Unlocking Fuel Cell Potential

Article 1: Electrochemical and Raman spectroscopic evaluation of catalyst durability for the start/stop operating condition of PEFCs

M. Hara, M. Lee, C.-H. Liu, B.-H. Chen, Y. Yamashita, M. Uchida, H. Uchida, M. Watanabe ^[1]

The selection and optimization of materials composing the cathodic catalysts are crucial aspects during the manufacturing of Polymer Electrolyte Fuel Cells (PEFCs).

The study further investigated the durability of several carbon-supported Pt catalysts (carbon black (CB) and graphitized carbon black (GCB)) thanks to electrochemical and Raman spectroscopic methods. During voltage step cycle testing, Pt/GCB samples notably exhibited longer half-lives (cycle numbers to reach half the original electrochemically active surface area (ECA) values) and smaller voltage decreases in the I–E measurements compared to Pt/CB. These results demonstrated a superior corrosion resistance of GCB over CB.

Complementarily, micro-Raman spectroscopy was employed to analyze the degree of degradation during the durability test by evaluating changes in the band area ratio of the G band (ca. 1575 cm⁻¹) to the D1 band (ca. 1325 cm⁻¹) for the different support carbons. Band areas were calculated from curve fitting of the Raman spectra. Comparison of the AD1-band/AG-band values before and after potential step cycles revealed differences in the degradation rates among GCB-supported catalysts. However, maintaining cell performance during voltage cycling was attributed to the high corrosion resistance of the support material and uniform dispersion of Pt nanoparticles on the support. Through this study it was revealed that the degradation of GCB support could be suppressed by heat treatment.

The degradation rates determined from the Raman spectra for GCB-supported catalysts were consistent with the results obtained from ECA and I–E measurements. The study demonstrated that micro-Raman spectroscopy is well-suited for investigating the corrosion of GCB-supported Pt catalysts, providing valuable insights into their degradation mechanisms.

Such insights are pivotal for advancing the development and optimization of fuel cell technology, offering pathways to more robust and efficient energy conversion devices.

Article 2: In situ optical measurements of solid oxide fuel cell electrode surfaces, and probing the oxidation of solid oxide fuel cell anodes using in situ Raman spectroscopy

Brightman, E.; Maher, R.; Offer, G.J.; Duboviks, V.; Heck, C.; Cohen, L.F.; Brandon, N.P. ^[2]

In this article, a miniaturized heated stage has been designed for in situ optical measurements of solid oxide fuel cell (SOFC) electrode surfaces. The aim is to enable a comprehensive investigation of the oxidation process of SOFC anodes using in situ and in operando Raman spectroscopy.

The presented optical cell prototype configuration provides the opportunity to combine electrochemical and optical spectroscopy measurements, for highly detailed study of electrochemical processes.

Enhancing the understanding of the electrochemical reaction mechanisms is crucial for addressing the significant challenges of durability and impurity tolerance in Solid Oxide Fuel Cell (SOFC) anodes. Major concerns include carbon deposition, sulfur poisoning, and redox cycling as primary causes of anode degradation. This paper presents a preliminary investigation into the oxidation state of a Ni-CGO cermet electrode during the reduction process.

Raman spectra centered on the NiO characteristic band were continuously collected during the reduction to monitor the oxidation state of the NiO at the surface. At the same time, the open circuit potential (OCP) of the cell was also monitored, overlaid with the integrated NiO Raman band intensity.

First, intensity of the NiO band initially exhibits an increasing trend simultaneously with the initial electrochemically measured reduction period. Subsequently, the band rapidly disappears, closely following a sharp drop in electrochemical potential. The OCP is influenced by the equilibrium of the redox reaction between H₂/H₂O at the working electrode and O₂/O₂₋ at the counter electrode. The reaction reaches equilibrium once an electrically conductive path is established at the anode, marked by the sharp drop in OCP observed. Moreover, the results have suggested that the formation of this electrically conductive path precedes the complete NiO reduction in the electrode.

The new apparatus resulting from the coupling with Raman spectroscopy, described in this paper, has great potential for a wide range of experiments, including investigation of reaction kinetics and mechanisms.

Article 3: Operando μ-Raman Study of the Actual Water Content of Perfluorosulfonic Acid Membranes in the Fuel Cell

Peng, Z.; Badets, V.; Huguet, P.; Morin, A.; Schott, P.; Tran, T. B. H.; Porozhnyy, M.; Nikonenko, V.; Deabate, S. ^[3]

Another capability of Raman microscopy is the ability to study the water content of polymer fuel cell membranes.

The electrolytic membrane is a hydrophobic perfluorinated carbon polymer, containing hydrophilic sulfonic acid groups (SO_3-). It allows H+ ions to circulate, but prevents electrons and gases from passing through. Water content and transport properties of perfluorosulfonic polymer electrolytes are strongly coupled features affecting each other. The humidification of this membrane, related to the hydration number (λ=[H₂O]/[SO_3-]), is a crucial factor for the proper functioning of fuel cells. When the membrane is hydrated, the protons attached to the sulfonic groups become mobile. If the water content of the membrane is too low, its conductivity decreases and its gas permeability increases. Drying out of the membrane causes its wear and reduces its performance. On the contrary, if its water content is too high, it is drowned, which causes it to swell and gives rise to mechanical tensions.

Results presented in this article demonstrate that Operando µ-Raman spectroscopy is a well-suited technique to probe/monitor the actual local water distribution across Nafion® and Aquivion™ membranes in the operating fuel cell. Different relative humidity, feed gases stoichiometry and current density conditions are investigated. During the variation of these different parameters, the Raman spectra presenting bands arising from vibrations of molecular groups belonging to the polymer phase and modes from sorbed water have been acquired. The integrated intensity signals of water and fluorinated matrix have been used to assess the water content evolution across the membrane. Water depth- profiles, obtained, appear almost linear, with larger water content at the cathode side. Moreover, the amount of sorbed water increases as expected at higher RH but decreases when the gas flow rate, and specially the current density, increases. This main phenomenon inducing membrane dehydration is the rising of the inner temperature with the current increase.

Insofar as the proton mobility across the electrolyte is related to the membrane water content and represents a main contribution to the FC performance, the previous results point out the concern of effective FC thermal management for improving the electrochemical system.

Concerning the differences observed between the (de)hydration behaviors of the different membranes studied in this work, Aquivion™ maintains during operation a water volume fraction definitively larger, thanks to the higher density of sulfonic groups and, presumably, the lower thickness decreasing ohmic losses (i.e. inner temperature) across the membrane at a given current density.

In this framework, operando µ-Raman represents a unique perspective tool to highlight several aspects of the membrane hydration behavior. In this sense, this technique should be able to aid considerably with the optimization of the cell operating conditions, as well as to assist the modeling of water transport processes.