Neutron Scattering Studies of Phosphate Proton Conductors

Proton ceramic fuel cells operating in the intermediate temperature range of 300-500 °C offer potentially revolutionary advantages over existing fuel cells because expensive noble metal catalysts would not be needed, and in situ reforming of liquid bio-fuels such as ethanol or methanol would be possible.The chief obstacle facing intermediate fuel cells is the lack of a suitable electrolyte in the operating temperature range. A good electrolyte is thermally and chemically stable, inexpensive, environmentally friendly, and has a proton conductivity on the order of 10^-2 S cm^-1 [Siemens per centimeter] at 400 °C. Acceptor-doped lanthanum orthophosphate is an attractive candidate electrolyte as it meets all of the above requirements except that its proton conductivity is 1-2 orders of magnitude too low. One of the motivations for the research presented here is to understand the microscopic mechanisms of proton transport in phosphate materials in order to suggest future synthetic approaches leading to higher performance materials.

Proton transfer in orthophosphates is known to involve both localized and long-range diffusion, but the energetically favored pathways and rate limiting steps of the proton transport are not well understood. We investigated acceptor-doped lanthanum orthophosphate by means of quasi-elastic neutron scattering (QENS), neutron powder diffraction, X-ray diffraction, and electrochemical impedance spectroscopy (EIS). The conductivity of the hydrated sample was determined in the temperature range 500-850 °C by EIS, and showed a clear proton-conductivity signature with activation energy of about 1.0 eV [electron volt]. The QENS experiment revealed a fast dynamical process below 500 °C that EIS did not observe. The fast proton diffusion’s activation energy is 0.09 eV in the temperature range from 150 °C to 500 °C. We determined the proton mean jump length, mean residence time, atomic displacement, and self-diffusion coefficient in order to characterize the proton dynamics. This work presents the first QENS investigation of proton dynamics in a rare earth phosphate. This work has also led to the development of a new sample cell environment, allowing QENS measurements to be performed under a humid or dry gas flow. Thus materials can be studied under conditions approximating those inside an operational fuel cell.