Platinum Monolayer Electrocatalysts

This book describes a science and technology of a new type of electrocatalysts consisting of a single atomic layer of platinum on suitable supports. This development helped overcome three major obstacles—catalysts‘ cost, activity, and stability—for a broad range of fuel cell applications. The volume begins with a short introduction to the science of electrocatalysis, covering four reactions important for energy conversion in fuel cells. A description follows of the properties of metal monolayers on electrode surfaces, and underpotential deposition of metals. The authors then describe the concept of Pt monolayer electrocatalysts and its implications and their synthesis by galvanic displacement of less-noble metal monolayers and other methods. The main part of the book presents a discussion of catalysts’ characterization and catalytic properties of Pt monolayers for the four main reactions of electrochemical energy conversion: oxygen reduction and oxidation of hydrogen, methanol and ethanol. The book concludes with a treatment of scale-up syntheses, fuel cell tests, catalysts’ stability and application prospects.

For Pt, the best single-element catalyst for many reactions, the question of content and loading is exceedingly important because of its price and availability. Using platinum as a fuel-cell catalyst in automotive applications will cause an unquantifiable increase in the demand for this metal. This big obstacle for using fuel cells in electric cars must be solved by decreasing the content of Pt, which is a great challenge of electrocatalysis Over the last several years we inaugurated a new class of electrocatalysts for the oxygen reduction reaction (ORR) based on a monolayer of Pt deposited on metal or alloy carbon-supported nanoparticles. The possibility of decreasing the Pt content in the ORR catalysts down to a monolayer level has a considerable importance because this reaction requires high loadings due to its slow kinetics. The Pt-monolayer approach has several unique features and some of them are: high Pt utilization, enhanced (or decreased) activity, enhanced stability, and direct activity correlations. The synthesis of Pt monolayer (ML) electrocatalysts was facilitated by our new synthesis method which allowed us to deposit a monolayer of Pt on various metals, or alloy nanoparticles [1, 2] for the cathode electrocatalyst. In this synthesis approach Pt is laid down by the galvanically displacing a Cu monolayer, which was deposited at underpotentials in a monolayer-limited reaction on appropriate metal substrate, with Pt after immersing the electrode in a K2PtCl4 solution.

Understanding oxidation/dissolution mechanisms of Pt is critical in designing durable catalysts for the oxygen reduction reaction (ORR), but exact mechanisms remain unclear. Our present work explores the oxidation/dissolution of Pt and Pt monolayer (ML) electrocatalysts over a wide range of applied potentials using cells that facilitate in situ measurements by combining X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements. Furthermore, the X-ray absorption near edge structure (XANES) measurement demonstrated that Pt nanoparticle surfaces were oxidized from metallic Pt to [alpha]-PtO2-type oxide during the potential sweep from 0.41 to 1.5 V, and the transition state of O or OH adsorption on Pt and the onset of the place exchange process were revealed by the delta mu ([Delta][mu]) method. Only the top layers of Pt nanoparticles were oxidized, while the inner Pt atoms remained intact. At a higher potential over 1.9 V, [alpha]-PtO2-type surface oxides dissolve due to local acidification caused by the oxygen evolution reaction and carbon corrosion. Pt oxidation of PtML on the Pd nanoparticle electrocatalyst is considerably hampered compared with the Pt/C catalyst, presumably because preferential Pd oxidation proceeds at the defects in Pt MLs up to 0.91 V and through O penetrated through the Pt MLs by the place exchange process above 1.11 V.

Fuel cells are one of the most promising clean energy conversion devices that can solve the environmental and energy problems in our society. However, the high platinum loading of fuel cells - and thus their high cost - prevents their commercialization. Non- or low- platinum electrocatalysts are needed to lower the fuel cell cost. Electrocatalysis in Fuel Cells: A Non and Low Platinum Approach is a comprehensive book summarizing recent advances of electrocatalysis in oxygen reduction and alcohol oxidation, with a particular focus on non- and low-Pt electrocatalysts. All twenty four chapters were written by worldwide experts in their fields. The fundamentals and applications of novel electrocatalysts are discussed thoroughly in the book. The book is geared toward researchers in the field, postgraduate students and lecturers, and scientists and engineers at fuel cell and automotive companies. It can even be a reference book for those who are interested in this area.

This book covers a significant number of R&D projects, performed mostly after 2000, devoted to the understanding and prevention of performance degradation processes in polymer electrolyte fuel cells (PEFCs). The extent and severity of performance degradation processes in PEFCs were recognized rather gradually. Indeed, the recognition overlapped with a significant number of industrial dem- strations of fuel cell powered vehicles, which would suggest a degree of technology maturity beyond the resaolution of fundamental failure mechanisms. An intriguing question, therefore, is why has there been this apparent delay in addressing fun- mental performance stability requirements. The apparent answer is that testing of the power system under fully realistic operation conditions was one prerequisite for revealing the nature and extent of some key modes of PEFC stack failure. Such modes of failure were not exposed to a similar degree, or not at all, in earlier tests of PEFC stacks which were not performed under fully relevant conditions, parti- larly such tests which did not include multiple on–off and/or high power–low power cycles typical for transportation and mobile power applications of PEFCs. Long-term testing of PEFCs reported in the early 1990s by both Los Alamos National Laboratory and Ballard Power was performed under conditions of c- stant cell voltage, typically near the maximum power point of the PEFC.

Meeting the need for a text on solutions to conditions which have so far been a drawback for this important and trend-setting technology, this monograph places special emphasis on novel, alternative catalysts of low temperature fuel cells. Comprehensive in its coverage, the text discusses not only the electrochemical, mechanistic, and material scientific background, but also provides extensive chapters on the design and fabrication of electrocatalysts. A valuable resource aimed at multidisciplinary audiences in the fields of academia and industry.

This book is a printed edition of the Special Issue "Electrocatalysis in Fuel Cells" that was published in Catalysts