Researchers at Jilin University in China have developed a new class of self-assembled crystalline porous organic salts (CPOSs) featuring high proton conductivity for applications such as proton-exchange membranes for fuel cells.

As reported in the journal Angewandte Chemie, the water content in the channels of the porous salts plays an important role in the proton conduction performance of the materials. The new porous salts feature high proton conductivity at ambient conditions and can reach as high as 2.2 x10-² S cm^-1 at 333 K (60 ˚C) and high humid conditions—among the best conductivity values reported yet for other porous materials.

Crystalline porous organic materials (e.g., covalent organic frameworks (COFs) covalent triazine frameworks (CTFs), etc.) are potentially useful for many applications, including catalytic systems, separation processes, and gas storage. Although these framework-like structures vary greatly, they have one thing in common: their components are connected through covalent bonds.

Porous organic salts, on the other hand, are a new class of materials with components held together by ionic bonds—the attractive forces between positive and negatively charged ions. They are challenging to produce because their pores usually collapse; the ionic bonds of previously known organic salts are not strong enough to stabilize a porous structure.

Crystal structure of four different CPOSs. Xing et al. Click to enlarge.

Researchers working with Teng Ben at Jilin University (Changchun, China) have now successfully combined organic bases and acids to produce salts with very strong bonds and defined crystalline structures that form stable pore systems.

Presenting high porosity, the crystalline salts contain polar 1-D channels for which it is easy to confine highly polar molecules e.g. water inside the channels. The porous organic salts capably reveal high proton conductivity which can be attributed to the water inside the pores of the salts. The key aspect of this study is to strengthen the ionic bond formed between acid and base so as to obtain stable porous organic salts.

—Xing et al.

These highly porous solids have the highest inner surface area ever found in an organic salt. The scientists demonstrated a significant correlation between the strength of the ionic bonds and the stability of the pore structure.

The pores in the salts form one-dimensional channels and can hold water. The water molecules are bound to each other and to the charged groups through hydrogen bonding. These aspects give the salts their unusually high proton conductivity.

Materials with high proton conductivity have become the focus of attention because they are good electrolytes for fuel cells. In a fuel cell, two half reactions of a chemical reaction occur while physically separated. The most popular version uses the reaction oxygen and hydrogen to form water. In this case, the two cells must exchange protons (positively charged hydrogen atoms) through an electrolyte—usually through a proton-conducting polymer membrane. Scientists have been searching for more efficient, robust electrolytes. These new salts may be candidates. They are very stable at higher temperatures and their proton conductivity increases as the temperature rises.

In conventional polymer membranes, proton transport occurs through water-containing channels through which the protons within the network are transferred from one molecule to the next through hydrogen bonded water molecules.

In the salts, the transport mechanism is different. Calculations indicate that the protons are sent through the channels by “courier”: A water molecule binds a proton and diffuses through the channel, releasing the proton on the other side.

Strategically designed to make space for water molecules inside the channels, CPOSs exhibit high porosity and are exceptionally promising as proton conducting materials. The proton conductivity values of the CPOSs are comparable to some organic materials reported …

We have focused in this contribution on how to increase the strength of the ionic bonds thereby enhancing the bonding energy between acid and base which helps yield stable crystalline salts with high porosities. Our laboratory is currently engaged in designing more space inside the CPOSs that can help to hold high polarity molecules thereby adding to reinforce the proton conductivity values of the CPOSs.

—Xing et al.

Resources

• Guolong Xing, Tingting Yan, Saikat Das, Teng Ben and Shilun Qiu (2018) “Synthesis of Crystalline Porous Organic Salts with High Proton Conductivity” Angewandte Chemie International Edition doi: 10.1002/anie.201800423