  Water is a life-giving source, fundamental to human existence, yet, over a
billion people lack access to clean drinking water. Present techniques for
water treatment such as piped, treated water rely on time and resource
intensive centralized solutions. In this work, we propose a decentralized
device concept that can utilize sunlight to split water into hydrogen and
hydrogen peroxide. The hydrogen peroxide can oxidize organics while the
hydrogen bubbles out. In enabling this device, we require an electrocatalyst
that can oxidize water while suppressing the thermodynamically favored oxygen
evolution and form hydrogen peroxide. Using density functional theory
calculations, we show that the free energy of adsorbed OH$^*$ can be used as a
descriptor to screen for selectivity trends between the 2e$^-$ water oxidation
to H$_2$O$_2$ and the 4e$^-$ oxidation to O$_2$. We show that materials that
bind oxygen intermediates sufficiently weakly, such as SnO$_2$, can activate
hydrogen peroxide evolution. We present a rational design principle for the
selectivity in electrochemical water oxidation and identify new material
candidates that could perform H$_2$O$_2$ evolution selectively.
