I used to titanium substrate. I passivated Ti surface with CV method in %5 H2SO4 solution. This way both cleaning and passivating Ti substrate. I think you can try this.
I'm not sure if you asked me, but this is what I know. I have never used HNO3. But it can be tried. I don't think we can know until we try it... As far as I know, sulfuric acid is more preferred in battery making. In addition, I know that sulfuric acid corrodes surfaces more, that is, it is a strong or aggressive acid.