Operando Magnetic Resonance Imaging for Visualizing Electrochemical Triple-Phase Boundary.

The triple-phase boundary (TPB) is a complex interface where gas, liquid, and solid phases converge, crucially regulating the efficiency and performance of many electrochemical devices such as fuel cells and batteries. However, conventional characterization techniques struggle to capture the dynamic processes and flooding at TPB. To address this, we develop operando electrochemical magnetic resonance imaging (EC-MRI), an inherently non-invasive technique sensitive to 1H, which probes both bulk and boundary regions, enabling real-time visualization of TPB evolution and a deeper understanding of its function at the device level under operational conditions. In a study of proton exchange membrane fuel cell (PEMFC), with a focus on the kinetically sluggish O2 reduction reaction in cathode, operando EC-MRI quantitatively illustrates the interplay between power output, water content, and TPB evolution. It reveals that the TPB maps undergo significant spatial and dynamic variations, with TPB deterioration, rather than apparent water accumulation, directly triggering flooding, as proved using catalysts with different water generation and adsorption capabilities. Our finding opens new perspectives on water management and TPB design, with potential applications in other critical electrochemical processes such as H2 oxidation, CO2 reduction, and N2 reduction, underscoring the value of operando EC-MRI for real-time diagnostics of electrochemical devices.