Influence of shear and compressive stress regimes on efficient NaBH₄ mechanochemical regeneration

Abstract

We compare the influence of tangential (shear) and normal (compressive) stress events on the mechanochemical regeneration of sodium borohydride NaBH₄ from hydrated sodium metaborate [Figure presented] and magnesium hydride MgH₂. Discrete element method (DEM) mechanical descriptors are used to design experiments that either maintain the mill at a constant rotational speed or maintain a constant total dissipation power, thereby separating stress distribution from net power input. Under constant power operation, a tangential rich regime achieves a record 94% conversion yield with 37.5% shorter milling time, 40% lower ball-to-powder ratio, and 34% reduced speed. However, this high yield requires such a substantial power consumption that the converted mass per Watt drops to only 0.090 gW⁻¹, below both balanced (0.113 gW⁻¹) and normal-bias (0.108 gW⁻¹) cases. By contrast, a tangential bias at half the power (3 W) still delivers 84% yield and peaks at 0.185 gW⁻¹, illustrating the often disregarded trade-off between absolute conversion and energetic productivity in mechanochemistry. Specific yield (conversion per Watt) likewise peaks at 0.28 W⁻¹ and declines linearly with fill ratio (R²>0.99). Mechanochemical energy leverage analysis reveals that, at most, 1.7–3.7% of input mechanical work is theoretically recoverable on an enthalpy basis, 2.1–4.4% on a Gibbs free energy basis, and 4–8.7% when considering the fuel value of all available hydrogen. Our mill-agnostic framework provides a transferable blueprint for cross-platform optimization of mechanochemical processes.