Comparative DEMS study on the electrochemical oxidation of carbon blacks

The commercial success of polymer electrolyte fuel cells (PEFCs) depends on the development of Pt-based oxygen reduction reaction (ORR) catalysts with greater activity and stability to reduce the amount of expensive noble metal per device. To advance toward this goal, we have tested a novel class of unsupported bimetallic alloy catalysts (aerogels) as the cathode material in PEFCs under two accelerated stress test conditions and compared it to a state-of-the-art carbon-supported benchmark (Pt/C). The investigated Pt 3 Ni aerogel shows little degradation under high potential conditions (> 1.0 V) which can occur during fuel starvation and start-up/shutdown of the cell. If tested under the same conditions, the Pt/C benchmark displays significant losses of electrochemical surface area and ORR activity due to carbon support corrosion as observed in cross section and transmission electron microscopy analysis. When testing the durability upon extended load cycling (0.6-1.0 V), Pt 3 Ni aerogel demonstrates less stability than Pt/C which is related to the severe Ni leaching from the alloy under such conditions. These findings highlight the advantages of using unsupported ORR catalysts in PEFCs and point to the reduction of non-noble metal dissolution as the next development step. Polymer electrolyte fuel cells (PEFCs) currently rely on large amounts of carbon-supported platinum (Pt/C) catalysts (≈0.4 mg Pt /cm 2 electrode ) to reduce the voltage losses due to the sluggish kinetics of the cathodic oxygen reduction reaction (ORR).1 Recent advancements in minimizing the Pt loading and associated costs were achieved by alloying platinum with other metals like Ni, Cu and Co which increases the Pt mass-specific ORR activity.2 On the other hand, these catalysts suffer from significant corrosion of the carbon support and Pt nanoparticles during PEFC operation which compromises their long-term efficiency and reliability.3 To specifically mitigate the issue of support stability, researchers have developed alternative corrosionresistant supports (e.g. conductive metal oxides 4-7 ), extended metal surfaces (e.g. 3 M nanostructured thin film catalysts 8 ) and unsupported materials (e.g. Pt-coated Ni, Co or Cu nanowires 9-11 ). Pursuing this last strategy, unsupported bimetallic Pt-Ni electrocatalysts with high specific surface area (≈30 m 2 /g Pt ) and nanochain network structure, referred to as aerogels, were synthesized in a previous work. 12These materials reach the U.S. Department of Energy target (DOE; i.e. 440 A/g Pt at 0.9 V vs. the reversible hydrogen electrode (V RHE )) for automotive PEFC application when tested as thin films by the rotating disk electrode (RDE) technique, 13 which is the standard tool employed by the majority of researchers in this field for initial assessment of catalyst activities. Considering that performance figures derived from such RDE experiments, often do not translate fully to the technical system, it is fundamental to also assess activity and durability in PEFCs to evaluate the real application p...

Section: Resultssupporting

confidence: 90%

Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes

Henning

Herranz

Ishikawa

et al. 2017

“…Interestingly decreasing the LPL even further (to 0.05 V RHE ), the CO 2 signal increases dramatically and a significant, almost constant CO 2 signal is observed even at the end of the test cycle. A similar effect was observed independently by Ashton et al 32 and Linse et al 37 Linse et al suggested that below 0.6 V RHE the changes in carbon corrosion behavior cannot be attributed to the catalytic effect of Pt, which is already reduced. Both authors agree that it can originate from modifications in carbon surface oxide composition.…”

Section: Resultssupporting

confidence: 78%

On the Need of Improved Accelerated Degradation Protocols (ADPs): Examination of Platinum Dissolution and Carbon Corrosion in Half-Cell Tests

Pizzutilo

Geiger

Grote

et al. 2016

136

150

In this work we employ the advanced scanning flow cell based analytical techniques, viz. inductively coupled plasma mass spectrometry (SFC-ICP-MS) and on-line electrochemical mass-spectrometry (SFC-OLEMS) to directly detect the amounts of dissolved platinum and evolved carbon dioxide in two protocols that are commonly used in the fuel cell community to simulate load cycle and start-stop conditions in proton exchange membrane fuel cells (PEMFCs). In contrast to previous assumptions, claiming a separation between carbon corrosion and platinum dissolution, in both standard protocols platinum dissolution and carbon corrosion are present at low rates, which is also reflected by a comparably low ECSA decrease. On the other hand, a huge increase in rate of both processes is observed during transitions from low to high potential regimes experienced by a PEMFC in operation, here studied in a third protocol covering the whole potential range from 0.6 to 1.5 V RHE . The latter is typically not addressed in literature. This finding is explained by taking into account platinum catalyzed carbon corrosion and transient platinum dissolution. Based on the obtained results, the question is raised on the practical adequacy of the standard protocols for differentiation of degradation processes and simulation of the degradation processes occurring in PEMFCs.

“…The fact that for a MEA with Pt black electrode and gold mesh contacting practically no CO 2 release was observed also is a strong indication that the start-stop cycle ADT does not significantly stress the ionomer neither in the membrane nor the electrode, A surprising result was the finding that the start-stop cycle ADT caused a destabilization of the carbon support, which lead to an increase of the carbon mass loss rates on subsequent operation even if no stress potentials were applied. This can go along with the finding by Ashton and Arenz 9,10 who found that transferring their carbon electrode to low potentials after excursion to high potential caused the formation of reduced oxygenated carbon surface groups which are easier to oxidize than the carbon itself. However, the potentials for which CO 2 release is observed in this work are even lower.…”

Section: Discussionsupporting

confidence: 77%

“…Ashton and Arenz reported on detailed studies of the carbon corrosion of high surface area carbon supports with and without graphitization treatments using DEMS. 9,10 They found that in particular for the non-graphitized support not the entire observed faradaic current caused the release of either CO 2 or O 2 . They therefore postulated that also a partial oxidation of the carbon surface can occur causing either the release of CO or the formation of oxygenated carbon groups at the carbon surface.…”

mentioning

confidence: 99%

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

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...