Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, ...

Emerging Two-Dimensional Nanomaterials for Electrocatalysis

Electrocatalytic transformation of carbon dioxide (CO2) and water into chemical feedstocks offers the potential to reduce carbon emissions by shifting the chemical industry away from fossil fuel dependence. We provide a technoeconomic and carbon emission analysis of possible products, offering targets that would need to be met for economically compelling industrial implementation to be achieved. We also provide a comparison of the projected costs and CO2 emissions across electrocatalytic, biocatalytic, and fossil fuel-derived production of chemical feedstocks. We find that for electrosynthesis to become competitive with fossil fuel-derived feedstocks, electrical-to-chemical conversion efficiencies need to reach at least 60%, and renewable electricity prices need to fall below 4 cents per kilowatt-hour. We discuss the possibility of combining electro- and biocatalytic processes, using sequential upgrading of CO2 as a representative case. We describe the technical challenges and economic barriers to marketable electrosynthesized chemicals.

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What would it take for renewably powered electrosynthesis to displace petrochemical processes

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

https://espace.library.uq.edu.au/view/UQ:df1181f/UQdf1181f_OA.pdf

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting.

Single-atom catalysts have emerged as an exciting paradigm with intriguing properties different from their nanocrystal counterparts Here we report Ni single atoms dispersed into graphene nanosheets, without Ni nanoparticles involved, as active sites for the electrocatalytic CO2 reduction reaction (CO2RR) to CO While Ni metal catalyzes the hydrogen evolution reaction (HER) exclusively under CO2RR conditions, Ni single atomic sites present a high CO selectivity of 95% under an overpotential of 550 mV in water, and an excellent stability over 20 hours’ continuous electrolysis The current density can be scaled up to more than 50 mA cm−2 with a CO evolution turnover frequency of 21 × 105 h−1 while maintaining 97% CO selectivity using an anion membrane electrode assembly Different Ni sites in graphene vacancies, with or without neighboring N coordination, were identified by in situ X-ray absorption spectroscopy and density functional theory calculations Theoretical analysis of Ni and Co sites suggests completely different reaction pathways towards the CO2RR or HER, in agreement with experimental observations

Isolated Ni single atoms in graphene nanosheets for high-performance CO2 reduction

A Two-Dimensional Porous Carbon-Modified Separator for High-Energy-Density Li-S Batteries

How to exert the energy density advantage is a key link in the development of lithium–sulfur batteries. Therefore, the performance degradation of high-sulfur-loading cathodes becomes an urgent problem to be solved at present. In addition, the volumetric capacities of high-sulfur-loading cathodes are still at a low level compared with their areal capacities. Aiming at these issues, two-dimensional carbon yolk-shell nanosheet is developed herein to construct a novel self-supporting sulfur cathode. The cathode with high-sulfur loading of 5 mg cm−2 and sulfur content of 73 wt% not only delivers an excellent rate performance and cycling stability, but also provides a favorable balance between the areal (5.7 mAh cm–2) and volumetric (1330 mAh cm–3) capacities. Remarkably, an areal capacity of 11.4 mAh cm–2 can be further achieved by increasing the sulfur loading from 5 to 10 mg cm–2. This work provides a promising direction for high-energy-density lithium–sulfur batteries. One of the challenges facing lithium-sulfur batteries is to develop cathodes with high mass and high volume loading. Here the authors show that two-dimensional carbon yolk-shell nanosheets are promising sulfur host materials, enabling stable battery cells with high energy density.

Self-supporting sulfur cathodes enabled by two-dimensional carbon yolk-shell nanosheets for high-energy-density lithium-sulfur batteries.

Electrochemical conversion of CO2 into fuels using electricity generated from renewable sources helps to create an artificial carbon cycle. However, the low efficiency and poor stability hinder the practical use of most conventional electrocatalysts. In this work, a 2D hierarchical Pd/SnO2 structure, ultrathin Pd nanosheets partially capped by SnO2 nanoparticles, is designed to enable multi-electron transfer for selective electroreduction of CO2 into CH3 OH. Such a structure design not only enhances the adsorption of CO2 on SnO2 , but also weakens the binding strength of CO on Pd due to the as-built Pd-O-Sn interfaces, which is demonstrated to be critical to improve the electrocatalytic selectivity and stability of Pd catalysts. This work provides a new strategy to improve electrochemical performance of metal-based catalysts by creating metal oxide interfaces for selective electroreduction of CO2 .

Electrochemical Reduction of Carbon Dioxide to Methanol on Hierarchical Pd/SnO2 Nanosheets with Abundant Pd–O–Sn Interfaces

In Situ Electrochemical Production of Ultrathin Nickel Nanosheets for Hydrogen Evolution Electrocatalysis

The electrochemical conversion of CO2 and H2O into syngas using renewably generated electricity is an attractive approach to simultaneously achieve chemical fixation of CO2 and storage of renewable energy. Developing cost-effective catalysts for selective electroreduction of CO2 into CO is essential to the practical applications of the approach. We report a simple synthetic strategy for the preparation of ultrathin Cu/Ni(OH)2 nanosheets as an excellent cost-effective catalyst for the electrochemical conversion of CO2 and H2O into tunable syngas under low overpotentials. These hybrid nanosheets with Cu(0)-enriched surface behave like noble metal nanocatalysts in both air stability and catalysis. Uniquely, Cu(0) within the nanosheets is stable against air oxidation for months because of the presence of formate on their surface. With the presence of atomically thick ultrastable Cu nanosheets, the hybrid Cu/Ni(OH)2 nanosheets display both excellent activity and selectivity in the electroreduction of CO2 to CO. At a low overpotential of 0.39 V, the nanosheets provide a current density of 4.3 mA/cm2 with a CO faradaic efficiency of 92%. No decay in the current is observed for more than 22 hours. The catalysts developed in this work are promising for building low-cost CO2 electrolyzers to produce CO.

Daohui Ou

Papers

A Two-Dimensional Porous Carbon-Modified Separator for High-Energy-Density Li-S Batteries

Self-supporting sulfur cathodes enabled by two-dimensional carbon yolk-shell nanosheets for high-energy-density lithium-sulfur batteries.

Electrochemical Reduction of Carbon Dioxide to Methanol on Hierarchical Pd/SnO2 Nanosheets with Abundant Pd–O–Sn Interfaces

In Situ Electrochemical Production of Ultrathin Nickel Nanosheets for Hydrogen Evolution Electrocatalysis

Ultrastable atomic copper nanosheets for selective electrochemical reduction of carbon dioxide