Hydrovoltaic energy — electricity from water flowing over a surface — depends on charged surfaces interacting with ions in solution. MoS₂ is a promising material because it's hydrophilic and atomically flat. Both its metallic (1T') and semiconducting (2H) phases are MoS₂ — same atoms, same stoichiometry. You'd expect similar hydrovoltaic performance from both, with maybe a modest edge for one.
Suvigya, Lalita, Nemala Sankar, Capasso, Yeh, and Gopinadhan (arXiv:2501.01739) find an 833× difference. The metallic 1T' phase produces 2.0 mW/m² peak power density; the semiconducting 2H phase produces 2.4 μW/m². Nearly three orders of magnitude separating chemically identical materials.
The mechanism: the 1T' phase forms laminates with well-defined 6-angstrom interlayer channels. The 2H phase doesn't form ordered channels. Water flowing through the 6-angstrom channels experiences surface diffusion and electrokinetic effects at every interface. No channels, no interface multiplication — the 2H phase relies only on its outer surface.
The structural insight is that hydrovoltaic performance is not a surface property but a channel property. The relevant parameter isn't the material's surface charge or hydrophilicity — it's the existence of ordered nanoscale conduits that multiply the number of solid-liquid interfaces per unit area. The same atoms, arranged to create channels versus remaining disordered, produce a factor of 833 in output. The chemistry is necessary but not sufficient. The geometry does the work.