Quantitative investigation of CeO₂ surface proton conduction in H₂ atmosphere

Abstract

This report is the first describing a study quantitatively analysing aspects of oxide surface protonics in a dry H₂ atmosphere. Elucidating surface protonics is important for electrochemical and catalytic applications. In this study, AC impedance spectroscopy was used to investigate surface conduction properties of porous CeO₂ at low temperatures (423–573 K) and in a dry H₂ atmosphere. Results demonstrated that the conductivity increased by several orders of magnitude when H₂ was supplied. Dissociative adsorption of H₂ contributes to conduction by forming proton–electron pairs. Also, H/D isotope exchange studies confirmed protons as the dominant conduction carriers. Furthermore, H₂ adsorption equilibrium modelling based on the Langmuir mechanism was applied to explain the H₂ partial pressure dependence of conductivity. For the first time, the obtained model explains the experimentally obtained results both qualitatively and quantitatively. These findings represent new insights into surface protonics in H₂ atmosphere.

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The phenomenon of surface protonics has potential applications in proton-conducting fuel cells,¹ proton exchange membranes² and electrochemical gas sensors.³ This phenomenon is defined as proton transfer at oxide surfaces. Moreover, surface protonics plays an important role in heterogeneous catalysis.^4–17 Surface proton conduction is observed in the presence of H₂O or H₂ at temperatures of room temperature to 773 K.^18–31 Because adsorption plays an important role, porous samples at low temperatures are preferred; for H₂O adsorption, multilayers are formed. It is noteworthy that no multilayer is formed in the adsorption of H₂, thereby leading to a difference in surface proton transport behaviour: in an H₂O atmosphere, free protons and mainly hydronium ions move through the chemisorbed and physisorbed water layers on the oxide, respectively according to the Grotthuss and vehicle mechanisms.²⁹ Which mechanism prevails depends on the water layer thickness in relation to temperature and relative humidity.²⁹ However, in an H₂ atmosphere, dissociative adsorption of H₂ produces proton–electron pairs. The produced protons are transferred on the surface lattice oxygen by a hopping mechanism.³¹ The presence of a metal (e.g. Pt) is regarded as necessary for H₂ dissociation and spillover.

The evaluation of surface proton conduction is of great importance. For that evaluation, AC impedance spectroscopy is a promising technique because it allows separation of the electrical properties into components (e.g. bulk and grain boundaries), and facilitates their subsequent quantified. To date, many reports have described studies using AC impedance measurements to characterise surface proton conduction properties in an H₂O atmosphere.^18–30 Earlier reports described that surface proton conductivity under an H₂O atmosphere can be quantified using an equivalent circuit suitable for the analysis.^28–30 The equivalent circuit included surface components parallel to the bulk and grain boundary components, thereby allowing direct assessment of surface proton transport in the adsorbed water layer.

However, the evaluation of surface proton conduction under a dry H₂ atmosphere has been reported only qualitatively.³¹ It has not yet reached the stage of quantitative evaluation. Therefore, a more general theory of surface proton conduction under a dry H₂ atmosphere must be found.

This study investigated surface proton conduction properties induced by dry H₂ supply on porous CeO₂ by AC impedance measurements. Then a model was proposed to interpret those properties in terms of H₂ partial pressure dependence. This study provides new insights into surface proton conduction in H₂ atmosphere on an oxide surface. As the sample to be measured, we selected CeO₂ (JRC-CEO-1), because surface protonic transport on CeO₂ has been widely studied^25–28 and CeO₂ as a surface proton-conducting oxide is important for catalysis and fuel cell applications.^4–12,27 Experimental procedures are described in ESI.†

First, the H₂ supply effects on conductivity for porous CeO₂ were investigated at various temperatures by AC impedance measurement. The obtained data were analysed using a parallel RQ equivalent circuit presented in Fig. S1 (ESI†). The temperature dependence of the CeO₂ conductivity with and without H₂ supply is presented in Fig. 1. From these data, we inferred that the H₂ supply increased the conductivity by several orders of magnitude. The marked increase in conductivity is not attributable to oxygen vacancy formation, as explained in the discussion of ESI† (see Fig. S2 and S3). A change in the activation energy was also observed before and after H₂ supply, as shown in Table 1, suggesting a change in the conduction mechanism or conductive carriers. According to earlier reports, porous samples with relative density of around 60% provide information related to conduction with adsorbed species.^28,31 As shown in Fig. S4 in ESI,† SEM observations confirmed the presence of numerous pores in samples with relative density of approximately 60%.

Fig. 1 Temperature dependence of conductivity for CeO₂ under Ar and 5%H₂/Ar atmospheres.

Table 1 Apparent activation energy of conduction for each condition

Condition (—)	Activation energy (eV)
Ar only (>573 K)	1.33
Ar only (<573 K)	0.18
5%H2/Ar (>500 K)	1.48
5%H2/Ar (<500 K)	0.24

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The increase in conductivity can therefore be attributed to the dissociative adsorption of H₂ and the formation of conducting species, as presented in eqn (1)–(3).

Therein, * denotes adsorption site. Hydrogen spillover has been reported earlier.^32,33

Dissociative adsorption of H₂ is thought to have occurred on the Pt electrode. It then spilled over onto CeO₂, giving rise to proton and electron pairs. On various reducible oxides (e.g. CeO₂, TiO₂, WO₃), protons are said to bind to surface lattice oxygen; electrons reduce metal cations near O–H bonds.³⁴ Protons move over the oxide surface, whereas electrons move via the conduction band.

Next, the H/D isotope effect was investigated to identify the dominant conducting carrier in the H₂ atmosphere. As a result, the H/D isotope effect was identified at each measured temperature, as shown in Table 2 and in Fig. S5 in ESI.† Those findings are explainable by the different barriers posed by the hopping mechanism against the transfer of protons and deuterons. The deuteron transfer barrier is well known to be higher than the proton transfer barrier because of the different energies of the O–H and O–D ground states. According to classical theory, the theoretical value of the H/D isotope effect on the hopping proton/deuteron conductivity σ_D⁺/σ_H⁺ is expected to be (≒0.71),^35,36 as calculated using the following equation.³⁷

Table 2 H/D isotope effects on conductivity at each temperature

Temp (K)	σ D/σH (—)
573	0.79
548	0.67
523	0.59
498	0.72
473	0.69
448	0.68
423	0.69

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In that equation, D denotes diffusivity; m stands for the mass of the diffusing species. As Table 2 shows, the theoretical value closely approximates the experimentally obtained values. It can therefore be inferred that protons are the dominant conduction carrier in the H₂ atmosphere and that they are transferred via the hopping mechanism. The possibility of proton conduction in the bulk was ruled out; in general, the diffusivity of hydrogen in CeO₂ is low,³⁸ and considerably high temperatures (above 1050 K) are required for CeO₂ to exhibit the bulk proton conduction.³⁹ So we attributed the measured proton conductivity only to the oxide surface and not to the bulk.

Next, to elucidate the surface protonics in an H₂ atmosphere further, the effects of the H₂ partial pressure dependence were investigated. As Fig. 2 shows, the conductivity increased concomitantly with increasing H₂ partial pressure. At the low H₂ partial pressure region, the slope was 0.13. It then saturated gradually with increasing H₂ partial pressure, indicating a relation between conductivity and H₂ coverage. Based on these results, an H₂ adsorption equilibrium model was proposed to interpret the H₂ partial pressure dependence of conductivity.

Fig. 2 H₂ partial pressure dependence of conductivity for CeO₂ at 473 K.

As explained in the preceding section, dissociative adsorption of H₂ and spillover lead to the formation of protons on the CeO₂ surface, as shown particularly by eqn (1)–(3). For simplicity, eqn (1) and (2) were combined. The reaction on the Pt electrode was omitted.

The total equation for eqn (3) and (5) and the equilibrium constant K_total can be written as

where θ denotes the coverage over CeO₂. Considering charge neutrality ([H⁺] = [e⁻]), the following equation is obtainable.

Then, rearranging eqn (8) yields the following equation.

This equation represents the H₂ partial pressure dependence of proton concentration. However, the relation between H₂ partial pressure and coverage is not considered. The H₂ partial pressure dependence is explainable by the Langmuir adsorption model. The Langmuir adsorption model includes the assumption that the adsorption sites are homogeneous without interactions and that no interaction exists between neighbouring adsorbed molecules. The model also describes the adsorption of molecules on a solid surface as a function of gas pressure at a constant temperature. Applying the Langmuir adsorption model to eqn (5) gives the following equation.

Therein, K_H2 denotes the Langmuir adsorption constant. The H₂ atom coverage represented in the eqn (10) can be assumed to be similar to θ. Then, substituting the coverage in the eqn (9) yields the following equation.

In that equation, K_H⁺ is the equilibrium constant for eqn (5). The carrier concentration such as that of protons is related to the conductivity.

σ = zFcμ (12)

Therein, z, F, c, and μ respectively denote the charge number, Faraday's constant, carrier concentration, and mobility.

Substituting the proton concentration expressed in eqn (11) into eqn (12) yields the following equation.

In this equation,

Therefore, the following can be inferred from eqn (13); (1) At low H₂ partial pressures, . Then the proton conductivity is proportional to the power of 1/4 of the H₂ pressure. (2) At high H₂ partial pressures, . Then the proton conductivity is independent of the H₂ partial pressure. These conclusions show close agreement with the experimentally obtained results. Furthermore, to explain the dependence of conductivity on H₂ partial pressure quantitatively rather than qualitatively, eqn (13) can be rearranged as shown below.

The values of A and K_H2 are obtainable by the slope and intercept of . The relation between is shown in Fig. 3.

Fig. 3 Relation between PH2−0.5 and σH+−2.

As expected, a linear relation was confirmed. The values of A and K_H2 were calculated respectively as 2.5 × 10⁻⁹ S cm⁻¹ and 4.8 × 10² atm⁻¹. Substituting these values in the eqn (13) enables calculation of the value of σ_H⁺ at the H₂ partial pressure. A comparison of the experimentally obtained data with the calculated data is presented in Fig. 4. The calculated σ_H⁺ showed good correspondence with the experimentally obtained results, thereby proving the model validity.

Fig. 4 H₂ partial pressure dependence of conductivity for CeO₂ at 473 K: experimentally obtained data and calculated data.

In summary, AC impedance measurements were taken to characterise the electrical properties of porous CeO₂ in a dry H₂ atmosphere. A marked increase in conductivity with H₂ supply was observed, suggesting the formation of conductive carriers (protons and electrons) by dissociative adsorption of H₂ as shown in Fig. 5. The H/D isotope effect was also observed, indicating protons as the dominant conductive carriers. Furthermore, the proposed H₂ adsorption equilibrium model quantitatively describes the H₂ partial pressure dependence of the conductivity. The model presented herein provides new insights into surface protonics in a dry H₂ atmosphere. The development of surface protonics in a dry H₂ atmosphere and the establishment of a method for measuring and evaluating such protonics are expected to be applied to various catalytic reactions such as ammonia synthesis in a dry H₂ atmosphere in the future.

Fig. 5 Schematic image of the surface proton conduction in a dry H₂ condition on CeO₂.

Conflicts of interest

The authors have no conflicts of interest.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cc03687h

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