Fig 1

Schematic diagram of different mechanism during alkaline HER reaction."

Fig 2

(a) Volcano-type dependence between HBE and exchange current densities log(i0) on monometallic surface 33; (b) steady state CVs of Pt(110) and Pt(100) in different pH electrolytes 34; (c) HBE on Pt(110) (solid symbols) and Pt(100) (empty symbols) surfaces obtained from CVs as a function of solution Pt 34; (d) a correlation between the exchange current density and Hupd peak position for supported Pt-group nanoparticles 35. (a) Adapted with permission from Ref. 33, Copyright 2013, Royal Society of Chemistry. (b, c) Adapted with permission from Ref. 34, Copyright 2015, Nature Publishing Group. (d) Adapted with permission from Ref. 35, Copyright 2016, American Association for the Advancement of Science."

Fig 3

(a) Schematic representation of the alkaline HER on a Ni(OH)2/Pt(111) heterointerface 46; (b) using Pt(553) as the H adsorption site, the volcanic diagram between OH binding energy and alkaline HER activity; (c) 3D volcano curves considering dissociation energy of H2O and adsorption energy of OHad and Had during alkaline HER process; (d) reaction energy diagram to illustrate the HER reaction mechanism 41. (a) Adapted with permission from Ref. 46, Copyright 2012, Nature Publishing Group; (b–d) Adapted with permission from Ref. 41, Copyright 2020, Nature Publishing Group."

Fig 4

(a) Schematic illustration showing reaction paths in alkaline HER on the surface of a Ni catalyst doped with oxophilic metal (M) atoms; (b) comparison plot of DFT calculation of hydrogen binding energy (ΔEH) and hydroxyl binding energy (ΔEOH) for Ni(111) and M—Ni(111) surfaces; (c) volcano plot of M—Ni and Ni catalysts with ?10 and Tafel slope as a function 49; (d) HER mechanism of PtSA-NiO/Ni 51"

Fig 5

(a) AEM pathway on single metal site. Metal site, the lattice oxygen and oxygen from the electrolyte are marked in yellow, green color and green ball, respectively. The black arrow means join, and the yellow arrow means release. (b–d) LOM mechanism with oxygen vacancy, single metal and dual-metal as active site, respectively. Chemically inert lattice oxygen, active lattice oxygen involving OER, and oxygen in the electrolyte are marked with green, red, and green spheres, respectively, and dotted lines represent oxygen vacancies."

Fig 6

(a) The scaling relationship between the free energy of the water dissociation on top of oxygen (ΔG*OOH ? ΔG*O (eV)) and the free energy of proton removal (ΔG*O ? ΔG*OH (eV)) 70; (b) The schematic representations of cation/anion redox chemistry guided by d–d Coulomb interaction (U) and charge transfer energy (Δ), which manifest conventional metal cation oxidation (left), oxygen anion oxidation (middle) and direct oxygen anion release (right) for OER, respectively; (c) Schematic formation of oxygen holes in |O 2p lone-pair states for NaxMn3O7; (d) Projected density of states of NaxMn3O7 slabs (x = 2, 1.5, 1, 0.5) 79; (e) Schematic energy bands of CoO2 and zinc-substituted CoO2 slabs in consideration of Mott-Hubbard splitting; (f) Transformation of OER mechanism due to Na+ introduction, including AEM (left) and LOM (right) 84. (a) Adapted with permission from Ref. 70, Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA; (c, d) Adapted with permission from Ref. 79, Copyright 2021, Nature Publishing Group; (e–f) Adapted with permission from Ref. 84, Copyright 2019, Nature Publishing Group."

Fig 7

(a) Schematic of the synthesis route of Co2P/CoN-NCNT prepared with two-step pyrolysis process; (b) HRTEM images of Co2P/CoN-NCNT core-shell structure; (c) Calculated free-energy diagram of different catalyst samples 104; (d) Synthesis process of Ru2B3@BNC catalysts; (e) HER activity of Ru2B3@BNC in 1 mol·L−1 KOH; (f) Calculated free-energy diagram of H adsorption 105; (g) Synthesis process, (h) TEM image and (i) HRTEM image of Ir-NBD-C; (j) LSV curves and Tafel slopes of Ir-NBD-C and other samples 112; (k) Synthesis and microstructure design of Ru1SACs@FeCo-LDH catalyst in alkaline; (l) FE-SEM image with the yellow dashed line showing the defect intensity curve; (m) Long-term stability measure of RuxSACs@FeCo-LDH catalyst during overall water splitting 119. (a–c) Adapted with permission from Ref. 104, Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA. (d–f) Adapted with permission from Ref. 105, Copyright 2020, Royal Society of Chemistry. (g–j) Adapted with permission from Ref. 112, Copyright 2022, Wiley-VCH Verlag GmbH & Co. KGaA. (k–m) Adapted with permission from Ref. 119, Copyright 2022, Royal Society of Chemistry."

Fig 8

(a–c) Schematic of the roll-to-roll system and design principle of catalyst prepared by electrophoretic deposition 130; (d) diagram of high-throughput production step and structure of MoS2-based ink-type electrocatalysts; (e) LSV curves of MoS2-based electrocatalysts 5; (f) synthesis and characterization of the h-NiMoFe catalyst; (g) LSV curves of h-NiMoFe, Ni foam and commercial Pt/C 126. (a–c) Adapted with permission from Ref. 130, Copyright 2015, American Chemical Society. (d, e) Adapted with permission from Ref. 5, Copyright 2020, Nature Publishing Group. (f, g) Adapted with permission from Ref. 126, Copyright 2021, Wiley-VCH Verlag GmbH & Co. KGaA."

Fig 9

(a) Diagram of the formation of bubbles on the catalyst surface; (b) Schematic diagram of synthesis steps and corresponding morphologies of FeCoNi-HNTAs catalyst; Superaerophobic and superhydrophilic measurements of (c) FeCoNi-HNTAs, (d) FeCoNi-LDH-HWAs, (e) MoS2/NF and (f) NF; (g) Comparison of overall water splitting performance of catalysts; (h) Long-term stability measurement of FeCoNi-HNTAs 134; (i) Illustration of synthesis of NiMoB-HF/NF hollow nanotube catalyst; Optical photos of H2 bubbles attached to Nickel foam (j-k) and NiMoB-HF/NF (m–n) at (j, m) low current density (20 mA∙cm−2) and (k, n) large current density (100 mA∙cm−2), respectivity; Schematic diagram of bubbles attached to Ni foam surface (l) and NiMoB-HF/NF (o). (p–r) Long-term stability measurement of NiMoB-HF/NF at different current densities 135. (b–h) Adapted with permission from Ref. 134, Copyright 2018, Nature Publishing Group. (i–r) Adapted with permission from Ref. 135, Copyright 2022, Nature Publishing Group."

Fig 10

(a) Schematic diagram of two types of electrodes (Conventional and unified electrode with the Ti paper); (b–d) AEMWE performance of LSV curves, Nyquist plot and stability 137. Adapted with permission from Ref. 137, Copyright 2022, American Chemical Society."

Table 1

Introduce of electrode performance of AEMWE."

Table 2

Introduction of common electrolytic cell characteristics."

Fig 11

(a) Schematic cross section of an AEM water electrolysis system; (b) AEM electrolyzer components; (c) plane diagram of PEM electrolyzer; (d) the components and assembly of the membrane-free flow electrolyzer 148. Adapted with permission from Ref. 148, Copyright 2021, Wiley-VCH Verlag GmbH & Co. KGaA."

Fig 12

The critical factors of designing alkaline water cell 149. (c, d) Adapted with permission from Ref. 149, Copyright 2016, Royal Society of Chemistry."

Table 3

The performance of industrial catalysts (≥ 500 mA∙cm?2) in alkaline media in the past five years."