Dehydrogenation of sodium borohydride and ammonia borane over cobalt-based catalysts: advances and prospects

Abstract

Chemical hydrogen storage is acknowledged as a promising approach for hydrogen storage, offering numerous advantages, such as high energy density, enhanced safety, and environmental adaptability, as well as potential economic benefits. Among the chemical hydrogen storage materials that have been reported, sodium borohydride and ammonia borane have attracted considerable scholarly interest due to their capacity to release hydrogen conveniently via solvolysis processes, such as hydrolysis and methanolysis, under ambient temperature conditions. Cobalt-based nanocatalysts, as representatives of non-noble metals, have been extensively investigated as cost-effective and efficient catalysts for hydrogen evolution from the solvolysis of sodium borohydride and ammonia borane. Nevertheless, a comprehensive review specifically focusing on cobalt-based catalysts for hydrogen production from sodium borohydride and ammonia borane has yet to be published. In this review, we provide a comprehensive summary of the historical development and recent advancements in cobalt-based catalysts for hydrogen generation from sodium borohydride and ammonia borane, encompassing synthesis methods, notable performances, and potential catalytic mechanisms. Our objective is to establish a reliable structure–property relationship and offer guidance for the future design of catalysts for hydrogen evolution from sodium borohydride and ammonia borane.

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Xiugang Li

Xiugang Li is a professor at the College of Material and Chemical Engineering, Tongren University. He received his Ph.D. degree from Jiangxi Normal University in 2022. His current research interest is in energy-storage materials for hydrogen generation.

Zhang-Hui Lu

Zhang-Hui Lu is a professor at Jiangxi Normal University and director of the Key Laboratory of Green Catalysis of Jiangxi Education Institutes. He obtained his Ph.D. from Kobe University in 2011. He received the first prize (2020) and second prize (2024) of the Jiangxi Provincial Natural Science Award. His research interests are centered on heterogeneous catalysis and electrocatalysis for clean energy and environmental clean-up.

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1. Introduction

The advancement of green sources of energy has emerged as a primary focus for the future, among which hydrogen, as the most abundant renewable energy source on Earth, exhibits significant potential for commercial utilizations.^1–5 Hydrogen has garnered unprecedented attention within the scientific community due to its numerous advantages, such as cleanliness, the absence of pollution, high calorific value upon combustion, and widespread accessibility.^6,7 In particular, the emergence of hydrogen fuel cells greatly broadens the range of potential uses of hydrogen by enabling the direct conversion of hydrogen energy into electrical energy.^8–10 The increasing utilization of hydrogen energy in portable electronic devices and vehicles underscores the need for the development of high-efficiency, secure, and portable hydrogen storage materials and hydrogen production technology.^11–14

Among the various available methods, chemical hydrogen storage presents notable advantages in terms of safety, convenience, and efficiency, thus rendering it the most promising option for extensive practical implementation on a large-scale.^15,16 In recent years, some promising hydrogen storage materials, for instance sodium borohydride (NaBH₄)^17–21 and ammonia borane (NH₃BH₃),^22–26 have garnered significant interest for potential applications in fuel cell vehicles owing to their high hydrogen content and good chemical stability (Fig. 1a). Sodium borohydride and ammonia borane are capable of generating hydrogen via pyrolysis.^27,28 Nevertheless, the elevated pyrolysis temperatures required for hydrogen production and the incomplete release of hydrogen significantly constrain their practical utility in fuel cells. An alternative method for the release of hydrogen from sodium borohydride and ammonia borane involves solvolysis in specific protic solvents, such as water (hydrolysis) and methanol (methanolysis). Given that solvolysis can occur at ambient temperature, this process has been considered as the most probable method for hydrogen production from sodium borohydride and ammonia borane. In order to facilitate the liberation of H₂ from these storage materials, exploiting appropriate catalysts is imperative. Although noble metal catalysts have exhibited conspicuous activities, scarcity and outrageous prices severely restrict their practical applications. Therefore, developing cost-efficient nanocatalysts is crucial for facilitating the rapid release of hydrogen from the solvolysis of sodium borohydride and ammonia borane in the liquid phase. With these points in mind, many recent works have focused on non-noble metal nanocatalysts, for instance Fe,^29–31 Co,^32,33 Ni^34,35 and Cu.^36,37 It is noteworthy that Co-based nanocatalysts with distinctive structures demonstrate superior catalytic properties and stability to those of other metals.^38–43 To date, a wide range of Co-based nanocatalysts, including monometallic Co, Co-based alloy, and Co X-ides (X = phosph, sulf, nitr, and carb) have been thoroughly studied as economical and efficient catalysts for hydrogen evolution from sodium borohydride and ammonia borane (Fig. 1b). However, to the best of our knowledge, no special reviews have been published on Co-based catalysts for hydrogen production from these hydrogen storage materials.

Fig. 1 (a) Applications of sodium borohydride and ammonia borane as chemical hydrogen storage materials; (b) the elements of the periodic table mentioned for Co-based nanocatalysts for hydrogen evolution from the solvolysis of sodium borohydride and ammonia borane.

This review aims to offer a comprehensive overview of recent advancements of Co-based nanocatalysts for catalytic hydrogen generation from sodium borohydride and ammonia borane. Firstly, different Co-based nanocatalysts, for instance monometallic Co, Co alloys, as well as Co X-ides, are systematically discussed with a focus on their application in catalytic hydrogen generation from the solvolysis of sodium borohydride and ammonia borane. Meanwhile, the interrelationships between synthesis method, material structure, composition, and catalytic activity are thoroughly analyzed. Then, the dehydrogenation mechanisms of these hydrogen storage materials over Co-based nanocatalysts are critically summarized. Finally, the challenges and potential opportunities of Co-based nanocatalysts are discussed for designing efficient Co-based catalysts for hydrogen generation from sodium borohydride and ammonia borane.

2. Performance evaluation methods

2.1 Hydrogen generation rate (HGR)

The hydrogen production rate (HGR) was firstly utilized as a metric for assessing the hydrolytic or methanolytic hydrogen production of sodium borohydride.^44,45 Presently, the HGR value has emerged as the predominant indicator for evaluating the hydrogen production efficacies of catalysts. The HGR value can be calculated based on the linear range of hydrogen evolution curves. The calculation formula is as follows:
where V(H₂) represents the volume number of hydrogen, m denotes the total weight of the catalyst or metal, and t signifies the hydrogen production time. The unit of HGR can be denoted as mL min⁻¹ g_Catalyst⁻¹ or mL min⁻¹ g_metal⁻¹. Generally, a higher HGR value indicates increased catalytic performance. It is important to emphasize that researchers are advised to maintain consistency in the quantity of NaBH₄ and experimental temperature when comparing the catalytic activity.

2.2. Turnover frequency

Turnover frequency (TOF) is an essential measure for evaluating the efficiency of hydrogen production from hydrogen storage materials.^46–49 The determination of the TOF value is contingent upon the linear segment of the hydrogen production curve, with the calculation formula outlined as follows:
where n(H₂) signifies the molar quantity of liberated hydrogen, n(metal) denotes the molar quantity of the metal employed, and t represents the duration of the reaction process. The unit of TOF is usually expressed in mol_H2 mol_metal⁻¹ min⁻¹ or mol_H2 mol_metal⁻¹ h⁻¹, which can be abbreviated as min⁻¹ or h⁻¹. The properties of heterogeneous nanocatalysts are primarily attributed to the surface atoms of metal NPs. Quantifying the exact number of metal surface atoms is challenging due to the variability in particle size of metal NPs. Therefore, it is imperative to further investigate a more comprehensive method for evaluating the metal in order to accurately determine TOF values for various catalysts and compare their catalytic activities.

3. Performance of Co-based nanocatalysts

Increasing attention on hydrogen energy has prompted an escalation in research on the dehydrogenation of sodium borohydride and ammonia borane since 2006. In the last 20 years (2005–2024), a total of 7519 articles have been published on catalytic hydrogen generation from these hydrogen storage materials with an upward trend year by year, highlighting the research hotspots in this field (Fig. 2a). Co-based nanocatalysts, as representatives of non-noble metals, have been the subject of extensive research due to their abundant reserves, low cost, and good catalytic properties for hydrogen generation from the solvolysis of sodium borohydride and ammonia borane. As illustrated in Fig. 2b, over the past five years, the number of scholarly articles addressing hydrogen production from sodium borohydride and ammonia borane over Co has markedly exceeded that of articles focused on Ni, Cu, and Fe, thereby underscoring the superiority of Co in facilitating hydrogen evolution from these hydrogen storage materials.

Fig. 2 (a) Number of articles regarding dehydrogenation with sodium borohydride and ammonia borane from 2005 to 2024; (b) percentage of metal-related articles on the solvolysis of sodium borohydride and ammonia borane in the past five years.

3.1. Sodium borohydride

Sodium borohydride (NaBH₄, SB) has garnered predominant interest as an exemplary hydrogen storage material, owing to its high hydrogen storage ability (10.8 wt%), exceptional stability, and non-toxic property.^8,25,50 Sodium borohydride can release hydrogen through hydrolysis (eqn (1)) or methanolysis (eqn (2)).

NaBH₄ + 2H₂O → NaBO₂ + 4H₂ (1)

NaBH₄ + 4CH₃OH → NaB(CH₃O)₄ + 4H₂ (2)

The hydrolysis and methanolysis of NaBH₄ each present distinct advantages for hydrogen production. The hydrolysis of NaBH₄ is widely recognized as a sustainable and cost-effective approach for hydrogen production due to the high hydrogen purity, controlled hydrogen production rate, and the exclusion of organic solvents.^51,52 On the other hand, the methanolysis of NaBH₄ exhibits unique characteristics, including enhanced reaction kinetics, reduced activation energy, and hydrogen production at temperatures below freezing.^53,54 NaBH₄ is capable of spontaneously liberating hydrogen via methanolysis at ambient temperature. Consequently, alkalis, for instance LiOH, NaOH, and KOH, are commonly utilized to impede the self-methanolysis of NaBH₄.⁵⁵ The development of suitable nanocatalysts is imperative for augmenting the efficiency of hydrogen production via the hydrolysis and methanolysis of NaBH₄.

3.1.1. Monometallic Co-based nanocatalysts for the dehydrogenation of NaBH₄. Schlesinger and colleagues were the first researchers to discover the potential of acids for facilitating the hydrolysis of NaBH₄.⁴⁴ Nevertheless, homogeneous catalysts present certain drawbacks, including challenges in separation and limited recyclability.⁵⁶ Therefore, numerous heterogeneous metal nanocatalysts have been devised as alternatives. Although noble metal nanocatalysts, such as Pt^51,57 and Ru,^58,59 have exhibited exceptional catalytic performance for NaBH₄ solvolysis, their exorbitant cost and limited availability impose restrictions on their practical implementation. As a result, researchers have directed their focus towards non-noble metal NP catalysts, particularly Co-based NPs, which have been extensively investigated for their conspicuous catalytic properties.^60–62 Nevertheless, the exothermic nature of the reduction reaction and the high surface energy contribute to the propensity of metal NPs to aggregate during the preparation and reaction processes, ultimately leading to a decrease in both catalytic efficacy and durability. Anchoring metal NPs on an appropriate carrier to prevent agglomeration is a potential solution to address the issue. In this regard, a series of porous materials, such as metal–organic frameworks (MOFs), graphene, porous carbon, and nitrogen-doped carbon materials are selected as supports to stabilize metal NPs owing to the expansive specific surface area, substantial porosity, and abundant anchoring sites.^63–65 Astruc et al. employed a classic MOF, ZIF-8, as the support to immobilize non-noble NPs (MNPs@ZIF-8, M = Fe, Co, Ni, Cu) for NaBH₄ hydrolysis (Fig. 3a).⁶⁶ Compared with FeNPs@ZIF-8, NiNPs@ZIF-8, and CuNPs@ZIF-8, CoNPs@ZIF-8 demonstrated superior catalytic performance for NaBH₄ hydrolysis under identical conditions, achieving a hydrogen generation rate (HGR) value of 14 023 mL g_Co⁻¹ min⁻¹ based on the total number of atoms within Co NPs at 303 K (Fig. 3b). Mahmood et al. fabricated a two-dimensional nitrogenated network polymer, which was used to encapsulate Co-oxide (Co@C₂N) through an in situ solvothermal method (Fig. 3c and d).⁶⁷ Highly crystalline Co oxide NPs exhibited a remarkable dehydrogenation property for NaBH₄ hydrolysis, achieving a maximum HGR value of 8903 mL min⁻¹ g_Catalyst⁻¹ at 303 K. Furthermore, Co@C₂N can reduce nitro compounds to aniline in situ with NaBH₄ as the reducing agent. The improved catalytic property and durability of Co@C₂N was attributed to the robust interactions between Co oxide NPs and the nitrogen-enriched C₂N framework. Sun et al. encapsulated Co NPs in a nitrogen-doped mesoporous graphitic carbon (NMGC) to fabricate core–shell structured Co@NMGC by carbonizing ethylenediaminetetraacetic acid (Fig. 3e).⁶⁸ Among the exploratory catalysts, Co@NMGC-500 demonstrated the optimum catalytic property (3575 mL min⁻¹ g_Catalyst⁻¹) for NaBH₄ hydrolysis at 298 K. Based on theoretical calculations, the prominent dehydrogenation property of Co@NMGC was ascribed to the redistribution of the electron potential within Co@NMGC, which enhanced the adsorption and dissociation of H atoms on Co NPs.

Fig. 3 (a) Synthetic route to MNPs@ZIF-8. (b) The corresponding hydrogen production curve from NaBH₄ hydrolysis over MNPs@ZIF-8. Reproduced with permission.⁶⁶ Copyright 2019, Wiley-VCH. (c and d) Morphology analysis of Co@C₂N nanocomposite. Reproduced with permission.⁶⁷ Copyright 2015, American Chemical Society. (e) Schematic illustration of Co@NMGC nanocomposite. Reproduced with permission.⁶⁸ Copyright 2020, Elsevier. (f) Schematic illustration of Co–CeO_x/NCNS. Reproduced with permission.⁶⁹ Copyright 2021, Royal Society of Chemistry.

In 2021, our group synthesized nitrogen-doped carbon nanosheets (NCNS) via carbonizing premixed ZIF-8 and potassium citrate.⁶⁹ Then Co–CeO_x nanocomposite was deposited on NCNS to fabricate the efficient and economical catalyst Co–CeO_x/NCNS for NaBH₄ hydrolysis (Fig. 3f). The optimized catalyst demonstrated an ultrahigh HGR value of 28 410 mL min⁻¹ g_Co⁻¹ for NaBH₄ hydrolysis. The remarkable hydrogen evolution performance of the catalyst derived from the diminutive dimensions of the Co–CeO_x nanocomposite, electron-rich Co NPs, and the robust synergistic interplay between Co–CeO_x and NCNS.

Monometallic Co catalysts have also been used to produce hydrogen via methanolysis of sodium borohydride. Caglar et al. synthesized activated carbon supported Co for the methanolysis of NaBH₄.⁷⁰ Optimal Co/AC with a metal loading of 1 wt% exhibited a HGR value of 7120.7 mL min⁻¹ g_Catalyst⁻¹ at 303 K. Demirci et al. fabricated TiO₂ immobilized Co nanocatalysts, which were employed for hydrogen evolution from the methanolysis of NaBH₄.⁷¹ 1 wt% Co/TiO₂ achieved the highest catalytic performance for the methanolysis of NaBH₄ (664 000 mL min⁻¹ g_Co⁻¹) at 298 K. Wang's group prepared a composite consisting of ZIF-67 and graphene oxide (GO) and then a Co-based catalyst was obtained for the methanolysis of NaBH₄ after the reduction of Co²⁺ in ZIF-67.⁷² ZIF-67@GO-2 with 6.0 wt% GO addition achieved the maximum HGR of 3200 mL min⁻¹ g_Catalyst⁻¹ for NaBH₄ methanolysis at 273 K.

3.1.2. Co-based alloy nanocatalysts for the dehydrogenation of NaBH₄. Alloys composed of diverse metals often demonstrate enhanced catalytic properties for hydrogen evolution from NaBH₄ owing to the synergistic interactions of the constituent metals.^73–75 Liu and colleagues synthesized a range of bimetallic M–Ru/C nanocomposites (M = Fe, Co, Ni, and Cu) through the straightforward process of alloying commercial Ru/C with non-noble metals.⁷⁴ They confirmed that Co–Ru/C demonstrated the optimal catalytic property for the dehydrogenation of NaBH₄, achieving a HGR value of 96 700 mL min⁻¹ g_Ru⁻¹, highlighting the significantly promoted effect of Co. Zabihi et al. prepared a magnetic activated carbon using wood as the precursor (MWAC). Then they deposited bimetallic Co–Ni on MWAC to fabricate alloy nanocatalysts via a co-deposition–precipitation method.⁷⁶ They found that the catalysts exhibited varying catalytic activities for NaBH₄ hydrolysis, with the order of Co/Ni/MWAC > Ni/Co/MWAC > Co–Ni/MWAC (prepared with various permutations of addition). Distinctly, Co/Ni/MWAC demonstrated the highest HGR value of 740.7 mL min⁻¹ g_Catalyst⁻¹ for NaBH₄ hydrolysis. Han and coworkers synthesized Co–Mo alloy NPs on three dimensional graphene oxide (3DGO) (Fig. 4a).⁷⁷ The resulting Co–Mo/3DGO catalyst with a Co : Mo molar ratio of 0.7 : 0.3 demonstrated optimal catalytic performance (7023.3 mL min⁻¹ g_Co⁻¹) for NaBH₄ hydrolysis. Guo's group impregnated Co and Zr precursors onto porous sheet-like activated carbon to obtain bimetal Zr–Co alloy NP catalysts by an impregnation–chemical reduction method.⁷⁸ They confirmed that the incorporation of Zr into Co resulted in heightened interactions between Co and the substrate. As a result, the Zr/Co/C catalyst demonstrated an enhanced dehydrogenation performance compared to that of Co/C for NaBH₄ hydrolysis.

Fig. 4 (a) Synthesis of the nanocatalyst Co–Mo/3DGO. Reproduced with permission.⁷⁷ Copyright 2019, Elsevier. (b and c) Morphology characterization of hollow Co–B nanospheres. Reproduced with permission.⁸⁴ Copyright 2008, Elsevier. (d) Synthetic route to Co–Ce–B/Chi-C. Reproduced with permission.⁶⁰ Copyright 2018, Elsevier. (e) Preparation of Fe–CoP/Ti and schematic illustration of hydrogen generation from NaBH₄ and H₂O. (f–j) Morphology features of the Fe–CoP/Ti nanoarray. Reproduced with permission.⁸⁶ Copyright 2016, Wiley-VCH.

For the methanolysis of NaBH₄, Figen et al. synthesized Ru–Co alloy NPs via a sol–gel method with citric acid as the gelling agent.⁷⁹ The resulting Ru–Co NPs provided a HGR of 1260 mL min⁻¹ g_catalyst⁻¹ for the methanolysis of NaBH₄ at 293 K. Wang's group fabricated a carbon nanotube-immobilized Ru–Co alloy nanocatalyst via microwave-assisted heating electroless plating.⁸⁰ The resulting Ru–Co/CNT catalyst demonstrated exceptional catalytic activity for NaBH₄ methanolysis (21 190 mL min⁻¹ g_catalyst⁻¹).

3.1.3. Co-based X-ide nanocatalysts for the dehydrogenation of NaBH₄. Co X-ides, such as Co phosphides, borides, oxides, and carbides, also exhibit outstanding catalytic activities due to their abundance of unsaturated sites, amorphous structures, and excellent chemical stability.^81,82 Schlesinger et al. first discovered that Co₂B had a preeminent catalytic performance for NaBH₄ hydrolysis in 1953. Since then, many researchers have focused on the study of Co–B complex catalysts. Liu and coworker confirmed that Co boride demonstrated better catalytic properties for NaBH₄ hydrolysis than Co powder.⁸³ The catalytic property of Co–B was observed to be influenced by the preparation conditions, for instance the pH value of NaBH₄ solution and the method of mixing the precursors. Chen and co-workers successfully synthesized hollow Co–B nanospheres using polystyrene as a template. Subsequent heat treatment led to the construction of porous walls with a thickness of approximately 10–15 nm in the Co–B hollow spheres (Fig. 4b and c).⁸⁴ As expected, the hollow Co–B complex demonstrated better catalytic performance compared to Co–B NPs for NaBH₄ hydrolysis. Li et al. synthesized a CoB/ZIF-8 catalyst by reducing the Co/Zn-ZIF-8 precursor in situ with NaBH₄ as the reducing agent.⁸⁵ The resulting CoB/ZIF-8 catalyst demonstrated good catalytic properties, providing an initial HGR value of 453.6 mL min⁻¹ g_Catalyst⁻¹ for NaBH₄ hydrolysis. Zou's group fabricated Ce-doped Co–Ce–B NPs using chitosan-derived carbon as the support (Fig. 4d).⁶⁰ The resulting Co–Ce–B/Chi-C catalyst exhibited significantly enhanced catalytic properties for NaBH₄ hydrolysis compared to unsupported Co–Ce–B NPs, achieving an ultrahigh HGR of 4760 mL⁻¹ min⁻¹ g_Catalyst⁻¹. Sun's group fabricated a Fe-doped CoP nanoarray on Ti foil (Fe–CoP/Ti) (Fig. 4e–g).⁸⁶ The identification of lattice fringes and ordered discrete spots suggested the existence of the CoP phase with Fe doping, as opposed to a blend of FeP and CoP (Fig. 4h–j). The resulting Fe–CoP/Ti catalyst displayed good catalytic properties and reusability for the hydrolytic dehydrogenation of NaBH₄, achieving a HGR value of 6060 mL min⁻¹ g_catalyst⁻¹, surpassing that of most Co-based catalytic systems.

Saka et al. employed the ZnCl₂-treated spirulina microalgal strain (SSMS-ZnCl₂) as the support to immobilize Co–B NPs.⁸⁷ The optimized SSMS-ZnCl₂–CoB gave maximal HGR values of 9266 and 36 366 mL min⁻¹ g_catalyst⁻¹ at 303 K and 333 K for NaBH₄ methanolysis. Zhu et al. deposited Co–P NPs on the dandelion-like CNT–Ni foam via a chemical vapor deposition (CVD) method.⁸⁸ At pH = 11 and NaH₂PO₂·H₂O concentration of 1.2 M in the plating solution, Co–P/CNT–Ni exhibited the highest catalytic performance (2430 mL min⁻¹ g_catalyst⁻¹) for the methanolysis of NaBH₄. The hydrogen evolution performances of NaBH₄ over different Co-based nanocatalysts are shown in Table 1.

Table 1 Catalytic properties of Co-based nanocatalysts for hydrogen evolution from NaBH₄

Catalysts	Active sites	Temp. (K)	Solvent	NaOH (wt%)	HGR (mL min^−1 g^−1)	Cycles, times and retained activity (%)	Ref.
a Based on Co metal weight.b Based on the catalyst weight.c Based on Ru metal weight.
Co	Co	295	H2O	8	—	—	89
Co/γ-Al2O3	Co	303	H2O	5	220^b	—	90
Co film/Cu foil	Co	298	H2O	0.8	1270^a	10, 90	43
p(HEMA)–Co	Co	303	H2O	5	1596^a	5, 57.7	91
p(SPM)–Co	Co	303	H2O	2.5	1288^a	—	92
Co NPs@ZIF-8	Co	303	H2O	1.6	14 023^a	—	66
Co/PGO	Co	298	H2O	1	5955^a	5, 73	93
Co@C2N	Co	303	H2O	5	8903^b	5, 56	67
Co@C	Co	303	H2O	6	1680^b	5, 93	94
Co@NMGC	Co	298	H2O	1	3575^b	20, 82.5	68
Co–CeOx/NCNS	Co	303	H2O	1.6	28 410^a	5, 12.5	69
Co-BTC@PUS	Co	303	H2O	3	37 640^b	6, 82.4	95
(Co-Ver)cat	Co	303	H2O	7	1991.4^a	10, 91.8	96
Co3O4-SCQD	Co	303	H2O	4	27 555^b	5, 30.6	97
Co/Al2O3	Co	298	CH3OH	—	2500^b	—	98
Co/AC	Co	303	CH3OH	—	7120.7^b	—	70
Co/TiO2	Co	298	CH3OH	—	664 000^a	—	71
ZIF-67@GO-2	Co	273	CH3OH	—	3200^b	5, 99	72
Ca0.8Co0.2CO3	Co	300	CH3OH	—	6666^b	5, 93	99
Co–Ru/C	CoRu	303	H2O	0.4	96 700^c	5, 80	74
Co–Fe/CQD	CoFe	308	H2O	4	20 220^b	4, 70	100
Ni–Co@3DG	CoNi	298	H2O	—	2041.5^b	5, 95.9	75
CoNi/Fe3O4@GO	CoNi	308	H2O	4	8200^b	5, 44.3	101
Co/Ni/MWAC	CoNi	303	H2O	5	740.7^b	6, 58.1	76
Co–Mo/3DGO	CoMo	298	H2O	1	7023.3^a	5, 56.3	77
ZnxCo1–Co@NC	ZnCo	303	H2O	7.3	1807^a	5, 32.5	102
Zr/Co/carbon	ZrCo	298	H2O	2	1708^b	—	78
Ru–Co	Ru–Co	293	CH3OH	10	1260^b	5, 50.8	79
Ru–Co/CNTs	Ru–Co	298	CH3OH	7	21 190^b	5, 75	80
Co–P	CoP	303	H2O	1	1647.9^b	11, 61	103
Co–P/Cu	CoP	303	H2O	1	954^b	—	104
Co–P/Cu sheet	CoP	303	H2O	10	5965^b	5, 40.8	105
Co–P/Cu sheet	CoP	303	H2O	1	1846^b	10, 66.8	106
Co–P/Ni foam	CoP	298	H2O	5	1490^b	—	107
Fe–CoP/Ti	CoP	298	H2O	1	6060^b	5, 99	86
Co–Ni–P/Pd–TiO2	CoNiP, Pd	328	H2O	10	3780^b	5, 86.4	108
NiCoP/Ni foam	CoNiP	298	H2O	1	3986^b	5, 76	109
Ni–Co–P/γ-Al2O3	CoNiP	328	H2O	4	6599.6^b	10, 37.3	110
Co–Ni–P/Cu sheet	CoNiP	303	H2O	10	3636^b	—	111
Co–W–P/Cu	CoWP	303	H2O	10	5000^b	5, 49	112
NiCoP/Ti	NiCoP	303	H2O	1	8010^b	8, 93	113
Co–W–P/CC	CoWP	303	H2O	2	4379^b	5, 60	114
Cu–Co–P/γ-Al2O3	CuCoP	298	H2O	5	1115^b	6, 66	115
CoNiFeP	CoNiFeP	303	H2O	1	1128^b	10, 70	116
Co–B–10CNTs	CoB	298	H2O	5	12 000^b	5, 64	117
Co–B/carbon	CoB	298	H2O	8	1128^b	—	118
Co–B/HPCM	CoB	298	H2O	5	3083.1^b	5, 91.7	119
Co–B/Ni foam	CoB	298	H2O	5	1640^b	—	120
Co–B/Ni foam	CoB	303	H2O	5	24 000^b	—	121
Co–B/Cu sheet	CoB	303	H2O	5	7935^b	5, 80	122
Co–B	CoB	303	H2O	5	2750^b	—	123
Co–B hollow spheres	CoB	298	H2O	8	3200^b	—	84
Mesoporous Co–B	CoB	298	H2O	—	3350^b	—	82
CoB/TiO2	CoB	303	H2O	3.8	12 503^b	—	124
Co–B/attapulgite clay	CoB	298	H2O	10	3350^b	9, 68.5	125
GO-modified Co–B	CoB	303	H2O	5	14 340^b	5, 81.5	126
CoB/sepiolite clay	CoB	303	H2O	1	1486^b	5, 31.6	127
Co2B–CoPOx	CoB, CoP	298	H2O	0.4	7716.7^b	5, 63.3	128
Co–Cu–B	CoCuB	293	H2O	7	2120^b	—	129
Co–Cu–B	CoCuB	298	H2O	1	3554^b	—	130
Co–Cr–B	CoCrB	298	H2O	0.1	3400^b	—	131
Co–Ni–B	CoNiB	301	H2O	15	2608^b	—	132
Co–Ce–B	CoCeB	303	H2O	5	4760^b	5, 81.4	60
Co–Mn–B	CoMnB	293	H2O	7.5	1440^b	—	133
Co–W–B/Ni foam	CoWB	303	H2O	5	15 000^b	5, 91	134
SSMS–ZnCl2–CoB	CoB	303	CH3OH	—	9266^b	5, 28.6	87
Co–P/CNT–Ni foam	CoP	298	CH3OH	1	2430^b	8, 65	88

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3.2. Ammonia borane

Ammonia borane (NH₃BH₃, AB) has garnered more and more attention as a prominent hydrogen storage material; this is attributed to the non-toxic property, extended stability, and notable gravimetric hydrogen density (19.6 wt%).^135,136 The initial discovery of ammonia borane's capacity to generate hydrogen via thermal decomposition upon melting was documented in 1987.^137,138 Nevertheless, the practical application of ammonia borane has been hindered by the need for high dehydrogenation temperatures and the formation of undesired byproducts of pyrolyzation.¹³⁹ An alternative method for releasing hydrogen is solvolysis, including hydrolysis (eqn (3)) or methanolysis (eqn (4)).^140–145

NH₃BH₃ + 2H₂O → NH₄BO₂ + 3H₂ (3)

NH₃BH₃ + 4CH₃OH → NH₄B(CH₃O)₄ + 3H₂ (4)

Numerous nanocatalysts have been explored for AB solvolysis, with noble metal-based nanocatalysts being recognized as the leading-edge in the field.^146–152 However, achieving the practical application of this system necessitates the development of cost-effective nanocatalysts to augment the kinetic performance. Numerous studies have been conducted on AB solvolysis over non-noble metal catalysts, among which Co-based NP catalysts, including monometallic Co NPs, Co-based alloy, and Co X-ides, exhibit higher catalytic activity than other non-noble metals.^153–155

3.2.1. Monometallic Co-based nanocatalysts for the dehydrogenation of NH₃BH₃. Monometallic Co nanocatalysts were first utilized for AB solvolysis. Metin and Özkar prepared poly(4-styrene sulfonic acid-co-maleic acid) immobilized Ni NPs (PSMA-Ni) and Co NPs (PSMA-Co) for AB hydrolysis.¹⁵⁶ According to transmission electron microscopy (TEM) characterization, the mean sizes of PSMA-Co and PSMA-Ni were measured as 5.3 nm and 2.1 nm, respectively. PSMA-Co demonstrated superior catalytic activity to that of PSMA-Ni as evidenced by a turnover frequency (TOF) of 25.7 min⁻¹ for AB hydrolysis. Nevertheless, these catalysts commonly demonstrated low cycling stability due to the dissolution of stabilizers. Yan devised a straightforward and effective approach for mitigating the deactivation of the Co nanocatalyst by means of the spontaneous and reversible metal-ion conversion of Co under ambient conditions (Fig. 5a).¹⁵⁷ The Co nanocatalyst exhibits exceptional activity even after an extended period of 73 days of recycling for AB hydrolysis.

Fig. 5 (a) Illustration of the sustainable synthesis of Co NPs and their application in catalytic hydrogen production from AB. Reproduced with permission.¹⁵⁷ Copyright 2013, Elsevier. (b) Illustration of the synthesis of KIT-6-confined Co NPs via single reducing agent, double reducing agent, and thermal reduction methods. Reproduced with permission.¹⁶¹ Copyright 2018, Elsevier. (c) Illustration of the synthesis of amorphous Co NP catalysts Co/MIL-101-1-U and Co/MIL-101-2-U via an in situ reduction method, and crystalline Co NP catalysts Co/MIL-101-1 and Co/MIL-101-2 via an ex situ reduction method. Reproduced with permission.¹⁶⁴ Copyright 2017, American Chemical Society. (d) Schematic illustration of photoactive Co/MIL-101(Cr)-NH₂ and the catalytic hydrolysis of AB under visible light irradiation. Reproduced with permission.¹⁶⁵ Copyright 2017, Elsevier. (e) The synthetic route to Co-g-C₃N₄/rGO. Reproduced with permission.¹⁶⁶ Copyright 2016, American Chemical Society.

In recent decades, various supports including porous carbon, silica, metallic oxide, and molecular sieves have been developed for the dispersion of Co-based metal NPs for the dehydrogenation of AB.^8,16,158 Xu et al. synthesized diverse non-noble NP catalysts utilizing SiO₂, γ-Al₂O₃, porous carbon, and MOFs as the support.¹⁵⁹ Notably, Co NPs immobilized on porous carbon exhibited optimal efficacy for AB hydrolysis. Conversely, porous carbon-supported Ni and Cu displayed low catalytic activities, while porous carbon-immobilized Fe NPs demonstrated negligible catalytic performance. Sullivan et al. synthesized SBA-15-immobilized Co NPs by a deposition impregnation method, and confirmed that the hydrogen evolution property of Co-SBA-15 for AB hydrolysis was influenced by counterions in the metal precursor, and the duration of activation for the reaction depended on the specific counterion.¹⁶⁰ The catalyst Co-SBA-15 derived from the reduction of Co(NO₃)₂ exhibited a prolonged activation period and lower activity energy for AB hydrolysis compared to those derived from the reduction of Co(BF₄)₂ or Co(CH₃COO)₂. Kao et al. effectively encapsulated ultra-small Co NPs in the mesopores of cubic silica KIT-6 through a straightforward chemical reduction method utilizing NaBH₄ and NH₃BH₃ as dual reducing agents (Fig. 5b).¹⁶¹ In comparison with the single reducing agent NaBH₄, Co@KIT-6 synthesized with double reducing agents exhibited superior activity, as evidenced by a TOF and activation energy of 20.05 min⁻¹ and 32.6 kJ mol⁻¹, respectively.

In recent years, organic porous polymers, for instance metal organic frameworks (MOFs), porous coordination cages (PCCs), porous aromatic frameworks (PAFs), covalent triazine frameworks (CTFs), covalent organic frameworks (COFs), and their derived porous carbons, are widely utilized as the carrier to stabilize metal NPs for the dehydrogenation of AB, owing to the large specific surface areas and abundant anchoring sites.^162,163 Gu et al. developed two preparation techniques for synthesizing amorphous or crystalline Co/MIL-101, focusing on the control and enhancement of catalyst performance (Fig. 5c).¹⁶⁴ Among them, Co/MIL-101 fabricated through an ultrasound-assisted in situ reduction strategy exhibited superior properties for AB hydrolysis, providing a TOF of 51.4 min⁻¹. The excellent hydrogen evolution property derived from the ultrafine and amorphous Co NPs. In the same year, the group synthesized a photoactive MOF material though the NH₂-functionalization of MIL-101, which was used to anchor Co NPs (Fig. 5d).¹⁶⁵ The resulting Co/MIL-101(Cr)-NH₂ catalyst demonstrated a conspicuous dehydrogenation property (117.7 min⁻¹) for AB hydrolysis under visible light irradiation. Li et al. fabricated graphitic carbon nitride (g-C₃N₄)-encapsulated Co NPs (Co@g-C₃N₄), and subsequently deposited this composite onto reduced graphene oxide (rGO) (Fig. 5e).¹⁶⁶ The g-C₃N₄ shells and rGO effectively inhibited the aggregation and leaching of Co NP cores. The optimized Co@g-C₃N₄–rGO catalyst demonstrated exceptional catalytic activity (15.4 min⁻¹) for AB hydrolysis.

He's group synthesized covalent triazine framework (CTF-1) immobilized Co NPs through an impregnation method and the optimal Co/CTF catalyst exhibited a notable dehydrogenation property (42.3 min⁻¹) for AB hydrolysis.¹⁶⁷ Zhou et al. fabricated porous coordination cage (PCC) encapsulated Co nanoclusters (NCs) via the Co²⁺ ion capture method (Fig. 6a).¹⁶⁸ The higher net charges of PCC-2a facilitated the dispersion of Co²⁺ cations, leading to the in situ formation of ultra-small Co NCs. In contrast, larger Co NPs were observed in PCC-2b with low net charges. Consequently, Co NCs supported on PCC-2a exhibited a superior hydrogen evolution property for AB hydrolysis to those supported on PCC-2b. Yang's group fabricated hierarchically porous carbon-confined Co NPs (Co/HPC) using Co/Zn-MOF-74 as the precursor via selective atom evaporation–isolation methods (Fig. 6b).¹⁶⁹ The inclusion of Zn atoms within Co/Zn-MOF-74 was found to be a significant factor in the generation of superfine Co NPs. The optimal Co/HPC-900 catalyst demonstrated a favorable hydrogen evolution property for AB hydrolysis with a TOF of 2.9 min⁻¹. Tao's group synthesized nitrogen-doped nanowires (NPCNW) through high-temperature calcination of the Co-MOF precursor. Ultrafine Co NPs immobilized on NPCNW (Co/NPCNW) demonstrated enhanced hydrogen evolution efficiency (12.2 min⁻¹) for AB hydrolysis, and maintained 94.6% of their initial property after ten cycles.⁶⁵

Fig. 6 (a) Synthetic route to Co nanoclusters encapsulated by PCCs. Reproduced with permission.¹⁶⁸ Copyright 2017, Wiley-VCH. (b) Schematic illustration of the fabrication of Co/HPC via the pyrolysis of Co-MOF. Reproduced with permission.¹⁶⁹ Copyright 2019, American Chemical Society. (c) The synthesis of multilayer benzidine-functionalized GO (M-BZ-GO), parallel nanocomposite (P-PAF-GO), and vertical nanocomposite (V-PAF-GO). Reproduced with permission.¹⁷⁰ Copyright 2023, Wiley-VCH.

Monometallic Co nanocatalysts also demonstrated significant catalytic activities in the methanolysis of AB. Our group developed a controllable step-growth strategy for fabricating vertical porous aromatic frameworks on graphene oxide (V-PAF-GO) (Fig. 6c). Subsequently, ultrafine Co NPs were encapsulated in the slit pores of the vertical nanocomposite.¹⁷⁰ The resulting Co/V-PAF-GO catalyst demonstrated an exceptional hydrogen evolution property (47.6 min⁻¹) for AB methanolysis. Experimental and theoretical studies demonstrated that electron transfer between Co and PAF contributed to the low activation energy of methanol decomposition, resulting in the preeminent catalytic performance. Oh's research team encapsulated ultrafine Co NPs in hierarchical Ni metal–organic framework (Ni-MOF) nanocolumn arrays (NCAs), which were subsequently utilized as catalysts for AB methanolysis.¹⁷¹ Attributed to the synergy between Co and Ni-MOF NCAs, Co@Ni-MOF NCA/NF displayed an outstanding catalytic activity for AB methanolysis with a TOF of 20.5 min⁻¹.

3.2.2 Co-based alloy for hydrogen generation from NH₃BH₃. Bimetallic or trimetallic alloy NP catalysts often exhibit significantly enhanced performance compared to monometallic catalysts due to electronic and geometric effects.²⁶ With these points in mind, many recent works have focused on Co-based heterometallic catalysts for promoting catalytic hydrogen production from AB. Xu et al. fabricated magnetic Au@Co NPs though a seeding–growth method (Fig. 7a).¹⁷² The Au@Co NPs demonstrated notable catalytic efficacy and enduring stability in AB hydrolysis. Li's research team successfully synthesized magnetically recyclable Au–Co alloy NPs on g-C₃N₄ (Au–Co@CN), which served as a photocatalyst for dehydrogenation through AB hydrolysis (Fig. 7b).¹⁷³ The optimal Au–Co@CN catalyst demonstrated a dramatically improved catalytic property (48.3 min⁻¹) under visible light than that without photoirradiation. The excellent catalytic property of Au–Co@CN derived from the synergistic interplay between Au and Co, along with the manifestation of the Mott–Schottky effects between metal and the carrier. Yu's group prepared various ultrafine and well-dispersed bimetallic alloy NPs using TiO₂ as the support though a step-by-step reduction strategy (Fig. 7c).¹⁷⁴ The incorporation of Co species substantially improved the properties of various noble metal NPs for AB hydrolysis. The optimal Pt_0.1%Co_3%/TiO₂ catalyst demonstrated an ultrahigh catalytic property (2250 min⁻¹) for the hydrolysis of AB. The interactions between Pt and CoO were shown to promote the adsorption and decomposition of H₂O according to density functional theory (DFT) calculations, leading to accelerated production of hydrogen. Our group encapsulated Au–Co NPs in SiO₂ nanospheres to fabricate core–shell Au–Co@SiO₂-RT at room temperature using metal–ammine crystal templates in the NP-5/cyclohexane reverse micellar system.¹⁷⁵ Then, Au–Co@SiO₂-RT nanospheres were treated at high temperature (573 K) to obtain Au–Co@SiO₂-HT. The resulting Au–Co@SiO₂-HT catalyst exhibited a better dehydrogenation property than that of Au–Co@SiO₂-RT, probably due to the reduced basic ammine content resulting from decomposition of the metal–ammine complex at high-temperature. In 2022, Lu et al. prepared CoRu nanoalloy catalysts using carbon quantum dots as the support via a simple synthesis method (CoRu/CQDs) (Fig. 7d–f).¹⁷⁶ The relationship between the tensile lattice strain of CoRu nanoalloys and their hydrogen evolution property for AB hydrolysis was systematically examined. According to theoretical calculations, the enhanced dehydrogenation property of CoRu_0.5/CQDs was ascribed to the favorable electron structure of the alloy resulting from the addition of trace amounts of Ru (Fig. 7g–i), which facilitated rapid interfacial electron transfer to intermediates, thereby enhancing the kinetics of H₂ evolution.

Fig. 7 (a) Schematic representation and color transformation during the fabrication of the Au@Co catalyst. Reproduced with permission.¹⁷² Copyright 2009, American Chemical Society. (b) Work functions and band structures of g-C₃N₄ and representative metals, and schematic illustration of the Mott–Schottky type contact of Au–Co alloy NPs. Reproduced with permission.¹⁷³ Copyright 2014, American Chemical Society. (c) Fabrication of TiO₂-supported bimetallic nanocatalysts. Reproduced with permission.¹⁷⁴ Copyright 2023, American Chemical Society. (d–f) Morphology characterization of carbon quantum dot-supported Ru and CoRu alloy NPs. (g–i) Integrated pixel intensities for the lattices of carbon quantum dot-supported Ru and CoRu alloy NPs. Blue spheres and yellow spheres represent Co and Ru, respectively. Reproduced with permission.¹⁷⁶ Copyright 2021, Wiley-VCH.

Xu's group successfully synthesized MIL-101-confined CuCo alloy NPs though a double-solvent strategy.¹⁷⁷ The resulting CuCo@MIL-101 catalyst displayed a superior catalytic property to that of monometallic Cu@MIL-101 and Co@MIL-101, because of the synergistic interactions between Cu and Co. In a follow-up study, the researchers found that another efficient means of averting particle agglomeration was to dope with inactive transition metals, for instance Mo and W. These metals exhibited high effectiveness in both metallic and oxidized states, with even low atomic concentrations leading to a substantial enhancement in catalytic surface area by preventing the agglomeration of metal NPs. Our group successfully synthesized noble metal-free CuCoMo alloy NPs using a straightforward co-reduction method, without the need for additional surfactants or carriers (Fig. 8a).¹⁷⁸ The optimal trimetallic CuCoMo NPs displayed notably improved catalytic performance compared to their bi-/monometallic counterparts. Furthermore, subsequent research demonstrated that the addition of NaOH prominently promoted the dehydrogenation rate of AB hydrolysis over CuCoMo NPs, achieving a nearly threefold increase in TOF compared to dehydrogenation without NaOH. Zhao et al. synthesized porous nitrogen-doped carbon immobilized NiCo alloy NPs using bimetallic ZnCo zeolitic imidazole frameworks as the precursor (Fig. 8b).¹⁷⁹ The enhanced catalytic property of the NiCo-NC nanocatalyst was attributed to its high nitrogen content and strong synergistic effect between the alloy components, which was evidenced by the elevated TOF value of 35.2 min⁻¹ achieved under alkaline conditions. Ma's group synthesized nearly atomically dispersed CoNi alloy on α-MoC and ascertained the interactions between Co and Ni utilizing real-space chemical mapping techniques at the atomic scale (Fig. 8c and d).¹⁸⁰ The Fourier transformed k²-weighted extended X-ray absorption fine structure (EXAFS) indicates that Co and Ni predominantly associate with C within the α-MoC matrix, as opposed to Mo. Furthermore, these spectra corroborate that Co and Ni are highly dispersed within the α-MoC structure without aggregating to form nanoparticles. 1.5Co1.5Ni/α-MoC demonstrated an optimal hydrogen evolution performance (321 min⁻¹) for AB hydrolysis than other Co/Ni ratios (Fig. 8e and f). Mechanistic investigations demonstrated that the enhanced catalytic performance derived from the collective abilities of α-MoC to facilitate water splitting and the synergistic interaction of the bimetallic species.

Fig. 8 (a) The enhanced catalytic property of AB hydrolysis with the assistance of a base over the CuCoMo catalyst. Reproduced with permission.¹⁷⁸ Copyright 2018, Royal Society of Chemistry. (b) Illustration of the synthesis of NiCo-NC. Reproduced with permission.¹⁷⁹ Copyright 2021, Elsevier. (c) Topography analysis of CoNi/α-MoC. (d) X-ray absorption near-edge characterization of CoNi/α-MoC. (e and f) Dehydrogenation curve and the corresponding TOF values for AB hydrolysis over CoNi/α-MoC with diverse molar ratios of Co : Ni. Reproduced with permission.¹⁸⁰ Copyright 2021, American Chemical Society.

Li et al. synthesized magnetically recyclable PVP-stabilized Co–Ni alloy nanocatalyst, and Co_0.7Ni_0.3 exhibited the highest hydrogen generation activity for AB methanolysis.¹⁸¹ Moreover, the catalyst retained 91.2% of its initial catalytic activity after eight cycles, demonstrating excellent stability. Sun's group fabricated monodisperse CoPd alloy NPs via the co-reduction of Co and Pd precursors in oleylamine and trioctylphosphine.¹⁸² Among all the tested catalysts, Co₄₈Pd₅₂ supported on C exhibited the highest catalytic activity for catalyzing AB methanolysis, providing a TOF of 27.7 min⁻¹. Rakap et al. synthesized a series of hydroxyapatite-immobilized Rh–M (RhCo, RhCu, RhFe) nanoclusters.¹⁸³ Rh₅₁Co₄₉@HA displayed the highest catalytic performance for AB methanolysis among all the catalysts examined.

3.2.3 Co-based X-ides for the dehydrogenation of NH₃BH₃. Transition metal X-ides, including borides, phosphides, and nitrides, are also utilized as nanocatalysts for hydrogen evolution from AB because of their superior corrosion resistance, excellent chemical stability, and high electrical conductivity.^14,184 Yu's group developed a hierarchical sandwich-like Co doped Ni nitride nanocomposite using 2D rGO as the support via the NH₃ gas topochemical nitridation method (Fig. 9a).¹⁸⁵ The rGO/CoNi–N composite demonstrated a significantly enhanced hydrogen generation performance (126.0 min⁻¹) in AB hydrolysis. The outstanding property of rGO/CoNi–N derived from the thorough exposure of active sites in metal NPs. Experimental and theoretical studies confirmed that the introduction of Co species modulated the electronic state of Ni–N by elevating the d-band center, resulting in the enhanced adsorption and lower dissociation energy barrier of H₂O. Zhong et al. synthesized a CoNiP nanobox supported on GO for AB hydrolysis (Fig. 9b).¹⁸⁶ Through the incorporation of a small amount (1.7 wt%) of doped Ni, the electronic structure of Co was effectively modulated, leading to a prominent enhancement of the hydrolysis property. As a result, the optimized CoNiP/GO catalyst demonstrated a record TOF of 134.6 min⁻¹ for AB hydrolysis. Based on X-ray absorption near-edge structure spectra (XANES), the catalytic performance primarily derived from the subtly altered chemical state of Co. Li's group fabricated a dual-active site B–Co–P complex with a typical atomic-bridge structure on boron nitride nanosheets (Fig. 9c).¹⁸⁷ The Co K-edge XANES revealed that the absorption of Co₃B–CoP/h-BN lay between that of CoO foil and Co₃O₄, and the coordination numbers of Co–B and Co–P were 4.3 and 3.9, which confirmed the existence of B–Co–P double sites of the atomic bridge structure. The resulting catalyst demonstrated remarkable kinetics (37 min⁻¹) and stability for AB hydrolysis. Experimental and theoretical investigations confirmed that the atomic-bridge structure in B–Co–P could effectively modify the electronic structure of Co and reduce the energy barrier of the hydrolysis reaction. Chen et al. engineered a three-dimensional porous CoP carbon-based framework (CoP@CNF) employing a bimetallic Zn/Co MOF as the precursor.¹⁸⁸ The optimal CoP@CNF catalyst exhibited an ultrahigh hydrogen generation property (165.5 min⁻¹) for AB hydrolysis, which was attributed to the excellent dispersion of CoP, large specific surface area, and favorable hydrophilicity and permeability.

Fig. 9 (a) Illustration of the synthesis of the rGO/CoNi–N nanocomposite. Reproduced with permission.¹⁸⁵ Copyright 2021, Elsevier. (b) Preparation of the CoNiP/GO nanocatalyst. Reproduced with permission.¹⁸⁶ Copyright 2021, Elsevier. (c) Preparation of the atomic-bridge structured B–Co–P complex. Reproduced with permission.¹⁸⁷ Copyright 2022, Elsevier.

Jagirdar et al. synthesized Co–Co₂B, Ni–Ni₃B and Co–Ni–B nanocomposites via the reduction of metal salts with ammonia borane.¹⁸⁹ The Co–Ni–B nanocomposite exhibited enhanced activity (10 min⁻¹) relative to the individual metal–metal boride nanocomposites. Kalu's group prepared Co–Ni–P composites using Pd-activated Al₂O₃ as the support.¹⁹⁰ Co–Ni–P/Pd-Al₂O₃ (Co : Ni = 86.4 : 13.6) emerged as a promising candidate for highly efficient hydrogen generation from AB methanolysis due to the high activity, stability, and reusability. The primary Co-based nanocatalysts for hydrogen evolution from AB and their corresponding efficiencies are presented in Table 2.

Table 2 Properties of different Co-based nanocatalysts for hydrogen evolution from AB

Catalyst	Active sites	Temp. (K)	Solvent	TOF (min^−1)	Cycles, times and retained activity (%)	Ref.
a Based on the moles of metal Co.b Based on the moles of alloy.c Based on the moles of all metals.
Co/γ-Al2O3	Co	298	H2O	2.3^a	—	159
In situ Co NPs	Co	298	H2O	44.1^a	10, 100	191
PSMA-Co	Co	298	H2O	25.7^a	—	156
Co/PEI-GO	Co	298	H2O	39.9^a	5, 65.6	192
Co/MIL-101-1-U	Co	298	H2O	51.4^a	5, 49.6	164
Co/NPCNW	Co	298	H2O	12.2^a	10, 94.6	193
Co@N–C-700	Co	298	H2O	5.6^a	10, 97.2	194
Co/HPC	Co	323	H2O	2.9^a	12, 90	169
Co/CTF	Co	298	H2O	42.3^a	5, 54.2	167
Co NCs@PCC-2a	Co	298	H2O	90.1^a	—	168
Co@C–N@SiO2-800	Co	298	H2O	8.4^a	5, 99	154
G6-OH(Co60)	Co	298	H2O	10.0^a	4, 46.2	195
Co–(CeOx)0.91/NGH	Co	298	H2O	79.5^a	5, 87	196
CoCl2	Co	298	CH3OH	3.7^a	—	197
Co/PAF-GO-V	Co	298	CH3OH	47.6^a	5, 53.6	170
Co@Ni-MOF NCA/NF	Co	298	CH3OH	20.5^a	5, 60.3	171
HCSS-Co3O4@CuO–NiO	Co	298	CH3OH	67.1^a	8, 83	198
RuCo/γ-Al2O3	RuCo	338	H2O	32.9^b	4, 21.2	199
AuCo@MIL-101	AuCo	298	H2O	23.5^b	5, 71.4	200
AgCo/PAMAM	AgCo	298	H2O	15.8^b	5, 60	201
Co35Pd65/C	CoPd	298	H2O	22.7^b	5, 75	202
Pd@Co@MIL-101	Pd, Co	303	H2O	51.0^b	5, 72.3	203
Fe0.3Co0.7 alloy	FeCo	293	H2O	13.9^b	—	204
CuCo/graphene	CuCo	293	H2O	9.18^b	3, 93	205
Cu0.2Co0.8/PDA–rGO	CuCo	303	H2O	51.5^b	5, 31.6	206
Cu0.5Co0.5@SiO2	CuCo	298	H2O	4.3^b	10, 93	207
Cu0.3Co0.7@MIL-101	CuCo	298	H2O	19.6^b	5, 81.8	177
Cu0.72Co0.18Mo0.1	CuCo	298	H2O	119.0^b	—	178
Cu0.72Co0.18Mo0.1	CuCo	298	H2O	46.0^b	3, 45.5	178
CuCo@MIL-101-1-U	CuCo	298	H2O	51.7^b	5, 46.2	164
Cu0.8Co0.2O–GO	CuCo	298	H2O	70.0^b	6, 94.7	208
PVP-stabilized Co0.7Ni0.3	CoNi	298	CH3OH	35.3^b	8, 91.2	181
Co48Pd52/C	CoPd	298	CH3OH	27.7^b	8, 98	182
Rh51Co49@HA	RhCo	298	CH3OH	193.7	5, 90	183
CoP@CNF	CoP	298	H2O	165.0^a	4, 90	188
Br1-CoP@C	CoP	298	H2O	67.3^a	5, 35	209
Co2P-CDs/mHNTs	CoP	298	H2O	178.0^a	5, 73	210
Fe–CoP@C	Fe–CoP	298	H2O	183.5^a	5, 68	211
B–Co–P	B–Co–P	303	H2O	37.0^a	5, 30	187
CoP–CoO/NCDS	CoP, CoO	298	H2O	89.6^a	5, 32.6	212
Ni0.23Co0.19P0.58@NHPC900	NiCoP	298	H2O	125.2^c	5, 75	213
rGO/CoNi–N	CoNiN	298	H2O	126.0^c	10, 40	185
Co–Co2B	Co, CoB	298	CH3OH	7.5^a	10, 99	189
Co–Ni–B	CoNiB	298	CH3OH	10.0^c	10, 99	189
Co–Ni–P	CoNiP	303	CH3OH	4.3^c	5, 65	190

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3.3. Other borohydrides and dimethylamine borane

Other borohydrides such as potassium borohydride, lithium borohydride, and the borane adduct dimethylamine borane are also employed for hydrogen production. However, due to their scarcity, there are few reports on their hydrogen production over Co-based catalysts. Büyükkanber's group synthesized carbon sphere-immobilized Co for the hydrolysis of KBH₄. The optimum catalyst, 10%Co/90%CS, gave a HGR of 7149.2 mL min⁻¹ g_cat⁻¹ for the hydrolysis of 1 wt% KBH₄ with 7 wt% KOH at 303 K.²¹⁴ The lack of micropores improved the performance by mitigating mass transfer limitations, particularly those associated with internal diffusion. Saka et al. fabricated caffeine carbon dot-immobilized Co catalysts in ethanol for the hydrolysis of KBH₄. The resulting Co@MOF-CQD (ethanol) catalyst displayed an ultrahigh HGR of 17 081 mL min⁻¹ g_cat⁻¹.²¹⁵ Oxygen atoms present on the catalyst surface promoted the generation of active sites on the cobalt substrate through the formation of hydrogen bonds with water molecules, consequently augmenting the catalytic activity.

Liu et al. synthesized a series of MoS₂ nanosheet-supported non-noble metal NPs, and found Co/MoS₂ was the most efficient catalyst for the hydrolysis of dimethylamine borane (DMAB), providing a TOF of 6.3 min⁻¹. Sen's group prepared RuCo alloy nanomaterials using functionalized multiwalled carbon nanotubes (MWCNTs) as the support via an ultrasonic double reduction strategy.²¹⁶ The RuCo@G-MWCNT catalyst obtained exhibited a superhigh hydrogen generation performance for the DMAB dehydrogenation reaction, accompanied by great reusability. Rakap synthesized hexadecyltrimethyl ammonium bromide-immobilized RhCo alloy nanoclusters; the optimal Rh_0.63Co_0.37@CTAB catalyst exhibited high efficiency and stability for the hydrolysis of DMAB, providing a TOF of 142.9 min⁻¹_.²¹⁷

4. Investigation of the catalytic mechanism

Comprehending the catalytic mechanism holds paramount significance in the design of the catalyst and for the improvement of the catalytic properties.^14,26,218 The study of catalytic mechanisms involves the breaking and formation of bonds, the adsorption of substrates, the dissociation of product hydrogen, and the energy barrier of the transition states. Isotope experiments and theoretical calculations are frequently utilized to explore the catalytic mechanism.

4.1. Mechanism of hydrogen production from NaBH₄

In 2019, Astruc's group first proposed an oxidative addition mechanism for NaBH₄ hydrolysis over CoNPs@ZIF-8.⁶⁶ They believed that the hydrogen bonding interactions between H₂O and borohydride effectively reduced the electron density of the O–H bond in H₂O, thus promoting the challenging oxidative addition of O–H bonds (Fig. 10a). Then, they investigated the isotopic effects of NaBH₄ hydrolysis using deuterium oxide (D₂O) instead of H₂O. A significant kinetic isotope effect was observed when utilizing D₂O (k_H/k_D = 6.85), suggesting that the cleavage of the O–H bonds in H₂O was the key to the rate-determining step (RDS) of NaBH₄ hydrolysis (Fig. 10b).

Fig. 10 (a) Oxidative addition mechanism of NaBH₄ hydrolysis over CoNPs@ZIF-8. (b) Hydrogen release of NaBH₄ in H₂O and D₂O over CoNPs@ZIF-8. Reproduced with permission.⁶⁶ Copyright 2019, Wiley-VCH. (c) Potential energy diagram associated with the dehydrogenation of NaBH₄ over Co/C. Reproduced with permission.²¹⁹ Copyright 2022, Elsevier. (d) Free energy diagram of NaBH₄ hydrolysis over Co@NMGC. (e) Electronic potential of Co@NMGC. Reproduced with permission.⁶⁸ Copyright 2020, Elsevier.

Dou et al. synthesized Co-inlaid carbon sphere catalysts and investigated the process of NaBH₄ hydrolysis.²¹⁹ They proposed that the initial formation of hydrogen molecules originated from NaBH₄, while the dissociation of water occurred during subsequent hydrogen molecule formation. The formation of NaOBO + H + H + 3H₂ exhibited the highest activation barrier (1.88 eV) among the reaction processes, suggesting that it served as the RDS of the hydrolysis reaction (Fig. 10c). Regarding the competing reaction, the energy barriers for the cleavage of the first and fourth B–H bonds in sodium borohydride exceed that of the O–H bond cleavage in water. Conversely, the energy barriers for the cleavage of the second and third B–H bonds are lower than that of the O–H bond cleavage in water. Sun's group investigated the adsorption behavior of BH₄⁻ on the surface of Co@NMGC, Co, as well as graphene, and calculated the Gibbs free energy of hydrogen adsorption (ΔGH*).⁶⁸ As shown in Fig. 10d, metallic Co exhibited a higher hydrogen adsorption energy compared to the graphene shell. After the introduction of encapsulated Co clusters, a significant reduction in ΔGH* of Co@NMGC was noted from 1.99 eV to 0.07 eV. Additionally, during the second step, Co@NMGC displayed the lowest activation barrier (0.22 eV), indicating a notable enhancement in hydrogen evolution efficiency. Analysis of the electronic properties of Co@NMGC indicated that the graphene shells displayed a higher proton affinity following adsorption compared to pristine graphene. The redistribution of electron potential within Co@NMGC, facilitated by the transfer of charges from the Co atom to the C atom, significantly improved the adsorption and dissociation of the H atom on the Co surface.

4.2. Mechanism of hydrogen production from NH₃BH₃

The initial reaction mechanism for the dehydrogenation of AB was primarily formulated through conjecture regarding the underlying reaction processes. Three potential mechanisms, the nucleophilic substitution mechanism, the activation of H₂O mechanism, and the oxidative addition mechanism, have been posited.^26,220,221 Lu's group presented a nucleophilic substitution mechanism for AB hydrolysis over Co–Co₃O₄/CDs.²²¹ They proposed that H₂O molecules adsorbed onto Co₃O₄ first dissociated to form OH* radicals (Fig. 11a). Consequently, the accumulation of OH* radicals on the Co surface facilitated the advancement of SN₂ reactions and enhanced the reaction rate of AB hydrolysis. Then, Li's group proposed the H₂O activation mechanism for AB hydrolysis (Fig. 11b–d).^153,222 They believed that the activation of B–H bonds of NH₃BH₃ took place on the Co–Co bonds of Co NPs, while H–O bonds in H₂O were activated by Co–O bonds. Ultimately, two activated hydrogen atoms combined to produce a hydrogen molecule. The activation process was pervasive within the whole reaction system, serving as a robust kinetic basis for achieving heightened catalytic activity. Astruc's group investigated the process of AB hydrolysis over Ni/ZIF-8 and proposed that water activation via the oxidative addition of O–H bonds resulted in the formation of adsorbed –OH and –H species (Fig. 11e).²²³

Fig. 11 (a) Nucleophilic substitution mechanism of AB hydrolysis over Co–Co₃O₄/CDs. Reproduced with permission.²²¹ Copyright 2019, Elsevier. (b and c) The activation mechanism of H₂O for AB hydrolysis catalyzed by Co-NC/NF₆₀₀. Reproduced with permission.²²² Copyright 2022, American Chemical Society. (d) The activation mechanism of H₂O for AB hydrolysis catalyzed by Co–CoO_x@NCS-II. Reproduced with permission.¹⁵³ Copyright 2019, American Chemical Society. (e) Oxidative addition mechanism for AB hydrolysis over Ni/ZIF-8. Reproduced with permission.²²³ Copyright 2017, American Chemical Society.

The breaking of B–N, B–H, and O–H bonds is involved in AB solvolysis. Isotope experiments serve as a valuable approach for elucidating the kinetics of these processes. Yu et al. performed measurements on the kinetic isotope effect during AB hydrolysis utilizing Pt_0.1%Co_3%/TiO₂ as the catalyst.¹⁷⁴ The dehydrogenation of NH₃BH₃ in D₂O exhibits an evident slow rate compared to that in H₂O, revealing the involvement of the breaking of O–H bonds in the RDS of AB hydrolysis (Fig. 12a). Astruc's group studied the kinetic isotope effect of hydrogen production from NH₃BH₃ and NH₃BD₃ in H₂O, and NH₃BH₃ in D₂O over Ni/ZIF-8 (Fig. 12b).²²³ The hydrolysis of NH₃BD₃ in H₂O over Ni/ZIF-8 exhibited a reduced reaction rate, and the kinetic isotope effect (KIE) was determined as 1.33, suggesting dehydrogenation behavior similar to that observed for NH₃BH₃ in H₂O. Conversely, a significant KIE value of 2.49 was detected during hydrogen generation in the NH₃BH₃–D₂O system, suggesting that the cleavage of O–H bonds in water could potentially serve as a slow step for the hydrolysis of AB. Our group thoroughly analyzed the kinetic isotope effects of AB methanolysis through the substitution of CH₃OH with CD₃OD and NH₃BH₃ with NH₃BD₃ and ¹⁵NH₃BH₃ (Fig. 12c).¹⁷⁰ The kinetic isotope effect constants were determined to be 1.05 for NH₃BD₃, 1.03 for ¹⁵NH₃BH₃, and 6.32 for CD₃OD, confirming the involvement of O–H bond breaking in the RDS.

Fig. 12 (a) H₂ generated from AB in H₂O and D₂O over Pt_0.25%Co_3%/TiO₂. Reproduced with permission.¹⁷⁴ Copyright 2023, American Chemical Society. (b) Hydrogen production from NH₃BH₃ and NH₃BD₃ in H₂O, and NH₃BH₃ in D₂O over Ni/ZIF-8. Reproduced with permission.²²³ Copyright 2017, American Chemical Society. (c) Isotopic experiments for AB methanolysis catalyzed by Co/V-PAF-GO. (d) Gibbs free-energy profiles and active energy diagrams of CH₃OH and NH₃BH₃ over Co, Co/GO, and Co/BZ-PAF. Reproduced with permission.¹⁷⁰ Copyright 2023, Wiley-VCH. (e) Spin-polarized partial density of states (PDOS) and energy diagrams of AB hydrolysis over CoRu_0.5/CQDs and bulk CoRu_0.5/CQDs. Reproduced with permission.¹⁷⁶ Copyright 2021, Wiley-VCH. (f) Energy profiles for the dissociation of H₂O and AB on Pt(111) and Pt₄@CoO(111). Reproduced with permission.¹⁷⁴ Copyright 2023, American Chemical Society.

Our group studied the adsorption and activation of AB and methanol molecules on GO-supported Co(111) (Co/GO), BZ-PAF-stabilized Co(111) (Co/BZ-PAF), and supported-free Co(111) by DFT calculations (Fig. 12d).¹⁷⁰ The activation barriers for CH₃OH molecules on the three catalysts were evidently higher than those for NH₃BH₃, thereby substantiating that breaking of the O–H bonds in CH₃OH served as the RDS of AB methanolysis. Lu's group investigated the PDOS and energy diagrams for AB hydrolysis over CoRu_0.5/CQDs and bulk CoRu_0.5/CQDs to provide a more comprehensive understanding of the reaction mechanism (Fig. 12e).¹⁷⁶ According to the partial density of states (PDOS) of H₂O adsorbed on CoRu_0.5/CQDs, they confirmed a higher degree of orbital overlap at the Ru sites compared to the Co sites in the presence of free H₂O. This observation indicated that Ru played a crucial part in the initial activation of H₂O on CoRu_0.5/CQDs. Furthermore, the PDOS analysis of relevant species during H₂O adsorption provided further evidence supporting the crucial role of Ru in facilitating electron transfer. The reaction pathway exhibited a consistent downward trend in the production of H₂, aligning with electronic structure calculations. The dissociation energy barrier of OH* to O* was calculated to be 0.43 eV, representing a critical determinant in the potential reaction. Additionally, a significant energy decrease of 3.69 eV was noted during the dissociation of AB and its subsequent combination with O*, highlighting the stronger binding of B–O compared to B–OH. Yu et al. constructed a Pt₄ cluster supported on CoO(111) (Pt₄@CoO(111)) and calculated the energy profiles for the dissociation of water and AB on the surface of Pt₄@CoO(111) and Pt(111) (Fig. 12f).¹⁷⁴ The dissociation of B–H bonds in AB on Pt₄@CoO(111) and Pt(111) exhibited low activation energies of 0.35 eV and 0.09 eV, suggesting a facile step for B–H bond dissociation. A significantly higher activation barrier (0.79 eV) for H₂O dissociation on Pt(111) was observed compared to that on Pt₄@CoO(111). The enhanced dehydrogenation property of Pt₄@CoO(111) for facilitating water dissociation was ascribed to the synergistic effects between Pt₄ and CoO(111).

The catalytic pathways for AB solvolysis catalyzed by Co-based X-ide catalysts are also investigated to explore the nature of the catalytic reactions. Li et al. constructed Ni₃N (110), Co–Ni₃N (110), Ni₃N (111), and Co–Ni₃N (111) modes to probe the reaction pathways of AB hydrolysis.²¹² According to Fig. 13a, the calculated E_ads(H2O) values for Ni₃N (110), Co–Ni₃N (110), Ni₃N (111), and Co–Ni₃N (111) were −0.186, −0.315, −0.291, and −0.418 eV, respectively. These results indicated that Co–Ni₃N exhibited a greater propensity for H₂O adsorption than Ni₃N, suggesting that Co doping effectively enhanced the binding affinity of H₂O. The results of the energy profiles for H₂O dissociation clearly demonstrated a decrease in activation energy (E_a) for H₂O dissociation on the surface of Co–Ni₃N (110) and Co–Ni₃N (111) by introducing Co species. This provided evidence that the incorporation of Co species in Ni₃N significantly enhanced the cleavage of H₂O, thereby remarkably enhancing the catalytic property of AB hydrolysis. Li's group investigated the adsorption and dissociation of AB and H₂O on CoP, Co₃B–CoP, and Co₃B–CoP/h-BN to reveal the potential mechanism of the B–Co–P site (Fig. 13b).¹⁸⁷ They confirmed that Co₃B–CoP/h-BN had the lowest dissociation barrier for AB, and the Co–P site was more efficient at activating the AB molecule than the Co–B site. In addition, the Co–P/Co–B sites on Co₃B–CoP/h-BN exhibited a lower energy barrier for H₂O dissociation than those on CoP, Co₃B, and Co₃B–CoP. Furthermore, the H₂O molecule proved to be more easily activated on the Co–B site than the Co–P site. Eventually, they revealed a possible mechanism for AB hydrolysis on B–Co–P sites (Fig. 13c). NH₃BH₃ and H₂O were first adsorbed on dual-active sites of atomic-bridge structured B–Co–P to form NH₃BH₃–[Co–P]* and H₂O–[Co–B]* species. Then, the H₂O–[Co–B]* species were dissociated to OH–[Co–B]* and H–[Co–B]*. On the other hand, NH₃BH₃–[Co–P]* cleaved to form NH₃BH₂–[Co–P]* and H–[Co–P]*. The combination of H–[Co–P]* and H–[Co–B]* resulted in the production of the H₂ molecule, which was subsequently liberated from the B–Co–P sites. The reaction continued until two additional H₂ molecules were generated through a similar reaction mechanism. Recently, the same group synthesized a Co₄N–Co₃O₄ interface structure catalyst for hydrogen generation from AB (Fig. 13d).²²⁴ They constructed Co₄N, Co₃O₄, and Co₄N–Co₃O₄ models and calculated the density of states of three catalysts and energy profiles for the adsorption and dissociation of AB and H₂O on these catalysts. The activation energy barriers for AB and H₂O on Co₃O₄–Co₄N (311) were significantly lower than those of Co₄N, Co₃O₄, and Co₃O₄–Co₄N. It was demonstrated that the construction of an interface structure between Co₃O₄ and Co₄N was crucial for enhancing the adsorption and activation of water and AB. An analysis of the aforementioned mechanism reveals that the energy barriers for O–H bond dissociation in water and methanol exceed that of B–H bond dissociation in AB during hydrolysis and methanolysis. This suggests that the B–H bond exhibits a higher propensity to dissociate in comparison with the O–H bond during AB solvolysis. Furthermore, the adsorption affinity of AB on the cobalt catalyst surface is greater than that of water and methanol, indicating the preferential adsorption of AB on the surface of cobalt-based catalysts.

Fig. 13 (a) The optimized structure modes for H₂O adsorption, and the corresponding E_ads(H2O), and activation energy diagram of water on the surface of Ni₃N, Co–Ni₃N, Ni₃N, and Co–Ni₃N.²¹² Copyright 2021, Elsevier. (b) Activation energy of AB and H₂O dissociation on Co–B and Co–P sites of CoP, Co₃B, Co₃B–CoP, and Co₃B–CoP/h-BN. (c) The proposed catalytic mechanism of AB hydrolysis over Co₃B–CoP/h-BN. Reproduced with permission.¹⁸⁷ Copyright 2022, Elsevier. (d) Density of states of Co₄N, Co₃O₄, and Co₄N–Co₃O₄ and the corresponding active energies of AB and H₂O adsorption and dissociation on these catalysts. Reproduced with permission.²²⁴ Copyright 2022, Wiley-VCH.

5. Conclusions and perspectives

In this review, we first introduced the characteristics of each chemical hydrogen storage material and proposed evaluation methods for their hydrogen evolution performance, followed by a conclusion on the catalytic performance of different Co-based nanocatalysts for the dehydrogenation of sodium borohydride and ammonia borane over the last 20 years. Then, we utilized numerous examples to demonstrate the benefits of different supports for catalyst design and their impact on the active center. Finally, we outlined the mechanisms and catalytic pathways of hydrogen production from these hydrogen storage materials, underscoring the significance of mechanistic investigations for catalyst design. Fig. 14 offers a thorough summary and future perspective on the dehydrogenation of sodium borohydride and ammonia borane with Co-based nanocatalysts. However, there remain several questions for both theoretical investigation and practical implementation of these hydrogen storage materials that require further attention. The following enumeration outlines the challenges and proposes potential predictions for future progress.

Fig. 14 Summary and outlook of dehydrogenation from sodium borohydride and ammonia borane over Co-based nanocatalysts.

5.1. Comparison of different hydrogen storage materials

Hydrogen production from sodium borohydride and ammonia borane has exhibited numerous advantages, including excellent stability, high safety for storage and transportation, as well as efficient and fast hydrogen production.^225–227 Nevertheless, there is limited research on the comparison of characteristics among these hydrogen storage materials, such as price, storage capacity, environmental stability, and regeneration performance. In light of this, a comprehensive analysis of these diverse hydrogen storage materials is provided in Table 3. The production of sodium borohydride, with sodium hydride as a raw material, is associated with high production costs.²²⁸ On the other hand, ammonia borane has not yet been produced on a large-scale, resulting in a high price.^226,229 For environmental storage, ammonia borane demonstrated greater stability as an ionic compound. Sodium borohydride undergoes deliquescence in the presence of air, resulting in the release of hydrogen.²²⁸ Regarding the hydrogen storage capacity, ammonia borane demonstrates the highest mass hydrogen storage density (19.4 wt%) among its counterparts. Nevertheless, hydrogen contained within the amino group remains unreleased during the processes of hydrolysis and methanolysis.

Table 3 Advantages and disadvantages of sodium borohydride and ammonia borane as hydrogen storage materials

Hydrogen storage materials	Advantages	Disadvantages
NaBH4	• Ambient operation temperature	• Expensive
	• Good kinetics	• Self-solvolysis
	• High hydrogen storage density (10.6 wt%)	• Hygroscopic
	• Good regenerability
NH3BH3	• Ambient operation temperature	• Expensive and scarce
	• Long-term stability	• Poor regenerability
	• High hydrogen storage density (19.4 wt%)	• Unreleased hydrogen from the NH3 group

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In terms of hydrogen production, sodium borohydride demonstrates superior kinetic properties, undergoing complete spontaneous methanolysis in methanol and 10% spontaneous hydrolysis in water at room temperature. Consequently, the addition of a base is frequently required to prevent spontaneous methanolysis or hydrolysis and facilitate consistent hydrogen generation within the fuel cell.^50,230 Ammonia borane can undergo hydrolysis or methanolysis at ambient temperature with appropriate catalysts, suggesting favorable kinetic behavior.

Additionally, the regeneration of sodium borohydride and ammonia borane is one of the factors that increases their cost. The regeneration methods for hydrogen storage materials are demonstrated in Table 4. The hydrolyzed derivative of sodium borohydride (NaBO₂) can be regenerated directly through the reduction of MgH₂, whereas the methanolysis derivative (NaB(OMe)₄) must first undergo hydrolysis to form NaBO₂ before being regenerated through the reaction with MgH₂.²³¹ The hydrolysis product of ammonia borane also yields NaBO₂, which can be subsequently reduced to NaBH₄ and then regenerated through reaction with an ammonium salt.²³² The methanolysis by-product, NH₄B(OMe)₄, can be directly regenerated through reduction with LiAlH₄.¹⁹⁷

Table 4 The regenerability of sodium borohydride and ammonia borane

Hydrogen storage materials	Regeneration method
NaBH4	• NaBO2 + 2MgH2 → NaBH4 + 2MgO
	• NaB(OMe)4 + 2H2O → NaBO2 + 4MeOH
	• NaBO2 + 2MgH2 → NaBH4 + 2MgO
NH3BH3	• NH4BO2 + NaOH → NaBO2 + NH3 + H2O
	• NaBO2 + 2MgH2 → NaBH4 + 2MgO
	• 2NaBH4 + (NH4)2SO4 → 2NH3BH3 + Na2SO4 + 2H2
	• NH4B(OMe)4 + NH4Cl + LiAlH4 → NH3BH3 + Al(OMe)3 + MeOH + H2 + LiCl + NH3

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5.2. Comparison of different Co-based nanocatalysts

A number of Co and Co-containing nanocatalysts have been exploited for liquid phase catalytic hydrogen generation from sodium borohydride and ammonia borane. Co-based catalysts exhibit cost-effectiveness and superior catalytic performance, rendering them a prominent avenue for the advancement of these hydrogen storage materials in portable electronic devices. Advantages and disadvantages of the previously discussed Co-based catalysts for the dehydrogenation of chemical storage materials are summarized in Table 5.

Table 5 Advantages and disadvantages of the Co-based catalysts discussed

Co-based catalysts	Hydrogen production application	Advantages	Disadvantages
Monometallic Co NPs or NCs	NaBH4, NH3BH3	• Facile fabrication	• Easy to aggregate
		• High dispersion	• Easily oxidized
		• Good property	• Poor durability and recyclability
		• Viable mass production
Co alloy NPs or NCs	NaBH4, NH3BH3	• Synergistic effect	• Preparation complexity
		• Good stability	• Uneven dispersion
		• Enhanced property
Co SACs	NaBH4, NH3BH3	• Ultrahigh atomic dispersion	• Preparation complexity
		• Ultrahigh atomic utilization	• Poor durability and recyclability
		• Unique adsorption	• Low metal loading
		• Ultrahigh property	• Need for stabilizer
Co X-ides	NaBH4, NH3BH3	• High thermal stability	• Preparation complexity
		• High oxidation stability	• Poor durability and recyclability
		• Unique adsorption	• Discharge of nitrogen and phosphoric wastewater
		• Good property

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Monometallic Co NPs or NCs have been extensively utilized in the solvolysis of sodium borohydride and ammonia borane for hydrogen production, owing to their high dispersity and facile preparation, underscoring the effectiveness of Co NPs or NCs in the breaking of B–H bonds. Nevertheless, these Co NPs and NCs exhibit a propensity to aggregate and oxidize due to inadequately stable ligands or confined protection, leading to a rapid deterioration in cycling stability. Compared to monometallic Co, Co alloy NPs or NCs displayed enhanced catalytic properties for the dehydrogenation of these hydrogen storage materials due to the alloying effect, which modified the electrical configuration and shifted the d-band center of the metals.^233,234 In addition, alloy NPs or NCs demonstrated better cycling stability than that of monometallic NPs or NCs, which was attributed to the partitioning effect between distinct metal components. However, the preparation process for alloy NPs or NCs is complex and it is necessary to prevent the inhomogeneity caused by segregation. Single-atom catalysts maximize the utilization of metal atoms, with each individual metal atom serving as a distinct active site for catalytic reactions.^235–238 Single-atom Co catalysts exhibit significant catalytic activity in the solvolysis of sodium borohydride and ammonia borane for hydrogen generation that is attributed to ultrahigh atomic utilization and unique adsorption forms.^239,240 Nevertheless, the elevated atomic dispersion rate induces a surge in surface energy, leading to a reduction in stability. In addition, this ultrahigh dispersion also results in the low metal loading of single-atom catalysts. Consequently, constructing specialized coordination groups or atoms to stabilize single atoms is an intricate and nuanced process. Co X-ide catalysts, such as borides, phosphides, and nitrides, are also used for hydrogen evolution from these hydrogen storage materials, due to excellent thermal stability, oxidation resistance, and unique adsorption and activation sites different from monometallic Co and its alloys.^241–243 After analyzing the properties of different Co X-ides, Co phosphates (Co–P) and nitrides (Co–N) were found to be more active than Co borides (Co–B);^244–246 this can potentially be attributed to more electron transfer, which is expected to be further investigated. Furthermore, Co X-ides demonstrated low cycling stability and complexity of preparation.^88,247 Therefore, the development of new supports or suitable synthesis methods to anchor or confine Co X-ides with high stability and durability will be a future development direction.

The primary objective of catalyst design is to facilitate their application in hydrogen production from chemical hydrogen storage materials. This necessitates the consideration of large-scale catalyst production, environmentally friendly synthesis, and industrial feasibility. In the context of Co-based catalysts, monometallic and alloy NPs are the most straightforward to produce industrially via a wet chemistry method, with the production process discharging only minimal amounts of B-containing wastewater, thereby largely achieving the aims of green synthesis. In contrast, metal clusters and single-atom catalysts demand stringent control over production conditions, posing challenges for large-scale production. Furthermore, the synthesis of Co X-ides involves lengthy preparation processes, requiring multiple reaction steps, and is associated with the discharge of nitrogenous and phosphorus wastewater.

5.3. Exploration of catalytic mechanisms and reaction pathways

The structure, composition, and physical characteristics of the support significantly influence the activity of metal active sites.^248–250 Investigating the influence of the support on the active sites is essential for the development of highly efficient and durable Co-based nanocatalysts. The high specific surface area of the support facilitates the dispersion of metal NPs, and the pore structure is conducive to the confinement of metal NPs.^251,252 The presence of heteroatoms and defects in the support can serve as anchoring sites for metal NPs, leading to a decrease in particle size and improvement in catalytic properties.^253–255 Furthermore, the specific crystal face of metal NPs significantly affects their properties, warranting further investigation.

The electronic structure of the support and the electronic interaction between the support and metal can also affect the hydrogen generation performance of NaBH₄ and NH₃BH₃ solvolysis.²⁵⁶ An electron-rich support (e.g. pyridine-rich, hydroxy-rich, and porphyrin-rich polymers) can interact with the metal precursor, enhancing the dispersibility of the metal, reducing its particle size, and consequently improving the catalytic activity.^257,258 In addition, the sharing of electrons between the electron-rich support and the vacant orbitals of Co leads to an accumulation of electrons on the surface of Co NPs, thereby creating numerous electron-rich sites that facilitate the activation of B–H.^66,170 On the other hand, an electron-deficient support (e.g. g-C₃N₄, MoO₃, and polymers containing nitro and cyano groups) facilitates the transfer of electrons from metallic Co to the support, resulting in the formation of electron-deficient Co. This electron-deficient state of Co subsequently promotes the adsorption of water on the Co surface, ultimately weakening the O–H bond. The influence of electron transfer between the support and Co on the adsorption and dissociation of B–H and O–H bonds remains unclear, meriting additional investigation by scholars.

Acid–base sites are crucial in the solvolysis of sodium borohydride and ammonia borane for hydrogen production. Brønsted acids have been demonstrated to catalyze the hydrolysis of sodium borohydride, facilitating the production of hydrogen in the absence of metal catalysts.^259,260 However, the solvolysis of sodium borohydride facilitated by Lewis acids has not been previously documented. Furthermore, it has been established that alkalis can inhibit the solvolysis process of sodium borohydride.⁵⁰ Research by Linares et al. demonstrated that both Lewis and Brønsted acid sites in zeolites facilitated the hydrolysis of ammonia borane, leading to enhanced hydrogen generation performance.⁸⁵ Specifically, Lewis acid sites slightly enhance the hydrogen production rate, while Brønsted acid sites increase it by 50%. On the other hand, the introduction of p-phenylenediamine base sites significantly enhances catalytic activity, with the effect being amplified as the base content increases. This finding suggests that base sites more effectively improve the hydrolytic hydrogen production of ammonia borane compared to acid sites. Our research group further substantiated that Brønsted bases, including NaOH, KOH, and Na₂CO₃, could significantly enhance the hydrolysis of ammonia borane, thereby facilitating hydrogen production.¹⁷⁸ Therefore, in-depth studies of the mechanism of acid and base promoted solvolysis of sodium borohydride and ammonia borane, and the strategic design of catalysts incorporating appropriate acid and base sites are expected to significantly enhance the performance of cobalt-based catalysts.

Based on statistical analysis, it was determined that Co exhibited superior performance for the solvolysis of NaBH₄ and NH₃BH₃ in comparison with Ni, Fe, and Cu.²⁵⁶ Subsequent investigations revealed that Co possessed the ability to decrease the energy barrier associated with the breaking of B–H bonds in NaBH₄ and NH₃BH₃ and O–H bonds in H₂O and CH₃OH.^68,170,261 Therefore, an in-depth investigation of the selectivity of Co in relation to the dehydrogenation of sodium borohydride and ammonia borane from an electronic structure perspective is crucial for the advancement of high-performance Co-based hydrogen production catalysts.

5.4. Prediction of future progress for hydrogen evolution from sodium borohydride and ammonia borane

Co-based supported nanocatalysts have attracted more and more attention for facilitating the dehydrogenation of sodium borohydride and ammonia borane, due to their unique metal active centers, strong metal–support interactions, and the promoting effects of specific sites in the support. The introduction of a support containing heteroatoms, such as N, B, O, S, and P, can enhance the interaction and electron transfer between the metal and the support, leading to a reduction in the size of metal NPs, alteration of the electronic structure of the metal, and enhancement of the catalytic performance of the catalyst.^262–266 Investigating the promoting effect of different heteroatoms on metal active sites is beneficial for designing metal nanocatalysts with high activity and stability. Based on the confinement effect of micropores and enhanced diffusion of substrates through mesopores and macropores, the hierarchical pore structure of the support can effectively increase the stability of metal NPs and improve their catalytic properties.^158,267 Furthermore, certain sites in the support can enhance the adsorption of substrates and improve their dehydrogenation performance. For instance, strongly acidic sites can improve the adsorption of sodium borohydride and ammonia borane.^268–270 Hence, the exploitation of a novel support to enhance the effectiveness of metal nanocatalysts holds significant importance in the development of these hydrogen storage materials.

Currently, the predominant focus of mechanistic research in the field of chemical hydrogen storage materials pertains to sodium borohydride and ammonia borane. In previous reports, research on the hydrogen production mechanism of sodium borohydride and ammonia borane primarily concentrated on the initial step of dehydrogenation, with limited exploration of the subsequent dehydrogenation process. Furthermore, investigations into the hydrogen production mechanism of sodium borohydride and ammonia borane predominantly relied on DFT calculations. The current model fails to accurately depict the catalyst structure, particularly in regard to the establishment of metal clusters, resulting in discrepancies between the atomic number and crystal structure in comparison with the actual metal structure. Therefore, it is imperative to investigate the mechanism from the perspective of theory and experiments, taking into consideration the actual structure of the catalyst.

The catalytic process consistently operates within a dynamic non-equilibrium state, posing challenges for directly observing the catalyst morphology and ascertaining the relationship between structure and activity. Operando characterization techniques can monitor the activity of catalysts in real time and help researchers understand the changes that occur during the reaction.^271–274 Operando techniques such as in situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) can be utilized to observe morphological changes to the catalysts, while in situ X-ray diffraction (XRD) can be employed to analyze the crystal phase structure and grain size alterations. Additionally, in situ mass spectrometry (MS), nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR) are valuable tools for characterizing the changes to intermediates during catalytic reactions. In situ characterization technology enables a comprehensive understanding of the structure, composition, reaction mechanism, activity, and lifespan of catalysts, thereby offering a theoretical foundation for catalyst design and optimization. In addition, X-ray absorption fine structure (XAFS) spectroscopy is a valuable technique for characterizing the valence state of Co, elucidating its binding interactions with the carrier material, and analyzing electron transfer dynamics, with a particular emphasis on the single-atom structural configuration. Spherical aberration corrected transmission electron microscopy (AC-TEM) serves as a powerful tool for identifying subnanometer Co clusters and Co single atoms, thereby elucidating the correlation between metal dispersion and catalytic activity. Surface enhanced Raman spectroscopy (SERS) provides valuable insights into dynamic surface phenomena, facilitating the monitoring of Co-based catalyst structures, adsorbent interactions of sodium borohydride and ammonia borane, as well as the reaction kinetics with exceptionally high spatial and temporal resolution.

In the process of developing catalysts to address urgent environmental concerns and energy demands, conventional design and optimization approaches frequently fall short of meeting the necessary criteria due to the intricate and vast nature of the catalyst parameters. The emergence of artificial intelligence (AI) has introduced a new epoch in catalyst optimization, presenting prospective solutions for the limitations of traditional methodologies.^275–277 Based on large language models (LLM), Bayesian optimization, and machine learning (ML), AI is anticipated to be employed in the screening of hydrogen production catalysts for chemical hydrogen storage materials, the optimization of catalytic hydrogen production processes, and the investigation of mechanisms.

The final industrialization goal of hydrogen generation systems based on sodium borohydride and ammonia borane over Co-based catalysts is the portable hydrogen storage system. The utilization of liquid hydrogen storage technology in the portable power supply system holds significant economic implications and research merit.²⁷⁸ This technology is expected to have broad applications in various industries, for instance fuel cell mopeds, hydrogen-powered drones, and large hydrogen-powered buses.

Data availability

All relevant data are within the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 22162014, 22162013, and U24A2044), the Guizhou Provincial Basic Research Program (Natural Science) (No. MS [2025]094), the Doctoral Talents Project of Tongren City (No. [2023]7), and the Doctoral Research Foundation Project of Tongren University (No. trxyDH2204).

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