Proton diffusion in the catalytic layer for high temperature polymer electrolyte fuel cells.

Area of Science:

• Materials Science
• Electrochemistry
• Neutron Scattering

Background:

• High-temperature polymer electrolyte fuel cells (HT-PEFCs) are crucial for clean energy.
• Understanding proton dynamics in the catalytic layer is key to improving HT-PEFC performance.
• Phosphoric acid (PA) is a common electrolyte in HT-PEFCs, but its behavior at the nanoscale is not fully understood.

Purpose of the Study:

• To investigate the nanosecond proton dynamics in phosphoric acid within the catalytic layer of HT-PEFCs.
• To understand the distribution and diffusion of phosphoric acid within the catalyst layer.
• To correlate proton dynamics with the overall conductivity of the fuel cell.

Main Methods:

• Quasielastic neutron scattering (QENS) using neutron backscattering spectroscopy.
• Studying proton dynamics at operating temperatures (300 K-430 K) on a local length scale.
• Analyzing catalysts doped with varying amounts of phosphoric acid.

Main Results:

• Proton diffusion in the catalytic layer is significantly slower than in bulk phosphoric acid due to adsorption on platinum particles.
• The proton dynamics can be accurately described by a random jump diffusion model with traps.
• The estimated conductivity, derived from the diffusion constant, is lower than expected for free phosphoric acid.

Conclusions:

• Adsorption of phosphoric acid on platinum catalyst surfaces hinders proton mobility.
• The 'trap model' provides a suitable framework for understanding proton diffusion in HT-PEFC catalytic layers.
• Optimizing phosphoric acid distribution and minimizing adsorption are critical for enhancing HT-PEFC conductivity and efficiency.