📄 chemistry

Ultrafast proton transport phosphate for protonic ceramic electrolysis cells oxygen electrodes

This study enhances the performance of protonic ceramic electrolysis cells by introducing a Sr3(PO4)2 phosphate pathway into perovskite oxygen electrodes, which overcomes intrinsic proton transport limitations through ultrafast migration between tetrahedra and achieves a 148% increase in current density at 600 °C.

Original authors: Zhenhua Wang, Chenhe Jia, Rongzheng Ren, Cheng Zou, Yuanyuan Lei, Pengbo Guo, Linlin Song, Xiaodan Yu, Wang Sun, Jinshuo Qiao, Kening Sun

Published 2026-08-20

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CC BY 4.0

Original authors: Zhenhua Wang, Chenhe Jia, Rongzheng Ren, Cheng Zou, Yuanyuan Lei, Pengbo Guo, Linlin Song, Xiaodan Yu, Wang Sun, Jinshuo Qiao, Kening Sun

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

Making clean hydrogen fuel from water is a cornerstone of a future energy system that does not rely on burning fossil fuels. One promising technology for this task is the protonic ceramic electrolysis cell, a device that splits water molecules into hydrogen and oxygen using electricity. These cells operate at moderate temperatures and can produce hydrogen that is already dry and pressurized, ready for use. However, for these devices to become practical on a large scale, they need to overcome a significant bottleneck: the speed at which protons, which are positively charged hydrogen atoms, can move through the material that acts as the oxygen electrode. In the materials currently used, the movement of these protons is often slowed down because they get stuck in the atomic structure, much like a traveler getting caught in a traffic jam caused by the very road they are trying to use.

Researchers at the Beijing Institute of Technology have found a way to bypass this traffic jam entirely. Instead of trying to fix the existing road, they introduced a completely new, ultra-fast pathway made of a specific type of phosphate material. By mixing this phosphate into the standard electrode material, they created a system where protons can zip through with almost no resistance. The result is a dramatic increase in the speed of the water-splitting reaction, allowing the device to produce hydrogen much more efficiently.

The core problem the team addressed lies in how protons usually travel inside the ceramic electrodes used in these cells. In traditional designs, protons move by hopping between oxygen atoms within a crystal structure known as a perovskite. To make this happen, the material must contain empty spaces, or vacancies, where water molecules can enter and break apart to release protons. However, the chemical tricks used to create these empty spaces also create electrical traps that catch the protons, holding them back and slowing down the entire process. It is a fundamental conflict: the very feature that allows protons to enter the material is also what prevents them from moving freely.

To solve this, the researchers turned to a material called strontium phosphate, which has a different atomic architecture built from tetrahedra, or four-sided shapes, rather than the octahedra found in the traditional ceramic. They developed a method to grow this phosphate material directly inside the pores of the standard electrode during a heating process. This created a composite material where the phosphate and the ceramic coexist side by side. The phosphate acts as a separate highway for the protons. Because this phosphate structure does not rely on the same empty spaces that trap protons in the ceramic, it offers a clear, unobstructed path.

The team tested this new composite using several different types of standard electrode materials. In every case, the addition of the phosphate phase significantly improved performance. When they measured how easily protons moved through the material, they found that the speed increased by nearly ten times compared to the unmodified ceramic. This was not because the material absorbed more water or created more empty spaces; in fact, the phosphate material barely absorbed water at all. Instead, the protons that did enter the phosphate structure moved with incredible ease.

Detailed analysis revealed why this new path was so fast. In the traditional ceramic, a proton must overcome a significant energy barrier to jump from one oxygen atom to the next. In the phosphate structure, this barrier is much lower, allowing the protons to move rapidly between neighboring phosphate groups. The researchers confirmed this by tracking how deuterium, a heavier version of hydrogen, moved through the material. The deuterium penetrated much deeper and faster in the phosphate-containing samples, proving that the transport mechanism was fundamentally different and much more efficient.

The practical impact of this discovery was immediate and substantial. In full-scale tests, the new electrodes allowed the device to produce hydrogen at a rate of two amperes per square centimeter at 600 degrees Celsius, a figure that represents a significant leap forward for this type of technology. The performance boost was consistent across different electrode compositions, suggesting that this strategy could be applied broadly to improve many existing designs. By introducing a material that operates independently of the limitations of the traditional ceramic, the researchers have provided a new way to accelerate the production of green hydrogen, moving the technology closer to the efficiency and scale needed for a clean energy future.

Technical Summary: Ultrafast Proton Transport Phosphate for Protonic Ceramic Electrolysis Cells Oxygen Electrodes

Problem Statement
Protonic ceramic electrolysis cells (PCECs) offer a pathway to produce dry, pressurized green hydrogen at intermediate temperatures with near-100% energy conversion efficiency. However, their large-scale deployment is currently hindered by the lack of suitable oxygen electrode materials. In conventional $ABO_3$ -type perovskite oxygen electrodes, proton transport relies on the hydration of oxygen vacancies, where protons hop between lattice oxygen ions. This mechanism creates an intrinsic coupling between oxygen vacancy chemistry and proton transport. Specifically, the negatively charged acceptor dopants required to create oxygen vacancies electrostatically associate with positively charged protonic defects, forming "proton traps." This trapping fundamentally restricts long-range proton migration, leading to sluggish electrode kinetics and limiting PCEC performance.

Methodology
To address this limitation, the authors introduced a vacancy-independent proton conductor, strontium phosphate ( $Sr_3(PO_4)_2$ , SPO), into perovskite oxygen electrodes. The study employed an in situ self-assembly strategy to incorporate SPO into four representative perovskite hosts: $Sr_2Fe_{1.5}Mo_{0.5}O_{6-\delta}$ (SFM), $SrFe_{0.9}Nb_{0.1}O_{3-\delta}$ (SFN), $SrCo_{0.5}Fe_{0.5}O_{3-\delta}$ (SCF), and $SrCo_{0.9}Nb_{0.1}O_{3-\delta}$ (SCN).

The research utilized a comprehensive suite of characterization techniques:

Structural Analysis: Powder X-ray diffraction (XRD) with Rietveld refinement, scanning transmission electron microscopy (STEM), and high-resolution TEM (HRTEM) confirmed the formation of a biphasic composite with direct interfaces between the perovskite and phosphate phases.
Electrochemical Testing: Symmetric and full electrolysis cells were tested to measure polarization resistance ( $R_p$ ) and current density across temperatures (500–700 °C).
Transport Mechanism Elucidation: Electrical conductivity relaxation (ECR) was used to decouple surface water exchange, oxygen diffusion, and proton diffusion. Isotope tracing (TOF-SIMS with $D_2O$ ), solid-state NMR ( $^1H$ and $^{31}P$ ), and synchrotron X-ray spectroscopy (O K-edge, Fe K-edge XANES/EXAFS) were employed to track proton populations and mobility.
Theoretical Modeling: Density Functional Theory (DFT) calculations were performed to determine oxygen vacancy formation energies, hydration energies, and proton migration barriers.

Key Contributions and Results

Decoupling Proton Transport from Oxygen Vacancies:
The study demonstrates that SPO provides a proton transport pathway independent of the perovskite host's oxygen vacancy chemistry. Unlike conventional perovskites where proton incorporation is coupled to vacancy formation, SPO's $PO_4$ tetrahedral framework does not rely on structural oxygen vacancies. DFT calculations revealed that the oxygen vacancy formation energy in SPO is extremely high (6.14 eV) compared to the benchmark proton conductor BZCYYb (2.26 eV), effectively preventing vacancy-mediated hydration within the phosphate phase.
Ultrafast Proton Migration via $PO_4$ Tetrahedra:
The authors identified that protons migrate rapidly between oxygen atoms on neighboring $PO_4$ tetrahedra. DFT calculations showed a proton migration barrier of only 0.15 eV in SPO, significantly lower than the ~0.35 eV barrier in the $BO_6$ octahedral framework of BZCYYb. This low barrier facilitates ultrafast proton hopping.
- Experimental Validation: TOF-SIMS depth profiling showed deeper deuterium penetration in SPO compared to BZCYYb, indicating faster isotope transport. Solid-state $^1H$ NMR revealed a narrower resonance for SPO, indicative of motional narrowing due to rapid local proton movement.
- Diffusion Coefficient: In the SFM-SPO composite, the proton chemical diffusion coefficient ( $D_H$ ) increased by nearly one order of magnitude (from the low $10^{-6}$ to the $10^{-5}$ cm $^2$ s $^{-1}$ range) compared to pristine SFM.
Enhanced Electrochemical Performance:
The introduction of SPO significantly improved the performance of PCEC oxygen electrodes:
- Reduced Polarization Resistance: At 600 °C, the polarization resistance of SFM-SPO was reduced by approximately 57% compared to SFM. Similar reductions (22–52%) were observed for SFN, SCF, and SCN hosts.
- Increased Current Density: The electrolysis current density of SFM-SPO increased by up to 148% at 600 °C. The best-performing electrode (SCN-SPO) achieved a current density of 2 A cm $^{-2}$ at 600 °C and 1.3 V.
- Stability: The SFM-SPO cell maintained stable electrolysis performance for 250 hours at 600 °C and 1.3 V without progressive degradation.
Universality of the Strategy:
The performance enhancement was demonstrated across four distinct perovskite compositions (Fe-based and Co-based), confirming that the in situ self-assembly of SPO tetrahedra is a general strategy for improving proton transport in conventional perovskite oxygen electrodes.

Significance
The paper claims that this work establishes a new structural basis for proton transport in PCEC oxygen electrodes. By introducing a phosphate phase with a $PO_4$ tetrahedral framework, the authors bypass the fundamental limitation of proton trapping inherent in acceptor-doped perovskite oxides. Rather than attempting to tune the vacancy-related traps within the oxide lattice, this strategy introduces an independent, ultrafast proton-conducting pathway. The results suggest that Sr $_3$ (PO $_4$ ) $_2$ acts as a highly effective proton conductor that operates outside the traditional oxygen vacancy chemistry, offering a viable route to overcome the kinetic bottlenecks currently limiting the scale-up of protonic ceramic electrolysis cells.

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