Review—Electromobility: Batteries or Fuel Cells?

The commercial feasibility of polymer electrolyte fuel cells (PEFCs) passes by the development of Pt-based, O 2 -reduction catalysts with greater activities and/or lower Pt-contents, as well as an improved stability. In an effort to tackle these requirements, unsupported bimetallic Pt-Ni nanoparticles (NPs) interconnected in the shape of nanochain networks (aerogels) were synthesized using a simple one-step reduction and gel formation process in aqueous solution. The products of this novel synthetic route were characterized by X-ray absorption spectroscopy to elucidate the materials' structure. Using electrochemical experiments, we probed the surface composition of the as-synthesized aerogels and of equivalent materials exposed to acid, and concluded that a Ni-(hydr)oxide side phase is present in the aerogel with a larger Ni-concentration. Regardless of this initial surface composition, the Pt-Ni aerogels feature a ≈3-fold increase of surface-specific ORR activity when compared to a commercial platinum-on-carbon catalyst, reaching the mass-specific requirement for application in automotive PEFCs. 1 to account for the large overpotential of the oxygen reduction reaction (ORR). Thus, Pt contributes significantly to the fuel cell system cost, and progress to reduce its loading is crucial to meet the long-term PEFC cost target of 40 $/kW set by the U. S. Department of Energy (DOE).2 One approach to reduce this excessive Pt-loading relies on increasing the catalysts' ORR activity, e.g. by alloying platinum with other metals like Ni, Cu and Co, to form materials which show up to one order of magnitude higher mass-specific activity than commercial Pt/C catalysts.3 On the other hand, these carbon-supported materials suffer from significant carbonand Pt-corrosion during the standard operation of PEFCs, gradually compromising their efficiency and reliability. 4 To partially overcome stability issues, research focuses on unsupported materials (e.g. Ptcoated Ni, Co or Cu nanowires 5-7 ) besides extended metal surfaces (e.g. 3 M nanostructured thin film catalysts 8 ) or alternative supports (e.g. conductive metal oxides 9-12 ). Naturally, those materials should be processable into catalytic layer architectures that provide reactant and product diffusion pathways similar to those in conventional Pt/C electrodes to guarantee high catalyst utilization and PEFC performance. 13To meet the requirements mentioned above, un-supported bimetallic electrocatalysts with high surface area (up to ≈80 m 2 /g metal ) and nanochain network structure, referred to as aerogels, have been synthesized. [14][15][16] The synthetic routes to prepare such materials vary, but generally involve the use of stabilizing surfactants and/or organic solvents [17][18][19] that can poison the catalyst's surface and decrease its activity. In contrast to those approaches, our groups have developed a facile one-step synthesis for mono-and bimetallic aerogels based on the reduction of metal salts by NaBH 4 in aqueous solution without addition of stabilizing surfac...

Section: Resultssupporting

confidence: 86%

Pt-Ni Aerogels as Unsupported Electrocatalysts for the Oxygen Reduction Reaction

Henning

Kühn

Herranz

et al. 2016

“…Recently, several studies have identified that the attainment of areal capacities as high as 4-8 mAh/cm 2 while minimizing the electrolyte content are the key factors in meeting these requirements. [1][2][3] The only currently commercialized Li-S battery has a significantly lower areal capacity of 2.5 mAh/cm 2 and operates in the presence of excess electrolyte, 4 necessitating significant technological breakthroughs to facilitate the possible use of Li-S batteries in the transportation sector. One of the main barriers to achieving such breakthroughs is the lack of fundamental understanding of the mechanism behind the operation of Li-S batteries.…”

mentioning

confidence: 99%

Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy

Gorlin

Patel

Freiberg

et al. 2016

Self Cite

116

134

Replacement of conventional cars with battery electric vehicles (BEVs) offers an opportunity to significantly reduce future carbon dioxide emissions. One possible way to facilitate widespread acceptance of BEVs is to replace the lithium-ion batteries used in existing BEVs with a lithium-sulfur battery, which operates using a cheap and abundant raw material with a high specific energy density. These significant theoretical advantages of lithium-sulfur batteries over the lithium-ion technology have generated a lot of interest in the system, but the development of practical prototypes, which could be successfully incorporated into BEVs, remains slow. To accelerate the development of improved lithium-sulfur batteries, our work focuses on the mechanistic understanding of the processes occurring inside the battery. In particular, we study the mechanism of the charging process and obtain spatially resolved information about both solution and solid phase intermediates in two locations of an operating Li 2 S-Li battery: the cathode and the separator. These measurements were made possible through the combination of a spectro-electrochemical cell developed in our laboratory and synchrotron based operando X-ray absorption spectroscopy measurements. Using the generated data, we identify a charging mechanism in a standard DOL-DME based electrolyte, which is consistent with both the first and subsequent charging processes. Lithium-sulfur (Li-S) batteries are an emerging battery technology that has the potential to meet the energy density and cost requirements of electric vehicles. Recently, several studies have identified that the attainment of areal capacities as high as 4-8 mAh/cm 2 while minimizing the electrolyte content are the key factors in meeting these requirements.1-3 The only currently commercialized Li-S battery has a significantly lower areal capacity of 2.5 mAh/cm 2 and operates in the presence of excess electrolyte, 4 necessitating significant technological breakthroughs to facilitate the possible use of Li-S batteries in the transportation sector. One of the main barriers to achieving such breakthroughs is the lack of fundamental understanding of the mechanism behind the operation of Li-S batteries. 1,5,6 In particular, it is not yet clear how the mechanism of discharge differs from the charge mechanism, 5 and if these two processes might change upon an increase in active material loading or reduction in electrolyte volume. 1 Consequently, there is a pressing need for performing operando characterization of Li-S batteries under a variety of conditions to identify fundamental aspects of the charging and discharging processes.One attractive but insufficiently explored system for a mechanistic characterization of Li-S batteries is the charging process of a Li 2 S cathode, a possible alternative to the conventional S 8 cathode, with a potential to enable batteries with silicon or tin rather than lithium anodes. 7,8 Specifically, it has been recently reported that a Li-S battery, which is assembled in a discharged s...

“…These developments are underway and have made significant progress. 5 Cost however is also closely tied to power density of the fuel cell stack. It is obvious that with higher power density less cells or an accordingly smaller cell area with the related reduced material use, is leading to a cost reduction if the high power density materials are of comparable cost.…”

mentioning

confidence: 99%

Operando Properties of Gas Diffusion Layers: Saturation and Liquid Permeability

Eller

Roth

Marone

et al. 2016

Polymer electrolyte fuel cells (PEFC) require a sophisticated water management to operate efficiently, especially at high current densities which are needed to reach system cost targets. The description of the complicated two-phase water transport remains a challenge in PEFC models and requires experimental validation on various length scales. In this work, operando X-ray tomographic microscopy (XTM) with scan times of 10 s was used to depict the liquid water at defined conditions at a technically relevant cell temperature of 80 • C. Cells with Toray TGP-H-060 gas diffusion layer (GDL) with microporous layer (MPL) and different rib width were operated with different feed gas humidifications (under-and oversaturated) and current densities between 0.75 to 3.0 A/cm 2 . Based on the quantification of the local and average saturation, the distribution of water cluster size is analyzed. Different categories of the water cluster connectivity are defined and quantified. The analysis is complemented with numerical simulations of the permeability in the liquid phase of the GDL that is correlated to saturation for the different GDL domains. The numerical simulations of the pressure drop of liquid water flow from the catalyst layer toward the gas channels in channel-rib repetition units allows for conclusions on cluster growth mechanisms. In the past two decades, large developments have lead hydrogen fed polymer electrolyte fuel cell (PEFC) technology to the brink of commercialization, e.g. in the stationary sector with more than 100000 deployments in the Enefarm activity 1 as well as in the mobile sector where major car manufacturers have presented first commercial vehicles 2,3 and niche-market commercialization for logistic vehicles. 4While for the automotive market also hydrogen infrastructure is a major barrier for widespread application, in both fields of application, cost of the fuel cell system remains the main hindrance for extensive spread of PEFC technology. Cost is closely tied to materials and manufacturing processes, such as more effective electrocatalyst and more durable membrane materials. These developments are underway and have made significant progress. 5 Cost however is also closely tied to power density of the fuel cell stack. It is obvious that with higher power density less cells or an accordingly smaller cell area with the related reduced material use, is leading to a cost reduction if the high power density materials are of comparable cost.Automotive fuel cells with state of the art materials and cell structures reach today more than 1 W/cm 2 with current densities up to 3 A/cm 2 .5 At such high current densities water management becomes more and more important and has to be properly designed on various scales from the system level to nanoscale structures, including all cell components as flow field plates, gas diffusion layers (GDL), the polymer membrane and the catalyst layer (CL). In order to minimize the ohmic losses, the polymer membrane needs to be well humidified, 6 however at high current den...