## 2 Protocols for Synchrotron X-Ray Experiments
### HAXPES
#### 2.3.2 Data Analysis Procedure  
HAXPES data analysis aims to extract critical parameters, such as binding energy peaks, full width at half maximum (FWHM), and d-band centers, which are pivotal for understanding the chemical and electronic properties of materials. The data analysis workflow is illustrated in Figure 6.  
(a) Energy Calibration  
Accurate energy calibration is fundamental to reliable HAXPES analysis, as it ensures the precise detection of binding energy shifts indicative of chemical and electronic variations. The binding energies (\(E_{\text{B}}\)) were calculated as follows:  
\[E_{B}=\hbar\
u-E_{k}-\phi \tag{3}\]  
where \(\hbar\
u\) is the incident X-ray energy, \(E_{\text{k}}\) is the kinetic energy of emitted photoelectrons, and \(\phi\) is the analyzer's work function. Calibration typically employs reference materials such as Au to align the Fermi level, which is calculated as:  
\[E_{B}=E_{Fermi}-E_{k} \tag{4}\]  
Figure 6: Workflow for HAXPES spectra analysis, illustrating the sequential steps from energy calibration to background correction and normalization. The calculation of d-band center for valence band spectra uses energy calibration and normalization, while peak positions with FWHM for core-level spectra also include background correction.  
In the case of charge-up effects, the C 1s peak serves as an alternative reference. However, the local chemical shifts can introduce inaccuracies. To mitigate this, a refined calibration approach is developed using the rising Fermi edge from a reference sample tailored for fuel-cell materials. An example of the refined calibration using the rising Fermi edge is shown in Figure 7. This method minimizes the errors associated with charge-up effects and structural or chemical variations, thereby ensuring robust calibration.  
Raw data from the HAXPES instruments at BL09 XU and BL46 XU, initially recorded in the kinetic energy format, are processed using a Python-based software system (Python 3.9) specifically developed for integration into the FC-BENTEN system. This approach streamlines energy calibration and converts data into binding energy profiles with minimal manual intervention, thereby enabling researchers to perform accurate and efficient analyses more easily. The protocol, validated at both beamlines, simplifies the data processing workflow and provides a robust framework for analyzing the chemical and electronic states of fuel-cell materials.  
(b) Background Correction and Normalization  
The binding energy spectra are analyzed using Origin 2023 software [16]. Background correction is performed using the Shirley function for the survey spectra and the Pt 4f, Pt 3d, and C 1s spectra. For the O 1s, Co 1s, and spectra with weak intensity or long asymmetric tails, an asymmetric least-squares smoothing function is applied. Normalization is subsequently performed over the range of 0-1. For the valence band spectra, only normalization is applied without background correction. Examples of the calibrated and normalized spectra are shown in Figure 8.  
Figure 7: Binding energy profiles of the valence band for the reference sample (10 V, red solid line) and the target sample (TEC36F52, black solid line), both provided by TANAKA Precious Metals. The intensity distributions are normalized. The interpolated binding energy is indicated by a green dashed line at an intensity of 0.4. The black dashed line represents the target sample data after energy correction, where the binding energy has been calibrated using the energy offset at the interpolated point, aligning it with the reference sample.