  Recently, superhydrides have been computationally identified and subsequently
synthesized with a variety of metals at very high pressures. In this work, we
evaluate the possibility of synthesizing superhydrides by uniquely combining
electrochemistry and applied pressure. We perform computational searches for
palladium superhydrides using density functional theory and particle swarm
optimization calculations over a broad range of pressures and electrode
potentials. We incorporate exchange-correlation functional uncertainty using
the Bayesian error estimation formalism to quantify the uncertainty associated
with the identified stable phases. Based on a thermodynamic analysis, we
construct pressure-potential phase diagrams and provide an alternate synthesis
concept, $\mathcal{P}^2$ (pressure-potential), to accessing novel phases having
high hydrogen content. Palladium-hydrogen is a widely-studied material system
with the highest hydride phase being Pd$_3$H$_4$. Most strikingly for this
system, at potentials above hydrogen evolution and $\sim$300 MPa pressure, we
find the possibility to make palladium superhydrides (e.g., PdH$_{10}$). We
demonstrate the generalizability of this approach for La-H, Y-H and Mg-H with
10-100 fold reduction in required pressure for stabilizing phases. In addition,
the $\mathcal{P}^2$ strategy allows stabilizing new phases that cannot be done
purely by either pressure or potential and is a general approach that is likely
to work for synthesizing other superhydrides at modest pressures.
