Disordered Au Nanoclusters for Efficient Ammonia Electrosynthesis.

The electrochemical nitrogen (N2 ) reduction reaction (N2 RR) under mild conditions is a promising and environmentally friendly alternative to the traditional Haber-Bosch process with high energy consumption and greenhouse emission for the synthesis of ammonia (NH3 ), but high-yielding production is rendered challenging by the strong nonpolar N≡N bond in N2 molecules, which hinders their dissociation or activation. In this study, disordered Au nanoclusters anchored on two-dimensional ultrathin Ti3 C2 Tx MXene nanosheets are explored as highly active and selective electrocatalysts for efficient N2 -to-NH3 conversion, exhibiting exceptional activity with an NH3 yield rate of 88.3±1.7 µg h-1 mgcat. -1 and a faradaic efficiency of 9.3±0.4 %. A combination of in situ near-ambient pressure X-ray photoelectron spectroscopy and operando X-ray absorption fine structure spectroscopy is employed to unveil the uniqueness of this catalyst for N2 RR. The disordered structure is found to serve as the active site for N2 chemisorption and activation during the N2 RR process.

Chemically coupling SnO2 quantum dots and MXene for efficient CO2 electroreduction to formate and Zn-CO2 battery.

Electrochemical conversion of CO2 into formate is a promising strategy for mitigating the energy and environmental crisis, but simultaneously achieving high selectivity and activity of electrocatalysts remains challenging. Here, we report low-dimensional SnO2 quantum dots chemically coupled with ultrathin Ti3C2Tx MXene nanosheets (SnO2/MXene) that boost the CO2 conversion. The coupling structure is well visualized and verified by high-resolution electron tomography together with nanoscale scanning transmission X-ray microscopy and ptychography imaging. The catalyst achieves a large partial current density of -57.8 mA cm-2 and high Faradaic efficiency of 94% for formate formation. Additionally, the SnO2/MXene cathode shows excellent Zn-CO2 battery performance, with a maximum power density of 4.28 mW cm-2, an open-circuit voltage of 0.83 V, and superior rechargeability of 60 h. In situ X-ray absorption spectroscopy analysis and first-principles calculations reveal that this remarkable performance is attributed to the unique and stable structure of the SnO2/MXene, which can significantly reduce the reaction energy of CO2 hydrogenation to formate by increasing the surface coverage of adsorbed hydrogen.

Bifunctional single-atomic Mn sites for energy-efficient hydrogen production.

The electrocatalytic hydrogen evolution reaction (HER) for H2 production is essential for future renewable and clean energy technology. Screening energy-saving, low-cost, and highly active catalysts efficiently, however, is still a grand challenge due to the sluggish kinetics of the oxygen evolution reaction (OER) in electrolyzing water. Herein, we present a single atomic Mn site anchored on a boron nitrogen co-doped carbon nanotube array (Mn-SA/BNC), which is perfectly combined with the hydrazine electrooxidation reaction (HzOR) boosted water electrolysis concept. The obtained catalyst achieves 51 mV overpotential at the current density of -10 mA cm-2 for the cathodic HER and 132 mV versus the reversible hydrogen electrode for HzOR, respectively. Besides, in a two-electrode overall hydrazine splitting (OHzS) system, the Mn-SA/BNC catalyst only needs a cell voltage of only 0.41 V to output 10 mA cm-1, with strong durability and nearly 100% faradaic efficiency for H2 production. This work highlights a low-cost and high-efficiency energy-saving H2 production pathway.

Isolated copper single sites for high-performance electroreduction of carbon monoxide to multicarbon products.

Electrochemical carbon monoxide reduction is a promising strategy for the production of value-added multicarbon compounds, albeit yielding diverse products with low selectivities and Faradaic efficiencies. Here, copper single atoms anchored to Ti3C2Tx MXene nanosheets are firstly demonstrated as effective and robust catalysts for electrochemical carbon monoxide reduction, achieving an ultrahigh selectivity of 98% for the formation of multicarbon products. Particularly, it exhibits a high Faradaic efficiency of 71% towards ethylene at -0.7 V versus the reversible hydrogen electrode, superior to the previously reported copper-based catalysts. Besides, it shows a stable activity during the 68-h electrolysis. Theoretical simulations reveal that atomically dispersed Cu-O3 sites favor the C-C coupling of carbon monoxide molecules to generate the key *CO-CHO species, and then induce the decreased free energy barrier of the potential-determining step, thus accounting for the high activity and selectivity of copper single atoms for carbon monoxide reduction.

Trifunctional Single-Atomic Ru Sites Enable Efficient Overall Water Splitting and Oxygen Reduction in Acidic Media.

Development of cost-effective, active trifunctional catalysts for acidic oxygen reduction (ORR) as well as hydrogen and oxygen evolution reactions (HER and OER, respectively) is highly desirable, albeit challenging. Herein, single-atomic Ru sites anchored onto Ti3 C2 Tx MXene nanosheets are first reported to serve as trifunctional electrocatalysts for simultaneously catalyzing acidic HER, OER, and ORR. A half-wave potential of 0.80 V for ORR and small overpotentials of 290 and 70 mV for OER and HER, respectively, at 10 mA cm-2 are achieved. Hence, a low cell voltage of 1.56 V is required for the acidic overall water splitting. The maximum power density of an H2 -O2 fuel cell using the as-prepared catalyst can reach as high as 941 mW cm-2 . Theoretical calculations reveal that isolated Ru-O2 sites can effectively optimize the adsorption of reactants/intermediates and lower the energy barriers for the potential-determining steps, thereby accelerating the HER, ORR, and OER kinetics.

Nitrogen doping and titanium vacancies synergistically promote CO2 fixation in seawater.

The electrocatalytic generation of useful chemicals from CO2, H2O, and sustainable energy resources offers a promising strategy for the carbon cycle. However, the current CO2 electrolysis system is mainly operated in artificial electrolytes (e.g. ionic liquids and inorganic salt solutions), of which the high cost and impractical working conditions hinder its large-scale development. In this case, seawater represents an attractive alternative due to its abundance and good conductivity. Herein, we show that N-doping and titanium vacancies (VTi) can be introduced in Ti3C2 MXene nanosheets via a facile NH3-etching pyrolysis approach. These nanosheets demonstrate impressive CO2 reduction reaction (CO2RR) performances in seawater with a remarkable 92% faradaic efficiency and a partial current density of -16.2 mA cm-2 for CO production, being close to those of noble metal electrodes. Mechanistic studies reveal that the existence of N dopants and VTi synergistically modulates the electronic structure of the active Ti site, on which the free energy barriers for the key *COOH formation and desorption of *CO are greatly reduced, thereby leading to a notable CO2RR improvement. This study provides an opportunity for developing an active and cost-effective CO2 electrolysis system by using seawater as the electrolyte.

Oxygen Doping Induced by Nitrogen Vacancies in Nb4 N5 Enables Highly Selective CO2 Reduction.

Surface vacancy engineering holds great promise for boosting the electrocatalytic activity for CO2 reduction reaction; however, the vacancies are generally unstable and may degrade into the inactive phase during electrolysis. Stabilizing the vacancy-enriched structure by heteroatoms can be an effective strategy to get a robust and active catalyst. Herein, a nitrogen-vacancy enriched Nb4 N5 on N-doped carbons is constructed, which is thereafter stabilized by a self-enhanced oxygen doping process. This oxygen-doped complex is used as an effective CO2 catalyst, which exhibits a maximum CO Faradaic efficiency of 91% at -0.8 V (vs reversible hydrogen electrode, RHE) and long-term stability throughout 30 h of electrocatalysis. Density function theory calculations suggest that the incorporation of oxygen in Nb4 N5 facilitates the formation of *COOH and thus promotes the CO2 reduction.

Single-Atom Catalysts for the Electrocatalytic Reduction of Nitrogen to Ammonia under Ambient Conditions.

Powered by renewable electricity, the electrochemical reduction of nitrogen to ammonia is proposed as a promising alternative to the energy- and capital-intensive Haber-Bosch process, and has thus attracted much attention from the scientific community. However, this process suffers from low NH3 yields and Faradaic efficiency. The development of more effective electrocatalysts is of vital importance for the practical applications of this reaction. Of the reported catalysts, single-atom catalysts (SACs) show the significant advantages of efficient atom utilization and unsaturated coordination configurations, which offer great scope for optimizing their catalytic performance. Herein, progress in state-of-the-art SACs applied in the electrocatalytic N2 reduction reaction (NRR) is discussed, and the main advantages and challenges for developing more efficient electrocatalysts are also highlighted.

Efficient Electroreduction CO2 to CO over MnO2 Nanosheets.

As a critical alternative step for the synthesis of important chemical feedstocks and complex carbon-based fuels, the electrochemical transformation of CO2 into CO holds great significance for the chemical industry. Here, MnO2 nanosheets array supported nickel foam has been synthesized and adopted as a binder-free catalyst for electrochemical CO2 reduction reaction (CO2RR). The well-distributed nanosheets of MnO2 impart a much higher density of accessible active sites for CO2RR, enabling the selective CO2 reduction to CO with a large current density (14.1 mA cm-2), excellent Faradaic efficiency (71%) and high electrochemical stability (10 h). This work first demonstrates the great potential of Mn-based oxides for electrocatalytic transformation of CO2 to valuable products.