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[Display omitted] • CO 2 to syngas with controllable H 2 /CO ratio was investigated using Ag/TiO 2 catalysts. • The H 2 /CO ratio was controlled by oxygen vacancies and phase difference of TiO 2. • The H 2 /CO ratio was increased by the oxygen vacancy defects of TiO 2 anatase. The direct conversion of CO 2 to syngas with controllable H 2 /CO ratio has been investigated using electrochemical reduction of CO 2 in aqueous media. Despite considerable progress on electrocatalytic syngas production, it remains challenging to generate a stable H 2 /CO ratio over a wide range of applied potentials. In this study, we investigated Ag/TiO 2 catalysts, by which we achieved a stable H 2 /CO ratio with high Faradaic efficiency (93–100%) and partial current densities (~164 mA·cm−2) for syngas production in a flow cell. The H 2 /CO ratio was controlled by changing the catalyst properties resulting from oxygen vacancies and phase difference of TiO 2. The H 2 /CO ratio of Ag/TiO 2 catalysts was increased by introducing oxygen vacancy defects in the bulk and on the surface of TiO 2 anatase. Furthermore, the H 2 /CO ratio was also increased by changing the TiO 2 phases from anatase to rutile, even if the rutile phase possessed fewer oxygen vacancies. The 40 wt% Ag/TiO 2 catalysts calcined in different gases (Ag/TiO 2 anatase-air, Ag/TiO 2 anatase-H 2 /Ar, and Ag/TiO 2 rutile-air) exhibited H 2 /CO ratios of 0.1–0.3, 0.5–1.1, and 0.5–1.5, respectively, within the range of potential from −0.35 to −0.65 V (vs. RHE). [ABSTRACT FROM AUTHOR]

[Display omitted] • A systematic study for the effect of porous structures on CO production was performed. • Different performance trends between H-cell and full cell analyses were observed. • A thin catalyst layer possessing mesopores promoted CO production in MEA condition. • The optimized catalyst exhibited the highest energy efficiency (55 %) at 265 mA cm−1. The introduction of a porous structure is a promising approach to promote the electrochemical reaction of catalysts, which can maximize the utilization of catalytic active sites and enhance mass transport. To fully understand the role of the porous structure, parallel studies on both half-cell and full-cell environments must be performed; however, few studies have reported electrochemical CO 2 conversion in a full-cell operation. In this work, we fabricated four types of porous Ni N C model catalysts designed to systematically investigate the relationship between porous structures and catalytic performances in a membrane electrode assembly (MEA) based catholyte-free CO 2 electrolyzer. The performance degradation of the microporous catalyst in the MEA resulted from low CO 2 accessibility due to small openings (<2 nm), and the absence of meso- or macropores that can facilitate mass transport in the catalyst layer. A thick catalyst layer developed a region in which H 2 evolution was dominant; the formation of this region degraded the CO 2 reduction efficiency in the MEA based on the macroporous catalyst. Consequently, mesoporous Ni-N x catalysts with small, uniform particles exhibited the highest efficiency in MEAs, because their appropriate pore size and catalyst layer thickness facilitated mass transport. The optimized catalyst achieved industry-relevant performance for CO production (265 mA cm−2 at 2.3 V) with a state-of-the-art energy efficiency of 55 % and excellent long-term stability in a full cell. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag/C composite and Ag electrocatalysts were synthesized by spray pyrolysis. • Composites had higher CO productivity and stability than Ag particles did. • Ag reaction sites on and within the carbon black support were used efficiently. • Carbon black support is a superior choice for CO 2 reduction by electrocatalyst. In this study, Ag/C composite catalysts with different weight ratios (20, 50, and 75 wt% Ag) were synthesized by spray pyrolysis to improve the performance of electrochemical CO 2 reduction for CO production. Among the electrocatalysts tested, the Ag75/C composite catalyst (75 wt% of Ag nanoparticles dispersed in carbon black support) showed the highest electrochemical CO 2 reduction performance, along with high stability that surpassed that of pure Ag particles. Above all, this is because Ag nanoparticles were dispersed on the surface of (and inside) the carbon black support. This enlarged the Ag surface area, thereby increasing the number of electrochemical CO 2 reduction sites. Additionally, the carbon black support has a hierarchically porous structure that improved the transport of the catholyte, reactant, and products, enabling enhanced electrochemical CO 2 reduction to CO. Furthermore, the use of carbon black support in the Ag/C composite catalysts allows the binder to bind more carbon microspheres than Ag nanoparticles, reducing the Ag surface area covered by the binder in comparison with that for pure Ag particles. Thus, the charge transfer resistance of a Ag/C composite catalyst was much lower than that of pure Ag particles due to the improved interconnection, contact, and number of electrochemical reaction sites provided by the combination of carbon black support and Ag nanoparticles. All of these interpretations imply that carbon black is an appropriate support able to play a key role in improving CO productivity via electrochemical CO 2 reduction. [ABSTRACT FROM AUTHOR]