The primary motivation for developing photo-electrochemical (PEC) hydrogen (H₂) systems is to reduce global greenhouse gas (GHG) emissions, as required by the 2015 Paris Agreement¹  and recently ratified by the threshold number of countries required for entry into force.²  While numerous approaches exist that can result in large GHG reductions,³  H₂ represents an energy carrier with many attractive features,⁴  including feedstocks for its production, relative ease of storage and transport, high efficiency of conversion into electricity, and clean combustion without any particulates or CO₂ emissions. Moreover, it is a useful intermediate chemical for many industrial purposes, including fuel upgrading or Fischer–Tropsch hydrocarbon synthesis,⁵  ammonia production,⁶  hydrochloric acid and methanol production,⁷  silicon manufacturing,⁸  and as a reducing agent for iron oxide.⁹  It can also be used in atomic hydrogen welding, as a rotor coolant in electric generators, and as a leak detector.⁷  Global H₂ production was 48 million metric tons (Mt) in 2006 and >50 Mt in 2014, with 40% produced in the United States.^10,11  Of the total US hydrogen production, petroleum refining consumed 64%, and chemical manufacturing (mainly ammonia production) consumed 33%. Global estimates indicate a potential demand of 300 Mt H₂ per year in 2050,¹²  with an energy equivalent of ∼300 billion gallons of gasoline, or ∼20% of global liquid fuel demand in 2013.¹³