Non-Pt catalysts

General Motors has assessed the required activity of a catalyst that costs less compared to the current state-of-the-art Pt activity based on these con-straints. i Assuming that the catalyst layer thickness could be increased to MOO pm from the currently used 10 pm, GM has calculated that the minimum volume activity (i.e., Acm ) for a cathode catalyst that costs less should be at least 10% of the current Pt activity. In reality, this seems rather generous, given the recent trend to reduce catalyst layer thicknesses to optimize high-current performances. The DoE has developed a series of volume activity targets for nonprecious metal catalysts, with the 10% of Pt activity target (300 Acm 3 at 0.8 V, H2/O2) necessary by 2015. 

There has long been interest in investigating Fe- and Co-based catalysts for oxygen reduction because of their role as highly effective enzymes for oxygen transport and conversion in biological systems. More recently, additional interest has been centered on alternative precious metals, metal oxides, and metal carbides and nitrides as possible oxygen reduction catalysts. 

Good progress has been made in improving the activity of non-Pt catalysts. The most promising systems will now be reviewed. However, very little work has been reported on the stability of these systems and virtually 

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As discussed previously, the DoE has set a target of catalyst activity of four times fhaf of pure Pt/carbon. This can be expressed as activity per mass of Ft or acfivify per cost equivalent. Three general approaches have been investigated to achieve this target Pt alloys, Pt core-shells, and non-Pt catalysts... 

It has been predicted that a Pd skin on a PdsFe core would sit close to the top of the activity/O binding energy "volcano" curve and thus have significantly higher activity than Pt and Pt/Pd core-shell materials. If these materials could be shown to be stable to long-term PEM conditions, then these could represent viable replacements for Pt. As with other alternative non-Pt catalysts, very few stability studies have been reported. 

Table 3.5. Turnover frequencies (TOFs) for O2 Reduction [e /site.s], Site Densities (SD in [sites/cm ]), and Their Product x 1.6 x 10 [C/e ] = A/cm of Supported Catalyst, at 800mVjj( f. ee for 47 wt% Pt Compared to Activity Requirements for a Costless Catalyst and the Experimental Absolute Activities of Non-Pt Catalysts from Selected References. TOFs are Shown Both at the Conditions of Measurement and Corrected for the Pt Reference Industrial Conditions, Assuming p02 to the First Order and = 54.7 kJ/mol O2, i.e., Corrected Using Parameters for Pt (Reproduced from Ref., with the Permission of Elsevier)...

Papers of 1056 dealed with non-Pt catalysts were found from 1964 to 2011. The amount of papers which treated organometaUic complexes was 69 % and that of chalcogenides is 12 %. However, we think that these catalysts have neither enough electrocatalytic activity for the ORR nor long-term stability. 

Poynton SD, Kizewski JP, Slade RCT, Varcoe JR (2010) Novel electrolyte membranes and non-Pt catalysts for low temperature fuel cells. Solid State Ionics 181(3-4) 219-222... 

Recent intensive research efforts have led to the development of less expensive and more abundant electrocatalysts for fuel cells. This book aims to summarize recent advances of electrocatalysis in oxygen reduction and alcohol oxidation, with a particular focus on low- and non-Pt electrocatalysts. The book is divided into two parts containing 24 chapters total. All the chapters were written by leading experts in their fields from Asia, Europe, North America, South America, and Africa. The first part contains six chapters and focuses on the electro-oxidation reactions of small organic fuels. The subsequent eighteen chapters cover the oxygen reduction reactions on low- and non- Pt catalysts. 

Chapter 1 discusses the current status of electrocatalysts development for methanol and ethanol oxidation. Chapter 2 presents a systematic study of electrocatalysis of methanol oxidation on pure and Pt or Pd overlayer-modified tungsten carbide, which has similar catalytic behavior to Pt. Chapters 3 and 4 outline the understanding of formic acid oxidation mechanisms on Pt and non-Pt catalysts and recent development of advanced electrocatalysts for this reaction. The faster kinetics of the alcohol oxidation reaction in alkaline compared to acidic medium opens up the possibility of using less expensive metal catalysts. Chapters 5 and 6 discuss the applications of Pt and non-Pt-based catalysts for direct alcohol alkaline fuel cells. 

Chapter 13 and 14 summarize the development of transitional metal oxides and transition metal chalcogenides for ORR, respectively. Chapter 15 is the only chapter in this book dedicated to the ORR catalysis of alkaline fuel cells. Electrocatalytic properties of various non-Pt catalysts including Ag, Pd, transition metal macrocycles, metal oxides, and multifunctional materials are presented. Fundamental issues related to the design of low-cost, high-performance electrocatalysts for alkaline fuel cells are discussed. Chapter 16 and 17 review the recent advances on the study of ORR on Au and Pd-based catalysts, respectively. 

Jiang et al. [15] reported that the ORR activities in alkaline medium of a Pd-coated Ag/C were three times higher than the corresponding activities on the Pt/C, as measured in their rotating disk electrode tests. Piana et al. [16] reported that the specific current of their K18 non-PGM catalyst is about three times higher than Pt/C and also its Tafel slope is lower, as it is observed for other non-Pt catalysts [17]. He et al. [18] reported that the kinetic current density of their non-noble metal catalyst based CuFe-Nx/C material was comparable or even higher than a commercial Pt/C catalyst. 

Non-Pt catalysts are extremely interesting, due to limited Pt sources and high cost. Pd-based [57,1011], Ag-based [55] and WC promoted [57] cathode catalysts... 

As stated at the beginning of this Section 9.5, a major driving force for the development of non-noble metal catalysts is flic tower-cost factor (along with potential mefiianol tolerance in the case of the DMFC system). But even if the cost of large-scale synthesis of these non-Pt catalyst formulations in a fuel cell stable... 

The use of non-Pt catalysts such as metal complexes with phthalocyanines and porphyrines (Zhang et al., 2009) or iron-based catalysts (Fe/N/C) (Lefevre... 

In the quest to prepare efficient, durable, inexpensive catalysts as alternatives to Pt and Pt-based materials, non-Pt catalysts may be a feasible way to permanently resolve the cost issue in PEM fuel cell commercialization. Although there is still a long way to go in making them comparable with Pt-based catalysts, significant progress in non-Pt catalysts has been reported in recent years. 

Two types of non-Pt catalysts are currently being explored non-Pt metals and N-containing transition metal macrocycle catalysts. 

Non-Pt metal-based catalysts can be used as both anode and cathode components. The addition of tungsten carbide has been reported to improve the catalytic ability of some non-Pt catalysts. Izhar and coworkers (2009) investigated carbon-supported cobalt-tungsten and molybdenum-tungsten carbides and their activities as anode catalysts, using a single fuel cell and half-cell RDE. The maximum power densities of their 873 K-carburized CoWC/KB and MoWC/KB were 15.7 and 12.0 mW cm 2, respectively, which were 14% and 11%, compared to a 20 wt% Pt/C catalyst. 

Research into such catalysts started in the 1960s, when Jasinski (1964) discovered that cohalt phthalo-cyanine could catalyze the ORR in an alkaline medium. Fe and Co are the most commonly used metals for these catalysts. The origin of the electrocataly tic activity of N-containing non-Pt catalysts was generally recognized to he the N4-chelates (or Nj-chelates) of transition metals, due to the simultaneous presence of metal precursors, active carbon, and a nitrogen source under pyrolysis conditions. Beck (1977) proposed that the mechanism of the ORR catalyzed by such catalysts was mainly involved hy a modified... 

In summary, while none of the durability issues experienced with Pt catalysts were expected to arise with N-containing non-Pt catalysts during long-term operation, due to their unique structure and reaction mechanism these complexes do not so far show many fundamental advantages in chemical/electro-chemical stability when used in PEM fuel cells. From a practical point of view, activity and stability are still significant challenges for state-of-the-art non-Pt catalysts in PEM fuel cell applications. 

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