Cs₃H₂[BOB(PO₄)₃] - the Best-Performing 1D Borophosphate Intermediate Temperature Proton Electrolyte

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Polyanionic borophosphates are an emerging class of intermediate temperature electrolytes.
• These materials offer high thermal stability, particularly in reducing hydrogen atmospheres.

Purpose of the Study:

• To investigate the electrolyte characteristics of rubidium borophosphate (Rb3H2[BOB-(PO4)3]) and cesium borophosphate (Cs3H2[BOB-(PO4)3]).
• To identify new members of the proton-conducting one-dimensional (1D) borophosphate polyelectrolyte family.
• To evaluate their potential as electrolytes for intermediate temperature proton conductors.

Main Methods:

• Synthesis and characterization of rubidium and cesium borophosphate compounds.
• Ab initio molecular dynamics simulations to understand proton (H+) mobility.
• Proton conductivity measurements under specific temperature and atmospheric conditions.

Main Results:

• Identified Rb3H2[BOB-(PO4)3] and the new compound Cs3H2[BOB-(PO4)3] as proton-conducting 1D borophosphate electrolytes.
• Ab initio simulations indicated high H+ mobility linked to borophosphate chain gyrational mobility.
• Cs3H2[BOB-(PO4)3] demonstrated the highest proton conductivity (10^-5 S/cm at 250°C) among studied borophosphates.
• Cs3H2[BOB-(PO4)3] exhibited excellent thermal stability and mechanical properties.

Conclusions:

• Cs3H2[BOB-(PO4)3] is the leading 1D borophosphate electrolyte due to its high conductivity, thermal stability, and mechanical properties.
• This advancement offers a promising pathway for improving electrolytes in intermediate temperature hydrogen fuel cells.
• The findings contribute significantly to the field of intermediate temperature proton conductors.