(Invited) Understanding Heterogeneous Functional Materials Via Scale-Bridging between Nano and Micro Transmission x-Ray Microscopy and Absorption Near Edge Structure

Abstract

Heterogeneous functional materials for energy-conversion and –storage, such as fuel cells, electrolyzers and batteries are hierarchical, porous, electroactive materials that rely on multi-scale imaging techniques for accurate morphological characterization. These materials are building blocks for the catalyst layers, gas diffusion layers and electrodes in fuel cells, porous transport layers (PTLs) in electrolyzers and porous electrodes within batteries. Functional materials electrochemical functionality can be assessed with various in-situ and operando techniques that probe chemical state during operation, such as x-ray absorption near edge structure (XANES). Synchrotron bright x-ray sources allow for nano-scale imaging combined with XANES, which state-of-the-art lab-scale systems are still lacking. Furthermore, these bright sources allow ultra-fast imaging on sub-second and micro-second temporal resolution. Optimization of temporal, spatial and chemical dimensions is critical to answer the morphological and chemical questions of interest [1]. Transmission x-ray microscopy (TXM) and its three-dimensional analog x-ray computed tomography (CT) allows 30 nm resolution with nano-CT set-up and 1 um resolution with micro-CT beamlines. The nano-CT beamlines have precise energy resolution to enable a combination of 3D imaging and XANES. The generated imaging data sizes exceed terabytes of space and requires modeling frameworks to interpret the findings. We have developed a scale-bridging algorithm to incorporate nano-scale findings into micro-scale using direct numerical simulations and coupling algorithms via effective properties and boundary conditions. In this work we will present several examples of operando data collected on fuel cells, electrolyzers and batteries using nano and micro x-ray CT and in some instances with XANES or with sub-second radiography. Full-field x-ray imaging beamline 18-ID at National Synchrotron Light Source II has achieved 3 minutes nano-CT scan, what enables full 3D XANES scan within reasonable time-frame. Some of the questions that we attempt to answer include activity and water management of NiCu/KB electrocatalyst used in alkaline fuel cells as anode; electrocatalyst activity and its interface with the PTL within operating electrolyzers, and Li metal dendrite growth in solid polymer electrolyte batteries during cycling. Scale-bridging framework is applied to fuel cell electrodes to understand oxygen transport and reaction-diffusion processes for these multi-scale catalyst layers.