Photon Factory Activity Report 2022 #40 (2023)

XAFS characterization for Cu-Fe alloy NRR electrocatalysts

BL-9C/2021G646

Kaichao ZHANG, Zixuan HE, Haoran XU, and Takehiko SASAKI*Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa,Chiba, 277-8561, Japan

2 Experiment
Alloy catalyst Cu1Fe2 sample was prepared by a wet
chemical method, where mixture of aqueous solutions ofprecursor nitrate salts are mixed and reduced with N2H4 ina Teflon autoclave at 120 ℃ for 6 hours. Extended X-ray
absorption fine structure (EXAFS) measurements at theCu-K edge and Fe-K edge were carried out in transmission
mode at room temperature at the BL9C station of thePhoton Factory at the Institute of Materials Structure
Science, High Energy Accelerator Research Organizationin Japan (KEK-IMSS-PF). The electron storage ring wasoperated at 2.5 GeV. EXAFS data were analyzed and fittedusing Athena and Artemis package based on IFFEFFIT
program Backscattering amplitude, and phase shift wereobtained by FEFF calculation.
3 Results and Discussion
Fig. 1 show the observed and fitted data for Fouriertransforms of (a) Cu K-edge and (b) Fe K-edge EXAFS
for Cu1Fe2 alloy sample. The NRR activity for this sample
was best among prepared alloy samples; NH3 yield rate 50
μg·h-1·mgcat-1 and Faradaic Efficiency 20 %.
Cu K-edge EXAFS and Fe K-edge EXAFS spectra
were analyzed and it was found that the sample consistedof Cu-Fe alloy component (Cu-Fe distance of 0.26 nm)
covered with oxide phase. Cu K-edge insitu XANES
spectra were measured during electro-catalytic reaction
and found that surface oxide phase was reduced during thereaction.

3FT[k χ(k)](a)

0123456456-1r / 10 nm(b)3FT[k χ(k)]1 Introduction
The electrochemical N2 reduction reaction (NRR) isone of the promising alternatives to generate ammonia
under ambient condition. The electricity for the reaction
can be provided by renewable source like solar or windenergy and it uses H2O as the hydrogen source instead ofgaseous hydrogen, indicating it is a cleaner and safer
method. However, the relatively low yield of ammonia andFaradaic efficiency (FE) of NRR restricts the further
application [1,2]. There are two reasons for suchdisadvantages. One reason is that the bond energy ofnitrogen molecular (N2) is so high that it is difficult tobreak under ambient condition. Another reason is thecompetition of hydrogen evolution reaction (HER). In
order to overcome these disadvantages, we aim to develop
binary alloy catalyst. XAFS analysis was adopted as acharacterization method. We have prepared Cu-Fe alloy
catalyst and achieved EXAFS measurements with various
relative ratios.

0123-1r / 10 nmFig. 1 Observed and fitted data for Fourier transformsof (a) Cu K-edge and (b) Fe K-edge EXAFS for Cu-2Fe
alloy sample. (Amplitude (solid curve) and imaginary
part (dotted curve), thick curve (observed) and thin curve(fitted)).
Table 1: Summary of the EXAFS fitting results
EdgePathR (10-1 nm)

CN

DW (10-5nm2)∆E0(eV)

Rf(%)

Cu K

Cu-O

1.915±0.029

6.6±0.913.1±4.1

5.1±3.7

0.4

Cu K

Cu-Fe

2.559±0.013

4.4±0.7

6.7±1.0

Cu K

Cu-Cu

2.982±0.098

8.1±2.7

7.3±0.7

a Fitting was conducted in the range Δk: 3–13.5 (10−1 nm) and ΔR: 1.2–3.0(10−1 nm). Amplitude reducing factor S02 = 0.95

References[1] H. D. Yang et al., ACS Energy Lett. 2018, 3,1219−1224.
[2] L. Zhang,et al., Adv. Mater. 2018, 30, 1800191.
* takehiko@k.u-tokyo.ac.jp