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A highly efficient approach for nitrate synthesis is developed under ambient conditions. The electrochemical generation of active chloride species by RuO2@TiO2/TP in chlorine evolution reaction effectively convert NO to nitrate in the electrolyte.

An Ag nanoparticle-decorated TiO2 nanoribbon array on a titanium plate performs efficiently in electrocatalytic NO2− reduction to NH3, achieving a large NH3 yield of 8743.1 μg h−1 cm−2 with a high faradaic efficiency of 96.4%.

RuO2 nanoparticles decorated TiO2 nanobelt on titanium plate is active for electrocatalytic hydrogen evolution, requiring overpotentials of 130, 143, and 380 mV to drive 100 mA cm−2 in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M PBS solution, respectively.

A Co-TiO2 nanoribbon array supported on a Ti plate is a high-efficiency catalyst for electrochemical NO3–-to-NH3 conversion, capable of attaining a large NH3 yield of 1127 μmol h−1 cm−2 and high Faradaic efficiency of 98.2%.

TiO2 decorated juncus effusus-derived carbon microtubes (TiO2/JE-CMTs) are highly active and stably for electrochemical N2 reduction to NH3, capable of attaining a large NH3 yield of 20.03 μg h−1 mg−1cat. with a high faradaic efficiency of 10.76%.

A CoP nanoneedle array supported on Ti mesh acts as a high-active electrocatalyst with a low onset potential toward NO-to-NH3 conversion, achieving a faradaic efficiency of 88.3% and a yield of 47.22 μmol h−1 cm−2.

Amorphous CoB nanoarray is a high-efficiency catalyst for electrocatalytic NO2−-to-NH3 conversion, capable of attaining a large NH3 yield of 233.1 μmol h−1 cm−2 and a high faradaic efficiency of 95.2% at −0.7 V in 0.1 M Na2SO4 with 400 ppm NO2−.

NiFe2O4/CC is a highly active electrocatalyst for the conversion of nitrate to ammonia under ambient conditions, achieving a remarkably high faradaic efficiency of 96.6% and a large yield of 10.3 mg h−1 cm−2 in 0.1 M PBS with 0.1 M NaNO3.

This review summarizes recent advancements in catalysts for NH3 electrosynthesis from NO3−, highlights various strategies to regulate their apparent or intrinsic activities, and proposes the perspective and challenges in this emerging area.

Electrochemical N2 reduction has emerged as an eco-friendly and sustainable alternative for industrial NH3 synthesis with the aid of efficient electrocatalysts under ambient conditions. Herein, we report that CuS concave polyhedral superstructures (CuS-CPSs) with high symmetry operate as capable electrocatalysts for artificial NH3 production with excellent selectivity. In 0.1 M HCl, CuS-CPSs achieve an appealing NH3 yield of 18.18 μg h−1 mg−1cat. and a faradaic efficiency of 5.63% at −0.15 V vs. reversible hydrogen electrode. Moreover, it also shows high electrochemical and structural stability. The catalytic mechanism of N2 reduction on CuS (103) is further discussed using density functional theory calculations.

Developing hierarchical electrocatalysts for the oxygen evolution reaction (OER) is of great importance for electrochemical hydrogen production. Here, we describe the development of a hierarchical CuO@NiCo layered double hydroxide core–shell nanoarray on copper foil (CuO@NiCo LDH/CF) as a 3D OER electrocatalyst. When tested in 1.0 M KOH, such CuO@NiCo LDH/CF offers superior catalytic activity with a geometrical catalytic current density of 20 mA cm−2 at an overpotential of only 256 mV. It also shows strong long-term electrochemical durability to retain its activity for at least 24 h.

Highly stable and efficient non-noble-metal electrocatalysts are strongly desired for the oxygen evolution reaction (OER) in alkaline media. In this study, we report the use of 4,4′-biphenyl dicarboxylic acid as an organic ligand to in situ develop an Ni-MOF nanosheet array on nickel foam (Ni-MOF/NF) as an OER catalyst. In 1 M KOH, such 3D Ni-MOF/NF drives a current density of 20 mA cm−2 at a low overpotential of 350 mV. Moreover, it exhibits strong long-term electrochemical stability for at least 24 h and achieves a high turnover frequency of 0.24 mol O2 per s at an overpotential of 400 mV.

The development of efficient electrocatalysts from Earth-abundant elements for the oxygen evolution reaction (OER) is highly desired. Here, we report the electrodeposition of a NiCo layered double hydroxide nanosheet array on graphite felt (NiCo LDHs/GF) as a 3D OER electrocatalyst. Such NiCo LDHs/GF exhibits superior electrocatalytic activity with the need for an overpotential of 249 mV to drive a current density of 20 mA cm−2 in 1.0 M KOH. It also shows strong long-term electrochemical durability with its activity being maintained for at least 24 h.

The Haber–Bosch process for industrial-scale NH3 production suffers from harsh reaction conditions with CO2 emission. Electrochemical N2 reduction has gained considerable recent interest as an eco-friendly and sustainable alternative for NH3 synthesis with the aid of efficient electrocatalysts for the N2 reduction reaction (NRR). Here, an Fe2O3 nanoparticle-reduced graphene oxide composite (Fe2O3-rGO) is reported to be an Earth-abundant NRR electrocatalyst, enabling efficient ambient N2-to-NH3 conversion. In 0.5 M LiClO4, this Fe2O3-rGO attains a large NH3 yield of 22.13 μg h−1 mg−1cat at −0.50 V and a high faradaic efficiency of 5.89% at −0.40 V (vs. RHE). It also demonstrates high electrochemical stability.

Electrocatalytic N2 reduction under ambient conditions is a promising alternative to the traditional Haber–Bosch process for environmentally benign and sustainable NH3 production but requires efficient electrocatalysts for the N2 reduction reaction (NRR). Here, we report that metal–organic framework-derived shuttle-like V2O3/C acts as an outstanding NRR electrocatalyst for N2-to-NH3 conversion with excellent selectivity under ambient conditions. In 0.1 M Na2SO4, such V2O3/C exhibits a remarkable NH3 yield of 12.3 μg h−1 mg−1cat. and a high faradaic efficiency of 7.28% at a potential of −0.6 V versus reversible hydrogen electrode, outperforming most of the reported aqueous-based NRR electrocatalysts. Notably, it also shows high electrochemical and structural stability.

Electrochemical reduction offers a carbon-neutral process for energy-saving NH3 synthesis under ambient conditions, but this process involves difficulty in breaking the strong NN bond of inert N2 and the challenge of N2 activation, underlining the need for high-performance electrocatalysts for the nitrogen reduction reaction (NRR). Herein, we report our recent effort in this direction by in situ hydrothermally growing a Nb2O5 nanowire array on carbon cloth (Nb2O5/CC) as a self-standing NRR catalyst electrode toward artificial N2 fixation with high selectivity under ambient conditions. In 0.1 M Na2SO4, such Nb2O5/CC achieves an NH3 formation rate of 1.58 × 10−10 mol s−1 cm−2 and a high faradaic efficiency of 2.26% at −0.60 V, rivaling the performances of most reported non-noble metal NRR electrocatalysts in neutral medium. Notably, this catalyst also shows high electrochemical stability during electrolysis and recycling tests. Density functional theory calculations reveal that a distal associative route is involved in the reaction pathways during the NRR over the Nb2O5 (001) facet. This work would open up an exciting new avenue to explore the design of self-standing 3D electrodes made of transition metal oxides for the NRR and other applications.

The development of efficient hydrogen evolution reaction (HER) catalysts is of great importance for electrolytic hydrogen production in alkaline media. In this communication, we report the development of an ultrafine PtNi nanoparticle-decorated Ni nanosheet array with ultralow Pt loading (7.7 wt%) as a superior electrocatalyst for the alkaline HER. Such a nanoarray drives a geometrical current density of 10 mA cm−2 at an overpotential as low as 38 mV in 0.1 M KOH, outperforming commercial 20 wt% Pt/C and all reported HER electrocatalysts. Remarkably, it also shows outstanding long-term electrochemical durability with a faradaic efficiency close to 100%. This work provides us with an attractive ultralow-Pt-content catalyst material in water-splitting devices toward high-performance and durable electrochemical production of hydrogen fuels.

Development of efficient and durable catalysts based on earth-abundant elements for oxygen evolution reactions (OER) is important for renewable energy storage and conversion technologies. Herein, we report the development of a Cu nanoparticle-embedded N-doped carbon nanowire array on copper foam (Cu–N–C NA/CF) via carbonization of a Cu(TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane) nanoarray. As a 3D OER electrode, this Cu–N–C NA/CF shows high catalytic activity, needing an overpotential of 314 mV to drive a geometrical current density of 20 mA cm−2 in 1.0 M KOH. It also shows strong long-term electrochemical durability. This suggested that CuO nanoparticles as active species were in situ electrochemically converted from Cu nanoparticles and stably dispersed in the carbon matrix during electrocatalysis.

Development of Earth-abundant oxygen evolution reaction (OER) electrocatalysts with high performance is essential for the practical application of hydrogen energy. Herein, we report a high-efficiency electrocatalyst for electrochemical water oxidation based on an Ni-MOF nanosheet array on Ni foam (Ni-MOF/NF). In 1.0 M KOH, this Ni-MOF/NF catalyst drives 100 mA cm−2 at an overpotential of 320 mV. Remarkably, it also demonstrates good electrochemical durability with the retention of its catalytic activity for at least 20 h and achieves a high turnover frequency of 0.25 mol O2 s−1 at an overpotential of 400 mV.

Developing selective and cost-effective CO2 reduction electrocatalysts is critical for electrochemical production of carbon-containing fuels. In this communication, we report on the in situ fabrication of a bromide-derived porous Ag nanowire film (BD-Ag) with adsorbed Br− anions via anodic oxidation of commerical Ag foil and subsequent electrochemical reduction. The resulting BD-Ag behaves as an efficient electrocatalyst for CO2 reduction to CO with high selectivity, with a low onset overpotential of only 296 mV and a high CO faradaic efficiency of 96.2% in 0.5 M KHCO3. Furthermore, it also shows excellent long-term electrochemical durability. This work offers an attractive catalyst material toward efficient and selective CO2 reduction for electrocatalytic applications.

It is highly desired to develop high-performance non-noble-metal electrocatalysts for the oxygen evolution reaction (OER) in alkaline solutions. In this communication, we report the hydrothermal synthesis of an Fe-MOF nanosheet array on nickel foam (Fe-MOF/NF) using aqueous Fe3+ and terephthalic acid as precursors. Such an Fe-MOF/NF shows superior electrocatalytic performance towards the OER in 1.0 M KOH with a low overpotential of 240 mV at 50 mA cm−2 and a small Tafel slope of 72 mV dec−1. Notably, this catalyst also exhibits high long-term electrochemical durability to maintain its activity for at least 30 h. Moreover, this catalyst electrode achieves a high turnover frequency of 0.74 mol O2 per s at an overpotential of 300 mV.

Ammonia borane (AB) has been regarded as a promising candidate for chemical hydrogen storage but needs an efficient catalyst for hydrolytic hydrogen generation. In this communication, a CoP nanoarray in situ grown on a Ti mesh (CoP NA/Ti) is reported as a robust non-noble-metal catalyst for effective hydrogen generation from AB hydrolysis. The initial turnover frequency and activation energy of CoP NA/Ti for AB hydrolysis are 42.8 molH2 molCoP−1 min−1 and 34.1 kJ mol−1, respectively. Moreover, this catalyst shows only 0.15% loss in catalytic activity after 10 cycles and its easy separation nature from fuel solution enables its attractive use as an on/off switch toward on-demand hydrogen generation. The amazing catalytic activity and high durability of such a catalyst are attributed to the fact that the 3D nanoarray configuration not only allows for more efficient diffusion of the fuel and hydrogen gas but favors the exposure of more active sites, and on the other hand, in situ growth of CoP on a Ti mesh also ensures strong adhesion between them.

It is highly desirable but still a challenging task to find a simple, fast and straightforward method to greatly improve the alkaline oxygen evolution reaction (OER) performance of a NiCo2O4 catalyst. In this communication, we demonstrate that developing an amorphous borate shell on a NiCo2O4 surface can boost its OER activity in alkaline media. As a 3D catalyst electrode, a NiCo2O4@Ni–Co–B nanoarray on carbon cloth needs an overpotential of only 270 mV to achieve a geometrical catalytic current density of 10 mA cm−2 in 1.0 M KOH, which is 100 mV less than that for a NiCo2O4 nanoarray. Notably, this electrode also demonstrates strong electrochemical durability, maintaining its activity for at least 100 h. The superior activity of NiCo2O4@Ni–Co–B is attributed to the amorphous Ni–Co–B shell on NiCo2O4 favoring the in situ electrochemical generation of more active species during water oxidation.

Developing highly efficient and non-noble-metal catalysts is of great importance for electrochemical energy storage and conversion. In this communication, we report the development of a porous Ni3N nanosheet array on carbon cloth (Ni3N NA/CC) as a high-performance and durable electrocatalyst for urea oxidation. To drive 10 mA cm−2, this Ni3N NA/CC only demands a potential of 1.35 V in 1.0 M KOH with 0.33 M urea. The high catalytic activity of the hydrogen evolution reaction enables Ni3N NA/CC as a bifunctional catalyst electrode for electrochemical hydrogen production and the two-electrode electrolyzer is capable of offering 10 mA cm−2 at a cell voltage of only 1.44 V, 120 mV less than that for the urea-free counterpart.

The development of earth-abundant electrocatalysts for efficient water oxidation under moderate conditions is highly desired but still a big challenge. In this communication, we demonstrate the topotactic conversion of a nickel phosphide nanoarray on carbon cloth into a nickel–borate–phosphate nanoarray (Ni–Bi–Pi/CC) by oxidative polarization in potassium borate water. When used as a 3D water oxidation catalyst, such Ni–Bi–Pi/CC shows high activity with a geometrical catalytic current density of 10 mA cm−2 at an overpotential of only 440 mV in 0.1 M K–Bi, rivaling the performances of the reported Ni-containing catalysts operated under benign conditions. Notably, this electrode also demonstrates strong long-term electrochemical durability with 100% Faradaic efficiency for oxygen evolution. All these features promise its use as an attractive low-cost catalyst electrode in water-splitting devices for mass production of hydrogen fuels under environmentally friendly conditions.

Energy-efficient electrolytic hydrogen generation is highly desired but still remains challenging. In this work, we demonstrate that a Cu3P nanoarray on copper foam (Cu3P/CF) behaves as a high-performance electrocatalyst for hydrazine oxidation reaction (HzOR) in alkaline electrolytes. A full electrolytic system employing Cu3P/CF as both an anode for HzOR and a cathode for hydrogen evolution reaction drives 100 mA cm−2 at a voltage of 0.72 V in 1.0 M KOH aqueous solution with 0.5 M hydrazine. It also demonstrates high long-term stability.

In this communication, we demonstrate the use of N-doped carbon dots (N-CDs) as a co-catalyst for hematite (α-Fe2O3) nanorod arrays on a Ti sheet for photoelectrochemical water oxidation. Under simulated light irradiation, the photocurrent density of the resulting N-CDs@α-Fe2O3/Ti as a photoanode exhibits a 3.4 times increase with a 280 mV cathode shift of the onset potential relative to that of pristine α-Fe2O3/Ti, with excellent photostability. The up-conversion fluorescent properties of N-CDs enable the absorption of longer wavelength irradiation with emission of short wavelength light to excite α-Fe2O3, leading to the generation of more electron–hole pairs.