The electrochemical production of hydrogen and hydrocarbons is considered to play a decisive role in the conversion and storage of excess amounts of renewable energy. The electrocatalysis of the oxygen evolution reaction (OER), however, faces significant challenges for practical implementation of electrolyzers. In this work, a comparative study on the activity and stability of oxidized polycrystalline noble metals during the OER is presented. All studied metals exhibit transient and steady-state dissolution. Transient dissolution takes place during oxide formation and reduction. Steady-state dissolution depends on the OER mechanism on each surface: On metals such as Ru and Au, for which oxygen from the oxide participates in the OER, the Tafel slope is low and the dissolution rate is high. In contrast, on metals for which oxygen evolves directly from adsorbed water, such as Pt and presumably Pd, the Tafel slopes are high and the dissolution rates are low. This should be considered in the design of optimal OER catalysts.The conventional energy strategy based on the deployment of fossil fuels comprises two serious drawbacks: depletion of irretrievable fuels and environmental concerns upon their exploitation. As a consequence, the growing general interest in the concept of sustainable renewable energy has boosted the implementation of alternative energy supply technologies over recent years. [1] Particularly, the utilization of energy from sun and wind is considered as an adoptable scenario in the energy paradigm shift, as each of them is capable of covering the global energy demand, which is estimated to be 17 TW in 2030. [2] Unlike traditional energy supplies, however, the production of electricity from sun and wind is intermittent, which creates a terawatt-scale challenge: fluctuations in energy supply by variation of seasonal weather conditions must be efficiently leveled off. Consequently, besides production itself, conversion and storage of enormous amounts of energy during peak production hours as well as its constant distribution to end users during downtime is an essential piece of the puzzle. One approach capable of handling the terawatt scale is the conversion of electrical energy and storage in the form of chemical bonds as, for example, hydrogen (through water reduction) or hydrocarbons (through carbon dioxide reduction). [3] For instance, these fuels can be produced electrochemically in electrolyzers [4] and then utilized at local fuel cell factories or transported to consumers on demand. [5] The essential counterreaction in an electrolyzer for either technology (water or CO 2 reduction) will be the oxygen evolution reaction (OER). However, the high energy required for the rearrangement of the chemical bonds of water during the OER is actually a bottleneck in the production costs compared to other technologies such as reforming. [3a, 6] Namely, sluggish electrode kinetics and detachment of evolved gas bubbles can severely restrict efficiency. [7] Moreover, the catalyst activity needs to be sustained...

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 V RHE 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.

This manuscript investigates the degradation of a Pt/Vulcan fuel cell catalyst under simulated start–stop conditions in an electrochemical half-cell. Identical location transmission electron microscopy (IL-TEM) is used to visualize the several different degradation pathways occurring on the same catalyst material under potential cycling conditions. The complexity of degradation on the nanoscale leading to macroscopic active surface area loss is demonstrated and discussed. Namely, four different degradation pathways at one single Pt/Vulcan aggregate are clearly observed. Furthermore, inhomogeneous degradation behavior for different catalyst locations is shown, and trends in degradation mechanisms related to the platinum particle size are discussed in brief. Attention is drawn to the vast field of parameters influencing catalyst stability. We also present the development of a new technique to study changes of the catalyst not only with 2D projections of standard TEM images but also in 3D. For this purpose, identical location tomography (IL-tomography) is introduced, which visualizes the 3D structure of an identical catalyst location before and after degradation.

SummaryPlatinum and Pt alloy nanoparticles supported on carbon are the state of the art electrocatalysts in proton exchange membrane fuel cells. To develop a better understanding on how material design can influence the degradation processes on the nanoscale, three specific Pt/C catalysts with different structural characteristics were investigated in depth: a conventional Pt/Vulcan catalyst with a particle size of 3–4 nm and two Pt@HGS catalysts with different particle size, 1–2 nm and 3–4 nm. Specifically, Pt@HGS corresponds to platinum nanoparticles incorporated and confined within the pore structure of the nanostructured carbon support, i.e., hollow graphitic spheres (HGS). All three materials are characterized by the same platinum loading, so that the differences in their performance can be correlated to the structural characteristics of each material. The comparison of the activity and stability behavior of the three catalysts, as obtained from thin film rotating disk electrode measurements and identical location electron microscopy, is also extended to commercial materials and used as a basis for a discussion of general fuel cell catalyst design principles. Namely, the effects of particle size, inter-particle distance, certain support characteristics and thermal treatment on the catalyst performance and in particular the catalyst stability are evaluated. Based on our results, a set of design criteria for more stable and active Pt/C and Pt-alloy/C materials is suggested.

No abstract

The durability of electrode materials is a limiting parameter for many electrochemical energy conversion systems. In particular, electrocatalysts for the essential oxygen reduction reaction (ORR) present some of the most challenging instability issues shortening their practical lifetime. Here, we report a mesostructured graphitic carbon support, Hollow Graphitic Spheres (HGS) with a specific surface area exceeding 1000 m(2) g(-1) and precisely controlled pore structure, that was specifically developed to overcome the long-term catalyst degradation, while still sustaining high activity. The synthetic pathway leads to platinum nanoparticles of approximately 3 to 4 nm size encapsulated in the HGS pore structure that are stable at 850 °C and, more importantly, during simulated accelerated electrochemical aging. Moreover, the high stability of the cathode electrocatalyst is also retained in a fully assembled polymer electrolyte membrane fuel cell (PEMFC). Identical location scanning and scanning transmission electron microscopy (IL-SEM and IL-STEM) conclusively proved that during electrochemical cycling the encapsulation significantly suppresses detachment and agglomeration of Pt nanoparticles, two of the major degradation mechanisms in fuel cell catalysts of this particle size. Thus, beyond providing an improved electrocatalyst, this study describes the blueprint for targeted improvement of fuel cell catalysts by design of the carbon support.

No abstract

The near-surface ion distribution at the solid-liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H(+) in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.