Electrocatalyst screening

There are many opportunities to be explored in the area of binary, ternary and quaternary anode catalyst formulations for methanol oxidation. Most of the studies to date, with the exception of the work by Gurau et al. on PtRuIrOs [122], explored a rather limited number of catalyst compositions, typically less than five. In order to search and discover the optimum formulation and composition, fast and efficient electrocatalyst screening techniques are required that are able to provide electrochemieal performance evaluations under conditions directly applicable to... 

The electrochemical activity of fuel cell electrodes may be investigated by ex situ and in situ CV experiments. Ex situ method is a relatively fast method for electrocatalysts screening but in situ method allows the electrocatalysts investigation in the fuel cell which provides a more realistic approach of the catalyst activity. In the in situ method, CV of the fuel cell can be performed in one electrode at a time, by making the other a pseudo-hydrogen reference electrode. Thus, the working electrode is the cathode as it is... 

Table 16.2 summarizes the different array substrates and methods of preparation that have been used in SECM electrocatalyst screening. Practical SECM high-throughput screening requires arrays of samples that can be prepared rapidly, inexpensively, and in an automated way. An array should cover a wide range of compositions with good reproducibility of test spot compositions across the array. 

The TG/SC scheme for FAOR (e.g., the reaction in DFAFCs) electrocatalyst screening is shown in Figure 16.9a." The tip is a Au UME modified with an electrodeposited Hg thin layer. A constant potential is applied to the tip to generate a constant flux of HCOOH from the reduction of CO2 dissolved in the KHCO3 electrolyte. HCOOH is then oxidized at the electrocatalyst compositions on the GC substrate. Figure 16.9b shows the SECM images of HCOOH oxidation... 

Once we have developed our basic model and shown how it may be used to estab-hsh trends in electrochemical reactivity, we will take the further step of applying it to the identification of new bimetallic electrocatalysts. We will introduce simple procedures to rapidly screen bimetallic alloys for promising electrocatalytic properties, and we will demonstrate the importance of including estimates of the alloys stabihty in the screening procedure. Finally, we will give examples of successful apphcation of this method to specific problems in the area of electrocatalyst development. 

The experimental results are in complete agreement with the predictions of our computational screening approach the annealed BiPt sample shows enhanced HER activity compared with pure Pt. As mentioned above, this result is rather counterintuitive, given that Bi itself is a notoriously poor electrocatalyst for the HER [Trasatti, 1972]. Hence, it appears that our computational, combinatorial screening procedure is capable of identifying improved catalysts for electrochemical reactions that are not immediately apparent from simple intuitive arguments. 

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. 

Fernandez JL, White JM, Sun YM, Tang WJ, Henkelman G, Bard AJ. 2006. Characterization and theory of electrocatalysts based on scanning electrochemical microscopy screening methods. Langmuir 22 10426-10431. 

Guerin S, Hayden BE, Lee CE, Mormiche C, Owen JR, Russell AE, Theobald B, Thompsett D. 2004. Combinatorial electrochemical screening of fuel cell electrocatalysts. J Comb Chem 6 149-158. 

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. 

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. 

Schematic of a 64-electrode cell for electrochemical screening of electrocatalysts. (Reprinted with permission from Journal of Combinatorial Chemistry, 6,149 (2004). Copyright 2004 American Chemical Society.)...

A typical approach is to utilise a substrate which when hydrolysed by the enzyme gives rise to a product which can be easily detected elect-rochemically. Thiocholine can be easily detected using screen-printed carbon electrodes doped with cobalt phthalocyanine (CoPC) [18,19], which acts as an electrocatalyst for the oxidation of thiocholine at a lowered working potential of approximately +100 mV (vs. Ag/AgCl) [18,19], thereby minimising interference from other electroactive compounds ... 

Combinatorial Synthesis and High-Throughput Screening of Fuel Cell Electrocatalysts... 

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

Two general techniques have been described for the high-throughput screening of electrocatalyst libraries optical screening and electrochemical screening. 

A third screening method for arrays of electrocatalysts was recently introduced by Hillier and coworkers [15, 29, 30]. Using a scanning electrochemical microscope (SECM), a microelectrode tip is moved over an electrocatalyst array. The resulting electrochemical feedback currents are measured and used to generate an activity map of the electrocatalyst library. This method does not require individual electronic addressability for each electrocatalyst... 

Structural and compositional characterization of individual elements of a combinatorial library can be important for the initial validation of a particular combinatorial synthesis method. Many earlier reports on combinatorial synthesis and screening of electrocatalysts fall short of reporting the complete structural and compositional characterization of individual library elements of interest. The workflow described here includes catalyst characterization before and after screening, thereby establishing an activity-composition-structure-stability relationship for electrocatalysts. This can be relevant in light of the extreme conditions present in a conventional fuel cell environment. 

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