The production of hydrogen by water electrolysis
suffers from the kinetic barriers in the oxygen evolution reaction (OER) that
limits the overall efficiency. As spin-dependent kinetics exist in OER, the
spin alignment in active OER catalysts is critical for reducing the kinetic
barriers in OER. It is effective to facilitate the spin polarization in
ferromagnetic catalysts by applying external magnetic field, which increases
the OER efficiency. However, more active OER catalysts tend to have dynamic
open-shell orbital configurations with disordered magnetic moments, without
showing an apparent long-range interatomic ferromagnetism; thus controlling the
spin alignment of these active catalysts is challenging. In this work, we
report a strategy with spin pinning effect to make the spins in active
oxyhydroxides more aligned for higher intrinsic OER activity. Such strategy bases
on a controllable reconstruction: ferromagnetic oxides with controlled
sulfurization can evolve into stable oxide<sub>FM</sub>/oxyhydroxide
configurations with a thin oxyhydroxide layer under operando condition. The
spin pinning effect is found at the interface of oxide<sub>FM</sub>/oxyhydroxide.
The spin pinning
effect can promote spin selective electron transfer on OER intermediates to
generate oxygens with parallel spin alignment, which facilitates the production
of triplet oxygen and increases the intrinsic activity of oxyhydroxide by ~ 1
order of magnitude. Under spin pinning, the spins in oxyhydroxide can become
more aligned after magnetization as long-range ferromagnetic ordering is
established on the magnetic domains in oxide<sub>FM</sub>. The OER kinetics are
facilitated accordingly after magnetization, implying that the spin pinning
effect is involved in the rate-determining step and this step is spin
dependent. The spin polarization process in OER under spin pinning is also
believed to be sensitive to the existence of active oxygen ligand (O(-)) in
oxyhydroxide. When the O(-) is created in 1<sup>st</sup> deprotonation step under
high pH, the spin polarization
of ligand oxygens will be facilitated, which reduces the barrier for subsequent
O-O coupling and promotes the O<sub>2</sub> turnover.
We have measured both the longitudinal and transverse spin diffusion coefficients, D~~a nd D~, in a saturated mixture of 3He in He ( He concentration x3=6.4%) in a magnetic field of 8.8 T in the temperature range 11~T~500 mK. Consequently we are able to provide direct evidence for the anisotropy in spin diffusion predicted by Meyerovich. Below -30 mK, D~~v aries as T in accordance with the predictions of Landau Fermi-liquid theory; but D~becomes smaller than D~~a t the lowest temperatures resulting in an anisotropy factor D~~/D~o f 4 at T=11 mK. We believe that this large anisotropy is due to a breakdown of the standard Landau Fermi-liquid theory in concentrated mixtures of He in He. Finally we use our measurements of the spin rotation parameter, p,MO, along with results from previous experiments at various He concentrations to deduce a value for the quasiparticle scattering length a = -0.88~0.05 A.
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