Ever‐increasing fossil‐fuel combustion along with massive CO2 emissions has aroused a global energy crisis and climate change. Photocatalytic CO2 reduction represents a promising strategy for clean, ...cost‐effective, and environmentally friendly conversion of CO2 into hydrocarbon fuels by utilizing solar energy. This strategy combines the reductive half‐reaction of CO2 conversion with an oxidative half reaction, e.g., H2O oxidation, to create a carbon‐neutral cycle, presenting a viable solution to global energy and environmental problems. There are three pivotal processes in photocatalytic CO2 conversion: (i) solar‐light absorption, (ii) charge separation/migration, and (iii) catalytic CO2 reduction and H2O oxidation. While significant progress is made in optimizing the first two processes, much less research is conducted toward enhancing the efficiency of the third step, which requires the presence of cocatalysts. In general, cocatalysts play four important roles: (i) boosting charge separation/transfer, (ii) improving the activity and selectivity of CO2 reduction, (iii) enhancing the stability of photocatalysts, and (iv) suppressing side or back reactions. Herein, for the first time, all the developed CO2‐reduction cocatalysts for semiconductor‐based photocatalytic CO2 conversion are summarized, and their functions and mechanisms are discussed. Finally, perspectives in this emerging area are provided.
Active and stable CO2‐reduction cocatalysts can obviously enhance the efficiency, selectivity, and stability of semiconductor‐based photocatalytic CO2 reduction. All of the developed CO2‐reduction cocatalysts are summarized, and their functions and insightful mechanisms are discussed. This can pave new avenues to the exploration of novel highly active and selective cocatalysts, toward high‐performance solar fuel production.
Compared to modern fossil‐fuel‐based refineries, the emerging electrocatalytic refinery (e‐refinery) is a more sustainable and environmentally benign strategy to convert renewable feedstocks and ...energy sources into transportable fuels and value‐added chemicals. A crucial step in conducting e‐refinery processes is the development of appropriate reactions and optimal electrocatalysts for efficient cleavage and formation of chemical bonds. However, compared to well‐studied primary reactions (e.g., O2 reduction, water splitting), the mechanistic aspects and materials design for emerging complex reactions are yet to be settled. To address this challenge, herein, we first present fundamentals of heterogeneous electrocatalysis and some primary reactions, and then implement these to establish the framework of e‐refinery by coupling in situ generated intermediates (integrated reactions) or products (tandem reactions). We also present a set of materials design principles and strategies to efficiently manipulate the reaction intermediates and pathways.
The concept of the electrocatalytic refinery (e‐refinery) is an intrinsically sustainable strategy to convert renewable feedstocks and energy sources to transportable fuels and value‐added chemicals. This Review describes the concept, fundamentals, and framework of e‐refinery processes with some game‐changing reactions and innovative catalyst design strategies.
The hydrogen evolution reaction (HER) is a fundamental process in electrocatalysis and plays an important role in energy conversion for the development of hydrogen‐based energy sources. However, the ...considerably slow rate of the HER in alkaline conditions has hindered advances in water splitting techniques for high‐purity hydrogen production. Differing from well documented acidic HER, the mechanistic aspects of alkaline HER are yet to be settled. A critical appraisal of alkaline HER electrocatalysis is presented, with a special emphasis on the connection between fundamental surface electrochemistry on single‐crystal models and the derived molecular design principle for real‐world electrocatalysts. By presenting some typical examples across theoretical calculations, surface characterization, and electrochemical experiments, we try to address some key ongoing debates to deliver a better understanding of alkaline HER at the atomic level.
Focusing on the long‐lasting debates surrounding the activity descriptor for the electrocatalytic hydrogen evolution reaction in alkaline conditions, some fundamental studies, from theoretical computations and surface electrochemistry on single crystal models, to practical electrocatalysts with large surfaces, are summarized.
Electrocatalytic production of hydrogen from seawater provides a route to low‐cost and clean energy conversion. However, the hydrogen evolution reaction (HER) using seawater is greatly hindered by ...the lack of active and stable catalysts. Herein, an unsaturated nickel surface nitride (Ni‐SN@C) catalyst that is active and stable for the HER in alkaline seawater is prepared. It achieves a low overpotential of 23 mV at a current density of 10 mA cm−2 in alkaline seawater electrolyte, which is superior to Pt/C. Compared to conventional transition metal nitrides or metal/metal nitride heterostructures, the Ni‐SN@C has no detectable bulk nickel nitride phase. Instead, unsaturated NiN bonding on the surface is present. In situ Raman measurements show that the Ni‐SN@C performs like Pt with the ability to generate hydronium ions in a high‐pH electrolyte. The catalyst operation is then demonstrated in a two‐electrode electrolyzer system, coupling with hydrazine oxidation at the anode. Using this system, a cell voltage of only 0.7 V is required to achieve a current density of 1 A cm−2.
An unsaturated nickel surface nitride exhibits efficient and stable performance for hydrogen evolution from seawater. The origin of the activity and stability of the unique surface nitride is investigated using in situ Raman spectroscopy. By coupling with hydrazine oxidation at the anode, a flow‐electrolyzer is assembled that can deliver a current density of 1 A cm−2 with a small cell voltage of 0.7 V.
Transitional metals are widely used as co‐catalysts boosting photocatalytic H2 production. However, metal‐based co‐catalysts suffer from high cost, limited abundance and detrimental environment ...impact. To date, metal‐free co‐catalyst is rarely reported. Here we for the first time utilized density functional calculations to guide the application of phosphorene as a high‐efficiency metal‐free co‐catalyst for CdS, Zn0.8Cd0.2S or ZnS. Particularly, phosphorene modified CdS shows a high apparent quantum yield of 34.7 % at 420 nm. This outstanding activity arises from the strong electronic coupling between phosphorene and CdS, as well as the favorable band structure, high charge mobility and massive active sites of phosphorene, supported by computations and advanced characterizations, for example, synchrotron‐based X‐ray absorption near edge spectroscopy. This work brings new opportunities to prepare highly‐active, cheap and green photocatalysts.
Density functional calculations were used to direct the design of phosphorene as a metal‐free co‐catalyst promoting photocatalytic H2 production in metal sulfide photocatalyst systems. The enhanced photocatalytic performance arises from the pronounced electronic coupling between metal sulfides and phosphorene, together with its advantageous band structure and excellent charge carrier mobility.
The establishment of electrocatalysts with bifunctionality for efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic environments is necessary for the development ...of proton exchange membrane (PEM) water electrolyzers for the production of clean hydrogen fuel. RuIr alloy is considered to be a promising electrocatalyst because of its favorable OER performance and potential for HER. Here, the design of a bifunctional electrocatalyst with greatly boosted water‐splitting performance from doping RuIr alloy nanocrystals with transition metals that modify electronic structure and binding strength of reaction intermediates is reported. Significantly, Co‐RuIr results in small overpotentials of 235 mV for OER and 14 mV for HER (@ 10 mA cm−2 current density) in 0.1 m HClO4 media. Therefore a cell voltage of just 1.52 V is needed for overall water splitting to produce hydrogen and oxygen. More importantly, for a series of M‐RuIr (M = Co, Ni, Fe), the catalytic activity dependence at fundamental level on the chemical/valence states is used to establish a novel composition‐activity relationship. This permits new design principles for bifunctional electrocatalysts.
Transition‐metal‐doped RuIr alloy nanocrystals are synthesized as bifunctional overall water splitting electrocatalysts for acidic environments. Due to both increased active oxygen species concentration and modified surface site valence states, the dopants result in significantly high electrocatalytic performance. A novel composition–activity relationship is established. This can be applied to the rational design of bifunctional electrocatalysts.
Research interest and achievements in zinc aqueous batteries, such as alkaline Zn//Mn, Zn//Ni/Co, Zn–air batteries, and near‐neutral Zn‐ion and hybrid ion batteries, have surged throughout the world ...due to their features of low‐cost and high‐safety. However, practical application of Zn‐based secondary batteries is plagued by restrictive energy and power densities in which an inadequate output plateau voltage and sluggish kinetics are mutually accountable. Here, a novel paradigm high‐rate and high‐voltage Zn–Mn hybrid aqueous battery (HAB) is constructed with an expanded electrochemical stability window over 3.4 V that is affordable. As a proof of concept, catalyzed MnO2/Mn2+ electrolysis kinetics is demonstrated in the HAB via facile introduction of Ni2+ into the electrolyte. Various techniques are employed, including in situ synchrotron X‐ray powder diffraction, ex situ X‐ray absorption fine structure, and electron energy loss spectroscopy, to reveal the reversible charge‐storage mechanism and the origin of the boosted rate‐capability. Density functional theory (DFT) calculations reveal enhanced active electron states and charge delocalization after introducing strongly electronegative Ni. Simulations of the reaction pathways confirm the enhanced catalyzed electrolysis kinetics by the facilitated charge transfer at the active O sites around Ni dopants. These findings significantly advance aqueous batteries a step closer toward practical low‐cost application.
A paradigm high‐rate and high‐voltage affordable Zn–Mn hybrid aqueous battery (HAB) is constructed with an electrochemical stability window over 3.4 V. Imposing power of 19 kW kg−1 and energy of 650 Wh kg−1 are demonstrated. Spectra techniques and theoretical calculations reveal the origin of the concomitant power and energy densities, via catalyzed kinetics in an aqueous battery.
Hollow structures exhibit fascinating and important properties for energy‐related applications, such as lithium‐ion batteries, supercapacitors, and electrocatalysts. Sodium‐ion batteries, as analogs ...of lithium‐ion batteries, are considered as promising devices for large‐scale electrical energy storage. Inspired by applications of hollow structures as anodes for lithium‐ion batteries, the application of these structures in sodium‐ion batteries has attracted great attention in recent years. However, due to the difference in lithium and sodium‐ion batteries, there are several issues that need to be addressed toward rational design of hollow structured sodium anodes. Herein, this research news article presents the recent developments in the synthesis of hollow structured anodes for sodium‐ion batteries. The main strategies for rational design of materials for sodium‐ion batteries are presented to provide an overview and perspectives for the future developments of this research area.
Hollow‐structured electrode materials exhibit fascinating properties as anodes for sodium‐ion batteries. Recent developments of hollow structured anodes for sodium‐ion batteries are summarized. Additionally, different rational design structures according to the features of sodium‐ion batteries are presented.
Monitoring and controlling the reconstruction of materials under working conditions is crucial for the precise identification of active sites, elucidation of reaction mechanisms, and rational design ...of advanced catalysts. Herein, a Bi‐based metal–organic framework (Bi‐MOF) for electrochemical CO2 reduction is selected as a case study. In situ Raman spectra combined with ex situ electron microscopy reveal that the intricate reconstruction of the Bi‐MOF can be controlled using two steps: 1) electrolyte‐mediated dissociation and conversion of Bi‐MOF to Bi2O2CO3, and 2) potential‐mediated reduction of Bi2O2CO3 to Bi. The intentionally reconstructed Bi catalyst exhibits excellent activity, selectivity, and durability for formate production, and the unsaturated surface Bi atoms formed during reconstruction become the active sites. This work emphasizes the significant impact of pre‐catalyst reconstruction under working conditions and provides insight into the design of highly active and stable electrocatalysts through the regulation of these processes.
The reconstruction of Bi‐MOFs prior to CO2 electroreduction can be decoupled into two steps: 1) electrolyte‐mediated conversion of Bi‐MOFs to Bi2O2CO3 by HCO3−‐initiated ligand substitution, and 2) potential‐mediated reduction of Bi2O2CO3 to Bi. The first step controls the final morphology and structure, whilst the second step determines the final composition and valence states.
Antisolvent addition has been widely studied in crystallization in the pharmaceutical industries by breaking the solvation balance of the original solution. Here we report a similar antisolvent ...strategy to boost Zn reversibility via regulation of the electrolyte on a molecular level. By adding for example methanol into ZnSO4 electrolyte, the free water and coordinated water in Zn2+ solvation sheath gradually interact with the antisolvent, which minimizes water activity and weakens Zn2+ solvation. Concomitantly, dendrite‐free Zn deposition occurs via change in the deposition orientation, as evidenced by in situ optical microscopy. Zn reversibility is significantly boosted in antisolvent electrolyte of 50 % methanol by volume (Anti‐M‐50 %) even under harsh environments of −20 °C and 60 °C. Additionally, the suppressed side reactions and dendrite‐free Zn plating/stripping in Anti‐M‐50 % electrolyte significantly enhance performance of Zn/polyaniline coin and pouch cells. We demonstrate this low‐cost strategy can be readily generalized to other solvents, indicating its practical universality. Results will be of immediate interest and benefit to a range of researchers in electrochemistry and energy storage.
Water activity and Zn2+ solvation in an ZnSO4 electrolyte are regulated by adding methanol as antisolvent. Methanol gradually interacts with the free and coordinated water in the Zn2+ solvation sheath in the electrolyte, to suppress side reactions and enhance the Zn2+ transference number. Concomitantly, Zn2+ deposition orientation is changed, resulting in dendrite‐free Zn deposition and boosted Zn reversibility.
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