Hydrogen dynamics in proton conducting materials
|Developing efficient, sustainable, and economic alternative energy systems is currently of great political, technical, and scientific interest.1,2 Hydrogen-oxygen fuel cells are a possible solution for applications requiring a portable energy source. Central to proton-based fuel cell operation is the proton exchange membrane (PEM), which simultaneously provides electronic insulation between the fuel cell’s anode and cathode, and allows proton conduction. In order to improve fuel cell efficiencies, it is important to develop PEM materials with high proton conductivity and low electronic conductivity in the temperature range from 100 °C to 300 °C. 3|
|One such class of materials is the recently discovered alkali thio-hydroxogermanate system, (MS)xGe(OH)4-x ∙ yH2O, where M=Na, K, Rb, Cs, and x=1, 2, 3, 4. The structural and physical properties of these materials have been studied by many techniques.3--7 In this study, we focus on the system with the largest reported conductivity, namely the (NaS)xGe(OH)4-x ∙ yH2O system for x=2, 3.|
|Understanding the hydrogen dynamics in the region of high conductivity before the onset temperature is of considerable interest. Two principal mechanisms for proton conduction exist: the vehicle mechanism, in which the proton is part of an ion--the “vehicle”--which diffuses through the solid; and the Grotthuss mechanism, in which the vehicle resides at fixed sites and transfers protons from one molecule to another by coordinated rotations and proton transfers through hydrogen bonds.8 In many cases, Grotthuss mechanisms are progressively dominated by vehicle mechanisms with increasing temperature.8|
In proton conductors containing waters of hydration, relevant molecular species often include H3O+ and H2O, and the local rotational and hopping motions of these charged and neutral molecules have a significant influence on the long range translational motion of protons through the material.9
The underlying proton conduction mechanism at the point of maximum conductivity in the x=2, 3 samples is thought to consist of hydrogen jumps between neighboring molecules through hydrogen bonds, followed by molecular rotations.3,5,7 It has been proposed5 that the proton may reside at an oxygen on the thio-oxoanion, or on a water molecule, forming H3O+.
The conduction mechanism might then consist of a rotation of an H3O+, followed by a hydrogen jump along the axis of a hydrogen bond to a neighboring molecule. Nuclear magnetic resonance (NMR) is a well established tool for probing molecular motion. 10--13 Considerable literature exists on the application of NMR to crystals with waters of hydration14 as well as to proton conducting materials.9,15--20 In particular, NMR has been used to distinguish between different proton conduction mechanisms.15 The variation of the NMR spectrum with temperature can yield dynamic information about specific nuclear groups, and nuclear relaxation times probe nuclear dynamics over time scales ranging from tens of kHz to tens of MHz.
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