Mixed matrix membranes for hydrogen separation: a comprehensive review and performance analysis

Hydrogen has emerged as one of the cleanest energy vectors that can support the transition into a green economy and thus can facilitate the transition to a carbon-neutral environment. Common hydrogen production methods include coal gasification, steam reforming, methane pyrolysis, and water electrolysis. All the hydrogen production methods produce a mixture of H₂ and other products such as CO₂, N₂ and CH₄ depending on the method. To separate hydrogen from other molecules, common methods such as cryogenic distillation and pressure swing adsorption have been used widely. In addition to these methods, membranes can be used which offer energy efficiency compared to the previously mentioned methods. The widely used membranes for H₂ separation are metallic membranes such as palladium-based membranes. Despite their high separation performance, they are not cost-effective. Another type of membrane that can address cost-efficiency, energy consumption, and performance limitations is the polymeric membrane. Moreover, polymeric membranes are also solution-processable and thus offer another advantage from a fabrication point of view. However, polymeric membranes usually suffer from a permeability-selectivity trade-off. Therefore, there is a need to improve the hydrogen separation performance of polymeric membranes, and one effective strategy is to form mixed matrix membranes (MMMs). MMMs are composite membranes composed of at least two components: polymers and fillers. The presence of fillers in this type of membrane is important to improve the separation performance of polymeric membranes. This review then aims to provide an overview of MMMs used for hydrogen separation, starting from their fabrication strategies until thorough discussions and assessments of different fillers. Moreover, this article also comprehensively evaluates the performance of MMMs by assessing their improvement in the separation performance and scrutinizing the impact of the filler's physical properties on the MMM performance. Lastly, the outlook of the field is also given to direct future research in this field.