Synchrotron Operando Spectroscopy for Electrochemical reduction of Single-Atom Alloy Ca talysts

2021 Fiscal Year Annual Research Report

• Project/Area Number: 21F41066
• Allocation Type: Single-year Grants
• Research Institution: The University of Tokyo
• Principal Investigator: 原田 慈久 東京大学, 物性研究所, 教授 (70333317)
• Co-Investigator(Kenkyū-buntansha): LIU DAOBIN 東京大学, 物性研究所, 外国人特別研究員
• Project Period (FY): 2021-09-28 – 2024-03-31
• Keywords: Ruthenium based catalyst / Acidic OER / Acidic OER / XAS / RIXS / Operando measurement

Outline of Annual Research Achievements

The electrochemical CO2 reduction (ECR) reaction is alternative to accomplish the closed-loop anthropogenic carbon cycle. It is usually coupled with the anodic oxygen evolution reaction (OER) in assembled devices. In the research, we developed an efficient Ru1-xIrxO2 solid solution catalyst for acidic OER and investigated the catalytic mechanism and stability behavior. The research achievements are summarized in the below.
(1) The synthesis of iridium-doped ruthenium oxide (Ru1-xIrxO2). We prepared the Ru1-xIrxO2 catalyst by using a molten salt assisted approach, where Ir and Ru precursors are dissolved in NaNO3 or LiNO3 salt and followed by thermal treatment at 400 degC with a quick cool-down.
(2) The acidic OER performance evaluation of Ru1-xIrxO2. We evaluated the OER performance for the Ru1-xIrxO2 catalyst obtained from LiNO3 and NaNO3, where the activity and stability of the two catalysts are almost same. Ru0.7Ir0.3O2 shows a comparable OER activity with as-prepared RuO2 and is much better that the commercial ones. Moreover, it has a long-time stability with low degradation rate in acidic electrolytes.
(3) Some basic characterizations on Ru1-xIrxO2. For the crystal structure, Ru1-xIrxO2 is a solid solution phase, which consists of small nanoparticles with the amount of grain boundaries and Ru vacancies. For the electronic structure, it shows a nonreversible oxidation with increasing the applied potential and the lattice oxygen oxidation mechanism (LOM) may not participate in OER.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

Currently, we have completed the most of experiments, including the synthesis of Ru1-xIrxO2 catalysts, acidic OER performance evaluation and their optimization and some physical characterizations. Based on these results, we may draw a deduction on that (1) the catalytic role of incorporated Ir species is to keep the high oxidation state of Ru species that are more active for OER. (2) The incorporation of Ir species into RuO2 crystal structure is indeed increasing the OER stability of Ru1-xIrxO2 in acidic electrolyte. (3) The LOM catalytic mechanism may not involve in the OER process. We will continue to do measurements (e.g. operando XAS and FTIR) for clarifying the points (1) and (3).

Strategy for Future Research Activity

As mentioned, we need to perform some measurements for further clarifying the acidic OER catalytic mechanism of Ru1-xIrxO2. To end of the research, we make a plan from the following three aspects.
(1) Operando XAS measurement for investigating the structure evolution of Ru1-xIrxO2 under the reactive OER conditions. Firstly, it needs to acquire evidence about the difference in coordination and distortion of Ru1-xIrxO2 compared to the bulk counterparts. Secondly, the structure information of Ru1-xIrxO2 under different operating potentials will be collected, which can give a deep insight into the possible structure reconstruction for Ru1-xIrxO2 towards the acidic OER reaction. We have submitted a proposal for XAS beamline time. The measurement can be done after the proposal is approved.
(2) The oxygen isotope labeling and operando ATR-FTIR measurements for unraveling the key intermediates and the origin of oxygen. The obtained O K edge XAs/RIXS spectra indicates the LOM mechanism may not participate in OER. We will further confirm the origin of oxygen by the isotope labeling DEMS. Moreover, operando ATR-FTIR measurement will be performed to acquire the key reaction intermediates for a clear understanding of OER mechanism. We have designed a special electrochemical cell, and will complete the measurement soon.
(3) The electrochemical device will be assembled by using the Ru1-xIrxO2 OER catalyst as the anode and the metal phthalocyanines/graphene (MePc/G) ECR catalyst as the cathode.