Dopants fixation of Ruthenium for boosting acidic oxygen evolution stability and activity

Designing highly durable and active electrocatalysts applied in polymer electrolyte membrane (PEM) electrolyzer for the oxygen evolution reaction remains a grand challenge due to the high dissolution of catalysts in acidic electrolyte. Hindering formation of oxygen vacancies by tuning the electronic structure of catalysts to improve the durability and activity in acidic electrolyte was theoretically effective but rarely reported. Herein we demonstrated rationally tuning electronic structure of RuO2 with introducing W and Er, which significantly increased oxygen vacancy formation energy. The representative W0.2Er0.1Ru0.7O2-δ required a super-low overpotential of 168 mV (10 mA cm−2) accompanied with a record stability of 500 h in acidic electrolyte. More remarkably, it could operate steadily for 120 h (100 mA cm−2) in PEM device. Density functional theory calculations revealed co-doping of W and Er tuned electronic structure of RuO2 by charge redistribution, which significantly prohibited formation of soluble Rux>4 and lowered adsorption energies for oxygen intermediates.

The mass and specific activities were obtained from the current densities at ɳ = 275 mV. Supplementary

ICP analysis of the concentration of each element in solution after OER.
Supplementary

Supplementary Note 1: The models for adsorbate evolution way toward OER
Besides the models for adsorbate evolution way toward OER, the corresponding model structures of lattice oxygen oxidation way toward OER were also established. The simulation of the two lattice oxygen atoms in W 0.2 Er 0.1 Ru 1 O 2-δ participating in the reaction was established. It is expected that the blue -OH is first adsorbed on O, which was connected with Ru active site. Then, one O 2 is generated to run away and leave an oxygen vacancy. After the optimization, -OH cannot exist stably at this position and is transferred to Ru, W or Er (Supplementary Figure 2). Therefore, the lattice oxygen in W 0.2 Er 0.1 Ru 1 O 2-δ will not participate in the reaction here. This result also proves that introducing W and Er would suppress the lattice oxygen participating in the reaction, in turn decreasing the dissolution rate in acidic electrolyte.

Supplementary Note 2: The compariosn for various doping locations of W in RuO 2
Theoretical calculations for different doping styles of W in RuO 2 were established and calculated.
According to calculation and comparison, it can be seen that the energy barrier of PDS in W 0.2 Ru 0.8 O 2-δ -1 was the smallest in these models (Supplementary Figure 5).

Supplementary Note 3: The comparison for various doping locations of Er in RuO 2
Theoretical calculations for different doping styles of Er in RuO 2 were established and calculated. The In the second step, one H could be taken off from the active sites. Simultaneously, the OER reaction could also be occurred on the active sites closing to the active site, which has been adsorbed -OH (Supplementary Figure 11). According to calculation, the energy barrier for PDS decreased, compared with the traditional models. Therefore, the results also indicated that the neighboring intermediates around active sites could contribute to enhancing the activity. Moreover, the free energy were calculated for the established models at U = 0, 1.23, and 1.4 V, respectively. Due to the influence of the neighboring intermediates around active sites, we only calculate limited models. In the maintext and Supplementary Figure 3-8, the calculated free-binding energies ignored the influence of the neighboring intermediates on the energetics.

Supplementary Note 6: The LSV error bars
The error bars of the current density measurement for these prepared electrocatalysts were also calculated. Firstly, each error bar represents the error among each repeated LSV curve for each sample at 10 mA cm -2 after 500 th cycle. It can be seen from Figure S21, the LSV curves revealed that the catalysts almost kept stable after 500 th cycle (Scan rate: 5 mV·s -1 ). The mean value of the overpotentials at 10 mA·cm -2 for these LSV curves were calculated from the 1 st , 100 th , 300 th , and 500 th circle. Simultaneously, the LSV curves shown in our main-text were all obtained after stable operation. In this part, we need to emphasize that the overpotentials at 10 mA cm -2 from the LSV curves need to be iR corrected. The value of R for correction was the same value. Sometimes, according to strict procedures, after testing each LSV curve, it is necessary to test its impedance value.
The calculated method for the error bars was illustrated as follows: Firstly, the overpotentials of 100 th , 300 th , and 500 th LSV curves minus the overpotential of the 1 st LSV curve at 10 mA·cm -2 . Then, the average of the difference was considered as the error (Supplementary Figure 22).

Supplementary Note 6: ECSA for bare carbon paper
ECSA for bare carbon paper (CP) was estimated by CV curves (Supplementary Figure 23). According to calculation, the ECSA for bare CP occupies 0.036% of the ECSA for the prepared RuO 2-δ @CP (Supplementary Figure 24). Simultaneously, the ECSA for the prepared RuO 2-δ is the smallest one among these prepared electrocatalysts. Therefore, the influence of the carbon paper on enhancing the ECSA for these prepared electrodes could be ignored.

Supplementary Note 7: ECSA for electrocatalysts on GC
The CV curves and ECSA for these prepared electrocatalysts loading on glassy carbon (GC) electrodes were also tested (Supplementary Figure 26, 27). The detailed data of C dl for these prepared electrodes was shown in Supplementary Figure 27. According to Supplementary Figure 27, it can be seen that the C dl of electrocatalysts on GC was almost consistent with that the electrocatalysts on carbon paper (Supplementary Figure 21 and Figure 6c).

Supplementary Note 8: Stability of electrocatalysts on GC
The stability of the prepared W 0. Simultaneously, it could be seen that the stability of W 0.2 Er 0.1 Ru 0.7 O 2-δ on GC electrode was very close to the stability of W 0.2 Er 0.1 Ru 0.7 O 2-δ on carbon paper in the main-text.