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First principles investigation of water adsorption, dopant segregation and surface thermodynamics of Mg and Sr substituted CaO(001) for Ca(OH)2/CaO thermochemical energy storage

This study uses density functional theory to demonstrate that while Mg and Sr dopants minimally affect the bulk equilibrium of the Ca(OH)₂/CaO thermochemical cycle, their strategic placement on the CaO(001) surface significantly enhances water binding energy and desorption temperatures, with surface Mg and Sr acting through distinct mechanisms to tune water uptake independently of the storage temperature.

Original authors: Abdul Rauf, Abdul Shakoor Sabir, Muhammad Danish

Published 2026-10-28

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Abdul Rauf, Abdul Shakoor Sabir, Muhammad Danish

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where we could capture the sun's heat during the day and hold it in a solid material, releasing it later to warm our homes or power factories. This is the promise of thermochemical energy storage, a method that relies on reversible chemical reactions between a solid and a gas. One of the most promising candidates for this job is a simple, abundant material: calcium oxide, commonly known as quicklime. When this material meets water vapor, it transforms into calcium hydroxide, a process that releases a significant amount of stored heat. To get the energy back out, the material is heated to drive the water away, returning it to its original state. The cycle is clean and the materials are cheap, but there is a catch. The speed at which the material absorbs water—and thus how quickly it can deliver heat—depends entirely on what happens at the very surface of the solid. If the surface is sluggish, the entire system slows down, especially as the temperature rises or the steam pressure drops.

Researchers have long known that adding small amounts of other elements, called dopants, can change how materials behave. In a recent study, scientists investigated two specific elements, magnesium and strontium, to see if they could make the surface of calcium oxide more eager to grab water. They used powerful computer simulations to watch, atom by atom, how a single water molecule lands on the surface of calcium oxide and breaks apart. The goal was to understand if these additives could speed up the reaction without messing up the delicate balance of temperatures required for the energy storage cycle to work efficiently.

The study began by building a digital model of the calcium oxide surface, a flat plane of atoms arranged in a grid. The researchers placed a single water molecule on this surface and watched how it settled. They found that the water molecule does not stay whole. Instead, it splits apart almost immediately. One part of the water, a hydroxyl group, attaches itself to two metal atoms on the surface, while the other part, a hydrogen atom, jumps onto a nearby oxygen atom on the surface. This split state is the most stable way for the water to exist on the surface, and it is the starting point for the material to turn into calcium hydroxide.

The team then replaced some of the calcium atoms in their model with magnesium or strontium to see how these changes affected the water. They discovered that both elements made the surface hold onto the water more tightly, but they did so in completely different ways. Magnesium, which is smaller than calcium, worked by stabilizing the hydrogen atom that had jumped onto the surface. It acted like a strong magnet for that specific hydrogen, pulling it firmly into place. Strontium, which is larger than calcium, worked differently. It provided a better spot for the hydroxyl group to attach itself, making that part of the split water molecule feel more comfortable.

When the researchers tried using both magnesium and strontium together, they expected the benefits to add up, creating an even stronger grip on the water. However, the results showed that the two elements did not work as a team. Because both elements were trying to stabilize the same hydrogen atom that had jumped onto the surface, their efforts overlapped. The extra strength gained from having both was less than the sum of their individual strengths. In fact, the larger effect of the two dominated the result, meaning that adding the second element provided very little extra benefit for the surface reaction.

Perhaps the most surprising finding concerned where these elements actually sit on the material. In a solid block of calcium oxide, magnesium prefers to hide just below the surface, while strontium naturally floats to the top. The researchers found that when water is present, magnesium is tempted to move up to the surface to help hold the water. However, this move only happens at lower temperatures. As the temperature rises toward the operating range of the energy storage system, magnesium retreats back below the surface, leaving the surface mostly to the calcium and strontium. Strontium, on the other hand, stays at the surface regardless of the water or the temperature.

This distinction is crucial for the future of energy storage. The study showed that while magnesium can make the surface hold water much more strongly, it is unreliable because it tends to hide when the system gets hot. Strontium, by contrast, stays right where it is needed and consistently improves the surface's ability to grab water, raising the temperature at which the water would otherwise evaporate by a significant margin. This means that strontium can make the hydration process faster and more efficient without changing the fundamental temperature at which the energy storage cycle operates.

The researchers also found a way to predict how strongly the water would bind just by looking at the vibration of the new chemical bonds formed on the surface. They discovered a direct link between the strength of the bond and the frequency at which the new surface hydroxyl group vibrates. This suggests that scientists could use infrared spectroscopy, a technique that measures these vibrations, to test how well a real-world material is performing without needing to run complex chemical tests.

Ultimately, the study reveals that the surface of the material is a distinct world from the bulk material inside. While adding magnesium or strontium barely changes the overall temperature balance of the energy storage cycle, their presence at the surface can dramatically alter how the material interacts with water. By choosing the right element and ensuring it stays at the surface, engineers can tune the speed of the reaction almost independently of the storage temperature. Strontium emerges as the most dependable choice for this task, offering a steady improvement in performance that could help make thermochemical energy storage a more viable solution for capturing and using renewable heat.

Technical Summary: First Principles Investigation of Water Adsorption, Dopant Segregation, and Surface Thermodynamics of Mg and Sr Substituted CaO(001)

Problem Statement
Thermochemical energy storage (TCES) based on the reversible hydration of calcium oxide ( $Ca(OH)_2 \rightleftharpoons CaO + H_2O$ ) is a promising technology due to the abundance and low cost of materials. However, the rate of heat release during discharge is limited by the kinetics of water hydration, which initiates at the $CaO$ surface. While additives are commonly used to improve performance, the specific role of isovalent dopants like Magnesium ( $Mg^{2+}$ ) and Strontium ( $Sr^{2+}$ ) at the surface level remains unclear. Previous companion studies indicated that these dopants have a negligible effect on the bulk equilibrium temperature of the cycle (shifting it by only $-5$ to $+25$ K). The central question addressed here is whether these dopants influence the critical first step of the process: the adsorption and dissociation of water on the $CaO(001)$ surface, and whether they segregate to the surface under operating conditions.

Methodology
The study employs Density Functional Theory (DFT) using the PBE functional within the Quantum ESPRESSO framework.

Models: $CaO(001)$ surfaces were modeled using a four-layer slab in a $(2 \times 2)$ unit cell. Dopants ( $Mg$ , $Sr$ , or both) were placed in the top layer or the second layer to simulate various segregation scenarios.
Adsorption: Water adsorption was investigated at a coverage of 1/4 monolayer (one molecule per surface cell). Multiple initial configurations (intact vs. split) were optimized to find the global minimum energy state.
Thermodynamics: Adsorption free energies ( $\Delta G_{ads}$ ) were calculated by including zero-point energy (ZPE) and vibrational contributions. Two entropy models were compared: a harmonic oscillator model and the Campbell and Sellers (CS) correlation. Desorption temperatures ( $T_{des}$ ) were defined where $\Delta G_{ads} = 0$ .
Segregation: Segregation energies ( $E_{seg}$ ) were computed for clean surfaces and surfaces with adsorbed water. An effective segregation energy ( $E^{eff}_{seg}$ ) was derived to determine dopant preference in equilibrium with water vapor across a range of temperatures and pressures.
Analysis: Electronic structure was analyzed via projected density of states (PDOS) and charge density differences. Vibrational frequencies of the surface hydroxyls were computed to identify spectroscopic signatures.

Key Results

Adsorption Mechanism: On all surfaces (pure and doped), the most stable state involves the dissociation of the water molecule. The resulting hydroxyl bridges two surface cations, while the proton attaches to a neighboring surface oxygen.
- Binding Energies: Relative to pure $CaO$ ( $E_{ads} = -0.84$ eV), $Mg$ in the top layer increases binding to $-1.17$ eV, and $Sr$ increases it to $-0.98$ eV. When both are present, the binding is $-1.18 $eV.$ Mg$ in the second layer slightly weakens binding ($-0.81$ eV).
- Non-Additivity: The effects of $Mg$ and $Sr$ are not additive. The combined gain is smaller than the sum of individual gains because both dopants act on the same shared proton and hydroxyl system; once the proton is strongly stabilized by $Mg$ , the additional stabilization from $Sr$ is diminished.
Mechanistic Roles:
- $Mg$ : Acts by stabilizing the surface oxygen that accepts the proton. It does not significantly stabilize the hydroxyl site itself.
- $Sr$ : Acts by providing a more favorable site for the bridging hydroxyl, consistent with the stronger water binding observed on pure $SrO$ .
Vibrational Signature: A strong linear correlation ( $r = -0.96$ ) exists between the adsorption energy and the stretching frequency of the newly formed surface hydroxyl ( $\nu(O_sH)$ ). As binding strengthens, the frequency increases from 2197 $cm^{-1}$ (weakest) to 2693 $cm^{-1}$ (strongest). This suggests infrared spectroscopy could be used to monitor binding strength.
Segregation Behavior:
- Clean Surface: $Sr$ (larger ion) strongly prefers the top layer ( $+0.46$ eV), while $Mg$ (smaller ion) prefers the second layer ($-0.29$ eV).
- With Adsorbed Water: The presence of water reverses the preference for $Mg$ , making the top layer energetically favorable ( $+0.08$ eV). However, in equilibrium with water vapor, this preference is only maintained below specific crossover temperatures (530–790 K at 1 bar, depending on the thermodynamic model). Near the bulk equilibrium temperature of the cycle, $Mg$ is expected to remain in the second layer in harmonic models.
Impact on Desorption Temperature:
- Surface doping significantly raises the temperature at which water desorbs. $Mg$ in the top layer raises $T_{des}$ by 190–250 K, and $Sr$ by 65–105 K.
- Crucially, these surface effects are decoupled from the bulk equilibrium. While the bulk equilibrium temperature shifts by only $-5$ to $+25$ K with these dopants, the surface water retention is enhanced by an order of magnitude more.

Significance and Claims
The paper concludes that surface doping offers a pathway to tune the hydration kinetics of the $Ca(OH)_2/CaO$ cycle independently of the storage temperature level.

Strontium ( $Sr$ ): Identified as the most reliable dopant for surface enhancement. It naturally segregates to the surface and increases water retention without significantly altering the bulk equilibrium temperature.
Magnesium ( $Mg$ ): Offers a stronger enhancement of water binding but is conditional. Its benefit is realized only if it can be maintained at the surface, which is thermodynamically unfavorable near the cycle's equilibrium temperature in the absence of kinetic barriers or specific operating conditions (e.g., lower temperatures or high steam pressures).
Codoping: Combining $Mg$ and $Sr$ does not yield additive benefits for the surface step due to the non-additive nature of their interaction with the dissociated water species.

The authors emphasize that these findings are thermodynamic limits. The actual performance in a real reactor will depend on kinetic factors such as dopant diffusion rates, the presence of defects (steps, vacancies), and higher water coverages, which were not fully addressed in this study. The strong correlation between binding energy and hydroxyl stretching frequency provides a specific, testable prediction for experimental validation via infrared spectroscopy.

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