Search

Chemical synthesis of platinum-rare earth metal (Pt-RE) nanoalloys, one of the most active catalysts for the oxygen re-duction reaction, has been a formidable challenge, mainly due to the vastly different standard reduction potentials of the two metals and high oxophilicity of the latter. Here we report a universal chemical process to prepare Pt-RE nanoalloys with tunable compositions and particle sizes. Pt and RE metal ions from the most common hydrated metal salts are first atomically embedded into an in-situ formed C-N network, yielding a stable compound insensitive to O2 and H2O. The Pt-RE nanoalloys are subsequently obtained by heating the compound under a mild reducing atmosphere (e.g. 3.3% H2/Ar). The key intermediate step of the process is the formation of RE carbodiimides (RE2(CN2)3) along with Pt particles. This synthesis mechanism suggests an efficient strategy to prepare nanoalloys with highly reactive metals.

Phosphoric acid is amphoteric and has been used to dope basic polymer, e.g. polybenzimidazole, membranes and acidic polymer, e.g. perfluorosulfonic acid, membranes. Both membrane systems exhibit high proton conductivities at temperatures above 100 °C under anhydrous conditions. The former has been developed into a commercial fuel cell technology while the latter can only deliver a few mA cm−2 in fuel cells and hydrogen pumping cells. In this work, it is experimentally verified that the current window of acid doped perfluorosulfonic acid membranes in electrochemical cells under dry conditions is limited by the migration of H4PO4 + species in combination with the slow H3PO4 diffusion. The phosphoric acid dynamics were monitored in a cell equipped with integrated reference electrodes in the electrolyte membrane, which allowed for quantification of the phosphoric acid in different membrane segments. From the time-resolved measurements, the H4PO4 + transference number was found to be as high as 52% under dry conditions. In combination with the slow H3PO4 back-diffusion, which was 5–6 orders of magnitude lower than that of water, the migration of H4PO4 + towards the cathode results in rapid resistance increase at the anode-membrane interface, ultimately leading to the cell failure.

Phosphoric acid is the common electrolyte for high-temperature polymer electrolyte fuel cells (HT-PEMFCs) that have advantages such as enhanced CO tolerance and simplified heat and water management. The currently used rotating disk electrode technique is limited to tests in dilute solutions at low temperatures and hence is not suitable for catalyst evaluation for HT-PEMFCs. In this study, we have designed and constructed a half-cell setup to measure the intrinsic activities of catalysts towards the oxygen reduction reaction (ORR) in conditions close to HT-PEMFC cathodes. By optimization of the hydrophobic characteristics of electrodes and the catalyst layer thickness, ORR activities of typical Pt/C catalysts are successfully measured in concentrated phosphoric acid at temperatures above 100 °C. In terms of mass-specific activities, the catalyst exhibits about two times higher activity in the half-cell electrode than that observed in fuel cells, indicating the feasibility of the technique as well as the potential for further improvement of fuel cell electrode performance.

This work studies the long-term durability of high-temperature polymer electrolyte membrane fuel cells based on acid-doped polybenzimidazole membranes. The primary focus is on acid loss via the evaporation mechanism, which is a major cause of degradation in applications that involve long-term operation. Durability is assessed for 16 identically fabricated membrane electrode assemblies (MEAs), and evaluations are carried out using operating parameters as stressors with gas stoichiometries ranging from 2 to 25, current densities from 200 to 800 mA cm(-2), and temperatures of 160 or 180 degrees C. Cell diagnostics are composed of time resolved polarization curves, post mortem analysis, and in situ temperature measurements. A major part of the cell degradation during these steady-state tests can be ascribed to increasing area-specific series resistance. By means of post mortem acid-loss measurements, the degradation is correlated to the temperature and to the accumulated gas-flow volume. Such relations are indicative of acid loss via evaporation. Current density also plays a critical role for the acid loss and, thus, for the overall cell degradation. The effect of current is likely tied to mechanisms that involve water generation, migration of electrolyte ions, and locally elevated temperature inside the MEAs. (C) The Author(s) 2018. Published by ECS.

Long-term durability of HT-PEM fuel cells based on thermally crosslinked polybenzimidazole

Ni-YSZ (nickel - yttria stabilized zirconia) is a material widely used for electrodes and supports in solid oxide electrochemical cells. The mechanical and electrochemical performance of these layers, and thus the whole cell, depends on their microstructure. During the initial operation of a cell, NiO is reduced to Ni. When this process is conducted under external load, like also present in a stack assembly, significant deformations of NiO/Ni-YSZ composite samples are observed. The observed creep is orders of magnitude larger than the one observed after reduction during operation. This phenomenon is referred to as accelerated creep and is expected to have a significant influence on the microstructure development and stress field present in the Ni-YSZ in solid oxide electrochemical cells (SOCs), which is highly important for the durability of the SOC. In this work we present energy selective neutron imaging studies of the accelerated creep phenomenon in Ni/NiO-YSZ composite during reduction and also during oxidation. This approach allowed us to observe the phase transition and the creep behavior simultaneously in-situ under SOC operation-like conditions.

The current technologies of fuel cells and electrolzers are based on ionic conducting electrolyte materials exclusively operational either in the low (20 - 200ºC) or high (600 - 1000ºC) temperature ranges. The intermediate temperature window, especially between 200 and 400 ºC, is still only represented by early fundamental material research for ionic electrolytes. Such materials, most likely based on proton conductors, are expected to bring a new generation of the technologies: fuel cells by direct oxidation or internal splitting of biofuels such as methanol and ethanol, as well as efficient water electrolyzer, preferrably a CO2 co-electrolyzer for generation of organic liquid fuels. Such technologies are of essential simplicity and allow for kinetic enhancement so that the need for precious metal catalysts as in low temperature systems might be eliminated. At the same time, this temperature range is low enough to have a wide selection of materials for cell and stack construction, and with potential long-term durability. This talk will briefly outline the recent work at DTU based on acid-base complexes and metal phosphates

The incorporation of phosphotungstic acid functionalized mesoporous silica in phosphoric acid doped polybenzimidazole (PA/PBI) substantially enhances the durability of PA/PBI based polymer electrolyte membrane fuel cells for high temperature operation at 200°C.