In the last decade, a few of the early attempts to bring CFD-DEM of fluidized beds beyond the limits of small, lab-scale units to larger scale systems have become popular. The simulation capabilities of the Discrete Element Method in multiphase flow and fluidized beds have largely benefitted by the improvements offered by coarse graining approaches. In fact, the number of real particles that can be simulated increases to the point that pilot-scale and some industrially relevant systems become approachable. Methodologically, coarse graining procedures have been introduced by various groups, resting on different physical backgrounds. The present review collects the most relevant contributions, critically proposing them within a unique, consistent framework for the derivations and nomenclature. Scaling for the contact forces, with the linear and Hertz-based approaches, for the hydrodynamic and cohesive forces is illustrated and discussed. The orders of magnitude computational savings are quantified as a function of the coarse graining degree. An overview of the recent applications in bubbling, spouted beds and circulating fluidized bed reactors is presented. Finally, new scaling, recent extensions and promising future directions are discussed in perspective. In addition to providing a compact compendium of the essential aspects, the review aims at stimulating further efforts in this promising field.

Per- and polyfluoroalkyl substances (PFAS) have found wide application in proton exchange membrane fuel cells (PEMFCs) and water electrolysers (PEMELs), thanks to their exceptional chemical and thermal stability. However, their environmental persistence and growing regulatory pressure—particularly from the European Union—have made the transition to PFAS-free components a priority. This work reviews current advancements in alternative materials that can guarantee the same performance or maybe improve it. Although several non-fluorinated materials have demonstrated initial performance close to PFAS-based benchmarks, significant challenges remain. These include limited long-term stability, difficulties for new materials to fit into existing stack architectures, and the lack of standardized testing protocols. Nevertheless, recent efforts have successfully demonstrated a PFAS-free PEM electrolyser stack at TRL 4, validating the technical feasibility of full PFAS substitution. Achieving commercial readiness will require parallel progress in materials development and industrial scalability. This review highlights the possibility that hydrogen technologies, such as fuel cells and electrolysers, which are called upon to support the energy transition towards a more sustainable future, are themselves truly environmentally friendly, thus making their use as green as possible.