Synthesis of Type II Ge and Ge–Si Alloyed Clathrates Using Solid-State Electrochemical Oxidation of Zintl Phase Precursors

Abstract

Germanium clathrates with the type II structure are openframework materials that show promise for various applications, but the difficulty of achieving phase-pure products via traditional synthesis routes has hindered their development. Herein, we demonstrate the synthesis of type II Ge clathrates in a two-electrode electrochemical cell using Na 4 Ge 4−y Si y (y = 0, 1) Zintl phase precursors as the working electrode, Na metal as the counter/reference electrode, and Na-ion conducting β″-alumina as the solid electrolyte. The galvanostatic oxidation of Na 4 Ge 4 resulted in voltage plateaus around 0.34−0.40 V vs Na/Na + with the formation of different products depending on the reaction temperature. When using Na 4 Ge 3 Si as a precursor, nearly phase-pure, alloyed type II Ge−Si clathrate was obtained at 350 °C. The Na atoms in the large (Ge,Si) 28 cages of the clathrate occupied off-centered positions according to Rietveld refinement and density functional theory calculations. The results indicate that electrochemical oxidation of Zintl phase precursors is a promising pathway for synthesizing Ge clathrates with type II structure and that Si alloying of the Zintl phase precursor can promote selective clathrate product formation over other phases.

Electrochemical Flux Synthesis of Type-I Na8Si46 Clathrates: Particle Size Control Using a Solid or Molten Na–Sn Mass Transport Mediator

“…The high Na + migration barriers 36 and porous nature of the formed clathrate products could explain why this process was associated with high polarization as seen in our previous work describing the solid-state electrochemical conversion of Zintl phase precursors to clathrates without a Na−Sn mediator. 10,23 The observation that the morphology and size of the clathrate crystals depend on whether Si mass transfer in the Na−Sn phase is possible at a given reaction temperature is consistent with results in conventional clathrate syntheses as well. For example, oxidation of Na 4 Si 4 under vacuum results in polycrystalline powders while oxidation under applied pressure 14,15 or in a Na−Sn flux results in single crystals.…”

“…After product formation, continued desodiation of Na 4 Si 4 must rely on Na + transport through the formed clathrate phase. The high Na + migration barriers and porous nature of the formed clathrate products could explain why this process was associated with high polarization as seen in our previous work describing the solid-state electrochemical conversion of Zintl phase precursors to clathrates without a Na–Sn mediator. , …”

“…Compared to the electrochemical synthesis method described in our previous work, , which used a solid-state conversion reaction between the Zintl phase precursor and clathrate (depicted in Figure c), the approach using the Na–Sn mass transport mediator has several advantages. First, the reactions using the mediator displayed constant polarization that remained low over the whole course of the reactions.…”

“…For instance, while the Na evaporation rate from Na 4 Si 4 in the conventional synthesis method is closely tied to the temperature of the reaction, an electrochemical cell decouples these variables, which can result in a wider range of possible reaction conditions. Our team has used such electrochemical techniques to demonstrate the synthesis of type-I silicon clathrates and type-II germanium and alloyed silicon/germanium clathrates from the respective Zintl phase precursors in cells containing Na β″-alumina as a solid electrolyte. , In those studies, the Zintl phase precursors were prepared as pellets and pressed against the solid electrolyte. Due to the nature of the solid-state conversion reaction, the resulting clathrate products are polycrystalline and made of particles typically on the order of a few microns or smaller in size.…”

“…19,21 Recently, it was demonstrated that electrochemical oxidation of Zintl phase precursors can be utilized to form clathrates through desodiation processes that are analogous to the reactions that rely on Na evaporation. 10,22,23 Electrochemical methods have the advantage of more control over the reaction kinetics (i.e., through modulation of the current) and the ability to observe the electrode potential in situ, which can be helpful for understanding the reaction mechanisms. For instance, while the Na evaporation rate from Na 4 Si 4 in the conventional synthesis method is closely tied to the temperature of the reaction, an electrochemical cell decouples these variables, which can result in a wider range of possible reaction conditions.…”