For proton-exchange-membrane fuel cells (PEMFCs) to become a mainstream energy source, significant improvements in their performance, durability, and efficiency are necessary. To improve their durability, there must be a solid understanding of how the structural and electrochemical processes are affected during operation to propose mitigation strategies. To this aim, in situ and operando characterization techniques locally identify structural and electrochemical processes, which cannot be captured using conventional techniques. Nevertheless, linking these properties in the same geometric area has been challenging due to its inherent limitations, such as sample size and imaging resolution. This has created a knowledge gap in structure-to-electrochemical performance relationships as operation and degradation unevenly affect different areas of the cell.

The hot pressing process for fabricating membrane electrode assemblies (MEAs) has been widely adopted, yet little is known of its effects on the microstructural properties of the different components of the MEA. In particular, the interaction of the electrolyte, electrode and gas diffusion layer (GDL) due to lamination is difficult to probe as conventional imaging techniques cannot access the internal structure of the MEA. Here, a novel approach is used, which combines characterisation of hot-pressed membrane electrode assemblies using X-ray computed tomography, thermogravimetric analysis, differential scanning calorimetry and atomic force microscopy, with electrochemical performance measurements from polarisation curves and high-frequency impedance spectroscopy. Membrane electrode assemblies hot pressed at 100 o C, 130 o C and 170 o C reveal significant differences in microstructure, which has a consequence for the performance. When hot pressed at 100 o C, which is lower than the glass transition temperature of Nafion (123 o C), the catalyst only partially bonds with the Nafion membrane, leading to increased Ohmic resistance. At 170 o C, the Nafion membrane intrudes into the electrode, forming pinholes, degrading the catalyst layer and filling pores in the GDL. Finally, at 130 o C, the interfacial contact is optimum, with similar roughness factor between the catalyst and Nafion membrane surface, indicating effective lamination of layers.

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Catalytic synergy is an unresolved activity descriptor in energy conversion reactions. Here, this study reports the synergistically enhanced intrinsic oxygen evolution reaction activity of a FeNi model catalyst modified with molybdate oxo‐anions via concertedly boosted surface/interface interactions such as interfacial charge transfer, surface intermediate adsorption, in situ phase transformation, and gas bubble evolution. Capturing such cosynergistic impact reveals the accelerated transition of oxygen deprotonation to oxyhydroxide state, ready‐to‐function Fe and Ni active sites, the instant transformation of Fe and Ni local microenvironments to γ ‐FeOOH and β  ‐NiOOH phases, and ultrafast gas bubble growth and release. The accelerated oxygen bubble evolution dynamic is monitored in operando with a decreased dynamic variation of the interfacial Faradaic resistance. Such cosynergistic molybdate modification results in a tenfold increase in oxygen evolution turnover frequency as well as a 55 mV decrease in the overpotentials to deliver 10 and 1000 mA cm−2 with respect to the FeNi‐model catalyst.

2 Highlights First description of in-plane water distribution using neutron imaging in an aircooled, open-cathode fuel cell. High water content identified under cathode land area, whereas anode water content is relatively homogeneous. Water distribution in anode GDL directly linked to dispersion of PTFE. Anode GDL composition shown to affect water content and distribution in cathode. Combined X-ray computed tomography, SEM/EDS and TGA used to characterise GDL structure and composition. AbstractIn-situ diagnostic techniques provide a means of understanding the internal workings of fuel cells under normal operating conditions so that improved designs and operating regimes can be identified. Here, an approach is used which combines exsitu characterisation of two anode gas diffusion / microporous layers (GDL-A and GDL-B) with X-ray computed tomography and in-situ analysis using neutron imaging of operating fuel cells. The combination of TGA, SEM and X-ray computed tomography reveals that GDL-A has a thin microporous layer with 26 % PTFE covering a thick diffusion layer composed of 'spaghetti' shaped fibres. GDL-B is covered by two microporous media (29 % and 6.5 % PTFE) penetrating deep within the linear fibre network. The neutron imaging reveals two pathways for water management underneath the cooling channel, either diffusing through the cathode GDL to the active channels, or diffusing through the membrane and towards the 3 anode. Here, these two behaviours are directly affected by the anode gas diffusion PTFE content and porosity. KeywordsGas diffusion layer; air-cooled open-cathode; X-ray computed tomography; neutron imaging; water management. IntroductionPolymer electrolyte fuel cells (PEFC) fuelled with hydrogen are among the most promising energy conversion technologies for a broad range of applications, including portable, stationary and automotive power delivery. However, understanding the cell water management is crucial for performance optimisation.Flooding impedes reactant transport (water mainly concentrating at the cathode) and reduces the surface area of the catalyst, causing significant if not catastrophic decay in cell performance, and dehydration can lead to cracks and irreversible damage [1][2][3]. The gas diffusion layer (GDL) provides a pathway for electron transport, ensures even reactant delivery and helps water management within each cell. The water balance between flooding and membrane dehydration is a function of the GDL's structure, porosity and PTFE (hydrophobic) content. Here, two commercial GDLs with microporous layers are characterised ex-situ by capturing the design and structure via X-ray computed tomography (CT), along with its polytetrafluoroethylene (PTFE) / carbon distribution via SEM/EDS analysis and thermogravimetric analysis (TGA); in-situ 'visualisation' of the water distribution in the in-plane orientation was performed using neutron radiography. These techniques can be correlated with one another to gain new insights into the water management role of the GDL in fuel c...

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Surface chemistry is a pivotal prerequisite beside the catalyst composition toward advanced water electrolysis. Here, an evident enhancement of oxygen evolution reaction (OER) is demonstrated on a vanadate-modified iron-nickel catalyst synthesized by a successive ionic layer adsorption and reaction method, which demonstrates ultralow overpotentials of 274 and 310 mV for delivering large current densities of 100 and 400 mA cm -2 , respectively in 1ᴍ KOH, where vigorous gas bubble evolution occurs. Vanadate modification augments the OER activity by (i) increasing the electrochemical surface area and intrinsic activity of the active sites, (ii) having an electronic interplay with Fe and Ni catalytic centers, and (iii) inducing a high surface wettability and a low-gas bubble-adhesion for accelerated mass transport and gas bubble dissipation at large current densities. Ex situ and operando Raman study reveals the structural evolution of β-NiOOH and γ-FeOOH phases during the OER through vanadate-active site synergistic interactions. Operando dynamic specific resistance measurement evidences an accelerated gas bubble dissipation by a significant decrease in the variation of the interfacial resistance during the OER for the vanadate-modified surface.Achievement of a high catalytic turnover of 0.12 s -1 suggests metallic oxo-anion modification as a versatile catalyst design strategy for advanced water oxidation.