Table 1.

Summary of major oxygen‐deficient Co‐based TMOs electrocatalysis for OER.

Synthesis method	Material	Method	Overpotential (b.t.) [mV]	Overpotential (a.t.) [mV]	Tafel slope (b.t.) [mV dec−1]	Tafel slope (a.t.) [mV dec−1]	KOH [m]	Ref.
thermal treatment	B‐CoO nanowires	CoO precursor was obtained by hydrothermal reaction of [Co (Ac)2 ⋅ 4H2O], (NH4HB4O7 ⋅ 3H2O) and water at 200 °C for 10 h, was annealed in Ar at 500 °C for 2 h	400 at 10 mA cm−2	280 at 10 mA cm−2	71	86	1	[24]
thermal treatment	Ni−Co/Co hydroxides	pristine Ni−Co/Co hydroxides were annealed in air at 300 °C	353 at 10 mA cm−2	320 at 10 mA cm−2	57	30	1	[25]
thermal treatment	PrBaCo2O5.75 PrBaCo2O5.5 perovskite	polycrystalline powders of PrBaCo2O6‐δ were annealed in N2 atmosphere at 250 and 600 °C for 30 min	–	360 (PrBaCo2O5.75) 420 (PrBaCo2O5.5) at 10 mA cm−2	–	70 (PrBaCo2O5.75) 80 (PrBaCo2O5.5)	1	[26]
hydrogen treatment	Fe2O3/CoOx	pristine Fe2O3/CoOx composite was annealed in 10 %H2/Ar at 150–300 °C for 30 min	–	316 at 10 mA cm−2	–	56	1	[27]
hydrogen treatment	Co3O4 quantum dots	pristine Co3O4 quantum dots were reduced 5 %H2/95 %Ar at 170 and 200 °C, 1 h	370 at 10 mA cm−2	315 (Co3O4‐200) 348 (Co3O4‐170) at 10 mA cm−2	72	49 (Co3O4‐200) 66 (Co3O4‐170)	1	[28]
hydrogen treatment	Ca2Mn2O5 perovskite	pristine CaMnO3 was annealed in 5 %H2 in Ar at 350 °C for 3 h	500 at 0.5 mA cm−2	380 at 1 mA cm−2	197	149	0.1	[23]
wet‐chemical reduction	Co3O4 mesoporous wires	pristine mesoporous Co3O4 NWs were reduced with NaBH4 solution at RT for 1 h	370 at 1.8 mA cm−2	370 at 13.1 mA cm−2	82	72	1	[16b]
wet‐chemical reduction	FexCoy‐ONS nanosheets	NaBH4 aqueous solution was added to a solution containing [Co(NO3)2, Fe(NO3)3, cetyltrimethylammonium bromide] at RT for 5 min	40 at 10 mA cm−2	308 at 10 mA cm−2	71.7	36.8	0.1	[29]
wet‐chemical reduction	Co2AlO4 nanosheets	pristine Co2AlO4 nanosheets were treated in a homogeneous solution of NaOH and ethylene glycol at 140 °C for 12 h	360 at 10 mA cm−2	280 at 10 mA cm−2	79.98	70.98	1	[30]
wet‐chemical reduction	CoFe2O4/graphene composite	pristine CoFe2O4/graphene was reduced with NaBH4 solution for 2 h	330 at 10 mA cm−2	300 at 10 mA cm−2	67	68	1	[31]
plasma treatment	Co3O4‐CoO nanosheets	Co3O4 nanosheets were treated by Ar–H2 plasma	330 at 10 mA cm−2	270 at 10 mA cm−2	49.3	49	1	[32]
plasma treatment	N‐doped Co3O4 nanosheets	pristine Co3O4 nanosheets were treated with N2 plasma at 300 W, 40 Pa for 60–80 s	560 at 10 mA cm−2	310 at 10 mA cm−2	234	59	0.1	[33]
plasma treatment	CoFe LDHs nanosheets	pristine CoFe LDHs were treated with H2O plasma with a dielectric barrier discharge (DBD) plasma reactor for 2–15 min	332 at 10 mA cm−2	290 at 10 mA cm−2	52	36	1	[34]
hydro‐thermal treatment	P‐doped Co3O4 nanosheet	Co3O4 and NaH2PO2⋅H2O with different weight ratios (2, 15, 30) were annealed in Ar atmosphere at low static pressure and at 250 °C for 1 h	395 at 20 mA cm−2	338 at 20 mA cm−2	62	52	1	[35]
laser irradiation	L–CoOOH	Co target was immersed in KOH solution was irradiated by nanosecond laser for 20 min at RT	410 at 10 mA cm−2	330 at 20 mA cm−2	75.4	63.2	1	[36]
laser irradiation	R‐FeCo2O4 nanoparticles	a water suspension of M‐FeCo2O4 microspheres was irradiated by laser for 30 min at RT	297 at 10 mA cm−2	276 at 10 mA cm−2	72.1	70.8	0.1	[37]
plasma treatment	La0.7Sr0.3CoO3‐δ	pristine La0.7Sr0.3CoO3‐δ was treated in Ar plasma for 10 min at 300 W and 70 Pa	433 at 10 mA cm−2	326 at 10 mA cm−2	92.1	61.6	1	[38]
thermal treatment	B‐doped CoO	a mixture of Co (Ac)2 and NH4HB4O7 (hydrothermally treated at 200 °C for 10 h) was annealed at 500 °C for 2 h in Ar	400 at 10 mA cm−2	280 at 10 mA cm−2	86	71	1	[24]
wet‐chemical reduction	CoFe2O4 nanosheets	pristine CoFe2O4 hollow nanospheres were reduced with NaBH4 solution for 1 h at RT	450 at 10 mA cm−2	320 at 10 mA cm−2	94	48	1	[39]
hydrogen treatment	reduced CaMn0.5Nb0.25O3‐δ	pristine CaMn0.5Nb0.25O3‐δ was annealed under 30 % H2/N2 at 350 °C for 2 h	300 at 0.5 mA cm−2	280 at 0.5 mA cm−2	140	98	0.1	[40]
ball milling	(Co0.2Mn0.2Ni0.2Fe0.2Zn0.2) Fe2O4	CoO, MnO, NiO, ZnO, and Fe3O4 powders were ball milled at a speed of 600 rpm for 65 h at a ball/powder ratio of 10 : 1	–	326 at 10 mA cm−2	–	53.6	1	[41]

b.t.=before treatment; a.t.=after treatment.