The electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) is among the most promising approaches used to transform greenhouse gas into useful fuels and chemicals. However, the reaction ...suffers from low selectivity, high overpotential, and low reaction rate. Active site identification in the CO2RR is vital for the understanding of the reaction mechanism and the rational development of new electrocatalysts with both high selectivity and stability. Herein, in situ characterization monitoring of active sites during the reaction is summarized and a general understanding of active sites on the various catalysts in the CO2RR, including metal‐based catalysts, carbon‐based catalysts, and metal‐organic frameworks‐based electrocatalysts is updated. For each type of electrocatalysts, the reaction pathway and real active sites are proposed based on in situ characterization techniques and theoretical calculations. Finally, the key limitations and challenges observed for the electrochemical fixation of CO2 is presented. It is expected that this review will provide new insights and directions into further scientific development and practical applicability of CO2 electroreduction.
Electrochemical reduction of CO2 to useful fuels and chemicals is of fundamental significance for carbon recycling but remains a great challenge. A comprehensive review of the essential advances in electrochemical reduction of CO2 is provided. This review will provide new insights into the further in situ characterization development and active sites identification of CO2 electroreduction.
Over the years, cobalt phosphates (amorphous or crystalline) have been projected as one of the most significant and promising classes of nonprecious catalysts and studied exclusively for the ...electrocatalytic and photocatalytic oxygen evolution reaction (OER). However, their successful utilization of hydrogen evolution reaction (HER) and the reaction of overall water‐splitting is rather unexplored. Herein, presented is a crystalline cobalt phosphate, Co3(OH)2(HPO4)2, structurally related to the mineral lazulite, as an efficient precatalyst for OER, HER, and water electrolysis in alkaline media. During both electrochemical OER and HER, the Co3(OH)2(HPO4)2 nanostructure was completely transformed in situ into porous amorphous CoOx
(OH) films that mediate efficient OER and HER with extremely low overpotentials of only 182 and 87 mV, respectively, at a current density of 10 mA cm−2. When assemble as anode and cathode in a two‐electrode alkaline electrolyzer, unceasing durability over 10 days is achieved with a final cell voltage of 1.54 V, which is superior to the recently reported effective bifunctional electrocatalysts. The strategy to achieve more active sites for oxygen and hydrogen generation via in situ oxidation or reduction from a well‐defined inorganic material provides an opportunity to develop cost‐effective and efficient electrocatalysts for renewable energy technologies.
A crystalline lazulite cobalt phosphate is identified as a low‐cost preelectrocatalyst for generating remarkably active and durable electrocatalysts for unifying the hydrogen evolution reaction, oxygen evolution reaction, and overall water‐splitting in alkaline media. Under oxidizing and reducing electrochemical environments, the restructuring (corrosion) of highly crystalline particles results in two different in situ‐generated amorphously active phases, yielding low overpotentials and cell potential.
Oxygen evolution reaction (OER) is an essential electrochemical reaction in water-splitting and rechargeable-metal-air-batteries to achieve clean energy production and efficient energy-storage. At ...first, this review discusses about the mechanism for OER, where an oxygen molecule is produced with the involvement of four electrons and OER intermediates but the reaction pathway is influenced by the pH. Then, this review summarizes the brief discussion on theoretical calculations, and those suggest the suitability of NiFe based catalysts for achieving optimal adsorption for OER intermediates by tuning the electronic structure to enhance the OER activity. Later, we review the recent advancement in terms of synthetic methodologies, chemical properties, density functional theory (DFT) calculations, and catalytic performances of several nanostructured NiFe-based OER electrocatalysts, and those include layered double hydroxide (LDH), cation/anion/formamide intercalated LDH, teranary LDH/LTH (LTH: Layered-triple-hydroxide), LDH with defects/vacancies, LDH integrated with carbon, hetero atom doped/core-shell structured/heterostructured LDH, oxide/(oxy)hydroxide, alloy/mineral/boride, phosphide/phosphate, chalcogenide (sulfide and selenide), nitride, graphene/graphite/carbon-nano-tube containing NiFe based electrocatalysts, NiFe based carbonaceous materials, and NiFe-metal-organic-framework (MOF) based electrocatalysts. Finally, this review summarizes the various promising strategies to enhance the OER performance of electrocatalysts, and those include the electrocatalysts to achieve ~1000 mA cm−2 at relatively low overpotential with significantly high stability.
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•NiFe based earth-abundant-electrocatalysts for OER in alkaline medium are reviewed.•Strategies used to achieve enhanced OER performance of electrocatalysts are reviewed.•The nanostructured electrocatalysts facilitate the gas evolution.•The electrocatalysts can generate active sites with optimal binding energy.
Water oxidation, or the oxygen evolution reaction (OER), which combines two oxygen atoms from two water molecules and releases one oxygen molecule, plays the key role by providing protons and ...electrons needed for the hydrogen generation, electrochemical carbon dioxide reduction, and nitrogen fixation. The multielectron transfer OER process involves multiple reaction intermediates, and a high overpotential is needed to overcome the sluggish kinetics. Among the different water splitting devices, proton exchange membrane (PEM) water electrolyzer offers greater advantages. However, current anode OER electrocatalysts in PEM electrolyzers are limited to precious iridium and ruthenium oxides. Developing highly active, stable, and precious‐metal‐free electrocatalysts for water oxidation in acidic media is attractive for the large‐scale application of PEM electrolyzers. In recent years, various types of precious‐metal‐free catalysts such as carbon‐based materials, earth‐abundant transition metal oxides, and multiple metal oxide mixtures have been investigated and some of them show promising activity and stability for acidic OER. In this review, the thermodynamics of water oxidation, Pourbaix diagram of metal elements in aqueous solution, and theoretical screening and prediction of precious‐metal‐free electrocatalysts for acidic OER are first elaborated. The catalytic performance, reaction kinetics, and mechanisms together with future research directions regarding acidic OER are summarized and discussed.
Water oxidation, also known as the oxygen evolution reaction (OER), plays the key role by providing protons and electrons needed for hydrogen generation and the carbon dioxide reduction. The thermodynamics, Pourbaix diagram, theoretical screening and prediction, catalytic performance, as well as reaction kinetics and mechanisms of precious‐metal‐free electrocatalysts for acidic OER are comprehensively summarized.
This report presents the interplay between the synthesis parameters, physicochemical properties, and electrochemical performance of core-shell low-Pt electrocatalysts (ECs) for the oxygen reduction ...reaction (ORR) based on PtxNi active sites stabilized on a carbon nitride shell. The impact of the pyrolysis temperature (Tf), of the support core (H) composition and of an electrochemical dealloying-activation step on the EC morphology and on the accessibility and stability of the active sites are studied in detail. Three supports are employed based on carbon nanoparticles and/or graphene platelets. The ORR performance of activated ECs measured by cyclic voltammetry with the thin-film rotating ring-disk electrode approach is strongly affected by Tf and H. The best performing ECs are tested in single proton exchange membrane fuel cells under operating conditions. The simultaneous presence of graphene and carbon in H improves the dispersion of active sites, resulting in a vastly improved mass activity and durability in comparison with a benchmark state-of-the-art Pt/C EC.
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•Hierarchical core-shell Pt–Ni ORR electrocatalysts (ECs) are obtained.•Interplay between synthesis, properties, and performance of ECs is discussed.•Hierarchical Pt–Ni ECs exhibit mass activities surpassing DOE targets.•Hierarchical Pt–Ni ECs are more durable than Pt/C reference ECs.
Carbon material is a promising electrocatalyst for the oxygen reduction reaction (ORR). Doping of heteroatoms, the most widely used modulating strategy, has attracted many efforts in the past decade. ...Despite all this, the catalytic activity of heteroatoms‐modulated carbon is hard to compare to that of metal‐based electrocatalysts. Here, a “double‐catalysts” (Fe salt, H3BO3) strategy is presented to directionally fabricate porous structure of crystal graphene nanoribbons (GNs)/amorphous carbon doped by pyridinic NB pairs. The porous structure and GNs accelerate ion/mass and electron transport, respectively. The N percentage in pyridinic NB pairs accounts for ≈80% of all N species. The pyridinic NB pair drives the ORR via an almost 4e− transfer pathway with a half‐wave potential (0.812 V vs reversible hydrogen electrode (RHE)) and onset potential (0.876 V vs RHE) in the alkaline solution. The ORR catalytic performance of the as‐prepared carbon catalysts approximates commercial Pt/C and outperforms most prior carbon‐based catalysts. The assembled Zn–air battery exhibits a high peak power density of 94 mW cm−2. Density functional theory simulation reveals that the pyridinic NB pair possesses the highest catalytic activity among all the potential configurations, due to the highest charge density at C active sites neighboring B, which enhances the interaction strength with the intermediates in the p‐band center.
The dominantly pyridinic NB pair is rationally fabricated on hierarchically structure of crystal graphene nanoribbons/amorphous carbon by a “double‐catalysts” strategy. As experiments and density functional theory demonstrated, the pyridinic NB plays a key role for the highly catalytic performance (half‐wave potential, 0.812 V vs reversible hydrogen electrode, electron transfer number, ≈3.94), comparable to commercial Pt/C and superior to most of carbon‐based electrocatalysts.
Electrocatalytic nitrogen fixation under ambient conditions represents an energy-saving sustainable alternative strategy to the energy-consuming traditional Haber–Bosch process toward ammonia ...synthesis. However, the traditional electrocatalysts for nitrogen reduction reaction (NRR) often suffer low selectivity and low activity. From quantum-mechanical calculations, we obtain a benefit clue that the Ni atom adsorbed by the first hydrogen ion in the catalyst exhibits selectivity for the adsorption of N2 and other H atoms, and it preferentially adsorbs N2 molecules. Thus, we propose an interfacial engineering strategy to simultaneously accelerate selectivity and activity using metal/metal hydroxide. The remarkable activity of metal/metal hydroxide originates from its synergized water dissociation and unique hydrogenation pathway of metal hydride. The priority absorption of the N2 suppresses the competitive hydrogen evolution reaction and accelerates the kinetics to generate ∗N2H: ∗H + N2 → ∗N2H, which a is rate-limiting step for NH3 synthesis. Using Ni/NiFe–OH as prototypes, here we show that selectivity and catalytic activity are simultaneously enhanced, surprisingly, in simple inorganic hybrid and confers exceptionally Faradaic efficiency of 23.34% and NH3 yield 19.74 μg h−1 cm−2 at −0.15 V versus reversible hydrogen electrode (RHE) in 0.5 M KOH electrolyte under ambient conditions. The long-term durability is also excellent. This work provides a possibility for the rational design of efficient electrocatalysts for N2 electrochemical reduction with a large-scale production.
A water dissociation happens in an alkaline electrolyte, which consumes a large amount of electrons, thereby delaying the hydrogen evolution reaction. Meanwhile, the generated H proton intermediates is adsorbed on the surface of neighboring Ni active sites to form Ni hydrides. Nickel hydride can not only inhibit the reaction of hydrogen evolution (Heyrovsky/Tafel step), but also promote the adsorption of nitrogen and activate nitrogen. Display omitted
•From quantum-mechanical calculations we obtain a benefit clue.•We propose an interfacial engineering strategy.•Using Ni/NiFe–OH as prototypes selectivity and catalytic activity are simultaneously enhanced.
Synergistic optimization of the elementary steps of water dissociation and hydrogen desorption for the hydrogen evolution reaction (HER) in alkaline media is a challenge. Herein, the Ru cluster ...anchored on a trace P‐doped defective TiO2 substrate (Ru/P‐TiO2) was synthesized as an electrocatalyst for the HER; it exhibited a commercial Pt/C‐like geometric activity and an excellent mass activity of 9984.3 mA mgRu−1 at −0.05 V vs. RHE, which is 34.3 and 18.7 times higher than that of Pt/C and Ru/TiO2, respectively. Experimental and theoretical studies indicated that using a rutile‐TiO2‐crystal‐phase substrate enhanced the HER activity more than the anatase phase. Rich surface oxygen vacancies on rutile‐TiO2 facilitated the adsorption and dissociation of water, while the partial substitution of Ti4+ with P5+ enhanced H2 generation by facilitating hydrogen spillover from the Ru site to the surface P site, synergistically enhancing the HER activity.
A ruthenium cluster anchored on a lightly P‐doped defective TiO2 catalyst was designed for the electrochemical hydrogen evolution reaction in alkaline electrolyte. The surface oxygen vacancies enhanced water dissociation and P‐doping facilitated the H2 generation by enabling hydrogen spillover from the Ru site to the surface P site, synergistically enhancing the HER activity.
Developing robust electrocatalysts and advanced devices is important for electrochemical carbon dioxide (CO2) reduction toward the generation of valuable chemicals. We present herein a ...carbon‐confined indium oxide electrocatalyst for stable and efficient CO2 reduction. The reductive corrosion of oxidative indium to the metallic state during electrolysis could be prevented by carbon protection, and the applied carbon layer also optimizes the reaction intermediate adsorption, which enables both high selectivity and activity for CO2 reduction. In a liquid‐phase flow cell, the formate selectivity exceeds 90 % in a wide potential window from −0.8 V to −1.3 V vs. RHE. The continuous production of ca. 0.12 M pure formic acid solution is further demonstrated at a current density of 30 mA cm−2 in a solid‐state electrolyte mediated reactor. This work provides significant concepts in the parallel development of electrocatalysts and devices for carbon‐neutral technologies.
A robust carbon‐covered indium oxide electrocatalyst demonstrates an enhanced and stable activity for the direct production of formic acid in a solid‐state carbon dioxide electrolyzer.
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