Fluorine Modification Promoted Water Dissociation into Atomic Hydrogen on a Copper Electrode for Efficient Neutral Nitrate Reduction and Ammonia Recovery

Electrocatalytic nitrate reduction to ammonia (NITRR) offers an attractive solution for alleviating environmental concerns, yet in neutral media, it is challenging as a result of the reliance on the atomic hydrogen (H*) supply by breaking the stubborn HO-H bond (∼492 kJ/mol) of H2O. Herein, we demonstrate that fluorine modification on a Cu electrode (F-NFs/CF) favors the formation of an O-H···F hydrogen bond at the Cu-H2O interface, remarkably stretching the O-H bond of H2O from 0.98 to 1.01 Å and lowering the energy barrier of water dissociation into H* from 0.64 to 0.35 eV at neutral pH. As a benefit from these advantages, F-NFs/CF could rapidly reduce NO3- to NH3 with a rate constant of 0.055 min-1 and a NH3 selectivity of ∼100%, far higher than those (0.004 min-1 and 9.2%) of the Cu counterpart. More importantly, we constructed a flow-through coupled device consisting of a NITRR electrolyzer and a NH3 recovery unit, realizing 98.1% of total nitrogen removal with 99.3% of NH3 recovery and reducing the denitrification cost to $5.1/kg of N. This study offers an effective strategy to manipulate the generation of H* from water dissociation for efficient NO3--to-NH3 conversion and sheds light on the importance of surface modification on a Cu electrode toward electrochemical reactions.

Lewis Acid Driving Asymmetric Interfacial Electron Distribution to Stabilize Active Species for Efficient Neutral Water Oxidation

Neutral oxygen evolution reaction (OER) with unique reactive environments exhibits extremely slow reaction kinetics, posing significant challenges in the design of catalysts. Herein, a built-in electric field between the tungstate (Ni-FeWO4 ) with adjustable work function and Lewis acid WO3 is elaborately constructed to regulate asymmetric interfacial electron distribution, which promotes electron accumulation of Fe sites in the tungstate. This decelerates the rapid dissolution of Fe under the OER potentials, thereby retaining the active hydroxyl oxide with the optimized OER reaction pathway. Meanwhile, Lewis acid WO3 enhances hydroxyl adsorption near the electrode surface to improve mass transfer. As expected, the optimized Ni-FeWO4 @WO3 /NF self-supporting electrode achieves a low overpotential of 235 mV at 10 mA cm-2 in neutral media and maintains stable operation for 200 h. Furthermore, the membrane electrode assembly constructed by such self-supporting electrode exhibits robust stability for 250 h during neutral seawater electrolysis. This work deepens the understanding of the reconstruction of OER catalysts in neutral environments and paves the way for development of the energy conversion technologies.

Creating Conjugated C-C Bonds between Commercial Carbon Electrode and Molecular Catalyst for Oxygen Reduction to Hydrogen Peroxide

Immobilizing molecular catalysts on electrodes is vital for electrochemical applications. However, creating robust electrode-catalyst interactions while maintaining good catalytic performance and rapid electron transfer is challenging. Here, without introducing any foreign elements, we show a bottom-up synthetic approach of constructing the conjugated C-C bond between the commercial Vulcan carbon electrode and an organometallic catalyst. Characterization results from FTIR, XPS, aberration-corrected TEM and EPR confirmed the successful and uniform heterogenization of the complex. The synthesized Vulcan-LN4 -Co catalyst is highly active and selective in the oxygen reduction reaction in neutral media, showing an 80 % hydrogen peroxide selectivity and a 0.72 V (vs. RHE) onset potential which significantly outperformed the homogenous counterpart. Based on single-crystal XRD and NMR data, we built a model for density functional theory calculations which showed a nearly optimal binding energy for the *OOH intermediate. Our results show that the direct conjugated C-C bonding is an effective approach for heterogenizing molecular catalysts on carbon, opening new opportunities for employing molecular catalysts in electrochemical applications.

Surface Reconstruction Enabled Efficient Hydrogen Generation on a Cobalt-Iron Phosphate Electrocatalyst in Neutral Water

Electrolytic hydrogen evolution reaction (HER) that can be performed efficiently in neutral conditions enables the direct splitting of seawater. However, the sluggish water dissociation kinetics in neutral media severely limits the practical deployment of this technology. Herein, we present a simple strategy to rationally design oxophilic and nucleophilic moieties through the in situ reconstruction of a free-standing bimetallic cobalt-iron phosphate electrode. Through an electrochemical reduction step, the electrode surface undergoes self-reconstruction to generate a thin (oxy)hydroxide layer, enabling a significantly improved HER activity in both buffered electrolyte and natural seawater. Our mechanistic investigations reveal the essential role of oxophilic (oxy)hydroxide species in improving the HER activity of nucleophilic bimetallic phosphate sites. In a buffer electrolyte (pH = 7), the resultant electrocatalyst only requires overpotentials of 97 and 198 mV to deliver a current density of 10 and 100 mA cm^-2, respectively, which outperforms that of the Pt benchmark. The in situ reconstruction strategy of active sites within such electrodes brings significant opportunity in developing active electrocatalysts that are capable of direct seawater splitting.

Sulfur dopant-enhanced neutral hydrogen evolution performance in MoO3nanosheets

Developing nonprecious-metal based catalysts with highly active and stable performance for hydrogen evolution reaction (HER) in neutral media is crucial points for realizing low-carbon economy because their practical use typically suffers from the slow kinetics. Herein, we developed S-doped MoO₃nanosheets toward neutral HER, fabricated by a versatile solvothermal and subsequently sulfuration processes. The obtained catalyst exhibits a small overpotential of 106 mV to reach 10 mA cm^-2in 1.0 M phosphate buffered saline, overwhelming most of recently reported catalysts. Meantime, it shows no notable deactivation after more than 60 h continuous electrolysis and 50 000 cycling tests. More importantly, the catalyst also can be applied in buffered seawater for electrocatalyzing HER, requiring 262 mV at 10 mA cm^-2and maintaining over 60 h. These findings open a new route for designing MoO₃-based catalysts for neutral hydrogen production.

Enhancing Electrochemical Hydrogen Evolution Performance of CoMoO4-Based Microrod Arrays in Neutral Media through Alkaline Activation

We present that activation of CoMoO₄-based microrod arrays in KOH (1.0 M, 2 h) allows us to significantly improve their electrochemical hydrogen evolution performance in phosphate buffer solution (1.0 M, pH = 7.1). The activation mechanism originates from the conversion of the surface layer of CoMoO₄ to Co(OH)₂ nanosheets, together with the release of Mo₃O₁₀^2- ions into the activation solution. Our experimental and calculated results suggest that the Co(OH)₂ nanosheets on the surface of the CoMoO₄-based microrod arrays show the ability to improve water molecule disassociation and stabilize the catalytic activity of the two-component catalysts by decreasing their overpotentials in the hydrogen evolution reaction. When extending this strategy to activate P-doped CoMoO₄ with a low hydrogen absorption free energy, we report the synthesis of a new class of superior neutral electrochemical hydrogen evolution catalysts of P-doped CoMoO₄-Co(OH)₂ microrod arrays. We show that a low overpotential of about 30 mV (obtained from bulk electrolysis) is required to deliver a current density of 10 mA cm^-2 in the neutral media. By making use of our catalyst and NiFe double hydroxide as cathodic and anodic electrodes, respectively, we fabricated a two-electrode electrolysis device for neutral overall water splitting. Our results showed a low cell voltage of 1.78 V (obtained from bulk electrolysis) that is needed for delivering a current density of about 10 mA cm^-2 in the neutral electrolyte, even outperforming the state-of-the-art catalyst combination of Pt/C∥RuO₂ in terms of catalytic activity and stability. These findings suggest that our strategy may be utilized as a facile but useful strategy toward the activation of molybdate catalysts to improve their HER performance in both basic and neutral media.

Oxygen Vacancy Engineering of Co3 O4 Nanocrystals through Coupling with Metal Support for Water Oxidation

Oxygen vacancies can help to capture oxygen-containing species and act as active centers for oxygen evolution reaction (OER). Unfortunately, effective methods for generating a high amount of oxygen vacancies on the surface of various nanocatalysts are rather limited. Here, we described an effective way to generate oxygen-vacancy-rich surface of transition metal oxides, exemplified with Co₃ O₄ , simply by constructing highly coupled interface of ultrafine Co₃ O₄ nanocrystals and metallic Ti. Impressively, the amounts of oxygen vacancy on the surface of Co₃ O₄ /Ti surpassed the reported values of the Co₃ O₄ modified even under highly critical conditions. The Co₃ O₄ /Ti electrode could provide a current density of 23 mA cm^-2 at an OER overpotential of 570 mV, low Tafel slope, and excellent durability in neutral medium. Because of the formation of a large amount of oxygen vacancies as the active centers for OER on the surface, the TOF value of the Co₃ O₄ @Ti electrode was optimized to be 3238 h^-1 at an OER overpotential of 570 mV, which is 380 times that of the state-of-the-art non-noble nanocatalysts in the literature.