The effect of platinum on carbon corrosion behavior in polymer electrolyte fuel cells

Differential electrochemical mass spectrometry as well as online mass spectrometry in combination with singe cell testing has been used to study carbon corrosion of typical carbon materials discussed as support for electro-catalyst in fuel cells, e.g. carbon blacks or carbon nanotubes. Beside standard tests used to study the stability under automotive LT-PEMFC conditions, additional tests were performed to try to test the stability under the operating conditions of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). It was shown that under LT-PEMFC conditions a strong catalytic effect of platinum on the carbon corrosion rate is observable. High temperatures in HT-PEMFC do accelerate the corrosion rate. It was further found that corrosion does not only occur at high potentials but also to a minor amount at lower potentials. This low potential corrosion is in particular observed after a prior potential excursion of the electrode indicating that such excursions do not only lead to the direct corrosion of the support but also to the formation of unstable surface groups, which can be removed subsequently. Throughout all tests, CNT exhibited a higher stability than the tested carbon blacks. In single cell testing, the contribution of other carbon materials in particular the MPL was evaluated. It was found that it is present but small compared to the catalyst support itself. In order to reduce the expenditure of platinum group metals in polymer electrolyte membrane fuel cells the dispersion of the catalyst on a conductive support is the current state-of the art. Only few examples do not follow this approach. Most prominent among the exemptions is the Nano-Structured Thin Film (NSTF) type of catalyst developed by 3M.1 For most applications, carbon has become, however, a sort of standard support material. To accomplish the goal to reduce costs for the fuel cells, inexpensive carbon black materials like Vulcan XC72R from Cabot are used most often. However, as carbon in presence of water is thermodynamically unstable for potentials higher than 0.207 V 2 carbon corrosion can become an issue if the oxidation of the carbon is not strongly kinetically hindered e.g. via high degrees of graphitization. Consequently, carbon materials exhibiting such high degrees of graphitization like carbon nanotubes or graphene have attracted attention as alternative supports.The focus of the research on carbon corrosion is on LT-PEMFC for automotive application. With respect to carbon corrosion the reversed current decay mechanism proposed by Reiser et al.3 is an accepted cause for this type of degradation processes. This is confirmed e.g. in the review of PEMFC degradation processes published by Yu et al. 4 The process which is induced by the formation of a hydrogen oxygen front on the anode side upon hydrogen admission after longer standstill can cause potential excursions of the cathode side to potentials between 1.4 and 1.75 V.5 Accordingly, the common tests suites published by US Department of Energy (DoE) and the Fuel Ce...

mentioning

confidence: 96%

DEMS and Online Mass Spectrometry Studies of the Carbon Support Corrosion under Various Polymer Electrolyte Membrane Fuel Cell Operating Conditions

Cremers

Jurzinsky

Meier

et al. 2018

“…− r gr = −dS C,gr /dt = k gr * S C,gr [7] − r ox = −dS C,ox /dt = k ox * S C,ox − k gr * S C,gr [8] Where r gr and r ox are the reaction rates (m Previous testing using Pt catalysts of different Pt/C ratios has shown that platinum catalyzes carbon oxidation and corrosion; 6,17,18 however, since this testing used the same Pt/C ratio an assumption of negligible platinum effect was made to simplify the model. Therefore, until improvements to the model are made to account for these phenomena, this model should only be used to compare catalysts with a similar Pt-C interface.…”

Section: Observations Inmentioning

confidence: 99%

A Semi-Empirical Two Step Carbon Corrosion Reaction Model in PEM Fuel Cells

Young

Colbow

Harvey

et al. 2013

The cathode CL of a polymer electrolyte membrane fuel cell (PEMFC) was exposed to high potentials, 1.0 to 1.4 V versus a reversible hydrogen electrode (RHE), that are typically encountered during start up/shut down operation. While both platinum dissolution and carbon corrosion occurred, the carbon corrosion effects were isolated and modeled. The presented model separates the carbon corrosion process into two reaction steps; (1) oxidation of the carbon surface to carbon-oxygen groups, and (2) further corrosion of the oxidized surface to carbon dioxide/monoxide. To oxidize and corrode the cathode catalyst carbon support, the CL was subjected to an accelerated stress test cycled the potential from 0.6 V RHE to an upper potential limit (UPL) ranging from 0.9 to 1.4 V RHE at varying dwell times. The reaction rate constants and specific capacitances of carbon and platinum were fitted by evaluating the double layer capacitance (Cdl) trends. Carbon surface oxidation increased the Cdl due to increased specific capacitance for carbon surfaces with carbon-oxygen groups, while the second corrosion reaction decreased the Cdl due to loss of the overall carbon surface area. The first oxidation step differed between carbon types, while both reaction rate constants were found to have a dependency on UPL, temperature, and gas relative humidity. Early polymer electrolyte membrane fuel cell (PEMFC) research used platinum black as the catalyst for both the cathode and anode electrodes. These electrodes were costly due to their very high Pt loadings (>>1.0 mg/cm 2 ). One of the ways of reducing the platinum loading was to replace platinum black with dispersed platinum nanoparticles on a carbon support. The smaller platinum particles on carbon enabled a reduced Pt loading of the cathode to less than 0.5 mg/cm 2 , while maintaining the required platinum surface area required for high fuel cell performance. Although cost was reduced, the durability of the fuel cell was negatively impacted. At potentials greater than 0.2 V RHE (reversible hydrogen electrode), the carbon support is thermodynamically unstable and able to oxidize to carbon dioxide (CO 2 ) and/or carbon monoxide (CO) [Eqs. 1-3], 1 leaving the platinum unsupported and inactive. Furthermore, due to the loss of support the platinum particles have been shown to agglomerate into larger particles, dissolve into the ionomer, or get washed out of the system.Even in the presence of platinum, the kinetics of carbon oxidation/corrosion is relatively slow; therefore, carbon is quite stable under normal PEMFC operating conditions. In practice, elevated cathode potentials of greater than 1.2 V RHE are required to oxidize carbon at reaction rates high enough to cause significant structural degradation. Normal PEMFC operation occurs between 0-1.0 V RHE ; however, upon fuel starvation or gas switching conditions (start-up and shutdown operation due to infiltration of oxygen into the normally hydrogen filled anode compartment during fuel cell off conditions) the cathode potential can exceed ...

“…[9][10][11] The COR is more intense during start/stop or fuel starvation events, for which high potential values are reached at the cathode side (E > 1.5 V vs. RHE). [15][16][17] Carbon corrosion occurring at the cathode side can therefore be severe.…”

mentioning

confidence: 99%

“…For example, a loss of electrochemical surface area (ECSA) is monitored over time, which is mainly caused by migration/aggregation of Pt nanoparticles on the carbon support, 3,4 Pt dissolution/redeposition (electrochemical Ostwald ripening) [5][6][7][8] and/or catalyst support corrosion leading to detachment of Pt nanoparticles. [9][10][11][12][13][14] The latter process, the electrochemical carbon oxidation reaction (COR), is thermodynamically favorable at potentials higher than 0.2 V vs. the reversible hydrogen electrode (RHE) according to Reaction 1: C + 2 H 2 O → CO 2 + 4 H + + 4 e − E • = 0.207 V vs. RHE [1] At temperatures below 100…”

mentioning

confidence: 99%

Sb-Doped SnO₂Aerogels Based Catalysts for Proton Exchange Membrane Fuel Cells: Pt Deposition Routes, Electrocatalytic Activity and Durability

Ozouf

Cognard

Maillard

et al. 2018

Tin dioxide is a promising catalyst support to improve the stability of proton-exchange membrane fuel cells (PEMFC) cathodes at high voltages. However, optimizing the catalytic activity for the oxygen reduction reaction (ORR) of tin dioxide based electrocatalyst still remains challenging. In this study, an antimony doped tin dioxide (ATO) aerogel featuring suitable physico-chemical properties for application at a PEMFC cathode was successfully synthetized. Two platinum nanoparticles deposition methods were tested and compared. One is a chemical reduction route (using ethylene glycol, EG), the other uses ultraviolet (UV) irradiation followed by different thermal post-treatments. Electrocatalysts structure and morphology were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments. The highest ORR mass activity measured in liquid electrolyte at 25 • C was obtained for Pt/ATO from EG method (32 A g Pt −1 versus 27 A g Pt −1 for a reference Pt/HSAC 40 wt%). Pt/ATO turned out to be more stable than Pt/HSAC during an accelerated stress test composed of 5,000 potential cycles between 1.0 and 1.5 V vs. RHE at 80 • C.