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W2C is trigonal omega structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one W2C sheet oriented in the (0, 0, 1) direction. W2+ is bonded in a distorted T-shaped geometry to three equivalent C4- atoms. All W–C bond lengths are 2.13 Å. C4- is bonded to six equivalent W2+ atoms to form edge-sharing CW6 octahedra.
W2C is beta Vanadium nitride-like structured and crystallizes in the orthorhombic Pbcn space group. The structure is three-dimensional. W2+ is bonded in a distorted T-shaped geometry to three equivalent C4- atoms. There are two shorter (2.12 Å) and one longer (2.14 Å) W–C bond lengths. C4- is bonded to six equivalent W2+ atoms to form a mixture of edge and corner-sharing CW6 octahedra. The corner-sharing octahedral tilt angles are 49°.
W2C is beta Vanadium nitride structured and crystallizes in the trigonal P-31m space group. The structure is three-dimensional. W2+ is bonded in a distorted T-shaped geometry to three C4- atoms. There are one shorter (2.11 Å) and two longer (2.13 Å) W–C bond lengths. There are two inequivalent C4- sites. In the first C4- site, C4- is bonded to six equivalent W2+ atoms to form corner-sharing CW6 octahedra. The corner-sharing octahedral tilt angles are 49°. In the second C4- site, C4- is bonded to six equivalent W2+ atoms to form a mixture of corner and edge-sharing CW6 octahedra. The corner-sharing octahedral tilt angles are 49°.
An overarching challenge of the electrochemical carbon dioxide reduction reaction (eCO 2 RR) is finding an earth-abundant, highly active catalyst that selectively produces hydrocarbons at relatively low overpotentials. Here, we report the eCO 2 RR performance of two-dimensional transition metal carbide class of materials. Our results indicate a maximum methane (CH 4 ) current density of -421.63 mA/cm 2 and a CH 4 faradic efficiency of 82.7% ± 2% for di-tungsten carbide (W 2 C) 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 CO 2 to produce CH 4 in a 700-h process under one sun illumination with a CO 2 RR 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 CO 2 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.