Iridium-containing water-oxidation catalysts in acidic electrolyte

doi:10.1016/S1872-2067(20)63722-6

Abstract

Abstract:

With the goal of constructing a carbon-free energy cycle, proton-exchange membrane (PEM) water electrolysis is a promising technology that can be integrated effectively with renewable energy resources to produce high-purity hydrogen. IrO₂, as a commercial electrocatalyst for the anode side of a PEM water electrolyzer, can both overcome the high corrosion conditions and exhibit efficient catalytic performance. However, the high consumption of Ir species cannot meet the sustainable development and economic requirements of this technology. Accordingly, it is necessary to understand the OER catalytic mechanisms for Ir species, further designing new types of low-iridium catalysts with high activity and stability to replace IrO₂. In this review, we first summarize the related catalytic mechanisms of the acidic oxygen evolution reaction (OER), and then provide general methods for measuring the catalytic performance of materials. Second, we present the structural evolution results of crystalline IrO₂ and amorphous IrO_x using in situ characterization techniques under catalytic conditions to understand the common catalytic characteristics of the materials and the possible factors affecting the structural evolution characteristics. Furthermore, we focus on three types of common low-iridium catalysts, including heteroatom-doped IrO₂ (IrO_x)-based catalysts, perovskite-type iridium-based catalysts, and pyrochlore-type iridium-based catalysts, and try to correlate the structural features with the intrinsic catalytic performance of materials. Finally, at the end of the review, we present the unresolved problems and challenges in this field in an attempt to develop effective strategies to further balance the catalytic activity and stability of materials under acidic OER catalytic conditions.

Key words: Electrocatalysis, Oxygen evolution reaction, Water splitting, Iridium, Proton exchange membrane electrolyzer

Yipu Liu, Xiao Liang, Hui Chen, Ruiqin Gao, Lei Shi, Lan Yang, Xiaoxin Zou. Iridium-containing water-oxidation catalysts in acidic electrolyte[J]. Chinese Journal of Catalysis, 2021, 42(7): 1054-1077.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(20)63722-6

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Figures/Tables 12

Fig. 1. Schematic illustration of the electrolyzer as a key part for constructing future hydrogen energy systems.

Fig. 2. Schematic illustration of (a) typical AEM scheme, (b) scaling relationship among M-OH, M-O and M-OOH, (c) a volcano plot using descriptor to represent the catalytic activity trend. (d) The volcano plot between catalytic activities and ΔGO - ΔGOH. Reproduced with permission [36]. Copyright 2011, Wiley-VCH. (e) The volcano plot between catalytic activities and eg electron. Reproduced with permission [38]. Copyright 2011, American Association for the Advancement of Science. (f) The relationship between p-state center and oxygen adsorption energy for cubic ABO3 oxides. Reproduced with permission. Copyright 2018, Royal Society of Chemistry [39].

Fig. 3. (a) Schematic illustration of three possible LOM schemes. (b) APT data of the near-surface region of hydrous Ir16Ox. (c) Local mass spectrum of oxide cluster in (b). (b,c) Reproduced with permission [42]. Copyright 2019, Royal Society of Chemistry. (d) Colored HRTEM image of La2LiIrO6 after 50 CV measurements. (e) XAS spectra of pristine La2LiIrO6, La2LiIrO6 after oxidation and IrO2. (d,e) Reproduced with permission [43]. Copyright 2017, Nature Publishing Group.

Fig. 4. (a) A polarization curve for OER. η10 is the overpotential at 10 mA/cm2. (b) A Tafel plot converted from (a). The red dashed line depicts the least potential deviation for determining the linear region. (c) Schematic diagrams of Tafel plot analysis for two specific catalysts. (d) CV plots with different scan rates under the non-Faradic range. (e) A linear trend conducted from the difference of ?j against different scan rates. (f) CV plot of SrZrO3-SrIrO3 solid solution at a potential region of 0.4-1.4 V vs. RHE.

Fig. 5. (a) The inverse relationship between the stabilities of monometallic oxides and their reactivities for OER. Reproduced with permission [56]. Copyright 2014, American Chemical Society. (b) Stability number presented for iridium containing catalysts in thin film or powder form. (c) Stability number plotted versus mass specific current density for some iridium containing powder catalysts. (b,c) Reproduced with permission [60]. Copyright 2018, Nature Publishing Group. (d) The activity stability factor (ASF) for Ir-poly and dft-Ir25Os75. Reproduced under the terms of the Creative Commons Attribution License.

Fig. 6. Iridium dissolution profiles for (a) Ir black (red), electrochemically oxidized iridium (blue) and (b) thermally prepared IrO2 when cycling the catalysts from 0.05 to 1.2 V. (a,b) Reproduced with permission [62]. Copyright 2017, American Chemical Society. (c) CV curves (left axis) and Ir oxidation state value at applied potentials (right axis) of EIROF. Reproduced with permission [65]. Copyright 2014, Royal Society of Chemistry. Ir 4f (d) and O 1s (e) XPS spectrum under open circuit and oxygen evolution conditions. (d,e) Reproduced with permission [66]. Copyright 2014, Wiley-VCH.

Fig. 7. (a) O K-edge spectra of IrO2 and amorphous IrOx. Reproduced with permission [67]. Copyright 2016, Royal Society of Chemistry. (b) O K-edge spectra of Ir-coated PEM during and after OER. (c) O K-edge spectra of Ir-coated PEM at successively applied potentials. (b,c) Reproduced with permission [69]. Copyright 2017, Royal Society of Chemistry.

Fig. 8. Dissolved Ru and Ir for (a) Ru@IrOx and (b) RuIrOx at different times. Reproduced with permission [71]. Copyright 2019, Elsevier. (c) Mass activity comparison between Ir0.7Ru0.3Ox (EC) and Ir0.7Ru0.3O2 (TT). Reproduced with permission [74]. Copyright 2017, Elsevier. (d) Schematic illustration of the Ni leaching from the surface of Ir-Ni mixed oxides. Reproduced with permission [75]. Copyright 2015, American Chemical Society. (e) Schematic illustration of catalytic sites in IrNiOx nanoparticles. Reproduced with permission [76]. Copyright 2018, Nature Publishing Group.

Fig. 9. (a) Crystal structure of 6H-SrIrO3. Reproduced with permission [87]. Copyright 2018, Nature Publishing Group. (b) Crystal structure of Ba3M’M’’2O9 triple perovskites. (c) XRD patterns of Ba3M’Ir2O9. (b,c) Reproduced with permission [91]. Copyright 2020, American Chemical Society. (d) The crystal structure and (e) electron location function (ELF) of 12L-perovskites (Ba4MIr3O12). (d,e) Reproduced with permission [90]. Copyright 2020, Elsevier. (f) The structure of Ruddlesden-Popper (RP) phases with (ABO3)n=1/(AO) formula. Reproduced with permission [93]. Copyright 2014, American Chemical Society. (g) Specific OER activity of Sr iridates in 0.1 M HClO4. Reproduced with permission [94]. Copyright 2017, American Chemical Society.

Table 1 List of structure information, measurement parameters and catalytic performances of complex iridium-based oxides.

Catalyst	Structural information	Electrolyte solution	Mass loading (mg/cm²)	η at j_geo=10 mA/cm_geo²(mV)	Specific activity	Mass fraction of dissolved ions	Ref.
IrO_x/SrIrO₃	Orthorhombic perovskite	0.5 M H₂SO₄	—	270-290 (AFM normalized)	1.53 V ~12 mA/cm_AFM²	CV @30 min (ICP) Sr-30%-50%	[79]
Ba₂PrIrO₆	Cubic perovskite	0.1 M HClO₄	0.95	~400 (~425 @1 h)	1.53 V ~1 mA/cm_ECSA²	1.55 V @1 h (ICP) Ba-14.2% Ir-0.8% Pr-11.3%	[82]
Pr₃IrO₇	Fluorite	0.1 M HClO₄	0.95	—	1.53 V ~0.1 mA/cm_ECSA²	—	[82]
SrCo_0.9Ir_0.1O_3-_δ	Orthorhombic perovskite	0.1 M HClO₄	0.255	~330	1.53 V ~10 mA/cm_BET²	10k cycles CV (ICP) Sr-64.11% Co-67.08% Ir-35.5%	[83]
Ir doped SrTiO₃	Cubic perovskite	0.1 M HClO₄	0.21	247	1.525 V 820 A/g_Ir 0.125 mA/cm_ECSA² 0.455 mA/cm_BET²	—	[84]
SrZrO₃-SrIrO₃ solid solution	Orthorhombic perovskite	0.1 M HClO₄	0.13	240	1.53 V 1540 A/g_Ir 0.22 mA/cm_ECSA² 1.2 mA/cm_BET²1.35 A/C	10 mA/cm_geo² @10h (ICP) Sr-12% Ir-1%	[53]
3C-SrIrO₃	Orthorhombic perovskite-3C	0.5 M H₂SO₄	0.9	270	1.525 V 38 A/g_Ir	10 mA/cm_geo² @30h (ICP) Sr-24%	[87]
6H-SrIrO₃	Monoclinic perovskite-6H	0.5 M H₂SO₄	0.9	248	1.525 V 76 A/g_Ir	10 mA/cm_geo² @30h (ICP) Sr-1%	[87]
Co-doped SrIrO₃ (6H)	Monoclinic perovskite-6H	0.1 M HClO₄	0.45	235	1.525 V 286.7 A/g_Ir	10 mA/cm_geo² @1h (ICP) Sr-12% Co-0.5% Ir-0.1%	[104]
BaIrO₃ polycrystalline particles	Monoclinic perovskite	0.5 M H₂SO₄	0.5	—	1.58 V 90 A/g_oxide ~4 A/F_ECSA	100-cycle CV (EDS) Ba-100%	[105]
La₂LiIrO₆	Monoclinic perovskite	0.1 M H₂SO₄	0.25	~300	1.53V ~40 A/g_oxide ~2.5 mA/cm_BET²	1.65 V @10 min (XPS) Ir-26.7% La-94.5% Li-100%	[43]
Ba₄PrIr₃O₁₂	Monoclinic perovskite-12L	0.1 M HClO₄	0.562	278	1.55 V 145 A/g_Ir 2.63 mA/cm_ECSA²	10 mA/cm_geo² @10 h (ICP) Ba-32% Pr-32% Ir-1.18%	[90]
Ba₃TiIr₂O₉	Hexagonal perovskite	0.1 M HClO₄	0.281	275	1.53 V 0.89 mA/cm_ECSA² 250 A/g_Ir	10 mA/cm_geo² @10 h (ICP) Ba-2.1% Ti-1.6%	[91]
Sr₂IrO₄	Tetragonal perovskite	0.1 M HClO₄	0.08 mg_Ir/cm²	286	1.55 V 394 A/g_Ir 6.1 mA/cm_BET²	10 mA/cm_geo² @6h (ICP) Sr-74% Ir-4%	[94]
Sr₄IrO₆	Hexagonal perovskite	0.1 M HClO₄	0.08 mg_Ir/cm²	287	1.55 V 274 A/g_Ir 0.7 mA/cm_BET²	10 mA/cm_geo² @6h (ICP) Sr-90% Ir-30%	[94]
Ca₂IrO₄	Hexagonal perovskite	0.1 M HClO₄	0.20	370	—	—	[95]
H_3.6IrO₄·3.7H₂O	Tetragonal perovskite	0.1 M HClO₄	0.255	—	1.53 V ~3.5 mA/cm_BET²	1 mA/cm_geo² @1 h (ICP) Ir-0.08%	[85]
Sr₂CoIrO₆	Orthorhombic perovskite	0.1 M HClO₄	0.255	330 (BET normalized)	1.55 V 3.5 mA/cm_BET²	1 mA/cm_BET² @24 h (ICP) Sr-5% Co-4% Ir-2.8%	[106]
Sr₂FeIrO₆	Triclinic perovskite	0.1 M HClO₄	0.255	420 (BET normalized)	1.55 V 1.8 mA/cm_BET²	1 mA/cm_oxide² @24 h (ICP) Sr-7% Fe-6% Ir-3%	[106]
Sr₂Fe_0.5Ir_0.5O₄	Tetragonal perovskite	0.1 M HClO₄	0.255	410 (BET normalized)	1.55 V 2.5 mA/cm_BET²	1 mA/cm_oxide² @24 h (ICP) Sr-4% Fe-6% Ir-8%	[106]
Pb₂Ir₂O_6.5	Cubic pyrochlore	0.1 M HClO₄	0.2	~470	1.55 V ~150 A/g_Ircm^-2	—	[98]
Bi₂Ir₂O₇	Cubic pyrochlore	0.1 M HClO₄	0.2		1.6 V ~30 A/g_Ircm^-2	—	[98]
Pr₂Ir₂O₇	Cubic pyrochlore	0.1 M HClO₄	0.285	~290	1.53 V 424.5 A/g_Ir	—	[99]
K_x_≈0.25IrO₂	Monoclinic hollandite	0.1 M HClO₄	0.2	350	1.53 V 12.2 A/g_Ir	—	[107]
CaIrO₃	Cubic perovskite	0.1 M HClO₄	0.2	—	1.55 V 30 A/g_Ir	—	[108]
β-H₂IrO₃	Orthorhombic	0.1 M H₂SO₄	0.5	345	1.53 V 1.0 mA/cm_BET²	50 h@1.55 V Ir-0.2 %	[109]

Table 1 List of structure information, measurement parameters and catalytic performances of complex iridium-based oxides.

Catalyst	Structural information	Electrolyte solution	Mass loading (mg/cm²)	η at j_geo=10 mA/cm_geo²(mV)	Specific activity	Mass fraction of dissolved ions	Ref.
IrO_x/SrIrO₃	Orthorhombic perovskite	0.5 M H₂SO₄	—	270-290 (AFM normalized)	1.53 V ~12 mA/cm_AFM²	CV @30 min (ICP) Sr-30%-50%	[79]
Ba₂PrIrO₆	Cubic perovskite	0.1 M HClO₄	0.95	~400 (~425 @1 h)	1.53 V ~1 mA/cm_ECSA²	1.55 V @1 h (ICP) Ba-14.2% Ir-0.8% Pr-11.3%	[82]
Pr₃IrO₇	Fluorite	0.1 M HClO₄	0.95	—	1.53 V ~0.1 mA/cm_ECSA²	—	[82]
SrCo_0.9Ir_0.1O_3-_δ	Orthorhombic perovskite	0.1 M HClO₄	0.255	~330	1.53 V ~10 mA/cm_BET²	10k cycles CV (ICP) Sr-64.11% Co-67.08% Ir-35.5%	[83]
Ir doped SrTiO₃	Cubic perovskite	0.1 M HClO₄	0.21	247	1.525 V 820 A/g_Ir 0.125 mA/cm_ECSA² 0.455 mA/cm_BET²	—	[84]
SrZrO₃-SrIrO₃ solid solution	Orthorhombic perovskite	0.1 M HClO₄	0.13	240	1.53 V 1540 A/g_Ir 0.22 mA/cm_ECSA² 1.2 mA/cm_BET²1.35 A/C	10 mA/cm_geo² @10h (ICP) Sr-12% Ir-1%	[53]
3C-SrIrO₃	Orthorhombic perovskite-3C	0.5 M H₂SO₄	0.9	270	1.525 V 38 A/g_Ir	10 mA/cm_geo² @30h (ICP) Sr-24%	[87]
6H-SrIrO₃	Monoclinic perovskite-6H	0.5 M H₂SO₄	0.9	248	1.525 V 76 A/g_Ir	10 mA/cm_geo² @30h (ICP) Sr-1%	[87]
Co-doped SrIrO₃ (6H)	Monoclinic perovskite-6H	0.1 M HClO₄	0.45	235	1.525 V 286.7 A/g_Ir	10 mA/cm_geo² @1h (ICP) Sr-12% Co-0.5% Ir-0.1%	[104]
BaIrO₃ polycrystalline particles	Monoclinic perovskite	0.5 M H₂SO₄	0.5	—	1.58 V 90 A/g_oxide ~4 A/F_ECSA	100-cycle CV (EDS) Ba-100%	[105]
La₂LiIrO₆	Monoclinic perovskite	0.1 M H₂SO₄	0.25	~300	1.53V ~40 A/g_oxide ~2.5 mA/cm_BET²	1.65 V @10 min (XPS) Ir-26.7% La-94.5% Li-100%	[43]
Ba₄PrIr₃O₁₂	Monoclinic perovskite-12L	0.1 M HClO₄	0.562	278	1.55 V 145 A/g_Ir 2.63 mA/cm_ECSA²	10 mA/cm_geo² @10 h (ICP) Ba-32% Pr-32% Ir-1.18%	[90]
Ba₃TiIr₂O₉	Hexagonal perovskite	0.1 M HClO₄	0.281	275	1.53 V 0.89 mA/cm_ECSA² 250 A/g_Ir	10 mA/cm_geo² @10 h (ICP) Ba-2.1% Ti-1.6%	[91]
Sr₂IrO₄	Tetragonal perovskite	0.1 M HClO₄	0.08 mg_Ir/cm²	286	1.55 V 394 A/g_Ir 6.1 mA/cm_BET²	10 mA/cm_geo² @6h (ICP) Sr-74% Ir-4%	[94]
Sr₄IrO₆	Hexagonal perovskite	0.1 M HClO₄	0.08 mg_Ir/cm²	287	1.55 V 274 A/g_Ir 0.7 mA/cm_BET²	10 mA/cm_geo² @6h (ICP) Sr-90% Ir-30%	[94]
Ca₂IrO₄	Hexagonal perovskite	0.1 M HClO₄	0.20	370	—	—	[95]
H_3.6IrO₄·3.7H₂O	Tetragonal perovskite	0.1 M HClO₄	0.255	—	1.53 V ~3.5 mA/cm_BET²	1 mA/cm_geo² @1 h (ICP) Ir-0.08%	[85]
Sr₂CoIrO₆	Orthorhombic perovskite	0.1 M HClO₄	0.255	330 (BET normalized)	1.55 V 3.5 mA/cm_BET²	1 mA/cm_BET² @24 h (ICP) Sr-5% Co-4% Ir-2.8%	[106]
Sr₂FeIrO₆	Triclinic perovskite	0.1 M HClO₄	0.255	420 (BET normalized)	1.55 V 1.8 mA/cm_BET²	1 mA/cm_oxide² @24 h (ICP) Sr-7% Fe-6% Ir-3%	[106]
Sr₂Fe_0.5Ir_0.5O₄	Tetragonal perovskite	0.1 M HClO₄	0.255	410 (BET normalized)	1.55 V 2.5 mA/cm_BET²	1 mA/cm_oxide² @24 h (ICP) Sr-4% Fe-6% Ir-8%	[106]
Pb₂Ir₂O_6.5	Cubic pyrochlore	0.1 M HClO₄	0.2	~470	1.55 V ~150 A/g_Ircm^-2	—	[98]
Bi₂Ir₂O₇	Cubic pyrochlore	0.1 M HClO₄	0.2		1.6 V ~30 A/g_Ircm^-2	—	[98]
Pr₂Ir₂O₇	Cubic pyrochlore	0.1 M HClO₄	0.285	~290	1.53 V 424.5 A/g_Ir	—	[99]
K_x_≈0.25IrO₂	Monoclinic hollandite	0.1 M HClO₄	0.2	350	1.53 V 12.2 A/g_Ir	—	[107]
CaIrO₃	Cubic perovskite	0.1 M HClO₄	0.2	—	1.55 V 30 A/g_Ir	—	[108]
β-H₂IrO₃	Orthorhombic	0.1 M H₂SO₄	0.5	345	1.53 V 1.0 mA/cm_BET²	50 h@1.55 V Ir-0.2 %	[109]

Fig. 10. (a) The crystal structure of pyrochlore. Reproduced with permission [97]. Copyright 2017, American Chemical Society. (b) Valence band spectra of Pb-Ir, Bi-Ir pyrochlores and IrO2. (c) Schematic illustration of the broadening Ir-5d band induced by the distortion of octahedra. (b,c) Reproduced with permission [98]. Copyright 2016, Nature Publishing Group. (d) Electronic phase diagram and corresponding band structure of Ir 5d orbitals. (e) O K-edge XAS of different pyrochlores. (d,e) Reproduced with permission [99]. Copyright 2018, Wiley-VCH. (f) Geometric current density and iridium mass activity of the different pyrochlores. Reproduced with permission [100]. Copyright 2019, American Chemical Society.

Fig. 11. Important perspectives for balancing the activity and stability of catalysts under acidic OER condition.

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