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Electroquímica del oxígeno en soluciones concentradas de NaOH: solubilidad, coeficientes de difusión y formación de
Cunzhong Zhang1, Fu-Ren F Fan, Allen J Bard
1Center for Electrochemistry, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-0165, USA.
Journal of the American Chemical Society
|December 10, 2008
Resumen
Las altas concentraciones de NaOH disminuyen la difusión y solubilidad de oxígeno, afectando el mecanismo de reacción de reducción de oxígeno. El número de electrones involucrados cambia de dos a uno con el aumento de la molaridad del NaOH.
Área de la Ciencia:
- La electroquímica es electroquímica.
- Química Física es la química física.
- Ciencia de los materiales Ciencia de los materiales.
Sus antecedentes:
- La comprensión de la reacción de reducción de oxígeno (ORR) es crucial para las tecnologías de conversión de energía.
- La influencia de los medios de alta resistencia iónica en los parámetros de ORR no se entiende completamente.
- Las soluciones acuosas de hidróxido de sodio (NaOH) presentan un entorno único debido al aumento de la viscosidad y la actividad del agua alterada.
Objetivo del estudio:
- Para investigar el coeficiente de difusión (D(O(2)), la solubilidad (C(O(2)), y el comportamiento electroquímico de la reducción de oxígeno en soluciones concentradas de NaOH.
- Para determinar el efecto de la concentración de NaOH en el número de electrones (n) involucrados en el paso inicial de la ORR.
- Para caracterizar el comportamiento electroquímico del producto ORR, el superóxido (O(2)(*-) utilizando microscopía electroquímica de barrido (SECM).
Principales métodos:
- Voltametría cíclica y amperometría transitorio en un ultramicroelectrodo de platino (Pt).
- Mediciones electroquímicas realizadas en soluciones acuosas de NaOH de entre 1 y 12 M.
- Microscopía electroquímica de barrido (SECM) para el análisis de productos.
Principales resultados:
- Tanto la D (O2) como la C (O2) disminuyen con el aumento de la concentración de NaOH, lo que se correlaciona con el aumento de la viscosidad de la solución (eta).
- Se obedece la relación Stokes-Einstein, lo que indica una constante O(2) radio molecular (2,8 Å).
- El número de electrones (n) en el primer paso ORR cambia de n=2 en 1-2 M NaOH a n=1 en >6 M NaOH, vinculado a cambios en la actividad del agua.
- El análisis SECM en 10 M NaOH reveló una constante de velocidad aparente de ~2.6 x 10(-4) cm/s para la pareja redox O(2)/O(2)(*-)
Conclusiones:
- El NaOH concentrado altera significativamente las propiedades físicas (viscosidad, actividad del agua) que afectan la cinética y el mecanismo de ORR.
- La vía ORR depende de la concentración de NaOH, pasando de un proceso de dos electrones a uno de un electrón.
- SECM es una herramienta valiosa para estudiar los productos ORR en electrolitos de alta concentración.
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