| Citation: |
Jonathan Ruiz Esquius, Alec P LaGrow, Haiyan Jin, Zhipeng Yu, Ana Araujo, Rita Marques, Adlio Mendes, Lifeng Liu. Mixed iridium-nickel oxides supported on antimony-doped tin oxide as highly efficient and stable acidic oxygen evolution catalysts[J]. Materials Futures, 2024, 3(1): 015102. DOI: 10.1088/2752-5724/ad16d2
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The global energy demand has been increasing exponentially since the 16th century, and it is set to keep rising at a similar rate in the future [1]. In this context, continuous consumption of fossil fuels becomes non-sustainable as this will not only deplete the limited reserves of fossil fuels but also cause ever-increasing environmental exacerbation [2]. Hence, there is a pressing need for widespread deployment of renewable energy that can fulfil the society’s energy demand, and concurrently reach the carbon-neutrality goals pledged by many countries worldwide. To this end, water electrolysis (WE) technologies and the associated electrocatalysts [3-7], have become increasingly important because they allow the surplus electricity obtained from intermittent renewable sources to be stored as hydrogen (commonly referred to as green’ hydrogen) [8], which can then be dispensed upon demand. Among the low-temperature WE technologies developed so far, proton exchange membrane WE (PEM-WE) has shown many advantages over the more mature alkaline WE (AWE), such as much lower gas crossover and accordingly higher purity of hydrogen, ability to operate at high current densities with lower ohmic losses, faster response under fluctuating power input (typical of renewable energy), ease of being pressurised, and more compact designs [9, 10]. However, the harsh oxidising conditions at the anode and the local strongly acidic environment at the electrode-electrolyte interface limit the catalysts candidates to virtually only iridium (Ir)-based materials. Ir is one of the scarcest and the most expensive metals [11], therefore, to enable PEM electrolysers to be deployed on multi-GW scale as planned to meet the demand for green hydrogen production [11-13], Ir must be more efficiently utilised without compromising its activity and stability towards the oxygen evolution reaction (OER).
A common strategy to increase the Ir mass activity is to disperse the noble metal onto a conductive support. Among many explored supports, different carbon materials have been broadly employed for a range of electrocatalytic applications. However, they generally suffer from electro-oxidation under harsh, corrosive, and oxidative conditions during acidic OER [14]. Besides various carbon materials, antimony-doped tin oxide (ATO) has been proposed as a better alternative for use as a support of Ir-based catalysts for its quasi-metallic n-type conductivity and significantly higher acid resistance than carbon materials [15-19]. Moreover, ATO is relatively cheaper than other conductive metal oxides such as indium-doped tin oxide (ITO) [11], and its synthesis is more sustainable than fluorine-doped tin oxide (FTO) [20, 21]. In addition to improving Ir dispersion, the use of ATO is also expected to meliorate the catalyst stability through the strong metal/metal-oxide support interaction [15, 22, 23].
Massu et al reported the synthesis of amorphous iridium oxy-hydroxides (Ir(O)x(OH)z, generally denoted as IrOx for simplicity) supported on ATO (35 wt.% Ir) [24], and they found that annealing leads to crystallisation of IrOx towards rutile IrO2, followed by a notable decrease in activity and faster degradation during the OER. This was attributed to the loss of surface -OH groups and the oxidation of Ir3+ to Ir4+ species. The loss of activity upon annealing agrees with the decrease in the electrochemically active surface area (ECSA) reported by Karimi et al over IrO2/ATO (20 wt.% Ir) catalysts [16], and it is also in line with several other reports on the effect of annealing over Ir-based catalysts for the OER [25-29]. The better activity of IrOx catalysts compared to crystalline rutile IrO2 towards the acidic OER [30-32] is usually attributed to their higher structural flexibility, the co-existence of Ir3+/Ir4+ species, and concomitant electrophilic O- sites prone to nucleophilic attack by water molecules and the formation of O-O bonds [33-39], but crystallisation would diminish these favourable properties of IrOx. Therefore, it is highly desirable to synthesise Ir-based OER catalysts in the form of IrOx and with improved catalytic stability by regulating the IrOx-support interaction.
Besides dispersion on supporting materials, the dilution of Ir with abundant transition metals in the form of secondary (or ternary) mixed oxides is another widely employed strategy to improve the Ir utilisation. To this end, nickel (Ni) has been widely employed as diluent [40], although other metals such as copper (Cu) [41-43] and cobalt (Co) [44-46] were also explored. The transition metal(s) introduced into the subsurface can modify the electronic structure of IrO2, and accordingly the absorption energy of oxygenate intermediates, leading to improved activity. For instance, Moghaddam et al observed a progressive increase in the Ir mass activity of IrOx colloids from 129 to 203 A gIr-1 (at 1.48 VRHE) by replacing 1/8 of the Ir atoms with Ni. However, a further increase in the Ni proportion resulted in a loss of mass activity [47]. During the OER, most of non-noble metals at the surface would leach out giving rise to an amorphous-like and hydroxide-rich surface with enhanced porosity, and hence, to an increase in the surface area. Meanwhile, the generated cationic vacancies in the lattice lead to the formation of oxygen 2p holes (electrophilic O- species) and the concomitant generation of Ir3+ species, associated with the higher activity observed for IrOx [48-51]. However, with excessive non-noble metal leaching, the generated under-coordinated IrOx species are also more prone to dissolution, compromising the overall catalyst stability [50]. Recently, Nong et al reported the loading of IrNix nanoparticles (NPs) on high-surface-area carbon, and an Ir-rich surface was obtained by electrochemical Ni dealloying, followed by surface oxidation. Compared to pure Ir NPs, a three-fold enhancement in the OER activity was observed for the IrNi3.3 counterparts. Yet, the authors only assessed the stability at 1 mA cm-2 to minimise carbon corrosion [52]. Later on, the same group further managed to load IrNi3 NPs on mesoporous ATO (20 wt.% Ir), followed by Ni dealloying and surface oxidation, and obtained a mass activity of ca. 87
Herein, we report the synthesis of ATO supported Ir-Ni oxy-hydroxide catalysts (IrNiy(O)x(OH)z/ATO, y = 0, 1, 2, and 3) through sequential precipitation of NiCl2 and IrCl3 in the form of oxy-hydroxides over commercially available ATO NPs. Different from previously reported synthetic approaches, such sequential precipitation would allow more Ir to appear at the surface of catalysts ensuring efficient usage of precious Ir. The influence of Ni content on the acidic OER was comprehensively investigated. For the sake of simplicity, the IrNiy(O)x(OH)z catalysts synthesised in this work will be denoted as IrNiyOx in the following of the paper. We found that the IrNi1Ox/ATO catalyst with an Ir loading of 19.7
Reagents: All chemicals and materials used in this work, including iridium chloride hydrate (IrCl3·xH2O, Alfa Aesar, 99%), nickel chloride hexahydrate (NiCl2·6H2O, Sigma Aldrich, 99%), lithium carbonate (Li2CO3, Merck, 99%), antimony-doped tin oxide (ATO, Sigma Aldrich, <50 nm), Nafion solution (Sigma Aldrich, 5 wt.%), isopropanol (Honeywell, 99%), and perchloric acid (HClO4, Sigma Aldrich, 70%), were commercially available and employed as received without further purification or treatment.
Unsupported IrOx was synthesised as a control when assessing the OER performance of IrNiyOx/ATO samples. The synthesis of IrOx was carried out as follows: IrCl3·xH2O (159 mg, 0.45 mmol) and four equivalents of Li2CO3 (133 mg, 1.8 mmol) were dissolved at room temperature under constant magnetic stirring for 16 h in 50 ml of deionised (DI) water (resistivity: 18.2 M cm). Under continuous stirring, the slurry was heated to reflux for 3 h. The mixture was then lifted from the oil bath and naturally cooled down to room temperature, and the precipitate was collected and washed with DI water by centrifuge (40 ml 6 times). The solid powder was subsequently placed in a filter paper and dried on a bench at room temperature for 16 h. ca. 100 mg of catalysts could be obtained in one synthetic batch.
The synthesis of IrNiyOx/ATO (25 wt.% (Ir+ Ni), y = 0, 1, 2, and 3) mixed oxides is exemplified through the description of IrNi1Ox/ATO catalysts. Specifically, IrCl3·xH2O (88 mg, 0.25 mmol) and Li2CO3 (74 mg, 1 mmol) were firstly dissolved in 25 ml of DI water under constant magnetic stirring for 16 h. In a separate flask, NiCl2·6H2O (59 mg, 0.25 mmol) and Li2CO3 (74 mg, 1 mmol) were dissolved in 25 ml of DI water. Under vigorous stirring, the NiCl2 solution was heated to 95 C for 1 h, and the IrCl3 solution was then added into the NiCl2 solution dropwise. Afterwards, the whole solution was stirred for 30 min. Subsequently, the ATO powders (188 mg) were added slowly into the solution under constant stirring. The slurry was then heated to reflux for 3 h and cooled naturally down to room temperature. The product was collected and washed in DI water by centrifuge (40 ml 6 times), and afterwards placed on a filter paper for drying at room temperature for 16 h. The amounts of precursors required for the synthesis of IrNi0Ox/ATO (denoted as IrOx/ATO hereafter), IrNi2Ox/ATO and IrNi3Ox/ATO are given in the Supporting Information (table S1). ca. 150-200 mg of catalysts could be collected in one synthetic batch.
X-ray powder diffraction (XRD) patterns were obtained on an X’Pert PRO diffractometer (PANalytical) working at 45 kV and 40 mA with Cu K
For electrochemical testing on a three-electrode configuration, the catalyst ink was prepared as follows: 2.5 mg of IrNiyOx/ATO (or unsupported IrOx) were dispersed in a mixture of 740
To assess the OER performance in a MEA, the catalyst ink was sprayed onto a commercial Nafion 115 membrane with a constant Ir loading of 0.5 mgIr cm-2. Catalyst inks were prepared by dispersing the required amount of catalysts (8.8, 20.0 and 22.0 mg for IrO2-Pk, IrOx and IrNi1Ox/ATO, respectively) in the mixture of H2O (4 ml), isopropanol (4 ml) and 5 wt.% Nafion solution (210
The IrNiyOx/ATO (y = 0, 1, 2, and 3) catalysts were synthesised through sequential precipitation of NiCl2 and IrCl3 in alkaline media, following a method adapted from previous reports [30, 55]. The Ir and Ni loading mass for the as-prepared samples was confirmed by ICP-AES (table S2). Commonly reported methods for Ir-based catalyst synthesis (excluding electrodeposition techniques) typically require a heat treatment step in order to generate the active phase (e.g. Adams fusion [25], Ir-based precursor’s decomposition [56-58], ligand or templating agent removal [28], and surface metal oxidation [27]). For the methods involving thermal treatment, the OER activity normally shows a volcano’-like trend with the heating temperature, with the peak relating to the formation of amorphous iridium oxy-hydroxide (Ir(Ox)(OH)y, hereafter denoted as IrOx). Lower temperatures do not accomplish the formation of preferable IrOx, whilst higher temperatures would lead to the crystallisation of IrOx to rutile IrO2, and accordingly the concomitant activity loss [24, 25], as mentioned in Introduction section. The same trend in OER activity with heating temperature was also reported for IrNi-based catalysts [40, 53]. Thus, direct precipitation of IrNiyOx phase without the need for any heat treatment, meanwhile employing commercially available precursors and following easily-scalable routes, is highly desired. Moreover, the sequential precipitation of NiCl2 and IrCl3 precursors enables to concentrate Ir at the catalyst surface, which allows for optimising Ir’s utilisation.
XRD was employed to assess the crystal phases of as-prepared IrNiyOx/ATO catalysts. Only peaks arising from the ATO (ICDD No. 01-075-8100) support were observed (figure S1(a)). No peaks related to crystalline Ir, Ni or IrNi phases were detected for any catalyst, suggesting that Ir and Ni are likely present in amorphous phases or as thin flakes/tiny aggregates beyond the detection limit of XRD (<2 nm). For comparison, the synthesised unsupported IrOx and commercial rutile IrO2 from Alfa Aesar (IrO2-AA) and Premetek (IrO2-Pk) were also characterised by XRD (figure S1(b)). No distinct features were observed for IrOx confirming its amorphous nature, in accordance with previous reports [30]. The XRD pattern for IrO2-AA is consistent with the standard diffraction pattern of rutile IrO2 (ICDD No. 00-015-0870); while for IrO2-Pk, a significant presence of rutile IrO2 was also observed, but the main crystalline phase seems to arise from metallic Ir (ICDD No. 00-006-0598).
The microscopic morphology of supported samples was examined by SEM. The as-received ATO support consists of aggregates with a rough surface and a wide range of particulate size from 1
The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high-resolution transmission electron microscopy (HRTEM) were carried out to unveil the morphology and atomic structure of IrOx and IrNiyOx/ATO catalysts. The ATO support was found to be highly crystalline with an average crystallite size of 9.8 5.2 nm, consistent with that calculated from the Scherrer equation (ca. 15 nm), and the Sb, Sn and O elements are uniformly distributed over individual particles (figure S4). According to HADDF-STEM, the unsupported IrOx is generally amorphous and made of particles in the 50 nm range. Nevertheless, the presence of some small IrO2 crystallites (1-2 nm) at the edge of the IrOx particle was observed by HRTEM (figure S5(d), not evident by HAADF-STEM), which very likely results from the re-crystallisation of amorphous IrOx upon the irradiation of electron beam. In fact, it is noticed that IrOx was very sensitive to the electron beam and changed dynamically during the TEM/STEM observation. Some structural damage was also found in IrOx NPs upon long-term electron beam irradiation (bright spots in figures S5(e)-(g)). For the IrOx/ATO catalyst, the higher molecular weight of Ir compared to Sn, Sb and O makes the IrOx phase stand out due to its higher brightness against the support (figures S6(a) and (b)). In addition, STEM-EDS elemental mapping revealed that although the IrOx phase is well dispersed onto the support, it does not cover the ATO surface entirely (figure S6(c)). High magnification HAADF-STEM images indicate that the IrOx-phase is distributed on the support as small crystallites (c.a. 1.5 0.4 nm). Further fast Fourier transform (FFT) analysis implies that the nanocrystallites are composed of metallic Ir (figures S7 and S8), which does not align with XRD, XPS and XAS results (as discussed below), suggesting that the observed nanocrystallites most probably result from electron beam induced reduction of IrOx and re-crystallisation.
The introduction of Ni in IrNi1Ox/ATO catalysts did not give rise to a significant change in the morphology, compared to that of IrOx/ATO. In this case, a granulated phase made of small nanoclusters was observed on the surface of the ATO support (figures 1(a), (b) and S9). Additionally, a good intercalation exists between these nanoclusters and the particulate ATO support, though the nanoclusters do not fully cover ATO. STEM-EDS elemental mapping revealed that Ir and Ni are homogeneously distributed on the surface of ATO (figure 1(c)). When the Ni content increases, a similar granular phase was observed in IrNi2Ox/ATO and IrNi3Ox/ATO. However, higher magnification STEM imaging and EDS elemental mapping indicate that Ir and Ni seem not to form a homogeneous phase (figures S10 and S11). In particular, high-contrast bright clusters and spots, most likely originating IrOx and atomically dispersed Ir atoms, were observed for IrNi3Ox (figures 2(a)-(d)), implying phase separation on both nanometre and atomic scales. To elucidate the nature of the nickel phase, an unsupported IrNi3Ox sample was synthesised following the same procedure but without adding ATO. The XRD pattern of this sample suggests that the Ni precursor precipitates as a low-crystallinity
The surface composition of catalysts was assessed by XPS. In this case, the powdery catalysts were mounted on a sticky carbon tape, which allows for calibration of all measured peaks against the C1s peak at 284.8 eV [60]. According to the survey spectra (figure S12), no chlorine (Cl) contamination was detected by XPS for any synthesised catalyst, indicating the complete conversion of metal chloride precursors into their corresponding metal oxy-hydroxides, and the efficient cleaning of the samples. The presence of lithium (Li) cannot be discerned from the survey spectra because the Li1s peak appears at a binding energy (BE) between the Ir4f and Ir4d peaks. Nevertheless, the high-resolution Ir4f spectra seem to indicate no Li contamination.
To elucidate the chemical nature of surface Ir sites on supported ATO, the Ir4f and O1s components of commercial IrO2 and unsupported IrOx references were firstly analysed. Ir4+ and O2- species were found to be exclusively present in commercial crystalline rutile IrO2, while co-existence of Ir3+/Ir4+ species were observed in amorphous IrOx, in agreement with previous reports [33]. However, quantification of the Ir3+/Ir4+ ratio from the Ir4f component turns out to be challenging due to the influence of the Ir5p1/2 peak, electron correlation and spin-orbit coupling on the electronic structure. Moreover, changes in the oxidation state of Ir-compounds typically only translate in a small variation in the BE of the Ir4f peak [61-63]. Hence, it is better to employ a model to fit the Ir4f peak highlighting changes in the peak shape without attempting to quantify the Ir3+/Ir4+ ratio, for non-synchrotron based XPS characterisation [61]. Typically, the co-existence of Ir3+/Ir4+ species associated with amorphous iridium oxy-hydroxide would result in a broadening of the Ir4f peak due to the variation in the atomic separation compared to rutile IrO2. In addition, the Ir4f spectrum of rutile IrO2 has a distinctive asymmetric shape, whilst a more symmetric peak envelope is the characteristic of IrOx that contains even minor Ir3+ sites [64]. In our experiments, the Ir4f peak for IrO2-AA is centred at 61.9 eV, within the range of BE reported for rutile IrO2 (61.7-61.9 eV) [34, 39, 64, 65], and it can be fitted with a model developed for commercial rutile IrO2 measured by laboratory XPS facilities (figure 3(a)) [64]. The Ir4f peak for IrO2-Pk shows a red shift by 0.2 eV compared to IrO2-AA (figure S13(a)), in agreement with the co-existence of IrO2 and metallic Ir in IrO2-Pk, as evidenced by XRD characterisation (figure S1(b)). Nevertheless, the O/Ir ratio obtained from XPS quantification for IrO2-AA (2.1) and IrO2-Pk (2.2) indicates that the surface of both samples consists of rutile IrO2, and that the metallic Ir in IrO2-Pk lies in the core of IrO2 NPs. Besides, the O1s XPS spectra of commercial IrO2 catalysts show the characteristic asymmetric peak centred at 530.1 eV (figure S13(b)), ascribed to the pure oxide nature of rutile IrO2. In contrast, the Ir4f peak for the unsupported IrOx catalyst appears more symmetric, exhibits a blue shift, and broadens compared to the IrO2-AA. Besides, the Ir4f peak can be well fitted with a model developed for commercial IrOx (figure 3(a)) [64], suggesting the co-existence of Ir3+/Ir4+ species commonly associated with amorphous iridium oxy-hydroxides [34, 39, 64, 65]. The presence of Ir3+ species is usually ascribed to high surface hydration, as indicated by the O1s peak centred at 531.2 eV (figure 3(b)) that confirms -OH groups dominate on the surface of IrOx. Additionally, physi-sorbed water was observed in the O1s XPS spectra of IrOx as a shoulder at 533.2 eV [62]. Quantitative XPS analysis revealed an O/Ir ratio of 4.3 for IrOx, corroborating the high hydration degree of IrOx compared to rutile IrO2 (O/Ir = 2).
Considering the overlap of the Ir4f and Ni3p peaks, the O1s and Sb3d5/2 peaks, and the fact that both the ATO support (SnO2 and Sb3O5) and metal oxy-hydroxides (IrOx and
Interpreting the Ni2p XPS spectra is challenging because multiple peaks are required to fit each oxidation state (e.g. 5 and 6 components for NiO and Ni(OH)2, respectively) [67]. Nevertheless, the chemical shift, indicative of the Ni species, can be used for rough analysis. For instance, Ni, NiO and Ni(OH)2 show a BE value of 852.6, 853.7 and 855.6 eV, respectively [60]. The Ni2p peak for IrNi1Ox/ATO is centred at 855.5 eV, indicating that Ni in IrNi1Ox/ATO may exist mainly in the form of Ni(OH)2 [67]. The predominant presence of Ni(OH)2 is in line with the high hydration degree observed in the Ir4f and O1s spectra, and is also reasonable considering the absence of heat treatment during the catalyst synthesis. In fact, Reier et al also observed that incorporating Ni into the IrO2 lattice in IrxNi1-xO films favoured the formation of surface hydroxide groups, instead of a metal oxide surface [50]. For IrNi2Ox/ATO and IrNi3Ox/ATO, no significant changes in the Ni2p peak were observed compared to the IrNi1Ox/ATO catalyst (figure 3(d)), implying that Ni in these samples is likely existent in Ni(OH)2 as well. This also agrees with XRD phase analysis of the unsupported IrNi3Ox reference (figure S2). The Ir4d and Ni2p peaks were employed to quantify the surface composition of the IrNiyOx phase. For all three IrNiyOx/ATO (y = 1, 2, and 3) samples, an excess of Ni compared to the nominal value on the surface of catalysts was observed (table S4). For IrNi2Ox/ATO and IrNi3Ox/ATO, such Ni enrichment may result from the exposure of subsurface Ni, given that HAADF-STEM imaging revealed a phase separation on nanometre/atomic scale, where IrOx/Ir is present in the form of tiny clusters or single atoms.
To further explore the electronic and structural differences among IrNiyOx/ATO catalysts, XAS examination was performed at the Ir-L3 edge (2p to 5d electronic transition). Information about the relative oxidation state can be obtained from the white line (WL) position, intensity and shape of the x-ray absorption near-edge structure (XANES). The adsorption edge energy was obtained from the first derivative (figure S16), and the XANES spectra were normalised using Athena from the Demeter software package [54]. The absorption edge is 11221.1 eV for IrO2-AA, which can be assigned to a formal bulk oxidation state of Ir4+, in line with previous reports on commercial crystalline IrO2 [33, 34] and in agreement with the above XPS and XRD results. The WL position of IrOx shifts towards lower energy compared to that of IrO2-AA, indicating an overall oxidation state lower than Ir4+. This confirms the coexistence of Ir3+ and Ir4+ species throughout the bulk of catalysts, consistent with our previous XPS analysis. However, the lack of Ir3+ standards prevents the exact determination of the average oxidation state for the synthesised samples. No shift in the WL position was observed for IrOx/ATO with respect to the unsupported IrOx, suggesting a comparable chemical environment and a similar Ir3+/Ir4+ ratio. In contrast, a blue shift in the absorption energy is observed in IrNi1Ox/ATO and IrNi2Ox/ATO relative to IrOx/ATO (figure 4(a)), manifesting that the introduction of Ni increases the overall bulk oxidation state of Ir and makes it closer to Ir4+. The difference in the WL peak intensity between IrO2-AA and IrNi2Ox/ATO may be related to variations in symmetry and ligand environment among samples, as suggested by different electron scattering paths observed in q-space (figure S17) [68]. The local environment of Ir sites was further investigated by the extended x-ray absorption fine structure (EXAFS) spectroscopy. Ir centres of all samples are expected to have an octahedral coordination, i.e. [IrO6], with O interconnected through corner- and edge-sharing positions. In rutile [IrO6], octahedra are interconnected in the most thermodynamically stable configuration leading to a rigid structure, whilst in amorphous samples, the [IrO6] octahedra are usually assembled in a more flexible configuration [31, 37]. The average Ir-O bond distance (Ir-Oav) was obtained by fitting the first coordination shell with a distorted octahedron containing two Ir-O axial (Ir-Oax) and four Ir-O equatorial (Ir-Oeq) bonds using IFEFFIT with Artemis from the Demeter software package (tables S5 and S6) [54]. An Ir-Oav bond distance of 1.97 and 2.01 was obtained for IrO2-AA and IrOx, respectively, in accordance with distances previously reported for rutile IrO2 and iridium oxy-hydroxides [62, 69]. The Ir-Oav bond distance for the supported IrOx/ATO is further elongated compared to the IrOx sample, which might be attributed to a wider Ir-O bond length distribution, although comparable Debye-Waller factors (
The acidic OER performance of unsupported IrOx and IrNiyOx/ATO was evaluated in a conventional three-electrode configuration in 0.1 M HClO4 electrolyte (pH = 1). Prior to each measurement, catalysts were conditioned by CV in the potential range of 0.8-1.4 VRHE at 50 mV s-1 for 20 cycles. The apparent activity was obtained from the reduction branch of CV curves (0.8-1.6 VRHE, 5 mV s-1) to avoid any overcompensation. As shown in figure 5(a), comparable apparent activity was obtained for IrOx, IrOx/ATO and IrNi1Ox/ATO catalysts (ca. 1.54 VRHE at 10 mA cm-2), even though the Ir loading on the electrode was reduced from 71.2
| catalyst | ERHE at 10 mA cm-2 | J (mA | TOF/s-1 at | Tafel slope/mV dec-1 | ECSA/cm2 |
| IrO2-Pk | 1.59 | 0.10 | 10 | 60 | 157 |
| IrOx | 1.54 | 0.29 | 34 | 50 | 197 |
| IrOx/ATO | 1.55 | 0.90 | 108 | 49 | 257 |
| IrNi1Ox/ATO | 1.54 | 1.02 | 123 | 50 | 191 |
| IrNi2Ox/ATO | 1.56 | 1.52 | 180 | 58 | 143 |
| IrNi3Ox/ATO | 1.57 | 1.26 | 152 | 62 | 180 |
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Tafel analysis was carried out to compare the OER kinetics of the synthesised catalysts. IrOx, IrOx/ATO and IrNi1Ox/ATO catalysts show a Tafel slope of ca. 50 mV dec-1 (figure 5(b) and table 1), in agreement with previous results reported for iridium oxy-hydroxides (35-55 mV dec-1) [31, 56, 65, 70]. In comparison, IrO2-Pk exhibits a Tafel slope of 60 mV dec-1, consistent with the values reported for rutile IrO2 (ca. 60 mV dec-1) [65, 70-73] and in line with the slower OER kinetics over crystalline IrO2 compared to amorphous iridium oxy-hydroxides. The Tafel slope of IrNi2Ox/ATO and IrNi3Ox/ATO is closer to 60 mV dec-1, which could be associated with the lower concentration of Ir species at the surface and the higher overall oxidation state close to Ir4+ compared to IrNi1Ox/ATO, as demonstrated by the above XPS and XAS results.
To assess the possible active sites involved during the OER, a CV scan in the 0.3-1.4 VRHE range was recorded (figure S20). The commercial IrO2-Pk showed no distinct redox transition as typically observed for crystalline IrO2 with a low OER activity [74]. By contrast, IrOx, IrOx/ATO and IrNi1Ox/ATO all exhibited typical redox pairs of Ir3+/Ir4+ at ca. 0.95 VRHE and Ir4+/Ir4+ at ca. 1.25 VRHE, associated with active amorphous iridium oxy-hydroxides [56, 63, 75], corroborating the presence of Ir3+ species as suggested by XPS and XAS. For IrNi2Ox/ATO and IrNi3Ox/ATO, a peak at ca. 0.65 VRHE was observed, which can be attributed to a redox process on Ni(OH)2 [76]. This indicates that the surface of these catalysts contains Ni species that is likely corroded during the acidic OER, which in turn would compromise the overall catalytic stability. To confirm that the OER activity only relates to Ir sites, in-situ XAS was conducted for the IrNi1Ox/ATO catalyst at the Ir-L3 edge and Ni-K edge. For the XAS spectra recorded at Ir-L3 edge, a blue-shift of 1.54 eV in the WL position was observed when increasing the anodic potential from 0.8 VRHE to 1.6 VRHE (figure 6(a)). This energy shift with the increasing potential into the OER regime relates to surface deprotonation and a concomitant increase in the oxidation state of Ir centres that promotes the OER process [31, 63, 65, 68]. Such a change in Ir oxidation state was found to be reversible, as confirmed by the recovery of the WL to its original position when the polarisation came back to 0.8 VRHE. In contrast, no evident changes in the Ni-K edge spectra were observed when the anodic potential increased to the OER regime (figure 6(b)). The XAS characterisation of the oxidation of Ni(OH)2 to NiOOH and NiO upon the OER has been well-documented previously [77, 78], so the fact that no change occurs at the Ni K-edge during the OER indicates that Ni centres were not involved during the reaction.
The ECSA of IrO2-Pk, IrOx, and IrNiyOx/ATO was estimated by electrochemical double-layer capacitance from CV measurements at different scan rates from 2 to 100 mVs-1 in a non-Faradaic potential region (0.47-0.57 VRHE). It is noted that such estimation varies largely depending on the reference specific capacitance (Cs) adopted and the presence of pseudo-capacitance and chemical capacitance [79-81]. Hence, it is only used to make comparison among the catalysts synthesised in this work (figures S21, S22 and table S7), but not to compare to other catalysts reported in the literature. Supporting IrOx onto ATO resulted in a higher ECSA, in agreement with the enhanced Ir dispersion; while for the IrNiyOx samples, a reduced ECSA was observed, which may be attributed to the inactive Ni introduced.
The robustness of catalysts is very important towards practical applications. Rutile IrO2 is reported to be the state-of-the-art catalyst for the acidic OER due to its outstanding stability against corrosion in acidic conditions. However, the stability comparison between crystalline IrO2 and IrOx is controversial [82]. Mom and co-workers, through online Ir dissolution studies combined with operando XAS and XPS characterisation, reported that during the OER the deprotonation of -OH intermediates, which precedes O-O coupling, occurs at the outermost surface over crystalline samples, while the deprotonation over amorphous IrOx happens on both outermost and sub-surface [61]. This can explain the higher activity usually observed for IrOx compared to rutile IrO2, but at the expenses of higher Ir dissolution [27, 61]. However, recent works also pointed out that the higher activity of IrOx is not always correlated with compromised stability [24, 31, 32, 83, 84]. The contradictory activity-stability trends reported earlier might correlate with the interplay of several Ir dissolution pathways [83, 85], the existence of different types of IrOx [30, 37], and other physical factors that influence O2 gas bubble formation and detachment [86, 87]. In our experiment, no evident rapid degradation was observed by CP (10 mA cm-2) during a 2 h test for IrOx, IrOx/ATO, IrNi1Ox/ATO and IrNi2Ox/ATO catalysts (figure 5(c)). In contrast, a gradual increase in anodic potential up to 1.9 VRHE was observed for the IrNi3Ox/ATO catalyst, indicative of catalyst degradation, which might be related to the presence of excess Ni that is prone to dissolution in acidic conditions. Furthermore, rapid degradation was also observed for IrO2-Pk, which mainly resulted from partial delamination of catalysts from the glassy carbon substrate. The catalytic stability was further appraised by comparing the polarisation curves before and after the CP test (figure S23). The initial polarisation curves of unsupported IrOx, IrOx/ATO and IrNiyOx/ATO (y = 1 and 2) nearly overlap with those after the 2 h CP test, demonstrating reasonably good catalytic stability. By comparison, a large positive shift of polarisation curves was observed for IrNi3Ox/ATO and IrO2-Pk after the CP test, which corroborates the instability of these catalysts. Given the observed good stability, unsupported IrOx, IrOx/ATO, IrNi1Ox/ATO and IrNi2Ox/ATO catalysts were further tested at 10 mA cm-2 for 15 h (figure S24). While unsupported IrOx, IrOx/ATO and IrNi1Ox/ATO all survived the long-term stability test, only IrNi1Ox/ATO showed insignificant sign of performance decay in the 15 h stress test. In contrast, IrNi2Ox/ATO became deactivated after 5 h.
To evaluate the potential of the synthesised catalysts for use in PEM-WE, a MEA was further fabricated using the IrOx/ATO and IrNi1Ox/ATO as the anode catalysts and commercial Pt/C (40 wt.% Pt) as the cathode catalysts, whose performance was tested in a single-cell electrolyser using the harmonised testing conditions proposed by the EU Joint Research Centre (60 C, 1 bar) [88]. For comparison, the catalytic activity of commercial IrO2-Pk was also assessed. At a comparable Ir loading of 0.5 mgIr cm-2, IrNi1Ox/ATO, IrOx/ATO and IrO2-Pk reached a current density of 1 A cm-2 at 2.03, 2.26 and 2.46 V, respectively (figure 7(a)). Assuming that the cell voltage is limited by the OER activity at the anode, the activity trend obtained in the MEA configuration aligns well with results obtained in the three-electrode configuration. The stability of MEAs was measured through chronopotentiometry at the current density initially required to reach 1.8 V (0.4 and 0.6 A cm2 for IrNi1Ox/ATO and IrOx/ATO, respectively). An upper limit of 2 V was applied to the cell voltage, according to the EU’s harmonised PEM-WE testing protocol [88]. Based on the first 60 h of accelerated stress test, a 0.7 mV h-1 degradation rate was observed for IrNi1Ox/ATO, whilst the degradation rate was 70% higher for IrOx/ATO (1.2 mV h-1). For both IrOx/ATO and IrNi1Ox/ATO catalysts, a sharp increase in cell voltage was seen after 70 and 90 h of operation (figure 7(b)), which might be attributed to a combination of catalyst degradation, agglomeration, delamination, membrane cracking and increased mass transport resistance as common causes reported for PEM-WE cell degradation [89, 90]. Further optimisation will be carried out to improve the performance of IrNi1Ox/ATO for PEM-WE.
In summary, we report a new sequential precipitation method for preparing IrNiyOx/ATO catalysts with an aim to reduce Ir utilisation while maintaining its good OER performance for PEM-WE. We systematically investigated how the Ni content influences the morphology, microstructure, surface chemistry and electrocatalytic performance using advanced physicochemical characterisation techniques. We found that phase separation exists in all IrNiyOx/ATO (y = 1, 2, and 3) samples, but IrNi1Ox/ATO shows a favourable Ir-rich surface and has a mixture of Ir3+ and Ir4+ species on its surface. As a result, IrNi1Ox/ATO exhibits reasonably good OER performance in terms of apparent activity, mass activity and turnover frequency. Importantly, it shows an outstanding catalytic stability compared to other reference catalysts under investigation. The good performance of IrNi1Ox/ATO has also been validated preliminarily in membrane electrode assemblies. Given the easy and potentially cost-effective synthetic approach, the IrNi1Ox/ATO shows substantial promise for use as high-performance alternative anode catalysts in PEM electrolysers.
Iridium is currently indispensable for use as anode catalysts in a practically usable PEM electrolyser. Although considerable research effort was also made to develop iridium- and even platinum group metal-free materials able to catalyse the anodic OER under acidic conditions, the unsatisfactory electrochemical stability of these materials in acid hardly allow them to be employed in practice. Hence, it is anticipated that iridium will still be a material of choice for industrial PEM electrolysers in the near to medium term. In this case, efficient usage of precious and scarce iridium becomes particularly important, considering the projection of large-scale deployment of PEM electrolysers in the coming decades for energy transition. Reducing iridium loading by introducing secondary inexpensive transition metals and enhancing effective exposure of active sites by loading iridium-based mixed oxides/oxy-hydroxides on acid-stable supports are promising approaches to improving the catalytic performance for PEM-WE and lowering materials costs. The powdery supported mixed oxide/oxy-hydroxide catalysts are also compatible with the fabrication techniques of MEAs currently being used to make PEM electrolysers. Notwithstanding some progress, the long-term stability of the supported mixed oxide/oxy-hydroxide catalysts should be further improved and their performance needs to be comprehensively assessed under various working conditions, before their usage in commercial PEM electrolysers.
This work was financially supported by the National Innovation Agency of Portugal through the project Baterias 2030 (Grant No. POCI-01-0247-FEDER-046109). J R E would like to acknowledge the Fundacin General CSIC’s ComFuturo programme which has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 101034263. The authors appreciate Dr Laura Simonelli and Dr Vlad Martin Diaconescu for their assistance in XAS measurements at the beamline BL22-CLSS, ALBA synchrotron (experiment AV-2022025706). R M is grateful to the Portuguese Foundation for Science and Technology (FCT) for the doctoral grant (Grant No. 2021.06496.BD). R M and A M are grateful for the financial support from: LA/P/0045/2020, UIDB/00511/2020 and UIDP/00511/2020, funded by the national funds through FCT/MCTES (PIDDAC). The materials characterisation was carried out using the Advanced Electron Microscopy, Imaging and Spectroscopy Facilities available at INL.
Author’s contribution
J R E was responsible for project management, catalyst synthesis, characterisation, testing and manuscript writing. A P L performed STEM characterisation. H J, Z P Y and A A contributed equally and helped in acquiring the XAS data, and they also ensured the smooth operation of the electrochemical test station, and the laboratory in general including the maintenance of working, counter and reference electrodes. R M and A M were responsible of MEA preparation and testing. L L proposed the concept, secured funding, supervised the research, and wrote the final version of the manuscript.
Author to whom any correspondence should be addressed.
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Figures(7) / Tables(1)
Copyright © 2021 Materials Futures Editorial Office
Supported by: Beijing Renhe Information Technology Co., Ltd. 
| catalyst | ERHE at 10 mA cm-2 | J (mA | TOF/s-1 at | Tafel slope/mV dec-1 | ECSA/cm2 |
| IrO2-Pk | 1.59 | 0.10 | 10 | 60 | 157 |
| IrOx | 1.54 | 0.29 | 34 | 50 | 197 |
| IrOx/ATO | 1.55 | 0.90 | 108 | 49 | 257 |
| IrNi1Ox/ATO | 1.54 | 1.02 | 123 | 50 | 191 |
| IrNi2Ox/ATO | 1.56 | 1.52 | 180 | 58 | 143 |
| IrNi3Ox/ATO | 1.57 | 1.26 | 152 | 62 | 180 |
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