A unified descriptor framework for hydrogen storage capacity and equilibrium pressure in interstitial hydrides
Pith reviewed 2026-05-10 15:13 UTC · model grok-4.3
The pith
Separate physical descriptors govern hydrogen storage capacity versus equilibrium pressure in interstitial hydrides
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using a curated database of pressure-composition-temperature measurements and white-box symbolic regression, the analysis reveals a clear separation of governing mechanisms, in which w is governed by geometric and lattice conditions, captured by the average atomic radius (⟨r_M⟩) and average thermal conductivity (⟨κ⟩), with an optimal regime of r_M ∼ 1.47 Å and relatively low ⟨κ⟩. In contrast, P_eq,RT is governed by elastic properties, captured by the average shear modulus (⟨G⟩) and average Poisson's ratio (⟨ν⟩), reflecting the role of lattice rigidity and mechanical compliance. These relationships are translated into compositional optimization pathways that follow the descriptor trends above
What carries the argument
White-box symbolic regression that isolates distinct averaged-property descriptor pairs for capacity (geometric/lattice) versus pressure (elastic), enabling direct compositional mapping
If this is right
- Compositional pathways can be followed to increase storage capacity w while maintaining equilibrium pressure near 0.1 MPa at room temperature
- Candidate alloys can be screened or designed by targeting the identified optimal ranges for radius, conductivity, shear modulus, and Poisson ratio
- The same descriptor separation supplies a template for physics-informed optimization of other interstitial compounds
- The framework yields an interpretable, general strategy for designing energy-storage materials without black-box models
Where Pith is reading between the lines
- The same separation of geometric versus elastic controls may appear in related interstitial systems such as nitride or carbide hosts for other gases
- High-throughput computational screening that enforces the 1.47 Å radius window could rapidly generate new test compositions
- If the radius optimum proves robust, it would constrain the periodic-table search space for future hydride discovery campaigns
Load-bearing premise
The assumption that the symbolic regression results reflect underlying physical mechanisms rather than statistical correlations in the specific dataset, and that the optimal regimes identified will generalize to new compositions
What would settle it
Synthesis and PCT measurement of an interstitial hydride alloy tuned to average metal radius near 1.47 Å and low thermal conductivity yet outside the suggested elastic-property window, checking whether capacity fails to rise while pressure remains near 0.1 MPa
read the original abstract
Hydrogen is a promising energy carrier, yet its practical deployment is limited by the lack of storage materials that simultaneously achieve high storage capacity ($w$) and practical equilibrium pressure at room temperature ($P_{\rm eq,RT}$). Interstitial metal hydrides offer fast kinetics and favorable thermodynamics (high $P_{\rm eq,RT}$) but suffer from intrinsically low w. Here, we establish a physically interpretable, data-driven framework to uncover descriptor-property relationships in interstitial hydrides using a curated database of pressure-composition-temperature measurements (Digital Hydrogen Platform, DigHyd) and white-box symbolic regression. Strikingly, the analysis reveals a clear separation of governing mechanisms, in which $w$ is governed by geometric and lattice conditions, captured by the average atomic radius ($\left\langle r_M \right\rangle$) and average thermal conductivity ($\left\langle\kappa\right\rangle$), with an optimal regime of $r_M \sim 1.47 \r{A}$ and relatively low $\left\langle\kappa\right\rangle$. In contrast, $P_{\rm eq,RT}$ is governed by elastic properties, captured by the average shear modulus ($\left\langle G \right\rangle$) and average Poisson's ratio ($\left\langle \nu \right\rangle$), reflecting the role of lattice rigidity and mechanical compliance. These relationships are translated into compositional optimization pathways that follow the descriptor trends above, enabling the design of candidate materials with enhanced w under practical equilibrium conditions ($P_{\rm eq,RT} \sim 0.1$ MPa). This work establishes a general, interpretable strategy for physics-informed design of energy materials systems.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces a data-driven framework for interstitial hydrides that uses white-box symbolic regression on the DigHyd database of pressure-composition-temperature data to identify descriptors for hydrogen storage capacity w and room-temperature equilibrium pressure P_eq,RT. It reports a separation in which w is controlled by geometric/lattice factors (average atomic radius ⟨r_M⟩ with optimum ~1.47 Å and average thermal conductivity ⟨κ⟩) while P_eq,RT is controlled by elastic factors (average shear modulus ⟨G⟩ and average Poisson's ratio ⟨ν⟩), and translates these into compositional design pathways aimed at simultaneous improvement of both properties.
Significance. If the reported descriptor separation proves robust and generalizable, the work would supply an interpretable, physics-informed route to hydride optimization that could help resolve the capacity-pressure trade-off in hydrogen storage. The explicit use of symbolic regression for transparency is a methodological strength. The significance is currently limited by the absence of independent validation that would distinguish physical mechanisms from statistical associations within the specific dataset.
major comments (2)
- [Abstract] Abstract: the claim of a 'clear separation of governing mechanisms' is load-bearing for the central contribution, yet the descriptors and optimal regimes (e.g., r_M ∼ 1.47 Å) are obtained directly from symbolic regression fits to DigHyd; without reported out-of-sample tests, ablation against alternative descriptor sets, or multicollinearity diagnostics among the averaged elemental properties, the mechanistic interpretation risks circularity.
- [Results] Results section on symbolic regression: the separation between geometric/lattice descriptors for w and elastic descriptors for P_eq,RT is presented as physically meaningful, but the manuscript does not provide the regression objective function, feature-importance ranking, or cross-validation metrics that would demonstrate the separation is not an artifact of the composition distribution in the database.
minor comments (2)
- [Notation] The averaging brackets ⟨ ⟩ for elemental properties should be defined at first use and the procedure for computing averages from elemental tables should be stated explicitly.
- [Figures] Any figures displaying regression surfaces or descriptor distributions would benefit from inclusion of quantitative goodness-of-fit statistics (R², MAE, or cross-validation scores) to allow readers to assess the strength of the reported relationships.
Simulated Author's Rebuttal
We thank the referee for the detailed and constructive report. The comments correctly identify the need for greater transparency and validation in our symbolic regression analysis to support the claimed separation of mechanisms. We have prepared revisions that directly address these points by adding the requested methodological details, validation metrics, and diagnostics. These changes will be incorporated into the revised manuscript to strengthen the robustness of our conclusions without altering the core findings.
read point-by-point responses
-
Referee: [Abstract] Abstract: the claim of a 'clear separation of governing mechanisms' is load-bearing for the central contribution, yet the descriptors and optimal regimes (e.g., r_M ∼ 1.47 Å) are obtained directly from symbolic regression fits to DigHyd; without reported out-of-sample tests, ablation against alternative descriptor sets, or multicollinearity diagnostics among the averaged elemental properties, the mechanistic interpretation risks circularity.
Authors: We agree that explicit validation is required to substantiate the mechanistic interpretation and to rule out circularity. In the revised manuscript we will add: (i) out-of-sample testing via a 70/30 train/test split of the DigHyd database with reported R² and RMSE on the held-out set; (ii) ablation experiments comparing the selected descriptor sets against alternative feature pools (e.g., replacing averaged elemental properties with composition-weighted alternatives); and (iii) multicollinearity diagnostics (variance inflation factors and pairwise correlation matrices) for the averaged properties ⟨r_M⟩, ⟨κ⟩, ⟨G⟩ and ⟨ν⟩. These additions will be placed in a new subsection of Results and referenced in the Abstract to support the separation claim. revision: yes
-
Referee: [Results] Results section on symbolic regression: the separation between geometric/lattice descriptors for w and elastic descriptors for P_eq,RT is presented as physically meaningful, but the manuscript does not provide the regression objective function, feature-importance ranking, or cross-validation metrics that would demonstrate the separation is not an artifact of the composition distribution in the database.
Authors: We acknowledge that these implementation details were omitted and that their absence leaves open the possibility of database-specific artifacts. The revised manuscript will include a dedicated “Symbolic Regression Methodology” subsection that specifies: the objective function (mean-squared error plus complexity penalty as implemented in the PySR library), feature-importance ranking derived from the frequency of each descriptor across the Pareto-optimal expressions, and k-fold (k=5) cross-validation scores with standard deviations. We will also report the distribution of training compositions to allow readers to assess potential bias. These additions will demonstrate that the geometric-versus-elastic separation persists under cross-validation and is not an artifact of the database composition. revision: yes
Circularity Check
Data-driven symbolic regression yields empirical descriptor trends; derivation is self-contained without reduction to inputs by construction
full rationale
The paper explicitly frames its contribution as a data-driven analysis: white-box symbolic regression is applied to the DigHyd database of PCT measurements to extract averaged elemental descriptors (⟨r_M⟩, ⟨κ⟩ for w; ⟨G⟩, ⟨ν⟩ for P_eq,RT) and identify an optimal regime. The claimed 'separation of governing mechanisms' and subsequent compositional optimization pathways are direct outputs of this regression rather than prior assumptions or self-referential definitions. No equations, uniqueness theorems, or ansatzes are imported via self-citation in a load-bearing way; the framework does not claim first-principles derivation but presents statistical relationships from the specific dataset as interpretable trends. Design suggestions follow the fitted descriptor trends by construction of the method, which is the standard and non-circular use of such models. The analysis is therefore self-contained as an empirical discovery procedure.
Axiom & Free-Parameter Ledger
free parameters (2)
- optimal average atomic radius r_M =
1.47 Å
- low average thermal conductivity threshold
axioms (2)
- domain assumption White-box symbolic regression can extract physically meaningful descriptors from property data
- domain assumption The Digital Hydrogen Platform (DigHyd) database accurately represents interstitial hydride behaviors
Forward citations
Cited by 1 Pith paper
-
Building a physics-aware AI ecosystem for solid-state hydrogen storage materials
A proposed physics-aware AI ecosystem combines organized data, physics-based models, and experimental feedback loops to enable consistent optimization and discovery of hydrogen storage materials.
Reference graph
Works this paper leans on
-
[1]
M.; Ekins, P.; McKenna, R.; Staffell, I
Johnson, N.; Liebreich, M.; Kammen, D. M.; Ekins, P.; McKenna, R.; Staffell, I. Realistic roles for hydrogen in the future energy transition. Nat. Rev. Clean Technol. 2025, 1, 351-371. DOI: 10.1038/s44359-025-00050-4
-
[2]
P.; Li, S.; Xie, D.; Wang, Y.; Li, Z.; Hu, P
Zhao, A. P.; Li, S.; Xie, D.; Wang, Y.; Li, Z.; Hu, P. J.-H.; Zhang, Q. Hydrogen as the nexus of future sustainable transport and energy system. Nat. Rev. Electr. Eng. 2025, 2, 447-446. DOI: 10.1038/s44287-025-00178-2
-
[3]
Gebretatios, A. G.; Banat, F.; Cheng, C. K. A critical review of hydrogen storage: Toward the nanoconfinement of complex hydrides from the synthesis and characterization perspectives. Sustain. Energ. Fuels. 2024, 8 (22), 5091-5130. DOI: 10.1039/d4se00353e
-
[4]
Materials for hydrogen storage
Züttel, A. Materials for hydrogen storage. Mater. Today 2003, 6 (9), 24-33. DOI: 10.1016/S1369-7021(03)00922-2
-
[5]
Bellosta von Colbe, J.; Ares, J.-R.; Barale, J.; Baricco, M.; Buckley, C.; Capurso, G.; Gallandat, N.; Grant, D. M.; Guzik, M. N.; Jacob, I.; Jensen, E. H.; Jensen, T.; Jepsen, J.; Klassen, T.; Lototskyy, M. V.; Manickam, K.; Montone, A.; Puszkiel, J.; Sartori, S.; Sheppard, D. A.; Stuart, A.; Walker, G.; Webb, C. J.; Yang, H.; Yartys, V.; Züttel, A.; Dor...
-
[6]
A.; Baricco, M.; Bellosta von Colbe, J.; Blanchard, D.; Bowman, R
Hirscher, M.; Yartys, V. A.; Baricco, M.; Bellosta von Colbe, J.; Blanchard, D.; Bowman, R. C.; Broom, D. P.; Buckley, C. E.; Chang, F.; Chen, P.; Cho, Y. W.; Crivello, J.-C.; Cuevas, F.; David, W. I. F.; de Jongh, P. E.; Denys, R. V.; Dornheim, M.; Felderhoff, M.; Filinchuk, Y.; Froudakis, G. E. Materials for hydrogen-based energy storage - Past, recent ...
-
[7]
Hydrogen in palladium and palladium alloys
Wicke, E.; Brodowsky, H.; Züchner, H. Hydrogen in palladium and palladium alloys. Hydrogen in Metals II. Topics in Applied Physics, vol 29; Alefeld, G.; Völkl, J., Eds.; Springer, 1978; pp 73-155. DOI: 10.1007/3-540-08883-0_19
-
[8]
A panoramic overview of hydrogen storage alloys from a gas reaction point of view
Sandrock, G. A panoramic overview of hydrogen storage alloys from a gas reaction point of view. J. Alloys Compd. 1999, 293-295, 877-888. DOI: 10.1016/S0925-8388(99)00384-9
-
[9]
Bowman Jr, R. C.; Fultz, B. Metallic hydrides I: Hydrogen storage and other gas-phase applications. MRS Bulletin 2002, 27, 688-693. DOI: 10.1557/mrs2002.223 21
-
[10]
Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Züttel, A.; Jensen, C. M. Complex hydrides for hydrogen storage. Chem. Rev. 2007, 107, 4111-4132. DOI: 10.1021/cr0501846
-
[11]
Metal hydride materials for solid hydrogen storage: A review
Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M. Metal hydride materials for solid hydrogen storage: A review. Int. J. Hydrogen Energy 2007, 32 (9), 1121-1140. DOI: 10.1016/j.ijhydene.2006.11.022
-
[12]
Jain, I. P.; Jain, P.; Jain, A. Novel hydrogen storage materials: A review of lightweight complex hydrides. J. Alloys Compd. 2010, 503 (2), 303-339. DOI: 10.1016/j.jallcom.2010.04.250
-
[13]
Jain, I. P.; Lal, C.; Jain, A. Hydrogen storage in Mg: A most promising material. Int. J. Hydrogen Energy 2010, 35 (10), 5133-5144. DOI: 10.1016/j.ijhydene.2009.08.088
-
[14]
A comprehensive review on metal hydrides- based hydrogen storage systems for mobile applications
Scarpati, G.; Frasci, E.; Di Ilio, G.; Jannelli, E. A comprehensive review on metal hydrides- based hydrogen storage systems for mobile applications. J. Energy Storage 2024, 102, 113934. DOI: 10.1016/j.est.2024.113934
-
[15]
Nemukula, E.; Mtshali, C. B.; Nemangwele, F. Metal hydrides for sustainable hydrogen storage: A review. Int. J. Energy Res. 2025, 6302225. DOI: 10.1155/er/6300225
-
[16]
Catalytic strategies and mechanisms for enhancing MgH2 solid-state hydrogen storage
Gao, Z.; Yang, X.; Zhuang, Z.; Zhang, Y.; Cai, J.; Li, Y.; Fu, W.; Li, H.; Yang, W. Catalytic strategies and mechanisms for enhancing MgH2 solid-state hydrogen storage. Chem Catal. 2026, 6, 101692. DOI: 10.1016/j.checat.2026.101692
-
[17]
Zhang, D.; Jia, X.; Tran, H. B.; Jang, S. H.; Zhang, L.; Sato, R.; Hashimoto, Y.; Sato, T.; Konno, K.; Orimo, S.; Li, H. “DIVE” into hydrogen storage materials discovery with AI agents. Chem. Sci. 2026, 17, 3031-3042. DOI: 10.1039/D5SC09921H
-
[18]
B.; Zhang, L.; Sato, R.; Hashimoto, Y.; Sato, T.; Konno, K.; Orimo, S.; Li, H
Jang, S.-H.; Zhang, D.; Jia, X.; Tran, H. B.; Zhang, L.; Sato, R.; Hashimoto, Y.; Sato, T.; Konno, K.; Orimo, S.; Li, H. Digital hydrogen platform (DigHyd): A rigorously curated database for hydrogen storage materials empowered by AI-assisted literature mining. arXiv, March 14,
-
[19]
DOI: 10.48550/arXiv.2603.14139
-
[20]
B.; Jia, X.; Konno, K.; Sato, R.; Orimo, S.; Li, H
Jang, S.-H.; Zhang, D.; Tran, H. B.; Jia, X.; Konno, K.; Sato, R.; Orimo, S.; Li, H. Physically interpretable descriptors drive the materials design of metal hydrides for hydrogen storage. Chem. Sci. 2025, 16, 23111-23120. DOI: 10.1039/D5SC07296D
-
[21]
GoodRegressor: A hierarchical inductive bias for navigating high-dimensional compositional space
Jang, S.-H. GoodRegressor: A hierarchical inductive bias for navigating high-dimensional compositional space. arXiv, February 20, 2026. DOI: 10.48550/arXiv.2510.18325 22
-
[22]
Beryllium: Health and safety guide ; World Health Organization, 1990
World Health Organization & International Programme on Chemical Safety. Beryllium: Health and safety guide ; World Health Organization, 1990. https://iris.who.int/handle/10665/40004
work page 1990
-
[23]
Metallic hydrides III: Body-centered-cubic solid-solution alloys
Akiba, E.; Okada, M. Metallic hydrides III: Body-centered-cubic solid-solution alloys. MRS Bull. 2002, 27 (9), 699-703. DOI: 10.1557/mrs2002.225
-
[24]
Cheng, B.; Kong, L.; Cai, H.; Li, Y.; Zhao, Y.; Wan, D.; Xue, Y. Exploring microstructure variations and hydrogen storage characteristics in TiVNbCrNi high-entropy alloys with different Ni incorporation. Int. J. Hydrog. Energy 2024, 72, 29-40. DOI: 10.1016/j.ijhydene.2024.05.317
-
[25]
Enblom, V.; Clulow, R.; Ha, T.-J.; Witman, M. D.; Way, L. E.; Han, S. J.; Brant Carvalho, P. H. B.; Stavila, V.; Suh, J.-Y.; Sahlberg, M.; Fadonougbo, J. O. A combined experimental and machine learning exploration of Ti2-xZrxMnCrFeNi high entropy Laves hydrides. Mater., 2025, 40, 102414. DOI: 10.1016/j.mtla.2025.102414
-
[26]
V.; Paul-Boncour, V.; Denys, R
Shtender, V. V.; Paul-Boncour, V.; Denys, R. V.; Crivello, J.-C.; Zavaliy, I. Y. TbMgNi4-xCox- (H,D)2 System. I: Synthesis, hydrogenation properties, and crystal and electronic structures. J. Phys. Chem. C 2020, 124 (1), 196-204. DOI: 10.1021/acs.jpcc.9b10252
-
[27]
Zhang, X. B.; Sun, D. Z.; Yin, W. Y.; Chai, Y. J.; Zhao, M. S. Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3-x(Al0.5Mo0.5)x (x = 0-0.4) hydrogen storage alloys. Electrochim. Acta. 2005, 50 (16-17), 3407-3413. DOI: 10.1016/j.electacta.2004.12.020
-
[28]
Effects of Co introduction on hydrogen storage properties of Ti-Fe-Mn alloys
Qu, H.; Du, J.; Pu, C.; Niu, Y.; Huang, T.; Li, Z.; Lou, Y.; Wu, Z. Effects of Co introduction on hydrogen storage properties of Ti-Fe-Mn alloys. Int. J. Hydrog. Energy 2015, 40 (6), 2729-
work page 2015
-
[29]
DOI: 10.1016/j.ijhydene.2014.12.089
-
[30]
Molinas, B.; Pontarollo, A.; Scapin, M.; Peretti, H.; Melnichuk, M.; Corso, H.; Aurora, A.; Gatttia, D. M.; Montone, A. The optimization of MmNi5-xAlx hydrogen storage alloy for sea or lagoon navigation and transportation. Int. J. Hydrog. Energy 2016, 41 (32), 14484-14490. DOI: 10.1016/j.ijhydene.2016.05.222
-
[31]
Zhou, C.; Wang, H.; Ouyang, L. Z.; Liu, J. W.; Zhu, M. Achieving high equilibrium pressure and low hysteresis of Zr-Fe based hydrogen storage alloy by Cr/V substitution. J. Alloys Compd. 2019, 806, 1436-1444. DOI: 10.1016/j.jallcom.2019.07.170
-
[32]
H.; Lombardo, L.; Girella, A.; Guzik, M
Jensen, E. H.; Lombardo, L.; Girella, A.; Guzik, M. N.; Züttel, A.; Milanese, C.; Whitfield, P.; Noréus, D.; Satori, S. The effect of Y content on structural and sorption properties of A2B7-type 23 phase in the La-Y-Ni-Al-Mn system. Molecules 2023, 28 (9), 3749. DOI: 10.3390/molecules28093749
-
[33]
Zhang, D.; Jia, X.; Wang, Y.; Liu, H.; Wang, Q.; Jang, S.-H.; Shah, D.; Ye, S.; Tran, H. B.; Li, H. Digital materials ecosystem: from databases to AI agents for autonomous discovery. Chem. Sci. 2026, 17, 5782-5804. DOI: 10.1039/D5SC09229A
-
[34]
Accelerating catalyst materials discovery with large artificial intelligence models
Zhang, D.; Chen, Y.; Liu, C.; Liu, Y.; Xin, H.; Peng, J.; Ou, P.; Li, H. Accelerating catalyst materials discovery with large artificial intelligence models. Angew. Chem. Int. Ed. 2026, e26150. DOI: 10.1002/anie.202526150
-
[35]
Wang, Q.; Yang, F.; Wang, Y.; Zhang, D.; Sato, R.; Zhang, L.; Cheng, J.; Yan, Y.; Chen, Y.; Kisu, K.; Orimo, S.; Li, H. Unraveling the complexity of divalent hydride electrolytes in solid- state batteries via a data-driven framework with large language model. Angew. Chem. Int. Ed. 2025, 64 (5), e202506573. DOI: 10.1002/anie.202506573
-
[36]
Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Cryst. 2011, 44, 1272-1276. DOI: 10.1107/S0021889811038970 24 ToC S1 Supplementary Information A unified descriptor framework for hydrogen storage capacity and equilibrium pressure in interstitial hydrides Seong-Hoon Jang*1, 2, Di Zhang1,3...
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.