Effect of the interfacial electric field on the HER on Pt(111) modified with iron adatoms in alkaline media

doi:10.1016/S1872-2067(22)64141-X

Abstract

Abstract:

The study of the hydrogen evolution reaction (HER) aimed to reach a deeper understanding of the parameters that control the rate of this reaction is of great importance given the technical relevance of hydrogen production as an energy vector in the so-called hydrogen economy. In previous works, laser-induced temperature jump (LITJ) experiments on Pt(111) modified with Ni(OH)₂ in alkaline media have revealed the importance of the interfacial electric field in the rate of the HER. It was hypothesised that small amounts of Ni(OH)₂ cause a decrease of the electric field because of a negative shift of the pzfc toward the onset of the hydrogen evolution. In this work, to test the validity of this hypothesis, the study has been extended to Pt(111) surfaces modified with Fe(OH)₂. The modified surfaces have been studied voltammetrically, and the voltammetric charges have been analysed. The voltammograms show a peak in the hydrogen evolution region that suggest the transformation in the adlayer from Fe(II) to Fe(0). In agreement with the coulometric analysis, the voltammetric features in the OH adsorption region would be related with the oxidation to the +3 valence state. The results obtained with LITJ method reflect the existence of a strong interaction of the Fe oxophilic species with the water molecules, shifting the potential of maximum entropy away from the onset of the HER. Hence, the most catalytic surface is the one with the lowest Fe coverage.

Key words: Hydrogen evolution reaction, Iron adatoms, Platinum single crystal, Pt(111), Laser induced temperature jump, Interfacial electric field, Potential of zero charge

Francisco J. Sarabia, Víctor Climent, Juan M. Feliu. Effect of the interfacial electric field on the HER on Pt(111) modified with iron adatoms in alkaline media[J]. Chinese Journal of Catalysis, 2022, 43(11): 2826-2836.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(22)64141-X

https://www.cjcatal.com/EN/Y2022/V43/I11/2826

Figures/Tables 14

Fig. 1. Cyclic voltammograms of the Pt(111) surface modified with different Fe(OH)2 coverages. Scan rate 50 mV s?1. 0.1 mol L?1 NaOH. Each one of these coverages 0.13, 0.23 and 0.38 were carried out after holding the working electrode under open circuit conditions during 35 s, 60 s, and 3 min respectively in an aqueous solution of 10-4 mol L?1 Fe(ClO4)2. Coverage is calculated according to Eq. (2).

Fig. 2. Evolution with the number of cycles of the voltammetric profile for the highest iron coverage. Scan rate 50 mV s?1. 0.1 mol L?1 NaOH.

Table 1 Average charge obtained from the anodic and cathodic peaks for each number of cycles from Fig. 2.

Number of cycle	Charge (µC cm^‒2)
1	246
2	248
4	264
7	269
12	280
30	288

Fig. 3. CO charge displacement curves for different iron coverages registered at 0.1 V (a) and their respective CO stripping recorded at 20 mV s?1 (b). Supporting electrolyte: 0.1 mol L?1 NaOH.

Fig. 4. Cyclic voltammogram for the Pt(111) modified with iron (θFe = 0.13 has been chosen as representative) before and after the CO stripping after the CO displacement at 0.1 V. Scan rate 50 mV s?1. Supporting electrolyte: 0.1 mol L?1 NaOH.

Fig. 5. CO charge displacement curves registered at 0.25 V for free Pt(111) and modified holding the electrode in 10-4 mol L?1 Fe(ClO4)2 during 35 s (a) and their respective CO stripping recorded at 20 mV s?1 (b). Supporting electrolyte: 0.1 mol L?1 NaOH.

Fig. 6. Cyclic voltammogram recorded after CO displacement at 0.25 V and CO stripping for the Pt(111) surface modified holding the electrode in 10-4 mol L?1 Fe(ClO4)2 during 35 s. Scan rate 50 mV s?1. Supporting electrolyte: 0.1 mol L?1 NaOH.

Fig. 7. HER of the Pt(111) modified with different iron coverages at 20 mV s?1 in alkaline media (0.1 mol L?1 NaOH) (a) and their respective Tafel plots (b). Tafel slopes measured in the linear region are indicated in Fig. (b). Coverages are calculated according to Eq. (2), except for the curve with θFe = 0.09, where the coverage was estimated from the hydrogen charge, according to Eq. (1). For the green and blue curves, the coverage is so low that cannot be estimated according to voltammetric measurements.

Fig. 8. Redox couple appearing during the HER in alkaline media (0.1 mol L?1 NaOH) of the Pt(111) surface modified with different iron amounts at 20 mV s?1.

Table 2 Involved charge under the peaks associated to the reduction and oxidation of the adsorbed iron species at -0.2 and -0.13 V for each iron coverage.

Time in 0.1 mmol L^‒1 Fe(ClO₄)₂	Cathodic peak (µC cm^‒2)	Anodic peak (µC cm^‒2)
35 s	184	63.5
60 s	328	112
3 min	432	181

Fig. 9. Plot of the hydrogen charge versus the iron charge at -0.13 V.

Fig. 10. Relationship between the charges involved in the OH region versus hydrogen charge (a) and iron charge (b).

Fig. 11. Laser ΔE vs. t transients recorded on the Pt(111) surface modified with different coverages of Fe(OH)2: << 0.09 (a), < 0.09 (b), 0.20 (c) and 0.40 (d). Laser beam energy: 0.8 mJ. Supporting electrolyte: 0.1 mol L?1 NaOH.

Fig. 12. ΔEpeak corresponding to each transient peak vs E for each iron coverage. (a) In case of the bipolar transient the value of zero has been given to ΔE, around which the peak potential is shown for each positive and negative contribution. (b) Plot of the thermal coefficients for each applied potential for the Pt(111) modified with different iron coverages. The corresponding values to the blank are taken from previous reports.

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