Cathodic catalysts

The catalyst inks were prepared by dispersing the catalyst nanoparticles into an appropriate amoimt of Millipore water and 5wt% Nafion solution. Then, both the anode and cathode catalyst inks were directly painted using a direct painting technique onto either side of a Nafion 117 membrane. A carbon cloth diffusion layer was placed on to top of both the anode and cathode catalyst layers [3-5]. The active cell area was 2.25cm. ... 

In this study, commercial catalysts were used. As the cathode catalyst Pt black was used. Different amounts of Pt black, Pt-Ru black, 10wt% Pt-Pd/C and 20wt% Pt-Pd/C catalysts were used for the anode. 

Fig. 2. Effects of ion dose density on i-V curves. Cathode catalyst loading = 0.2 mg-Pt/cm, H2/air.

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

Paradoxically, all these significant recent contributions to the theory of the ORR, together with most recent experimental efforts to characterize the ORR at a fuel cell cathode catalyst, have not led at aU to a consensus on either the mechanism of the ORR at Pt catalysts in acid electrolytes or even on how to properly determine this mechanism with available experimental tools. To elucidate the present mismatch of central pieces in the ORR puzzle, one can start from the identification of the slow step in the ORR sequence. With the 02-to-HOOads-to-HOads route appearing from recent DFT calculations to be the likely mechanism for the ORR at a Pt metal catalyst surface in acid electrolyte, the first electron and proton transfer to dioxygen, according to the reaction... 

What is behind the apparent disagreements between Tafel slopes and reaction orders reported from recent investigations of the ORR at PEFC cathode catalysts and the slopes and reaction orders measured earlier for model systems of low Pt surface area Is the ORR process at a dispersed Pt catalyst possibly different in nature from the ORR process at low-surface-area Pt ... 

The lesson to be taken from this report by Paik et al. [2004] is that a Pt catalyst in contact with a hydrous electrolyte is so active in forming chemisorbed oxygen at temp-eramres and potentials relevant to an operating PEFC, that the description of the cathode catalyst surface as Pt, implying Pt metal, is seriously flawed. Indeed, that a Reaction (1.4) acmally takes place at a Pt catalyst surface, exposes, Pt to be less noble than usually considered (although it remains a precious metal nevertheless. ..). Such a surface oxidation process, taking place on exposure to O2 and water and driven by electronically shorted ORR cathode site and metal anode site, is ordinarily associated with surface oxidation (and corrosion) of the less noble metals. 

A COMPREHENSIVE EXPRESSION FOR Jqrr WITH CONSIDERATION OF THE REAL NATURE OF THE CATHODE CATALYST SURFACE... 

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Campbell S. 2006. Ballard Power System. Development of transition metal/chalcogen based cathode catalysts for PEM fuel cells. DOE Hydrogen Program Review, May 16-19, Washington, DC. Available at http //www.hydrogen.energy.gov/annual review06 fuelcells.html (click on catalysts section). 

In addition to their proven capacity to catalyze a highly efficient and rapid reduction of O2 under ambient conditions (e.g., cytochrome c oxidase, the enzyme that catalyzes the reduction of >90% of O2 consumed by a mammal, captures >80% of the free energy of ORR at a turnover frequency of >50 O2 molecules per second per site), metalloporphyrins are attractive candidates for Pt-free cathodes. Probably the major impetus for a search for Pt-free cathodic catalysts for low temperature fuel cells is... 

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. 

Jasinski R. 1964. A new fuel cell cathode catalyst. Nature 201 1212. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

Fuel cell applications Manganese dioxide as a new cathode catalyst in microbial fuel cells [118] OMS-2 catalysts in proton exchange membrane fuel cell applications [119] An improved cathode for alkaline fuel cells [120] Nanostructured manganese oxide as a cathodic catalyst for enhanced oxygen reduction in a microbial fuel cell [121] Carbon-supported tetragonal MnOOH catalysts for oxygen reduction reaction in alkaline media [122]... 

Manganese dioxide as a new cathode catalyst in microbial fuel cells. Journal of Power Sources, 195, 2586-2591. 

Zinc-Air Sealant, cathode catalyst support and current collector... 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

Alternative cathode catalysts to platinum have been the focus of many researchers over the past four decades. Numerous reviews have been published on various aspects and types of PEM fuel cell cathode catalysts.2,7-21 In this work we review the major classes of non-noble metal ORR catalysts in acidic electrolytes. The techniques used to study the catalysts, a brief history of catalyst development including major breakthroughs, and possible future directions will be discussed. 

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