Piezoelectric energy harvesting is a process in which energy in the form of kinetic movements can be harvested and converted into useful electrical energy using piezoelectric materials. Metal-organic frameworks (MOFs) have a huge potential for piezoelectric energy harvesting owing to their high flexibility, structural tunability, and very low dielectric constants due to their high porosity. The piezoelectric constant d relevant for piezoelectric energy harvesting depends on the piezoelectric constant e and the flexibility of the structure (i.e. mechanical properties). The mechanical properties of MOFs have previously been extensively studied but the piezoelectric constant e was never explored for MOFs. In this work, we generate a database of piezoelectric properties, specifically e for around ∼1608 previously synthesized non-centrosymmetric MOF structures. The calculations were performed using the density functional perturbation theory (DFPT) method. The highest piezoelectric constant e obtained in this work is approximately ∼2.76 C m⁻², which is significantly higher than that of the flexible organic piezoelectric polymer polyvinylidene fluoride (PVDF) and its copolymers. In this work, we analyze and identify structural factors that influence the values of the piezoelectric constant for high-performing MOFs. Based on that, a series of guidelines for the design of MOF structures that can lead to a high piezoelectric constant e are presented. One class of high-performing piezoelectric MOFs is based on polar patterns of O—(short)—Mo—(long)—O unequal bond length, reminiscent of ferroelectric inorganic oxides. This class could have potential for ferroelectricity, meaning that the bond length pattern could be reversed by external electrical field. We substantiate this by showing experimentally via SHG-microscopy that the O—(short)—Mo—(long)—O unequal bond lengths are indeed malleable by external conditions.

Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are highly versatile materials based on inorganic modes connected via organic linkers or purely via the connection of organic building blocks, respectively. This results in 3-D nanoporous frameworks, which, due to their combination of high porosity and variability of building blocks, can exhibit exceptional properties that make them attractive. Certain applications (e.g., in electronics and as membranes) require a thin film or even a patterned morphology on various substrates. Inkjet printing of MOFs has emerged as a simple and effective technique for the scalable production of a wide range of MOF (gradient) films and patterns on a wide range of substrates according to specific requirements. This review comprehensively reviews the achievements in inkjet printing of both MOFs and COFs. We discuss the different substrates, ink formulation, and hardware intertwined requirements needed to achieve high-resolution printing and obtain desired properties such as porosity, physical-mechanical characteristics, and uniform thickness. Crucial aspects related to ink formulation, such as colloidal stability and size control of MOFs and COFs, are discussed. Additionally, we highlight potential opportunities for furthering the development of inkjet printing of MOFs/COFs and critically assess the reporting of the printing procedures and characterization of the resultant materials. In this manner, this review aims to contribute to the advancements in understanding and optimization of inkjet printing of MOFs and COFs, as this technique holds great potential for diverse applications and functionalization of MOF/COF films and patterns.

A key challenge in capturing CO2 from postcombustion gases is humidity due to competitive adsorption between CO2 and H2O. Multivariate (MTV) metal-organic frameworks (MOFs) have been considered a promising option to address this problem, e.g., combining CO2-affinitive and hydrophobic groups. Here, we synthesized a series of amine and methyl cofunctionalized MTV MIL-53(Al)-xNH2(1 - x)CH3 and their parent materials. All the mixed linker MIL-53(Al)-xNH2(1 - x)CH3 showed amino linker enrichment compared to the synthesis ratio, yet the linkers were distributed relatively homogeneously from the bulk to the surface. Material hydrophobicity or hydrophilicity varied with methyl or amino group content, respectively. The single-component adsorption indicated that certain mixed linker MIL-53(Al)-xNH2(1 - x)CH3 might outcompete the parent materials. In CO2-H2O competitive adsorption, however, the hydrophobic parental MIL-53(Al)-CH3 outperformed the mixed linker MOFs. CO2 adsorption capacities of 5.4, 4.9, and 3.6 wt % were found for 0.3 bar of CO2 under 0, 5, and 10% RH, respectively. The results highlight that materials with enhanced hydrophobicity and tight-fitting pores can outperform groups with high CO2 affinity in the CO2 capture under humid conditions.