Probing the proton exchange kinetics of BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ ceramic electrolyte by operando diffuse reflectance infrared Fourier transform spectroscopy

Abstract

Proton exchange kinetics plays an important role in governing the performance of intermediate-temperature protonic ceramic electrolysis cells (PCECs) for hydrogen production. Our understanding of the nature of the surface hydration reaction at the single-cell level, however, remains very limited, hampering further efficiency improvements. Here, we developed a custom operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) platform that operates under high temperature and steam conditions with applied bias. Quantitative investigations of surface H₂O/D₂O isotope exchange in a BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb1711) protonic electrolyte-based single cell were conducted under different applied voltages using this DRIFTS platform, to gain molecular-level insight into hydration kinetics. The findings show that the application of an external voltage significantly enhances the surface proton exchange rate, decreasing the apparent activation energy from 29.1 kJ mol⁻¹ at open-circuit voltage (OCV) to 6.8 kJ mol⁻¹ at 1.3 V. In addition, distinct voltage-induced spectral shifts in O–D vibrations point to dynamic changes in surface hydration. These findings demonstrate a sensitive spectroscopic platform for probing interfacial proton processes and reveal strong electrochemical control over surface proton kinetics, offering new opportunities for probing electrolyte hydration behavior in PCECs.

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Broader context

Protonic ceramic electrolysis cells (PCECs) are a promising technology for efficient hydrogen production at intermediate temperatures. A critical bottleneck lies in the hydration processes, which govern the concentration and mobility of protons and directly influence protonic conductivity and Faradaic efficiency. While recent studies have advanced materials and device performance, the fundamental understanding of surface hydration dynamics under realistic single-cell operation remains very limited, hampering further efficiency improvements. Protonic conduction mechanisms inferred from previous studies, mainly on electrolytes alone, may not accurately represent device conditions, leaving local proton dynamics largely unresolved. Thus, gaining direct insight into hydration and exchanging kinetics under practical cell operation is highly desirable. In this work, we developed a custom operando DRIFTS platform capable of operating under high temperature and steam conditions with applied bias. The platform also incorporated H₂O/D₂O isotope exchange for molecular-level insight into the hydration exchange kinetics. Our findings demonstrated that applied bias significantly accelerates surface hydration, providing direct evidence of how the electrical field impacts surface hydration reactions at the single-cell level. This discovery may have a profound impact on electrocatalysis and the use of PCECs in various energy-related applications.

Intermediate-temperature protonic ceramic electrolysis cells (PCECs), employing a protonic conductor as the electrolyte, represent a promising technology for efficient hydrogen production through steam electrolysis.^1–5 Currently, the acceptor-doped barium zirconates, typically formulated as BaZr_1−xM_xO_3−δ (M = trivalent cation, 0 < x < 1), have been intensively investigated as the electrolyte because of their higher protonic conductivity, good sinterability and chemical stability.^6,7 Proton carriers in these materials are introduced through a thermally activated hydration reaction, wherein water molecules react with oxygen vacancies to form hydroxide defects: .^8,9 At elevated temperatures, the electrolyte can facilitate proton diffusion through the hopping mechanism between neighboring oxygen ions in the presence of hydroxide defects .¹⁰ In other words, the protons are not stuck to any particular oxygens but are rather free to move from one oxygen to another, resulting in high proton mobility. Hydration in PCECs plays a central role in determining the concentration of mobile protons directly influencing proton conductivity and Faradaic efficiency.¹¹ While recent studies have demonstrated promising PCEC development, most have focused on materials development, fabrication, and device-level metrics with less attention to the fundamental hydration processes.^12–19 The limited understanding of surface hydration dynamics in practical single-cell operation, however, has hampered improvement of the electrolysis performance.

Proton conduction in ceramic oxides typically involves two coupled elementary steps: (i) proton transfer via hydrogen bonding between adjacent oxygens, and (ii) rotational reorientation of hydroxyl groups.²⁰ The rates of these processes are highly sensitive to the local bonding environment.²¹ Stronger hydrogen bonds may facilitate proton transfer but hinder OH group reorientation, introducing a trade-off that modulates macroscopic transport properties. Meanwhile, diverse surface hydration conduction phenomena were discussed over ceramic oxides in previous studies. For instance, by leveraging thermal desorption spectroscopy, Miyoshi et al.²² proposed that hydroxyl-terminated nanoparticle surfaces formed hydrogen bonds with water molecules, creating water-retaining nanochannels that enabled surface protonic conduction. Norby et al.²³ applied impedance spectra to simulate the equivalent circuit to separate the volume and surface protonic transportation. However, these protonic conduction mechanisms based on the electrolyte alone may not be accurately represented or adopted at the single-cell device level due to significantly different working conditions, leaving the local proton dynamics largely unresolved. Therefore, gaining direct insight into surface hydration and exchange kinetics under applied electrochemical bias in PCEC operation mode is highly desirable for the rational design of next-generation hydrogen production systems.^24,25

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is especially effective for detecting surface intermediate species under actual working conditions (i.e., operando).^26–29 While it has been widely used in room or low-temperature electrochemical systems, its application under intermediate-temperature conditions, particularly in the presence of steam and electrochemical bias, remains limited.^30–32 This gap is particularly significant for the investigation into the PCECs given that they are an emerging technology for hydrogen production.^33–35 For example, how surface hydration responds to electrochemical potential and how surface dynamics evolve under operando conditions have not been observed experimentally thus far. Therefore, coupling DRIFTS with electrochemical control and isotope labeling (e.g., H₂O/D₂O exchange) can enable real-time, molecular-level insight into surface reaction pathways and hydration kinetics in the protonic electrolyte of PCECs, unveiling key pieces of the puzzle. However, designing a high-temperature chamber that supports such electrochemical testing under humidified conditions while maintaining gas sealing, and thermal/electrical insulation is nontrivial, and requires extensive design effort and customized modification of standard testing fixtures.

To address the aforementioned challenges, we developed a custom operando DRIFTS platform tailored for intermediate-temperature PCECs. As shown in Fig. 1 and Fig. S1, the ceramic half-cell consisted of a thin, dense, single-phase BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb1711) electrolyte layer supported on a porous Ni–BZCYYb1711 hydrogen electrode that was prepared for operando DRIFTS experiments (Fig. S2 and S3). The cell was mounted inside the custom-designed high-temperature reaction chamber, with the electrolyte surface exposed to humidified air and aligned with the incident IR beam for direct spectral acquisition. The bottom hydrogen electrode was exposed to flowing dry hydrogen gas, thereby reproducing asymmetric gas conditions relevant to practical steam electrolysis operation. Silver paste was applied to both surfaces to serve as current collectors.

Fig. 1 (a) Photograph of the custom-built operando DRIFTS setup during operation. (b) Schematic illustration of the DRIFTS platform, highlighting its capability for simultaneous infrared spectroscopy and electrochemical measurements under controlled temperature and dual gas environments.

This platform enables time-resolved DRIFTS detection of hydration behavior under controlled temperature, distinct gas environments, and simultaneous electrochemical operation (Fig. 1b). By employing H₂O/D₂O isotope exchange, we monitored the interconversion between O–H and O–D vibrational modes and carried out quantitative kinetic analysis, providing direct insight into surface hydration exchange kinetics.^36,37 This integrated approach not only elucidates how electrochemical potential modulates interfacial proton exchange but also establishes a quantitative framework linking local surface chemistry with macroscopic electrochemical performance in PCECs. To the best of our knowledge, this is the first operando DRIFTS study to probe the hydration dynamics of a proton-conducting ceramic electrolyte operating in the single-device mode.

To validate the origin of the surface hydroxyl signals, DRIFTS were first collected under dry air at 600 °C. As shown in Fig. S4, the BZCYYb1711 electrolyte exhibits a weak yet distinct –OH stretching band centered around ∼3379 cm⁻¹,³⁸ even in the absence of externally introduced steam. No corresponding signal is observed from the ceramic sealing material (Fig. S5), confirming that the hydroxyl features originate intrinsically from the BZCYYb1711 surface.³⁰ While most previous studies have suggested that steam is required to initiate hydration, our results provide direct evidence that intrinsic hydration can occur without externally supplied H₂O. This observation underscores the electrolyte's strong hydration affinity, even under nominally dry conditions.

Time-resolved isotope exchange experiments were subsequently conducted by switching the feed gas from 3% H₂O to 3% D₂O in humidified air under open-circuit conditions. As shown in Fig. 2a and Fig. S6, the –OH band (∼3379 cm⁻¹) gradually decreases in intensity, while a corresponding –OD band (∼2500 cm⁻¹) simultaneously emerges and intensifies.³⁹ The absence of intermediate HDO features suggests a direct, one-to-one replacement of surface protons with deuterons.^40,41 Minor spectral features attributed to molecular H₂O (∼1650 cm⁻¹ and ∼3700 cm⁻¹) and CO₂ (∼2370 cm⁻¹) are also present but do not interfere with the analysis.⁴² It is notified that even after long-time equilibrium, a small residual –OH band remains, indicating that a fraction of strongly bonded hydroxyl species persist in the subsurface that are less accessible to D₂O molecules.⁴³ The normalized intensities of the –OH and –OD signals intersect at approximately 0.5, marking the crossover point. This crossover occurs more rapidly at elevated temperatures, indicating thermally activated surface exchange kinetics (Fig. 2b and Fig. S7). For example, such equilibrium is reached within 5 min at 600 °C but requires more than 25 min at 400 °C. To quantify the exchange kinetics, the natural logarithm of the normalized –OH intensity [ln(Rel. OH)] was plotted as a function of time (Fig. 2c).³⁶ The resulting linear trends demonstrate a first-order kinetic process, in which the exchange rate is directly proportional to the surface –OH concentration. We also conducted time-resolved isotope exchange experiments by switching the feed gas from 3% H₂O to 3% D₂O while monitoring the decay of the –OH band, and vice versa by tracking the decay of the –OD band, to determine the surface isotope exchange rate constants under each condition. As illustrated in Fig. 2d, the extracted rate constants were k (OH) = 0.1294 min⁻¹ and k (OD) = 0.0914 min⁻¹, yielding a kinetic isotope ratio of 1.41, closely matching the theoretical value of 1.4 based on the mass difference between H and D, i.e., (mD/mH)^1/2.³⁶ This agreement supports a proton hopping mechanism consistent with Grotthuss-like transport under open-circuit conditions.^22,44

Fig. 2 (a) Time-resolved DRIFTS of the BZCYYb1711 electrolyte during isotope exchange from 3% H₂O to 3% D₂O at 600 °C. Prior to the exchange, the sample was pre-equilibrated in 3% H₂O until the –OH intensity reached the steady state. The air flow rate was maintained at 50 sccm, and 3% H₂O or D₂O vapor was introduced using a controlled bubbling system; (b) normalized intensities of the –OH and –OD bands as a function of exchange time at different temperatures; (c) the plots of the natural logarithm of normalized –OH intensity [ln(Rel. OH)] versus time from (b). (d) Comparison of the –OH and –OD decay kinetics at 600 °C under OCV, obtained by switching the feed gas from 3% H₂O to 3% D₂O and vice versa. “Rel. OH” and “Rel. OD” represent the relative intensity of the OH and OD, respectively.

While open-circuit measurements provide fundamental insight into proton exchange, practical PCECs operate under applied voltage or current. To investigate how electrochemical bias affects surface hydration kinetics, operando DRIFTS isotope exchange experiments were conducted under varying applied potentials (Fig. 3). As shown in Fig. 3a, isotope exchange at 600 °C proceeds significantly faster under 1.3 V compared to OCV conditions, indicating accelerated surface proton exchange. This voltage-enhanced behavior persists across the full temperature range studied (Fig. S8). At 400 °C, the time to reach 90% isotopic equilibration is reduced by ∼73%, and even at 600 °C, a ∼19% reduction is observed (Fig. 3b and Fig. S9). This substantial improvement highlights the critical role of electrochemical potential in promoting surface proton exchange beyond thermal activation alone. Additionally, increasing the potential led to a distinct redshift and broadening of the –OD stretching band, along with the appearance of a low-wavenumber shoulder near ∼2450 cm⁻¹ (Fig. 3c), indicating a weakening of the –OD bond strength.^22,36,45 It was reported that the changes of hydrogen-bond interactions could promote reorientation of surface hydroxyl groups, thereby facilitating proton (or deuteron) transfer along the surface.²¹ Correspondingly, the surface exchange rate constant (k) increased monotonically with the applied voltage, reaching a maximum at 1.3 V (Fig. 3d). Beyond this potential, no further enhancement was observed, indicating a transition from voltage-limited to diffusion- or site-limited proton exchange, where proton availability might become the dominant constraint.

Fig. 3 (a) Time-resolved DRIFTS of BZCYYb1711 during H₂O/D₂O isotope exchange at 600 °C and 1.3 V. (b) Temperature dependence of isotope equilibration time (defined as the time to reach 90% exchange) under OCV and 1.3 V. (c) Equilibrium DRIFTS at 600 °C after H₂O-to-D₂O exchange under different applied voltages. (d) Surface exchange rate constant (k) as a function of applied voltage.

The activation energy (E_a) for proton exchange in PCECs is a critical descriptor that governs reaction kinetics and overall electrochemical performance. As shown in Fig. 4a, the apparent E_a under OCV is 29.1 kJ mol⁻¹. In stark contrast, application of a 1.3 V bias reduces E_a to just 6.8 kJ mol⁻¹, which represents a direct experimental quantification enabled by operando DRIFTS of voltage-induced enhancement in proton exchange dynamics. As summarized in Table 1, most previously reported isotope-exchange measurements were conducted at lower temperatures, whereas the current work demonstrates the feasibility of performing kinetic measurements at elevated temperatures (up to 600 °C). The surface E_a obtained under OCV in this work is comparable to values from in situ Raman methods, while its difference from the bulk conductivity method indicates the surface sensitivity of the operando DRIFTS. It should be noted that the nearly fourfold decrease in the E_a under 1.3 V highlights the significant role of electrochemical potential in accelerating surface hydration kinetics, providing new insights into practical PCEC operation enabled by such operando characterization. A mechanistic illustration is proposed in Fig. 4b to explain this behavior. Under OCV, surface proton exchange proceeds via a thermally activated Grotthuss-type mechanism, in which proton transfer is limited by local hydrogen bonding strength and OH group orientation. Upon applying a bias, the electric field induces charge redistribution at the surface, which in turn lowers the energy barrier for proton reorientation, thereby promoting faster proton exchange.

Fig. 4 (a) Apparent activation energy (E_a) for surface isotope exchange under open-circuit voltage (OCV) and 1.3 V. (b) Schematic illustration of the proposed voltage-enhanced proton exchange mechanism.

Table 1 Comparison of surface exchange constant (k) and activation energy (E_a) for proton-conducting oxides measured by different techniques

Materials	Atmosphere	Tools	k (min^−1)	E a (kJ mol^−1)	Ref.
BaZr0.1Ce0.7Y0.1Yb0.1O3−δ	3% D2O	Operando DRIFTS	0.1311 (600 °C);	29.1	This work
			0.1384 (600 °C and 1.3 V)	6.8 (1.3 V)
BaZr0.1Ce0.7Y0.1Yb0.1O3−δ	3% D2O	In situ Raman	0.0137 (300 °C)	21.0	46
SrCe0.95Yb0.05O3−δ	P H2O = 0.0004 Pa in vacuum	Elastic recoil detection	0.0084 (25 °C)	N/A	43
	P D2O = 0.0004 Pa in vacuum		0.00084 (25 °C)
SrZr0.9Sc0.1O3−δ	Saturated D2O	Conductivity measurement	N/A	59.8	47
	Saturated H2O			56.9
BaZr0.1Ce0.9Y0.1O3−δ	1.5% H2O	Conductivity measurement	N/A	56.9	48
	1.5% D2O		N/A	60.8
SrCe0.95Yb0.05O2.975	∼7% H2O	Conductivity measurement	N/A	74	49

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This operando DRIFTS study provides direct evidence that applied bias accelerates surface proton exchange in PCECs, while more in-depth kinetics and the proton conduction mechanism need further investigation. Looking ahead, the effects of ceramic structure such as grain size, and steam concentration on hydration reactions are not yet well understood. Another key step will be to couple these surface hydration dynamics with bulk proton conductivity to understand the transport pathways across the electrolyte. In addition, combining operando DRIFTS with artificial intelligence (AI) offers a promising way to predict interfacial behavior under diverse conditions. Beyond the electrolyte, this operando DRIFTS platform can also be extended to probe intermediate species in triple-conducting steam electrodes and electrocatalysts, paving the way for more efficient and durable systems for hydrogen production and beyond.

Author contributions

Y. M.: conceptualization, data curation, formal analysis, investigation, methodology and writing – original draft; F. L.: conceptualization, formal analysis, investigation, methodology, supervision, writing – original draft and writing – review and editing; M. L.: methodology, writing – original draft and writing – review and editing; Z. W.: methodology, visualization, writing – original draft and writing – review and editing; H. D.: methodology; Q. Z.: methodology; H. L.: methodology; W. W.: methodology; Q. S.: methodology; J. G.: methodology; Z. Z.: methodology; H. Z.: conceptualization, investigation, methodology, supervision and writing – review and editing; D. D.: conceptualization, funding acquisition, investigation, methodology, project administration, resources, supervision, writing – original draft, and writing – review and editing.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ee05957g.

Acknowledgements

This work is supported by the HydroGEN Advanced Water Splitting Materials Consortium, established as part of the Energy Materials Network under the U.S. Department of Energy (USDOE); the Office of Energy Efficiency and Renewable Energy (EERE); the Hydrogen and Fuel Cell Technologies Office (HFTO) under DOE Idaho Operations Office under contract no. DE-AC07-05ID14517.

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