A lithium-air battery based on lithium oxide (Li ₂ O) formation can theoretically deliver an energy density that is comparable to that of gasoline. Lithium oxide formation involves a four-electron reaction that is more difficult to achieve than the one- and two-electron reaction processes that result in lithium superoxide (LiO ₂ ) and lithium peroxide (Li ₂ O ₂ ), respectively. By using a composite polymer electrolyte based on Li ₁₀ GeP ₂ S ₁₂ nanoparticles embedded in a modified polyethylene oxide polymer matrix, we found that Li ₂ O is the main product in a room temperature solid-state lithium-air battery. The battery is rechargeable for 1000 cycles with a low polarization gap and can operate at high rates. The four-electron reaction is enabled by a mixed ion–electron-conducting discharge product and its interface with air.

An overarching challenge of the electrochemical carbon dioxide reduction reaction (eCO₂RR) is finding an earth-abundant, highly active catalyst that selectively produces hydrocarbons at relatively low overpotentials. Here, we report the eCO₂RR performance of two-dimensional transition metal carbide class of materials. Our results indicate a maximum methane (CH₄) current density of −421.63 mA/cm² and a CH₄ faradic efficiency of 82.7% ± 2% for di-tungsten carbide (W₂C) nanoflakes in a hybrid electrolyte of 3 M potassium hydroxide and 2 M choline-chloride. Powered by a triple junction photovoltaic cell, we demonstrate a flow electrolyzer that uses humidified CO₂ to produce CH₄ in a 700-h process under one sun illumination with a CO₂RR energy efficiency of about 62.3% and a solar-to-fuel efficiency of 20.7%. Density functional theory calculations reveal that dissociation of water, chemisorption of CO₂ and cleavage of the C-O bond—the most energy consuming elementary steps in other catalysts such as copper—become nearly spontaneous at the W₂C surface. This results in instantaneous formation of adsorbed CO—an important reaction intermediate—and an unlimited source of protons near the tungsten surface sites that are the main reasons for the observed superior activity, selectivity, and small potential.