Steady-state model of a high-temperature solid oxide fuel cell （SOFC） is considered, which refers to constant chemical potentials of incoming hydrogen fuel and oxidant. Lowering of the cell voltage below its reversi...Steady-state model of a high-temperature solid oxide fuel cell （SOFC） is considered, which refers to constant chemical potentials of incoming hydrogen fuel and oxidant. Lowering of the cell voltage below its reversible value is attributed to polarizations and imperfect conversions of reactions. An imperfect power formula summarizes the effect of transport laws, irreversible polarizations and efficiency of power yield. Reversible electrochemical theory is extended to the case with dissipative chemical reactions; this case includes systems with incomplete conversions, characterized by ＂reduced affinities＂ and an idle run voltage. Efficiency drop is linked with thermodynamic and electrochemical irreversibilities expressed in terms of polarizations （activation, concentration and ohmic）. Effect of incomplete conversions is modeled by assuming that substrates can be remained after the reaction and that side reactions may occur. Optimum and feasibility conditions are discussed for basic input parameters of the cell. Calculations of maximum power show that the data differ for power generated and consumed and depend on current intensity, number of mass transfer units, polarizations, electrode surface area, average chemical rate, etc.. These data provide bounds for SOFC energy generators, which are more exact and informative than reversible bounds for electrochemical transformation.展开更多
A fundamental understanding of the electrochemical reaction process and mechanism of electrodes is very crucial for developing high-performance electrode materials.In this study,we report the sodium ion storage behavi...A fundamental understanding of the electrochemical reaction process and mechanism of electrodes is very crucial for developing high-performance electrode materials.In this study,we report the sodium ion storage behavior and mechanism of orthorhombic V_(2)O_(5) single-crystalline nanowires in the voltage window of 1.0–4.0 V(vs.Na/Na+).The single-crystalline nanowires exhibit a large irreversible capacity loss during the first discharge/charge cycle,and then show excellent cycling stability in the following cycles.At a current density of 100 mA g^(−1),the nanowires electrode delivers initial discharge/charge capacity of 217/88 mA h g^(−1),corresponding to a Coulombic efficiency of only 40.5%;after 100 cycles,the electrode remains a reversible discharge capacity of 78 mA h g^(−1) with a fading rate of only 0.09%per cycle compared with the 2nd cycle discharge capacity.The sodium ion storage mechanism was investigated,illustrating that the large irreversible capacity loss in the first cycle can be attributed to the initially formed single-crystalline α′-Nax V_(2)O_(5)(0.02<x<0.88),in which sodium ions cannot be electrochemically extracted and the α′-Na0.88 V_(2)O_(5) can reversibly host and release sodium ions via a single-phase(solid solution)reaction,leading to excellent cycling stability.The Na^(+) diffusion coefficient in α′-Nax V_(2)O_(5) ranges from 10^(−12) to 10^(−11.5) cm^(2) s^(−1) as evaluated by galvanostatic intermittent titration technique(GITT).展开更多
文摘Steady-state model of a high-temperature solid oxide fuel cell （SOFC） is considered, which refers to constant chemical potentials of incoming hydrogen fuel and oxidant. Lowering of the cell voltage below its reversible value is attributed to polarizations and imperfect conversions of reactions. An imperfect power formula summarizes the effect of transport laws, irreversible polarizations and efficiency of power yield. Reversible electrochemical theory is extended to the case with dissipative chemical reactions; this case includes systems with incomplete conversions, characterized by ＂reduced affinities＂ and an idle run voltage. Efficiency drop is linked with thermodynamic and electrochemical irreversibilities expressed in terms of polarizations （activation, concentration and ohmic）. Effect of incomplete conversions is modeled by assuming that substrates can be remained after the reaction and that side reactions may occur. Optimum and feasibility conditions are discussed for basic input parameters of the cell. Calculations of maximum power show that the data differ for power generated and consumed and depend on current intensity, number of mass transfer units, polarizations, electrode surface area, average chemical rate, etc.. These data provide bounds for SOFC energy generators, which are more exact and informative than reversible bounds for electrochemical transformation.
基金financially supported by the National Natural Science Foundation of China (51664012)Guangxi Natural Science Foundation (2017GXNSFAA198117 and2015GXNSFGA139006)the Technology Major Project of Guangxi(AA19046001)
文摘A fundamental understanding of the electrochemical reaction process and mechanism of electrodes is very crucial for developing high-performance electrode materials.In this study,we report the sodium ion storage behavior and mechanism of orthorhombic V_(2)O_(5) single-crystalline nanowires in the voltage window of 1.0–4.0 V(vs.Na/Na+).The single-crystalline nanowires exhibit a large irreversible capacity loss during the first discharge/charge cycle,and then show excellent cycling stability in the following cycles.At a current density of 100 mA g^(−1),the nanowires electrode delivers initial discharge/charge capacity of 217/88 mA h g^(−1),corresponding to a Coulombic efficiency of only 40.5%;after 100 cycles,the electrode remains a reversible discharge capacity of 78 mA h g^(−1) with a fading rate of only 0.09%per cycle compared with the 2nd cycle discharge capacity.The sodium ion storage mechanism was investigated,illustrating that the large irreversible capacity loss in the first cycle can be attributed to the initially formed single-crystalline α′-Nax V_(2)O_(5)(0.02<x<0.88),in which sodium ions cannot be electrochemically extracted and the α′-Na0.88 V_(2)O_(5) can reversibly host and release sodium ions via a single-phase(solid solution)reaction,leading to excellent cycling stability.The Na^(+) diffusion coefficient in α′-Nax V_(2)O_(5) ranges from 10^(−12) to 10^(−11.5) cm^(2) s^(−1) as evaluated by galvanostatic intermittent titration technique(GITT).
