Periodic Reporting for period 1 - AdIrCAT (Atomically dispersed iridium catalysts for efficient and durable proton exchange membrane water electrolysis)
Periodo di rendicontazione: 2021-12-01 al 2023-11-30
To meet the United Nation’s Sustainable Development Goal (SDG7: Ensure access to affordable, reliable, sustainable and modern energy for all) and the ambitious goals of limiting global temperature increase set out in the Paris Agreement (keeping a global temperature rise this century well below 2 ˚C above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 ˚C) and by the UN’s Intergovernmental Panel on Climate Change (70–85% world electricity from renewable energy sources by 2050), renewable energy must be deployed on a greater scale than ever in the coming decades. Given the intermittent nature of renewable energy and the mismatch between availability and demand, for large-scale deployment, appropriate energy storage solutions are critically needed. Converting renewable energy into storable, dispensable hydrogen fuel through water electrolysis is by far one of the most promising solutions. The “green hydrogen” thus produced will play a vital role in the decarbonization of various sectors, particularly the heavy industry and freight road transport where electrification is impossible or too costly.
Studies have found that low-valent Ru plays a vital role in improving the activity and stability of catalysts in acidic media, however, there is rare reports about how to accurately control the content of Ru3+ in Ru-based oxides. In view of this, this work introduces halogen atoms (Cl, Br) and transition metal atoms (Mn, Fe) into Ru-based oxides to improve th activity and stability by regulating the catalyst preparation method and the content of Ru3+ in Ru-based oxides.
The current Ru oxides are mainly tested in alkaline solutions of half-cell devices. There are few devices derived from Ru oxides in acid solution, especially measured at high current density (1000 mA/cm2) on membrane electrode assemblies (MEAs) under industrial conditions. Therefore, the research plans to test the developed highly active and stable catalyst in MEA under industrially relevant conditions to explore the feasibility of this emerging OER catalyst in actual PEMWE and promote the transformation and industrial application of catalysts. In addition, we will use synchrotron radiation characterization, spherical aberration electron microscopy, and density functional theory to analyze the atomic structure of the catalyst and the corrosion mechanism during the reaction in detail, providing a strong theoretical basis for the synthesis of highly stable catalysts under large current density.
Studies have found that low-valent Ru plays a vital role in improving the activity and stability of catalysts in acidic media, however, there is rare reports about how to accurately control the content of Ru3+ in Ru-based oxides. In view of this, this work introduces halogen atoms (Cl, Br) and transition metal atoms (Mn, Fe) into Ru-based oxides to improve th activity and stability by regulating the catalyst preparation method and the content of Ru3+ in Ru-based oxides.
The current Ru oxides are mainly tested in alkaline solutions of half-cell devices. There are few devices derived from Ru oxides in acid solution, especially measured at high current density (1000 mA/cm2) on membrane electrode assemblies (MEAs) under industrial conditions. Therefore, the research plans to test the developed highly active and stable catalyst in MEA under industrially relevant conditions to explore the feasibility of this emerging OER catalyst in actual PEMWE and promote the transformation and industrial application of catalysts. In addition, we will use synchrotron radiation characterization, spherical aberration electron microscopy, and density functional theory to analyze the atomic structure of the catalyst and the corrosion mechanism during the reaction in detail, providing a strong theoretical basis for the synthesis of highly stable catalysts under large current density.
1. Communication and dissemination
Three articles with contributions from the AdIrCAT project will be published on international famous journals . Currently, one article is under review, one is submitted and one is in preparation as shown in the report. Also, part of this work has been present in INL annual research symposium and INL research open day.
2. The physicochemical characterization of the catalysts
The surface chemical information of Ru3.4MnOx RuOx and c-RuO2 catalysts are next investigated by XPS and XAFS in Figure 3. As shown in Table 1, more oxygen vacancies exist in Ru3.4MnOx than RuOx and c-RuO2. Significantly, the variation in the proportion of surface oxygen vacancy species and lattice oxygen (OV/OL) is observed from 0.66 to 0.76 after Mn doping, signifying an abundance of OV and thus excellent electronic capture and transfer properties of the nanosheets. The Ru 3p XPS spectrum of catalysts (Figure 3b) exhibits two predominant peaks located at 462.4 eV (Ru4+ species) and 464.5 eV (Ru3+ species). The integrated area of the Ru3+/Ru4+ signals is then calculated to examine the relative abundance of Ru3+ in the different catalysts. The larger value of the ratio of Ru3+/Ru4+ from Ru3.4MnOx than c-RuO2 indicates abundant three valence Ru generates when use NaCl as template.
As shown in the Normalized Ru K-edge XANES of Figure 3c, the absorption edge of Ru3.4MnOx is at slightly lower energy than that of c-RuO2, suggesting a slightly lower Ru valence state in Ru3.4MnOx. An average oxidation state of Ru species in Ru3.4MnOx was approximately +3.3 (Figure 3d), which is considered as the combination of pristine Ru4+ and Ru3+ cation. In the meantime, the presence of Ru3+ and VO defects is also found in the undoped RuOx catalyst, seemly caused by the catalyst synthesis method used NaCl as template. Thus, it can conclude that the Mn doping, in addition to the catalysis synthesis method, has induced the generation of VO defects and more low-valence Ru3+ specie, both of which would impact the activity and durability of Ru3.4MnOx for acidic OER.
3. The OER catalytic performance of catalysts
As shown in Figure 4a, the current density of Ru3.4MnOx is the largest at corresponding potentials among all catalysts and only require 193 mV overpotential to reach a current density of 10 mA cm-2, which is comparable, even superior to other reported catalysts. Obviously, the activity of Ru3.4MnOx is much higher than that of RuOx, indicating the promotional effect of the incorporation of Mn in Figure 4a. The mass activity of Ru3.4MnOx is 1636 A/g at a voltage of 1.5V 17 times larger than c-RuO2 in Figure 4b. Moreover, the Tafel slope of Ru3.4MnOx nanosheets are ≈59 mV dec−1 lower than those of RuOx (69 mV dec−1), and even the benchmark c-RuO2 (104 mV dec−1) (Fig. 3c). Such a low Tafel slope value signies the fast kinetic merit of Ru3.4MnOx for water oxidation. The electrochemical stability is evaluated by galvanostatic electrocatalysis. Obviously, Ru3.4MnOx maintains almost unchanged performance to the initial state for 700 h at a constant current density of 10 mA cm−2, superior to commercial RuO2 (it degrades totally in 0.5 h, Figure 4d).
4. The DFT calculation
The DFT calculations will be performed on the computer cluster with 160 cores available at the host institution using periodic, spin-polarized DFT as implemented in the Vienna ab initio program package (VASP). Based on the calculations, the optimal structure facilitating the OER will be identified and compared to the atomic structures of the catalysts obtained by HAADF-STEM (on-going).
Three articles with contributions from the AdIrCAT project will be published on international famous journals . Currently, one article is under review, one is submitted and one is in preparation as shown in the report. Also, part of this work has been present in INL annual research symposium and INL research open day.
2. The physicochemical characterization of the catalysts
The surface chemical information of Ru3.4MnOx RuOx and c-RuO2 catalysts are next investigated by XPS and XAFS in Figure 3. As shown in Table 1, more oxygen vacancies exist in Ru3.4MnOx than RuOx and c-RuO2. Significantly, the variation in the proportion of surface oxygen vacancy species and lattice oxygen (OV/OL) is observed from 0.66 to 0.76 after Mn doping, signifying an abundance of OV and thus excellent electronic capture and transfer properties of the nanosheets. The Ru 3p XPS spectrum of catalysts (Figure 3b) exhibits two predominant peaks located at 462.4 eV (Ru4+ species) and 464.5 eV (Ru3+ species). The integrated area of the Ru3+/Ru4+ signals is then calculated to examine the relative abundance of Ru3+ in the different catalysts. The larger value of the ratio of Ru3+/Ru4+ from Ru3.4MnOx than c-RuO2 indicates abundant three valence Ru generates when use NaCl as template.
As shown in the Normalized Ru K-edge XANES of Figure 3c, the absorption edge of Ru3.4MnOx is at slightly lower energy than that of c-RuO2, suggesting a slightly lower Ru valence state in Ru3.4MnOx. An average oxidation state of Ru species in Ru3.4MnOx was approximately +3.3 (Figure 3d), which is considered as the combination of pristine Ru4+ and Ru3+ cation. In the meantime, the presence of Ru3+ and VO defects is also found in the undoped RuOx catalyst, seemly caused by the catalyst synthesis method used NaCl as template. Thus, it can conclude that the Mn doping, in addition to the catalysis synthesis method, has induced the generation of VO defects and more low-valence Ru3+ specie, both of which would impact the activity and durability of Ru3.4MnOx for acidic OER.
3. The OER catalytic performance of catalysts
As shown in Figure 4a, the current density of Ru3.4MnOx is the largest at corresponding potentials among all catalysts and only require 193 mV overpotential to reach a current density of 10 mA cm-2, which is comparable, even superior to other reported catalysts. Obviously, the activity of Ru3.4MnOx is much higher than that of RuOx, indicating the promotional effect of the incorporation of Mn in Figure 4a. The mass activity of Ru3.4MnOx is 1636 A/g at a voltage of 1.5V 17 times larger than c-RuO2 in Figure 4b. Moreover, the Tafel slope of Ru3.4MnOx nanosheets are ≈59 mV dec−1 lower than those of RuOx (69 mV dec−1), and even the benchmark c-RuO2 (104 mV dec−1) (Fig. 3c). Such a low Tafel slope value signies the fast kinetic merit of Ru3.4MnOx for water oxidation. The electrochemical stability is evaluated by galvanostatic electrocatalysis. Obviously, Ru3.4MnOx maintains almost unchanged performance to the initial state for 700 h at a constant current density of 10 mA cm−2, superior to commercial RuO2 (it degrades totally in 0.5 h, Figure 4d).
4. The DFT calculation
The DFT calculations will be performed on the computer cluster with 160 cores available at the host institution using periodic, spin-polarized DFT as implemented in the Vienna ab initio program package (VASP). Based on the calculations, the optimal structure facilitating the OER will be identified and compared to the atomic structures of the catalysts obtained by HAADF-STEM (on-going).
Impact
The AdIrCAT project impacted significantly on the fellow’s future career prospects. The next career step of the researcher will apply for a permanent staff researcher position at other institutions. Undoubtedly, the participation in a European-funded fellowship, which was obtained by the fellow, has strengthened the fellow’s CV.
The work carried out addresses the long-term stability problem of Ru-based oxides in strong acid solution for water oxidation and also reduce the cost in the future industrial application.
Furthermore, the “green hydrogen” thus produced will play a vital role in the decarbonization of various sectors, particularly the heavy industry and freight road transport where electrification is impossible or too costly.
The AdIrCAT project impacted significantly on the fellow’s future career prospects. The next career step of the researcher will apply for a permanent staff researcher position at other institutions. Undoubtedly, the participation in a European-funded fellowship, which was obtained by the fellow, has strengthened the fellow’s CV.
The work carried out addresses the long-term stability problem of Ru-based oxides in strong acid solution for water oxidation and also reduce the cost in the future industrial application.
Furthermore, the “green hydrogen” thus produced will play a vital role in the decarbonization of various sectors, particularly the heavy industry and freight road transport where electrification is impossible or too costly.