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A porous crystal family has been explored as alternatives of Nafion films exhibiting super-proton conductivities of ≥10−2 S cm−1. Here, the proton-conduction natures of a solution-processed film of nanoparticles (NPs) have been studied and compared to those of a Nafion film. A mono-particle film of Prussian-blue NPs is spontaneously formed on a self-assembled monolayer substrate by a one-step solution process. A low-temperature heating process of the densely packed, pinhole-free mono-particle NP film enables a maximum 105-fold enhancement of proton conductivity, reaching ca. 10−1 S cm−1. The apparent highest conductivity, compared to previously reported data of the porous crystal family, remains constant against humidity changes by an improved water-retention ability of the film. In our proposed mechanism, the high-performing solution-processed NP film suggests that heating leads to the self-restoration of hydrogen-bonding networks throughout their innumerable grain boundaries. [ABSTRACT FROM AUTHOR]

Abstract: A specific electronic interaction among metal ions emerges in solution‐processed films of coordination polymer (CP) nanoparticles (NPs). The most classic CP, Prussian blue (PB), is alloyed with Ni to form FexNi3−x[Fe(CN)6]2 showing an identical X‐ray crystallographic phase in all composition ratios of 0≤x≤3. By surface modification using [Fe(CN)6]4−, the PB alloy NPs are dispersed into water, and spin‐coated films of the NPs adhere to indium‐tin‐oxide substrates via a low‐temperature heating process at 100 °C. It is revealed that systematic fine tuning of the electronic energy levels is optically and electrochemically possible in the films by controlling the composition ratios. This is possible due to the d(t2g)–π*(C≡N−) electronic conjugation among three redox‐active iron species, FeII−C≡N, FeIII−C≡N, and FeIII−N≡C, which coexist by alloying with Ni. The heating process conducts the restoration of discontinued d–π* bonding networks among the NPs. [ABSTRACT FROM AUTHOR]