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

Abstract

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...

Online Monitoring of Electrochemical Carbon Corrosion in Alkaline Electrolytes by Differential Electrochemical Mass Spectrometry

“…Carbon corrosion was intensively studied using various analytical techniques, including Raman spectroscopy, FT‐IR spectroscopy, X‐ray diffractometry, X‐ray photoelectron spectroscopy, and identical location transmission electron microscopy . Carbon becomes thermodynamically unstable at potentials higher than its equilibrium potential of 0.207 V versus reversible hydrogen electrode (RHE) . The consequences of carbon corrosion typically include a decrease of the electrochemically active surface area (ECSA) as well as the conductivity.…”

“…[14] Carbon becomes thermodynamically unstable at potentials higher than its equilibrium potential of 0.207 Vv ersus reversible hydrogen electrode (RHE). [15] Thec onsequences of carbon corrosion typically include ad ecrease of the electrochemically active surface area (ECSA) as well as the conductivity. Electrochemical oxidation of carbon leads to the formation of both soluble and insoluble organic and inorganic products in the electrolyte.T ypical products of carbon electrooxidation include CO and CO 2 , [6,16,17] [Eqs.…”

Temperature Effects in Polymer Electrolyte Membrane Fuel Cells

“…At temperatures closer to the operating temperature of a PEMFC, the degree of the support oxidation is shown against the cell potential in Figure 4b. [55] As it is seen in the Figure, both the higher cell potential and the temperature contribute to accelerated oxidation of carbon support. Moreover, by decreasing the cell temperature closer to the freezing point of water, higher durability of the support is reported in literature (Figure 4c).…”

Strategies for Engineering High‐Performance PGM‐Free Catalysts toward Oxygen Reduction and Evolution Reactions

“…To understand the catalysts in terms of the active sites, reaction mechanisms and degradation mechanisms, advanced physical characterizations, and related theoretical simulation are critical. [ 91,111 ] The advanced in situ and operando technologies (such as X‐ray absorption spectroscopy, [ 112 ] transmission electron microscopy, [ 113 ] mass spectroscopy, [ 114 ] etc.) are also powerful techniques to detect the intermediates during ORR and OER processes and capture the side products during degradation, [ 105,112,113,115 ] which is leveraged in mechanism study.…”