Electrocatalyst discovery

Most of the electrocatalysts we will discuss in this book are in the form of porous metal films deposited on solid electrolytes. The same film will be also used as a catalyst by cofeeding reactants (e.g. C2H4 plus 02) over it. This idea of using the same conductive film as a catalyst and simultaneously as an electrocatalyst led to the discovery of the phenomenon of electrochemical promotion. 

The above results demonstrate that computational screening is promising technique for use in electrocatalyst searches. The screening procedure can be viewed as a general, systematic, DFT-based method of incorporating both activity and stability criteria into the search for new metal alloy electrocatalysts. By suggesting plausible candidates for further experimental study, the method can, ultimately, result in faster and less expensive discovery of new catalysts for electrochemical processes. 

Chen GY, Delafuente DA, Sarangapani S, Mallouk TE. 2001. Combinatorial discovery of bifunctional oxygen reduction-water oxidation electrocatalysts for regenerative fuel cells. Catal Today 67 341-355. 

Reddington E, Sapienza A, Gurau B, et al. 1998. Combinatorial electrochemistry A highly parallel, optical screening method for discovery of better electrocatalysts. Science 280 1735-1737. 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

Advanced discovery of new electrocatalyst formulations is increasingly dominated by two techniques high-throughput screening of both model and practical catalyst materials and computational approaches to identify new active surfaces through theory. 

The development of new and improved electrocatalysts begins with the discovery of materials displaying improved intrinsic electrochemical activity. Intrinsic activity is best observed and compared in a well-controlled catalyst environment where variables that may disguise the intrinsic activity trends are minimized or absent. Particle size, particle size distribution, surface area, catalyst utilization and the distribution of crystallographic phases are parameters that are typically difficult to control. Vapor deposition of unsupported thin film electrocatalysts eliminates many of these variables. This method provides a controlled synthetic route to smooth, single-phase or multi-phase, ordered or disordered metal alloy phases depending on deposition and processing conditions. 

The complete primary screening workflow for the discovery of new fuel cell electrocatalysts is shown in Fig. 11.8. The individual components of this workflow are designed such that no bottleneck occurs. 

Significant advances have also been made in developing novel materials for oxygen reduction. For example, the use of Chevrel phases and chalcogenides as ORR electrocatalysts in aqueous electrochemistry has been significantly advanced since their discovery in 1986 by Alonso-Vante and Tributsch [175-177]. 

Sun, Y. P. Buck, H. Mallouk, T. E., Combinatorial discovery of alloy electrocatalysts for amperometric glucose sensors, Anal. Chem. 2001, 73, 1599-1604... 

Reviewing the work on tire Pt-Ru electrocatalysts is beyond the scope of this article. We will briefly comment on some key advances in this area. Although early discovery by Petrii, and Bockris and Wrob Io wa established the catalytic activity of Pt-Ru alloys for methanol oxidation, despite of active investigation that followed, even the optimum composition of Pt-Ru is yet to be firmly settled. An early explanation for the mechanism by which bimetallic catalysts improve upon the performance of pure Pt, that is, the bifunctional mechanism proposed by Watanabe and Motoo, was recently challenged. 

Fuel cell electrocatalysis also has advanced significantly with innovations in the preparation of active Pt-Ru catalysts. A new type of electrocatalyst was developed, consisting of a Pt submonolayer on Ru nanoparticles. It has high CO tolerance and a very low Pt content. Its synthesis was facilitated by the discovery of electroless deposition of Pt on Ru nanoparticles that can be controlled so that most (> 90%) Pt atoms become available for the catalytic reaction. The catalytic activity of PtRu2o prepared by this method affords considerable advantages in the oxidation of H2, CO, and CH3OH compared with commercial Pt-Ru alloys. 

The project goals are to significantly improve both the kinetic performance of the electrocatalyst powder at low noble metal loading and its utilization in the cathode layers through layer structure development. Limitations in the catalyst performance will be addressed through combinatorial discovery of supported catalyst compositions and microstructures. The discovery of these new catalyst formulations will be carried out under conditions that have been scaled for commercial powder production. A large variation of binary, ternary and quaternary noble metal -transition metal alloys and mixed metal-metal oxide catalyst compositions will be screened. To improve the utilization/performance of the catalyst in MEAs,... 

In order to execute a combinatorial approach for discovery of novel electrocatalyst materials, several key workflow components need to be in place, including the ability to generate a large mrmber of electrocatalyst powders with variation in the composition and microstructure and to test their activity in the ORR by a rapid screening technique. 

Recently, there has been an increasing interest in the development of fuel cells. However, the major problems of electrocatalyst in fuel cells are the high loading of Pt and deactivation of Pt electrocatalyst [1,2], On the other hand, since the discovery of carbon nanotubes (CNTs), extensive research in the fields of applied physics, chemistry, materials science and engineering has rapidly emerged. Formaldehyde, as one of the intermediate products of methanol oxidation, can be activated to decompose to smaller fragments, protons, electrons and CO2 at high efficiency. 

One method of combating poisoning of hydrogen electrodes by CO is to modify the catalyst using an approach in which the relative strength of the chemisorbed CO bond is reduced. It is more than 30 years since the discovery of Pt/Ru as an electrocatalyst which is relatively tolerant of CO (relative to pure Pt), and no significantly better electrocatal5dic system has yet been foimd. 

A key to widespread use of fuel cells as a power source is high-performance, low-cost manufacturable electrocatalyst. Ink-jet technology has been used in library preparation for methanol fuel cell catalysts discovery at Penn State University and Illinois Institute of Technology [34]. 

How, then, does one go about enhancing the activity of electrocatalysts There are essentially two approaches improving the intrinsic activity of the catalyst and increasing the accessible surface area of the catalyst. The former task is accomplished through discovery of new materials or surface modifications that improve the charge-transfer characteristics of the catalyst the latter is accomplished through design. In order to accomplish this task, one must assemble a catalyst layer that maximizes the activity and accessible surface... 

In the acid medium of PEM fuel cells that may also contain some levels of fluoride derived from membrane degradation, Pt cannot be replaced with a non-noble metal or a metallic oxide, as both will corrode in such environment. This chapter describes some of the efforts that have been made over 40 years to obtain other non-precious metal electrocatalysts for ORR in acidic medium. They aU started with the discovery in 1964 that C0N4 phthalocyanine was capable of oxygen reduction in an alkaline solution. ... 

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