Table 1 A brief summary of the representative experimental studies on NRR using various electrocatalytic heterogeneous catalysts

Fe-phthalocyanine

1.0 M KOH

25 °C

mol s⁻¹ cm⁻²

− 47.8 mA cm⁻²

Current density

1989 [59]

ZnS

1.0 M KOH

25 °C

mol s⁻¹ cm⁻²

− 0.1 V

vs RHE

1990 [60]

ZnSe

1.0 M KOH

25 °C

mol s⁻¹ cm⁻²

− 0.1 V

vs RHE

1990 [60]

Ti

0.2 M LiClO4/0.18 M ethanol in THF

25 °C

2.0 V

Cell Voltage

1994 [60]

Cu

0.2 M LiClO4/0.18 M ethanol in THF

25 °C

2.0 V

Cell Voltage

1994 [61]

Ru/C

2.0 M KOH

20 °C

mol s⁻¹ cm⁻²

− 1.10 V

vs Ag/AgCl

2000 [62]

90 °C

mol s⁻¹ cm⁻²

− 0.96 V

vs Ag/AgCl

2000 [62]

Polyaniline

methanol/LiCiO₄/H₂SO₄

25 °C

mol⁻¹ ml⁻¹

− 0.12 V

vs RHE

2005 [63]

30 wt % Pt/C

0.50 M H₂SO₄

RT

mol s⁻¹ cm⁻²

1.6 V

Cell Voltage

2013 [64]

H⁺/Li⁺/NH₄⁺ mixed electrolyte

80 °C

mol s⁻¹ cm⁻²

1.2 V

Cell Voltage

2013 [65]

Ru/Ti

0.50 M H₂SO₄

30 °C

mol s⁻¹ cm⁻²

− 0.15 V

vs NHE

N/A

2014 [66]

Rh/Ti

mol s⁻¹ cm⁻²

− 0.171 V

Ni wire

0.050 M H₂SO₄/0.1 M LiCl, EDA

RT

mol s⁻¹ cm⁻²

1.8 V

Cell Voltage

2016 [58]

Porous Ni

2-propanol/H₂SO₄

RT

mol s⁻¹ cm⁻²

0.5 mA cm⁻²

Current Density

2016 [67]

Mo nanofilm

0.010 M H₂SO₄

RT

mol s⁻¹ cm⁻²

− 0.49 V

vs RHE

0.72%(at − 0.29 V vs RHE)

2017 [54]

γ-Fe₂O₃

0.10 M KOH

65 °C

mol s⁻¹ cm⁻²

0 V

vs RHE

2017 [68]

Au nanorods

0.10 M KOH

RT

mol s⁻¹ cm⁻²

− 0.2 V

vs RHE

2017 [55]

Au/TiO2

0.10 M HCl

RT

mol s⁻¹ mg ⁻¹_cat

− 0.2 V

vs RHE

2017 [56]

Au-CeOx/RGO

0.10 M KOH

RT

mol s⁻¹ mg ⁻¹_cat

− 0.2 V

vs RHE

2017 [57]

30 wt % Fe₂O₃-CNT

0.50 M KOH

RT

mol s⁻¹ cm⁻²

− 2.0 V

vs Ag/AgCl

2017 [69]

PEBCD (poly N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide)/C

0.50 M Li₂SO₄

25 °C

mol s⁻¹ cm⁻²

− 0.5 V

vs RHE

2017 [70]

40 °C

mol s⁻¹ cm⁻²

− 0.5 V

MOF(Fe)(metal–organic-frameworks)

2.0 M KOH

90 °C

mol s⁻¹ cm⁻²

1.2 V

Cell Voltage

2017 [71]

Fe₂O₃-CNT

2.0 M NaHCO₃

RT

mol s⁻¹ cm⁻²

− 2.0 V

vs Ag/AgCl

0.15%(at − 1.0 V vs Ag/AgCl)

2017 [72]

Fe on stainless steel mesh

Ionic liquid

RT

mol s⁻¹ cm⁻²

− 0.8 V

vs NHE

2017 [73]

N-doped carbon

0.05 M H₂SO₄

RT

mol s⁻¹ mg⁻¹

− 0.9 V

vs RHE

2018 [32]

Ru nanosheets

0.10 M KOH

RT

mol s⁻¹ mg ⁻¹_cat

− 0.2 V

vs RHE

2018 [74]

Mo₂N nanorod

0.1 M HCl

25 °C

mol s⁻¹ mg ⁻¹_cat

− 0.3 V

vs RHE

2018 [75]

VN nanowire array

0.1 M HCl

25 °C

mol s⁻¹ cm⁻²

− 0.3 V

vs RHE

2018 [76]

Pt

6 M KOH/polymer gel

30 °C

mol s⁻¹ cm⁻²

0.5 V

Cell Voltage

2018 [77]

Ir

60 °C

mol s⁻¹ cm⁻²

0.25 V

Cell Voltage

Bi₄V₂O₁₁/CeOx

HCl, PH = 1

RT

mol s⁻¹ mg ⁻¹_cat

− 0.2 V

vs RHE

2018 [78]

Pore-size-controlled hollow gold nanocatalysts

0.1 M LiOH

RT

mol s⁻¹ cm⁻²

− 0.4 V

vs RHE

2018 [37]

Hollow gold nanocages

0.5 M LiClO₄

RT

mol s⁻¹ cm⁻² at − 0.5 V

− 0.4 V

vs RHE

2018 [79]

MoN Nanosheets

0.1 M HCl

RT

mol s⁻¹ cm⁻²

− 0.3 V

vs RHE

2018 [34]

N-doped carbon nanospikes

0.25 M LiClO₄

RT

mol s⁻¹ cm⁻²

− 1.19 V

vs RHE

2018 [36]

Vanadium Nitride Nanoparticles

3 M KOH

RT

mol s⁻¹ cm⁻²

vs RHE

2018 [35]

Chromium Oxynitride nanoparticles

Nafion Solution(5% wt)

RT

mol s⁻¹ cm⁻²

2.0 V

Cell Voltage

2018 [38]

MoS₂

0.1 M Na₂SO₄

RT

mol s⁻¹ cm⁻²

− 0.5 V

vs RHE

2018 [33]