Effect of Tin Doping on α-Fe₂O₃ Photoanodes for Water Splitting

Sputter-deposited films of α-Fe₂O₃ of thickness 600 nm were investigated as photoanodes for solar water splitting and found to have photocurrents as high as 0.8 mA/cm² at 1.23 V vs the reversible hydrogen electrode (RHE). Sputter-deposited films, relative to nanostructured samples produced by hydrothermal synthesis,, permit facile characterization of the role and placement of dopants. The Sn dopant concentration in the α-Fe₂O₃ varies as a function of distance from the fluorine-doped tin oxide (FTO) interface and was quantified using secondary ion mass spectrometry (SIMS) to give a mole fraction of cations of approximately 0.02% at the electrolyte interface. Additional techniques for determining dopant density, including energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), electrochemical impedance spectroscopy (EIS), and conductivity measurements, are compared and discussed. Based on this multifaceted data set, we conclude that not all dopants present in the α-Fe₂O₃ are active. Dopant activation, rather than just increasing surface area or dopant concentration, is critical for improving metal oxide performance in water splitting. A more complete understanding of dopant activation will lead to further improvements in the design and response of nanostructured photoanodes

Electrolyte Stability Determines Scaling Limits for Solid-State 3D Li Ion Batteries

Rechargeable, all-solid-state Li ion batteries (LIBs) with high specific capacity and small footprint are highly desirable to power an emerging class of miniature, autonomous microsystems that operate without a hardwire for power or communications. A variety of three-dimensional (3D) LIB architectures that maximize areal energy density has been proposed to address this need. The success of all of these designs depends on an ultrathin, conformal electrolyte layer to electrically isolate the anode and cathode while allowing Li ions to pass through. However, we find that a substantial reduction in the electrolyte thickness, into the nanometer regime, can lead to rapid self-discharge of the battery even when the electrolyte layer is conformal and pinhole free. We demonstrate this by fabricating individual, solid-state nanowire core–multishell LIBs (NWLIBs) and cycling these inside a transmission electron microscope. For nanobatteries with the thinnest electrolyte, ≈110 nm, we observe rapid self-discharge, along with void formation at the electrode/electrolyte interface, indicating electrical and chemical breakdown. With electrolyte thickness increased to 180 nm, the self-discharge rate is reduced substantially, and the NWLIBs maintain a potential above 2 V for over 2 h. Analysis of the nanobatteries’ electrical characteristics reveals space-charge limited electronic conduction, which effectively shorts the anode and cathode electrodes directly through the electrolyte. Our study illustrates that, at these nanoscale dimensions, the increased electric field can lead to large electronic current in the electrolyte, effectively shorting the battery. The scaling of this phenomenon provides useful guidelines for the future design of 3D LIBs