries are considered as key elements in the development of sustainable energy architecture for our future societies. For several of these electrochemical energy conversion devices, platinum – typically dispersed in the form of nanoparticles on a carbon support – serves as the state of the art electrocatalyst.[1] the broad commercial application of such devices, for instance the application of proton exchange membrane fuel cells (PEMFCs) in the automotive industry, has not been successful so far, mainly due to the lack of stability of the electrode materials, which is one of the most important challenges still to be met. Major progress has been made in the understanding of the degradation phenomena of platinum-based electrocatalysts occurring in a fuel cell under operation, which has led to signi cant improvements in stability.[2,3,4,5,6,7,8] In particular, platinum dissolution (and eventually successive re-deposition) was demonstrated to be of tremendous importance in the course of PEMFC catalyst degradation.[1,5,9,10,11,12] Even though platinum as a noble metal is more inert towards oxidation in air than other materials, thermodynamic considerations of Pourbaix already suggest that it can be oxidized and
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