Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study

Abstract

To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as "conditioning", but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt−Pt and Pt−Co coordination).

Hydrocarbon-based Pemion™ proton exchange membrane fuel cells with state-of-the-art performance

“…S3 †) following the approach of Gasteiger et al 52,57 The Tafel slopes of the Pemion™ and the PFSA MEA are similar and in accordance with Tafel slopes reported in literature for MEAs with PFSAbased ionomers ($70 mV dec À1 ). 39,52 While this is expected for the same catalyst, the Tafel slope of our Pemion™ MEA is considerably lower than typical values for hydrocarbon MEAs found in literature, 46,58,59 which shows the progress towards higher kinetic performance.…”

“…1) as both membrane and ionomer in the catalyst layer. PtCo/C was used in the catalyst layer as it was shown in recent publications 39,40 to enable very high mass activities and fuel cell performances. With a peak power density of 2.1 W cm À2 under H 2 /O 2 (95% RH, 80 C, ambient pressure), and 1.1 W cm À2 under H 2 /air (80% RH, 80 C, 250 kPa abs ), we signicantly exceed all reported values for wholly hydrocarbon fuel cells, and for the rst time reach performance comparable to state-of-theart PFSA-based fuel cells.…”

Highly durable electrocatalyst with low-loading platinum-cobalt nanoparticles dispersed over single-atom Co-N-Graphene nanofiber for efficient fuel cell

“…The accelerated stress test (AST) was conducted by square wave voltage cycling at 0.6 V for 3 s and 0.95 V for 3 s with 200 ml min -1 H 2 feeding into the anode and 75 ml min -1 N 2 feeding into the cathode based on DOE's AST protocol required for MEA durability evaluation5 ; the cell was set at 80 o C and 100 % RH and the back pressures of both cathode and anode were set at ambient (101 KPa). After completion of the voltage cycles, a voltage recovery (VR) process was performed: the cell was held at 0.1 V under H 2 -Air for 1 h at 40 o C, 150 % RH, and 150 kPa abs , followed by application of various voltages between 0.1 V and 0.85 V for different time periods30 . Current-voltage polarization was then recorded to measure changes in catalyst activity and durability.…”

Electrochemical Cleaning Stability and Oxygen Reduction Reaction Activity of 1‐2 nm Dendrimer‐Encapsulated Au Nanoparticles

“…[42,43] As alluded to earlier, however, accurate correlation of theory and experiment requires knowledge of NP size and structure during electrocatalytic reactions. The best way to do this involves operando measurements, [44][45][46][47][48] but this is often difficult or impossible to achieve. As an alternative, one can evaluate the size and structure of catalysts before and after electrocatalytic reactions.…”