Recent progress on earth abundant hydrogen evolution reaction and oxygen evolution reaction bifunctional electrocatalyst for overall water splitting in alkaline media

Journal of Power Sources 333 (2016) 213e236

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Review article

Recent progress on earth abundant hydrogen evolution reaction and oxygen evolution reaction bifunctional electrocatalyst for overall water splitting in alkaline media Mohammed Ibrahim Jamesh a, b, *, 1 a b

Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Applied and Plasma Physics, School of Physics (A28), University of Sydney, Sydney, NSW 2006, Australia

h i g h l i g h t s
Water-splitting of earth-abundant-bifunctional-HER-OER-electrocatalysts is reviewed.
The bifunctional-electrocatalysts exhibit better or well-comparable with Pt/C//RuO2.
Earth-abundant-bifunctional-HER-OER-electrocatalysts exhibit significant stability.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 July 2016 Received in revised form 27 September 2016 Accepted 28 September 2016

Electrochemical water-splitting is one of the promising ways for producing clean chemical fuel (Hydrogen) while cheap-earth-abundant-bifunctional-electrocatalyst is one of the possible way for improving the overall cost efficiency of water-splitting. This paper reviews the chemical state, hydrogen and oxygen evolution reaction activity in alkaline media, overall water-splitting performance in alkaline media, stability, and possible-factors for improving its efficiency of various kinds of recently reported electrocatalyst such as Ni-P, Co-P, Ni-Co-P, graphene-Co-P, O/N/C-Co/Ni, Ni-S, B-Ni/Co, Ni-Co, Mo, Se, Fe, Mn/Zn/Ti, and metal-free based earth-abundant-bifunctional-electrocatalyst. This paper also reviews and highlights the remarkable water splitting performance of the earth-abundant-bifunctionalelectrocatalyst those exhibit better or well comparable with Pt/C//RuO2. © 2016 Elsevier B.V. All rights reserved.

Keywords: Bifunctional HER plus OER electrocatalyst Earth abundant electrocatalyst Electrochemical water splitting Hydrogen energy

1. Introduction The energy demand increasing and depletion of fossil fuels are vital challenging issues that cause for the discovering alternative earth abundant energies and efficient designing of energy storage devices [1e3]. Renewable energy resources such as solar energy, wave power, and wind energy are alternative energy resources for energy demand issue but those are intermittent and hence electrochemical water splitting is one of a promising way to convert electrical energy (from intermittent renewable energy resources) into chemical energy (clean chemical fuel, hydrogen) and as a result hydrogen fuel can be used as non-intermittent clean energy

* Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail addresses: [email protected], [email protected]. 1 Alumini. http://dx.doi.org/10.1016/j.jpowsour.2016.09.161 0378-7753/© 2016 Elsevier B.V. All rights reserved.

resources [4e7]. Water splitting composed of two half reactions including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) but efficient water splitting can be achieved by efficient electrocatalyst to facilitate HER (2Hþ þ 2e / H2) at the cathode and OER (2H2O / O2 þ 4Hþ þ 4e) at the anode. Currently, Pt group metals and Ru/Ir-based compounds are the state-of-theart, noble, efficient HER and OER electrocatalysts respectively with low overpotential and Tafel slope [8]. However, these noble electrocatalyst suffer from high cost, scarcity that limiting its application to produce hydrogen resource economically by water splitting. Moreover, the cell potential of the commercial water electrolyzers (1.8e2.0 V) are about 570e770 mV higher than the theoretical minimum value (1.23 V) [6]. Since hydrogen gas (a clean chemical fuel) can be a more suitable solution for environmental emissions, sustainability, and energy security issues [4] but hydrogen gas is a very rare existence gas in earth naturally. Hence water electrolyzer using earth abundant (more available and hence cheaper material) efficient electrocatalyst can possibly be used for

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hydrogen gas production with cheap and efficiency that can possibly be alternative for water electrolyzer using scarceexpensive-noble electrocatalyst [8], commercial water electrolyzer with higher cell potential than theoretical value [6], and industrial hydrogen gas production using other methods which suffers high cost-low purity problem [9]. Hence development of efficient, earth abundant, cheap, function at less cell voltage, and stable electrocatalysts for water splitting (HER and OER) are very important. Various kinds of earth abundant electrocatalyst for HER such as MoS2 [10], Mo2C [11], Ni2P [12], Fe or Co or FeCo-N-carbon nanotube [13] (CNT), and Fe-Ni-S [14] in acid media and electrocatalyst for HER such as Ni/NiO [15], NiO/Ni [16], Ni-Mo [17], MoP [18], and CoS2 [19] in alkaline media but electrocatalyst for OER such as Ni-P [20], NiCoFe-layered double hydroxide [21] (LDH), NiFeMn-LDH [22], Co1-xFex(OOH) [23], FeOOH/Co/FeOOH [24], FeOOH/CeO2 [25], ZnxCo3-xO4 [26], ZnCo2O4/N-CNT [27], and nano C-O-Ti [28] in alkaline media have been reported. Most of the HER earth abundant electrocatalysts can perform well in both acid and alkaline media whereas most of the OER earth abundant electrocatalysts can be unstable in acid media but instead they can perform well in alkaline media for example electrocatalysts for HER such as CoMo, NiMo, NiMoCo, and NiMoFe requires overpotential (h) of <200 mV to achieve 10 mA cm2 in acid and alkaline media whereas electrocatalysts for OER such as Co/P, CoFe, NiCo, NiCr, NiFe, NiSn, NiZn, and NiMoFe required h of <400 mV to achieve 10 mA cm2 in alkaline media [8]. Integrating HER and OER in a single system using Ea-Bf-Ec//Ea-Bf-Ec two-electrode bifunctional water electrolyzer (Ea-Bf-Ec: Earth Abundant Bifunctional Electrocatalyst) where same kind of two earth abundant electrocatalysts can function as cathode for HER (when one same kind of Ea-Bf-Ec is connected to the negative terminal of the power supply) and anode for OER (when another same kind of Ea-Bf-Ec is connected to the positive terminal of the power supply) in a same electrolyte media (Fig. 1) can further reduce the cost and simplify the water splitting system [29e32]. Since water splitting in acid media requires stable, scarce, expensive, acid insoluble OER electrocatalyst and hence alkaline media is well compatible for earth abundant OER electrocatalyst [8] and also alkaline media is currently used in commercial water electrolyzer [6]. Hence developing effective-cheap-earthabundant HER and OER bifunctional electrocatalyst in alkaline media can significantly improve the efficiency of the overall water splitting. Electrochemical techniques are quite useful to study electrode/ electrolyte interface [33e36] and remarkable water splitting performance of the earth-abundant-bifunctional-electrocatalysts in alkaline media have been reported [29e32,37e39] using electrochemical techniques those are even better or well comparable with Pt/C//RuO2 and commercial water electrolyzer. Nevertheless review on remarkable water splitting performance and recent progress of the earth-abundant-bifunctional-electrocatalysts in alkaline media have been rarely reported. In this respect, the present paper reviews the chemical state, hydrogen and oxygen evolution performance in alkaline media, overall water-splitting performance in alkaline media, and stability of various kinds of recently reported electrocatalyst such as Ni-P, Co-P, Ni-Co-P, graphene-Co-P, O/N/CCo/Ni, Ni-S, B-Ni/Co, Ni-Co, Mo, Se, Fe, Mn/Zn/Ti, and metal-free based cheap-earth-abundant-bifunctional-electrocatalyst. The present paper also reviews and highlights the remarkable water splitting performance such as lower cell voltage and long term stability of the earth-abundant-bifunctional-electrocatalysts those exhibits better or well comparable with Pt/C//RuO2 and commercial water electrolyzer. This paper also further reviews and discusses the possible factors for improving overall water splitting efficiency of various earth-abundant-bifunctional-electrocatalyst and finally

the present paper also reviews the practical demonstration of earth-abundant-bifunctional-electrocatalyst for overall water splitting using single AA battery or solar panel. 2. Earth abundant HER and OER bifunctional electrocatalyst for overall water splitting in alkaline media 2.1. Ni-P based earth abundant HER and OER bifunctional electrocatalyst Tang et al. [40] have fabricated Ni-P nanoparticle film (Fig. 2a) as a bifunctional electrocatalyst on Ni foam using electrodeposition. The bare Ni foam exhibit overpotential (h) of 241 mV (Tafel slope: 120 mV/dec) for HER and potential of 1.637 V (Tafel slope: 84 mV/ dec) for OER whereas the Ni-P exhibit h of 80 mV (Tafel slope: 50 mV/dec) for HER and potential of 1.540 V (Tafel slope: 58 mV/ dec; h: 309 mV) for OER to achieve 10 mA cm2 in 1.0 M KOH which indicates the Ni-P exhibit enhanced HER and OER activity than bare Ni foam. The Ni-P catalyst contains Ni0 and Ni2þ possibly due to the formation of metallic Ni and Ni phosphide that exhibit enhanced HER and OER activity than bare Ni foam. Moreover the Ni-P//Ni-P two-electrode water electrolyzer exhibit reasonable stability with a decay of about 20 mV (from 1.67 V to 1.69 V) after 14 h of electrolysis in 1.0 M KOH. Ledendecker et al. [41] have fabricated Ni5P4 film as a bifunctional electrocatalyst on Ni foil via phosphorization treatment by heating red phosphorous with Ni foil at 550  C. The Ni5P4 exhibit enhanced HER (h: 150 mV for 10 mA cm2 with Tafel slope of 53 mV/dec) and OER (h: 290 mV for 10 mA cm2 with Tafel slope of 40 mV/dec) activity in 1.0 M KOH. The Ni5P4 catalyst contains crystalline Ni5P4 phase with sheet like morphology grown perpendicular to the Ni foil substrate with Ni/P ratio of 1.32 that exhibit enhanced HER and OER activity. The enhanced OER activity of the Ni5P4 film is also involved by the formation of NiOOH on the surface of Ni5P4 film during OER in 1.0 M KOH. Moreover the Ni5P4// Ni5P4 two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.70 V to achieve 10 mA cm2 in 1.0 M KOH and 72% efficiency is achieved. Wang et al. [42] have fabricated Ni-P (Fig. 2b) as bifunctional electrocatalyst on carbon fiber paper (CFP or CP) using electrodeposition followed by phosphorization treatment with red phosphorous at 500  C. The CP@Ni-P exhibit enhanced HER (h of 117, 150 and 250 mV for 10, 20 and 100 mA cm2) and OER (h of 190, 230 and 300 mV for 10, 20 and 50.4 mA cm2) activity in 1.0 M KOH. The CP@Ni-P catalyst contains (1100) and (2110) crystal planes of hexagonal single-crystalline Ni5P4 nanosheet, single-crystalline NiP2 and Ni2P that exhibit enhanced HER and OER activity. Moreover the CP@Ni-P exhibit long term stability to sustain 10 mA cm2 for OER with a negligible decay of about 150 mV (from ~1.35 V to ~1.50 V) after 180 h in 1.0 M KOH. The enhanced long term OER activity of the CP@Ni-P could be due to the formation of Ni-P/NiO/Ni(OH)x by the transformation of Ni-P into NiO wrapped with Ni(OH)x layer during OER in 1.0 M KOH. Moreover the CP@Ni-P//CP@Ni-P two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.63 and 1.73 V to achieve 10 and 20 mA cm2 respectively in 1.0 M KOH. At 10 mA cm2, 100% Faradaic efficiency is observed for volumes of the generated H2 and O2 gas at cathode and anode respectively and exhibit reasonable stability with 91.0% efficiency after 100 h of water splitting with negligible potential decay and negligible cathode surface damage while the anode surface Ni-P transformed into NiO but the underlying Ni-P anode retained. Li et al. [43] have fabricated NixPy as bifunctional electrocatalyst on CFP using solidphase reactions between NaH2PO2$H2O and b-Ni(OH)2 at different temperatures (275, 325, 375 and 475  C). The NixPy-325 exhibit enhanced HER (h of 160 mV with Tafel slope of 107.3 mV/ dec for 20 mA cm2) and OER (h of 320 mV with Tafel slope of

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Fig. 1. Schematic diagram illustrating the Ea-Bf-Ec//Ea-Bf-Ec two-electrode bifunctional water electrolyzer (Ea-Bf-Ec: Earth Abundant Bifunctional Electrocatalyst) where same kind of two earth abundant electrocatalysts can function as cathode for HER and anode for OER in a same electrolyte media.

72.2 mV/dec for 10 mA cm2) activity in 1.0 M KOH. The NixPy-325 catalyst contains cubic NiP2 phase ((210) crystal surface), hexagonal Ni5P4 phase ((201) crystal surface) and hexagonal Ni2P phase ((201) crystal surface) that exhibit enhanced HER and OER activity. The enhanced OER activity of the NixPy-325 is also involved by the formation of NiOOH on the surface of Ni-P during OER in 1.0 M KOH. Moreover the NixPy-325//NixPy-325 two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.57 V to achieve 10 mA cm2 in 1.0 M KOH with 100% Faradaic efficiency and reasonable stability is obtained for 60 h. Jiang et al. [44] have fabricated Ni-P (Fig. 2c) as bifunctional electrocatalyst on copper foil using electrodeposition method. The Ni-P exhibit enhanced HER (h of 93, 110, 160, and 219 mV for 10, 20, 100 and 500 mA cm2 respectively with Tafel slope of 43 mV/dec) and OER (h of 344, 399, and 460 mV for 10, 100, and 500 mA cm2 respectively with Tafel slope of 49 mV/dec) activity in 1.0 M KOH. The Ni-P catalyst is amorphous and contains metallic Ni (Ni0) and phosphide with Ni/P atomic ratio of 2 that exhibit enhanced HER and OER activity. The enhanced OER activity of the Ni-P is also involved by the formation of NiO/NiOOH on the surface of Ni-P during OER in 1.0 M KOH. Moreover the Ni-P//Ni-P two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.67 V to achieve 10 mA cm2 in 1.0 M KOH and reasonable stability is obtained after 1000 cycles continuous potential scan with negligible potential decay. Stern et al. [39] have fabricated Ni2P as bifunctional

electrocatalyst on glassy carbon electrode (GCE)/Ni foam using heat treatment. The Ni-P exhibit enhanced HER (h of 220 mV for 10 mA cm2) and OER (h of 290 mV for 10 mA cm2 with Tafel slope of 59 mV/dec) activity in 1.0 M KOH. The Ni2P catalyst is polydispersed crystalline nanoparticle (Fig. 2d) with average particle size of about 50 nm contains predominant Ni2P phase and minor Ni(PH2O2)2$(H2O)6 phase that exhibit enhanced HER and OER activity. The enhanced OER activity of the Ni2P is also involved by the formation of NiOx nanoparticle (~2e3 nm) on the surface of Ni-P during OER in 1.0 M KOH. Moreover the Ni2P//Ni2P two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.63 V to achieve 10 mA cm2 in 1.0 M KOH and reasonable stability is obtained after 10 h with negligible potential decay. You et al. [38] have fabricated Ni2P/Ni as bifunctional electrocatalyst on Ni foam using template free electrodeposition followed by phosphodization. The Ni2P/Ni exhibit enhanced HER (h of 98 and 120 mV for 10 and 20 mA cm2 respectively with Tafel slope of 72 mV/dec) and OER (h of 200, 268, and 375 mV for 10, 100, and 1000 mA cm2) activity in 1.0 M KOH. The Ni2P/Ni catalyst is urchin-like Ni2P having porous (primary macropores of 100e350 mm and complementary macropores of ~10 mm size) 3D hierarchically superstructures contains Ni2P and Ni phase that exhibit enhanced HER and OER activity. The enhanced OER activity of the Ni2P/Ni is also involved by the formation of NiO/NiOOH predominantly NiO on the surface of Ni2P/Ni during OER in 1.0 M

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Fig. 2. Ni-P based HER and OER bifunctional electrocatalyst (reproduced with permission and labeling used in this review are presented in italic format): (a) SEM image of Ni-P nanoparticle film (inset: corresponding cross sectional image) [40], (b) SEM image of CP@Ni-P (nano sheet morphology) [42], (c) SEM image of Ni-P film [44], (d) TEM image of Ni2P nanoparticles [39], (e) SEM images of Ni8P3 (nanoflocs-like morphology) [47] and (f) HRTEM image of Ni8P3 (inset: corresponding SAED pattern) [47].

KOH. Moreover the Ni2P/Ni//Ni2P/Ni two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.49 and 1.68 V to achieve 10 and 100 mA cm2 respectively in 1.0 M KOH and exhibit reasonable stability for 40 h with negligible potential decay. Wang et al. [45] have fabricated porous Ni-P as bifunctional electrocatalyst on Ni foam by phosphorization with red phosphorous at 500  C. The porous Ni-P exhibit enhanced HER (h of ~130 mV for 10 mA cm2) and OER (h of 350 mV for 191 mA cm2) activity in 1.0 M KOH. The porous Ni-P catalyst contains skeletons Ni2P wrapped with vertically aligned single crystalline hexagonal Ni5P4 nanosheet and NiP2 nanosheet that exhibit enhanced HER and OER activity. The enhanced OER activity of the porous Ni-P is also involved by the formation of NiO/Ni(OH)x on the surface of porous Ni-P during OER in 1.0 M KOH. Moreover the porous Ni-P// porous Ni-P two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.64 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit 90.2% efficiency at 10 mA cm2 which also exhibit 72.2% efficiency even at 100 mA cm2 and exhibit a long term stability by sustain 10 and 20 mA cm2 for 1000 h with negligible potential decay. Liu et al. [46] have fabricated Ni-P as bifunctional electrocatalyst on copper foil using electrodeposition method. The Ni-P nanoparticle film exhibit enhanced HER (h of 98 mV for

10 mA cm2) and OER (h of 325 mV for 10 mA cm2) activity in 1.0 M KOH. Moreover the Ni-P//Ni-P two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.68 V to achieve 10 mA cm2 in 1.0 M KOH. Chen et al. [47] have fabricated Ni8P3 as bifunctional electrocatalyst by acid activation of Ni foam followed by air oxidation (3 days) followed by phosphorization with NaH2PO2 at 250  C. The Ni8P3 exhibit enhanced HER (h of 130 and 160 mV for 10 and 30 mA cm2, respectively with Tafel slope of 58.5 mV/dec) and OER (h of 270 mV for 30 mA cm2 with Tafel slope of 73.2 mV/dec) activity in 1.0 M KOH. For HER at h of 120 mV, the Ni8P3 exhibit reasonable stability for 24 h with negligible current decay while for OER at potential of 1.55 V, the Ni8P3 exhibit reasonable stability with current gain from 41.2 to 48.9 mA cm2. The Ni8P3 having 3D dispersed nanoflocs-like morphology (Fig. 2e) and contains rhomohedral Ni8P3 phase (Fig. 2f) and also contains NiO (due to oxidation in air) that exhibit enhanced HER and OER activity. The enhanced OER activity are also due to the formation of NiO on the surface of Ni8P3 during OER and due to the 3D morphology with facilitate gas evolution. Moreover the Ni8P3// Ni8P3 two-electrode bifuntional water electrolyzer requires reasonable voltage of ~1.61 V to achieve 10 mA cm2 in 1.0 M KOH with near 100% Faradic efficiency and exhibit reasonable stability

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for 4 h. The potential for overall water splitting of the various kinds of reported Ni-P//Ni-P two-electrode earth abundant bifunctional electrocatalysts requires to achieve 10 mA cm2 in 1.0 M KOH are in the following order: 1.49 V (Ni2P/Ni//Ni2P/Ni; You et al. [38]) < 1.57 V (NixPy-325//NixPy-325; Li et al. [43]) < ~1.61 V (Ni8P3//Ni8P3; Chen et al. [47]) < 1.63 V (CP@Ni-P//CP@Ni-P; Wang et al. [42]) z 1.63 V (Ni2P//Ni2P; Stern et al. [39]) < 1.64 V (Ni-P//NiP; Wang et al. [45]) < 1.67 V (Ni-P//Ni-P; Tang et al. [40]) z 1.67 V (Ni-P//Ni-P; Jiang et al. [44]) < 1.68 V (Ni-P//Ni-P; Liu et al. [46]) < ~1.70 V (Ni5P4//Ni5P4; Lendendecker et al. [41]) that are far better or well comparable with 1.71 V of Pt/C//RuO2 [48]. For twoelectrode bifuntional water electrolyzer, Ni2P/Ni//Ni2P/Ni [38] exhibit remarkably much lower potential of 1.49 V and the remarkably much higher catalytic activity are due to the following factors: (1) The Ni2P/Ni catalyst is urchin-like Ni2P having porous (primary macropores of 100e350 mm and complementary macropores of ~10 mm size) 3D hierarchically superstructures contains Ni2P and Ni phase and the 3D porous structure can provide enhanced electroactive sites (for HER and OER) and can facilitate the gas evolution (H2 and O2), and (2) The OER activity is further enhanced by the in-situ formation of NiO/NiOOH predominantly NiO on the surface of Ni2P/Ni during OER in 1.0 M KOH. 2.2. Co-P based earth abundant HER and OER bifunctional electrocatalyst Jiang et al. [49] have fabricated Co-P as bifunctional electrocatalyst on copper foil using electrodeposition method. The Co-P exhibit enhanced HER (h of 94, 115 and 158 mV for 10, 20 and 100 mA cm2 with Tafel slope of 42 mV/dec) and OER (h of 345, 413 and 463 mV for 10, 100, and 500 mA cm2 with Tafel slope of 72.2 mV/dec) activity in 1.0 M KOH. The thickness of the noncrystalline Co-P film (Fig. 3a) is 1e3 mm that contains metallic Co (Co0), Co phosphate and Co phosphide with 6.98 of Co/P molar ratio that exhibit enhanced HER and OER activity. The Co-P film contains metallic Co, Co phosphate and Co phosphide while after HER, the Co-P film exhibit metallic Co and Co phophide with 10.50 of Co/P molar ratio with negligible structural change whereas after OER, the Co-P film exhibit Co3O4 along with metallic Co and Co phosphate with 9.74 of Co/P molar ratio with obvious structural change. Moreover the Co-P//Co-P two-electrode bifuntional water electrolyzer requires reasonable voltage of about ~1.65 V to achieve 10 mA cm2 in 1.0 M KOH with 100% Faradaic efficiency and exhibit reasonable stability for 24 h electrolysis. Vigil et al. [50] have fabricated Co-P as bifunctional electrocatalyst on GCE by hydrothermal treatment followed by phosphorization of spinel Co3O4 at 300  C. The Co-P exhibit enhanced HER (h of 200 mV for 10 mA cm2 with Tafel slope of 60 mV/dec) and OER (h of 430 mV for 10 mA cm2 with Tafel slope of 83 mV/dec) activity in 1.0 M KOH. The Co-P nano-particles having thin nanorod like morphology (Fig. 3b) contains about 80% of CoP and 20% of Co2P phase that exhibit enhanced HER and OER activity. Moreover the Co-P//Co-P two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.86 VDE (DE ¼ EOEReEHER; E is potential) to achieve 10 mA cm2 in 1.0 M KOH. Chang et al. [51] have fabricated Co-P (Fig. 3c) nano-sheet (NS) as bifunctional electrocatalyst on GCE using a green approach by phosphodization of Co3O4 NS at 300  C. The Co-P NS/C exhibit enhanced HER (h of 111, 139 and 191 mV for 10, 20 and 50 mA cm2 with Tafel slope of 70.9 mV/dec) and OER (h of 277 mV for 10 mA cm2 with Tafel slope of 85.6 mV/dec) activity in 1.0 M KOH. In addition to the physical mixture of C for enhanced conductivity, the Co-P NS is nanoporous and mesoporous in nature with ~3.3 nm height having specific surface area of 95.1 m2/g (Brunauer Emmett-Teller (BET)) and it contains orthorhombic CoP and cubic CoO phases with crystal size of 13.0 ± 0.5 and

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19.8 ± 1.8 nm, respectively along with phosphide and orthophosphate with 47.61:38.03:14.36 of Co:P:O atomic ratio that exhibit enhanced HER and OER activity. Moreover the Co-P NS//Co-P NS two-electrode bifuntional water electrolyzer using membrane electrode assembly (MEA) requires reasonable voltage of 1.54 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 24 h electrolysis with negligible potential of about 10 mV decay and the electrolyzer requires only 1.8 V to achieve 335 mA cm2. The Co-P NS contains CoP, CoO, phosphide and orthophosphate while after water electrolysis, the Co-P NS cathode (HER side) retain the same chemical phase with negligible structural change whereas the Co-P NS anode (OER side) exhibit Co3O4 along with CoP and CoO with obvious structural change suggesting the robustness of the Co-P NS for HER and slight degradation for OER during water electrolysis. Wang et al. [52] have fabricated Co-P as bifunctional electrocatalyst on carbon cloth by hydrothermal synthesis of Co(CO3)0.5(OH)$0.11H2O followed by phosphidation with PH3 at 300  C. The Co-P exhibit enhanced HER (h of 95 mV for 10 mA cm2 with Tafel slope of 60 mV/dec) and OER (h of 281 mV for 10 mA cm2 with Tafel slope of 62 mV/dec) activity in 1.0 M KOH. The Co-P catalyst is polycrystalline with nano-needle morphology (Fig. 3d) vertically oriented on carbon cloth having 1e5 mm in length with 200e300 nm in diameter contains predominant CoP and minor Co2P phases and the Co-P nano-needle comprises of inner orthorhombic CoP, middle Co(POx)y and outer spinal Co3O4 layer with Co:P atomic ratio of 1.11:1 that exhibit enhanced HER and OER activity. The middle Co(POx)y and outer Co3O4 layer are increased along with formation of few Co(OH)2 nano-particle with the Co:P atomic ratio 2.81:1 on the Co-P occurs by oxidation during OER. Moreover the Co-P//Co-P two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.61 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 72 h electrolysis with negligible potential of 14 mV decay. Zhu et al. [53] have fabricated Co-P mesoporous nanorod arrays (MNA) as bifunctional electrocatalyst on Nickel foam using electrodeposition method. The Co-P MNA exhibit enhanced HER (h of 54, 121, and 235 mV for 10, 100, and 800 mA cm2 with Tafel slope of 51 mV/ dec) and OER (potential of 1.52 V for 10 mA cm2 with Tafel slope of 65 mV/dec) activity in 1.0 M KOH. The Co-P MNA having vertically oriented nanorods (Fig. 3e) with 50e120 nm in diameter, high BET specific surface area of 148 m2/g and pore volume of 0.224 cm3/g contains orthorhombic CoP phase that exhibit enhanced HER and OER activity. The enhanced OER activity of the Co-P MNA is also involved by the formation of crystalline Co oxo/hydroxo species on the surface of Co-P MNA during OER in 1.0 M KOH. Moreover the Co-P MNA//Co-P MNA two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.62 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 32 h electrolysis with negligible potential of about 30 mV decay. Yang et al. [54] have fabricated Co-P nano-wire (NW) array as bifunctional electrocatalyst on Ti mesh that Co-P exhibit enhanced HER (h of 72 mV for 10 mA cm2) and OER (h of 310 mV for 10 mA cm2) activity in 1.0 M KOH. Moreover the Co-P NW//Co-P NW two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.64 V to achieve 10 mA cm2 in 1.0 M KOH. Chang et al. [55] have fabricated Co-P nano-rod (NR) as bifunctional electrocatalyst using hydrothermal method. The Co-P NR/C exhibit enhanced OER (h of 320 mV for 10 mA cm2 with Tafel slope of 71 mV/dec) activity in 1.0 M KOH. The BET specific surface area of Co-P NR is 87.2 m2/g and contains orhtorhobic CoP and cubic CoO phases along with Co phosphate and phosphide with 44.65:41.83:13.50 atomic ratio of Co:P:O that exhibit enhanced OER activity. Moreover the Co-P NR// Co-P NR two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.59 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 24 h electrolysis. Jin et al. [37]

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Fig. 3. Co-P based HER and OER bifunctional electrocatalyst (reproduced with permission and labeling used in this review are presented in italic format): (a) SEM image of Co-P nanoparticle film (inset: corresponding cross sectional image) [49], (b) TEM dark field image of Co-P (nanoparticle having nanorod-like morphology) [50], (c) HRTEM image of Co-P (nanosheet) [51], (d) SEM image of Co-P (nano needle) [52], (e) SEM image of Co-P (mesoporous nanorod arrays) [53], (f) TEM image of Co2P nanowire (inset: corresponding SAED pattern) [37], (g) Two electrode water electrolysis (polarization curve) of Co2P nanowire//Co2P nanowire in 1 M KOH [37], (h) SEM image of CoP/CoxPO4 (porous) [56], and (i) SEM image of Co-P (nanosheet like morphology) [57].

have fabricated Co2P NWs as bifunctional electrocatalyst by microwave assisted synthesis (energy saving, toxic PH3 emission free, and rapid synthetic method). The Co2P NWs exhibit enhanced HER (h of ~139 mV for 10 mA cm2) and OER (h of 260 and 290 mV for 10 and 20 mA cm2 with Tafel slope of 52 mV/dec) activity in 1.0 M KOH. Co2P NWs exhibits metallic characteristic with more conductivity than other allotropes and the Co2P NWs is crystalline with the growth orientation of (112) and (020) lattice planes and the NW (Fig. 3f) is 5 nm in diameter that exhibit enhanced HER and OER activity. The enhanced OER activity of the Co2P NWs is also involved by the formation of conductive Co oxo/hydroxide on the surface of Co2P NWs during OER in 1.0 M KOH. Moreover the Co2P NWs//Co2P NWs two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.44 V (Fig. 3g) to achieve 10 mA cm2 in 1.0 M KOH and requires only 1.60 V to achieve 100 mA cm2 and exhibit reasonable stability for about 6 h electrolysis and the catalyst exhibit 84% and 79% high efficiency to achieve 100 and 200 mA cm2. Yang et al. [56] have fabricated CoP/CoxPO4 as bifunctional electrocatalyst on Cr/Au substrate using sputtering Cr/ Au as a substrate on glass followed by electrodeposition of Co layer followed by anodic treatment for porous Co layer followed by phosphodization at 300  C for porous CoP/CoxPO4. The CoP/CoxPO4 exhibit enhanced HER (h of 430 mV for 30 mA cm2) and OER (h of 330 mV for 30 mA cm2 with Tafel slope of 65 mV/dec) activity in 1.0 M KOH and exhibit reasonable stability. The CoP/CoxPO4 is

porous (Fig. 3h) with average pore size of ~80 nm and contains Co phosphide (d spacing of ~0.247 nm) and Co phosphate phases (d spacing of ~0.296 nm) that exhibit enhanced HER and OER activity. Moreover the CoP/CoxPO4//CoP/CoxPO4 two-electrode bifuntional water electrolyzer requires reasonable voltage of ~1.91 V to achieve 10 mA cm2 in 1.0 M KOH. Vigil et al. [57] have fabricated nanosheet like Co-PP (Co phosphide/phosphate) film (Fig. 3i) as bifunctional electrocatalyst on Au substrate by electrodeposition of Co hydroxide film followed by heat treatment at 300  C followed by phophorization of spinel Co3O4. The Co-P exhibit enhanced HER (h of 196 mV for 10 mA cm2 with Tafel slope of 67 mV/dec) and OER (h of 340 mV for 10 mA cm2) activity in 1.0 M KOH. Ryu et al. [58] have fabricated Co-P nanoparticle as bifunctional electrocatalyst on carbon by thermal decomposition. The Co-P exhibit enhanced HER (h of 250 mV for 20 mA cm2) and OER (h of 360 mV for 10 mA cm2 with Tafel slope of 66 mV/dec) activity in 0.1 M KOH. The Co-P nanoparticle contains crystalline orthorhombic CoP phase surrounded by a ~5 nm thick amorphous layer containing Co oxo/ hydroxo species and Co metaphophate mostly in þ2 state that exhibit enhanced HER and OER activity. The potential for overall water splitting of the various kinds of reported Co-P//Co-P twoelectrode earth abundant bifunctional electrocatalysts requires to achieve 10 mA cm2 in 1.0 M KOH are in the following order: 1.44 V (Co2P NWs//Co2P NWs; Jin et al. [37]) < 1.54 V (Co-P NS//Co-P NS; Chang et al. [51]) < 1.59 V (Co-P NR//Co-P NR; Chang et al.

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[55]) < 1.61 V (Co-P//Co-P; Wang et al. [52]) < 1.62 V (Co-P MNA// Co-P MNA; Zhu et al. [53]) < 1.64 V (Co-P NW//Co-P NW; Yang et al. [54]) < ~1.65 V (Co-P//Co-P; Jian et al. [49]) < 1.86 V (Co-P//Co-P; Vigil et al. [50]) < ~1.91 V (CoP/CoxPO4//CoP/CoxPO4; Yang et al. [56]) that are far better or well comparable with 1.71 V [48] of Pt/C// RuO2. For two-electrode bifuntional water electrolyzer, Co2P NWs// Co2P NWs [37] (fabricated by toxic PH3 emission free, energy saving, and rapid micro-wave assisted synthetic method) exhibit remarkably much lower potential of 1.44 V and the remarkably much higher catalytic activity are due to the following factors: (1) Among allotropes, Co2P exhibits metallic characteristic with more conductivity, (2) The crystalline Co2P NWs having the growth orientation of (112) and (020) lattice planes possess nano-wire like morphology having about 5 nm in diameter (Fig. 3f), and (3) The insitu formation of conductive Co oxo/hydroxide species on the surface of Co2P NWs during OER in 1.0 M KOH enhanced further OER activity. 2.3. Ni containing Co-P based earth abundant HER and OER bifunctional electrocatalyst Vigil et al. [50] have fabricated Ni-Co-P as bifunctional electrocatalyst by hydrothermal treatment (for spinel Co3O4 preparation) followed by ion exchange process (for NixCo3-xO4 preparation) followed by phosphorization (for Ni-Co-P preparation). The Co-P exhibit enhanced HER (h of 180 mV for 10 mA cm2 with Tafel slope of 63 mV/dec) and OER (h of 360 mV for 10 mA cm2 with Tafel slope of 82 mV/dec) activity in 1.0 M KOH. The Ni-Co-P is nano-particles or clusters of ~5e10 nm in size contains about 80% of CoP and 20% of Co2P phase and the incorporation of Ni in Co-P (NiCo-P) lead to ~22 and ~2.1 times higher electrochemically active surface area (ECSA) and BET surface area, respectively than Co-P. As a result, the Ni-Co-P exhibit enhanced and better HER and especially OER activity than Co-P. Moreover the Ni-Co-P//Ni-Co-P twoelectrode bifuntional water electrolyzer requires reasonable voltage of 1.77 VDE to achieve 10 mA cm2 in 1.0 M KOH and the cell potential of Ni-Co-P//Ni-Co-P (1.77 VDE) is lower than the Co-P//CoP (1.86 VDE). 2.4. Graphene containing Co-P based earth abundant HER and OER bifunctional electrocatalyst Jiao et al. [48] have fabricated Co-P/reduced graphene oxide (rGO) as bifunctional electrocatalyst by pyrolysis followed by phosphodization. The Co-P/rGO exhibit enhanced HER (h of 150 mV for 10 mA cm2 with Tafel slope of 38 mV/dec) and OER (h of 340 mV for 10 mA cm2 with Tafel slope of 66 mV/dec) activity in 1.0 M KOH and exhibit reasonable stability for 22 h. The Co-P/rGO is porous and crystalline with sheet like morphology with thickness of ~200 nm contains orthorhombic CoP phase that exhibit enhanced HER and OER activity. The enhanced OER activity of the Co-P/rGO is also involved by the formation of Co3O4 on the surface of Co-P/rGO during OER in 1.0 M KOH. Moreover the Co-P/rGO//CoP/rGO two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.70 V to achieve 10 mA cm2 in 1.0 M KOH with 100% Faradaic efficiency. Wang et al. [59] have fabricated CoP2/ reduced graphene oxide (RGO) as bifunctional electrocatalyst by phosphodization at 600  C. The CoP2/RGO exhibit enhanced HER (h of 88 and 106 mV for 10 and 20 mA cm2 respectively) and OER (h of 300 and 330 mV for 10 and 20 mA cm2 respectively) activity in 1.0 M KOH. Moreover the CoP2/RGO//CoP2/RGO two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.56 V to achieve 10 mA cm2 in 1.0 M KOH. Huang et al. [60] have fabricated CoP/graphene (G) as bifunctional electrocatalyst by phophodization of Co3O4/G at 300  C. The CoP/G exhibit enhanced

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and better HER (h of 154 mV for 10 mA cm2) and OER (h of 292 mV for 10 mA cm2 with Tafel slope of 80 mV/dec) activity than CoP (HER: h of 201 mV for 10 mA cm2 and OER: h of 342 mV for 10 mA cm2 with Tafel slope of 90 mV/dec) in 1.0 M KOH. The CoP/G is honeycomb-like morphology contains both CoP and graphene phases while CoP is aggregate like morphology contains only CoP phase and hence CoP/G exhibit better HER and OER activity than CoP due to the dual effect (CoP for electroactive sites and G for enhancing charge transfer) of CoP/G. Moreover the CoP/G//CoP/G two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.626 and 1.885 V to achieve 10 and 100 mA cm2, respectively in 1.0 M KOH and exhibit 76% efficiency after 10 h electrolysis at 1.7 V. Yu et al. [61] have fabricated hollow CoP nanoparticles (CoPh)/N-doped graphene (NG) as bifunctional electrocatalyst by thermal treatment followed by phosphodization at 350  C. The CoPh/NG exhibit enhanced and better HER (h of 83 mV for 10 mA cm2 with Tafel slope of 57 mV/dec) and OER (h of 262 and 343 mV for 10 and 100 mA cm2, respectively with Tafel slope of 54 mV/dec) activity than CoPh/G (HER: h of 109 mV for 10 mA cm2 with Tafel slope of 78 mV/dec and OER: h of 292 and 409 mV for 10 and 100 mA cm2, respectively with Tafel slope of 98 mV/dec) in 1.0 M KOH. The CoPh/NG exhibit better HER and OER activity than CoPh/G due to the dual effect (N-doping in CoPh/G) of CoPh/NG. Moreover the CoPh/NG//CoPh/NG two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.58 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 65 h electrolysis and CoPh/NG//CoPh/NG exhibit lower voltage of 1.58 V than 1.651 V of CoPh/G//CoPh/G to achieve 10 mA cm2. The potential for overall water splitting of the various kinds of reported G containing CoP//G containing CoP two-electrode earth abundant bifunctional electrocatalysts requires to achieve 10 mA cm2 in 1.0 M KOH are in the following order: 1.56 V (CoP2/RGO//CoP2/RGO; Wang et al. [59]) < 1.58 V (CoPh/NG//CoPh/NG; Yu et al. [61]) < 1.626 V (CoP/G//CoP/G; Huang et al. [60]) < 1.70 V (Co-P/ rGO//Co-P/rGO; Jiao et al. [48]) that are far better than 1.71 V of Pt/ C//RuO2 [48]. 2.5. O/N/C containing Co or Ni based earth abundant HER and OER bifunctional electrocatalyst You et al. [62] have fabricated polyhedron-like morphology CoP/NC (Fig. 4a) as bifunctional electrocatalyst on GCE by carbonization at high temperature followed by phosphodization at 300  C. The Co-P/NC exhibit enhanced HER (h of 154, 173, and 234 mV for 10, 20 and 100 mA cm2 with Tafel slope of 51 mV/dec) and OER (h of 319 mV for 10 mA cm2 with Tafel slope of 52 mV/dec) activity in 1.0 M KOH. The Co-P/NC having encapsulation configuration and 3D interconnected mesoporous structure with high BET surface area of 183 m2/g, pore volume of 0.276 cm3/g, mesoporous size of >2.0 nm, high electrochemical double-layer capacitance (Cdl: 19.1 F/g) and Co-P/NC contains CoP and Co2P phases with ~33 nm crystal size along with pyridinic N and graphitic N that exhibit enhanced HER and OER activity. Moreover the Co-P/NC//Co-P/NC two-electrode bifuntional water electrolyzer requires reasonable voltage of 2.0 V to achieve 165 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 24 h electrolysis. Hou et al. [63] have fabricated CoPcarbon nanotube (CNT) as bifunctional electrocatalyst by phosphodizaiton of Co3O4-CNT at 300  C. The CoP-CNT exhibit enhanced and better HER (h of 215 mV for 10 mA cm2 with Tafel slope of 56 mV/dec) and OER (h of 330 mV for 10 mA cm2 with Tafel slope of 50 mV/dec) activity than CoP (HER: h of 465 mV for 10 mA cm2 with Tafel slope of 121 mV/dec and OER: h of 400 mV for 10 mA cm2 with Tafel slope of 80 mV/dec) in 0.1 M NaOH. The CoPCNT contains CoP nanoparticles (orthorhombic CoP Phase) of 1.5e2 nm in size well distributed on the CNT of diameter 20e40 nm

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Fig. 4. Reproduced with permission and labeling used in this review are presented in italic format. ONC-Co/Ni based HER and OER bifunctional electrocatalyst: (a) SEM image of CoP/NC (polyhedron-like morphology) [62], (b) SEM image of PNC/Co (irregular rhombic dodecahedral morphology) [65], and (c) TEM image of Ni@NC [69]. Ni-S based bifunctional electrocatalyst: (d) SEM images of Ni9S8 (nanoparticles cluster-like morphology) [47]. Ni-Co based HER and OER bifunctional electrocatalyst: (e) FESEM image of NiCo2O4 hollow microcuboids [77], (f) SEM images of NiCo2S4 nanowires [79], and (g) SEM images of Ni2.3%-CoS2 nanowires [80].

and the CoP-CNT exhibit better HER and OER activity than CoP due to the dual effect (CoP for electroactive sites and CNT for enhancing charge transfer) of CoP-CNT. Moreover CoP-CNT exhibit better OER activity than CoP due to the higher Cdl (ECSA) and the Cdl of postOER CoP-CNT is 9.0 mF cm2 which is 39 times higher than Cdl of post OER CoP (0.23 mF cm2). CoP-CNT exhibit nearly 100% Faradic efficiency and reasonable stability for HER and OER. Jin et al. [64] have fabricated Co-CoOx/N-doped carbon (NC) as bifunctional electrocatalyst by thermal treatment of melamine, CoNO3.6H2O, and D(þ)-glucosamin hydrochloride at 800  C in N2 atmosphere. The Co-CoOx/NC catalyst on Ni foam electrode exhibit enhanced HER (h of 134 and 200 mV for 20 and 83 mA cm2) and OER (h of 260 mV for 10 mA cm2) activity in 1.0 M KOH. The Co-CoOx/NC catalyst contains graphitic C, metallic Co (Co0), CoO, and cubic Co3O4 spinel phases and having flake-like morphology with Co nanoparticles dispersed uniformly on N-doped C sheet and contains CoOx nanoparticles of 12.3 nm in size. As a result of the synergistic effect of metallic Co (Co0) and CoOx, the presence of electron-rich N, the stability of cobalt nanoparticles encapsulated in C, the high charge transfer of C, BET surface area of 311 m2/g and pore volume of 0.71 cm3/g that exhibit enhanced HER and OER activity. Li et al. [65] have fabricated porous N rich carbon (PNC)/Co of irregular rhombic dodecahedral morphology (Fig. 4b) as bifunctional electrocatalyst by calcined ZIF-67 under Ar at 600  C. The Co-P exhibit enhanced HER (h of 298 mV for 10 mA cm2 with

Tafel slope of 131 mV/dec) and OER (h of 370 mV for 10 mA cm2 with Tafel slope of 76 mV/dec) activity in 1.0 M KOH and exhibit reasonable stability after 10 h electrolysis. The PNC/Co catalyst contains ~10 nm in size Co nanoparticles ((111) plane of crystalline Co) well dispersed in porous rhombic dodecahedral carbon ((002) plane of graphitic C) frame work along with a number of CNTs (~14 nm in diameter) extended from C framework surface and the PNC/Co further contains Co-Nx, graphitic-N, pyridinic-N, pyrrolic-N, metallic Co (Co0), Co2þ and Co3þ (Co2þ and Co3þ are due to surface oxidation in air) that exhibit enhanced HER and OER activity. The BET surface area of ZIF-67 (>1500 m2/g) is relatively much higher than PNC/Co (91 m2/g) but the PNC/Co catalyst exhibit enhanced and better OER activity than ZIF-67 due to the relatively higher ECSA (higher Cdl of 40.8 mF cm2) and the formation of Co3O4 on Co during OER of PNC/Co. Moreover the PNC/Co//PNC/Co twoelectrode bifuntional water electrolyzer requires reasonable voltage of 1.64 V to achieve 10 mA cm2 in 1.0 M KOH. Wang et al. [66] have fabricated Co nanoparticles encapsulated in N-doped carbon (Co@NC) as bifunctional electrocatalyst by pyrolysis followed by acid leaching. The Co-P exhibit enhanced HER (h of 210 mV for 10 mA cm2) and OER (h of 400 mV for 10 mA cm2) activity in 1.0 M KOH. The Co@NC catalyst having the stability of cobalt nanoparticles encapsulated in C, the presence of electronrich N and the high conductivity of C that enhanced HER and OER activity. Wang et al. [67] have fabricated carbon paper(CP)/carbon

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tubes (CTs)/Co-S as bifunctional electrocatalyst by electrodeposition method. The CP/CTs/Co-S exhibit enhanced HER (h of 190 mV for 10 mA cm2 with Tafel slope of 131 mV/dec) and OER (h of 306 mV for 10 mA cm2 with Tafel slope of 72 mV/dec) activity in 1.0 M KOH. The CP/CTs/Co-S catalyst having 3D array morphology with increased accessibility and exposure of active sites, enhanced gaseous products release, increased vectorial electron transport process and higher ECSA (higher Cdl of 103.7 mF cm2) that exhibit enhanced HER and OER activity. After HER, the sheet-like morphology of the catalyst is almost retained with small S loss (Co/P ratio of ~1.6) whereas after OER, the catalyst surface exhibit structural change (formation of many regular plate-like morphology) with large S loss (Co/P ratio of ~5) and the catalyst surface contains Co(OH)2 formation. Moreover the CP/CTs/Co-S//CP/ CTs/Co-S two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.743 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability after 1000 continuous cycles of potential scan. Du et al. [68] have fabricated Co3O4 nanocrystals (NCs) as bifunctional electrocatalyst on CFP by decomposition of the [Co(NH3)n]2þeoleic acid complex thermally followed by spray deposition. The Co3O4 NCs exhibit enhanced HER (h of 155 mV for 10 mA cm2) and OER (h of 380 mV for 10 mA cm2) activity in 1.0 M KOH. The Co3O4 NCs having exposed electroactive sites and enhanced charge transfer that exhibit enhanced HER and OER activity. Ren et al. [69] have fabricated Ni supported N-doped C (Ni@NC) (Fig. 4c) as bifunctional electrocatalyst by soaking cellulose filter paper into a nickel/phenanthroline solution followed by carbonized at 800  C. The Ni@NC exhibit enhanced HER (h of 190 mV for 10 mA cm2) and OER (h of 390 mV for 10 mA cm2 with Tafel slope of 44 mV/dec) activity in 0.1 M KOH. The Ni@NC contains graphitic carbon phase, face centered cubic of metallic Ni (Ni0) phase, Ni0, Ni2þ, Ni3þ, Ni, NiO, NiO(OH), pyridinic-N (C-N) and oxidized-N (O-N) that exhibit enhanced HER and OER activity. The Ni@NC exhibit h of 430 mV to achieve about 25 mA cm2 and exhibit reasonable stability after 10 h with 5% current decay. Moreover the Ni@NC//Ni@NC two-electrode bifuntional water electrolyzer requires reasonable voltage of about 1.81 VDE to achieve 10 mA cm2 in 0.1 M KOH. Xi et al. [70] have fabricated Nibased nanosheets with thin C coating (Ni@C-400 NSs) as bifunctional electrocatalyst on nickel foam by carbothermal reduction. The Ni@C-400 NSs exhibit enhanced HER (h of 110 mV for 10 mA cm2 with Tafel slope of 95 mV/dec) and OER (h of 310 mV for 10 mA cm2 with Tafel slope of 95 mV/dec) activity in 1.0 M KOH and exhibit reasonable stability after 500 cycles of potential scan with slight potential decay. The Ni@C-400 NSs contains metallic Ni (Ni0) phase, NiO phase, pyridinic-N, pyrolic-N with the BET surface area of 109.51 m2/g and pore diameter of 10.08 nm and the nanosheet is madeup of NiO and Ni nanoparticles covered in thin graphitic C. As a result the Ni@C-400 NSs exhibit enhanced HER and OER activity. Moreover the Ni@C-400 NSs//Ni@C-400 NSs twoelectrode bifuntional water electrolyzer requires reasonable voltage of 1.64 V to achieve 10 mA cm2 in 1.0 M KOH. 2.6. Ni-S based earth abundant HER and OER bifunctional electrocatalyst You et al. [31] have fabricated h-NiSx as bifunctional electrocatalyst on Ni foam by electrodeposition of porous Ni microsphere arrays followed by sulfurization at 400  C. The h-NiSx exhibit enhanced HER (h of 60 and 89 mV for 10 and 20 mA cm2 respectively with Tafel slope of 99 mV/dec) and OER (h of 180, 217, and 316 mV for 10, 100, and 500 mA cm2 with Tafel slope of 96 mV/dec) activity in 1.0 M KOH. The h-NiSx is porous 3D hierarchically morphology contains NiS2 and NiS phases with molar percentage of ~20% and 80% respectively and contains large

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mesopores and numerous micropores and the microparticle of NiSx is 4e10 mm in size that exhibit enhanced HER and OER activity. After 10 h HER, the h-NiSx exhibit reasonable stability with about 10 mV of potential decay while the structure and composition are retained. After 10 h OER, the h-NiSx exhibit reasonable stability with about 18 mV of potential decay and the structure is retained while the enhanced OER activity of the h-NiSx is also involved by the formation of NiO on the surface of Ni-S during OER. Moreover the h-NiSx//h-NiSx two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.47 V and 1.53 V to achieve 10 and 20 mA cm2, respectively in 1.0 M KOH and exhibit reasonable stability for 5 h and subsequent 5 h electrolysis to achieve 10 and 20 mA cm2, respectively. Feng et al. [71] have fabricated Ni3S2 nanosheet array as bifunctional electrocatalyst by sulfidization of Ni foam using thiourea at 150  C in hydrothermal system. The Ni3S2 exhibit enhanced HER (h of 223 mV for 10 mA cm2) and OER (h of 260 mV for 10 mA cm2) activity in 1.0 M KOH and exhibit 100% faradic efficiency and also exhibit reasonable stability after 200 h. The Ni3S2 catalyst contains (021) and (003) planes of hexagonal Ni3S2 phase with nanosheet morphology and the nanosheet grown vertically over Ni foam having ~2 mm of height and 10e15 nm of thickness with the exposed facet of nanosheet is {210}. As a result of the {210} high index facets of nanosheet, Ni3S2 with metallic character which facilitate electron access to active sites, nanosheet morphology for large active sites and self supporting Ni foam to enhance charge transfer that exhibit enhanced HER and OER activity. Moreover the Ni3S2//Ni3S2 two-electrode bifuntional water electrolyzer requires reasonable voltage of ~1.76 V to achieve ~13 mA cm2 in 1.0 M KOH and exhibit reasonable stability after 150 h electrolysis. Zhu et al. [72] have fabricated NiS as bifunctional electrocatalyst by sulfurization of Ni foam with S powder at 350  C. The NiS microsphere film exhibit enhanced HER (h of 158 mV for 20 mA cm2) and OER (h of 335 mV for 50 mA cm2) activity in 1.0 M KOH and exhibit reasonable stability after 1000 cycles of potential scan. Moreover the NiS//NiS two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.64 V to achieve 10 mA cm2 in 1.0 M KOH. Ouyang et al. [73] have fabricated Ni3S2 as bifunctional electrocatalyst by pretreatment of Ni foam with HCl followed by hydrothermal treatment. The hierarchically porous Ni3S2 nanorod array exhibit enhanced HER (h of 200 mV for 10 mA cm2) and OER (h of 217 mV for 10 mA cm2) activity in 1.0 M KOH. Chen et al. [47] have fabricated Ni9S8 as bifunctional electrocatalyst by acid activation of Ni foam followed by air oxidation (3 days) followed by sulfurization with S powder at 350  C. The Ni9S8 exhibit enhanced HER (h of 230 and 280 mV for 10 and 30 mA cm2, respectively with Tafel slope of 123.3 mV/dec) and OER (h of 340 mV for 30 mA cm2 with Tafel slope of 109.8 mV/ dec) activity in 1.0 M KOH. For HER at h of 180 mV and OER at potential of 1.55 V, the Ni9S8 exhibit reasonable stability for 24 h with negligible current decay. The Ni9S8 is polycrystalline having 3D nanoparticles cluster-like morphology (Fig. 4d) with average size of 4e5 mm in diameter and contains orthorhombic Ni9S8 phase and also contains NiO (due to oxidation in air) that exhibit enhanced HER and OER activity. The enhanced OER activity are also due to the formation of NiO on the surface of Ni9S8 during OER and due to the 3D morphology with facilitate gas evolution. Moreover the Ni9S8//Ni9S8 two-electrode bifuntional water electrolyzer requires reasonable voltage of ~1.61 V to achieve 1.7 mA cm2 in 1.0 M KOH. Sivanantham et al. [74] have fabricated Ni3S2 as bifunctional electrocatalyst on Ni foam using two step (in-situ growth followed by anion-exchange reaction) hydrothermal method. The Ni3S2 exhibit enhanced HER (h of 310 mV for 10 mA cm2 with Tafel slope of 96 mV/dec) and OER (h of 300 mV for 10 mA cm2 with Tafel slope of 51 mV/dec) activity in 1.0 M KOH. The Ni3S2 having partially grown nanowire array morphology and contains

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rhombohedral phase that exhibit enhanced HER and OER activity. Moreover the Ni3S2//Ni3S2 two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.73 V to achieve 10 mA cm2 in 1.0 M KOH. 2.7. B containing Co or Ni based earth abundant HER and OER bifunctional electrocatalyst Liang et al. [75] have fabricated Ni-B as bifunctional electrocatalyst using electroless deposition method by dipping of Ni foam for 10 s alternatively into Ni precursor (NiCl2.6H2O) and reducing solutions (NaBH4 and NaOH). The Ni-B exhibit enhanced HER (h of 125 and 186 mV for 20 and 100 mA cm2, respectively with Tafel slope of 93 mV/dec) and OER (h of 360 mV for 100 mA cm2 with Tafel slope of 76 mV/dec) activity in 1.0 M KOH. The amorphous NiB nanoparticle is ~50 nm in size contains 93% (atomic) of Ni and further contains metallic Ni and oxidized (due to air) Ni and B that exhibit enhanced HER and OER activity. After 10 h HER at h of 172 mV, the Ni-B nanoparticles aggregate into bigger particles while retained its chemical state and exhibit reasonable stability with negligible current decay. After 10 h OER at h of 318 mV, the NiB nanoparticles aggregate into nanosheet with bigger in size while the enhanced OER activity of the Ni-B is also involved by the formation of NiOOH on the surface of Ni-B during OER and exhibit reasonable stability with slight current decay. Moreover the Ni-B// Ni-B two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.69 V to achieve 15 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 10 h with slight current density decay. Masa et al. [76] have fabricated Co2B-500 as bifunctional electrocatalyst by reduction of CoCl2 chemically with NaBH4 followed by annealing. The Co2B-500 exhibit enhanced OER (h of 380 mV for 10 mA cm2 with Tafel slope of 45 mV/dec) activity in 0.1 M KOH. The Co2B-500 exist as distinct particles in widespread amorphous layer and contains nanocrystalline phases and exhibit crystals with lattice space of 2.1 Å along with zone axis of [210] for tetragonal structure of Co2B and the Co 2P3/2 of Co2B-500 exhibit a 0.45 eV of positive chemical shift due to Co2B structure. As a result, Co2B-500 exhibit enhanced HER and OER activity. The enhanced OER activity of the Co2B is also involved by the formation of CoOOH on the surface of Co2B during OER in 0.1 M KOH. Moreover the Co2B//Co2B serve as a bifuntional electrocatalyst and requires h of 0.280 mV for HER and potential of 1.61 V for OER to achieve 10 mA cm2 in 1.0 M KOH. 2.8. Ni and Co based earth abundant HER and OER bifunctional electrocatalyst Gao et al. [77] have fabricated NiCo2O4 hollow microcuboids as bifunctional electrocatalyst by solvothermal method for Ni/Co based precursor preparation followed by annealing the precursor at 350  C. The NiCo2O4 hollow microcuboids exhibit enhanced HER (h of 110 and 245 mV for 10 and 100 mA cm2, respectively with Tafel slope of 49.7 mV/dec) and OER (potential of 1.52 V for 10 mA cm2 with Tafel slope of 53 mV/dec) activity in 1.0 M NaOH and exhibit reasonable stability for 32 h with slight potential decay (~15 mV for HER and ~10 mV for OER). The NiCo2O4 having 3D hierarchically hollow microcuboid morphology (Fig. 4e) composed of 1 D nanowires having mesopores well distributed over the shell and the NiCo2O4 contains (311) and (220) planes of crystalline NiCo2O4 phase with BET surface area of 69.6 m2/g and pore size of 17.43 nm and the NiCo2O4 further contains Co-O and Ni-O bonds that exhibit enhanced HER and OER activity. Moreover the NiCo2O4//NiCo2O4 hollow microcuboids two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.65 V and 1.74 V to achieve 10 and 20 mA cm2, respectively in 1.0 M NaOH and exhibit reasonable

stability for 36 h electrolysis. Liu et al. [78] have fabricated Ni-Co-S nanosheet (NS) as bifunctional electrocatalyst on copper foam using electrodeposition method. The Ni-Co-S with 3D NS morphology exhibit enhanced HER (h of 140 mV for 10 mA cm2) and OER (h of 363 mV for 100 mA cm2) activity in 1.0 M KOH. Moreover the NiCo-S NS//Ni-Co-S NS two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.67 V to achieve 10 mA cm2 in 1.0 M KOH. Sivanantham et al. [74] have fabricated NiCo2S4 nanowire (NW) as bifunctional electrocatalyst on Ni foam using two step (in-situ growth followed by anion-exchange reaction) hydrothermal method. The NiCo2S4 NW exhibit enhanced HER (h of 210 mV for 10 mA cm2 with Tafel slope of 58.9 mV/dec) and OER (h of 260 mV for 10 mA cm2 with Tafel slope of 40.1 mV/dec) activity in 1.0 M KOH. After HER for 50 h at static potential of 0.323 V, the NiCo2S4 NW exhibit reasonable stability to achieve 18 mA cm2 with ~13% current decay while morphology and chemical state are retained. After OER for 50 h at static potential of 1.527 V, the NiCo2S4 NW exhibit reasonable stability to achieve 10 mA cm2 with ~15% current decay and the morphology and chemical state are changed. The polycrystalline NiCo2S4 nanowires (with sharp tips) is ~70 nm in diameter and ~1 mm in length oriented nearly vertical to the Ni foam and contains predominant NiCo2S4 cubic phase and minor Ni3S2 phase and further contains Ni2þ, Ni3þ, Co2þ, Co3þ and S2 that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH and Co(OH)2 on the surface of NiCo2S4 NW during OER in 1.0 M KOH. Moreover, the NiCo2S4 NW (HER: h of 210 mV and OER: h of 260 mV) exhibit enhanced and better HER and OER activity than Ni3S2 (HER: h of 310 mV and OER: h of 300 mV) and NiCo2O4 NW (HER: h of 310 mV and OER: h of 330 mV). Moreover the NiCo2S4 NW//NiCo2S4 NW two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.63 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 50 h of electrolysis with ~10% potential decay at 10 mA cm2. Moreover, the NiCo2S4 NW// NiCo2S4 NW (1.63 V) two-electrode bifuntional water electrolyzer requires lower cell voltage to achieve 10 mA cm2 in 1.0 M KOH than Ni3S2 (1.73 V) and NiCo2O4 NW (1.84 V). Liu et al. [79] have fabricated NiCo2S4 NW (nanowire) as bifunctional electrocatalyst on carbon cloth by hydrothermal treatment along with annealing (for NiCo2O4 NW preparation) followed by sulfurization with sulfur powder at 450  C. The NiCo2S4 NW exhibit enhanced HER (h of 305 mV for 100 mA cm2 with Tafel slope of 141 mV/dec) and OER (h of 340 mV for 100 mA cm2 with Tafel slope of 89 mV/dec) activity in 1.0 M KOH. For HER, the morphology and chemical state of the NiCo2S4 NW are retained after 500 cycles potential scan with negligible current decay. For OER, the morphology of the NiCo2S4 NW is retained while chemical state is changed after 500 cycles potential scan with slight current decay. The NiCo2S4 nanowires (Fig. 4f) is 50e100 nm in diameter and about few micrometers in length and the NiCo2S4 NW consists of NiCo2S4 nanoparticles and contains NiCo2S4 phase and further contains Ni2þ, Ni3þ, Co2þ, Co3þ and S2 with 1:2.1:4.2 atomic ratio of Ni:Co:S that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH and Co(OH)2 on the surface of NiCo2S4 NW during OER in 1.0 M KOH. Moreover the NiCo2S4 NW//NiCo2S4 NW two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.68 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability at 14 mA cm2 for 10 h with negligible potential decay. Fang et al. [80] have fabricated Ni2.3%-CoS2 NW (nanowire) as bifunctional electrocatalyst on carbon cloth by two step hydrothermal treatment. The Ni2.3%-CoS2 NW exhibit enhanced HER (h of 231 mV for 100 mA cm2 with Tafel slope of 106 mV/dec) and OER (h of 370 mV for 100 mA cm2 with Tafel slope of 119 mV/dec) activity in 1.0 M KOH. After 12 h of HER, the Ni2.3%-CoS2 NW exhibit reasonable stability while the chemical

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state is retained. After 12 h of OER, the Ni2.3%-CoS2 NW exhibit reasonable stability while the chemical state is changed. The Ni2.3%CoS2 nanowires (Fig. 4g) is 50e100 nm in diameter and about few micrometers in length and the Ni2.3%-CoS2 NW consists of Ni2.3%CoS2 nanoparticles and the Ni2.3%-CoS2 NW (Ni-doped CoS2) contains CoS2 phase with 0.05:1:2 atomic ratio of Ni:Co:S that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH and Co(OH)2 on the surface of Ni2.3%-CoS2 NW during OER in 1.0 M KOH. Moreover the Ni2.3%-CoS2 NW//Ni2.3%-CoS2 NW two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.66 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 12 h electrolysis. Yin et al. [81] have fabricated NiCo2O4 nanowires with Co0.57Ni0.43 layered oxide nanosheets as bifunctional electrocatalyst on CFP that exhibit enhanced HER in acid medium and OER (h of 340 mV for 10 mA cm2 with Tafel slope of 63 mV/dec) activity in 0.1 M KOH with reasonable stability and the bifuntional catalyst (act as both cathode and anode) exhibit water electrolysis by a single cell battery. 2.9. Mo based earth abundant HER and OER bifunctional electrocatalyst Tian et al. [82] have fabricated NiMo hollow nanorod array (HNRs) as bifunctional electrocatalyst on Ti mesh by template (ZnO array) assisted electrodeposition method. The NiMo HNRs exhibit enhanced HER (h of 92 and 200 mV for 10 and 100 mA cm2 with Tafel slope of 76 mV/dec) and OER (h of 310 mV for 10 mA cm2 with Tafel slope of 47 mV/dec) activity in 1.0 M KOH. For HER at static overpotential of 140 mV, the NiMo HNRs exhibit reasonable stability for 15 h while for OER at static overpotential of 350 mV, the NiMo HNRs exhibit reasonable stability for 20 h to achieve 50 mA cm2 with negligible current decay (~4 mA cm2). Moreover the NiMo HNRs (HER: h of 200 for 100 mA cm2 and OER: h of 310 mV for 10 mA cm2) exhibit enhanced and better HER and OER activity than NiMo nanoparticles (NPs) (HER: h of 275 for 100 mA cm2 and OER: h of 350 mV for 10 mA cm2). The NiMo HNRs having nanorod of 500 nm in diameter consists of intercrossed nanosheet forms porous nanostructure and contains Ni4Mo phase with 4:1 atomic ratio of Ni:Mo that exhibit enhanced HER and OER activity. Moreover the NiMo HNRs//NiMo HNRs twoelectrode bifuntional water electrolyzer requires reasonable voltage of 1.64 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 10 h. In addition NiMo HNRs//NiMo HNRs water electrolyzer requires lower cell voltage of 1.64 V than 1.70 V of NiMo NPs//NiMo NPs to achieve 10 mA cm2. Jin et al. [83] have fabricated porous MoO2 nanosheet (NS) as bifunctional electrocatalyst on Ni foam by hydrothermal method followed by annealing. The MoO2 NS exhibit enhanced HER (h of 25 mV for 10 mA cm2 with Tafel slope of 41 mV/dec) and OER (potential of 1.49 V for 10 mA cm2 with Tafel slope of 54 mV/dec) activity in 1.0 M KOH and exhibit nearly 100% Faradic efficiency and exhibit reasonable stability for 12 h. Moreover MoO2 NS (HER: h of 25 mV for 10 mA cm2 and OER: potential of 1.49 V for 10 mA cm2) exhibit enhanced and better HER and OER activity than compact MoO2 (HER: h of 124 mV for 10 mA cm2 and OER: potential of 1.56 V for 10 mA cm2). The MoO2 NS have porous nanosheet like morphology and contains MoO2 phase that exhibit enhanced HER and OER activity and the MoO2 NS (Cdl: 422 mF cm2) has higher ECSA than compact MoO2 (Cdl: 31 mF cm2). Moreover the MoO2 NS//MoO2 NS two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.53 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 24 h and the MoO2 NS//MoO2 NS two-electrode bifuntional water electrolyzer requires lower voltage of 1.53 V than 1.73 V of compact MoO2//compact MoO2 to

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achieve 10 mA cm2. Zhang et al. [84] have fabricated MoS2/Ni3S2 heterostructures (hs) as bifunctional electrocatalyst on Ni foam by solvothermal method. The MoS2/Ni3S2 hs exhibit enhanced HER (h of ~110 mV for 10 mA cm2 with Tafel slope of 83 mV/dec) and OER (h of 218 mV for 10 mA cm2 with Tafel slope of 88 mV/dec) activity in 1.0 M KOH. The MoS2/Ni3S2 hs composed of Ni3S2 nanoparticles (several hundred nm in size) and MoS2 nanosheets (30e130 nm in size and 5e15 nm in thickness) and contain macropores and contain MoS2 and Ni3S2 phases that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiO on the surface of MoS2/Ni3S2 hs during OER in 1.0 M KOH. Moreover the MoS2/Ni3S2 hs//MoS2/Ni3S2 hs two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.56 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 100 h of water electrolysis at higher current density of 500 mA cm2. Yan et al. [85] have fabricated CoO/MoOx as bifunctional electrocatalyst from CoMoO4 by hydrogenation. The CoO/MoOx exhibit enhanced HER (onset h of 40 mV) and OER (onset h of 230 mV) activity in 1.0 M KOH. The CoO/MoOx having crystalline CoO embedded in an MoOx amorphous matrix with nanorods on nanosheet morphology that exhibit enhanced HER and OER activity. The potential for overall water splitting of the various kinds of reported Mo based//Mo based two-electrode earth abundant bifunctional electrocatalysts requires to achieve 10 mA cm2 in 1.0 M KOH are in the following order: 1.53 V [83] (Porous MoO2 NS//Porous MoO2 NS) < 1.56 V [84] (MoS2/Ni3S2 hs// MoS2/Ni3S2 hs) < 1.64 V [82] (NiMo HNRs//NiMo HNRs) < 1.70 V [82] (NiMo NPs//NiMo NPs) < 1.73 V [83] (Compact MoO2//Compact MoO2) that are far better or well comparable with 1.71 V [48] of Pt/ C//RuO2. For two-electrode bifuntional water electrolyzer, Porous MoO2 NS//Porous MoO2 NS [83] exhibit much lower potential of 1.53 V and the much higher catalytic activity are due to the following factors: (a) Porous MoO2 NS have porous nanosheet like morphology and contains MoO2 phase, and the porous structure can provide enhanced electroactive sites (for HER and OER) and can facilitate the gas evolution (H2 and O2), and (b) the Porous MoO2 NS (Cdl: 422 mF cm2) has higher ECSA. 2.10. Se based earth abundant HER and OER bifunctional electrocatalyst Xu et al. [86] have fabricated Ni3Se2 (nanoforest) Nfor as bifunctional electrocatalyst on Ni foam using solution chemical route. The Ni3Se2 Nfor exhibit enhanced HER (h of 203 and 279 mV for 10 and 100 mA cm2 with Tafel slope of 79 mV/dec) and OER (h of 239, 242, and 353 mV for 10, 20, and 100 mA cm2, respectively) activity in 1.0 M KOH. The Ni3Se2 Nfor having 3D hierarchically porous nanoforest (Fig. 5a) like morphology composed of main trunk (2e5 mm), branch (500e1000 nm) and secondary leaf-like (100e300 nm) structures and the Ni3Se2 Nfor contains rhombohedral Ni3Se2 phase and having metallic character, hydrophilic and aerophobic surface that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH on the surface of Ni3Se2 Nfor during OER in 1.0 M KOH. Moreover the Ni3Se2 Nfor//Ni3Se2 Nfor two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.612 V to achieve 10 mA cm2 in 1.0 M KOH with nearly 100% Faradaic efficiency and exhibit reasonable stability for 140 h. Kwak et al. [87] have fabricated NiSe2 (nanocrystals) NCs as bifunctional electrocatalyst by photo induced cation exchange reaction of GeSe2 in aqueous solution. The NiSe2 NCs having NiSe2 phase with nanocrystal size of about 20 nm that exhibit enhanced HER and OER (h of 250 mV for 10 mA cm2 with Tafel slope of 38 mV/dec) activity in 1.0 M KOH. The enhanced OER activity is also involved by the formation of NiOOH on the surface of NiSe2 NCs during OER in 1.0 M KOH.

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Fig. 5. Reproduced with permission and labeling used in this review are presented in italic format. Se based HER and OER bifunctional electrocatalyst: (a) SEM image of Ni3Se2 (nanoforest) [86], (b) TEM image of Co0.13Ni0.87Se2 (nanoparticles) [88], (c) SEM image of a-CoSe (nanoparticles) [90], (d) SEM images of Ni3Se2 [91], and (e) SEM image of NiSe nanowire [30]. Fe based bifunctional electrocatalyst: (f) SEM image (inset: TEM image) of Fe2Ni2N (nanoplate arrays) [32], (g) SEM image of NiFe (nanosheet) [102], and (h) SEM images of nanoporous-(Co0.52Fe0.48)2P (inset: corresponding cross sectional image) [103].

Moreover the NiSe2 NCs//NiSe2 NCs two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.60 V to achieve 10 mA cm2 in 1.0 M KOH. Liu et al. [88] have fabricated Co0.13Ni0.87Se2 as bifunctional electrocatalyst on Ti plate by electrodeposition method. The Co0.13Ni0.87Se2 exhibit enhanced HER (h of 64 mV for 10 mA cm2 with Tafel slope of 63 mV/dec) and OER (h of 320 mV for 100 mA cm2 with Tafel slope of 94 mV/dec) activity in 1.0 M KOH. The Co0.13Ni0.87Se2 composed of Co doped into crystalline NiSe2 phase and the film having nanoparticle (Fig. 5b) morphology with 15e20 nm in diameter that exhibit enhanced HER and OER activity. For HER, the Co0.13Ni0.87Se2 exhibit reasonable stability for 60 h with negligible current density decay and without chemical state change. For OER, the Co0.13Ni0.87Se2 exhibit reasonable stability for 10 h with slight current density decay and with structural and chemical state change. The enhanced OER activity is also involved by the formation of NiOOH on the surface of Co0.13Ni0.87Se2 during OER in 1.0 M KOH. Moreover the Co0.13Ni0.87Se2//Co0.13Ni0.87Se2 two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.62 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit nearly 100% Faradaic efficiency and exhibit reasonable stability for 10 h with negligible potential decay. Pu et al. [89] have fabricated NiSe2 as bifunctional electrocatalyst on Ti plate by electrodeposition method. The NiSe2 exhibit enhanced HER (h of

96 mV for 10 mA cm2 with Tafel slope of 82 mV/dec) and OER (h of 295 mV for 20 mA cm2 with Tafel slope of 82 mV/dec) activity in 1.0 M KOH. The NiSe2 contains nanoparticles of 5e20 nm in size and contains cubic pyrite type NiSe2 phase that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH on the surface of NiSe2 during OER in 1.0 M KOH. Moreover the NiSe2//NiSe2 two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.66 V to achieve 10 mA cm2 in 1.0 M KOH with nearly 100% Faradaic efficiency and exhibit reasonable stability for 16 h. Liu et al. [90] have fabricated amorphous CoSe (a-CoSe) as bifunctional electrocatalyst on Ti mesh by electrodeposition method. The a-CoSe exhibit enhanced HER (h of 121 mV for 10 mA cm2 with Tafel slope of 84 mV/dec) and OER (h of 292 mV for 10 mA cm2 with Tafel slope of 69 mV/dec) activity in 1.0 M KOH. The a-CoSe is amorphous nanoparticles (Fig. 5c) with Co:Se atomic ratio of 1.09:1 that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of CoOOH on the surface of a-CoSe during OER in 1.0 M KOH. Moreover the a-CoSe//a-CoSe two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.65 V to achieve 10 mA cm2 in 1.0 M KOH with nearly 100% Faradaic efficiency and exhibit reasonable stability for 27 h with about 40 mV potential decay. Shi et al. [91] have fabricated Ni3Se2 as bifunctional

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electrocatalyst on copper foam by electrodeposition method. The Ni3Se2 exhibit enhanced HER (h of 100 mV for 10 mA cm2 with Tafel slope of 98 mV/dec) and OER (h of 340 mV for 50 mA cm2 with Tafel slope of 80 mV/dec) activity in 1.0 M KOH. The Ni3Se2 contains trigonal Ni3Se2 phase and contains microparticles (Fig. 5d) that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH on the surface of Ni3Se2 during OER in 1.0 M KOH. Moreover the Ni3Se2//Ni3Se2 twoelectrode bifuntional water electrolyzer requires reasonable voltage of 1.65 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 12 h. Tang et al. [30] have fabricated NiSe nanowire (NW) as bifunctional electrocatalyst on Ni foam by hydrothermal treatment using NaHSe. The NiSe NW exhibit enhanced HER (h of 96 mV for 10 mA cm2 with Tafel slope of 120 mV/dec) and OER (h of 270 mV for 20 mA cm2 with Tafel slope of 64 mV/ dec) activity in 1.0 M KOH. The NiSe NW having nanowire (Fig. 5e) like morphology with 20e80 nm in diameter and several micrometers in length and contains NiSe phase along with NiO (due to surface oxidation) that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH on the surface of NiSe NW during OER in 1.0 M KOH. Moreover the NiSe NW//NiSe NW two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.63 V to achieve 10 mA cm2 in 1.0 M KOH with nearly 100% Faradaic efficiency and exhibit reasonable stability for 20 h. Wu et al. [92] have fabricated Ni0.85Se as bifunctional electrocatalyst on graphite substrate by electrodeposition followed by calcination. The Ni0.85Se contains hexagonal Ni0.85Se phase that exhibit enhanced HER (h of 200 mV for 10 mA cm2) and OER (h of 302 mV for 10 mA cm2) activity in 1.0 M NaOH. Moreover the Ni0.85Se//Ni0.85Se two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.73 V to achieve 10 mA cm2 in 1.0 M NaOH and exhibit reasonable stability for 48 h. The potential for overall water splitting of the various kinds of reported Se based two-electrode earth abundant bifunctional electrocatalysts requires to achieve 10 mA cm2 in 1.0 M KOH are in the following order: 1.60 V [87] (NiSe2 NCs//NiSe2 NCs) < 1.61 V [86] (Ni3Se2 Nfor//Ni3Se2 Nfor) < 1.62 V [88] (Co0.13Ni0.87Se2//Co0.13Ni0.87Se2) < 1.63 V [30] (NiSe NW//NiSe NW) < 1.65 V [90] (a-CoSe//a-CoSe) z 1.65 V [91] (Ni3Se2// Ni3Se2) < 1.66 V [89] (NiSe2//NiSe2) that are far better than 1.71 V [48] of Pt/C//RuO2. For two-electrode bifuntional water electrolyzer, NiSe2 NCs//NiSe2 NCs [87] exhibit much lower potential of 1.60 V and the much higher catalytic activity are due to the following factors: (a) The NiSe2 NCs having NiSe2 phase with nanocrystal size of about 20 nm, and (b) The OER activity is further enhanced by the in-situ formation of NiOOH on the surface of NiSe2 NCs during OER in 1.0 M KOH. 2.11. Fe based earth abundant HER and OER bifunctional electrocatalyst Zhang et al. [93] have fabricated Ni0.9Fe0.1/N doped nanocarbon (NC) as bifunctional electrocatalyst by pyrolyzing a Ni(CH3COO)2$4H2O and FeCl3$6H2O with urea. The Ni0.9Fe0.1/NC exhibit enhanced HER (h of 85 mV for 10 mA cm2) and OER (h of 270 mV for 10 mA cm2) activity in 1.0 M KOH. The Ni0.9Fe0.1/NC having NieFe alloy nanoparticles of 10e50 nm in size encapsulated or well dispersed in bamboo like nanotube (graphitic C) and contains CeO/ CeN, C]N, and C]C/CeC along with pyridinic, pyrolic, graphitic, and oxidized pyridinic N and having BET surface area of 153.7 m2/g that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of NiOOH and Fe3þ species (possibly due to the formation of Ni1-xFexOOH) on the surface of Ni0.9Fe0.1/NC during OER in 1.0 M KOH. Moreover the Ni0.9Fe0.1/NC// Ni0.9Fe0.1/NC two-electrode bifuntional water electrolyzer requires

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reasonable voltage of 1.58 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 24 h. Jia et al. [94] have fabricated Ni3FeN (nanoparticles) NPs as bifunctional electrocatalyst by reverse microemulsion method (for preparation of ultrathin NieFe Layered Double Hydroxide (LDH) nanosheet) followed by thermal ammonolysis at 500  C. The Ni3FeN-NPs exhibit enhanced HER (h of 158 and 416 mV for 10 and 200 mA cm2 with Tafel slope of 42 mV/ dec) and OER (h of 280 mV for 10 mA cm2 with Tafel slope of 46 mV/dec) activity in 1.0 M KOH. The Ni3FeN-NPs having nanoparticles of ~100 nm in size contains Ni3FeN phase and further contains predominant Ni0 and Fe0 along with minor Ni2þ and Fe3þ (due to surface oxidation) that exhibit enhanced HER and OER activity and exhibit reasonable stability for HER and OER (retained 94% current density) for 9 h. Hou et al. [95] have fabricated electrochemically exfoliated graphene foil (EG)/Co0.85Se/NiFe-LDH as bifunctional electrocatalyst by insitu growth of Co0.85Se on EG followed by deposition (hydrothermal treatment) of NiFe-LDH on EG/ Co0.85Se. The EG/Co0.85Se/NiFe-LDH exhibit enhanced HER (h of 260 mV for 10 mA cm2 with Tafel slope of 160 mV/dec) and OER (potential of 1.50 and 1.51 V for 150 and 250 mA cm2 with Tafel slope of 57 mV/dec) activity in 1.0 M KOH. After HER and OER, the EG/Co0.85Se/NiFe-LDH exhibit structural and chemical state stability. The EG/Co0.85Se/NiFe-LDH composed of NiFe-LDH (~10 nm in thickness) oriented on EG/Co0.85Se (Co0.85Se nanosheet array of ~30 nm in thickness and few micrometers in length vertically oriented on EG) and the EG/Co0.85Se/NiFe-LDH contains hexagonal Co0.85Se, NiFe-LDH, and graphitic carbon phases and having BET surface area of 156 m2/g with highly hydrophophic surface (~0.80 ) that exhibit enhanced HER and OER activity. Moreover the EG/ Co0.85Se/NiFe-LDH//EG/Co0.85Se/NiFe-LDH two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.67 and 1.71 V to achieve 10 and 20 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 10 h. Ma et al. [96] have fabricated Ni2/3Fe1/ 3-reduced graphene oxide (rGO) as bifunctional electrocatalyst from rGO and NiFe-LDH nanosheets. The Ni2/3Fe1/3-rGO (composed of NiFe-LDH and rGO nanosheets) exhibit enhanced HER and OER (h of 210 mV for 10 mA cm2 with Tafel slope of 40 mV/dec) activity in 1.0 M KOH. The catalytic activity was increased with increase in Fe content of NiFe system. Moreover the Ni2/3Fe1/3-rGO// Ni2/3Fe1/3rGO two-electrode bifuntional water electrolyzer operated by AA battery of 1.5 V. Jiang et al. [32] have fabricated Fe2Ni2N (nanoplate arrays) NPA as bifunctional electrocatalyst on Ni foam by hydrothermal treatment (for NiFe-LDH preparation) followed by calcinations at 380  C with NH3. The Fe2Ni2N NPA exhibit enhanced HER (h of 180 mV for 10 mA cm2 with Tafel slope of 101 mV/dec) and OER (onset potential of ~1.47 V with Tafel slope of 34 mV/dec) activity in 1.0 M KOH and exhibit reasonable stability for HER (retained 95.6% current density) and OER (retained 91.7% current density) for 10 h. The Fe2Ni2N NPA having nanoplate (Fig. 5f) like morphology with 500 nm in size and 20 nm in thickness and contains Fe2Ni2N phase that exhibit enhanced HER and OER activity. Moreover the Fe2Ni2N NPA//Fe2Ni2N NPA two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.65 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 10 h with 94.5% current density retained. Luo et al. [29] have fabricated NiFe-LDH as bifunctional electrocatalyst on Ni foam by one-step hydrothermal growth method. The NiFe-LDH exhibit enhanced HER (h of 210 mV for 10 mA cm2) and OER (h of 240 mV for 10 mA cm2) activity in 1.0 M NaOH. Moreover the NiFe-LDH// NiFe-LDH two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.70 V to achieve 10 mA cm2 in 1.0 M NaOH with nearly 100% Faradaic efficiency and exhibit reasonable stability for 10 h with small potential decay. Zhu et al. [97] have fabricated Ni2.5Co0.5Fe-LDH as bifunctional electrocatalyst on Ni foam by electrodeposition method. The Ni2.5Co0.5Fe-LDH exhibit

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enhanced HER (h of 200 mV for 40 mA cm2 with Tafel slope of 78 mV/dec) and OER (potential of 1.54 V for 40 mA cm2 with Tafel slope of 52 mV/dec) activity in 1.0 M KOH. The ultrathin hexagonal crystalline Ni2.5Co0.5Fe-LDH sheets having 100 nm of lateral size, contains Ni2þ, Co2þ, Fe3þ, and having 1.90:0.62:1.00 atomic ratio of Ni:Co:Fe with monolithic structure, low activation energy of 21.0 kJ/ mol, moderate Co dopant (formation of CoOOH during oxidation on Ni2.5Co0.5Fe-LDH) for enhanced conductivity, and having superaerophobic surface along with macropores for facilitate rapid gas evolution that exhibit enhanced HER and OER activity. Moreover the Ni2.5Co0.5Fe-LDH//Ni2.5Co0.5Fe-LDH two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.62 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability with 87% retained current density after 10, 000 s electrolysis. Xiao et al. [98] have fabricated NiFe/NiCo2O4 as bifunctional electrocatalyst on Ni foam by hydrothermal and electrodeposition method. The NiFe/ NiCo2O4 exhibit enhanced HER (h of 105 mV for 10 mA cm2 with Tafel slope of 88 mV/dec) and OER (h of 340 mV for 1200 mA cm2 with Tafel slope of 38.8 mV/dec) activity in 1.0 M KOH. The crystalline 3D hierarchical porous NiFe/NiCo2O4 composed of three layers that are NiFe-(oxy)hydroxide mesoporous nanosheet of ~5 nm (topmost layer), vertically oriented macroporous NiCo2O4 nanoflake of ~500 nm (intermediate layer), and supermacroporous Ni foam of ~500 mm (bottom layer). The 3D structure for accelerated gas dissipation with more exposed electroactive surface along with enhanced charge transfer by conductive Ni foam of NiFe/NiCo2O4 exhibit enhanced HER and OER activity. Moreover the NiFe/ NiCo2O4//NiFe/NiCo2O4 two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.67 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability at 20 mA cm2 for 10 h. Zhang et al. [99] have fabricated Fe10Co40Ni40P as bifunctional electrocatalyst by electrodeposition of FeCoNi-LDH nanosheet on Ni foam followed by phosphidization with NaH2PO2 at 300  C. The Fe10Co40Ni40P exhibit enhanced HER (h of 68 mV for 10 mA cm2) and OER (h of 250 mV for 10 mA cm2) activity in 1.0 M KOH. The 3D structured Fe10Co40Ni40P with 18:47:25 mol percent of Fe:Co:Ni exhibit enhanced HER and OER activity. Moreover the Fe10Co40Ni40P//Fe10Co40Ni40P two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.57 V to achieve 10 mA cm2 in 1.0 M KOH. Yan et al. [100] have fabricated Fe-P (nanotubes) NTs as bifunctional electrocatalyst by growing ZnO nanowires on carbon cloth using wet chemical process followed by converting ZnO nanowires into Fe(OH)3 NTs using sacrificial template accelerated hydrolysis method followed by phosphidation with NaH2PO2 at 300  C. The Fe-P NTs exhibit enhanced HER (h of 120 mV for 10 mA cm2 with Tafel slope of 59.5 mV/dec) and OER (h of 288 mV for 10 mA cm2 with Tafel slope of 43 mV/dec) activity in 1.0 M KOH. Karger et al. [101] have fabricated CoFe2O4 nanoparticles (NPs) as bifunctional electrocatalyst by hydrothermal method and deposit the CoFe2O4 NPs on carbon fiber paper by spin coating. The CoFe2O4 NPs exhibit enhanced HER (h of 356 mV for 10 mA cm2) and OER (h of 378 mV for 10 mA cm2 with Tafel slope of 73 mV/ dec) activity in 1.0 M NaOH. For OER, the CoFe2O4 NPs exhibit reasonable stability for 40 h and retained chemical state and morphology after 15 h. The CoFe2O4 NPs having nanoparticles with average size of 69 nm and contains CoFe2O4 and graphite phases that exhibit enhanced HER and OER activity. Luo et al. [102] have fabricated NiFe nanosheet (NS) as bifunctional electrocatalyst on Ni foam by electrodeposition method. The NiFe NS exhibit enhanced HER (h of 139 mV for 10 mA cm2 with Tafel slope of 112 mV/dec) and OER (h of 264 mV for 20 mA cm2 with Tafel slope of 51 mV/ dec) activity in 1.0 M KOH. The NiFe NS is crystalline having nanosheet (Fig. 5g) like morphology and contains NiFe alloy phase along with Ni0, Ni2þ and Fe oxide (due to surface oxidation) that exhibit enhanced HER and OER activity. The enhanced OER activity

is also involved by the formation of possibly NiOOH/FeOOH on the surface of NiFe NS during OER in 1.0 M KOH. Moreover the NiFe NS// NiFe NS two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.64 V to achieve 10 mA cm2 in 1.0 M KOH with nearly 100% Faradaic efficiency and exhibit reasonable stability for 14 h with negligible potential decay. Tan et al. [103] have fabricated nanoporous-(Co0.52Fe0.48)2P as bifunctional electrocatalyst by metallurgical alloy design followed by electrochemical etching. The nanoporous-(Co0.52Fe0.48)2P (Fig. 5h) exhibit enhanced HER (h of 79 mV for 10 mA cm2 with Tafel slope of 40 mV/dec) and OER (h of 270 mV for 10 mA cm2 with Tafel slope of 30 mV/dec) activity in 1.0 M KOH. The nanoporous-(Co0.52Fe0.48)2P is crystalline and having orthorhombic phosphide structure with optimal Co/Fe ratio to achieve Gibbs free energy of H* absorption jDGH*j close to zero and comparable with 0.09 eV of jDGH*j for Pt that exhibit enhanced HER and OER activity. Moreover the nanoporous(Co0.52Fe0.48)2P//nanoporous-(Co0.52Fe0.48)2P two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.53 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit reasonable stability for 50 h at static potential of 1.55 V with negligible current density decay. The potential for overall water splitting of the various kinds of reported Fe-based//Fe-based two-electrode earth abundant bifunctional electrocatalysts requires to achieve 10 mA cm2 in 1.0 M KOH are in the following order: 1.53 V [103] (nanoporous(Co0.52Fe0.48)2P//nanoporous-(Co0.52Fe0.48)2P) < 1.57 V [99] (Fe10Co40Ni40P//Fe10Co40Ni40P) < 1.58 V [93] (Ni0.9Fe0.1/NC// Ni0.9Fe0.1/NC) < 1.62 V [97] (Ni2.5Co0.5Fe-LDH//Ni2.5Co0.5FeLDH) < 1.64 V [102] (NiFe NS//NiFe NS) < 1.65 V [32] (Fe2Ni2N NPA// Fe2Ni2N NPA) < 1.67 V [98] (NiFe/NiCo2O4//NiFe/NiCo2O4) z 1.67 V [95] (EG/Co0.85Se/NiFe-LDH//EG/Co0.85Se/NiFe-LDH) that are far better than 1.71 V [48] of Pt/C//RuO2. For two-electrode bifuntional water electrolyzer, nanoporous-(Co0.52Fe0.48)2P//nanoporous(Co0.52Fe0.48)2P [103] exhibit much lower potential of 1.53 V and the much higher catalytic activity are due to the following factors: (a) The nanoporous-(Co0.52Fe0.48)2P having nanoporous morphology and the nanoporous morphology can provide enhanced electroactive sites (for HER and OER), and (b) The nanoporous(Co0.52Fe0.48)2P is crystalline and having orthorhombic phosphide structure with optimal Co/Fe ratio to achieve Gibbs free energy of H* absorption jDGH*j close to zero and comparable with 0.09 eV of jDGH*j for Pt. 2.12. Mn/Zn/Ti based earth abundant HER and OER bifunctional electrocatalyst Liang et al. [104] have fabricated Zn0.76Co0.24S/CoS2 nanowire (NW) as bifunctional electrocatalyst by preparing ZnCo2O4 NW precursor on Ti mesh (using hydrothermal and thermal method) followed by sulfidization with S powder at 500  C. The Zn0.76Co0.24S/CoS2 NW exhibit enhanced HER (h of 238 mV for 20 mA cm2 with Tafel slope of 164 mV/dec) and OER (h of 330 mV for 20 mA cm2 with Tafel slope of 79 mV/dec) activity in 1.0 M KOH and exhibit reasonable stability for 10 h. The crystalline 3D porous Zn0.76Co0.24S/CoS2 NW having nanowire like morphology with nanowire of ~7 mm in length and contains Zn0.76Co0.24S and CoS2 phases that exhibit enhanced HER and OER activity. The enhanced OER activity is also involved by the formation of Co(OH)2 on the surface of Zn0.76Co0.24S/CoS2 NW during OER in 1.0 M KOH. Moreover the Zn0.76Co0.24S/CoS2 NW//Zn0.76Co0.24S/CoS2 NW twoelectrode bifuntional water electrolyzer requires reasonable voltage of 1.66 V to achieve 10 mA cm2 in 1.0 M KOH. Li et al. [105] have fabricated CoMnO@CN superlattice as bifunctional electrocatalyst on Ni foam by solvent evaporation induced self assembly method. The CoMnO@CN superlattice exhibit enhanced HER (h of 71 mV for 20 mA cm2 with Tafel slope of 152 mV/dec) and OER

M.I. Jamesh / Journal of Power Sources 333 (2016) 213e236

(potential of 1.65 V for 308 mA cm2 with Tafel slope of 97 mV/dec) activity in 1.0 M KOH. The CoMnO@CN superlattice composed of Ndoped C framework continuously coated on Co-Mn oxide nanoparticles having uniform size with 1.4:1 atomic ratio of Co:Mn and the CoMnO nanoparticles play a key role for OER activity while CN framework play a key role for HER activity and the ordered superlattice structure provides enhanced reactive sites with efficient charge transfer and stable structural integrity that exhibit enhanced HER and OER activity. Moreover the CoMnO@CN superlattice//CoMnO@CN superlattice two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.7 V and 1.8 V to achieve 54 and 108 mA cm2, respectively in 1.0 M KOH and exhibit reasonable stability. Zhang et al. [106] have fabricated TiN@Ni3N nanowire (NW) as bifunctional electrocatalyst by fabrication of TiN NW on Ti foil (using hydrothermal method for TiO2 NW formation followed by annealing in NH3 at 800  C for TiN NW formation) followed by fabrication of Ni3N on TiN-Ti foil (using chemical bath deposition method and annealing in air at 350  C for NiO NW formation followed by annealing in NH3 at 450  C for Ni3N NW formation). The TiN@Ni3N NW exhibit enhanced HER (h of 21, 34, 56, and 62 mV for 10, 20, 60, and 80 mA cm2, respectively with Tafel slope of 42.1 mV/dec) and OER (potential of 1.58 and 1.65 V for 10 and 50 mA cm2, respectively with Tafel slope of 93.7 mV/dec) activity in 1.0 M KOH and exhibit nearly 100% faradic efficiency. The TiN@Ni3N NW having myriophyllum-like morphology with ~20 mm in length and ~150 nm in diameter and composed of Ni3N nanoparticles dispersed on polycrystalline TiN NW with 50e200 nm in diameter and contains TiN and Ni3N phases and further contains Niþ, Ti-O, Ti-N, Ti-N-O, and Ni(OH)2 (due to oxidation in air) that exhibit enhanced HER and OER activity. The enhanced HER activity is also involved by the formation of NiO/Ni while the enhanced OER activity is also involved by the formation of NiOOH on the surface of TiN@Ni3N NW during HER and OER, respectively in 1.0 M KOH. Moreover the TiN@Ni3N NW//TiN@Ni3N NW two-electrode bifuntional water electrolyzer requires reasonable voltage of 1.64, 1.69, and 1.89 V to achieve 10, 20, and 100 mA cm2, respectively in 1.0 M KOH and exhibit reasonable stability for 16 h with current density retention of 63.8% at 1.62 V. Danilov et al. [107] have fabricated Fe/ TiO2 as bifunctional electrocatalyst on mild steel by electrodeposition using methane sulfonate iron plating bath containing colloidal TiO2 particles. The Fe/TiO2 exhibit enhanced HER (h of 470 mV for 250 mA cm2 with Tafel slope of 106 mV/dec) and OER (h of 480 mV for 250 mA cm2 with Tafel slope of 116 mV/dec) activity in 1.0 M NaOH. The Fe/TiO2 having flake-like TiO2 particles agglomerated on Fe matric and the increase in Ti content exhibit increase in HER and OER activity. 2.13. Metal free earth abundant HER and OER bifunctional electrocatalyst Lai et al. [108] have fabricated O, N, and P tri-doped porous graphite C (ONPPGC) as metal free bifunctional electrocatalyst by polymerization of aniline on oxidized carbon cloth (OCC) using phytic acid followed by hydrothermal treatment and pyrolysis. The ONPPGC/OCC exhibit enhanced HER (h of 446 mV for 10 mA cm2 with Tafel slope of 154 mV/dec) and OER (h of 410 mV for 10 mA cm2 with Tafel slope of 83 mV/dec) activity in 1.0 M KOH. The ONPPGC/OCC composed of porous layer (O, N, and P tri-doped porous graphite C of 5 nm in thickness) on OCC with C, O, N, and P atomic percent of 82.8, 16.4, 0.46, and 0.32%, respectively and contains graphitic C phase and further contains graphitic, pyrrolic, and pyridinic N along with PeC, PeC bonds and eCeO, eC]O, and eCOOR functional groups and having BET surface area of 94 m2/g and pore volume of 0.071 cm3/g that exhibit enhanced HER and OER activity. Moreover the ONPPGC/OCC//ONPPGC/OCC two-

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electrode bifuntional water electrolyzer requires reasonable voltage of 1.66 V to achieve 10 mA cm2 in 1.0 M KOH and exhibit nearly 100% Faradaic efficiency and exhibit reasonable stability for 10 h with 10 mV potential decay. 2.14. Comparison of h for HER and OER of earth abundant bifunctional electrocatalyst Comparison of h to achieve 10 mA cm2 for HER of various kinds of reported earth abundant bifunctional electrocatalysts with Pt/C in 1.0 M KOH alkaline electrolyte is presented in Table 1. For HER in 1.0 M KOH to achieve 10 mA cm2, TiN@Ni3N NW (21 mV) [106] and Porous MoO2 NS (25 mV) [83] exhibit remarkably much lower h and those are even 9 mV and 5 mV respectively lower than Pt/C (30 mV) [80]. For HER in 1.0 M KOH to achieve 10 mA cm2, TiN@Ni3N NW [106] exhibit remarkably much lower h of 21 mV and the remarkably much higher catalytic activity are due to the following factors: (a) The TiN@Ni3N NW having myriophyllum-like morphology with ~20 mm in length and ~150 nm in diameter and composed of Ni3N nanoparticles dispersed on polycrystalline TiN NW with 50e200 nm in diameter and contains TiN and Ni3N phases and further contains Niþ, Ti-O, Ti-N, Ti-N-O, and Ni(OH)2 (due to oxidation in air), and (b) The HER activity of TiN@Ni3N NW is further enhanced by the in-situ formation of NiO/Ni on the surface of TiN@Ni3N NW during HER in 1.0 M KOH. For HER in 1.0 M KOH to achieve 10 mA cm2, Porous MoO2 NS [83] exhibit remarkably much lower h of 25 mV and the remarkably much higher catalytic activity are due to the following factors: (a) Porous MoO2 NS have porous nanosheet like morphology and contains MoO2 phase, and (b) the Porous MoO2 NS (Cdl: 422 mF cm2) has higher ECSA. For HER in 1.0 M KOH to achieve 10 mA cm2, CoP-MNA (54 mV) [53], h-NiSx (60 mV) [31], Co0.13Ni0.87Se2 (64 mV) [88], Fe10Co40Ni40P (68 mV) [99], CoP NW (72 mV) [54], nanoporous-(Co0.52Fe0.48)2P (79 mV) [103], NiP (80 mV) [40], CoPh/NG (83 mV) [61], Ni0.9Fe0.1/NC (85 mV) [93], CoP2/RGO (88 mV) [59], NiMo HNRs (92 mV) [82], Ni-P (93 mV) [44], Co-P (94 mV) [49], CoP (95 mV) [52], NiSe2 (96 mV) [89], NiSe NW (96 mV) [30], and Ni2P/Ni (98 mV) [38] exhibit much lower h and those are even lesser than 100 mV and even only about 24e68 mV higher than Pt/C (30 mV) [80]. Comparison of h to achieve 10 mA cm2 for OER of various kinds of reported earth abundant bifunctional electrocatalysts with RuO2 in 1.0 M KOH alkaline electrolyte is presented in Table 2. For OER in 1.0 M KOH to achieve 10 mA cm2, h-NiSx (180 mV) [31], CP@Ni-P (190 mV) [42], and Ni2P/Ni (200 mV) [38] exhibit remarkably much lower h and those are even 110 mV, 100 mV, and 90 mV, respectively lower than RuO2 (290 mV) [40]. For OER in 1.0 M KOH to achieve 10 mA cm2, h-NiSx [31] exhibit remarkably much lower h of 180 mV and the remarkably much higher catalytic activity are due to the following factors: (a) The h-NiSx is porous 3D hierarchically morphology contains NiS2 and NiS phases with molar percentage of ~20% and 80% respectively and contains large mesopores and numerous micropores and the microparticle of NiSx is 4e10 mm in size and the 3D porous structure can provide enhanced electroactive sites (OER) and can facilitate the gas evolution (O2), and (b) The OER activity of h-NiSx is further enhanced by the in-situ formation of NiO on the surface of Ni-S during OER in 1.0 M KOH. For OER in 1.0 M KOH to achieve 10 mA cm2, CP@Ni-P [42] exhibit remarkably much lower h of 190 mV and the remarkably much higher catalytic activity are due to the following factors: (a) The CP@Ni-P catalyst contains (1100) and (2110) crystal planes of hexagonal single-crystalline Ni5P4 nanosheet, single-crystalline NiP2 and Ni2P, and (b) The OER activity of CP@Ni-P is further enhanced by the in-situ formation of Ni-P/NiO/Ni(OH)x by the transformation of Ni-P into NiO wrapped with Ni(OH)x layer during OER in 1.0 M KOH. For OER in 1.0 M KOH to achieve 10 mA cm2, Ni2P/Ni [38] exhibit remarkably

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Table 1 Comparison of overpotential (hHER) to achieve 10 mA cm2 for hydrogen evolution reaction (HER) of various kinds of reported earth abundant bifunctional electrocatalysts with Pt/C in 1.0 M KOH alkaline electrolyte. Table 1 (Part 1)

Table 1 (Part 2)

Table 1 (Part 3)

Bifuntional Catalyst

E

hHER (mV)

Ref.

Bifuntional Catalyst

E

hHER (mV)

Ref.

Bifuntional Catalyst

E

hHER (mV)

Ref.

*

NF Ti-f NF NF NF Ti-pl NF Ti-m FS NF CP NF GC Ti-m CF* CF CC Ti-pl NF

30 21 25 54 60 64 68 72 79 80 83 85 88 92 93 94 95 96 96

[40] [106] [83] [53] [31] [88] [99] [54] [103] [40] [61] [93] [59] [82] [44] [49] [52] [89] [30]

Ni2P/Ni Ni3Se2 NiFe/NiCo2O4 Ni@C-400 NSs MoS2/Ni3S2 hs CoP NS/C CP@NieP FeeP NTs a-CoSe Ni8P3 Co2P NWs NieCoeS NS Ni5P4 CoP/rGO-400 CoeP/NC CoP/G Co3O4 NCs Ni3FeN-NPs NieCoeP np

NF CF NF NF NF GC CFP CC Ti-m NF GC CF Ni-f GC GC GC CFP GC GC

98 100 105 110 ~110 111 117 120 121 130 ~139 140 150 150 154 154 155 158 180

[38] [91] [98] [70] [84] [51] [42] [100] [90] [47] [37] [78] [41] [48] [62] [60] [68] [94] [50]

Fe2Ni2N NPA CP/CTs/CoeS Co-PP CoeP np Ni3S2 Ni3Se2 Nfor Co@NC NiCo2S4 NW Ni2P Ni9S8 Co-CoOx/NC EG/Co0.85Se/NiFe-LDH PNC/Co CoP/CoxPO4 ONPPGC/OCCMF

NF CP Au GC NF NF CP NF GC NF GC EG GC Au OCC

180 190 196 200 200 203 210 210 220 230 232 260 298 ~380 446

[32] [67] [57] [50] [73] [86] [66] [74] [39] [47] [64] [95] [65] [56] [108]

Pt/C TiN@Ni3N NW Porous MoO2 NS CoP-MNA h-NiSx Co0.13Ni0.87Se2 Fe10Co40Ni40P CoP NW nanoporous-(Co0.52Fe0.48)2P NiP CoPh/NG Ni0.9Fe0.1/NC CoP2/RGO NiMo HNRs NieP CoeP CoP NiSe2 NiSe NW

hHER ¼ Overpotential to achieve 10 mA cm2 for HER in 1 M KOH; E ¼ Electrode; * ¼ Non-earth-abundant catalyst; rGO/RGO ¼ Reduced Graphene Oxide; G ¼ Graphene; x ¼ Number; EG ¼ Electrochemically exfoliated graphene foil; LDH ¼ Layered Double Hydroxide; MF ¼ Metal-Free; NF ¼ Nickel Foam; Ni-f ¼ Ni foil; FS ¼ Free-Standing; GC ¼ Glassy-Carbon; Ti-m ¼ Ti mesh; Ti-pl ¼ Ti plate; Ti-f ¼ Ti foil; NWs ¼ Nano-Wires; NS ¼ Nano Sheet; NCs ¼ Nano Crystals; Nfor: Nano Forest; NPA ¼ Nano Plate Array; np/ NP ¼ Nano Particle; NTs ¼ Nano Tubes; MNA ¼ Mesoporous Nanorod Arrays; CF]Cu Foam; CF* ¼ Cu Foil; CC ¼ Carbon Cloth; CFP/CP ¼ Carbon Fiber Paper; CP ¼ Carbon Paper; CTs: Carbon Tubes; NC]N doped Nano Carbon; PGC ¼ Porous Graphite Carbon; OCC ¼ Oxidized Carbon Cloth; hs ¼ Heterostructures; P ¼ Porous; a ¼ amorphous; h ¼ hollow; HNRs ¼ Hollow Nanorod Array; PP¼Phosphide/Phosphate.

Table 2 Comparison of overpotential (hOER) to achieve 10 mA cm2 for oxygen evolution reaction (OER) of various kinds of reported earth abundant bifunctional electrocatalysts with RuO2 in 1.0 M KOH alkaline electrolyte. Table 2 (Part 1)

Table 2 (Part 2)

Table 2 (Part 3)

Bifuntional Catalyst

E

hOER (mV)

Ref.

Bifuntional Catalyst

E

hOER (mV)

Ref.

Bifuntional Catalyst

E

hOER (mV)

Ref.

*

NF NF CFP NF GC NF NF NF NF GC GC NF NF CP FS NF GC GC CC

290 180 190 200 210 217 218 239 250 250 260 260 260 262 270 270 277 280 281

[40] [31] [42] [38] [96] [73] [84] [86] [99] [87] [37] [64] [74] [61] [103] [93] [51] [94] [52]

FeeP NTs Ni5P4 Ni2P CoP/G a-CoSe CoP2/RGO CP/CTs/CoeS NiP CoP/CoxPO4 Ni@C-400 NSs CoP NW NiMo HNRs CoeP/NC NixPy-325 CoP NR Co-PP CoP/rGO-400 NieP CoeP

CC Ni-f GC GC Ti-m GC CP NF Au NF Ti-m Ti-m GC CFP GC Au GC CF* CF

288 290 290 292 292 300 306 309 ~310 310 310 310 319 320 320 340 340 344 345

[100] [41] [39] [60] [90] [59] [67] [40] [56] [70] [54] [82] [62] [43] [55] [57] [48] [44] [49]

TiN@Ni3N NW NieCoeP np PNC/Co Co3O4 NCs Co@NC ONPPGC/OCCMF CoeP np

Ti-f GC GC CFP CP OCC GC

~350 360 ~370 380 400 410 430

[106] [50] [65] [68] [66] [108] [50]

RuO2 h-NiSx CP@NieP Ni2P/Ni Ni2/3Fe1/3-rGO Ni3S2 MoS2/Ni3S2 hs Ni3Se2 Nfor Fe10Co40Ni40P NiSe2 NCs Co2P NWs Co-CoOx/NC NiCo2S4 NW CoPh/NG nanoporous-(Co0.52Fe0.48)2P Ni0.9Fe0.1/NC CoP NS/C Ni3FeN-NPs CoP

hOER ¼ Overpotential to achieve 10 mA cm2 for OER in 1 M KOH; E ¼ Electrode; * ¼ Non-earth-abundant catalyst; rGO/RGO ¼ Reduced Graphene Oxide; G ¼ Graphene; x/ y ¼ Number; MF ¼ Metal-Free; NF ¼ Nickel Foam; Ni-f ¼ Ni foil; FS ¼ Free-Standing; GC ¼ Glassy-Carbon; Ti-m ¼ Ti mesh; Ti-f ¼ Ti foil; NWs ¼ Nano-Wires; NR ¼ Nano Rod; NS ¼ Nano Sheet; NCs ¼ Nano Crystals; NPA ¼ Nano Plate Array; np/NP ¼ Nano Particle; NTs ¼ Nano Tubes; CF ¼ Cu Foam; CF* ¼ Cu Foil; CC ¼ Carbon Cloth; CFP/CP ¼ Carbon Fiber Paper; CP ¼ Carbon Paper; CTs: Carbon Tubes; NC ¼ N doped Nano Carbon; PGC ¼ Porous Graphite Carbon; OCC ¼ Oxidized Carbon Cloth; hs ¼ Heterostructures; P ¼ Porous; a ¼ amorphous; h ¼ hollow; HNRs ¼ Hollow Nanorod Array; PP ¼ Phosphide/Phosphate.

much lower h of 200 mV and the remarkably much higher catalytic activity are due to the following factors: (a) The Ni2P/Ni catalyst is urchin-like Ni2P having porous (primary macropores of 100e350 mm and complementary macropores of ~10 mm size) 3D hierarchically superstructures contains Ni2P and Ni phase and the 3D porous structure can provide enhanced electroactive sites (OER) and can facilitate the gas evolution (O2), and (b) The OER activity is further enhanced by the in-situ formation of NiO/NiOOH predominantly NiO on the surface of Ni2P/Ni during OER in 1.0 M KOH. For

OER in 1.0 M KOH to achieve 10 mA cm2, Ni2/3Fe1/3-rGO (210 mV) [96], Ni3S2 (217 mV) [73], MoS2/Ni3S2 hs (218 mV) [84], Ni3Se2 Nfor (239 mV) [86], Fe10Co40Ni40P (250 mV) [99], NiSe2 NCs (250 mV) [87], Co2P NWs (260 mV) [37], Co-CoOx/NC (260 mV) [64], NiCo2S4 NW (260 mV) [74], CoPh/NG (262 mV) [61], nanoporous(Co0.52Fe0.48)2P (270 mV) [103], Ni0.9Fe0.1/NC (270 mV) [93], CoP NS/ C (277 mV) [51], Ni3FeN-NPs (280 mV) [94], CoP (281 mV) [52], and Fe-P NTs (288 mV) [100] exhibit remarkably lower h and those are even about 80 to 2 mV lower than RuO2 (290 mV) [40]. For OER in

M.I. Jamesh / Journal of Power Sources 333 (2016) 213e236

1.0 M KOH to achieve 10 mA cm2, Ni5P4 (290 mV) [41], and Ni2P (290 mV) [39] exhibit remarkably lower h those are even equal to RuO2 (290 mV) [40]. For OER in 1.0 M KOH to achieve 10 mA cm2, CoP/G (292 mV) [60], a-CoSe (292 mV) [90], CoP2/RGO (300 mV) [59], CP/CTs/Co-S (306 mV) [67], NiP (309 mV) [40], CoP/CoxPO4 (~310 mV) [56], Ni@C-400 NSs (310 mV) [70], CoP NW (310 mV) [54], NiMo HNRs (310 mV) [82], Co-P/NC (319 mV) [62], NixPy-325 (320 mV) [43], CoP NR (320 mV) [55], Co-PP (340 mV) [57], CoP/ rGO-400 (340 mV) [48], Ni-P (344 mV) [44], Co-P (345 mV) [49], TiN@Ni3N NW (~350 mV) [106], Ni-Co-P np (360 mV) [50], PNC/Co (~370 mV) [65], and Co3O4 NCs (380 mV) [68] exhibit much lower h and those are even only about 2e90 mV higher than RuO2 (290 mV) [40]. 2.15. Comparison of potential for overall water splitting of earth abundant bifunctional electrocatalyst Comparison of potential to achieve 10 mA cm2 for overall water splitting of various kinds of reported earth abundant bifunctional electrocatalysts with Pt/C//RuO2 in 1.0 M KOH alkaline electrolyte is presented in Table 3. For two-electrode bifuntional water electrolyzer (overall water splitting) in 1.0 M KOH to achieve 10 mA cm2, Co2P NWs//Co2P NWs (1.44 V) [37], h-NiSx//h-NiSx (1.47 V) [31], and Ni2P/Ni//Ni2P/Ni (1.49 V) [38] exhibit remarkably much lower potential and those are even 270 mV, 240 mV and 220 mV, respectively lower than Pt/C//RuO2 (1.71 V) [48] and those are even about 360 to 310 mV lower than commercial water electrolyzer (1.8e2.0 V) [6]. For two-electrode bifuntional water electrolyzer, the remarkably much lower potential of 1.44 V (remarkably much higher catalytic activity) exhibit by Co2P NWs//Co2P NWs [37] (fabricated by energy saving, toxic PH3 emission free, and rapid

229

micro-wave assisted synthetic method) are due to the following factors: (a) Co2P exhibits metallic characteristic with more conductivity than other allotropes, (b) The crystalline Co2P NWs with the growth orientation of (112) and (020) lattice planes have nanowire like morphology with about 5 nm in diameter (Fig. 3f), and (c) The OER activity is further enhanced by the in-situ formation of conductive Co oxo/hydroxide species on the surface of Co2P NWs during OER in 1.0 M KOH. For two-electrode bifuntional water electrolyzer, the remarkably much lower potential of 1.47 V (remarkably much higher catalytic activity) exhibit by h-NiSx//hNiSx [31] are due to the following factors: (a) The h-NiSx is porous 3D hierarchically morphology contains NiS2 and NiS phases with molar percentage of ~20% and 80% respectively and contains large mesopores and numerous micropores and the microparticle of NiSx is 4e10 mm in size and the 3D porous structure can provide enhanced electroactive sites (for HER and OER) and can facilitate the gas evolution (H2 and O2), and (b) The OER activity of h-NiSx is further enhanced by the in-situ formation of NiO on the surface of Ni-S during OER in 1.0 M KOH. For two-electrode bifuntional water electrolyzer, the remarkably much lower potential of 1.49 V (remarkably much higher catalytic activity) exhibit by Ni2P/Ni// Ni2P/Ni [38] are due to the following factors: (a) The Ni2P/Ni catalyst is urchin-like Ni2P having porous (primary macropores of 100e350 mm and complementary macropores of ~10 mm size) 3D hierarchically superstructures contains Ni2P and Ni phase and the 3D porous structure can provide enhanced electroactive sites (for HER and OER) and can facilitate the gas evolution (H2 and O2), and (b) The OER activity is further enhanced by the in-situ formation of NiO/NiOOH predominantly NiO on the surface of Ni2P/Ni during OER in 1.0 M KOH. For two-electrode bifuntional water electrolyzer in 1.0 M KOH to achieve 10 mA cm2, nanoporous-(Co0.52Fe0.48)2P//

Table 3 Comparison of potential to achieve 10 mA cm2 for overall water splitting of various kinds of reported earth abundant bifunctional electrocatalysts with Pt/C//RuO2 in 1.0 M KOH alkaline electrolyte. Table 3 (Part 1)

Table 3 (Part 2)

Table 3 (Part 3)

Bifuntional Catalyst (work as anode as well as cathode)

E

Vws

Ref.

Bifuntional Catalyst (work as anode as well as cathode)

E

*

Pt/C//RuO2 Co2P NWs

NF NF

1.71 1.44

[48] [37]

Ni2.5Co0.5Fe-LDH Co0.13Ni0.87Se2

h-NiSx

NF

1.47

[31]

CP@NieP

NF 1.62 Ti- 1.62 pl CFP 1.63

Ni2P/Ni nanoporous-(Co0.52Fe0.48)2P Porous MoO2 NS CoP NS

1.49 1.53 1.53 1.54M

[38] [103] [83] [51]

Ni2P CoP/G NiCo2S4 NW NiSe NW

NF NF NF NF

CoP2/RGO MoS2/Ni3S2 hs NixPy-325 Fe10Co40Ni40P CoPh/NG Ni0.9Fe0.1/NC

NF FS NF CP/ Ti-fm GC NF CFP NF CP NF

1.56 1.56 1.57 1.57 1.58 1.58

[59] [84] [43] [99] [61] [93]

Porous NieP PNC/Co TiN@Ni3N NW NiS Ni@C-400 NSs CoP NW

CoP NR NiSe2 NCs

Ti-ft GC

1.59 1.60

[55] [87]

NiFe NS NiMo HNRs

Ni3Se2 Nfor CoP Ni8P3

NF CC NF

1.61 [86] 1.61 [52] ~1.61 [47]

CoeP Fe2Ni2N NPA a-CoSe

CoP-MNA

NF

1.62

Ni3Se2

NF GC Ti-f NF NF Tim NF Tim CF NF Tim CF

[53]

Vws

Ref.

Bifuntional Catalyst (work as anode as well as cathode)

E

Vws

Ref.

[97] [88]

Ni2.3%-CoS2 NiSe2

1.66 1.66

[80] [89]

[42]

Zn0.76Co0.24S/CoS2

1.66

[104]

1.63 1.63 1.63 1.63

[39] [60] [74] [30]

ONPPGC/OCCMF NiP NieP NieCoeS NS

CC Tipl Tim OCC NF CF* CF

1.66 1.67 1.67 1.67

[108] [40] [44] [78]

1.64 1.64 1.64 1.64 1.64 1.64

[45] [65] [106] [72] [70] [54]

NiFe/NiCo2O4 EG/Co0.85Se/NiFe-LDH NiCo2S4 Ni5P4 CoP/rGO-400 CP/CTs/CoeS

NF EG CC Ni-f GC CP

1.67 1.67 1.68 ~1.7 1.7 1.74

[98] [95] [79] [41] [48] [67]

1.64 1.64

[102] NieCoeP np [82] CoeP np

GC GC

1.77DE [50] 1.86DE [50]

GC Au

1.89DE [76] ~1.91 [56]

~1.65 [49] 1.65 [32] 1.65 [90] 1.65

Co2B CoP/CoxPO4

[91]

Vws ¼ Potential in voltage for overall water splitting in 1 M KOH; E ¼ Electrode; * ¼ Non-earth-abundant and non-bifuntional catalyst; M ¼ Membrane Electrode Assembly; rGO/RGO ¼ Reduced Graphene Oxide; G ¼ Graphene; x/y-Number; EG ¼ Electrochemically exfoliated graphene foil; LDH ¼ Layered Double Hydroxide; MF ¼ Metal-Free; NF ¼ Nickel Foam; Ni-f ¼ Ni foil; FS ¼ Free-Standing; GC ¼ Glassy-Carbon; Ti-m ¼ Ti mesh; Ti-pl ¼ Ti plate; Ti-f ¼ Ti foil; Ti-fm ¼ Ti foam; Ti-ft ¼ Ti felt; NWs ¼ Nano-Wires; NR ¼ Nano Rod; NS ¼ Nano Sheet; NCs ¼ Nano Crystals; Nfor: Nano Forest; NPA ¼ Nano Plate Array; np ¼ Nano Particle; MNA ¼ Mesoporous Nanorod Arrays; CF ¼ Cu Foam; CF* ¼ Cu Foil; CC ¼ Carbon Cloth; CFP/CP ¼ Carbon Fiber Paper; CP ¼ Carbon Paper; CTs: Carbon Tubes; NC ¼ N doped Nano Carbon; PGC ¼ Porous Graphite Carbon; OCC ¼ Oxidized Carbon Cloth; hs ¼ Heterostructures; P ¼ Porous; a ¼ amorphous; h ¼ hollow; HNRs ¼ Hollow Nanorod Array; DE ¼ EOEReEHER, where E is potential.

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nanoporous-(Co0.52Fe0.48)2P (1.53 V) [103], Porous MoO2 NS//Porous MoO2 NS (1.53 V) [83], CoP NS//CoP NS (1.54 VMEA) [51], CoP2/RGO// CoP2/RGO (1.56 V) [59], MoS2/Ni3S2 hs//MoS2/Ni3S2 hs (1.56 V) [84], NixPy-325//NixPy-325 (1.57 V) [43], Fe10Co40Ni40P//Fe10Co40Ni40P (1.57 V) [99], CoPh/NG//CoPh/NG (1.58 V) [61], Ni0.9Fe0.1/NC// Ni0.9Fe0.1/NC (1.58 V) [93], CoP NR//CoP NR (1.59 V) [55], and NiSe2 NCs//NiSe2 NCs (1.60 V) [87] exhibit remarkably much lower potential and those are even about 180 to 110 mV lower than Pt/C// RuO2 (1.71 V) [48] and those are even about 270 to 200 mV lower than commercial water electrolyzer (1.8e2.0 V) [6]. For twoelectrode bifuntional water electrolyzer in 1.0 M KOH to achieve 10 mA cm2, Ni3Se2 Nfor//Ni3Se2 Nfor (1.61 V) [86], CoP//CoP (1.61 V) [52], Ni8P3//Ni8P3 (~1.61 V) [47], CoP-MNA//CoP-MNA (1.62 V) [53], Ni2.5Co0.5Fe-LDH//Ni2.5Co0.5Fe-LDH (1.62 V) [97], Co0.13Ni0.87Se2//Co0.13Ni0.87Se2 (1.62 V) [88], CP@Ni-P//CP@Ni-P (1.63 V) [42], Ni2P//Ni2P (1.63 V) [39], CoP/G//CoP/G (1.63 V) [60], NiCo2S4 NW//NiCo2S4 NW (1.63 V) [74], NiSe NW//NiSe NW (1.63 V) [30], Porous Ni-P//Porous Ni-P (1.64 V) [45], PNC/Co//PNC/Co (1.64 V) [65], TiN@Ni3N NW//TiN@Ni3N NW (1.64 V) [106], NiS//NiS (1.64 V) [72], Ni@C-400 NSs//Ni@C-400 NSs (1.64 V) [72], CoP NW// CoP NW (1.64 V) [54], NiFe NS//NiFe NS (1.64 V) [102], NiMo HNRs// NiMo HNRs (1.64 V) [82], Co-P//Co-P (~1.65 V) [49], Fe2Ni2N NPA// Fe2Ni2N NPA (1.65 V) [32], a-CoSe//a-CoSe (1.65 V) [90], Ni3Se2// Ni3Se2 (1.65 V) [91], Ni2.3%-CoS2//Ni2.3%-CoS2 (1.66 V) [80], NiSe2// NiSe2 (1.66 V) [89], Zn0.76Co0.24S/CoS2//Zn0.76Co0.24S/CoS2 (1.66 V) [104], ONPPGC/OCCMF//ONPPGC/OCCMF (1.66 V) [108], NiP//NiP (1.67 V) [40], Ni-P//Ni-P (1.67 V) [44], Ni-Co-S NS//Ni-Co-S NS (1.67 V) [78], NiFe/NiCo2O4//NiFe/NiCo2O4 (1.67 V) [98], EG/ Co0.85Se/NiFe-LDH//EG/Co0.85Se/NiFe-LDH (1.67 V) [95], NiCo2S4// NiCo2S4 (1.68 V) [79], Ni5P4//Ni5P4 (~1.7 V) [41], and CoP/rGO-400// CoP/rGO-400 (1.7 V) [48] exhibit remarkably lower potential and those are even about 100 to10 mV lower than Pt/C//RuO2 (1.71 V) [48] and those are even about 190 to 100 mV lower than commercial water electrolyzer (1.8e2.0 V) [6]. For two-electrode bifuntional water electrolyzer in 1.0 M KOH to achieve 10 mA cm2, CP/CTs/Co-S//CP/CTs/Co-S (1.74 V) [67], and Ni-Co-P np//Ni-Co-P np (1.77DE V) [50] exhibit much lower potential and those are even only about 30e60 mV higher than Pt/C//RuO2 (1.71 V) [48] and those are even about 60 to 30 mV lower than commercial water electrolyzer (1.8e2.0 V) [6]. For two-electrode bifuntional water electrolyzer in 1.0 M KOH to achieve 10 mA cm2, Co-P np//Co-P np (1.86DE V) [50], Co2B//Co2B (1.89DE V) [76], and CoP/CoxPO4//CoP/CoxPO4 (~1.91 V) [56] exhibit lower potential and those are only about 150e200 mV higher than Pt/C// RuO2 (1.71 V) [48] and those are even well comparable with commercial water electrolyzer (1.8e2.0 V) [6]. 2.16. Stability of earth abundant bifunctional electrocatalyst for overall water splitting The long term stability of the electrode material is critically important to improve the long term stability of the water electrolyzer by reducing the maintenance and operating costs economically [6]. Comparison of long term stability for overall water splitting of various kinds of reported earth abundant bifunctional electrocatalysts with Pt//Ir/C in 1.0 M KOH alkaline electrolyte is presented in Table 4. For two-electrode bifuntional water electrolyzer (overall water splitting) in 1.0 M KOH, Porous Ni-P//Porous NiP [45] exhibit reasonable stability for 1000 h of water electrolysis with negligible potential decay at 10 and 20 mA cm2 and that is even remarkably much higher long term stability than Pt//Ir/C [32] (after only 10 h of electrolysis, Pt//Ir/C retained only 7.6% efficiency of initial current density at 1.8 V). For two-electrode bifuntional water electrolyzer, the remarkably much higher long term stability (reasonable stability for 1000 h of water electrolysis) exhibit by

Porous Ni-P//Porous Ni-P [45] are due to the following factors: (a) The porous Ni-P catalyst contains skeletons Ni2P wrapped with vertically aligned single crystalline hexagonal Ni5P4 nanosheet and NiP2 nanosheet, and the porous structure can provide enhanced electroactive sites (for HER and OER) and can facilitate the gas evolution (H2 and O2), and (b) The OER activity of Porous Ni-P is further enhanced by the in-situ formation of NiO/Ni(OH)x on the surface of Porous Ni-P during OER in 1.0 M KOH. For two-electrode bifuntional water electrolyzer in 1.0 M KOH, Ni3S2//Ni3S2 [71] exhibit reasonable stability for 150 h of water electrolysis at 1.76 V, Ni3Se2 Nfor//Ni3Se2 Nfor [86] exhibit reasonable stability for 140 h (Fig. 6a) of water electrolysis at 1.7 V, CP@Ni-P//CP@Ni-P [42] exhibit reasonable stability with 91.0% efficiency after 100 h (Fig. 6b) of water electrolysis with negligible potential decay at 10 mA cm2, and MoS2/Ni3S2 hs//MoS2/Ni3S2 hs [84] exhibit reasonable stability for 100 h of water electrolysis at higher current density of 500 mA cm2 and those are even remarkably higher stability than Pt//Ir/C [32] (7.6% efficiency at 1.8 V for 10 h). For twoelectrode bifuntional water electrolyzer in 1.0 M KOH, Co-P//Co-P [52] exhibit reasonable stability for 72 h (Fig. 6c) of electrolysis with negligible potential of ~14 mV decay, CoPh/NG//CoPh/NG [61] exhibit reasonable stability for 65 h of water electrolysis, NixPy325//NixPy-325 [43] exhibit reasonable stability for 60 h of electrolysis at 1.57 V, NiCo2S4 NW//NiCo2S4 NW [74] exhibit reasonable stability for 50 h of electrolysis with ~10% potential decay at 10 mA cm2, and nanoporous-(Co0.52Fe0.48)2P//nanoporous(Co0.52Fe0.48)2P [103] exhibit reasonable stability for 50 h (Fig. 6d) of electrolysis at 1.55 V with negligible current density decay, and Ni2P/Ni//Ni2P/Ni [38] exhibit reasonable stability for 40 h of electrolysis with negligible potential decay and those are even much higher stability than Pt//Ir/C [32] (7.6% efficiency at 1.8 V for 10 h). For two-electrode bifuntional water electrolyzer in 1.0 M KOH, CoPMNA//CoP-MNA [53] exhibit reasonable stability for 32 h of electrolysis with ~30 mV potential decay at 10 mA cm2, a-CoSe//aCoSe [90] exhibit reasonable stability for 27 h of electrolysis with ~40 mV potential decay at 10 mA cm2, Co-P//Co-P [49] exhibit reasonable stability for 24 h of electrolysis, CoP NS//CoP NS [51] exhibit reasonable stability for 24 h of electrolysis with ~10 mV potential decay at 10 mA cm2, CoP NR//CoP NR [55] exhibit reasonable stability for 24 h of electrolysis at 10 mA cm2, Co-P/ NC//Co-P/NC [62] exhibit reasonable stability for 24 h of electrolysis, Porous MoO2 NS//Porous MoO2 NS [83] exhibit reasonable stability for 24 h of electrolysis, Ni0.9Fe0.1/NC//Ni0.9Fe0.1/NC [93] exhibit reasonable stability for 24 h of electrolysis with ~30 mV potential decay at 10 mA cm2, NiSe NW//NiSe NW [30] exhibit reasonable stability for 20 h of electrolysis with negligible potential decay at 20 mA cm2, NiSe2//NiSe2 [89] exhibit reasonable stability for 16 h of electrolysis at 20 mA cm2, TiN@Ni3N NW//TiN@Ni3N NW [106] exhibit reasonable stability for 16 h of electrolysis with 63.8% efficiency at 1.62 V, Ni-P//Ni-P [40] exhibit reasonable stability for 14 h of electrolysis with 20 mV potential decay at 10 mA cm2, NiFe NS//NiFe NS [102] exhibit reasonable stability for 14 h of electrolysis with negligible potential decay at 10 mA cm2, Ni2.3%-CoS2 NW//Ni2.3%-CoS2 NW [80] exhibit reasonable stability for 12 h of electrolysis, and Ni3Se2//Ni3Se2 [91] exhibit reasonable stability for 12 h of electrolysis at 10 mA cm2 and those are even higher stability than Pt//Ir/C [32] (7.6% efficiency at 1.8 V for 10 h). For two-electrode bifuntional water electrolyzer for 10 h of electrolysis in 1.0 M KOH, Fe2Ni2N NPA//Fe2Ni2N NPA [32] exhibit reasonable stability with 94.5% efficiency at 1.8 V, Ni2P//Ni2P [39] exhibit reasonable stability with negligible potential decay, CoP/ G//CoP/G [60] exhibit reasonable stability with 76% efficiency at 1.7 V, h-NiSx//h-NiSx [31] exhibit reasonable stability, Ni-B//Ni-B [75] exhibit reasonable stability, NiCo2S4 NW//NiCo2S4 NW [79] exhibit reasonable stability with negligible potential decay, NiMo

M.I. Jamesh / Journal of Power Sources 333 (2016) 213e236

231

Table 4 Comparison of long term stability for overall water splitting of various kinds of reported earth abundant bifunctional electrocatalysts with Pt//Ir/C in 1.0 M KOH alkaline electrolyte. Bifuntional Catalystac

CA

CP

D (h)

Remark after stability test

Ref.

*

Yes NA Yes Yes NA NA NA NA Yes NA Yes NA NA NA Yes NA NA Yes NA NA NA NA Yes NA NA NA NA Yes NA Yes NA Yes NA NA NA NA NA NA NA NA Yes

NA Yes NA NA Yes Yes Yes Yes NA Yes NA Yes Yes Yes NA Yes Yes NA Yes Yes Yes Yes NA Yes Yes Yes Yes NA Yes NA Yes NA Yes Yes Yes Yes Yes Yes Yes Yes NA

10 1000 150 140 100 100 72 65 60 50 50 40 32 27 24 24 24 24 24 24 20 16 16 14 14 12 12 10 10 10 10 10 10 10 10 10 10 10 6 4 ~2.77

7.6% E at 1.8 V NPD at 10 and 20 mA cm2 RS at 1.76 V RS at 1.7 V 91% E at 10 mA cm2 RS at 500 mA cm2 ~14 mV PD RS RS at 1.57 V ~10% PD at 10 mA cm2 RS at 1.55 V NPD ~30 mV PD at 10 mA cm2 ~40 mV PD at 10 mA cm2 RS ~10 mV PD at 10 mA cm2 RS at 10 mA cm2 RS RS ~30 mV PD at 10 mA cm2 NPD at 20 mA cm2 RS at 20 mA cm2 63.8% E at 1.62 V 20 mV PD at 10 mA cm2 NPD at 10 mA cm2 RS RS at 10 mA cm2 94.5% E at 1.8 V NPD 76% E at 1.7 V RS RS NPD RS RS at 20 mA cm2 RS at 20 mA cm2 RS at 20 mA cm2 10 mV PD 80% E at 100 mA cm2 RS at 10 mA cm2 87% E

[32] [45] [71] [86] [42] [84] [52] [61] [43] [74] [103] [38] [53] [90] [49] [51] [55] [62] [83] [93] [30] [89] [106] [40] [102 [80] [91] [32] [39] [60] [31] [75] [79] [82] [88] [95] [98] [108] [37] [47] [97]

Pt//Ir/C Porous NieP Ni3S2 Ni3Se2 Nfor CP@NieP MoS2/Ni3S2 hs CoeP CoPh/NG NixPy-325 NiCo2S4 NW nanoporous-(Co0.52Fe0.48)2P Ni2P/Ni CoP-MNA a-CoSe CoeP CoP NS CoP NR CoeP/NC Porous MoO2 NS Ni0.9Fe0.1/NC NiSe NW NiSe2 TiN@Ni3N NW NieP NiFe NS Ni2.3%-CoS2 NW Ni3Se2 Fe2Ni2N NPA Ni2P CoP/G h-NiSx NieB NiCo2S4 NW NiMo HNRs Co0.13Ni0.87Se2 EG/Co0.85Se/NiFe-LDH NiFe/NiCo2O4 ONPPGC/OCCMF CoP NW Ni8P3 Ni2.5Co0.5Fe-LDH

Bifuntional Catalystac ¼ Same kind of two earth abundant electrocatalysts can function as cathode for HER and anode for OER; CA ¼ Chronoamperometry; CP ¼ Chronopotentiometry; D ¼ duration for stability test; * ¼ Non-earth-abundant and non-bifuntional catalyst; NA ¼ Not Applicable; E ¼ Efficiency; NPD ¼ Negligible Potential Decay; RS ¼ Reasonable Stability; PD ¼ Potential Decay; G ¼ Graphene; x/y ¼ Number; EG ¼ Electrochemically exfoliated graphene foil; LDH ¼ Layered Double Hydroxide; MF ¼ Metal-Free; NW ¼ Nano-Wire; NR ¼ Nano Rod; NS ¼ Nano Sheet; Nfor ¼ Nano Forest; NPA ¼ Nano Plate Array; MNA ¼ Mesoporous Nanorod Arrays; CP ¼ Carbon Fiber Paper; NC]N doped Nano Carbon; PGC ¼ Porous Graphite Carbon; OCC ¼ Oxidized Carbon Cloth; hs ¼ Heterostructures; a ¼ amorphous; h ¼ hollow; HNRs ¼ Hollow Nanorod Arrays.

HNRs//NiMo HNRs [82] exhibit reasonable stability, Co0.13Ni0.87Se2// Co0.13Ni0.87Se2 [88] exhibit reasonable stability at 20 mA cm2, EG/ Co0.85Se/NiFe-LDH//EG/Co0.85Se/NiFe-LDH [95] exhibit reasonable stability at 20 mA cm2, NiFe/NiCo2O4//NiFe/NiCo2O4 [98] exhibit reasonable stability at 20 mA cm2, and ONPPGC/OCCMF//ONPPGC/ OCCMF [108] exhibit reasonable stability with 10 mV potential decay, and those are even higher stability than Pt//Ir/C [32] (7.6% efficiency at 1.8 V for 10 h). The long term stability including corrosion stability of the electrode material is critically important to improve the long term stability of the water electrolyzer by reducing the maintenance and operating costs economically [6,109]. Nevertheless, corrosion characteristic of the bifunctional earth abundant electrocatalyst constituting the electrodes in alkaline environment, the shelf life of the electrodes and cells during active and inactive conditions have been rarely reported. The formation of corrosion products on the electrode surface during immersion in electrolyte at different immersion time intervals can alter the corrosion rate of the electrode [33,110]. Hence, the corrosion characteristic of the electrode during active condition (with water electrolysis) and inactive conditions

(without water electrolysis, i.e. only immersion) could be different because during inactive conditions, the corrosion products formed on the electrode surface could also involve in the corrosion rate of the electrode [111]. Corrosion stability is a part of long term stability of the water electrolyzer. Hence, for two-electrode bifuntional water electrolyzer (overall water splitting), the long term stability of various kinds of reported earth abundant bifunctional electrocatalysts in 1.0 M KOH alkaline electrolyte (Table 4) is also indicate its corrosion stability during active conditions. 2.17. Possible factors of earth abundant bifunctional electrocatalyst for efficient overall water splitting The remarkably much lower potential, stability and efficiency of earth abundant electrocatalyst for two-electrode bifuntional water electrolyzer in alkaline media could be possibly due to the insitu formation of surface layer by oxidation on the electrocatalyst surface of the anode (OER) during water electrolysis, surface morphology, conductive electrode, electrochemically active surface area (ECSA), and metallic character. The insitu formation of surface

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M.I. Jamesh / Journal of Power Sources 333 (2016) 213e236

Fig. 6. Reproduced with permission and labeling used in this review are presented in italic format. Long term stability for two electrode water electrolysis in 1 M KOH: (a) Chronoamperometric curve of Ni3Se2 nanoforest//Ni3Se2 nanoforest at 1.70 V for 140 h [86], (b) Chronopotentiometric curve of CP@Ni-P//CP@Ni-P at 10 and 20 mA cm2 for 100 h [42], (c) Chronopotentiometric curve of Co-P//Co-P at 10 mA cm2 for 72 h [52], and (d) Chronoamperometric curve of nanoporous-(Co0.52Fe0.48)2P//nanoporous-(Co0.52Fe0.48)2P at 1.55 V for 50 h [103].

layer by oxidation on the electrocatalyst surface of the anode (OER) surface such as formation of NiO [39,47] or NiOOH [41,43] or NiO/ NiOOH [38,44] or NiO/Ni(OH)x [42,45] on Ni-P based bifunctional electrocatalyst, formation of Co3O4 [40] or CoOOH [37,53] or Co(POx)y/Co3O4/Co(OH)2 [52] on Co-P based bifunctional electrocatalyst, formation of NiOOH [30,86e89,91] on Ni-Se based bifunctional electrocatalyst, formation of CoOOH [90] on Co-Se based bifunctional electrocatalyst, formation of Co3O4 [65] on Co based bifunctional electrocatalyst, formation of NiO [84] on Ni-S based bifunctional electrocatalyst, formation of Co(OH)2 [104] on Co-S based bifunctional electrocatalyst, formation of NiOOH and Co(OH)2 [79,80] on Ni-Co-S based bifunctional electrocatalyst, formation of NiOOH [75] on Ni-B based bifunctional electrocatalyst, formation of CoOOH [76] on Co-B based bifunctional electrocatalyst, and formation of NiOOH and Fe3þ (possibly Ni1-xFexOOH) [93] or NiOOH/FeOOH [102] on Ni-Fe based bifunctional electrocatalyst during water electrolysis could possibly enhance the OER activity and thus possibly enhances the efficiency of overall water splitting and this process is illustrated as schematic diagram in Fig. 7. The surface morphology of the electrocatalyst such as nanowire, nano-porous, porous nano-sheet, nano-rod, nano-forest, mesoporous nano-rod array, nano-plate array and so on of Co2P NWs [37], nanoporous-(Co0.52Fe0.48)2P [103], Porous MoO2 NS [83], CoP NR [55], Ni3Se2 Nfor [86], CoP-MNA [53], Fe2Ni2N NPA [32], and so on exhibit enhanced electroactive sites for HER and OER and thus

possibly enhances the efficiency of overall water splitting. Another advantage of the surface morphology is higher surface wettability and facilitating gas releasing electrode surface. Ni3Se2 Nfor [86] exhibit hydrophilic surface that possibly lead to aerophobic property and as a result, enhanced electrocatalyst/electrolyte interaction along with efficient detachment of bubbles (bubbles generated in smaller size and rapidly evolved from the surface due to weaker adhesive force) during electrolysis thus possibly enhances electron transfer thus possibly enhances the efficiency of overall water splitting. Surface wettability and facilitating gas releasing electrode surface is one of an important factor for efficient electrolysis and similar effects have been demonstrated earlier [10,112] for other systems. Lu et al. [10] demonstrated that nanostructured MoS2 exhibit enhanced HER activity than flat MoS2 because of the higher surface wettability and facilitating gas releasing electrode surface (superaerophobic surface) due to bubbles generated in smaller size and rapidly evolved from the surface with weaker adhesive force. The contact angle, bubble adhesive force, and bubble releasing size of nanostructured MoS2 are 52.3 ± 2.2, 10.8 ± 1.7 mN, and <100 mm, respectively while the contact angle, bubble adhesive force, and bubble releasing size of flat MoS2 are 77.1 ± 3.4, 124.8 ± 6.1 mN, and >400 mm, respectively. The effects (surface wettability and facilitating gas releasing electrode surface) not only improve the electrolysis efficiency of cheap-earth-abundant catalyst but also play a vital role for improving electrolysis efficiency of noble expensive-

M.I. Jamesh / Journal of Power Sources 333 (2016) 213e236

233

Fig. 7. Schematic diagram illustrating the involvement of insitu formed surface layer on bifunctional electrocatalyst surface (anode) during OER in water electrolysis in alkaline media.

non-earth-abundant state-of-art catalyst such as Pt. Li et al. [112] demonstrated that pine shaped Pt nanoarray exhibit enhanced HER activity than Pt flat due to the higher surface wettability and facilitating gas releasing electrode surface (superaerophobic surface) due to bubbles generated in smaller size and rapidly evolved from the surface with weaker adhesive force. The contact angle, bubble adhesive force, and bubble releasing size of Pt nanoarray are 29.3 ± 2.5, 11.5 ± 1.2 mN, and <50 mm, respectively while the contact angle, bubble adhesive force, and bubble releasing size of Pt flat are 81.4 ± 2.4, 145.6 ± 2.1 mN, and ~500 mm, respectively. Conductive electrodes (listed in Tables 1e3 such as Ni foam, glassy

carbon, carbon cloth, graphene foil, Cu foam, Ti mesh, and so on) are also one of the factor could possibly improve the efficiency of overall water splitting. For two-electrode bifuntional water electrolyzer in 1.0 M KOH to achieve 10 mA cm2, Porous MoO2 NS// Porous MoO2 NS [83] (having higher ECSA of Cdl: 422 mF cm2) exhibit lower cell voltage of 1.53 V than 1.73 V of compact MoO2// compact MoO2 (having lower ECSA of Cdl: 31 mF cm2). For twoelectrode bifuntional water electrolyzer in 1.0 M KOH to achieve 10 mA cm2, Ni-Co-P//Ni-Co-P [50] (the incorporation of Ni in Co-P (Ni-Co-P) lead to ~22 times higher ECSA) exhibit lower cell voltage of 1.77 VDE than 1.86 VDE of Co-P//Co-P (having ~22 times lower

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Fig. 8. Reproduced with permission and labeling used in this review are presented in italic format. Practical demonstration of overall water splitting in 1.0 M KOH of (a) NiMo HNRs//NiMo HNRs [82], (b) Porous MoO2 NS//Porous MoO2 NS [83], and (c) Co2P NWs//Co2P NWs [37] using a single AA/AAA battery of ~1.5 V.

ECSA). The relatively higher ECSA of the bifunctional electrocatalyst can possibly leads to enhanced HER and OER activity and thus can possibly enhances the efficiency of overall water splitting. Co2P NWs [37], Ni3Se2 Nfor [86], and Ni3S2 nanosheet array [71] exhibit metallic character that can possibly lead to more conductivity that can facilitate electron access to the electro-active sites during electrolysis thus can possibly enhances the efficiency of overall water splitting. 2.18. Practical application of earth abundant bifunctional electrocatalyst for overall water splitting Earth abundant bifunctional electrocatalyst serves as cathode (HER) and anode (OER) for overall water splitting which is a complete application. More practical application have also been demonstrated for overall water splitting in 1.0 M KOH using earth abundant bifunctional electrocatalysts such as NiMo HNRs//NiMo HNRs [82] (Fig. 8a), Porous MoO2 NS//Porous MoO2 NS [83] (Fig. 8b), Co2P NWs//Co2P NWs [37] (Fig. 8c), and Ni2/3Fe1/3-rGO//Ni2/3Fe1/3rGO [96] those exhibit overall water splitting using a single AA/AAA battery of ~1.5 V while NiCo2S4 NW//NiCo2S4 NW [74] exhibit overall water splitting using a GaAs thin film solar cell panel combined with 18 W LED desk night lamp power source. 3. Conclusion Chemical state, hydrogen and oxygen evolution performance in alkaline media, overall water-splitting activity in alkaline media, and stability of different kinds of recently reported electrocatalyst such as Ni-P, Co-P, Ni-Co-P, graphene-Co-P, O/N/C-Co/Ni, Ni-S, B-Ni/ Co, Ni-Co, Mo, Se, Fe, Mn/Zn/Ti, and metal-free based cheap-earth-

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