Bingzi Zhang

Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes

Tertiary sulfonium is introduced as the cationic functional group for hydroxide exchange membranes (HEMs). The methoxyl-substituted triarylsulfonium functionalized HEM (i.e., PSf-MeOTASOH) exhibits excellent thermal stability (TOD: 242 °C), acceptable hydroxide conductivity (15.4 mS cm−1 at 20 °C), and good chemical stability. Our work shows that, similar to nitrogen and phosphorus, a sulfur element with designed side groups can also be used to construct HEM cationic functional groups.

Permethyl Cobaltocenium (Cp*2Co+) as an Ultra-Stable Cation for Polymer Hydroxide-Exchange Membranes.

Hydroxide (OH−)-exchange membranes (HEMs) are important polymer electrolytes enabling the use of affordable and earth-abundant electrocatalysts for electrochemical energy-conversion devices such as HEM fuel cells, HEM electrolyzers, and HEM solar hydrogen generators. Many HEM cations exist, featuring desirable properties, but new cations are still needed to increase chemical stability at elevated temperatures. Here we introduce the permethyl cobaltocenium [(C5Me5)2Co(III)+ or Cp*2Co+] as an ultra-stable organic cation for polymer HEMs. Compared with the parent cobaltocenium [(C5H5)2Co(III)+ or Cp2Co+], Cp*2Co+ has substantially higher stability and basicity. With polysulfone as an example, we demonstrated the feasibility of covalently linking Cp*2Co+ cation to polymer backbone and prepared Cp*2Co+-functionalized membranes as well. The new cation may be useful in designing more durable HEM electrochemical devices.

Stabilizing the Imidazolium Cation in Hydroxide-Exchange Membranes for Fuel Cells

Stable and able: The hydroxide-conducting cationic functional group used in the hydroxide-exchange membranes of fuel cells is key to controlling chemical stability and solubility. A new imidazolium cation, 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl)imidazolium, is designed to take advantage of both strong electron-donation properties and steric hindrance. Synergy between these two effects leads to an efficient hydroxide-exchange membrane, with increased alkaline stability and improved OH(-) conductivity.

Structure–Property Relationships in Hydroxide‐Exchange Membranes with Cation Strings and High Ion‐Exchange Capacity

A series of poly(2,4-dimethyl-1,4-phenylene oxide) hydroxide-exchange membranes (HEMs) with cation strings containing a well-defined number of cations (CS-n) and similar, high ion-exchange capacities are synthesized to investigate the effect of cation distribution on key HEM properties. As the number of cations on each string grows, the size of the ionic clusters increases from 10 to 55 nm. Well-connected ion pathways and a hydrophobic framework are observed for n≥4. The enhanced phase segregation increases the hydroxide conductivity from CS-1 to CS-6 (30 to 65 mS cm(-1) ) and suppresses the water uptake (from 143 % to 62 %). Moreover, molar hydroxide conductivities for CS-n membranes show two distinctive stages as n increases: ∼23 S cm(2)  mol(-1) for n≤3; and ∼34 cm(2)  mol(-1) for n≥4.

A New Alkali‐Stable Phosphonium Cation Based on Fundamental Understanding of Degradation Mechanisms

Highly alkali-stable cationic groups are a critical component of hydroxide exchange membranes (HEMs). To search for such cations, we studied the degradation kinetics and mechanisms of a series of quaternary phosphonium (QP) cations. Benzyl tris(2,4,6-trimethoxyphenyl)phosphonium [BTPP-(2,4,6-MeO)] was determined to have higher alkaline stability than the benchmark cation, benzyl trimethylammonium (BTMA). A multi-step methoxy-triggered degradation mechanism for BTPP-(2,4,6-MeO) was proposed and verified. By replacing methoxy substituents with methyl groups, a superior QP cation, methyl tris(2,4,6-trimethylphenyl)phosphonium [MTPP-(2,4,6-Me)] was developed. MTPP-(2,4,6-Me) is one of the most stable cations reported to date, with <20 % degradation after 5000 h at 80 °C in a 1 m KOD in CD3 OD/D2 O (5:1 v/v) solution.

Relating alkaline stability to the structure of quaternary phosphonium cations

Alkali-stable quaternary phosphonium (QP) is a type of cationic group for hydroxide exchange membranes (HEMs). To elucidate the relationship between structure and alkaline stability, we investigated the kinetics and degradation mechanism of a series of QP cations by both experiment and computation, and established a semi-empirical formula based on the Taft equation to directly estimate alkaline stability of QP cations from the 31P NMR chemical shift δ and the steric substituent constant Es, facilitating the search for QP cations with improved alkaline stability.