Green Hydrogen

1. Background and state-of-the-art

Green hydrogen (H₂) is produced by splitting of water and is considered as one of the most carbon-neutral sources of renewable energy. Water-splitting can be achieved by several means but the three methods below remain the most popular forms for green H₂ generation:

• photoelectrochemical (PEC),
• photocatalytic (PC),
• electrocatalytic (EC).

In PEC and PC, solar energy is utilized for water-splitting and both the approaches are based on the same mechanism and impose the same thermodynamic and kinetic requirements to involved semiconductor materials. EC utilizes the electrical energy from intermittent renewable sources (such as solar and wind) to drive the water-splitting process, while the input feedstock can either be ultrapure water or low-grade water (such as seawater).

In contrast to EC and PEC, where evolved H₂ and O₂ are separated due to their evolution on different (photo)electrodes, in PC, a mixture of H₂ and O₂ are obtained as water splitting reaction takes place in the suspension of photocatalyst particles. The main disadvantages of PC with respect to EC/PEC are related to the costs for H₂ and O₂ separation and reduced H2 yield due to the thermodynamically favourable backward reaction. However, PC is regarded as the cheapest approach and offers several advantages including possibility for easy increase of photocatalysts’ effective surface area and no need for electrical connections. Additionally, in dual-functional photocatalysis, H₂ evolution can be combined with oxidative degradation of toxic organic compound or some other oxidation reaction, enabling formation of added-value products.¹ Our research explores all the three methods of water-splitting, i.e. PEC, PC and EC.

PEC water-splitting

In spite of the high promises of PC and PEC, several challenges and issues related to the efficiency of solar light absorption, costs, stability and sustainability have to be solved that both processes become economically viable for H₂ production. Progress in the fields can be made by designing of new systems, materials and interfaces based on the understanding of the interfacial phenomena, thermodynamic and kinetics criteria. In the development of PEC cells, the integration of narrow band-gap semiconductors (Si, Ge, GaAs) with thin metal oxide protective layer has recently attracted great scientific attention.^2,3,4 Si, Ge, GaAs semiconductors, which are characterized by good absorption of the visible and infrared part of the solar light, high electron-hole mobilities and conduction band (CB) energy more negative than hydrogen redox (EH+/H₂) potential, are not chemically stable in the relevant solution environments during long term operations. The protection of the semiconductor against corrosion can be achieved by the epitaxially grown protective oxide material with good lattice match with the semiconductor and appropriate conduction band alignment that facilitate electron transport and reduce the recombination at the interface. Si is a narrow band gap semiconductor (1.1 eV) and exhibit (001) surface unit cell (3.84 Å) that matches well with the lattice constant of a cubic perovskite SrTiO₃ (STO) (3.905 Å).² Additionally, the STO/Si heterojunction is characterized by nearly zero conduction-band offset. Although STO was proved as appropriate protective oxide layer for Si-based photocatode for PEC H₂ evolution, there are still unresolved issues related to the influence of the STO/Si interface and characteristics of the epitaxial STO layer on the solar-to-hydrogen (STH) efficiency. We propose a radically new approach for controlling of the interface between semiconductor (i.e. Si) and protective oxide layer (i. e. STO) by graphene oxide (GO), that enables remote epitaxial growth of STO on Si substrate (Figure 1). Insertion of graphene oxide between STO and Si was found to result in sharp, atomically defined interface.

Figure 1: (a) schematic representation of rGO and STO protected Si as photocathode for green hydrogen evolution, (b) linear sweep voltametry measurement, (c) stability of photocathode.

PC water-splitting:

Like in PEC, the efficiency of PC depends on controlling of all steps in the (photocatalytic) process, that are: (i) photon absorption, (ii) photoexcited charge (electron (e^-) and hole (h⁺) separation, (iii) charge diffusion and transport, (iv) catalytic reaction on the catalyst’s active site, and (v) mass transfer.⁵
Promotion of each step has to be considered in designing of highly effective photocatalyst particles. The basic thermodynamic requirement that has to meet each water-splitting photocatalyst are suitable band gap (>1,23 eV, preferably 1.6-2.4 eV)⁶, CB more negative than water reduction potential ((H⁺/H₂; 0^-0.059 pH, V versus NHE) and valence band (VB) more positive than water oxidation potential (H2O/O2; 1,23-0.059 pH, V versus NHE).⁷ In our research we focus on engineering of water-splitting photocatalysts based on MTiO₃ (M= Sr, Ba, Ca) perovskite particles (Figures 2-4), that meet the aforementioned thermodynamic requirements and concomitantly offer several possibilities for tailoring of chemical compositions, defects and morphologies. In designing of efficient water-splitting photocatalyst particles special attention is paid to enhancement of light absorption by narrowing of the band gap through doping, defects (i.e. oxygen vacancies (Ov)), creation of heterojunctions (i.e. SrTiO₃/TiO₂, MTiO₃/Bi₄Ti₃O₁₂) and improvement of light harvesting ability by fabrication of hierarchical microporous and mesoporous architectures. For diminishing of e^- and h⁺ recombination we propose exploiting of the internal electric fields, arising in the photocatalyst particle due to ferroelectricity (Figure 3), different facets polarities resulting from different orientations/terminations.8 In the particles with different types of exposed facets several mechanisms can also co-exist and collectively contribute to the formation of differently charged surfaces. In particular, the ferroelectricity in the particles creates relatively strong internal electric field, that separate e- and h+, and also influence the adsorption characteristics of the surface for reactant molecules.⁹ Our aim is to explore the contribution of these mechanisms to selective spatial reactivity of the particles’ surfaces and to improvement of the STH efficiency.

Figure 2: Scanning electron microscope (SEM) images of ferroelectric (001) oriented BaTiO3 plates (left) and nanoblocks (right), prepared by topochemical conversion from Bi4Ti3O12 plates (Source: M. Maček Kržmanc, Cryst. Growth Des. 17 (2017) 3210).

Figure 3: Ferroelectric characteristics of the BaTiO3 nanoblocks (4-nanoblocks assembly): (a) Topography (height image), PFM out-of-plane (b) amplitude and (c) phase images, (d) Local hysteresis loops: amplitude (below) and phase (above) measured in the spot marked with a cross in Fig. (a). (Source: M. Maček Kržmanc et al. Cryst. Growth Des. 17 (2017) 3210-3220.31).

Figure 4: STEM image (left) and high resolution (HR) STEM image (right) of a part of a (100) oriented mesocrystalline SrTiO₃ plate.

In PC, we also work on layered semiconductors such as graphitic carbon nitride (gC₃N₄), which has been one of the most popular 2D photocatalyst owing to its layered morphology, excellent chemical and redox stability, narrow bandgap, low-cost and facile synthesis strategies. Furthermore, it has a conduction band minimum (-1.3 V vs NHE) enough to drive the HER. Despite such a promising set of optoelectronic properties, the reported photocatalytic efficiencies of gC₃N₄ are still limited and reasonable reaction rates are observed only when co-catalysts based on platinum group metals (PGMs, such as Pt, Pd, Rh) are incorporated.10 Our research is focused on the development of non-PGM HER co-catalysts (such as Co-B, Co-Mo) and their integration with gC3N4 nanosheets, eventually understanding the reaction and degradation mechanism of such photocatalyst composites. We have been successful in fabrication of ultra-thin nanosheets of gC₃N₄ embedded with small-sized co-catalyst nanoparticles (< 5nm) through controlling the synthesis conditions and reaction precursors (Figure 5).¹¹ Our aim is to gain a fundamental understanding of the changes in co-catalyst properties during photocatalytic redox processes in such gC₃N₄ composites, which will eventually guide us to improve their efficiencies on par with the PGM co-catalysts.

Figure 5: (a) Low magnification bright field (BF) and dark field (DF) micrographs of the CoB-gC3N4 composite; (b) atomic-scale HAADF-STEM image of a CoB NP embedded in gC3N4 nanosheet with (c) corresponding FFT image indicating that the NP is oriented along the [111] zone axis. (Source: S. Gupta, et al, Int. Journal of Hydrogen Energy, 60, 1288–1298, 2024)

EC water-splitting

In EC, anion-exchange membrane water electrolyzers (AEMWEs) have rapidly emerged as a new technology that overcomes the challenges of incumbent alkaline water electrolyzers (AWEs) and acidic proton-exchange membrane water electrolyzers (PEMWEs), making it the most ideal technology for upscaling.¹² However, the voltage efficiency of state-of-the-art (SoA) commercial AEMWEs is still below 70%, resulting in a higher cost of H2 production. Moreover, the commercial AEMWEs have stack lifetimes of >20000 h but most of the reported electrocatalysts for AEMWE are stable only for 100 – 1000 h.¹² Our research is aimed at overcoming these challenges of electrode performance and stability through designing new electrodes based on non-noble metal catalysts. Our catalysts belong to the class of metal X-ides (where X = B, P and S) that show ‘bifunctional’ characteristics, rendering them useful for both cathode and anode applications.¹³ Over the years, we have developed special expertise in development and understanding of first-row transition-metal borides (such as CoB, NiB, FeB) and fabrication of multi-component electrocatalysts (such as CoMoB, CoMoPB, CoNiFePB, etc.) with different dimensionalities (0D nanoparticles, 1D nanowires, 2D nanosheets).^14-17 Additionally, the emphasis lies on the performance evaluation of catalysts under single-cell electrolyzers, to establish their performance in commercially relevant testing conditions.

In addition to conventional EC water-splitting, we also work on development of cathode and anode catalysts for direct seawater electrolysis. We use our knowledge in multi-metallic systems to fabricate electrocatalysts with improved functionalities when used under saline conditions. Instead of ultrapure water, our electrolytes are based on synthetic seawater (such as ‘Absolute Ocean’) or alkaline simulated seawater (KOH + NaCl) or even real Adriatic seawater. The research is also focused on designing strategies to prevent chloride-induced corrosion and selectively produce oxygen (and hydrogen) from seawater.18 We lay special stress on understanding the catalyst degradation mechanism under such corrosive conditions, which eventually guides our design strategies for fabrication of better electrocatalysts.

2. Objectives, originality and impact on new research approaches

In the field of PEC the main objectives are:

(i) To understand key parameters for higher STH efficiency on atomically defined oxide-semiconductor heterostructures.
(ii) To scan for novel protective oxides with engineered band offset and to maximize STH values after their integration with semiconductor photoelectrodes.
(iii) Integration of oxide layer with semiconductor using pulse-laser deposition (PLD).

Our group already developed new approach for successful epitaxial integration of STO on Si using PLD.^19,20 Additionally, we also demonstrated that GO nanosheets can effectively direct the growth of STO on Si- substrate (Figure 1). The concept of applying GO as an integration-enabling agent is radically new, fully scalable and will enable a breakthrough for the forthcoming hydrogen economy. The results are anticipated to bring a significant advancement in the understanding of the influence of interface and epitaxial oxide-layer characteristics on STH efficiency.

In the field of PC the main objectives are:

(i) Developing the strategy for controlling the shape, preferential orientation, ferroelectric polarization, type of exposed facets, termination of various perovskite-based particles, including (nano)cubes, (nano)plates, polyhedral micro(nano) crystals, and/or mesocrystalline and heterostructural particles (Figures 2, 3, 4, 6-7).
(ii) Defect engineering (Ov) of developed particles.
(iii) Assessment of the photocatalytic performance of the perovskite particles for H2 evolution and evaluation of the contribution of particular aspects (i.e. ferroelectricity, spatial selective photochemical reactivity (different facets/polarities)) to the enhanced photocatalytic performance (in cooperation with the projects partners).
(iv) Fabrication and integration of non-PGM co-catalysts in gC3N4 nanosheets to improve the overall PC efficiency for HER (Figure 5).
Based on the thorough studies of crystal growth and reaction mechanisms we have already developed several procedures for controlling of perovskite particles morphologies and heterostructures (Figures 2, 4, 6-7).^21-25

Figure 6: STEM image of SrTiO₃/Bi₄Ti₃O₁₂ heterostructural plate (Bi₄Ti₃O₁₂ inside, SrTiO₃ outer part of the plate).

Figure 7: Polyhedral BaTiO3 particles (left) and ferroelectric heterostructure of BaTiO3 nanorods/Bi4Ti3O12 plate (right).

In the field of EC the main objectives are:

1. To understand the structure-activity-stability relationships in our electrocatalysts, enabling us to tailor them and extract maximum electrocatalytic performance and stability
2. Understand the catalyst activation and degradation mechanisms in complex multi-component systems when used in single-cell electrolyzers
3. Design measures to protect electrodes against corrosion and deactivation in seawater-based electrolytes

Our group has proposed the hypothesis explaining the higher hydrogen and oxygen evolution rates in amorphous metal borides, compared to conventional metallic and metal oxide catalysts.^26,27 We have discovered several new and active configurations of metallic borides and phospho-borides for not just conventional water electrolysis, but also for seawater electrolysis and urea electrolysis.^28,29 

3. Unique methodology

Attainment of full epitaxy using PLD requires careful monitoring and studies of the film growth and interfaces. In addition to reflection high-energy electron diffraction (RHEED), which is used during film growth, the films are characterized by XPS, Joule-Thomson scanning tunneling microscope. The composition and structural properties of the films are analyzed using Rutherford back-scattering spectrometry, aberration-corrected scanning transmission electron microscope, and high-resolution X-ray diffractometer suited for thin-film studies. Raman spectroscopy are used in operando to determine structural variations of strain engineered films during H₂ evolution reaction.

The syntheses of pre-defined perovskite particles morphologies include topochemical conversions of templates or modification of the crystal facets’ energy by additives, which selectively adsorb on particular facets and retard their growth-rate. Information about the type of exposed facets and termination will be obtained by High-Angle Annular Dark-Field Scanning Transmission Electron Microscope (HAADF-STEM), XPS, Time of Flight–Secondary Ion Mass spectrometry (ToF-SIMS). In an event these techniques do not provide straightforward result of surface termination, employing of Low Energy Ion Scattering (LEIS) analysis. Characterization of electronic nature of crystal defects are performed by Electron Energy Loss Spectroscopy (EELS), Photoluminiscence (PL) and electron paramagnetic resonance (EPR).

REFERENCES:

1 Kampouri, S., et al.; ACS Catal 9 (2019) 4247-70.
2 Ji, L., et al.; Nat. Nanotechnol. 10 (2015) 84-90.
3 Stoerzinger, K.A., et al.; MRS Communications 8 (2018) 446-52.
4 Kornblum, L., et al.; Energy Environ. Sci. 10 (2017) 377-82.
5 Afroz, K., et al.; J Mater Chem A 6 (2018) 21696-718.
6 Zhao, Y., et al.; Nano Energy 30 (2016) 728-44.
7 Chen, S.S., et al.; Nat. Rev. Mater. 2 (2017) 17.
8 Zhu, Y.S., et al.;Phys. Chem. Chem. Phys. 19 (2017) 7910-18.
9 Cui, Y., et al.; Chem Mater. 25 (2013) 4215-4223.
10 Wang, Y., et al.; Nat Energy 4, (2019) 746–760.
11 Gupta, S., et al.; Int. J. Hydrogen Energy, 60, (2024) 1288–1298
12 Du, N., et al.; Chem. Rev. 122, (2022) 11830.
13 Kawashima, K.; et al.; Chem. Rev. 123, 23, (2023) 12795–13208
14 Bhide, A.; Gupta, S.; et al.; Materials Today Energy, 44, (2024) 101638
15 Gupta, S.; et al.; Nanoscale, 10, (2018) 8806-8819
16 Gupta, S.; et al.; Advanced Functional Materials, 30, (2020) 1906481
17 Chunduri, A.; Gupta, S.; et al.; ChemSusChem, 13, (2020) 6534 - 6540
18 Gupta, S.; et al.; ACS Applied Energy Materials, 3, 8, (2020) 7619–7628
19 Klement, D., et al.; Appl. Phys. Lett. 106 (2015) 071602.
20 Diaz-Fernandez, D., et al.; RSC Advances 7 (2017) 24709-17.
21 Maček Kržmanc, M., et al.; Cryst. Growth Des. 17 (2017) 3210-3220.
22 Maček Kržmanc, M., et al.; Ceram. Int. 41 (10) (2016) 15128-15137.
23 Maček Kržmanc, M., et al.; J. Am. Ceram. Soc. 96 (2013) 3401-3409.
24 Maček Kržmanc, M., et al.; Ceram. Int. 44 (2018) 21406-21414.
25 Čontala, A., et al.; Acta Chim. Slov., 65(2018) 630-637.
26 Gupta, S.; et al.; Journal of Power Sources, 279, (2015) 620-625
27 Gupta, S.; et al.; ACS Sustain. Chemistry & Engineering, 7, 19, (2019) 16651
28 Kanwar, T.; et al.; J. Power Sources, 633, (2025) 236427
29 Bhabhal, R.; Gupta, S.; et al.; Int. J. Hydrogen Energy, 116, (2025) 299-311 

Green Hydrogen - Researchers involved

Senior Researchers:
Prof. Dr. Matjaž Spreitzer
Dr. Marjeta Maček Kržmanc
Asst. Prof. Suraj Gupta
Dr. Darinka Primc

Postdoctoral Researchers:
Dr. Jyoti Gupta

PhD Students:
Joyal Johny
Lucija Bučar
Subhashish Rooj