Interdependence of electroformation and hydrogen incorporation in   titanium dioxide
  It is shown that in nanoporous titanium dioxide films, sensitivity to atmospheric hydrogen exposure and electroforming can coexist and are interdependent. The devices work as conventional hydrogen sensors below a threshold electric field while above it, the well-known electroforming is observed. Offering hydrogen in this regime accelerates the electroforming process, and in addition to the usual reversible increase of the conductance in response to the hydrogen gas, an irreversible conductance decrease is superimposed. The behavior is interpreted in terms of a phenomenological model where current carrying, oxygen-deficient filaments with hydrogen-dependent conductivities form inside the titanium dioxide matrix. 
M. Strungaru, M. Cerchez, S. Herbertz, T. Heinzel, M. El Achhab, and   K. Schierbaum
cond-mat.mtrl-sci