This report builds on our recent disclosure of a fully-integrated, photoelectrochemical (PEC) device for hydrogen evolution using a structure incorporating a III–V triple-junction photovoltaic (PV) embedded in a Nafion membrane. Catalyst integration is realized by compression of catalyst-modified, carbon electrodes against the front and back PV contacts, resulting in a wireless, monolithic PEC assembly. Using this device architecture, we demonstrate significant enhancements in device stability and longevity, by transitioning from a liquid-water to water-vapor anode. Our use of a gas-fed anode enables 1000 h of cumulative device operation at a peak solar-to-hydrogen efficiency of 14%, during simulated, solar illumination at 1 sun and outdoor, diurnal cycling. Vapor-fed water oxidation is shown to reduce drops in device performance by mitigating the corrosion effects that are commonly associated with full-aqueous immersion of the electrochemical and photovoltaic elements in PEC devices.

remain intractable as atmospheric CO 2 concentrations continue to rise.An appreciation for this reality has motivated the development of carbonneutral/negative processes, such as CO 2 -derived fuels/materials production, capable of displacing their carbon-positive counterparts across all economic sectors. CO 2 reduction reactions generally enable a wide range of mixed products, [4] mostly depending on the catalyst, its surface structure, and the electrolyte, [5][6][7][8][9][10][11][12][13][14] often requiring downstream separation. However, carbon monoxide (CO) and its mixture with hydrogen (H 2 ), called synthesis gas (syngas), may be obtained at a selectivity higher than 90%. [15] Syngas is an important precursor in the chemical industry for the production of alcohols, [16] acetic acid, [17] and synthetic hydrocarbon fuels [18,19] but is typically produced from fossil fuels. Alternatively, CO 2 reduction may be driven by electricity from renewable sources, such as solar cells, significantly lowering its carbon footprint. [20][21][22] In solar-driven CO 2 electrolysis (artificial photosynthesis), [23] the solar-to-fuel (STF) efficiency marks a critical figure-of-merit, describing the efficiency with which incident solar energy is incorporated into the chemical bonds of a particular fuel or fuel mixture. Recently, STF efficiencies of 19% at 1 sun illumination intensity have been achieved in devices featuring physically separated photovoltaic (PV) and electrolyzer components. [24][25][26][27][28][29] However, the state of monolithic, photoelectrochemical (PEC) devices has lagged behind the performance of PV-electrolyzers, with monolithic architectures to date displaying peak STF efficiencies of 4.6% for solar CO 2 conversion. [30][31][32] Monolithic designs do not require any wiring between PV and electrocatalytic components during unbiased, PEC operation. [33] The challenge of constructing robust, monolithic PEC devices is compounded by the requisite integration of the PV in the electrolysis compartment, limiting the choice of photoabsorber materials to those resistant to corrosion by the alkaline electrolytes generally employed in CO 2 electrolyzers. [34] Despite such constraints, PEC monoliths may benefit from a more compact device design, enabling lower charge transport losses, and favorable heat-exchange between the PV and the electrolyzer, [33] especially under light concentration. [35] This work describes a monolithic PEC device that converts CO 2 to syngas at a record STF efficiency above 10% (combined solar-to-CO and solar-to-H 2 efficiency). This benchmark represents a doubling in the reported peak efficiency for such Increasing anthropogenic carbon dioxide emissions have prompted the search for photoelectrochemical (PEC) methods of converting CO 2 to useful commodity products, including fuels. Ideally, such PEC approaches will be sustained using only sunlight, water, and CO 2 as energetic and reactant inputs. However, low peak conversion efficiencies (<5%) have made commercialization of fully-integrated...

Photoelectrochemical (PEC) conversion of carbon dioxide into valuable chemicals and fuels represents a promising path towards combating anthropogenic CO2 emissions. However, the limited conversion efficiencies, operation lifetimes and CO2 utilization...