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Photocatalytic production of solar fuels from CO2 is a promising strategy for addressing global environmental problems and securing future energy supplies. Although extensive research has been conducted to date, numerous impediments to realizing efficient, selective, and stable CO2 reduction have yet to be overcome. This comprehensive review highlights the recent advances in CO2 photoreduction, including critical challenges such as light-harvesting, charge separation, and the activation of CO2 molecules. We present promising strategies for enhancing the photocatalytic activities and discuss theoretical insights and equations for quantifying photocatalytic performance, which are expected to afford a fundamental understanding of CO2 photoreduction. We then provide a thorough overview of both traditional photocatalysts such as metal oxides and state-of-the-art catalysts such as metal–organic frameworks and 2D materials, followed by a discussion of the origin of carbon in CO2 photoreduction as a means to further understand the reaction mechanism. Finally, we discuss the economic viability of photocatalytic CO2 reduction before concluding the review with proposed future research directions.

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

If we wish to sustain our terrestrial ecosphere as we know it, then reducing the concentration of atmospheric CO2 is of critical importance. An ideal pathway for achieving this would be the use of sunlight to recycle CO2, in combination with water, into hydrocarbon fuels compatible with our current energy infrastructure. However, while the concept is intriguing such a technology has not been viable due to the vanishingly small CO2-to-fuel photoconversion efficiencies achieved. Herein we report a photocatalyst, reduced blue-titania sensitized with bimetallic Cu–Pt nanoparticles that generates a substantial amount of both methane and ethane by CO2 photoreduction under artificial sunlight (AM1.5): over a 6 h period 3.0 mmol g−1 methane and 0.15 mmol g−1 ethane are obtained (on an area normalized basis 0.244 mol m−2 methane and 0.012 mol m−2 ethane), while no H2 nor CO is detected. This activity (6 h) translates into a sustained Joule (sunlight) to Joule (fuel) photoconversion efficiency of 1%, with an apparent quantum efficiency of φ = 86%. The time-dependent photoconversion efficiency over 0.5 h intervals yields a maximum value of 3.3% (φ = 92%). Isotopic tracer experiments confirm the hydrocarbon products originate from CO2 and water.

With the rapid increase in energy consumption worldwide, the development of renewable and alternative energy sources can sustain long-term development in the energy field. Hydrogen (H2), which is one of the clean chemical fuels, has the highest weight energy density and its combustion byproduct is only water. Among the various methods of producing hydrogen source, water electrolysis is a process that can effectively produce H2. However, it is difficult for commercialization of water electrolysis for H2 production due to the high cost and low abundance of noble metal-based cathodic electrode used for highly efficiency. Several studies have been conducted to reduce noble metal loading and/or completely replace them with other materials to overcome these obstacles. Among them, stainless steel contains many components of transition metals (Ni, Cr, Co) but have sluggish reaction kinetics and small active surface area. In this study, the problem of stainless steel was to be solved by utilizing the electrocatalytic properties of silver nanoparticles on the electrode surface, and electrodes were easily fabricated through the electrodeposition process. In addition, the surface shape, elemental properties, and HER activity of the electrode was analyzed by comparing it with the commercialized silver nanoparticle-coated invasive electrodes from Inanos (Inano-Ag-IE) through the plasma coating process. As a result, silver nanoparticle-coated conventional electrode (Ag-CE) fabricated through electrodeposition confirmed high HER activity and stability. However, the Inano-Ag-IE showed low HER activity as silver nanoparticles were not found. We encourage further research on the production process of such products for sustainable energy applications.

Cu-based photocatalysts are widely used for the photocatalytic reduction of CO2 owing to their non-toxicity, earth abundance, extended light absorption, suppressed charge recombination, and good catalytic performance. In addition to low cost, abundant availability, and the ease of synthesis, they exhibit unique features, such as broad optical absorption, reaction intermediates stabilization, and C–C coupling ability, which lead to the formation of C2 + products. The photocatalytic activity of Cu-based photocatalysts is essentially linked to the optical absorption and interfacial charge transfer at the junction of Cu and the semiconductor substrate. However, the poor resistance of Cu to oxidation seriously perturbs the effective utilization of its unique features in practical applications. To date, various approaches, such as the use of metal/non-metal co-catalysts, Z-scheme heterostructures, and hole scavengers, have been proposed to improve its photocatalytic performance and maintain its stability in prolonged reactions. In addition to these approaches, as single metal atom catalysts, atomically dispersed Cu photocatalysts have gained immense attention because they can regain the desired oxidation state, and hence exhibit good stability. The designation of suitable oxidation states of Cu for various CO2 reduction reaction steps is challenging because of the rapid change in its oxidation states under ambient/irradiated environments. However, in situ spectroscopic analyses have nominated Cu2+ for CO2 adsorption, Cu1+ for the photoreduction reaction, and Cu0 for effective charge separation. In this review, the recent advancements in the photocatalytic activity, selectivity, and stability of Cu-based photocatalysts are discussed systematically. Certain concepts and mechanisms related to the photocatalytic performance of Cu-based catalysts have also been discussed. Finally, the future research directions are discussed based on the available relevant literature.

A desire for renewable alternatives to fossil fuels can be achieved by utilizing CO2, H2O, and solar energy to generate solar fuels. A novel N-doped graphene oxide enfolded reduced titania (NGO-RT) composite was demonstrated for photocatalytic CO2 reduction into CH4. Later, a small amount of Pt NPs was deposited on NGO-RT that increases the catalytic performance towards CH4 formation. The optimized Pt1.0%-NGO-RT catalyst displayed a selective visible-light CO2 reduction into CH4 using a flow reactor system with ≈12 and ≈2 times higher activity than pristine RT and NGO-RT, respectively. The catalyst demonstrated long-term stability over 35 h. The photo-induced CO2 reduction mechanism was first validated through the electron transfer process, where charge trapping by Ti3+ states near the conduction band of RT plays a vital role in the selective CH4 evolution. These trapped electrons transfer from RT to the closely connected interface of N-doped graphene oxide and Pt NPs to restrict the recombination of electron/hole pair. The improved catalytic performance can be attributed to RT’s downward band bending at the NGO-RT interface, where electron transfer from RT to NGO decreases the charge recombination.

Technological development after the industrial revolution has improved the quality of human life, but global energy consumption continues to increase due to population growth and the development of fossil fuels. Therefore, numerous studies have been conducted to develop sustainable long-term and renewable alternative energy sources. The anodic electrode, which is one of the two-electrode system components, is an essential element for effective energy production. In general, precious metal-based electrocatalysts show high OER reactions from the anodic electrode, but it is difficult to scale up due to their low abundance and high cost. To overcome these problems, transition metal-based anodic electrodes, which exhibit advantages with respect to their low cost and high catalytic activities, are in the spotlight nowadays. Among them, stainless steel is a material with a high ratio of transition metal components, i.e., Fe, Ni, and Cr, and has excellent corrosion resistance and low cost. However, stainless steel shows low electrochemical performance due to its slow sluggish kinetics and lack of active sites. In this study, we fabricated surface modified electrodes by two methods: (i) anodization and (ii) hydrogen peroxide (H2O2) immersion treatments. As a result of comparing the two methods, the change of the electrode surface and the electrochemical properties were not confirmed in the H2O2 immersion method. On the other hand, the porous electrode (PE) fabricated through electrochemical anodization shows a low charge transfer resistance (Rct) and high OER activity due to its large surface area compared to the conventional electrode (CE). These results confirm that the synthesis process of H2O2 immersion is an unsuitable method for surface modification. In contrast, the PE fabricated by anodization can increase the OER activity by providing high adsorption of reactants through surface modification.

Photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels is a promising approach for storing solar energy while reducing greenhouse gas emissions. However, still certain issues including low product yields, limited photocatalyst stability and relatively high cost have hampered practical implementation of this technology. In the present work, a unique strategy is adopted to synthesize a stable, and inexpensive photocatalyst comprised of earth-abundant materials: a reduced titania-Cu2O Z-scheme heterostructure. Under illumination for 6 h, the optimized reduced titania-Cu2O photocatalyst enables 0.13 % photoreduction of highly diluted CO2 with water vapors to 462nmol g−1 of CH4 while showing excellent stability over seven testing cycles (42 h). Our studies show the Z-scheme inhibits Cu2O photocorrosion, while its synergistic effects with reduced titania result in sustained CH4 formation in a continuous flow photoreactor. To the best of our knowledge stability exhibited by the reduced titania-Cu2O Z-scheme is the highest for any Cu-based photocatalyst.

Photocatalytic CO2 reduction is emerging as an affordable route for abating its ever increasing concentration. For commercial scale applications, many constraints are still required to be addressed. A variety of research areas are explored, such as development of photocatalysts and photoreactors, reaction parameters and conditions, to resolve these bottlenecks. In general, the photocatalyst performance is mostly adjudged in terms of its ability to only produce hydrocarbon products, and other vital parameters such as light source, reaction parameters, and type of photoreactors used are not normally given appropriate attention. This makes a comprehensive comparison of photocatalytic performance quite unrealistic. Hence, probing the photocatalytic performance in terms of apparent quantum yield (AQY) with the consideration of certain process and experimental parameters is a more reasonable and prudent approach. The present brief review portrays the importance and impact of aforementioned parameters in the field of gas phase photocatalytic CO2 reduction.

A microbial electrolysis cell (MEC) consumes the chemical energy of organic material producing, in turn, hydrogen. This study presents a new hybrid MEC design with improved performance. An external TiO2 nanotube (TNT) array photoanode, fabricated by anodization of Ti foil, supplies photogenerated electrons to the MEC electrical circuit, significantly improving overall performance. The photogenerated electrons help to reduce electron depletion of the bioanode, and improve the proton reduction reaction at the cathode. Under simulated AM 1.5 illumination (100 mW cm&minus, 2) the 28 mL hybrid MEC exhibits a H2 evolution rate of 1434.268 &plusmn, 114.174 mmol m&minus, 3 h&minus, 1, a current density of 0.371 &plusmn, 0.000 mA cm&minus, 2 and power density of 1415.311 &plusmn, 23.937 mW m&minus, 2, that are respectively 30.76%, 34.4%, and 26.0% higher than a MEC under dark condition.