Porous electrodes are prevalent in electrochemical devices. Electrochemical impedance spectroscopy (EIS) is widely used as a noninvasive, in situ characterization tool to investigate multi-phase (electronic, ionic, gaseous) transport and coupling interfacial reactions in porous electrodes. Interpretation of EIS data needs model and fitting which largely determine the type and amount of information that could possibly be obtained, and thereby the efficacy of the EIS method. This review focuses on physics-based models, as such models, compared to electrical circuit models, are more fundamental in our understanding of the porous electrodes, hence more reliable and more informative. Readers can have a glimpse of the long history of porous electrode theory and in particular its impedance variants, acquaint themselves with the celebrated de Levie model and a general theoretical framework, retrace the journey of extending the de Levie model in three directions, namely, incorporating new physico-chemical processes, treating new structural effects, and considering high orders. Afterwards, a wealth of impedance models developed for lithium-ion batteries and polymer electrolyte fuel cells are introduced. Prospects on remaining and emerging issues on impedance modelling of porous electrodes are presented. When introducing theoretical models, we adopt a “hands-on” approach by providing substantial mathematical details and even computation codes in some cases. Such an approach not only enables readers to understand the assumptions and applicability of the models, but also acquaint them with mathematical techniques involved in impedance modelling, which are instructive for developing their own models.
Three-dimensional macroporous carbon materials with hierarchical pore structures (3D MPC) have wide applications, but the scale-up synthesis is limited by the cumbersome procedures of template formation and removal. Herein, we show that NaCl crystallites, which form in situ in a lyophilizing process of a NaCl solution containing a carbon precursor and are removable simply through water washing, can act as templates to grow 3D MPC materials with graphene-like ultrathin and mesoporous walls through pyrolitic carbonization. Further, by use of a nitrogen (N)-rich polymer (polyvinylpyrrolidone, PVP) as the carbon precursor and introduction of Fe salt in the precursor, an MPC catalyst with high Fe/N doping content is achieved due to the NaCl crystallites serving as confining agents to simultaneously prevent the large weight loss and N evaporation, a severe problem in usual pyrolytic syntheses of Fe-N-C catalysts. Benefiting from the mass transport convenience of the macropores as indicated by the impedance spectroscopy results, the Fe/N-doped 3D MPC exhibits high catalytic performance toward the oxygen reduction reaction. The dual functionality, facile formation and removal, and reusability of NaCl make the present method a promising way to gain cost-effective porous Fe-N-C catalysts.
Carbon defects tune the formation of NiFe LDH nanodots confined in the mesopores of a macro–mesoporous carbon substrate, forming a hybrid electrocatalyst with excellent bifunctional performance for oxygen evolution and reduction reactions.
Nitrogen (N)‐doped carbons are potential nonprecious metal catalysts to replace Pt for the oxygen reduction reaction (ORR). Pyridinic‐N‐C is believed to be the most active N group for catalyzing ORR. In this work, using zinc phthalocyanine as a precursor effectively overcomes the serious loss of pyridinic‐N, which is commonly regarded as the biggest obstacle to catalytic performance enhancement upon adopting a second pyrolysis process, for the preparation of a 3D porous N‐doped carbon framework (NDCF). The results show only ≈14% loss in pyridinic‐N proportion in the Zn‐containing sample during the second pyrolysis process. In comparison, a loss of ≈72% pyridinic‐N occurs for the non‐Zn counterpart. The high pyridinic‐N proportion, the porous carbon framework produced upon NaCl removal, and the increased mesoporous defects in the second pyrolysis process make the as‐prepared catalyst an excellent electrocatalyst for ORR, exhibiting a half‐wave potential (E1/2 = 0.88 V) up to 33 mV superior to state‐of‐the‐art Pt/C and high four‐electron selectivity (n > 3.83) in alkaline solution, which is among the best ORR activities reported for N‐doped carbon catalysts. Furthermore, only ≈18 mV degradation in E1/2 occurs after an 8000 cycles' accelerating stability test, manifesting the outstanding stability of the as‐prepared catalyst.
Heteroatoms doping is able to produce catalytic sites in carbon materials for oxygen reduction reaction (ORR); while hierarchically porous structure is necessary for efficient exposure and accessibility of the usually limited catalytic sites in such activated carbon catalysts. This work reports an in situ generated dual-template method to synthesize the Fe/N/S co-doped hierarchically porous carbon (FeNS/HPC), with NaCl crystallites formed during the precursor lyophilization process as the primary template to generate ∼500 nm macropores with ultrathin graphene-like carbon-layer walls, and FeO nanoparticles formed during the high-temperature carbonization process as the secondary template to produce mesopores on the walls of macropores. As well as the coexistence of graphitic-N, pyridinic-N, and thiophene-S which are beneficial to ORR, the as prepared FeNS/HPC possesses a highly graphitized and interconnected hierarchical porous structure, giving a specific surface area as high as 938 m g. As a consequence, it exhibits excellent four-electron oxygen reduction performance in both alkaline and acid electrolytes. The in situ generation and facile solution removal make the present template method a promising way for scale-up preparation of active porous carbon materials for various applications.
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