Invited Paper EH-TuE3
Soft X-ray Operando Spectroscopy for Polymer Electrolyte Fuel Cells and Li Ion Batteries

Tuesday, December 9, 2014, 6:20 pm, Room Lehua

In order to meet strong demands for electronic structure analysis of green devices, namely 1) power generation devices such as polymer electrolyte fuel cells (PEFC), 2) power efficient devices such as graphene FET and Resistive RAM, and 3) energy storage devices such as Li ion battery (LIB), we have developed two soft X-ray nano-spectroscopy systems at the SPring-8 University-of-Tokyo (UT) outstation. One is operando soft-X-ray emission spectroscopy (XES)¹⁾ forPEFC cathode catalysts and Li ion battery, and the other is scanning photoelectron microscopy with 70 nm spatial resolution²⁾, which has been used to analyze graphene FET and organic FET in operando.

First, we analyzed electronic structures of carbon-related catalysts alternative to Pt for PEFC in order to elucidate the oxygen reduction reaction (ORR) mechanism. We prepared metal phthalocyanine-based carbon catalysts with 1-2% nitrogen and less than 1 % of Fe for PEFC. Photoelectron spectroscopy and first principles calculation revealed that zigzag edge carbons with neighboring graphite-like nitrogen are ORR active sites. B ased on these analyses, we fabricated fuel cell stack for PEFC which showed comparable performance to Pt catalysts. Furthermore, we have taken operando XES spectra of Fe 2p-3d transition during power generation, revealing that Fe impurity may act as an ORR catalyst³⁾.

Next, we analyzed the change of Fe 3d states accompanied with the Li intercalation/ deintercalation process by resonant photoemission spectroscopy. Since the battery voltage should reflect the energy difference between Li 2s and Fe 3d down-spin state, we measured the change in Fe 3d down-spin states for LiFePO₄ (3.4V) and Li₂FeP₂O₇ (3.6V) and found that 0.2 eV shift from PO₄ to P₂O₇ poly-anions directly reflects battery voltage. Furthermore, the operando XES method was applied to cathode materials LiMn₂O₄ in Li ion battery to reveal the electronic structure change of Mn with changing OCV (open circuit voltage). It was demonstrated that the Mn³⁺ and Mn⁴⁺ states are successfully distinguished using high-energy-resolution resonant XES⁴⁾. Multiplet calculations⁵⁾ have been performed to determine the electronic structures in comparison with operando XES spectra for both Fe and Mn chemical states in FC and LIB, respectively.

This work has been done in collaboration mainly with Y. Harada, H. Niwa,T. Aoki, Y. Nabae, Y. Nanba and D. Asakura.

References

1) Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012). 2) K. Horiba et al., Rev. Sci. Instrum. 82,113701(2011). 3) H. Niwa et al., Electrochemistry Com. 35, 57 (2013). 4) D. Asakura et al., SPring-8 BL07LSU Activity Report 2012. 5) Y. Nanba et al., PCCP 14, 7031 (2014).