Polyoxometalates (POMs) as Proton Conductors

The development of solid-state proton conductors (SSPCs) is of significant interest for their deployment as proton exchange membranes in fuel cell (PEMFC) technology. The aqueous medium synthesis with ample intrinsic proton sources has made crystalline polyoxometalates (POMs) promising SSPCs over others. Herein, we aim to showcase the solitary POMs and their hybrids (with polymers and MOFs/COFs) as SSPCs by organizing them based on the approaches taken up (intrinsic or extrinsic) to install various protonic sources while positioning them within specific components of the POM frameworks. Particular attention is paid with a critical discussion on whether the conductivity is purely protonic or a combination of protonic and other ionic conductivity (majorly originating from charge balancing counterions) and thus recommend the terminology to be used. The "Critical Discussion" section provides in-depth insights which are often overlooked, while the future recommendations are made in "Future Outlook".