Transporting the hydrogen anion Hydrogen cation (H ⁺ ) transport is common in both biological systems and engineered ones such as fuel cells. In contrast, the transport of hydrogen anions (H ⁻ ) is far less common and is usually coupled with or compromised by the parallel transport of electrons. Kobayashi et al. examined the transport of H ⁻ in a series of rare-earth lithium oxyhydrides (see the Perspective by Yamaguchi). They prevented electronic conduction by using Li ⁺ as a countercation. In an electrochemical cell, the oxyhydride material acted as a solid-state electrolyte for H ⁻ , which suggests an alternative avenue for developing energy storage devices. Science , this issue p. 1314 ; see also p. 1262