Thermodynamically favored stable hydrogen storage reversibility of NaBH4 inside of bimetallic nanoporous carbon nanosheets

Abstract

Sodium borohydride (NaBH₄) has been widely regarded as a potential hydrogen storage material due to its high gravimetric and volumetric capacity. Its practical application, however, is hindered by the high operating temperature of over 500 °C and extremely poor reversibility. Herein, thermodynamically favored reversible hydrogen storage performance of NaBH₄ is developed inside of bimetallic (NiCo) nanoporous carbon nanosheets. Benefiting from the synergetic effect of nanoconfinement and the catalytic role of NiCo alloys, the apparent activation energy for H₂ desorption from NaBH₄@NiCo-NC is reduced to 36.8 kJ mol⁻¹ with the complete H₂ desorption at only 400 °C, resulting in the simultaneous formation of NiB and CoB as the dehydrogenation products. More interestingly, the reversible transformation between NiB/CoB and Ni₂B/Co₂B that are in situ formed during the subsequent re-/dehydrogenation process acts as a B reservoir, leading to a significant decrease of Gibbs free energy change for the reversibility of NaBH₄ to 1.41 eV, 0.62 eV lower than that of pure NaBH₄. As a result, coupled with the catalytic role of in situ formed Co₂B/Ni₂B in promoting the H₂ desorption performance of NaBH₄, a stable reversible capacity of 9.0 wt% is realized for NaBH₄ at a temperature as low as 400 °C for 5 cycles.