High hydrogen evolution activity and suppressed H₂O₂ production on Pt-skin/PtFe alloy nanocatalysts for proton exchange membrane water electrolysis

Abstract

We, for the first time, demonstrate high electrocatalytic activity for the hydrogen evolution reaction (HER) on PtFe alloy nanoparticles with stabilized Pt-skin layers supported on carbon black (Pt_xAL–PtFe/C), which allows the reduction of Pt loading to be lowered to 1/20 compared with a conventional Pt black cathode in proton exchange membrane water electrolysis (PEMWE). The area-specific HER activity of Pt_xAL–PtFe/C was found to be ca. 2 times higher than that of commercial Pt/C at 80 °C and −0.02 V vs. RHE. Pt_xAL–PtFe/C exhibited the additional important advantage of suppressed H₂O₂ production during the HER in the presence of O₂, which inevitably diffuses from the anode in PEMWE. Both the excellent HER performance and low H₂O₂ production are attributed to the lower adsorption energies of atomic hydrogen on Pt-skin surfaces, as revealed by DFT calculations.

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High purity hydrogen is produced by water electrolysis with renewable electric power (such as photovoltaics or wind power systems), which levels the output fluctuations when combined with stationary fuel cells. Proton exchange membrane water electrolyzers (PEMWEs) show advantages such as high energy conversion efficiency at high current densities, as well as a compact system with easy maintenance and start-up/shut-down. However, one of the disadvantages of conventional PEMWEs is the use of large amounts of noble metal catalysts such as (Pt + Ir) black at the oxygen-evolving anode (≥2 mg_Pt+Ir mg cm⁻²) and Pt black at the hydrogen-evolving cathode (≥2 mg_Pt cm⁻²).^1–6

Our recent research has aimed to develop new electrocatalysts that can reduce the amount of noble metals down to 1/10 while maintaining high voltage efficiency ε_V exceeding 90%. We have recently succeeded in preparing new anode catalysts, IrO_x nanoparticles on doped SnO₂ supports with a fused-aggregate structure, having the potential of decreasing the Ir loading to 0.1 mg cm⁻², while maintaining ε_V = 90% at 1 A cm⁻² and 80 °C.⁷

At the cathode side, Pt catalysts have been commonly used which have high activity for the hydrogen evolution reaction (HER) in acidic media. The use of Pt nanoparticles supported on carbon black (Pt/C) can reduce the Pt loading markedly, compared with Pt black,⁸ but Pt black has still been predominantly employed in practical PEMWEs in order to ensure a long lifetime of the membrane-electrode assembly (MEA). During water electrolysis, O₂ generated at the anode diffuses (“crosses over”) through the PEM to the cathode, resulting in the production of H₂O₂via a two-electron oxygen reduction reaction (ORR), as well as possibly via a chemical reaction of O₂ with adsorbed H atoms (or H₂ molecules). H₂O₂ can diffuse into both the PEM and the catalyst layer. Most of the H₂O₂ can be decomposed by Pt cathode catalysts. However, OH radicals are formed in the presence of impurities such as Fe²⁺.⁹ The HO˙ attacks the PEM, as evidenced by decreases in the thickness of the PEM after long-term operation.^8,10,11 It is essential to clarify whether Pt/C may intrinsically produce more H₂O₂, or it may produce more HO˙ since carbon supports might take part in the radical formation.¹² Therefore, we have focused on developing highly active, highly durable catalysts for the HER with suppressed production of H₂O₂ in the presence of crossover O₂.

Recently, we have found that Pt–M (M = Fe, Co, Ni) alloy catalysts, having a stabilized Pt skin (Pt_xAL, one to two atomic layers), supported on carbon black (Pt_xAL–Pt–M/C) exhibited higher activity for the hydrogen oxidation reaction (HOR) in 0.1 M HClO₄ solution at 70 to 90 °C than that of commercial Pt/C (c-Pt/C). Both the kinetically-controlled area-specific activity j_k and mass activity MA_k for the HOR on the Pt_xAL–PtFe/C at 20 mV vs. reversible hydrogen electrode (RHE) were about 2 times higher than those of c-Pt/C.¹³ In the present research, considering the general trend that Pt-based catalysts that display high activity for the HOR also display high activity for the HER,¹⁴ we examine the activity for the HER on Pt_xAL–PtFe/C, together with clarifying the H₂O₂ formation rate during the HER in the presence of dissolved O₂, compared with those on c-Pt/C and Pt black.

The Pt_xAL–PtFe/C catalyst was prepared in the same manner as that described previously.¹⁵ The specific surface area of the carbon black support was 780 m² g⁻¹. Two commercial catalysts, c-Pt/C (46.1 mass% Pt, TEC10E50E, TKK) and Pt black (TPT-200, Tokuriki Honten Co., Ltd), were used for comparison. XRD patterns, elemental distributions, and TEM images of the catalysts are shown in Fig. S1–S3 (ESI†). Electrochemical experiments were performed using a channel flow double electrode (CFDE) cell in 0.1 M HClO₄ at 80 °C,¹⁶ while the working electrode (WE) employed herein was a glassy carbon (GC) substrate in order to minimize the H₂O₂ production. The reference electrode used was a reversible hydrogen electrode (RHE), and all of the electrode potentials in this paper were given versus the RHE.

Fig. 1a shows iR-free polarization curves for the HER on Pt_xAL–PtFe/C, c-Pt/C, and Pt black in H₂-saturated 0.1 M HClO₄ at 80 °C, in which the current is shown as the apparent mass activity (MA). The value of MA at all potentials increased in the order Pt black < c-Pt/C < Pt_xAL–PtFe/C. The apparent area-specific activities (j_s, based on the electrochemically active area, ECA) at −0.02 V on these catalysts are shown in Fig. 1b. Since the value of j_s on c-Pt/C is comparable to that of Pt black, the higher MA on c-Pt/C than that of Pt black is well explained by its larger ECA (69.7 m² g_Pt⁻¹vs. 6.8 m² g_Pt⁻¹). Notably, the value of j_s on Pt_xAL–PtFe/C was about 2 times higher than that of c-Pt/C, providing high MA, e.g., the MA of Pt_xAL–PtFe/C at −0.02 V was −1.62 A mg_Pt⁻¹, which is about 1.6 times higher and 20 times higher than that of c-Pt/C (−1.01 A mg_Pt⁻¹) and Pt black (−0.08 A mg_Pt⁻¹), respectively. In addition, an accelerated durability test of the catalysts was conducted by potential cycling between −0.03 and 0.95 V at 80 °C in N₂-purged 0.1 M HClO₄, which simulates the start–stop cycles for PEMWEs (see Fig. S4, ESI†). As shown in Fig. 2, the HER MA loss of Pt_xAL–PtFe/C at −0.03 V after 5000 cycles was only 8%, whereas c-Pt/C showed a larger loss of 20%. The increased durability of Pt_xAL–PtFe/C was attributed to the uniform particle size of Pt_xAL–PtFe, as well as the increased rigidity of the Pt skin/alloy structure.^13,17 Such superior properties (HER activity and durability) of Pt_xAL–PtFe/C at the practical operating temperature would make it possible to reduce the Pt loading for the PEMWE.

Fig. 1 (a) HER polarization curves (iR-free) of Nafion-coated electrodes in H₂-saturated 0.1 M HClO₄ (mean flow rate of 111 cm s⁻¹) at 80 °C with potential scan rate of 5 mV s⁻¹, and (b) apparent Pt area-specific activity (j_s) at −0.02 V vs. RHE.

Fig. 2 Changes in MA values for the HER at −0.03 V and 80 °C measured after 5000 potential cycles (hatched bars) between −0.03 and 0.95 V in N₂-purged 0.1 M HClO₄ with the scan rate of 0.1 V s⁻¹ (see Fig. S4, ESI†).

Next, we have measured the H₂O₂ production rates at these catalysts during the HER. H₂O₂ emitted from the WE of the CFDE was detected as the oxidation currents at the collecting electrode (CE) while hydrogen was also being generated at the WE. We note that the H₂ generated at the WE would also, in principle, be detected as HOR current at the CE, but, at the high potential (1.4 V) of the CE, the HOR is experimentally found to be suppressed.¹⁸ Raw data are shown in Fig. S5 (ESI†). In any case, the small background current is subtracted. Fig. 3 shows the potential dependence of the H₂O₂ oxidation current density at the CE (a measure of H₂O₂ production rate, r(H₂O₂) at the three catalysts during the HER) measured in O₂-saturated 0.1 M HClO₄ solution. A general trend for all catalysts is that the r(H₂O₂) increases gradually from 0 to ca. −0.06 V. The r(H₂O₂) on Pt black was nearly identical with that of Pt/C between 0 to −0.03 V, but it became lower at increasingly negative potentials, which may be the reason for the use of Pt black so far. The r(H₂O₂) on Pt_xAL–PtFe/C was clearly lower than those on Pt black or Pt/C. Below −0.06 V, the r(H₂O₂) steeply decreased for Pt black and Pt/C. Such trend was also seen for Pt_xAL–PtFe/C, but the peak potential shifted negatively (−0.07 V). The H₂O₂ yields P(H₂O₂), i.e., the percentage of H₂O₂ production versus the total current at the WE, are also shown in Fig. S6 (ESI†). The values of P(H₂O₂) on these catalysts decreased monotonically with decreasing potential. Pt_xAL–PtFe/C exhibited substantially lower r(H₂O₂) than those of Pt black and Pt/C, especially in the potential range from 0 to −0.06 V. The suppression of H₂O₂ production at the cathode, as seen for the lower values of P(H₂O₂) and r(H₂O₂), can contribute with certainty to mitigating the chemical degradation of membranes or MEAs.

Fig. 3 Potential-dependent H₂O₂ oxidation current density for various catalysts measured in O₂-saturated 0.1 M HClO₄ solution at 80 °C.

Here, we propose that both the increase and decrease of r(H₂O₂) are due to reactions involving adsorbed hydrogen, specifically, involving O₂ to produce H₂O₂, and, in turn, its further reduction. It is well known for Pt single crystal electrodes that H₂O₂ can be produced quantitatively on the Pt(111) surface in the low-potential region where the coverage of underpotentially deposited hydrogen atoms θ(H_UPD) is high.¹⁹ This effect also can operate with Pt/C catalysts.^20,21 Thus, H₂O₂ is produced predominantly on (111) facets of the Pt nanoparticles due to end-on adsorption of O₂, with the neighboring sites blocked by H_UPD. It has been clarified that the adsorption of H_UPD on the (111) face of Pt–Ni or Pt–Co alloy is weaker and the θ(H_UPD) is lower than that of Pt(111).^22,23 From the cyclic voltammograms for the three catalysts in N₂-saturated solution (Fig. S7, ESI†), it is clear that the H_UPD adsorption on Pt_xAL–PtFe/C is shifted toward less positive potentials, suggesting that the surface sites for O₂ adsorption would be less blocked by H_UPD, reducing the end-on adsorption of O₂ and subsequent H₂O₂ production.

In order to understand the details of the increased rate for the HER and suppressed rate of H₂O₂ production on Pt skin–PtFe/C, density functional theory (DFT) calculations were carried out based on the stepped (221) surface, which includes (110) steps and (111) terraces, similar to those adopted in our previous work.¹³ In the present work, we consider that the steps are active sites for the coupled association of hydrogen atoms and desorption of H₂, commonly referred to as the Tafel step, as described by Santana et al. in their DFT study of the HOR and HER on Pt(110).²⁴ As shown in Table 1, the H adsorption energy at the (110) step edge of Pt skin/PtFe(221) is smaller than that of Pt(221). The activation energy for the Tafel step calculated is closely related to the H adsorption energy: Pt skin/PtFe(221) exhibited a smaller activation energy by 0.42 eV, compared with Pt(221) (see Fig. S8, ESI†). Thus, the higher rate for the HER on Pt_xAL–PtFe/C is reasonably explained by the decreased H adsorption energy.

Table 1 Adsorption energies (eV) of 2H and O₂ on Pt(221) and Pt skin/PtFe(221) surfaces

Adsorption site	Pt(221)	Pt skin/PtFe(221)
(110) step edge-2H	−1.37	−0.94
(110) step edge-O2	−1.71	−1.10
(111) terrace-2H	−0.91	−0.08

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Interestingly, on the (111) terraces of Pt skin/PtFe(221), the H adsorption energy was found to be even more significantly decreased, being smaller by 0.83 eV than that for Pt(221). This is consistent with lower θ(H_UPD) on the Pt skin/PtFe than those of Pt/C or Pt black, as shown in Fig. S7 (ESI†). In order to calculate the adsorption energy of H_UPD as a function of coverage, we have used flat (111) models. As shown in Fig. S9 (ESI†), a honeycomb structure of the bridging H could be simulated at potentials below 0 V.^25,26 As summarized in Table 2, the adsorption energies are similar to those for the (111) terraces on the (221) surfaces (see Table 1), especially for Pt(111). Irrespective of θ(H_UPD), Pt skin/PtFe(111) showed significantly lowered H adsorption energies than those for Pt(111).

Table 2 Adsorption energies (eV) of H on Pt(111) and Pt skin/PtFe(111) surfaces at θ(H_UPD) = 1/9, 1 and θ(H_UPD) + θ(H_OPD) = 4/3

θ(HUPD)	Pt(111)	Pt skin/PtFe(111)
1/9	−1.05	−0.38
1	−0.99	−0.36
4/3	−0.86	−0.17

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It is also important to discuss the role of the overpotentially deposited hydrogen H_OPD in these reactions. The H_OPD has been proposed to form below 0.1 V and to be involved in the HER. However, it differs from the H adsorbed at (110) steps discussed above, which is associated with a CV peak often observed at ca. 0.1 V.²⁷ Specifically, we have reported that an intermediate of the HER on Pt is atomic H in the atop configuration, based on the in situ FTIR study, i.e., a peak of 2080–2090 cm⁻¹ observed at potentials less positive than ca. 0.1 V was assigned to the H_OPD on a polycrystalline Pt film electrode.²⁸ Such an assignment has been validated by DFT studies on Pt(111).^25,26 The adsorption energies of the H_OPD are smaller than those of H_UPD, resulting in lower average adsorption energy (Table 2 and Fig. S10, ESI†). The very weak adsorption of H means that, after its formation, it is very reactive, as discussed below.

The H_OPD could chemically reduce O₂ to form H₂O₂ (step 1 in Fig. S11, ESI†), and subsequently reduce H₂O₂ further to H₂O (step 2 in Fig. S8 and S11, ESI†). The gradual increase of r(H₂O₂) at potentials between 0 to −0.06 V, and rapid decreases in r(H₂O₂) at less positive potentials on all three catalysts (Fig. 3) can be explained well by the stepwise reduction of O₂, based on our FTIR result that the coverage of H_OPD increases gradually below 0.1 V and increases steeply with further increase in the cathodic overpotential.²⁸ Thus far, we have explained the potential-dependence of H₂O₂ formation shown in Fig. 3. Further, the DFT calculations suggests a more facile reduction of H₂O₂ with H_OPD on Pt skin/PtFe(111) in comparison with Pt(111) (Fig. S12, ESI†), by 0.8 eV more exothermic, which can delay or slower the formation of H₂O₂, as seen for the less positive peak potential in Fig. 3. Thus, based on both indirect experimental and DFT results, we conclude that the weaker adsorption of both H_UPD and H_OPD, affording reduced end-on adsorption of O₂, as well as easier H₂O₂ reduction by H_OPD on the Pt_xAL–PtFe surface, is responsible for the smaller H₂O₂ production rate. However, obtaining direct experimental evidence addressing this point is challenging and will be the subject of ongoing research.

In addition, it was found that the HER rates on all catalysts were suppressed by the presence of O₂ (Fig. S5, ESI†). The suppression was comparable for Pt_xAL–PtFe/C and c-Pt/C (by 13%) and somewhat larger for Pt black (by 20%) at −0.1 V in 0.1 M HClO₄ solution saturated with O₂ (1 atm). This is consistent with the strong adsorption of O₂ at the HER-active step sites (Table 1). However, such a high O₂ concentration would not normally exist at the cathode catalyst layer/PEM interface, and thus, the depression of the HER current would be much lower. In any case, it is necessary to examine the actual oxygen concentration in the operating cell and its effects.

We have also examined the performance and durability of the Pt_xAL–PtFe/C cathode catalyst (0.2 mg_Pt cm⁻², 1/10 that of standard Pt black) and a conventional anode (IrO₂ + Pt black) in a PEMWE single cell with a reference electrode (RHE). The polarization curves recorded for cell potential and the electrode potentials vs. RHE are shown in Fig. 4a. The cell potential E_cell was 1.57 V (voltage efficiency of 94.3%) at 1 A cm⁻² and 80 °C. The Pt_xAL–PtFe/C cathode was found to operate stably for continuous operation at 1 A cm⁻² and 80 °C for 1000 h (Fig. 4b). In particular, we found that the membrane thickness was virtually unchanged (about 50 μm) after the durability test for 1000 h (Fig. S13, ESI†), suggesting that the formation of H₂O₂ near the Pt skin–PtFe/C cathode was possibly suppressed, which would be consistent with the CFDE results presented above. To further confirm that decreased H₂O₂ production results in increased cell stability, a cell stability test with a cathode consisting of a commercial Pt/C catalyst will be carried out and reported elsewhere.

Fig. 4 (a) Polarization curves for PEMWE single cell with Pt_xAL–PtFe/C cathode (0.2 mg_Pt cm⁻²) and IrO₂ + Pt black anode (0.92 mg_Ir+Pt cm⁻²) at 80 °C showing cell potential, anode potential and cathode potential from top to bottom. The electrolyte membrane was NRE212 (50 μm thick). (b) Potential variations during the long-term cell test at 1 A cm⁻² and 80 °C.

In conclusion, we report herein a new hydrogen evolution electrocatalyst having a Pt-skin surface on PtFe alloy nanoparticles dispersed on a carbon support (Pt_xAL–PtFe/C). It exhibited ca. 2-times higher HER specific activity than commercial Pt catalysts, together with lower H₂O₂ production during the HER in the presence of O₂. DFT results suggest that weak H adsorption on the Pt skin/PtFe surface boosts the HER while suppressing H₂O₂ production. This work contributes to expanding the understanding of HER electrocatalysis with accompanying H₂O₂ formation, as well as the cost reduction of PEM electrolysis devices, which should promote larger scale application.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by “Fundamental Research on Highly Efficient Polymer Electrolyte Water Electrolyzers with Low Noble Metal Electrocatalysts” from Grant-in-Aid No. 17H01229 for Scientific Research (A) from the Japan Society for the Promotion of Science.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supplementary figures. See DOI: 10.1039/c8cp06825a

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