Monolithic Photoelectrochemical CO₂ Reduction Producing Syngas at 10% Efficiency

Abstract

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...

Powering the World with Solar Fuels from Photoelectrochemical CO2 Reduction: Basic Principles and Recent Advances

“…A new design of selfbiased PEC device with a flow-cell configuration using gas-diffusion electrodes was developed, which resembles commercial electrolyzers. [115,116] Agbo's group [116] designed a PV-integrated membrane (PIM) and sandwiched it between an Au-coated and an Ir-coated gas-diffusion electrode (Figure 10c). Note that a square window was introduced on the gas-diffusion electrodes, ensuring the light absorption of the PV cell.…”

“…[ 110 ] Copyright 2021, Wiley‐VCH; c) Reproduced with permission. [ 116 ] Copyright 2021, Wiley‐VCH; d) Reproduced with permission. [ 119 ] Copyright 2020, Elsevier.…”

“…Copyright 2019, Royal Society of Chemistry; b) Reproduced with permission [110]. Copyright 2021, Wiley-VCH; c) Reproduced with permission [116]. Copyright 2021, Wiley-VCH; d) Reproduced with permission [119].…”

Photo‐Electrochemical Conversion of CO2 Under Concentrated Sunlight Enables Combination of High Reaction Rate and Efficiency

“…The same definition applies to recent reports of photocathode or photoanode in contact with the electrolyte but featuring a buried PV junction. [5][6][7] In the majority of PEC reports, CO was the targeted product through Equation 1 resulting from Equation 1a,b halfcell reactions, with the EC part being either a Hcell, [8][9][10][11][12][13][14][15][16] a one compartment cell (no separator), [17,18] a two compartments flow cell (membrane separator), [4,[19][20][21][22][23][24] a monolithic PEC cell, [25][26][27] a proton exchange membrane based (PEM) zero gap cell, [28] or an anion exchange membrane based (AEM) zerogap stack. [29] All these studies were performed at irradiation intensity of 1 sun (1 kW m −2 , see Discussion S1 (Supporting Information) for detailed defini tion) [4,[8][9][10][11][12][13][14][15][16]18,19,[21][22][23][24][25][26]…”

“…Solar‐to‐CO conversion efficiency and CO partial current density in the electrolyzer as a function of incoming solar power for previously reported light‐driven P‐EC reduction of CO 2 into CO experiments. [ 4,8–29 ] Solar concentration ≥ 1 sun. Electrolyzer part: H‐cell (square), flow cell (diamond), monolithic stack (circle), zero‐gap (triangle) and no membrane cell (star).…”