The extensive consumption of fossil fuels has caused the rapid increase in the CO2 level in the atmosphere, forcing people to find a clean and efficient technology of CO2 conversion to alleviate CO2 emissions and develop value-added products. Among various CO2 conversion systems, electroreduction of CO2 to value-added chemicals is a feasible way for practical applications. Copper, the only metal that can catalyze CO2 reduction to multi-carbon products, has attracted the most attention among various catalysts. However, slow reaction kinetics, low product selectivity, as well as poor stability are the main drawbacks of single metallic Cu-based catalysts. Such issues can be addressed by introducing second metal in Cu-based catalysts. Here, we summarize the recent progress relating to the Cu-based bimetallic electrocatalysts for CO2 reduction, and discuss the composition and structure effects on the activity and selectivity of electrochemical CO2 reduction. Last, we outline the challenges and perspectives on electrocatalysts for this field. We expect that this review can provide new insights into the further development of Cu-based bimetallic electrocatalysts for CO2 reduction.

Water oxidation is an important reaction for multiple renewable energy conversion and storage-related devices and technologies. High-performance and stable electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Bimetallic (oxy)hydroxides have been widely used in alkaline OER as electrocatalysts, but their activity is still not satisfactory due to insufficient active sites. In this research, A unique and efficient approach of sacrificial W to prepare CoFe (oxy)hydroxides with abundant active species for OER is presented. Multiple ex situ and operando/in situ characterizations have validated the self-reconstruction of the as-prepared CoFeW sulfides to CoFe (oxy) hydroxides in alkaline OER with synchronous W etching. Experiments and theoretical calculations show that the sacrificial W in this process induces metal cation vacancies, which facilitates the in situ transformation of the intermediate metal hydroxide to CoFe-OOH with more high-valence Co(III), thus creating abundant active species for OER. The Co(III)-rich environment endows the in situ formed CoFe oxyhydroxide with high catalytic activity for OER on a simple flat glassy carbon electrode, outperforming those not treated by the sacrificial W procedure. This research demonstrates the influence of etching W on the electrocatalytic performance, and provides a low-cost means to improve the active sites of the in situ self-reconstructed bimetallic oxyhydroxides for OER.

Precious metals (like Ir, Ru, and Pt) and their derivatives are the benchmark catalysts for water splitting in acidic media due to their high stability and activity. However, the high cost and scarcity of these materials hamper the large-scale applications. To solve this issue, construction of catalysts containing low content of precious metals with high intrinsic activity can be an efficient strategy, which expectedly can decrease the cost but meanwhile preserve the activity. Herein, we synthesized an IrW/WO₃ array catalyst by in situ formation of IrW alloy on hierarchical WO₃ nanosheet arrays. With extremely low Ir content of 1.25 wt % in 0.5 M H₂SO₄, this composite catalyst not only shows superior water oxidation activity (the overpotential at 10 mA cm^-2 is only 229 mV, significantly lower than that of the commercial IrO₂ (358 mV)) but also exhibits excellent proton reduction performance (the overpotential at -10 mA cm^-2 is 49 mV, close to that of commercial Pt/C catalyst (42 mV)), showing promising bifunctionality for the overall water splitting. As a result, only 1.5 V is needed to drive the overall water splitting at 10 mA cm^-2 with a good long-term stability under acidic conditions. These remarkable features can be ascribed to the abundant active sites exposed by the three-dimensional nanostructure, and the high intrinsic activity per Ir site. The theoretical calculation verifies that Ir sites in IrW surface after oxidation have a higher intrinsic activity than IrO₂ for water oxidation. We believe this research can supply a strategy to design highly active and stable catalysts with low loading of noble metals for acidic water splitting.

Solar water splitting is one of the most efficient technologies to produce H-2, which is a clean and renewable energy carrier. Photoanodes for water oxidation play the determining roles in solar water splitting, while its photoelectrochemical (PEC) performance is severely limited by the hole injection efficiency at the interface of semiconductor/electrolyte. To address this problem, in this research, by employing BiVO4 as the model semiconductor for photoanodes, we develop a novel, facile, and efficient method, which simply applies K cations in the preparation process of BiVO4 photoanodes, to in situ induce a crystalline-amorphous heterophase junction by the formation of an amorphous BiVO4 layer (a-BiVO4) on the surface of the crystalline BiVO4 (c-BiVO4) film for PEC water oxidation. The K cation is the key to stimulate the formation of the heterophase, but not incorporated in the final photoelectrodes. Without sacrificing the light absorption, the in situ formed a-BiVO4 layer accelerates the kinetics of the hole tranfer at the photoanode/electrolyte interface, leading to the significantly increased efficiency of the surface hole injection to water molecules. Consequently, the BiVO4 photoanode with the crystalline-amorphous heterophase junction (a-BiVO4/c-BiVO4) exhibits almost twice the photocurrent density at 1.23 V (vs reversible hydrogen electrode) for water oxidation than the bare c-BiVO4 ones. Such advantages from the crystalline-amorphous heterophase junction are still effective even when the a-BiVO4/c-BiVO4 is coated by the cocatalyst of FeOOH, reflecting its broad applications in PEC devices. We believe this study can supply an efficient and simple protocol to enhance the PEC water oxidation performance of photoanodes, and provide a new strategy for the potential large-scale application of the solar energy-conversion related devices.

Electrocatalytic water splitting is an efficient means of producing energy carriers, such as H2. The hydrogen evolution reaction (HER) requires high-efficiency electrocatalysts. Understanding the active site structures of the HER electrocatalysts is essential for the rational design and development of water splitting devices. In this study, porous CoW sulfides were employed as model electrocatalysts for pH-universal HER. Multiple characterization studies, such as X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and operando X-ray diffraction, were systematically used to investigate the reconstruction of the active species at the surface and in the bulk. The results show that during the HER, the structural transformation of the species CoW sulfides is strongly dependent on the pH of the electrolyte. Electrolytes of varying pH lead to varied reconstruction and influence the true catalytically active species responsible for the HER. The surface and the bulk of the electrocatalysts transform to different oxides/hydroxides when subjected to the HER. This is the first time that the pH-dependent bulk and surface structural evolution in the HER has been revealed. This study reveals the reconstruction and potential active site evolution of mixed-metal sulfides for the HER. We believe that the present study not only provides an idealized "pre-catalyst"for pH-universal highly-efficient HER, but also provides a thorough understanding about the identification of the real active sites and the mechanism of the structural evolution of the electrocatalysts during hydrogen evolution.

The development of highly active and durable catalysts for water oxidation under acidic conditions is necessary but challenging for renewable energy conversion. Ir-based catalysts are highly efficient for water oxidation in acid, but their large scale application is hindered by the high cost and scarcity of iridium. Herein, we use an amorphous WO3 induced lattice distortion (AWILD) strategy to reduce the Ir content to only 2 wt% in the final material. The optimized hybrid nitrogen-doped carbon (NC)/WO3/IrO2 can efficiently catalyze water oxidation with a low overpotential of 270 mV at 10 mA cm(-2) current density (eta (10)) and a high turnover frequency of over 2 s(-1) at 300 mV overpotential in 0.5 M H2SO4, a performance that surpasses that of commercial IrO2 significantly. Introducing the layer of amorphous WO3 between IrO2 nanoparticles and NC can distort the lattice of IrO2, exposing more highly active sites for water oxidation. The AWILD effect compensates for the lower Ir content and dramatically reduces the cost of the catalyst without sacrificing the catalytic activity. Additionally, this catalyst also exhibits high activity in acid for hydrogen evolution with only 65 mV of eta (10) attributed to the AWILD effect, exhibiting efficient bifunctionality as a Janus catalyst for overall water splitting. The AWILD approach provides a novel and efficient strategy for low-cost and highly efficient electrocatalysts for acidic overall water splitting with an extremely low content of noble metals.

As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It has been reported that electron-chemical conversion of metal OER chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoSx into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoSx to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH)(2) intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoSx has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.

Noble-metal-free bimetal-based electrocatalysts have shown high efficiency for water oxidation. Ni and/or Co in these electrocatalysts are essential to provide a conductive, high-surface area and a chemically stable host. However, the necessity of Ni or Co limits the scope of low-cost electrocatalysts. Herein, we report a hierarchical hollow FeV composite, which is Ni- and Co-free and highly efficient for electrocatalytic water oxidation with low overpotential 390 mV (10 mA cm(-2) catalytic current density), low Tafel slope of 36.7 mV dec(-1), and a considerable durability. This work provides a novel and efficient catalyst, and greatly expands the scope of low-cost Fe-based electrocatalysts for water splitting without need of Ni or Co.

The use of cobalt porphyrin complexes as efficient and cost-effective molecular catalysts for water oxidation has been investigated previously. However, by combining a set of analytical techniques (electrochemistry, ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and synchrotron-based photoelectron spectroscopy (SOXPES and HAXPES)), we have demonstrated that three different cobalt porphyrins, deposited on FTO glasses, decompose promptly into a thin film of CoOx on the surface of the electrode during water oxidation under certain conditions (borate buffer pH 9.2). It is presumed that the film is composed of CoO, only detectable by SOXPES, as conventional techniques are ineffective. This newly formed film has a high turnover frequency (TOF), while the high transparency of the CoOx-based electrode is very promising for future application in photoelectrochemical cells.

Ru-bda based molecular water oxidation catalysts 1 and 2 (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) containing a thiophene group are attached to the surfaces of electrodes by the method of electropolymerization. The Ru-bda molecular catalyst functionalized graphite carbon electrode can catalyze water oxidation efficiently under a overpotential of ca 500 mV to obtain current density of 5 mA cm(-2); and the similarly functionalized photoelectrode based on alpha-Fe2O3 (hematite) film can work as an photoanode for light driven water splitting.