# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 3 Applications
### Proton Conduction  
As a final application in which porous metal phosphonates can have a particular benefit, proton conduction is discussed. A summary of the proton conducting frameworks has been listed in Table 3. The framework built-up by zinc precursor and BTP as the phosphonate ligand (denoted as PCMOF-3) was evaluated for proton conduction. The aquo ligands on zinc were ascribed as the main source of protons to be transported. The proton conductivity of the framework was strongly dependent on the humidity, with \(4.5\times 10^{-8}\) S/cm at 44% RH (25 \({}^{\circ}\)C) and \(1\times 10^{-5}\) S/cm at 98% RH (25 \({}^{\circ}\)C). The activation energy (E\({}_{\text{a}}\)) for the proton transfer -0.17 eV was derived from the bulk conductivity of the PCMOF-3 framework. In comparison, the activation energy for 1 mol/L HCl has been reported as 0.11 eV and that of Nafion as 0.22 eV. Thus, PCMOF-3 represents a proton-conducting metal phosphonate framework in which the conductivity does not involve secondary SO\({}_{3}\)H sites. The polar interlayer of the framework serves as a scaffold for highly ordered water molecules that form a Grothuss proton transfer pathway, as evidenced by the low E\({}_{\text{a}}\). The study concluded with the evidence that both the phosphonate 'O' atoms and Zn aquo ligands contributed to the proton transfer process [122]. In another study, the \(\beta\)-PCMOF-2 framework, prepared by sodium and phloroglucinol in the presence of chlorosulfonic acid was applied. This sulfonate framework displayed a proton conductivity of \(1.3\times 10^{-3}\) S/cm at 85 \({}^{\circ}\)C and 90% RH. In order to improve the conductivity, an isomorphic ligand exchange was performed with a BTP ligand to replace part of the aryl sulfonic moieties with the BTP ligands, forming a new mixed PCMOF-2\({}^{1}\)\({}_{/}\) framework. This resulted in an order of magnitude with improved conductivity to values of \(2.1\times 10^{-2}\) at 85 \({}^{\circ}\)C and 90% RH. This conductivity was the highest equilibrated MOF proton conduction reported, exceeding the sulfuric acid modified MIL-101 framework. The activation energy from the Arrhenius plot was 0.21 eV, indicative for the Grothuss mechanism of proton conduction. The conductivity in this framework was extremely dependent on the RH and dropped significantly to \(2.4\times 10^{-5}\) S/cm at 20 \({}^{\circ}\)C and 50% RH [123]. In another study, a magnesium phosphonate framework was made from Mg(NO\({}_{3}\))\({}_{2}\) and octamethylane diamine-N,N,N',N'-tetrakis-methylenephosphonic acid (H\({}_{8}\)ODTMP) and evaluated for proton conductivity. This magnesium phosphonate framework was possessing 1D channels filled with water molecules running along the c and b axes. The P-OH groups in the framework were found to be pointing inwards to the channel, and thus allowing proton transport to take place, which results in conductivity. The highest proton conductivity for this framework was \(1.6\times 10^{-3}\) S/cm at 19 \({}^{\circ}\)C, 100% RH. The proton conductivity in the framework was attributed to the proton transfer in the framework via the Grothuss hoping mechanism [124]. The influence of metal choice was studied with hybrid metal phosphonates, MFM-Ni and MFM-Co were studied for their proton conductivity properties. At 25 \({}^{\circ}\)C and 98% RH, the proton conductivities of Ni and Co hybrids were \(4.5\times 10^{-4}\) and \(4.4\times 10^{-5}\) S/cm, respectively. The differences in conductivities between the isostructural materials arose from differences in the bond strength between the coordination water molecules and the metal cations. The activation energy for proton conduction in the Ni-hybrid was 0.43 eV, this value is at the boundary of the two mechanisms responsible for proton conduction (Grothuss \(<0.4\) eV and vehicular \(>0.4\) eV) [125].