Elucidating the Complex Oxidation Behavior of Aqueous H3PO3 on Pt Electrodes via In Situ Tender X-ray Absorption Near-Edge Structure Spectroscopy at the P K-Edge

In situ tender X-ray absorption near-edge structure (XANES) spectroscopy at the P K-edge was utilized to investigate the oxidation mechanism of aqueous H3PO3 on Pt electrodes under various conditions relevant to high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) applications. XANES and electrochemical analysis were conducted under different tender X-ray irradiation doses, revealing that intense radiation induces the oxidation of aqueous H3PO3 via H2O yielding H3PO4 and H2. A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation-induced effects, enabling a more accurate determination of the aqueous H3PO3 oxidation mechanism. In situ XANES studies of aqueous 5 mol dm–3 H3PO3 on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H3PO3 to H3PO4. This oxidation is enhanced upon applying a positive potential to the Pt electrode or raising the electrolyte temperature, the latter being corroborated by complementary ion-exchange chromatography measurements. Notably, all of these oxidation processes involve reactions with H2O, as further supported by XANES measurements of aqueous H3PO3 of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H2O content. The significant role of water in the oxidation of H3PO3 to H3PO4 supports the reaction mechanisms proposed for various chemical processes observed in this work and provides valuable insights into potential strategies to mitigate Pt catalyst poisoning by H3PO3 during HT-PEMFC operation.


Current and potential profile for the electrodeposition of Pt black, estimation of Pt black maximum thickness
The current and potential profile for the electrodeposition of Pt black is shown in Figure S1.
Figure S1.Current and potential profiles during the electrodeposition of Pt black.The electrodeposition was performed at room temperature using a solution containing 2 mol dm -3 HCl + 2 wt% H2PtCl6.
The estimation of the electrodeposited Pt black thickness was performed using Faraday's law for electrolysis, as shown in Eq.S1:  =   Pt ∆  Pt  geo   Eq.S1 I is the current drawn to the working electrode (I ~ -4.40 mA), MPt is the molar mass of Pt (195 g mol -1 ), Δt is the electrodeposition time (53 s), Pt corresponds to the density of Pt (21.45 g cm -3 ), Ageo is the geometrical area of the electrode in contact with the electrolyte (0.502 cm 2 ), F is Faraday constant (F= 9.648 × 10 4 C mol -1 ), and z is the number of electrons transferred per Pt atom in the electrodeposition process, and is equal to z = 4, for the following reaction of Pt deposition given in Eq.S2.

H +
(aq) + [PtCl6] 2- (aq) + 4 e -⇆ Pt(s) + 6 HCl(aq) Eq.S2 From Eq. S1, it was estimated that the maximum thickness of the electrodeposited Pt black is 10 nm.It is important to note that the presented value is slightly overestimated due to the assumption of 100 % efficiency for the electrodeposition.In reality, a part of the current was used for the hydrogen evolution reaction (occurring in EWE ≤ 0 V vs. RHE).The production of hydrogen on the Pt electrode surface during the electrodeposition (i.e., bubble formation on the electrode surface) facilitates a rougher formation of the electrodeposited Pt black.

SEM images and electrochemically active surface area (ECSA) determination of planar Pt electrode and Pt black electrode, comparison with commercial Pt/C catalysts
To confirm the increased surface roughness of the electrodeposited Pt black scanning electron microscopy (SEM), and hydrogen underpotential deposition (HUPD) measurements were conducted on both the planar Pt and Pt black electrodes, as illustrated in Figure S2.The HUPD technique was employed to determine the electrochemically active surface area (ECSA) of these electrodes.SEM image of Pt black indicates an increase in surface roughness compared to planar Pt.For ECSA determination, the total charge of the underpotentially deposited hydrogen was first determined by using Eq.S3 in the following: Eq. S3 QUPD is the charge of the underpotentially deposited hydrogen monolayer (in C), ʋ corresponds to the scan rate (50 mV s -1 ), Ageo is the geometrical area of the working electrode (0.502 cm 2 ), and the term inside the integral corresponds to the area of the shaded region in Figure S2.B and S2.D.

S5
Subsequently, the total charge was normalized by the specific charge of the underpotentially deposited hydrogen monolayer on Pt (θPt= 210 µC cm -2 , see Ref. 1,2 ).The total charge of hydrogen monolayer was found to be 149.36µC and 750.50 µC, for planar Pt and Pt black, respectively.Using this method, the ECSA of planar Pt and Pt black was estimated to be 0.71 cm 2 and 3.57 cm 2 , respectively.Subsequently, the roughness factor was determined by normalizing the ECSA with the geometrical surface area (Ageo= 0.502 cm 2 for both planar Pt and Pt black electrodes), yielding a roughness factor of 1.41 for planar Pt and 7.10 for Pt black.In this comparison, it is shown that Pt black exhibited approximately 5 times larger surface area than planar Pt.
To further confirm the increase in the surface roughness of the Pt electrode, AFM was performed on both electrodes (see AFM images in Figure 1.D and 1.E in the main text).AFM revealed an estimated surface roughness of (0.9 ± 0.1) nm for planar Pt and (4.6 ± 0.3) nm for Pt black.The AFM-derived surface roughness supports the ECSA observation that the Pt black possesses approximately 5 times larger surface area than planar Pt.
To assess whether the Pt electrodes employed in this study exhibit similar electrochemical behavior to a commercial Pt/C catalyst commonly used in fuel cell applications, cyclic voltammetry (CV) was conducted using a catalyst ink prepared with the commercial Pt/C catalyst.For this purpose, a Pt/C catalyst ink was prepared by mixing 2.5 mg of Pt/C catalysts (HiSPEC® 4000, 40 wt.%Pt, Johnson-Matthey) with 30 µl of an isopropanol solution containing 5 wt.% nafion (LIQUion(™) Solution LQ-1005, 1000 EW @ 5% weight).The solution was stirred and sonicated for approximately 20 minutes, following the procedure outlined in Ref. 3 .Then 15 µl of the Pt/C ink was dropcasted onto Au substrate (Ageo = 0.75 cm 2 ) and was dried for roughly 30 minutes.SEM images were then captured, and CV experiments were performed, as illustrated in Figure S3.

Comparison between XANES spectra of 5 mol dm -3 H3PO3 recorded with rapid beam blocking method and with a radiation-attenuating filter
Two different approaches of minimizing irradiation doses were used for the XANES experiments: (i) rapid blocking of incoming X-rays when no signal is recorded, and (ii) by utilizing an irradiation attenuating filter.The procedure for the first approach is detailed in the main text.For the second approach, a 36 µm thick Kapton® membrane served as a filter to attenuate the incoming X-rays to ≈   Each measurement is separated by a distance larger than the beam spot.The fluorescence maps were obtained with an incoming X-ray energy of 2152.5 eV, corresponding to the white line energy position of H3PO4, ensuring high sensitivity to the generated H3PO4 resulting from the oxidation of H3PO3.S9

Considerations for the energy resolution of the in situ P K-edge XANES measurements
In this study, the experimental energy resolutions were approximated by the square root summation of: (i) the energy resolution of the beamline, (ii) the natural width of the probed transition, and (iii) the experimental energy steps, as presented in the SI of Ref. Eq. S4 The beamline energy resolution is ≈ 0.25 eV, for the excitation energy of 2.14 keV.The natural width of the P K-edge transition is 0.53 eV 5 .The experimental energy step is 0.25 eV.This translates to the energy resolution of 0.64 eV for the in situ P K-edge XANES experiments.

Sequential P K-edge XANES of planar Pt|5 mol dm -3 H3PO3 without electrolyte flow at different measurement positions
To investigate the influence of irradiation to the P K-edge XANES of 5 mol dm -3 H3PO3, sequential P K-edge XANES measurements on planar Pt|5 mol dm -3 H3PO3 were performed using low irradiation dose without electrolyte flow on two different positions, illustrated in Figure S7.For measurements performed at the same measurement position, there is a progressive increase of spectral weight corresponding to P compounds with the oxidation state of (+5) (e.g., H3PO4-like compounds), likely indicating the formation of H3PO4 over time due to the radiation-induced oxidation of aqueous H3PO3 to H3PO4.Details on radiation-induced oxidation of aqueous H3PO3 is discussed in section 3.2. of the main text.
Subsequently, when the measurement is conducted at a different spot (position 1 → position 2), there is a significant decrease in spectral weight corresponding to the P (+5) compound compared to the last XANES recorded at position 1 (see dark blue curve and light red curve at Figure S7).Yet, the first XANES spectrum at position 2 (light red curve, measured at t = 18 minutes) seems to display a slightly higher spectral weight corresponding to the P (+5) compound compared to the first XANES spectrum recorded at position 1 (light blue curve, measured at t = 1 minute).This might potentially correspond to the diffusion of H3PO4 formed from the previous XANES experiment at position 1.
irradiation (e.g., H3PO4), for the remainder of this study the sequential XANES was conducted on three different positions at a distance larger than the beam spot with constant electrolyte flow of 0.05 ml min -1 throughout the experiments (by the use of syringe pump Legato110, KD Scientific).This ensures that the electrolyte in the ~0.75 ml reactor chamber is renewed after three sequential XANES scans.The sequential XANES measurements recorded with this approach (shown in Figure 2.B in the main text, for XANES of planar Pt|5 mol dm -3 H3PO3) exhibit a very small standard deviations over multiple measurements, which suggest that this approach of minimizing the radiation dose is effective in sufficiently suppressing undesirable radiation-induced effects.Two syringe pumps (Legato110, KD Scientific) were utilized in a push-pull mode with an electrolyte flow rate of 0.05 ml min -1 to prevent exposure of the electrolytes to the outer environment, thereby avoiding potential contamination from air dissolution (O2, CO2, etc.).The chosen flow rate served two purposes: (1) to ensure complete regeneration of the electrolyte volume after three sets of XANES measurements, as a minimum of three XANES scans were taken for each experimental condition, and (2) to prevent excessive pressure on the X-ray transparent membrane, which could lead to membrane breakage during measurements at higher flow rates.
For temperature control, heating wires (Ni-wire, Heraeus Hanau) were applied to raise the temperature of the inlet electrolyte (H1) and outlet electrolyte (H3), and are sealed with thermal insulating tape (K-Flex ST).Additional heating elements were placed on the cell body (H2) to maintain a stable temperature during measurements.
Temperature sensors T1 and T6 monitored the heating cable's temperatures.T2 and T5 were designated for monitoring the inlet and outlet electrolyte temperatures, respectively.T3 provided temperature readings of the electrolyte within the reactor chamber (where the electrode is in contact with the electrolyte).Lastly, T4 was used to monitor the temperature of the three-electrode flow cell body.In our setup, the incoming X-ray probes the electrode|electrolyte interface at an angle nearly perpendicular to the sample surface, while the fluorescence photons are recorded at an angle of θ = 45° relative to the sample surface.Thus, the incoming X-rays probe farther into the sample than the depth from which X-ray fluorescence photons can be detected.This is because the detected X-ray fluorescence photons needs to travel a longer path in the material (as illustrated in Figure S9).As a result, the detected X-ray fluorescence will likely experience higher attenuation compared to the incoming X-ray.For this reason, given that the XANES measurement is conducted in FY-mode (i.e., by recording the X-ray fluorescence from the sample), considerations for effective detection depth in this investigation is based on the effective attenuation length of detected photons.
In this study, the 'effective attenuation length of detected photons' is defined as the length at which the intensity of the X-ray fluorescence has been attenuated to (1/e = 0.367) of their initial intensity (I0) as it probes through the electrolyte, Pt electrode, Ti adhesion layer, and Kapton window, as shown in

Figure S9
. The attenuation of photon intensity through the different layers can be described by Lambert-Beer's Law (() =  0  − ∑(µ  (,)∆  ) =  0 ∑   (, , )) (see Ref. 6 ).By using Lambert-Beer's Law, the thickness of the electrolyte that the fluorescence X-rays need to pass through until the intensity is attenuated to (1/e) of its initial value can be determined by Eq.S5.
Eq. S5 Here, µ, d, and T represent the absorption coefficient, distance travelled by the photons, and transmittance of each material probed by the X-rays, respectively.Kpt, Ti, Pt, and elec.stands for the Kapton membrane, Ti adhesion layer, Pt electrode, and electrolyte, respectively.dKpt, dTi, and dPt represents the distance that the fluorescence photons need to travel through these materials before the photons reach the detectors.Approximately, this distance is given by   = ℎ   cos  , which for θ = 45° corresponds to 16.9 µm, 7.01 nm, and 21.2 nm for dKpt, dTi, and dPt, respectively.The transmittances for 16.9 µm Kapton, 7.01 nm Ti, and 21.2 nm Pt are 0.46, 0.97, and 0.88 at the X-ray energy of 2139 eV (the energy of P Kß1 X-ray fluorescence, as detailed in Ref. 7 ), respectively, as determined from LBL (Lawrence Berkeley Laboratory) X-ray filter transmission database 8 , which is based on Ref 9 .Here considerations were conducted with P Kß1 fluorescence, since this X-ray fluorescence line has higher energy compared to P Kα1 line or P Kα2 (~2014 eV or 2013 eV 7 ), therefore the P Kß1 fluorescence determined the 'maximum' detection limit among all detected the X-ray fluorescence photons.By incorporating these transmittance values into Eq.S5, it can be shown that the electrolyte needs to possess a transmittance of 0.40 to attenuate the X-ray fluorescence intensity to (1/e) of its initial intensity.Given that H3PO3 in 5 mol dm -3 H3PO3 electrolyte possesses the density of: ρ5MH3PO3= c × MH3PO3 = 5 mol dm -3 × 81.99 g mol -1 = 409.95g dm -3 , using the same X-ray transmission database, this transmittance corresponds to approximately 5.5 µm of the electrolyte layer.
Hereafter, the electrolyte thickness required to attenuate the X-ray fluorescence to this value will be referred to as the 'effective attenuation length of detected photons'.Beyond the attenuation length of detected photons, the generation of X-ray fluorescence still occurs as the incoming X-ray still probes deeper into the electrolyte.However, the intensity of X-ray fluorescence that reaches the detector beyond this point is very small compared to the total intensity of X-ray fluorescence that is recorded, and therefore it is not considered.Hence, in this study, the length at which the probing photons (i.e., the incoming X-rays) need to travel in the electrolyte, until they reach the point corresponding to the attenuation length of detected photons, is considered as the 'effective detection depth in the electrolyte'.
The effective detection depth in the electrolyte can be estimated by the multiplication of the 'effective attenuation length of detected photons' with the cosines of the angle between the detector and the sample (here θ = 45°).This corresponds to an effective detection depth of ~3.8 µm for the planar Pt electrode (deposited on Kapton|Ti layer).Please note however, that this value is overestimated, since the estimation was performed by approximating the transmittance of electrolyte layer with the transmittance of H3PO3 in 5 mol dm -3 H3PO3 electrolyte only.In reality, the 5 mol dm -3 H3PO3 electrolyte also consist of H2O (71 wt.% H2O), and as a result, more X-rays are absorbed by the H2O, leading to a smaller effective attenuation length.For instance, when the same considerations are made using the transmittance of H2O (ρH2O ≈ 1000 g cm -3 ), this corresponds to an effective probing depth in the electrolyte of ~1.41 µm.In this study, considerations are made with the transmittance of H3PO3 in the 5 mol dm -3 aqueous electrolyte, to provide a value of 'maximum' detection depth in the electrolyte.
Using similar procedures, the effective detection depth in the electrolyte can be approximated for XANES measurements using ~10 nm Pt black electrode (deposited on Kapton|Ti|planar Pt).This estimation leads to the effective detection depth in the electrolyte of approximately 0.71 µm for Pt black electrode.It is important to note that this approximation was made with the assumption of a flat layer, and therefore, it might not be highly accurate for the Pt black electrode with high surface roughness.
However, despite its limitations, this estimation provides an approximate value for the different effective detection depths between the electrolyte, as the observability of the effects from Pt surface significantly depends on the effective detection depth in the electrolyte.
For 12 µm thick 'Pt free' Kapton substrate (without Ti|planar Pt layer), more careful considerations are needed.Using a similar procedure for estimation leads to an effective detection depth of ~15 µm.
However, in the case of the 12 µm thick Kapton and the experimental geometry described earlier, the 'effective attenuation length of probing photons' is approximately 8.8 µm, which is lower than the 'effective attenuation length of detected photons' (Considerations for the 'effective attenuation length of probing photons' are detailed in the section S12).This is not the case for the planar Pt electrode and Pt black electrode, where the effective length of probing photons is much higher than the detected photons (approximately 7 µm and 6 µm for planar Pt and Pt black, respectively, as detailed in section S12).
Therefore, for the 'Pt free' Kapton case, the effective detection depth in the electrolyte is determined by the 'effective attenuation length of probing photons' (~8.8 µm), as the intensity of X-ray fluorescence generated beyond this length is very small compared to the total intensity of recorded photons.
In this estimation of effective detection depth, the bending angle of Kapton|Ti|planar Pt membrane, due to the pressure difference between the reactor chamber (at approximately 10 3 mbar) and the UHV condition in the beamline (at the pressure of ~9 × 10 -8 mbar or less), is not considered, as the bending angle under these experimental conditions is very small (< 1°).The bending angle can be estimated using the following approximation.The shear stress of the sample can be expressed by the resultant of pressure experienced by the sample: In this experiment, ∑  ≈ 10 3 mbar.Since Kapton layer is thicker (12 µm) than the Pt layer (15 nm) and the Ti layer (5 nm), the bending is mostly determined by the Kapton.Given that Kapton® possess a Young Modulus of 2.5 GPa 10 , the bending angle is ≈ 4 × 10 -5 °.Given this small bending angle, the effect of the Kapton|Ti|planar Pt membrane bending on the angle of the detector for the approximation of effective detection depth is not considered.Additionally, the effect of electrolyte flow to the bending of the membrane is also not considered, since low flow rate is used (0.05 ml min -1 ) and the electrolyte flow vertically from the bottom of the reactor to the top of the reactor, i.e., parallel to the Kapton|Ti|Pt electrode membrane.

Theoretical estimation of the probed electrode-surface-to-electrolyte-volume ratio for the P K-edge XANES experiments
To evaluate the relative contribution of the XANES signal arising from the probed Pt electrode surface compared to the overall XANES signal originating from the entire electrolyte volume, a theoretical estimation is conducted.This estimation was made by comparing the number of probed H3PO3 molecules on the electrode surface to the total number of H3PO3 molecules in the entire electrolyte volume probed by XANES.
In this estimation, it is assumed that the probed electrode surface is composed of metallic Pt, for metallic Pt possesses a high catalytic activity towards the oxidation of aqueous H3PO3, as observed in the previous study 11 .Consideration is made for a static system, in which molecules do not move in the electrolytes.Furthermore, this estimation was made with the assumption that the H3PO3 in contact with the electrode surface will undergo an oxidation process, thereby contributing to a change in the XANES spectra.Thus, this estimation provides a lower limit of observed H3PO4 due to the catalyzed oxidation of H3PO3 by Pt.
The number of Pt atoms in the monolayer surface area probed by XANES can be estimated using Eq.S6.
The ratio of the XANES signal arising from H3PO3 on the electrode surface to the total signal originating from H3PO3 in the entire probed electrolyte can be estimated by dividing the number of Pt atoms covering the monolayer surface (determined using Eq.S6, assuming that one Pt atom is in contact with one H3PO3 molecule) to the number of probed H3PO3 molecules (approximated using Eq.S7).
For 5 mol dm -3 H3PO3 electrolyte, the probed-surface-to-volume ratio is approximately 4.4% for Pt black and about 0.2% for planar Pt.In the assumption of a static condition (i.e., assuming no diffusion) S18 and that one Pt atom is oxidizing one H3PO3 molecule in contact with it, at the OCP, it is within reason to assume that the number of H3PO4 formed by Pt-catalyzed oxidation of aqueous H3PO3 is similar to the probed-electrode-surface-to-electrolyte-volume-ratio.This supports the significant increase of spectral weight corresponding to P compounds with oxidation state of (+5), such as H3PO4-like compounds, observed in the XANES of 5 mol dm -3 H3PO3 on Pt black (see Figure 1.C of the main text).
Additionally, it is important to note that while a significant portion of the XANES signal arises from the X-ray fluorescence generated from the bulk electrolyte volume irradiated with the X-rays, the recorded intensity of X-ray fluorescence generated from the electrolyte near the surface is expected to be higher than the recorded intensity of X-ray fluorescence from the bulk solution, even though the intensity from this near-surface layer only makes up a small portion of the total recorded intensity.
This phenomenon is illustrated in Figure S11, modelling the X-ray absorption and fluorescence by layers of electrolyte as the incoming X-rays penetrate deeper into the electrolyte layer.Initially, incoming X-rays with the initial intensity of I0 are attenuated by the Kapton, Ti, and Pt layer by a factor of:  0  −(µ kpt  kpt + µ Ti  Ti + µ Pt  Pt ) =  0  kpt  Ti  Pt , as they reach the electrolyte.Here µi, di, and Ti Where θ is the angle between the sample and the photon detector and Ti' corresponds to the transmission of material i for the thickness of (di /cos θ).Now, consider that most of the intensity of the incoming X-rays is not absorbed by the first layer of the electrolyte but rather travels through a thickness of the electrolyte layer (Δx) before some portion of the Here, TΔx' corresponds to the transmission of the electrolyte layer for the thickness of (Δx/cos θ).
This process occurs many times (n times) for each part of the electrolyte layer (with a thickness of Δx) until the X-rays reach the effective detection depth in the electrolyte.After the effective detection depth, S19 this process still occurs, but the intensity of X-ray fluorescence that reaches the detectors is very small compared to the intensity of X-ray fluorescence that is generated within the effective detection depth and therefore is not considered.The incoming X-rays that reach the last layer of electrolyte (at the  ).The estimations were performed using the model presented

S21
in Figure S11, with an electrolyte-volume-elements with a thickness of 0.05 µm (each element possesses a transmittance of TΔx = 99.7% and TΔx'= 99.8% for X-ray energy of 2156.5 eV and 2139 eV, respectively).The x-axis represents the position along the electrolyte layer, where "0" corresponds to the region in direct contact with the electrode (i.e., closer to the incoming X-rays from the beamline), while the maximum distance represents the effective detection depth in electrolyte (approximately 0.71 µm, 3.88 µm, and 8.8 µm for Pt black, planar Pt, and Pt free Kapton, respectively, as detailed in section S8).
As illustrated from Figure S12, the intensity of X-ray fluorescence originating from the first 50 nm layer of the electrolyte, contributes approximately 7.4 %, 1.6 %, and 0.9 % of the total recorded intensity for Pt black, planar Pt, and Kapton, respectively.Notably this Figure shows that, Pt black exhibits a significantly higher contribution of X-ray fluorescence from the electrolyte layer near the surface compared to the other electrodes.Furthermore, this graph emphasizes that although the intensity contribution from the layer closest to the electrode is relatively higher, for the XANES recorded on planar Pt and Pt free Kapton, it still constitutes only a small portion of the overall recorded intensity.
For this approximation, however, it is noteworthy that the estimation made on the rough Pt black might be less accurate than for the other electrodes.Such is the case, since the approximation of the X-ray fluorescence ratio is made until the effective detection depth of the electrolyte.As discussed in section S9, the consideration for effective detection depth in the electrolyte is made with the assumption of a flat layer.As a result, the estimation of effective detection depth in the electrolyte is less accurate for the rough Pt black than for the other electrodes.Consequently, the approximation or X-ray fluorescence ratio is also less accurate for the Pt black electrode.
Nevertheless, with this approximation, it can be shown that the intensity of X-ray fluorescence originating from the electrolyte layer close to the Pt black electrode is considerably higher for Pt black than planar Pt.Planar Pt also displays a slightly higher ratio compared to 'Pt free' Kapton.This difference in ratio aligns well with the XANES measurement results presented in Figure 1.C in the main text, which show an increase in white line intensity ratio related to the features of the P (+5) white line and to the P (+3) white line in the order of 'Pt free' Kapton, planar Pt, and (strong increase) with Pt black.

Detection of H2 upon the oxidation of aqueous H3PO3 to H3PO4
To confirm that upon the oxidation of aqueous H3PO3 there is a formation of H2 alongside H3PO4, complementary gas chromatography measurements were conducted on aqueous H3PO3 solution before and upon dispersion of Pt/C catalysts into the solution.
In this experiment 125 ml of a 0.03 mol dm -3 aqueous H3PO3 solution was placed in a magnetically stirred three-necked round-bottom flask and maintained at room temperature (~25°C).The solution is continuously purged with Ar (flow rate: 25 ml min -1 ), which also serves as a GC carrier and reference gas.The outlet gas from the flask passed through a molecular sieve (3 Å, Alfa Aesar) drier and was sampled to a GC (Focus GC, Thermo Fisher) equipped with a HP-PLOT Molsieve (19095P-MS6) column connected to a Thermal Conductivity Detector (TCD), for characterization of the outlet gas' composition.
Prior to the addition of Pt/C, no measurable amount of H2 gas was generated in the reaction mixture; only the presence of N2 and O2, originating from contamination from air, was visible on the chromatogram (see Figure S13).Subsequently, upon the addition of 48 mg of Pt/C catalyst (Hispec4000, Johnson-Matthey, UK) into the solution, a new peak appeared on the chromatogram at the retention time of 1.65 min., corresponding to that of H2 during calibration (as illustrated in Figure S13).This confirms the formation of H2 due to oxidation of aqueous H3PO3 to H3PO4, as discussed in the main text.The production of H2 in the system was followed for around 3 hours (180 min.)during which the H3PO3 concentration decreased by about 17%.As illustrated in Figure S14, the rate of H2 production during the experiment decreased slightly from about 200 to 160 μmol min -1 gPt -1 and seems to be limited by the amount of the catalyst (Pt).

Estimation of irradiation dose absorbed by the electrolyte during the XANES experiments
The total radiation dose absorbed by the sample during the XANES experiment was estimated by Eq.S8 Here, DT represents the total radiation dose to the sample (in Gy), Dr corresponds to the radiation rate (in Gy s -1 ), as given by Ref. 12 .Δt is the exposure time (in s), E represents the incoming X-ray energy (in J).N0 is the number of photons per unit time at the solution interface (in s -1 ), and m corresponds to the mass of the electrolyte solution absorbing the synchrotron radiation (in kg).N0 was determined just before XANES measurement, by using a photodiode (ODD-AXU-010, Optodiode) placed in the incoming X-rays trajectory, in the vicinity of probed Pt|aqueous electrolyte interface.
To estimate the mass of the electrolyte absorbing the synchrotron irradiation, the probed electrode|electrolyte interface irradiated by the X-rays was modelled.The model is depicted in In this model, the incoming X-ray probes the sample at angle nearly perpendicular to the sample as a representation of the experimental geometry.The volume of the sample probed by the X-rays was modeled as an elliptical cylinder.The base of the cylinder corresponds to the approximate size of the beam spot (237 µm × 37 µm, as determined before the experiment), and the height represents the 'effective attenuation length of probing photons'.In this model, the 'effective attenuation length of probing photons' is defined as the distance at which the incoming photon intensity has been attenuated to (1/e = 0.367) of the initial incoming intensity (I0) as it probes through the Kapton, Ti adhesion layer, Pt electrode, and electrolyte.It is important to note that the estimation of the irradiation dose absorbed by the sample is made by considering the 'effective length of probing photons' and not the 'effective detection depth of the sample' (which is approximated in detail in section S8).This is because the irradiation dose corresponds to the mass of the sample that absorbed the radiation.Thus, the estimation of irradiation dose absorbed by the electrolyte is based on the attenuation length of probing X-ray and not the detected photons.
By using Lambert-Beer's Law, the thickness of the electrolyte that X-rays need to pass through until the intensity is attenuated to (1/e) of its initial value can be determined by using Eq.S5.The transmittances for 12 µm kapton, 5 nm Ti, and 15 nm Pt are 0.58, 0.99, and 0.91 at the X-ray energy of 2156.5 eV (the maximum energy of the XANES scans), respectively, as determined from LBL (Lawrence Berkeley Laboratory) X-ray filter transmission database 8 .By incorporating these transmittance values into Eq. S5, it can be shown that the electrolyte needs to possess a transmittance of 0.69 to attenuate the incoming X-ray intensity to (1/e) of its initial intensity.Considering the H3PO3 density of (ρ5MH3PO3= c × M = 5 mol dm -3 × 81.99 g mol -1 = 409.95g dm -3 ) for 5 mol dm -3 H3PO3 electrolyte, this transmittance corresponds to approximately 7.0 µm of the electrolyte layer.
Hereafter, the electrolyte thickness required to attenuate the X-rays to this value will be referred to as the "effective attenuation length of probing photons".Please note that this value is slightly overestimated since it approximates the transmittance of the electrolyte layer only with the transmittance of H3PO3 for 5 mol dm -3 H3PO3 electrolyte.In reality, the electrolyte consists of a mixture of H3PO3 and H2O, and as a result, more X-rays are absorbed by the H2O.This leads to a smaller effective attenuation length.
Please also note that the estimation was performed for a 15 nm planar Pt electrode.For the Pt black electrode, which is approximately 10 nm thicker than planar Pt, the effective attenuation length in the electrolyte is ~6.0 µm, slightly lower than the effective attenuation length with planar Pt.Similarly, for XANES recorded on the Pt free Kapton (without planar Pt layer and Ti layer), the effective attenuation length of the probing photon is ~8.8 µm.Note that the estimation is made under assumption of flat Pt layer, and thus the estimation is less accurate for Pt black with high surface roughness.It is also worth noting, that for 5 mol dm -3 H3PO4 or 5 mol dm -3 H3PO2, the effective detection depth is slightly different than the H3PO3 (by around ± 0.2 µm), given that the density and the transmittance of both solutions differs slightly from H3PO3.Furthermore, it is important to emphasize that the effective attenuation length of probing photons is different from the 'effective attenuation length of detected photons', or effective detection depth in electrolyte.Considerations for effective detection depth in the electrolyte is discussed in section S9.
Table S2.Estimated radiation dose absorbed by electrolyte for each in situ P K-edge XANES shown in  incoming photon fluxes from the beamline as measured by the photodiode.Please note that the photon fluxes needed to calculate the radiation dose absorbed by the electrolyte (using Eq.S8), are not the incoming photon flux from the beamline, but photon fluxes that reach the electrolyte.This corresponds to the incoming photon flux from beamline (I0) multiplied by the X-ray transmittance of the attenuating layer before photons reach electrolyte**.For instance, transmittance of the attenuating layer in the XANES of planar Pt|5 mol dm -3 H3POx is given by 12 µm Kapton, 5 nm Ti, and 15 nm Pt electrode, which corresponds to (0.58×0.99×0.91×I0= 0.52 I0), as previously discussed.# For measurements with the same photon fluxes, the ratio of radiation dose is similar to the ratio of exposure time.Note that the ratio is calculated for the same electrolyte (e.g., the ratio of irradiation dose for planar Pt|H3PO3 with low irradiation dose is made in comparison to the planar Pt|H3PO3 with high irradiation dose).
In Table S2, it is shown that the mass of electrolyte absorbing irradiation in the XANES measurements for different electrodes is comparable.For instance, the percentage difference between the radiation dose absorbed by the electrolyte during XANES measurements with planar Pt and Pt black is (  ( ) +   ( ) ) 2 ⁄ ) × 100% = 10.1%.This similarity arises because, although the effective attenuation length of probed photons differs for various electrodes (resulting in different mass of electrolyte that is absorbing the irradiation), the intensity of photons reaching the electrolyte also varies among electrodes.For instance, despite the smaller effective attenuation length of probing photons in the XANES measurements recorded on Pt black compared to planar Pt, the intensity of photons reaching the electrolyte is also smaller in XANES measurements with Pt black.This is due to thicker Pt black attenuating more incoming X-rays than the thinner planar Pt, as reflected in the transmittance of the attenuating layer column in Table S2.
Additionally, please note that in these experiments, shorter time exposure for several of the XANES measurements with low dose was achieved by rapid opening and closing of a valve during the XANES experiments (to be specific: valve is closed during the deadtime of the XANES measurements).It is interesting to note that the recorded electrode EOCP can be used to determine the radiation exposure time during the experiment with low dose achieved with the aforementioned method.

S29
During the XANES (i.e., synchrotron radiation illumination), there is a general drop of EOCP, hinting at H2 generation, as is detailed in the main text.In between XANES measurements, the valve was closed as the measurement spot was changed and the monochromator energy was set back to the starting energy of the XANES scan.During this time (i.e., ~265 s to 380 s and 610 s to 675 s), the EOCP increased, since H2 was not generated anymore, while the H2 previously generated during illumination, diffuse away from the Pt electrode.For a similar reason, during the XANES, EOCP repeatedly increases and decreases, as the valve rapidly opens and closes, as shown in the inset plot of Figure S16.This observation indirectly shows the time for which the sample was exposed to radiation (i.e., the time in which EOCP is decreasing) and the time in which radiation was blocked off the sample (the time in which EOCP is increasing).This EOCP observation was used to estimate exposure time for the estimation of the radiation dose.
In addition, a comparison of the irradiation dose between the current experiment and two previous investigations (Ref. 4and Ref. 11 ), is provided in the following.
For the XANES results of aqueous H3PO3 presented in Ref. 4 , measurements were conducted at the HiKE end-station 13 located at the bending magnet beamline of KMC-1 14 , using a Si (111) double crystal monochromator.With this configuration, the incoming photon fluxes to the end station were approximately 2×10 10 photons s -1 at the excitation energy of 2.1 keV.The scan duration for XANES scans (from 2120 eV to 2200 keV) was approximately 40 minutes (and only ~15 minutes for XANES scan to reach the energy of 2160 eV, which is used for this experiment).Based on the estimation of dose using Eq.S8, the dose for the measurements in the mentioned study was approximately 1.1 × 10 5 kGy, which is only 65% of the dose used for the "low dose" XANES experiment used in the current study.
For the in situ AP-HAXPES data presented in Ref. 11 , experiments were performed at the SpAnTeX end-station 15 , which is also located at the bending magnet KMC-1 beamline.A Si (111) crystal monochromator was used for these experiments, resulting in incoming photon fluxes of approximately 9×10 10 photons s -1 at the excitation energy of 3 keV.This flux counts for roughly 12% of the incoming fluxes used in the current XANES study using the CPMU17 EMIL undulator beamline (~7.7 × 10 11 photons s -1 ).Furthermore, at this excitation energy, the X-ray transmittance of H2O and H3PO3 is much larger than the transmittance at 2.15 keV.The transmittance for 1 µm layer of H2O and H3PO3 at 3 keV are 0.98 and 0.91, respectively, whereas at 2.16 keV, the transmittance for H2O and H3PO3 are 0.95 and 0.8 respectively.Given that the photon absorbance is proportional to (1transmittance), the X-ray absorbance by the H3PO3 at 3 keV is approximately 9%, while the X-ray absorbance at 2.15 keV is ~20%.This indicate that lower dose of X-ray is absorbed by the electrolyte for the in situ AP-HAXPES experiment (i.e., at 3 keV) compared to the current XANES experiments (i.e., at ~2.15 keV).Additionally, this measurement is performed at ambient pressure between 18 mbar to 22 mbar of water vapor.Hence, some of the incoming X-rays will also be absorbed by the water vapor in the chamber, resulting in even lower photon flux to the sample.Consequently, this translates to a during the in situ AP-HAXPES experiments were ~0.4 VRHE and 0.47 VRHE, for 5 mol dm -3 and 1 mol dm -3 electrolytes, respectively 4 .This value is very similar to the EOCP of the same system measured without irradiation (see the initial EOCP given in Figure 2.D in the main text).If a strong radiationinduced effect occurs, then the EOCP should decrease to a lower value (e.g., closer to ~0.1 VRHE), as observed in the current experiment.Therefore, these EOCP values further indicate that very small radiation effects occurred during the in situ AP-HAXPES experiment.

Additional P K-edge XANES of aqueous H3PO3 with different incoming photon fluxes
To further validate the dependency of the increase in spectral weight corresponding to P(+5) compounds observed on the P K-edge XANES of 5 mol dm -3 with different radiation doses H3PO3 (as shown in  As observed in Figure S17, with an increase in photon flux, a higher spectral weight corresponding to P compounds with an oxidation state of P (+5) is evident, consistent with the trend revealed in the main text.Additionally, to confirm that EOCP of planar Pt|5 mol dm -3 H3PO3 is increasing when radiation is blocked right after irradiation is applied to the system (towards the initial EOCP without irradiation), EOCP of planar Pt electrode|5 mol dm -3 H3PO3 under repeated application of synchrotron irradiation and without irradiation, is recorded.This experiment is shown in Figure S19.  2 =  ( °− ≅ )   (  + ) 2 Eq.S9 aH2 corresponds to the activity of H2.EEq represents the equilibrium potential of the reaction, in this case, the EOCP after the drop where a quasi-steady-state is obtained (e.g., when the EOCP is almost constant, in this case, EOCP recorded at t = 300 s).E ° is the standard redox potential of the H + /H2 (0 VRHE), R is the universal gas constant (R = 8.31 J K -1 mol -1 ).T corresponds to the temperature (~ 297 K).F is the Faraday constant (F = 9.648 × 10 4 C mol -1 ) and z is the number of transferred electrons (z = 2 for HER/HOR).aH+ corresponds to the activity of H + in the electrolyte.Given that the 5 mol dm -3 H3PO3 possesses a pH of 0.32, the activity of H + was determined to be:   + = 10 − = 0.47.

Additional
The activity of gasses is correlated to the gasses' partial pressure via Eq.S10: where p(H2) is the partial pressure of H2 (in Pa) and p standard is the standard pressure (10 5 Pa).
Table S3 shows the estimation of partial pressure H2 during experiments with 5 mol dm -3 H3PO3 and 5 mol dm -3 H3PO4, determined from Eq. S9 and S10.Table S3.Estimated partial pressure of H2 [p(H2)] during the EOCP recording of 5 mol dm -3 H3PO3 with different irradiation doses, calculated from Eq. S9 and S10.*Please note that this estimation was made given the assumption that the EOCP drop solely corresponds to the presence of H2 and subsequent H + /H2 equilibrium at the Pt surface.

Sample
As shown in Table S3, the p(H2) in 5 mol dm -3 H3PO3 under a high irradiation dose is ~ 4 × 10 3 times bigger than the p(H2) under a low irradiation dose.This confirms that the H2 generation under high irradiation doses is indeed higher than the H2 generation in low doses.
However, the p(H2) that were estimated for the experiment with 5 mol dm -3 H3PO4 electrolyte, are significantly lower than the corresponding values in 5 mol dm -3 H3PO3.Especially for the low dose measurement, the estimated p(H2) is very low, and as such, its physical meaning is questionable.For the measurement with 5 mol dm -3 H3PO3 electrolyte, there is likely a high formation of H2.This leads to the a strong drop of EOCP in 5 mol dm -3 H3PO3 electrolyte, since the EOCP is then predominantly influenced by the 2 H + + 2 e -⇆ H2 reaction.For measurement in the 5 mol dm -3 H3PO4 electrolyte, the effect from the aforementioned reaction is not the dominant factor, as the presence of H2 is likely much lower than that in the experiment with H3PO3.Thus, the estimated p(H2) value is only "accurate" for the 5 mol dm -3 H3PO3 electrolyte, where 2 H + + 2 e -⇆ H2 reaction majorly influences the EOCP, and is less accurate for the 5 mol dm -3 H3PO4 electrolyte.
Yet, even though the estimation of p(H2) for the aqueous H3PO4 electrolyte is imprecise, it is important to note that the high drop of EOCP in 5 mol dm -3 H3PO3 compared to 5 mol dm -3 H3PO4 (in both irradiation doses, see Figure S20), indicates a much stronger activity of H2 in H3PO3 electrolyte (i.e.much higher H2 formation).Assuming that the drop of EOCP in 5 mol dm -3 H3PO4 was caused by the H2 formation due to the radiolysis of water (as detailed in the main text), a similar drop of EOCP should be seen in 5 mol dm -3 H3PO3, given that both solutions have the same concentration of H2O.This suggests that another process is also taking place in the aqueous H3PO3 electrolyte under irradiation, which results in H2 generation.This further indicates the possibility of radiation-induced oxidation of H3PO3 to H3PO4, generating H2 alongside it (see details in the main text).As shown in Table S4, there is a significantly faster conversion of H3PO2 to H3PO4 in an aqueous solution, compared to the conversion rate of aqueous H3PO3 to H3PO4 at the same concentration.This highlights the inherent instability of aqueous H3PO2 in contrast to H3PO3.These findings further support the observed high spectral weight corresponding to P(+5) compounds (e.g.H3PO4) in the XANES spectra of aqueous H3PO2 compared to H3PO3 (as shown in Here F is the Faraday constant.z is the number of electrons transferred in the reaction and is equal to z = 2 for the electrochemical oxidation of H3PO3, as given by Eq. 5 in the main text.jgeo corresponds to the geometrical current densities recorded in this experiment (shown in the inset plot of   This estimation shows that at more positive potentials, a higher formation of H3PO4 is observed, which aligns with the increasing spectral weight corresponding to P (+5) compounds in XANES (Figure 3.B in the main text).Please note that this estimation was made under the assumption of 100% Faradaic efficiency of the electrochemical oxidation of H3PO3 to H3PO4, as given by Eq.S11.Additionally, it is noteworthy that the assumption was made in static condition (i.e., no diffusion).Moreover, the electrolyte was continuously flown during measurements (flow rate: 0.05 ml min -1 , in the reactor chamber of ~750 µl).Thus, this approximation shows the maximum generation of H3PO4.  .The spectra of electrolytes containing higher concentrations of H2O (i.e., higher H2O wt.%), exhibit a higher spectral weight corresponding to P compounds with the oxidation state of (+5).For visualization, each spectrum is normalized to the maximum intensity of the spectrum.Measurements were performed at the temperature of 25 °C.
Although higher radiation doses led to more pronounced oxidation overall, a similar trend was observed compared to XANES spectra recorded with low radiation doses (Figure 5 of the main text): Spectra recorded with higher water concentration (i.e., higher H2O wt.%) exhibited prominently stronger spectral weight corresponding to P(+5) compounds (e.g., H3PO4).

Figure S2 .
Figure S2.(A) SEM image and (B) CV of planar Pt electrode.(C) SEM image and (D) CV of Pt black electrode.SEM image on planar Pt (panel A) was taken before the in situ P K-edge XANES experiments, while the image on Pt black (panel C) was taken post-mortem after all of the in situ XANES experiments were performed (after the beamtime campaign).CVs were recorded for the determination of ECSA via HUPD.CVs are recorded with the starting potential of +0.035 VRHE in the positive-going potential sweep in N2 purged 0.1 mol dm -3 (0.1 M) H2SO4 using the scan rate of 50 mV s -1 , with two different upper limit potentials.

Figure
Figure S3.(A) SEM image and (B) CV of a catalysts ink prepared from commercial Pt/C catalysts.CVs were recorded with the starting potential of +0.0 VRHE in the positive-going potential sweep in N2 purged 0.1 mol dm -3 (0.1 M) H2SO4 using a scan rate of 50 mV s -1 .

Figure S4 .
Figure S4.Schematic presentation of the three-electrode flow cell used for the in situ P K-edge XANES experiments at the OAESE end-station.
21.2% of the original intensity.The filter was positioned after the last optical elements of the EMIL beamline.To precisely measure the incoming X-ray photon flux (I0), a photodiode (ODD-AXU-010, Optodiode) was placed in the trajectory of the incoming X-rays near the probed Pt|aqueous electrolyte interface.Figure S5 illustrate a comparison of the P K-edge XANES of planar Pt|5 mol dm -3 H3PO3 obtained with these different methods.

Figure S5 .
Figure S5.Comparison of 5 mol dm -3 H3PO3 XANES spectra recorded with the rapid beam blocking methods and with a radiation-attenuating filter.Irradiation dose was estimated by using Eq.S8, as detailed in section S12.The XANES experiment with rapid beam blocking was conducted with the incoming photon flux of ~ 7.7 × 10 11 photons s -1 , while the XANES experiment with radiation attenuating filter was performed with the incoming photon flux of ~1.6 × 10 11 photons s -1 .The radiation exposure time to the sample were: 163 seconds and 265 seconds, for the XANES experiment exploiting the rapid beam blocking and the radiation attenuation filter, respectively.

Figure 5 .Figure S6 .
FigureS5demonstrates a higher signal-to-noise ratio for the XANES measurement recorded with rapid beam blocking compared to that recorded with an irradiation attenuating filter.Therefore, for the bulk of the study, measurements were conducted using the rapid beam blocking method.

Figure S7 .
Figure S7.Sequential P K-edge XANES measurements of planar Pt|5 mol dm -3 H3PO3 recorded as a function of time at two different measurement spots separated by a distance larger than the beam spots.The measurements were conducted without electrolyte flow at room temperature (≈ 25 °C), at the open circuit potential (OCP).

8.
Figure S8.Illustration of the heating and temperature monitoring system for the in situ P K-edge XANES investigation.The components labeled S, T, and H represent the syringe pumps, thermocouples, and heating elements, respectively.

S13 9 .
Considerations for effective detection depth in the electrolyte for the in situ P K-edge XANES experimentsTo estimate the effective detection depth in the electrolyte for the in situ P K-edge XANES experiments, the electrode|electrolyte interface irradiated by the X-rays was modelled as depicted in FigureS9.

Figure S9 .
Figure S9.Illustration of the experimental setup depicting the electrode|electrolyte interface probed by the X-rays.The incoming X-rays probed the sample at angle close to 90°, while the X-ray fluorescence are detected at an angle of  = 45° relative to the sample surface.Note that the length shown in the illustration is not up to scale.Further details regarding the estimation of the 'effective attenuation length of probing photon's and the 'effective detection depth in electrolyte' are provided in the following text.

Figure S10 .
Figure S10.Schematic illustration of (A) the pressure difference between the reactor chamber and UHV condition in the beamline and (B) the bending of Kapton/electrodes due to pressure difference between the pressure difference.Here  represents the elongation due to the pressure difference and  corresponds to the bending angle.Note that the bending illustration is exaggerated for visualization.
represent the absorption coefficient, thickness, and transmittance of the attenuating layer I, respectively.Upon reaching the first electrolyte layer next to the electrode surface, absorption occurs, leading to the fluorescence of the electrolyte layer.The intensity of X-ray fluorescence (If) is proportional to the incoming X-ray intensity (Ii) reaching the sample, following the relationship µ ∝     (see Ref. 6 ).Thus, the fluorescence intensity is: If = Cf Ii = Cf I0 (Tkpt TTi TPt), where Cf represents a proportionality constant correlating the generated fluorescence to the incoming intensity (note that Cf < 1).The fluorescence Xrays then travel back through the Kapton, Ti, and Pt layers where they are attenuated, and upon reaching the detector, they possess the intensity of:   ′ =    − incoming X-rays is absorbed by the electrolyte and induces fluorescence.The incoming intensity at this point is I= I0 (Tkpt TTi TPt) (TΔx), where TΔx is the transmittance of the electrolyte layer with the thickness of Δx.This process leads to fluorescence with the intensity of If = I0 Cf (Tkpt TTi TPt)(TΔx).Then the fluorescence will pass through the electrolyte layer of thickness (Δx/cos θ), layers of Pt, Ti, and Kapton before reaching the detector, where the intensity becomes If'= I0 Cf (Tkpt TTi TPt) (Tkpt' TTi' TPt') (TΔx TΔx').

Figure S12 .
Figure S12.Theoretically estimated ratio of recorded fluorescence intensity from each electrolyte layer, i (If,i'), compared to the total fluorescence originating from the entire electrolyte layer within the effective detection depth in the electrolyte (∑  , ′

Figure S13 .
Figure S13.Gas chromatograms of: (black) mixture of H2, O2, and N2 in Ar used during calibration; Ar with gas phase above the reaction mixture, (red) before the addition of Pt/C, and (green, blue) at different reaction times following the addition of Pt/C catalysts.

Figure S14 .
Figure S14.Rate of H2 production during first 3 hours following the dispersion of Pt/C catalysts in 125 ml of 0.03 mol dm -3 aqueous H3PO3, due to the Pt-catalyzed reaction between the H3PO3 and H2O.

Figure S15 .
Figure S15.Illustration of the model depicting the electrode|electrolyte interface probed by the X-rays.Note that the length shown in the illustration is not up to scale.Further details regarding the estimation of the 'effective attenuation length of probing photon' are provided in the following text.

Figure 1 .
Figure 1.B, 1.C, 2.A, 2.B,and 2.C. in the main text.Estimations were conducted with Eq.S8, with the incoming photon energy of 2156.5 eV (maximum energy of the XANES scan).Note that for XANES of 'Pt free' Kapton|(5 mol dm -3 ) H3PO3 with low dose, the incoming photon fluxes are different, since the measurement was performed in a different beamtime campaign with lower photon fluxes.Yet, due to the longer exposure time for that experiment, the radiation dose remains comparable to other measurements with lower radiation doses (e.g., XANES of planar Pt|(5 mol dm -3 ) H3PO3).
Figure S16 shows the example EOCP recording of Pt black|5 mol dm -3 during three sequential XANES experiments.

Figure S16 .
Figure S16.EOCP recording during three sequential XANES scans of Pt black|(5 mol dm -3 ) H3PO3.The inset plot provides a magnified view of the EOCP during XANES, showing rapid fluctuations as the valve is quickly closed and opened to minimize the radiation dose.Measurements were conducted at the temperature of 25 °C.

Figure 2 .
Figure 2.B in the main text), additional XANES measurements were conducted with varying incoming photon fluxes (which are proportional to photon fluxes reaching electrolyte solution, as indicated by Eq.S8), as depicted in Figure S17.
photon fluxes and to reduce the incoming photon fluxes for these experiments.

Figure S19 .
Figure S19.EOCP of planar Pt electrode|5 mol dm -3 (5M) H3PO3 under repeated application of synchrotron irradiation (SR, region shaded in gray) and without irradiation (illustrated by green arrow).An increase of EOCP is observed in the period without irradiation.The experiment was performed at a temperature of 25 °C.

Figure 15 .
Figure S19 illustrates the reproducible increase of EOCP (towards the initial EOCP of the measurement, as given in Figure 2.D in the main text) at the period at which radiation is blocked, right after synchrotron irradiation to the planar Pt|5 mol dm -3 H3PO3.

S36 16 .
Stability assessment of aqueous H3PO2 with and without the presence of Pt through ion-exchange chromatographyTo evaluate the stability of aqueous H3PO2, ion-exchange chromatography (IEC) experiments were conducted on a set of 10 mmol dm -3 H3PO2 that had been aged for various durations, both in the presence and absence of Pt catalysts, as depicted in FigureS21.The experimental setup used for these investigations was the same as described in Section 2.3 of the main text, with the eluent of 20 mmol dm -3 KOH being employed.

Figure S21 .
Figure S21.Ion exchange chromatography (IEC) performed on a 10 mmol dm -3 (10 mM) H3PO2 electrolyte that has been aged for a specific duration () at 25 °C.In (A), no Pt/C catalysts were dispersed in the electrolyte before aging, while in (B), 50 mg of Pt/C catalysts (40 wt% Pt) were dispersed before aging.(C) Concentration of H3PO2 and H3PO4, obtained from the IEC results in panels (A) and (B).A pronounced oxidation of H3PO2 to H3PO4 is observed after just 1 minute of aging.

FigureFigure 4
FigureS21illustrates a significant oxidation of H3PO2 to H3PO4 within 1 minute of aging in the aqueous 10 mmol dm -3 H3PO2.By comparing the IEC results of 10 mmol dm -3 H3PO2 with those of 10 mmol dm -3 H3PO3 (see Figure4.E in the main text), it becomes evident that the conversion of H3PO2 to H3PO4 occurs at a faster rate compared to the conversion of H3PO3 to H3PO4 in electrolytes of the same concentration.TableS4shows a comparison of the electrolyte aging time until 90% of the initial

Figure 2 .S38 17 . 5 (
B and 2.C in the main text).Theoretical estimation of H3PO4 generation in 5 mol dm -3 H3PO3 during positive potentials application on the Pt black electrodesFor further insight into the increasing XANES spectral weight corresponding to P(+5) compounds of Pt black|(5 mol dm -3 ) H3PO3 at various positive electrode potentials, a rough estimation of the maximum number of H3PO4 moles generated by electrochemical oxidation of H3PO3 to H3PO4 via Eq. in the main text) during XANES experiments was made.Estimations were made by the Faraday law of electrolysis for the reaction product's number of moles, given in the following Eq.S11.

Figure 3 .
A in the main text), and Ageo represents the geometrical area of the electrode in contact with the electrolyte (Ageo = 0.502 cm 2 ).

Figure
Figure S22illustrates the number of moles of product generated during XANES with Pt black, as estimated with Eq.S11.

Figure S22 .
Figure S22.Estimation of the number of mole of oxidation products: H3PO4, generated during P K-edge XANES with Pt black|(5 mol dm -3 ) H3PO3.The estimation assumes 100% Faradaic efficiency for the electrochemical oxidation of H3PO3 to H3PO4 (see Eq. 5 in the main text)

18 .
Figure S23.(A) CV and (B) Chronoamperometry (CA) profile of the planar Pt|(5 mol dm -3 ) H3PO3 system at 25 °C, during the in situ P K-edge XANES measurements.pα denotes the potential corresponding to the maximum current density of electrochemical oxidation of H3PO3 to H3PO4 via Eq. 5 given in the main text.The green arrows illustrate the current density response during the positivegoing potential sweep, while the orange arrows show the current density response during the going potential sweep.The slight oscillation response of current density observed during the CA is attributed to the rapid valve opening/closing utilized during the XANES experiment to minimize irradiation dose.(C) Comparison of the amount of theoretically estimated oxidation products, i.e., H3PO4, generated during XANES with 5 mol dm -3 H3PO3 on planar Pt vs Pt black.Estimations were made by the method detailed in section 12 of the SI.(D) Respective P K-edge XANES spectra for the different potential values shown in panel (A).A negligible spectral change is observed between the XANES spectra recorded with different potentials.

Figure S23 .
Figure S23.A and S23.B present the cyclic voltammetry (CV) and chronoamperometry (CA) profiles during XANES of 5 mol dm -3 H3PO3 on a planar Pt electrode.The observed current density is much lower compared to the measurements on Pt black, primarily due to the smaller surface area of the planar Pt electrode (around 5 times smaller, as detailed in Section S2).The corresponding theoretical estimation of generated H3PO4 during these measurements indicates that the production of H3PO4 in these experiments is about 20 times lower than in experiments with Pt black.The theoretical estimation is performed using the same method detailed in Section S17.The respective P K-edge XANES spectra 4, i.e., by the following equation:

Table S1
illustrates the mass of probed electrolyte for each electrode and electrolyte solutions, that is

Table S1 .
Estimated electrolyte mass that absorbs irradiation during in situ P K-edge XANES scans