Boron from net charge acceptor to donor and its effect on hydrogen uptake by novel Mg-B-electrochemically synthesized reduced graphene oxide

Abstract

Hydrogen uptake (H-uptake) is studied in ball milled Mg-B-electrochemically synthesized reduced graphene oxide (erGO) nanocomposites at PH2 ≈ 15 bar, ~ 320 °C. B/C (weight ratio): 0, ~ 0.09, ~ 0.36, ~ 0.90 are synthesized maintaining erGO≈10wt %. B occupies octahedral interstices within Mg unit cell—revealed by electron density maps. Persistent charge donations from Mg and B to C appear as Mg-C (~ 283.2 eV), B-C (~ 283.3–283.9 eV) interactions in C-1s core X-ray photoelectron spectroscopy (XPS) at all B/C. At B/C > 0.09, charge reception by B from Mg yields Mg-B interaction. This net charge acceptor role of B renders it electron-rich and does not alter Mg unit cell size significantly. Despite charge donation to both C and B, the Mg charge is < + 2, resulting in long incubation times (> 5 h) at B/C > 0.09. At B/C≈0.09 the minimal Mg-B interaction renders B a charge donor, resulting in Mg-B repulsion and Mg unit cell expansion. Mg-C peak shift to lower binding energies (C-1s XPS), decreases incubation time to ~ 2.25 h and enhances H-uptake kinetics. Various atomic interactions influence the reduction of incubation time in H-uptake and increase its kinetics in the order: (Mg → C; B → C)B/C≈0.09, B: donor > (Mg → C)B/C=0 > (ternary Mg → B → C)B/C>0.09, B: acceptor.

Nanointerface Engineering of Metal Hydrides for Advanced Hydrogen Storage

“…The catalytic effects of subsurface carbon on the Mg (0001) surface were explored by ab initio DFT calculations. 93 The Mg−B−erGO nanocomposite was prepared by ball-milling with Mg, B, and electrochemically synthesized reduced graphene oxide (erGO). The authors explained that with a low boron concentration the nanocomposite readily absorbs hydrogen due to the repulsion between B and Mg in the lattice, while a high boron concentration boosts the electron transfer from Mg to B, which causes the lattice to contract and consequently makes hydrogen absorption difficult (Figure 8a).…”

“…The additionally introduced element can influence the interfacial interaction even though it is not directly bonded to the carbon substrate. Aditya et al demonstrated through experiments and calculations that the effect of charge transfer by the interstitial boron on Mg changed with the boron concentration . The Mg–B–erGO nanocomposite was prepared by ball-milling with Mg, B, and electrochemically synthesized reduced graphene oxide (erGO).…”

“…(a) Changes in the Mg lattice structure with the addition of boron and the consequent changes in the Mg–C interaction during hydrogen absorption. Reproduced with permission from ref . Copyright 2021 Springer Nature.…”

“…In this regard, the host–guest interaction can be another tool for altering the kinetic or thermodynamic barrier of nanoconfined hydrides and can be tuned via a proper modification. Indeed, some recent studies show the additional reduction of thermodynamic or kinetic barriers of nanoconfined hydride systems by introducing foreign elements onto the surfaces of scaffold materials, simultaneously exploiting the reported benefits of foreign element introduction and nanoconfinement. ,,,,− Moreover, such changes induced by the introduction of doping elements upon changing the nanoconfined hydride–scaffold interface have been reported for both binary metal hydrides ,,,, and complex hydrides. − …”

Recent Development in Nanoconfined Hydrides for Energy Storage

“…NaBH4 [49,74,216,236,241], Mg(BH4)2 [42,55,61,70,105,126,132,151,159,207,216,220,224,236,242,243], Ca(BH4)2 [116,216,236,243] and (TM)(BH4)x [150,216], ammonia-borane NH3.BH3 [36,38,63,64,75,85,86,88,90,139,[140][141][142][143][144]153,154,156,162,171,175,197,213,[244]…”