Hydrogen evolution reactions using 3D printed composites of copper with graphene and hexagonal boron nitride

Abstract

The design of scalable, efficient electrocatalysts is essential for green hydrogen production. Here, we employed direct ink writing (DIW) 3D printing to fabricate Cu-based composites with graphene (Gr) and hexagonal boron nitride (h-BN), utilizing precise porosity control and interface engineering for enhanced hydrogen evolution reaction (HER) performance. The DIW-printed Cu–Gr composite outperforms Cu-hBN and pristine Cu, achieving an overpotential of 129 mV at 20 mA cm⁻², a Tafel slope of 125 mV dec⁻¹, and excellent stability over 10 hours. Improved conductivity, charge transport, and active site exposure drive superior catalytic activity. Computational studies confirm that Gr (or h-BN) enhances the adsorption enthalpy, promoting catalytic interactions. This work highlights DIW-printed Cu–Gr composites as scalable, self-standing, and sustainable HER electrocatalysts.

---

Molecular hydrogen is a highly efficient renewable fuel, offering a sustainable solution to global energy demands due to its high energy density, long-term storage capability, and minimal environmental impact.¹ Various methods exist for hydrogen production, including coal conversion, methanol or ammonia decomposition, natural gas steam reforming, and water electrolysis. Among these, hydrogen production by water electrolysis is particularly advantageous owing to its abundance, high energy purity and cost-effectiveness, occurring in acidic, neutral and alkaline media.² The process involves two half-reactions: the hydrogen evolution reaction (HER) at the cathode (2H₂O + 2e⁻ → H₂ + 2OH⁻) and the oxygen evolution reaction (OER) at the anode (4OH⁻ → O₂ + 2H₂O + 4e⁻). Alkaline water electrolysis offers high stability, product purity, and low cost while mitigating electrode corrosion and dissolution.³ However, its slow kinetics result in higher overpotential requirements, limiting efficiency. While Pt/C catalysts demonstrate excellent HER activity with an overpotential of just 29 mV at 10 mA cm⁻² in 1M KOH,³ their high cost and scarcity hinder large-scale application. Recently, 3D electrodes have offered an effective solution for boosting the HER efficiency, thanks to their high surface area, tunable porosity, and improved mass transport.^4,5 These features promote efficient gas diffusion, enhance active site accessibility, and support long-term stability.

In this context, additive manufacturing (AM), known as the 3D printing method, has gained significant attention for electrode fabrication due to its advantages, including rapid prototyping, easy processing, and cost-effectiveness. Among various AM techniques, direct ink writing (DIW) stands out as an attractive 3D printing method, enabling direct material deposition with a binder followed by sintering. DIW offers a broad material selection, tunable solid structures, single-step processing, and cost efficiency. Additionally, it allows control over surface morphology and surface area while ensuring uniform particle distribution via differential stress during extrusion, both of which are crucial for efficient HER performance.⁶ In this technique, earth-abundant Cu and Cu-based composites are a great choice owing to their low toxicity, low cost, ease of structural stability, high thermal and electrical conductivity and high strength compared to other transition metals. However, existing synthesis methods for Cu-based HER catalysts are often complex and lack scalability.^7,8 In this context, the DIW approach offers a solution by enabling the fabrication of self-supported catalysts, preventing material detachment from the substrate, and enhancing electrode stability. Furthermore, the incorporation of 2D layered materials such as graphene (Gr) and hexagonal boron nitride (hBN), known for their excellent catalytic activity, is effective in improving HER performance.⁹

Inspired by these advantages, this study explores two types of DIW-printed Cu composites incorporating varying ratios of Gr (1, 2, 3 wt%) and hBN (0.5, 1, 2 wt%) as secondary phases. Cuboid geometries were used for Cu-hBN and trapezoid geometries for Cu–Gr to assess the effects of optimized printing parameters on HER performance, microstructural uniformity, and phase distribution. After sintering and optimizing the efficiency of these DIW-printed green bodies, pristine Cu, Cu-2wt%Gr (Cu–Gr), and Cu-2wt%hBN (Cu-hBN) composites have been characterized by optical, SEM-EDAX, and transmission electron microscopy to understand the hBN and Gr distribution in the Cu matrix. As expected, due to the high active surface area, tunable porosity, high electrical conductivity, exposed number of active sites, homogeneous distribution, fast ion transport and chemical stability, the DIW printed Cu–Gr composite exhibited outstanding HER performance over the DIW printed Cu-hBN and pristine Cu-achieving an overpotential of 129 mV at 20 mA cm⁻², a Tafel slope of around 125 mV dec⁻¹, an electrochemical surface area of 6.03 cm², and stability even at 22 mA cm⁻² with negligible potential decay for 10 h, which outperforms most of the reported electrocatalysts. This work opens a new frontier for developing sustainable, scalable, highly efficient electrocatalysts for green hydrogen production.

Fig. 1a illustrates the schematic of ink preparation and DIW printing, with raw powder morphologies shown in Fig. S1 (ESI†). The optimized DIW process ensures uniform secondary phase distribution, as shown in a previous study,¹⁰ resulting in consistent material properties and enhanced catalytic performance. Optimized rheology, aided by CMC as a binder, ensures smooth extrusion and improved packing density, with analysis (Fig. S2, ESI†) revealing superior printability of Cu-2wt%Gr inks compared to Cu-2wt%hBN. Sintering at 950 °C promotes strong Cu particle bonding, eliminates binders, and preserves structural integrity and functional properties of Gr and hBN due to their high thermal stability. Details are in the ESI.† The X-ray diffractograms (Fig. 1b) show the appearance of peaks at 42.94, 50.07, 73.75, 89.59, and 94.79, designated to (111), (200), (220), (311), and (222) crystallographic planes of FCC Cu. No Gr or hBN peaks are visible in the XRD patterns, suggesting good dispersion and effective sintering. Optical (Fig. 1c–e) and SEM images (Fig. 1f–h) reveal uniform porosity and compact microstructures in the composites, with Gr and hBN contributing to enhanced packing density compared to DIW Cu. Colour contrast in the micrographs distinguishes the Cu matrix (grey) from secondary phases or pores (dark regions). The porosity is generated during printing and subsequent sintering processes. The uniform distribution of hBN and Gr within the Cu-matrix is clearly observed in Fig. 1g and h and EDS elemental mapping of the composites further confirms their successful incorporation (Fig. S3, ESI†). Additional investigation of the interfacial interaction between the secondary phase and Cu matrix was conducted using transmission electron microscopy (TEM) analysis, as depicted in Fig. S4 (ESI†). The findings reveal uniform distribution and strong interfacial bonding between the Cu matrix and secondary phases (hBN and Gr), resulting in denser, well-aligned composite structures. Compared to pristine Cu, the composites exhibit reduced porosity (Fig. S5, ESI†), highlighting the role of secondary phases in enhancing matrix bonding. This optimized microstructure is essential for enhancing thermal and electrical conductivity, as well as catalytic efficiency.

Fig. 1 (a) Schematic representation of ink preparation and DIW printing; (b) XRD patterns of representative composites with DIW processed Cu before electrocatalysis; (c)–(e) optical surface morphology and (f)–(h) SEM morphology of DIW Cu, Cu-hBN and Cu–Gr before electrocatalysis, respectively.

The HER activity of DIW-printed Cu and its composites with Gr and hBN was investigated using different electrochemical experiments in N₂-saturated 1.0 M KOH, where they were directly used as working electrodes. Investigation of varying Gr and h-BN contents indicates that 2 wt% achieves the highest HER efficiency in both cases (Fig. S6, ESI†), enabling a direct comparison with neat Cu to assess performance enhancements. Interestingly, linear sweep voltammetry (2 mV s⁻¹, iR-compensated) highlights the significant impact of graphene incorporation and DIW-enabled geometry in the 3D-printed Cu structure (Fig. 2a). The Cu–Gr composite exhibits an exceptionally low overpotential of 20 mV at 10 mA cm⁻², as shown in the inset of Fig. 2a, closely approaching commercial Pt/C (29 mV)³ and outperforming Cu-hBN (268 mV) and pristine Cu (404 mV). This remarkable performance stems from the synergy between graphene's high conductivity and large surface area and the designed geometry enabled by DIW, which promotes effective mass transport and active site exposure. In contrast, the Cu-hBN composite shows limited improvement, as h-BN's wide band gap and insulating nature restrict charge transfer, impeding HER kinetics. Comparative analysis of overpotentials at 10 and 20 mA cm⁻² (Fig. 2b) confirms that the 3D-printed Cu–Gr composite exhibits the lowest values (20 mV and 129 mV, respectively), outperforming both Cu-hBN and pristine Cu, as well as reported Cu rods.¹¹ Notably, η₁₀ of Cu–Gr is found to be lower compared to Cu/CoFe (171 mV) and CF/Cu₂Se (212 mV) hybrid catalysts and even lower than poly polished Cu (320 mV).^11–13 These results underscore the critical role of graphene incorporation in enhancing HER activity through improved electrical conductivity, large surface area, and synergistic interactions with the Cu matrix. Additionally, DIW-induced porosity facilitates better ion diffusion and electrode–electrolyte interaction.

Fig. 2 HER catalytic performance of 3D printed Cu–Gr, Cu-hBN and Cu catalysts at a scan rate of 2 mV s⁻¹ in 1.0 M KOH electrolyte solution: (a) polarisation curves; inset: zoomed-in view of the polarisation curves; (b) comparison of overpotentials at 10 mA cm⁻² and 20 mA cm⁻² current densities; (c) comparison of turnover frequencies (TOFs) at various overpotentials; (d) Tafel plots; (e) Nyquist plots; (f) average of capacitive current density plots against scan rate (10, 20 30, 50, 70 and 100 mV s⁻¹). The linear slope is equivalent to the double-layer capacitance (C_dl); (g) chronoamperometric measurements of the HER at an overpotential of 159 mV and LSV curves of the Cu–Gr free-standing catalyst before and after 300 cycles. (h) Comparison of the HER activity of recently published catalysts in terms of overpotential and Tafel slope with a DIW printed Cu–Gr composite.^5,8,14–25

Turnover frequency (TOF), another key metric for evaluating electrocatalytic activity, quantifies the rate of H₂ evolution per unit time. Detailed TOF calculations are provided in the ESI,† with comparative plots for each catalyst shown in Fig. 2c. These findings imply that DIW printed Cu–Gr possesses the highest TOF of 0.065 s⁻¹ at 700 mV overpotential, suggesting the faster kinetics and enhanced intrinsic activity of Gr-reinforced DIW printed Cu catalysts. Tafel analysis (Fig. 2d) shows that Cu–Gr has the lowest slope (∼125 mV dec⁻¹), significantly lower than Cu-hBN (385 mV dec⁻¹) and pristine Cu (404 mV dec⁻¹), following a Volmer–Tafel mechanism, while Cu-hBN follows the Heyrovsky–Tafel pathway. The HER mechanism in an alkaline medium is explained in the ESI† (Fig. S7). Exchange current density (J_o) is another useful parameter for deciding the intrinsic catalytic activity of pristine Cu and its composites under equilibrium conditions. J_o is obtained through the extrapolation of the linear fit to the X-axis, which intersects at a point near the equilibrium potential (zero potential). As shown in Fig. S8 (ESI†), Cu–Gr exhibits the highest exchange current density (1.27 mA cm⁻²), surpassing Cu-hBN by 0.741 mA cm⁻² and pristine DIW Cu by 0.92 mA cm⁻². This highlights the role of uniformly distributed conductive Gr in enhancing electrode/electrolyte contact, improving electron transfer, lowering the activation energy, and boosting the HER efficiency. Moreover, EIS analysis (100 kHz–0.1 Hz, 30 mV AC) reveals that Cu–Gr exhibits the lowest charge transfer resistance (1.06 Ω) and more vertical Warburg resistance (Fig. 2e), indicating improved electron transfer and ion diffusion. The high porosity and conductivity of Cu–Gr provide large surface areas and active sites and fast charge transfer, enhancing the HER activity. The reduced R_ct suggests that the 3D porous Cu–Gr structure facilitates faster water adsorption and dissociation, boosting the HER kinetics. To gain further insight into the HER catalytic activity, double-layer capacitance (C_dl) was evaluated to determine the electrochemical surface area (ECSA) (Fig. 2f). Fig. S9a–c (ESI†) presents CV curves at all scan rates in the non-faradaic region for all catalysts. Cu–Gr exhibited the highest C_dl (0.24113 mF cm⁻²), indicating strong electrolyte interaction, enhanced water adsorption, and faster HER kinetics. In contrast, Cu-hBN showed a much lower C_dl (0.01641 mF cm⁻²), comparable to pristine Cu (0.02815 mF cm⁻²), due to hBN's low conductivity, which hinders charge transport. As ECSA = C_dl/C_s (C_s = 40 μF cm⁻² in 1 M KOH), Cu–Gr exhibited the highest ECSA (6.03 cm²), significantly surpassing Cu-hBN (0.4102 cm²) and DIW Cu (0.704 cm²), confirming its superior ion-accessible surface area and HER activity. The increased surface area contributes to an increase in the absorbed hydrogen atoms at the electrode surface (Volmer step). Fig. 2g presents chronoamperometric curves at 159 mV, confirming Cu–Gr's superior durability with a stable 22 mA cm⁻² current density for 10 h, while Cu-hBN exhibited a lower current density of 10.6 mA cm⁻², almost similar to pristine Cu, with significantly reduced stability.

Moreover, minimal LSV changes after 300 cycles (inset, Fig. 2g) highlight Cu–Gr's stability due to the synergistic effect of conductive Gr and porous Cu, enabling high surface area, fast charge transport, and efficient water adsorption–dissociation. Printing parameters also influence secondary phase distribution and porosity, enhancing HER performance. Fig. 2h compares the HER activity of reported Cu-based catalysts, demonstrating DIW Cu–Gr's potential for scalable, durable electrocatalysts in green H₂ production. Post-HER analysis (Fig. S10 and S11, ESI†) confirms Cu₂O and Cu(OH)₂ formation in Cu–Gr, enhancing the active sites and surface area and modifying the electronic structure for improved conductivity and catalytic activity. Detailed illustrations are in the ESI.†

To understand the enhanced HER activity of the DIW-printed composites, Grand Canonical Monte Carlo (GCMC) simulations were conducted for hydrogen adsorption on Cu, Cu–Gr, and Cu-hBN (Fig. 3). The adsorption enthalpy increased from −2.60 kJ mol⁻¹ (Cu) to −4.79 kJ mol⁻¹ (Cu–Gr) and −4.90 kJ mol⁻¹ (Cu-hBN), indicating stronger interactions (Fig. 3d). Snapshots (Fig. 3a–c) show increased hydrogen adsorption near Gr/hBN, suggesting improved catalytic performance.⁶ Notably, the Gr-based catalysts outperform the others due to their large surface area and high conductivity, while h-BN's insulating nature limits its activity. Although GCMC simulations and enthalpy models simplify reality, they still support the superior performance of Cu/Gr composites. Methodological details are provided in the ESI.†

Fig. 3 Structures used to perform the Monte Carlo simulations. (a) Pristine copper, (b) copper + graphene; (c) copper + hBN. The location of the hydrogen molecules indicates adsorption sites. (d) Enthalpy of adsorption for hydrogen molecules interacting with the three structures at a pressure of 1 atm and temperature of 300 K.

We report scalable DIW-printed Cu-based composites with Gr and h-BN for efficient HER in alkaline media. Rationally designed through tuned porosity and optimized interface engineering, Cu–Gr outperforms Cu-hBN and pristine Cu, achieving 129 mV at 20 mA cm⁻², a ∼125 mV dec⁻¹ Tafel slope, high ECSA (6.03 cm²), and excellent stability at high current density. The enhanced HER activity stems from Gr's uniform distribution, high electrical conductivity, rapid charge transport, and large surface area. Theoretical findings confirm stronger catalytic interactions and increased H₂ adsorption near 2D materials. This work opens a new frontier for developing sustainable, scalable, highly efficient electrocatalysts for green hydrogen production.

C. S. T. acknowledges DAE Young Scientist Research Award (DAEYSRA), and AOARD (Asian Office of Aerospace Research and Development) grant no. FA2386-21-1-4014, Naval Research Board, AMT and Energy & Water Technologies of TMD Division of DST for funding supports.

Data availability

All data supporting the findings of this study are available within the article and its ESI.† Additional data are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. C. Juan, B. Lan, C. Zhao, H. Zhang, D. Li and F. Zhang, Chem. Commun., 2023, 59, 6187–6190.
2. D. Zhu, L. Wang, M. Qiao and J. Liu, Chem. Commun., 2020, 56, 7159–7162.
3. A. Zheng, Y. Wang, F. Zhang, C. He, S. Zhu and N. Zhao, iScience, 2021, 24, 103430.
4. A. P. Tiwari, G. Bae, Y. Yoon, K. Kim, J. Kim, Y.-L. Lee, K.-S. An and S. Jeon, ACS Sustainable Chem. Eng., 2023, 11, 5229–5237.
5. A. P. Tiwari, M. S. Rahman and W. J. Scheideler, Adv Mater. Technol., 2024, 9, 2400160.
6. Y. Lu, B. Li, N. Xu, Z. Zhou, Y. Xiao, Y. Jiang, T. Li, S. Hu, Y. Gong and Y. Cao, Nat. Commun., 2023, 14, 6965.
7. C. Lu, J. Wang, S. Czioska, H. Dong and Z. Chen, J. Phys. Chem. C, 2017, 121, 25875–25881.
8. J. Tian, Q. Liu, N. Cheng, A. M. Asiri and X. Sun, Angew. Chem., 2014, 126, 9731–9735.
9. J. Joyner, E. F. Oliveira, H. Yamaguchi, K. Kato, S. Vinod and D. S. Galvao, et al. , ACS Appl. Mater. Interfaces, 2020, 12, 12629–12638.
10. R. Das, N. K. Katiyar, S. Sarkar, S. Sarkar, V. Mishra and C. S. Tiwary, ACS Appl. Eng. Mater., 2024, 2, 1234–1244.
11. P. Farinazzo, B. D. Martins, P. Papa Lopes, E. A. Ticianelli, V. R. Stamenkovic, N. M. Markovic and D. Strmcnik, Electrochem. Commun., 2019, 100, 30–33.
12. L. Yu, H. Zhou, J. Sun, F. Qin, D. Luo, L. Xie, F. Yu, J. Bao, Y. Li, Y. Yu, S. Chen and Z. Ren, Nano Energy, 2017, 41, 327–336.
13. S. Anantharaj, T. S. Amarnath, E. Subhashini, S. Chatterjee, K. C. Swaathini, K. Karthick and S. Kundu, ACS Catal., 2018, 8, 5686–5697.
14. B. Ma, Z. Yang, Z. Yuan and Y. Chen, Int. J. Hydrogen Energy, 2019, 44, 1620–1626.
15. A. I. Inamdar, H. S. Chavan, B. Hou, C. H. Lee, S. U. Lee, S. Cha, H. Kim and H. Im, Small, 2020, 16, 1905884.
16. L. Ye and Z. Wen, Chem. Commun., 2018, 54, 6388–6391.
17. L. Yu, H. Zhou, J. Sun, F. Qin, F. Yu, J. Bao, Y. Yu, S. Chen and Z. Ren, Energy Environ. Sci., 2017, 10, 1820–1827.
18. A. Han, H. Zhang, R. Yuan, H. Ji and P. Du, ACS Appl. Mater. Interfaces, 2017, 9, 2240–2248.
19. B. Zhou, H. Hu, Z. Jiao, Y. Tang, P. Wan, Q. Yuan, Q. Hu and X. J. Yang, Int. J. Hydrogen Energy, 2021, 46, 22292–22302.
20. S. Li, M. Li and Y. Ni, Appl. Catal., B, 2020, 268, 118392.
21. H. Sun, Q. Li, Y. Lian, C. Zhang, P. Qi, Q. Mu, H. Jin, B. Zhang, M. Chen, Z. Deng and Y. Peng, Appl. Catal., B, 2020, 263, 118139.
22. A. Kumar, V. Q. Bui, J. Lee, A. R. Jadhav, Y. Hwang, M. G. Kim, Y. Kawazoe and H. Lee, ACS Energy Lett., 2021, 6, 354–363.
23. C. Cheng, F. Zheng, C. Zhang, C. Du, Z. Fang, Z. Zhang and W. Chen, J. Power Sources, 2019, 427, 184–193.
24. S. Lv, J. Li, B. Zhang, Y. Shi, X. Liu and T. Wang, Int. J. Hydrogen Energy, 2022, 47, 9593–9605.
25. D. T. Tran, V. H. Hoa, S. Prabhakaran, D. H. Kim, N. H. Kim and J. H. Lee, Appl. Catal., B, 2021, 294, 120263.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc01464f

‡ R. D and R. B contributed equally.

---