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. 2024 Apr 17;146(18):12496–12510. doi: 10.1021/jacs.4c00436

Applying Nuclear Forward Scattering as In Situ and Operando Tool for the Characterization of FeN4 Moieties in the Hydrogen Evolution Reaction

Nils Heppe †, Charlotte Gallenkamp †,‡, Rifael Z Snitkoff-Sol §, Stephen D Paul †, Nicole Segura-Salas †, Hendrik Haak †, Dominik C Moritz ∥, Bernhard Kaiser ∥, Wolfram Jaegermann ∥, Vasily Potapkin †, Atefeh Jafari ⊥, Volker Schünemann #, Olaf Leupold ⊥, Lior Elbaz §, Vera Krewald ‡, Ulrike I Kramm †,*
PMCID: PMC11082898  PMID: 38630640

Abstract

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Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and is applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron–nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate-determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal–nitrogen–carbon (MNC) catalysts.

Introduction

The Hydrogen Economy scheme is based on the idea that surplus energy from intermittent sustainable sources could be used to generate hydrogen as a green energy carrier, which can be used for a wide variety of applications. At the heart of this scheme are electrolyzer technologies which electrocatalytically produce hydrogen from water for later use in power to X1,2 and power to metal3 applications, or in fuel cell electric vehicles.4,5 A group of catalysts that holds great promise for fuel cells6−11 and water electrolysis12−14 are metal–nitrogen–carbon (MNC; M = Co, Fe, Ni, Mo, Mn) catalysts. Structural characterization led to the conclusion that metal centers coordinated by four pyrrolic nitrogen atoms act as catalytically active sites for the hydrogen evolution reaction (HER).15−17 However, MNC catalysts often contain multiple metal speciation. For example, depending on the preparation, metal particles, oxides, or sulfides are found.18 The overlay of several compounds hinders structure–activity correlations, which in turn impedes the development of more advanced catalysts.19,20

Metallo porphyrins immobilized on high-surface-area carbons (M-Porph/C) have structural resemblance to MNCs, and thus could be used to mimic the molecular MN4 centers embedded in the amorphous carbon backbone.21−23 While there are several publications available in which the HER was explored in homogeneous systems, i.e., porphyrins dissolved in an organic solvent,24,25 reports on heterogeneous HER electrocatalysis with immobilized porphyrins are rare. For heterogeneous catalysis systems in an aqueous environment, HER electrocatalysis is described by two consecutive proton-coupled electron transfer (PCET) steps. In alkaline electrolyte, water acts as a proton source and is bound at the MN4 sites to become stepwise reduced under release of hydroxide ions.

In our previous work, we explored the effect of substituents on the porphyrin ring on the HER activity of carbon-supported iron porphyrin systems.23Ex situ structural characterization along with electrochemical measurements and theoretical calculations indicated that improved performance might be achieved when a ferrous iron ion with a porphyrin radical ligand [(P•)Fe(II)]− is formed (later denoted D) during the reaction, rather than the generally accepted PFe(I) state. We suggested that higher turnover frequency (TOF) is enabled by a shift in electron density from the iron center to one of the nitrogen ligands, forming a structure that facilitates O–H bond breaking in the adsorbed water molecule.23

Herein, we used in situ and operando nuclear forward scattering (NFS) to obtain new in-depth insights into the HER mechanism with a Fe porphyrin. Our insights point out opportunities to further develop this class of HER catalysts and even MNCs in general. NFS was first demonstrated in 1985 by Gerdau et al.26 This synchrotron-based technique can be regarded as Mössbauer spectroscopy (MS) in the time domain. Both NFS and MS are based on the nuclear resonance effect, which is highly sensitive to chemical changes in the environment of individual metal sites. In NFS, only the radiation that is elastically and coherently scattered in resonance with the Mössbauer nuclei is registered. In the most common transmission mode of MS, the energy of the incoming radiation from a radioactive source is varied around the nuclear resonance energy using the Doppler effect, and all of the radiation transmitted through the sample is registered. Nuclear resonance vibrational spectroscopy (NRVS)27 uses the fact that a nuclear transition can also occur when the energy of the incoming synchrotron radiation and that of a vibrational mode match exactly the nuclear transition energy. The same holds when the synchrotron radiation has exactly the energy to excite both the nucleus and a vibrational mode during the scattering process. While in principle more than two dozen isotopes are known to be applicable for these techniques,28 laboratory-based MS is mainly used to study Fe-57 and Sn-119 isotopes. Both NFS and NRVS can be applied at brilliant synchrotrons such as PETRA III, ESRF, Spring-8, or APS.29−37 The use of synchrotron radiation opens the window to a wider range of elements of interest for electrocatalysis or battery research, e.g., Ni-61,33 Ru-99,31 and Ir-193.38 NRVS provides information on vibrational modes and was used for freeze quenched studies to identify intermediate species in enzymes,39 the operando characterization of sulfide HER catalysts,40 but also in FeNC catalysts19,41 and their model systems.23 Reports on NFS for energy applications have been limited,42 and to the best of our knowledge, the method was not applied yet in electrocatalysis, while specifically for the discrimination of iron sites in FeNC catalysts, it holds great potential. We therefore first present the similarities and differences with MS as a baseline for the discussion of the work presented here and later showcase the use of NFS to elucidate the HER mechanism with an iron porphyrin as a model system for FeNCs.

Results and Discussion

Power of Deciphering Iron Site Distributions by MS versus NFS

In both NFS and MS, it is the nuclear transition between the ground state (Ig = 1/2) and the excited state (Ie = 3/2) that is used to extract information on the hyperfine interaction parameters (HIPs) of iron (isotope selective on Fe-57). Theoretical modeling suggests these HIPs to be especially sensitive toward changes in FeN4 environments compared to other spectroscopic parameters.43,49 In Figure 1, we summarize the impact of various hyperfine interactions on the energy levels in this nuclear transition and compare how it affects the MS and the corresponding NFS profiles.

Figure 1.

Figure 1

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Illustration of the impact of nuclear transitions (a–c) on resulting Mössbauer spectra (d–f) and nuclear forward scattering profiles (g–i). The top row visualizes the impact of an electric monopole (a), electric quadrupole (b), and magnetic hyperfine interaction (c) on the energetic levels of ground and excited state in the absorber (the following abbreviations are used: Ie, Ig: energetic levels of the nuclear excited and ground state; MHF: magnetic hyperfine field; τ1/2: half-life time of the excited state; CS: chemical shift; ΔEQ: quadrupole splitting; fwhm: full line width at half-maximum). Assuming thin layer absorber approximation, the simulated Mössbauer spectra and NFS profiles associated with a singlet I (d, g, CS = −0.05 mm s–1, fwhm = 0.4 mm s–1), a doublet II (e, h, CS = 0.17 mm s–1, EQ = 0.97 mm s–1, fwhm = 0.6 mm s–1), and a sextet III (f, i, CS = 0.0 mm s–1, H0 = 33 T, fwhm = 0.3 mm s–1) are compared. Note, the values that are used for the simulation are associated with common species found in FeNC catalysts,48 namely, superparamagnetic α-Fe, a FeN4 environment, and bulk α-Fe.

In MS, the sample is irradiated continuously with 14.4 keV γ-quanta that are formed in the radioactive decay of Co-57.48 After absorption of the γ-quanta in the sample, the excited state can relax back to the ground state by incoherently emitting another γ-quant. This absorption and emission process is termed resonance scattering. Due to different chemical environments, the energy of the emitted γ-quanta associated with the decay from the Co-57 source differs slightly from the energy associated with this transition in the sample. This shift in energy induced by the electric monopole interaction is indicated in Figure 1a by the arrows of different lengths. To bring both energy transitions into resonance, the emitted photon energy is altered by a Doppler movement of the source. The resulting spectrum is given in the energy domain (as the Doppler velocity). In the predominant transmission geometry, nuclear resonance scattering then appears as a single absorption line (Figure 1d). Using MS as an in situ or operando tool for FeNC characterization requires long measurement times of 8–48 h even with fully enriched samples and high electrode loadings.8,11

Using NFS, the sample is irradiated with short pulses (50–100 ps) of 14.4 keV photons. These pulses are absorbed in the sample by the Fe-57 nuclei, which brings them to the excited state indiscriminate of their chemical environment. Due to the short pulse compared to the nuclear lifetime of the excited Ie = 3/2 state in Fe-57, a nuclear exciton (coherent superposition of excited states over the resonant atoms) is formed.44 Consecutively, prior to the next pulse, the decay mode of the delayed forward scattered photons is measured in the time domain while the nuclei return to their ground states.45 The case of only an electric monopole interaction is shown in Figure 1d that gives a continuous decay in NFS. In the presence of more than one discrete resonance transition, the exciton decay leads to a characteristic interference profile in the time domain which is caused by quantum mechanical interference of the coherently emitted photons.44 The interference pattern, referred to as quantum beats, reflects the energetic differences between the resonance transitions in the time domain.46,47

An electric quadrupole interaction (Figure 1b) shows two absorption lines (doublet) in MS and periodic quantum beats in NFS (Figure 1e,h). An external applied magnetic field or an internal magnetic field induces a magnetic hyperfine field (MHF, see Figure 1c) leading to six allowed resonance transitions, which show up as a sextet in MS and as intricate interference patterns in NFS (Figure 1f,i).

Changing the mode of measurement from the energy (MS) to the time domain (NFS) enables significantly faster measurements with a lower sample quantity and the discrimination of inorganic minor phases from FeN4 moieties even at room temperature. This is elaborated by simulated spectra in Figure 2a–f, and further proven for a FeNC catalyst in the Supporting Information, Figure S1.

Figure 2.

Figure 2

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Illustration of Mössbauer spectra and NFS profile resulting from the overlay of two components systems. (a–f) Advantage of NFS over conventional MS: (a–c) exemplary Mössbauer spectra resulting from the overlay of two components, using the species from Figure 1 in a 1:1 area ratio. The envelope (i.e., sum spectrum of the subspecies) is shown in black. In comparison, (d–f) shows the corresponding simulations of NFS profiles. In (b), an example with two very similar doublet sites is shown (II: see above, IV: CS = 0.41 mm s–1, EQ = 0.96 mm s–1, fwhm = 0.6 mm s–1)—a situation expected, e.g., by the overlay of superparamagnetic iron oxide nanoparticles and FeN4 moieties, as often found in FeNC catalysts measured by RT Mössbauer spectroscopy.11 The corresponding simulation of the NFS profile in (e) shows the difference in the emission spectra when the fit of the two individual components is assumed vs a fit using one broad doublet that characterizes the sum spectrum of II + IV (V: CS = 0.29 mm s–1, EQ = 0.96 mm s–1, fwhm = 0.7 mm s–1).

Figure 2a–c showcases how the presence of two species alters the respective energy and time spectra. Figure 2b shows a practical example from MS (D1a and D1b from Ni et al.11) of two species with similar quadrupole splitting (ΔEQ) and slightly different chemical shifts (CS).

In MS, this could easily be misinterpreted as a single broad doublet species (as indicated from the envelope), specifically if further sites additionally overlay, which is often the case for FeNC. In contrast, the NFS profiles reflect the interference of resonance emissions, which permits easier discrimination of similar but not identical environments as demonstrated in Figure 2e.

Figure S1 shows the fitted data of the room-temperature and low-temperature Mössbauer spectra of a FeNC catalyst. At room temperature, the spectrum is composed of three doublet sites. There is no indication of inorganic impurities. Going down to 5.5 K gives clear evidence of the additional inorganic impurities that show up as sextet components (Figure S1e). Thus, in the case of MS, the measurement at low temperatures is required to enable magnetic ordering to discriminate between FeN4 moieties and inorganic impurities. In contrast, trying to fit the NFS profile (obtained at room temperature) with only the three doublets provides a worse fit that only improves a little by allowing the HIPs to change (Figures S1b,d). Only by adding magnetic components can a good fit quality be achieved (Figure S1f) that is in good agreement with the relative contributions and HIPs of iron sites in the low-temperature MS measurement.

It is important to note that NFS cannot provide a direct measure of the CS values. Fortunately, if more than one iron environment contributes to the profile, the difference in chemical shift (ΔCS) between the iron sites is obtained.45 Thus, in the case one component of known HIPs is present, it can act as an internal reference for the calibration of the chemical shift (CS*). In the given case (Figure S1) the D1 doublet can act as a refence (indicated by the dashed line at ΔCS = 0 mm s–1) that leads to full alignment of the HIPs for both room-temperature data sets. The larger CS values obtained at 5.5 K are in agreement with the expected change based on the second-order Doppler shift.

After the advantages of NFS are highlighted, it needs to be stated, however, that the fitting procedure is more complex and relies on libraries of existing and likely components for benchmarking obtained by MS or from theoretical calculations.

After having clarified the enormous benefits of NFS vs MS, in the following, we present a proof-of-concept study for exploring the HER mechanism with an iron porphyrin immobilized on carbon by in situ and operando NFS at different potentials. Each measurement took approximately 1 h, significantly faster than MS would have taken (in our previous work 48 h were required per in situ condition for a heat-treated Fe-Porph/C that comes closest in its morphology and iron content50). These NFS profiles are fitted and interpreted on the basis of MS obtained ex situ and calculated HIPs of relevant intermediates. We used these methods in combination with Fourier-transformed alternating current Voltammetry (FTacV) to assist in deciphering the HER mechanism.

Basic Characterization by Electrochemistry and Post Mortem Experiments

An iron porphyrin, 5,10,15,20-tetrakis(4-cyanophenyl)-21,23H-porphyrin-iron(III) chloride (PFe-Cl), is immobilized on carbon black (CB) (CB|PFe-Cl) and used as a model FeNC HER catalyst. To study its HER activity, cyclic voltammetry is conducted at a large potential window (Figure 3a), and additional CVs and RDE measurements are presented in Figure S2.

Figure 3.

Figure 3

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Electrochemical characterization with an indication of the in situ and operando conditions as well as MS data obtained at cond. 1 and 5. (a) RDE results of CB|PFe(III)-Cl in 0.1 M KOH: Cyclic voltammetry and HER Linear sweep voltammetry measured at 1600 rpm with a sweep rate of 10 mV s–1; the inset shows the porphyrin structure, as discussed in the main text, the dashed lines and numbers indicate at which potentials the NFS spectra are acquired during 1 h CA measurements (for detailed measurement sequence, see Table S4). (b) FTacV: Second and third harmonic components and the total current (inset) generated in the FTacV measurements (To increase visibility, only one in four sinusoidal segments is shown for the total current. The complete raw data used for analysis can be found in Figure S3). Parameters: frequency, 0.977 Hz; amplitude, 250 mV; scan rate, 4.882 mV s–1; initial potential, −0.1 V; end potential, −0.5 V. Mössbauer spectra taken at 295 K of (c) cond. 1, the pristine CB|PFe(III)-Cl powder and (d) post mortemcond. 5, a carbon cloth electrode coated with CB|PFe(III)-Cl after electrochemical testing.

The catalyst shows the expected capacitance from the high-surface-area CB support and a reversible Fe(II/III) redox couple at 0.38 V vs RHE. At −0.2 V vs RHE, the reduction current slightly increases, followed by a steep increase at about −0.5 V vs RHE, and reaches a current density of −20 mA cm–2 at 0.8 V vs RHE, attributed to the electrocatalysis of the HER.

Previous works show that for homogeneous catalysis (i.e. dissolved porphyrin in org. electrolyte) the formation of a PFe(I) is assumed to be the rate-determining step (RDS) for HER, while our previous work on heterogeneous HER electrocatalysis indicates that the reduction to PFe(II) is the RDS, with a significant contribution from the porphyrinic N atoms to the catalytic reaction.23−25 Capturing the Fe(I/II) redox couple in standard electrochemical experiments such as CV is problematic, as the potentials for this redox reaction overlap with the commencement of the HER. To understand the reaction mechanism and to be able to conclude if the porphyrin is reduced to PFe(I), we use FTacV to distinguish between Faradaic and chemical steps, and thus obtain better resolution on this possible transition. FTacV is well-known for its high kinetic sensitivity and ability to discriminate between irreversible catalytic currents and underlying reversible Faradaic processes.51−55

Figure 3b shows the second and third harmonic components generated from the FTacV measurements, conducted at an applied sinewave frequency and amplitude of 0.977 Hz and 250 mV, respectively, in the potential range of −0.1 to −0.5 V vs RHE. Further data at more negative potentials are given in Supporting Figure S3. The typical signal generated due to the presence of a reversible Faradaic species is lacking from the harmonic components.56 Measurements at higher frequencies (up to 5 Hz) did not result in any detectable signal, indicating that PFe(II) is not reduced to PFe(I) at this potential window.

The conditions for in situ and operando NFS measurements are selected on the basis of the electrochemical data. In the given potential window, we decided to probe four different potentials (labeled cond. Two −5, see top x-axis) that are representative for different catalysis conditions, which are compared to the initial ex situ state, cond. 1 (catalyst powder). This is followed (in order of time) by in situ measurements at open-circuit potential (OCP = 920 mV vs RHE, cond. 2) to probe the sample interaction with the electrolyte, then at 0.0 V vs RHE, which is above the onset for the HER but below the Fe(II/III) redox transition (cond. 3), and operando at −0.8 V vs RHE, which is in the regime of HER and below typical Fe(II) → Fe(I) reduction potentials (cond. 4). A final in situ measurement is made at the OCP (cond. 5) to check for reversibility of iron signatures. The differentiations of in situ and operando conditions are made according to M. Bañares, namely, operando refers to experimental conditions where the catalytic reaction takes place, while in situ is associated with the effect of the applied potential.57,58

The ex situ room-temperature (RT) Mössbauer spectra of cond. 1 and cond. 5 are given in Figure 3c,3d. The main findings derived from these measurements are summarized here as inputs for the fitting of NFS data. In the Supporting Information, further data are given and the fit development is discussed in detail (Supporting Note 1, Figures S4, S5 and Table S1). Note, these ex situ measurements are conducted prior to cond. 2 and post mortem for cond. 3 & 5, using the water rinsed and dried electrodes.

The initial catalyst powder cond. 1 shows an asymmetric doublet D1, which is typical for halide-binding ferric porphyrins.59−61 Also in the as-prepared electrode (cond. 2), D1 is the only detected site. Thus, the iron sites are not affected by the mixing with Nafion. The situation is changed for the post mortem electrode cond. 5. Here, 67% can be assigned to D1. However, two further doublets appear in the Mössbauer spectrum, D3b and D4. They are assigned to a ferric high spin site (D3b, 22%, CSRT = 0.39 mm s–1, ΔEQ,RT = 0.52 mm s–1) and a ferrous high spin site (D4, 11%, CSRT = 0.81 mm s–1, ΔEQ,RT = 0.68 mm s–1). For D4 the HIPs at RT are likely related to a Fe1–xO phase,62 indicating a demetalation of part of the iron-N4 moieties and subsequent oxide formation.

To further validate this conclusion, post mortem X-ray photoelectron spectroscopy (XPS) measurements are conducted. XPS is performed ex situ similar to MS on the pristine catalyst (cond. 1), the as-prepared electrode (cond. 2), and post mortemcond. 3 (0.0 V) and cond. 5 (OCPend). Figure 4 shows high-resolution scans of the N 1s (a) and Fe 3p (b) signals. Again, we note that the most relevant aspects for NFS interpretation are summarized here, and a detailed discussion is given in the Supporting Note 2 (and related Supporting Figures S6 and S7).

Figure 4.

Figure 4

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Photoelectron spectroscopy to verify the chemical states of iron and nitrogen. XPS results of the pristine powdered catalyst (cond. 1) and the prepared electrode layers after different treatments (cond. 2 as-prep. electrode prior to OCP, post mortemcond. 3 and post mortemcond. 5), for the high-resolution (a) N 1s and (b) Fe 3p regions.

The integration of the different N 1s species is done following established models.63−66 The results shown in Figure 4a indicate that the FeN4 moieties undergo partial demetalation during the HER and contribute to the rising of a N–H signal. However, an exact quantification is impeded by the overlay with cyanophenyl substituents that also undergo a reaction. In the Fe 3p spectrum associated with cond. 1, a broad peak is found that reflects the P(III)Cl state. The peak maximum is located at 56 eV. For all conditioned electrodes, the signal becomes broader and shifts slightly by 0.35 eV to lower BE. This is interpreted by a small contribution of Fe(II) to the spectra (as indicated in the spectrum of cond. 3). Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements of the electrolyte after testing cond. 5 let us estimate a leaching of only 2% of total iron from the electrode. Thus, XPS and ICP-OES of cond. 5 support our assignment of Fe1–xO formation by MS.

Nuclear Forward Scattering–Fit Model

The conclusions drawn from MS and XPS assist in the development of the fitting model for our NFS profiles. The site D1 associated with the halide bound PFe(III)-Cl is present under all MS and XPS probed conditions and thus similarly assumed in the NFS profiles of all samples. This enables us to translate the relative chemical shift (ΔCS) to a chemical shift CS* referenced to α-Fe, considering the CS of D1 vs α-Fe (CSD1 = 0.30 mm s–1), thus CS* is calculated as CS* = ΔCS + CSD1. Figure 5a shows the NFS profiles for the conditions indicated in Figure 3a. The corresponding electrochemical data of the potential hold, CVs and LSVs prior to and after cond. 4 can be found in the SI, Figure S8. Figure 5b compares the HIPs obtained from the NFS fits and Figure 5c, which is the ratio of each component. The fitting parameters are listed in Table 1, and in the Supporting Information, the general Fit model (Supporting Note 3) and the Fit development (Supporting Note 4) for each condition are given.

Figure 5.

Figure 5

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Summary of the in situ and operando nuclear forward scattering data. (a) NFS profiles of cond. 1–5 with the corresponding fit, the description, and validation of the related fits are given in the Supporing Note 4 and (b) HIPs of the identified iron sites as derived from the fit model and (c) their relative abundance for the given conditions.

Table 1. Summary of the Hyperfine Interaction Parameters Obtained from In Situ and Operando Nuclear Forward Scattering Experimentsa.

sample signal ratio % CS*mm s–1 ΔEQ mm s–1 MHF/rrelax assigned species
cond. 1 powder D1 100 0.30b 0.61 ±33b T / 78 A
cond. 2 BoTc, OCP D1 100 0.30b 0.77 ±33b T / 91 A
cond. 3 0 V D1 93 0.30b 0.77b ±33b T / 91b A
D2a 7 0.58 1.38   B
cond. 4 –0.8 V D1 74 0.30b 0.77b ±33b T / 91b A
D2b 21 0.65 1.42   C
D3a 6 0.41 1.63   E
cond. 5 EoTd, OCP D1 84 0.30b 0.77b ±33b T / 91b A
D2a 9 0.57 1.39   B
D3b 7 0.47 0.49   F
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a

HIPs as extracted from the fitting of NFS profiles, CS* reported vs α-Fe via internal referencing on D1, the magnetic hyperfine field (MHF) in T and the relaxation rate (rrelax) in number of relaxations per nuclear lifetime.

b

Parameter was fixed to a defined value.

c

BoT: beginning of test.

d

EoT: end of test.

NFS profiles of cond. 1 and cond. 2 are well fitted assuming only the presence of the same and asymmetric doublet D1 (SI, see Figure S9 for a direct comparison of the measurements and Fit 1.3 and Fit 2.1 for the individual fits). Upon applying 0.0 V vs RHE (cond. 3) a second species D2a is required to obtain a good fit (see comparison of fit development in the SI, Fits 3.1 and 3.2). On the basis of the electrochemical data provided in Figure 3 and the abundance of the species shown in Figure 5c, it is concluded that D2a (CS* = 0.58 mm s–1, ΔEQ = 1.38 mm s–1) is likely a ferrous species resulting from the reduction of the iron porphyrin. At cond. 4 the HIPs of the second doublet D2b (CS* = 0.65 mm s–1, ΔEQ = 1.42 mm s–1) and its share in the fit change and also a third component (D3a, CS* = 0.41 mm s–1, ΔEQ = 1.63 mm s–1) is required (see fits with one–three iron environments in SI, Fits 4.1–4.3). While at cond. 5 the NFS profile flattens, the beating patterns of doublet sites are still significant. Indeed, besides D1, D2a (CS* = 0.57 mm s–1, ΔEQ = 1.39 mm s–1) and D3b (CS* = 0.47 mm s–1, ΔEQ = 0.49 mm s–1) are required to reasonably fit the NFS profile (see the SI, Fits 5.1–5.3). The observation of D3b is well in line with the Mössbauer measurements (see above).

Assignment of Iron Species by Comparison to DFT

To identify the involved iron sites, HIPs are calculated using DFT for the starting compound and possible intermediates of the reduction cycle. The confidence level of HIPs can be extracted from our previous calibration study,67 the specific DFT methodology was described previously by Heppe et al. and the predictions were validated using NRVS and electrochemical measurements.23 All relevant HIPs obtained from DFT are listed in Supporting Table S2 and are directly compared with the HIPs from MS and NFS experimental results in Figure 6. For this comparison, the HIPs as determined by MS at RT as well as at 5.5 K (LT) are listed in Figure 6a. Due to the second-order Doppler shift (SOD) the CS at RT is shifted to lower values. Note, DFT does not account for temperature effects and therefore represents 0 K HIPs. Thus, the calculated CSs appear at higher values than the values derived from NFS (referenced by PFe(III)-Cl (D1) at RT) and RT MS. Figure 6c shows a direct overlay of DFT- and NFS-derived HIPs, from which the doublets are assigned to the following sites

Figure 6.

Figure 6

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Comparison of hyperfine interaction parameters obtained by different experimental techniques and theoretical calculations. HIPs as determined by (a) MS at 295 K (RT) and 5.5 K (LT), the numbers indicate the condition the species were detected at (error bars refer to 95% confidence interval), and (b) calculated by DFT, the trust region of the calculation is given by the gray cross-hair (B3LYP: CS ± 0.065 mm s–1, ΔEQ ± 0.18 mm s–1), the letters indicate the species the parameter set was calculated for and (c) a comparison of NFS-derived parameters (as previously shown separately in Figure 5b) with selected DFT data sets.

D1 must correspond to the signature of the initial iron porphyrin, PFe(III)-Cl (A), and D2a matches nicely with the HIPs of PFe(II)–OH (B) when considering the temperature effect caused by the SOD displayed in Figure 6a. D2b can be connected to PFe(II)–OH2 (C) and is thus further protonated; as discussed below, D can be excluded, since it represents an intermediate. Note that for D2a and D2b, the shifts in ΔEQ and CS*, respectively, fit nicely to the trend of the DFT values for B and C. The observed parameters of D3a are close to the calculated values of the intermediate spin (IS) PFe(III)–OH (E). Considering the difference in CS between experiment and theory observed for the previous species, it can be expected that the applied potential of −0.8 V vs RHE in the experiment leads to a higher CS for D3a than would be observed without an applied potential. This assignment is strengthened when considering that the ΔEQ values, which are less prone to errors, are in excellent agreement with the DFT prediction. The HIPs of D3b (CS* = 0.47 mm s–1, ΔEQ = 0.49 mm s–1) are indicative of an HS Fe(III) species. We exclude inorganic minor phases (see Supporting Table S3 and Supporting Note 1) and assign this site to PFe(III)–OH (F). No SOD shift is seen in the post mortem measurement cond. 5 (Figure 6a). For the sake of completeness, we note that E and F have the same chemical composition with two different spin states that have a small energy difference (see Supporting Table S2). While the multireference character of these species can thus not be excluded, the influence of the applied potential and putative changes in the environment such as hydrogen bonding may have a substantial influence on the ligand field splitting that can stabilize either the intermediate spin (E) or high spin (F) forms. With the given iron species and the limited changes expected during HER, a chemically sensible assignment of the iron sites formed in the experimental sequence to specific predicted species can be made.

HER Mechanism

Having identified the iron signatures under the various conditions, the HER mechanism can be elaborated. Figure 7 shows a simplified reaction scheme in which the iron sites identified in this work are highlighted, the detailed postulated scheme based on ex situ results is shown in Figure S10.23 When the electrode is swept below the Fe(II/III) redox potential, some of the pristine Fe(III) porphyrin (A) undergoes reduction (eq 1) to a Fe(II) state that quickly binds a hydroxide ion from the electrolyte forming (PFe(II)–OH)− (B).

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Figure 7.

Figure 7

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Deduced catalytic cycle of the hydrogen evolution reaction mechanism on molecular FeN4 sites. Reaction scheme of the HER on the investigated porphyrin system, highlighting the iron environments that are identified by post mortem MS and in situ and operando NFS. Reaction A → B was observed at cond. 3 (0.0 V), reaction B → C is assumed to start between 0.0 and −0.8 V (prior to cond. 4), reaction C → D is assumed to be the rate-determining step (RDS) of the cycle leading to the accumulation of D2b at operando condition.

Increasing the cathodic overpotential leads to a nonelectrochemical ligand exchange to PFe(II)–OH2 (C)

graphic file with name ja4c00436_m002.jpg 2

Since this PFe(II)–OH2 species accumulates at operando conditions, it is assumed to be the reactant of the RDS. In general, the intensity of the catalytic cycle intermediates (as fraction of the catalytically active sites) is governed by the quasi-stationary concentrations as defined by the reaction kinetics, with the species prior to the slowest step likely having the highest concentration. In our previous work,23 we found indications that the second reduction leading to a pseudo-Fe(I) species (D) might be related to the RDS. Based on the comparison of iron porphyrins with different substituents it was concluded that the higher the electron density on the N atoms of intermediate species D, the higher the turnover frequency for the HER.23 This intermediate species D is proposed to maintain the metal center in a Fe(II) state while the additional electron is delocalized in the porphyrin π-system, which is in agreement with investigations of pure reduced iron porphyrins by Römelt et al.68 PFe(I) states are reported to be inherently instable69 even though often proposed in homogeneous catalysis.24,70 The performed FTacV measurements do not show a Faradaic signal associated with the second electron transfer (Fe(I/II)) strengthening our hypothesis that it happens in conjunction with a fast irreversible nonelectrochemical reaction. Thus, the formation of species D is assumed to be an internal base reaction, deprotonating the water molecule of intermediate species C and forming a hydroxo ligand in the axial position on the Fe center and binding H to one of the four porphyrinic N atoms. It is assumed that after this step, an oxidative deprotonation leads to the formation of species E while releasing H2 to the electrolyte. These results are in line with operando X-ray absorption fine structure results published earlier by Cao et al. on a CoNC system investigated in an alkaline electrolyte,71 underlining the similar behavior of the porphyrinic and the MNC system and thereby the relevance of our data to single-atom catalysis. Moreover, with our operando NFS data, we can identify the intermediate C that is formed prior to the RDS as well as other species (B, E, and F) that belong to the reaction mechanism.

Based on the identified reactant for the RDS and the absence of a Faradaic signal, a multistep reaction (eq 3) through a Fe(I) species seems unlikely. Additionally, this multistep reaction would not agree with the PCET assumption discussed earlier.

graphic file with name ja4c00436_m003.jpg 3

On the basis of the potential where species E (PFe(III)–OH (IS)) is formed, it is most likely a stable state, which forms prior to the start of the next reaction cycle, that is initiated with the first reduction. The presence of this state can be linked to the assumption that part of the D2b-related iron species are at cond. 4 is associated with a partially higher contact resistance leading to a slower reduction process. A detailed discussion can be found in Supporting Note 3.

In regard to the reaction mechanism, this would mean that the oxidized PFe(III)–OH (HS) species F observed under cond. 5 is probably the same fraction that was already reduced at cond. 3. This observation is supported by in situ XAS investigations suggesting the formation of Fe(III)–OH species of phthalocyanines and porphyrins alike at similar potentials even in acidic electrolyte.72,73 Therefore, this part is postulated to contribute mostly to the observed activity by following the reaction scheme, as depicted in Figure 6. The fact that D3b can be observed at cond. 5 by in situ NFS as well as post mortem (MS) further strengthens the assignment of PFe(III)–OH (HS) (F) as well as the validity of the fit model. The fact that PFe(II)–OH (B) cannot be found ex situ is in agreement with the identification of a Fe(II) oxide species found in the related Mössbauer spectra (Figure S4c). This is supported by the fact that ex situ measurements on hydroxo iron porphyrins are rare and usually only Fe(III) species are described.74−76 It is to be assumed that PFe(II)–OH (B) can only be observed under in situ conditions and that the lack of potential and exposure to air lead to the degradation of this site, which is possibly associated with iron oxide formation as has been found in the post mortem Mössbauer spectrum of cond. 5 (D4).

Conclusions

In this work, in situ and operando nuclear forward scattering (NFS) is demonstrated as a powerful technique to follow structural changes under in situ and operando conditions. We explore the HER with a model catalyst, iron porphyrin supported on carbon. Our results demonstrate that based on the fit model developed from NFS and DFT different intermediates of the reaction mechanism can be assigned.

Specifically, we identify four species with different local environments associated with iron-N4 species found in the catalytic cycle that contribute in different relative quantities to the NFS profiles depending on the potential condition. This is proved here using a combination of NFS with XPS, MS, and FTacV. With HIPs predicted by DFT, an assignment to specific iron environments was made. We identify the species formed prior to the RDS and can assign it to a ferrous porphyrin complex with water as an axial ligand (C). The fact that this species is only present at operando conditions underlines the importance of in situ and operando NFS to confirm our previously postulated internal base mechanism (from theory and ex situ experiments).23 The knowledge of the species which is formed prior to the rate-determining step is important for further optimization and catalyst engineering. The in situ and operando results as well as the proven differentiability of FeN4 moieties from impurities, show that NFS has great potential for the in situ and operando characterization of FeNC catalysts or in general for the characterization of any electrocatalysts that contain a Mössbauer active isotope, which goes far beyond iron.

Experimental Section

Materials

For MS and NFS experiments fully enriched 57Fe porphyrin was used while commercial 1,2,3,4-tetrakis(4-cyanophenyl)porphyrin iron(III) chloride (TPP(CN)4-FeCl or short PFe(III)-Cl) was used for RDE, FTacV and XPS measurements. The preparation of the enriched porphyrin and its impregnation on a carbon support is documented elsewhere.23 In short, a free-base porphyrin gets metalated by heating it with an excess of freshly prepared 57FeCl3 anhydrous in DMF under N2 conditions. In contrast to the above-given reference, herein the product was precipitated from solution by slowly adding water until a clear yellow solution was obtained. The catalyst was prepared by impregnating a carbon support (63.4 mg BlackPearls2000) with one monolayer of PFe(III)-Cl (60 mg) using a DCM solution (15.3 mmol L–1) to disperse the support in it. By allowing the solvent to evaporate while treated in an ultrasonic bath an even distribution should be achieved.

General Electrochemistry

Electrode material and geometry as well as the catalyst loading were adapted for each spectroscopic characterization, to guarantee optimal spectroscopic results. Details on the preparation of the working electrodes are given in the corresponding subsections (see below, RDE end of this part). For all electrodes used in post mortem, in situ, or operando methods the obtained HER current densities were usually smaller than in the RDE because the working electrode was kept static. On the electrodes adapted for spectroscopy also the characteristic features of the CV got indistinct to a certain degree, as shown in Supporting Figure S2.

A PARSTAT 3000A DX potentiostat was used for the standard electrochemical investigations. All measurements were done in a three-electrode setup using a Hg|HgO|1 M NaOH reference electrode from ALS. This was calibrated every day vs a HydroFlex reversible hydrogen electrode (RHE) from Gaskatel to reference the given potentials to RHE. Typically, this resulted in a potential difference of around 907 mV, which is in good agreement with the theoretical expectation. As the counter electrode, glassy carbon rods purchased from HTW-Germany were used. A 0.1 KOH electrolyte was prepared by dissolving KOH pellets in ultrapure water (18.2 Ω). The catalyst layer was deposited on the electrodes by drop casting. For the ink preparation, 5 mg of catalyst was dispersed in 142 μL of water and then subsequently 25 μL of 5 wt % Nafion solution, 83.3 μL of isopropanol, and 125 μL of 0.1 M H2SO4 were added. For ideal dispersion, the mixture was treated 3 times successively in an ultrasonic bath (USB) for 45, 15, and 1 min, respectively. In between, the vial was placed on an orbital shaker for 1 min. To obtain the different loadings of the various electrodes, the amount of ink deployed was varied. Close to complete drying of the catalyst layer, the process is quenched by a drop of electrolyte to reach a homogeneous catalyst layer. The obtained working electrode was positioned in the electrolyte filled with N2-saturated premixed electrolyte. The measurements were performed under constant saturation of the electrolyte with N2. Regardless of the used working electrode type, the same electrochemical testing protocol was with an emphasis on the in situ and operando measurements. The detailed protocol is given in Supporting Table S4.

For the rotating disk electrode (RDE) measurements, a ring disk electrode with a glassy carbon disk electrode (ddisk = 5.5 mm, Adisk = 0.2376 cm2) was loaded with 0.5 mg cm–2 of catalyst and mounted on a Pine Research MSR 636A rotator. The HER cycles were performed at 1500 rpm.

Fourier Transform ac Voltammetry Measurements (FTacV)

The high kinetic sensitivity of FTacV was used to probe the catalysis of the HER. The working electrode was a rotating disc electrode (RDE, E5T, Pine) with a glassy carbon disc (0.196 cm2). Analogous to standard RDE, similar ink and same loading (0.5 mg cm–2) were used. The counter electrode was a glassy carbon rod, and the reference electrode was a homemade reversible hydrogen electrode. The electrolyte was 0.1 M KOH. The solution was purged with pure Ar for 10 min before the measurement started. A Biologic SP-300 potentiostat was used in all related measurements.

Inductive Coupled Plasma Optical Emission Spectroscopy

ICP-OES was used to quantify the iron concentration in the electrolyte of the electrodes used for MS experiments before and after the EC test protocol. Therefore, a defined mass of electrolyte (ca. 70 mL) was inserted in the cell. After finishing the test protocol and removing the electrode from the cell, conc. HCl was added to the electrolyte to reach pH 1 in order to prevent Fe(OH)3 precipitation. Afterward, the electrolyte was diluted to a defined volume (250 mL) and without further treatment measured in a SPECTRO GENESIS FES 27 instrument using the primary spectral iron emission of 239.562 nm. The external 5-point calibration was done using a 1 g L–1 Fe standard from Carl Roth. The average electrolyte Fe concentration was 60 μg L–1 before and 71 μg L–1 after the EC test. This corresponds to an iron leaching in terms of absolute quantities of 2.6 μgFe in reference to 125 μgFe in the catalyst layer.

X-ray Photoelectron Spectroscopy

XPS powder samples were prepared by pressing powder onto an 8 mm × 8 mm piece of indium foil, which was then mounted on a stainless-steel sample holder. For ex situ measurements of electrodes, 8 mm × 8 mm GC plates were coated on an area of (4 × 8) mm2 with 0.21 mg cm–2 catalyst and tested as described above. To mount the electrodes on the stainless-steel sample holder, the GC plates were rinsed with water and left to dry under ambient conditions. Afterward, the sample was introduced into ultrahigh vacuum (UHV) of the Darmstadt Integrated System for Fundamental Research (DAISY FUN).77 It is equipped with a PHOIBOS 150 hemispherical analyzer, CEM 9 Channeltron analyzer, and an XR50 M X-ray source, all three from SPECS Surface Nano Analysis GmbH. The XPS measurements were performed at pressures below 1 × 10–9 mbar with monochromatic excitation of Al Kα (1486.74 eV) radiation. The pass energy was set to 20 eV for the survey scans as well as high-resolution (HR) Fe 3p scans and to 10 eV for all other HR scans. In the case of iron, Fe 3p was chosen instead of Fe 2p, as a significantly better signal-to-noise ratio was apparent. A comparison of Fe 2p and Fe 3p can be found in Supporting Figure S7 for the powder sample. The energy scale of the setup is calibrated using Cu 2p, Ag 3d, and Au 4f core levels and Fermi edges. The evaluation software used was Igor Pro. The obtained spectra were analyzed with CasaXPS. The background in HR XPS was determined as Shirley type, to accommodate for inelastic electron scattering. The signals were fitted with a Gaussian/Lorentzian (70/30) line shape model. Only the Fe 3p was fitted with a linear background and a 60/40 Gaussian/Lorentzian line shape according to the fit model provided by Yamashita et al.78 In contrast to the fit of Yamashita no asymmetry in line shape was needed to fit the Fe 3p signals. The electrodes of cond. 3 and cond. 5 showed minor systematically increased BE of all HR scans. Therefore, the BE was referenced to the F 1s signal from HR scans to be 688.3 eV as determined for the as-prepared (cond. 2) electrode. The sample labeled as cond. 3 was quenched, by removing the electrode rapidly from the electrolyte while the potential of 0.0 V was still applied.

57Fe Mössbauer Spectroscopy

Powder samples were prepared by filling a pouch between two Kapton foils by using a brass sheet with a 15 mm hole as a spacer. MS electrodes were prepared by inserting a strip of carbon cloth with epoxide resin except for a 15 mm in diameter part where the catalyst was deposited with a loading of 1.0 mg cm–2. The spectra of cond. 2 were acquired from a stack of three freshly prepared electrodes without further treatment, i.e., prior to immersing in the electrolyte. For the post mortem (cond. 5) measurements, three freshly prepared electrodes were used to run the EC test protocol. Afterward, the electrodes were rinsed with deionized water, dried in air, and then inserted as a stack into the spectrometer. The room-temperature measurements were conducted in a home-build setup, with a velocity drive from Halder Instruments a proportional counter as detector, preamplifier, amplifier, and a CMCA-500 from Wissel. The 57Co/Rh-source (initial activity 100 mCi) is kept at room temperature and accelerated in a triangular waveform. Measurements at 5.5 K were performed in a similar setup in combination with a JANIS SHI-850–5 closed-cycle He cryostat. During the measurement, the cryostat is filled with 25 mbar He as heat exchange gas. All spectra were calibrated against a 25 mm Fe foil (Goodfellow) with natural 57Fe abundance at room temperature. Data treatment was done using the MossA software package.79

Nuclear Forward Scattering

The measurements were conducted at the P01 High Resolution Dynamics beamline at PETRA III (Deutsches Elektronen Synchrotron, DESY) during the beamtime I-20200863.29,30 The synchrotron operated with a 40 bunches filling mode and a photon energy of 14.4 keV (57Fe resonance) is selected at the P01 beamline. In between bunches, the delayed photons were detected by an avalanche photodiode detector (APD), yielding the nuclear forward scattering profiles. Data was treated using the CONUSS software package.80 In the program, the profiles can either be plotted in the time domain (as used in this work) or plotted as a function of t/τ0. The lifetime τ0 of the Ie = 3/2 state of Fe-57 is 97.8 ns. For in situ and operando measurements a glassy carbon electrode plate of 30 mm × 12 mm × 3 mm coated with a loading of 1 mg cm–2 catalyst on an active area of (10 × 5) mm2 was mounted in a trough-like cell. Maximizing the count rate was done by placing the electrode nearly parallel to the beam direction, to irradiate the full width of the catalyst layer. The optimal alignment was checked by an angular optimization of the electrode prior to running the test protocol. The electrochemical testing was conducted using the protocol given in the Supporting Information, Table S4. An error value for each individual parameter was estimated by varying it until the fit deviated significantly from the measured NFS profile. The deviation was deemed significant when χ2 increases by 10%.

Density Functional Theory

A detailed explanation and discussion of the theoretical model can be found in our previous work.23 All calculations were carried out using unrestricted Kohn–Sham density functional theory in version 4.2.1 of the ORCA suite of programs.81 For geometry optimizations and subsequent frequency calculations, the TPSS82 density functional was used with the Ahlrichs’ basis sets def2-SVP83 for the description of C and H atoms and def2-TZVP83 for all other atoms (Fe, O, and Cl). The Split-RI-J approximation was employed using the def2/J basis set.84 The convergence criteria for SCF and geometry optimization were set to “tight” in the ORCA nomenclature, and the size of the radial and angular grid was set to 6.0. Dispersion correction by Grimme with Becke-Johnson damping (D3BJ) was employed85,86 and water was used as an implicit solvent within the SMD model.87 For the calculation of HIPs, single-point calculations using the B3LYP88,89 density functional were performed on the optimized geometries, as previously calibrated by the authors.67 The electronic structures and relative spin state energies were evaluated using the same settings as for the HIP predictions, but with the OLYP density functional89,90 and excluding dispersion corrections.

Acknowledgments

Financial support from the Deutsche Forschungsgemeinschaft (DFG PAK981 with KR3980/8-1 and JA 859/34-1) is gratefully acknowledged. C.G. acknowledges support from Merck’sche Gesellschaft für Kunst und Wissenschaft e.V. V.S., U.I.K., and V.P. acknowledge financial support by the German Federal Ministry of Education and Research (05K22UK2 (V.S.), 05K19RD1 (U.I.K., V.P.)). U.I.K. and V.K. further acknowledge financial support by the CRC1487 Iron, upgraded! (DFG, Project number 443703006). The authors acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III, and the authors thank Ilya Sergueev, Réne Steinbrügge, and Deepak Prajapat for assistance in using P01 High Resolution Dynamics Beamline. Beamtime was allocated for proposals I-20200863 and I-20221267. C.G. and V.K. acknowledge computing time for this research project on the HHLR Lichtenberg high-performance computer at TU Darmstadt. The authors acknowledge support by the mass spectrometry core facility team of the Chemistry Department (TU Darmstadt) for measurements of the ESI/APCI spectra.

Data Availability Statement

The experimental data associated with this work will be uploaded to the TuDatalib repository of TU Darmstadt and be made available upon request to the corresponding author.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c00436.

  • Contains supporting notes on MS; XPS and NFS (description of fit model and fit development); supporting figures (comparison MS at RT and LT vs NFS of a FeNC catalyst; additional EC experimental data as well as MS; XPS and a detailed reaction mechanism); supporting tables (Mössbauer parameters, HIPs from DFT; HIPs of various inorganic iron compounds and EC testing protocol applied for in situ/operando NFS); and supporting references (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja4c00436_si_001.pdf (2MB, pdf)

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Supplementary Materials

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Data Availability Statement

The experimental data associated with this work will be uploaded to the TuDatalib repository of TU Darmstadt and be made available upon request to the corresponding author.

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