The NaBH₄ electro-oxidation kinetics in a molten NaOH-KOH eutectic mixture (0.585:0.415 mole fractions) is investigated on Ni, electrochemically oxidized Ni and Pt electrodes in conjunction with the performance of a novel NaBH₄/KIO₄ battery [1, 2]. The oxidized Ni shows excellent electrocatalytic activity towards BH₄^- oxidation at 458 K, with apparent exchange current density, Tafel slope and electron transfer coefficient of 22.54 mA cm^-2, 127 mV dec^-1 and 0.7 respectively. The activation energy of the BH₄^- oxidation reaction is 16.3 kJ mol^-1. The BH₄^- diffusion coefficient was determined using a chronoamperometric technique. X-ray photoelectron spectroscopy (XPS) analysis of the anodically oxidized Ni reveals the formation of NiO, which is responsible for the increased catalytic activity towards BH₄^- oxidation compared to the untreated Ni electrode. A compact molten NaOH-KOH eutectic electrolyte battery with dissolved NaBH₄ at the anode, immobilized KIO₄ at the cathode and sintered Ni separator, was assembled and investigated for the first time. An oxidized Ni gauze served as the anode, whereas the cathode was Pt mesh containing the immobilized KIO₄. Figure 1 a and b, show the NaBH₄/KIO₄ battery polarization curves as a function of NaBH₄ molal concentration and the discharge cell voltage profile over time at 160 mA cm^-2, respectively.

Fig. 1: a) Polarization curves of the NaBH₄/KIO₄ battery at 458 K in the NaOH-KOH eutectic mixture, b) Constant current discharge at 160 mA cm^-2 with 500 milimolal NaBH₄. Oxidized Ni gauze anode; Pt gauze cathode with 2.5 g cm^-2 loading of KIO₄; electrolyte: NaOH-KOH eutectic mixture with mole fractions 0.515:0.485.

Factorial design of experiments revealed the main and interaction effects influencing the power output of the NaBH₄/KIO₄ battery. Temperature and cathodic KIO₄ loading have the most significant main effects on the power density, whereas the NaBH₄ concentration in the molal concentration range of 50 to 500 milimolal, has a comparatively smaller effect. A maximum power density of 210 mW cm^-2 is achieved at 458 K. With further research and development it is proposed that the NaBH₄ – KIO₄ battery could be used together with solar – thermal energy storage systems.

References:

1. A.Wang and E.L. Gyenge J.Power Sources 282 (2015) 169-173.
2. A. Wang and E.L. Gyenge, manuscript in preparation (2016).