Nitrogen-doped carbon-based catalysts are increasingly being studied as Pt-free electrodes for oxygen reduction in polymer electrolyte membrane fuel cells. Here, we study the oxygen reduction activity of stoichiometric carbon nitride, which has much higher nitrogen content and is synthesized at lower temperatures, without using ionic or metallic iron. Carbon nitride was studied and characterized via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, BET specific surface area analysis, and thermogravimetric analysis. Rotating electrode voltammetry in oxygen-saturated sulfuric acid was used to determine the catalytic activity. The onset potential for oxygen reduction by carbon nitride electrodes was 0.69 V (vs NHE) compared to 0.45 V for a carbon black reference electrode. However, the current density was low, possibly due to the low surface area of the material. Blending the carbon nitride with a high surface area carbon black support resulted in a significant improvement in current density and in an increase in onset potential to 0.76 V. The role of surface area was elucidated via cyclic voltammetry. This work confirms that stoichiometric carbon nitride has improved inherent oxygen reduction activity compared to pure carbon, and suggests that Fe coordination sites are not essential for electrochemical oxygen reduction in nitrogen-containing carbon materials.

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Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % 13 C-labeled atacticpolyacrylonitrile (a-PAN) heat-treated at various temperatures (T s ) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) 13 C NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused 13 C− 13 C INADEQUATE and 1 H− 13 C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different T s values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through this work, the stabilization index (SI) was defined on the basis of the quantified C-1 and C-3 DP/MAS spectra. The former reached 0.87 at T s = 370°C, and further higher T s values did not affect SI; however, the latter continuously increased up to 0.66 at T s = 450°C. All of the experimental results indicated that oxygen plays a vital role on the whole reaction process as well as the final products of stabilized a-PAN.

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In this paper, the electrochemical properties and performances of all-solid-state lithium polymer batteries (LPBs) using standard PEO-based solid-state polymer electrolytes (SPEs) are reported and discussed. The assembled cell showed stable charge-discharge cycles (>150 cycles) at 30 C. This is due to desirable solid electrolyte interface (SEI) film formation at the SPE | cathode interface at the first cycle indicated by activation energy measurements for interfacial Li ion exchange reaction. However, sudden capacity fading for prolonged electrochemical cycles was indicated by an accelerated aging test at higher current density (1 C) and temperature conditions (60 C), accompanied by an increase of electrochemical polarization. This degradation phenomenon may be fatal for practical usage of large-scale batteries which requires extremely long-time durability. Two sequential factors affecting the capacity fading are proposed through the studies of in situ 19 F-NMR imaging, real-time monitoring of the total cell thickness, and electrochemical measurements such as AC impedance. One factor is degradation of the cathode sheet or cathode composite assembly, owing to cyclic volumetric change from the two-phase LiFePO 4 -FePO 4 reaction. Such degradation leads to uneven electric contact at the electrode | electrolyte interface, thereby enhancing local electrochemical polarization. The second factor, namely, Li salt decomposition, is triggered by this local polarization, giving rise to the continuous capacity fading and the increase of polarization. This degradation scenario can be general enough to include the full range of all-solid-state LPB devices, since the trigger of degradation owes to non-fluidity of solid | solid contact, or solid electrolytes cannot immerse into the cavities caused by pulverization of cathode particles unlike liquid electrolytes. On the basis of these results, we attempted to improve the mechanical properties of the binder materials of cathode sheets, and demonstrated improved cyclic durability.

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Solid‐state lithium polymer secondary batteries (LPB) are fabricated with a two‐electrode‐type cell construction of Li|solid‐state polymer electrolyte (SPE)|LiFePO4. Plasticizers of poly(ethylene glycol) (PEG)‐borate ester (B‐PEG) or PEG‐aluminate ester (Al‐PEG) are added into lithium‐conducting SPEs in order to enhance their ionic conductivity, and lithium bis‐trifluoromethansulfonimide (LiTFSI) is used as the lithium salt. An improvement of the electrochemical properties is observed upon addition of the plasticizers at an operation temperature of 60 °C. However, a decrease of discharge capacities abruptly follows after tens of stable cycles. To understand the origin of the capacity fading, electrochemical impedance techniques, ex‐situ NMR and scanning electron microscopy (SEM)/energy dispersive X‐ray spectroscopy (EDS) techniques are adopted. Alternating current (AC) impedance measurements indicate that the decrease of capacity retention in the LPB is related to a severe increase of the interfacial resistance between the SPE and cathode. In addition, the bulk resistance of the SPE film is observed to accompany the capacity decay. Ex situ NMR studies combined with AC impedance measurements reveal a decrease of Li salt concentration in the SPE film after cycling. Ex situ SEM/EDS observations show an increase of concentration of anions on the electrode surface after cycling. Accordingly, the anions may decompose on the cathode surface, which leads to a reduction of the cycle life of the LPB. The present study suggests that a choice of Li salt and an increase of transference number is crucial for the realization of lithium polymer batteries.