Application of Copper-Based Compounds in Energy Conversion and Catalysis

Abstract

Due to the crisis of energy consumpticon and environmental pollution, developing high-efficiency and low-cost catalysts is especially crucial and demanded, and the related research is increasing rapidly. Between them, copper and copper-based compounds are broadly investigated, due to their excellent properties, including ability of absorbing visible light, electronic tunability through adjusting the type and ratio of the bonded element, high catalytic efficiency and recycling property, abundant in the earth, low cost and valuable facet engineering. In this chapter, we will first introduce the crystal and electronic structure of pure copper, including the bulk and various surfaces. Then, the electronic structure of copper-based compounds will be introduced, including CuO_x, CuN_x, CuSi_x, and so on, whose band structure can be tuned from metal to semiconductor, topological semimetal, and even superconductor. At last, the application and mechanism in catalysis will be introduced, including plasmonic catalysis, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), carbon dioxide reduction reaction (CO₂RR), and single-atom catalysis (SAC). We found that Cu element can be incorporated into a broad type of materials with novel electronic structures. Furthermore, Cu-based materials play a vital role in energy conversion and catalysis.

1. Introduction

In the twenty-first century, the growing global energy demand is juxtaposed against the traditional energy supply system, which is heavily reliant on fossil fuels. The consumption of these fuels produce considerable amounts of greenhouse gases and pollutants, posing significant threats to both the environment and human health [1, 2, 3]. The rising consumption of fossil energy and the resulting CO₂ emissions not only degrade air quality but also contribute to the exacerbation of global warming (Figure 1) [4]. To address this challenge, developing clean and sustainable energy resources is crucial. Solar and hydrogen energy are considered as promising alternative energy sources, and they can even be integrated into the prospective energy supply system [5].

Figure 1.

Global energy-related CO₂ emissions of recent years [4].

Solar energy is a kind of clean, abundant, and widely available energy source. As depicted in Figure 2, the solar spectra cover a wide range from ultraviolet (UV) to near-infrared (NIR), with visible light occupying 43% [6]. Therefore, optimizing energy utilization in the visible region is imperative for enhancing solar energy utilization [7], including photovoltaic cells and solar-thermal systems, which transfer sunlight into electricity and heat. Despite achieving a certain level of solar energy utilization, these technologies face ongoing challenges, including limited conversion efficiency, high cost, and energy storage issues [8, 9].

Figure 2.

Solar energy spectrogram [6].

Hydrogen (H₂), serving as a clean energy carrier, can be exclusively yielded from the consumption of water, without producing pollutants [10]. Additionally, its high-energy density renders hydrogen suitable for energy storage, transportation, and efficient utilization [11]. Hydrogen can be generated through diverse methods, encompassing water electrolysis, fossil fuel reforming, and biomass conversion. Between them, water electrolysis stands out as the cleanest method for hydrogen production, albeit being less energy-efficient and more costly [12]. Consequently, improving the energy-transfer efficiency and reducing the cost of water splitting constitute a pivotal role in maximizing the utility of hydrogen energy [13]. In addition, how to reduce the harmful gas CO₂ and NO_x is important for environmental protection. Then, convert them into useful nitric and carbonous chemicals, such as NH₃ and C₂H₄, utilizing solar energy through photocatalysis or photoelectrocatalysis could be a preferable route.

In recent years, significant progress has been achieved in the research on the utilization of solar energy. As illustrated in Figure 3, the publication number of the related research has increased continually over the past two decades. However, there is still an ongoing need to improve solar energy conversion efficiency and catalysis, specifically in the visible-light range.

Figure 3.

Trends of published papers on solar energy utilization over the last two decades.

This chapter will first introduce the crystal and electronic structure of pure copper in Section 2, covering the bulk and low-index surfaces. In Section 3, the crystal and electronic structures of various copper-based compounds will be briefly summarized, encompassing semiconductors, alloys, topological semimetals, and superconductors. In Section 4, the main applications and mechanisms of Cu-based catalysis and energy conversion will be introduced. The overarching goal is to facilitate readers’ comprehension of the crystal structure, electronic structure, applications and mechanisms of copper and copper-based compounds in energy conversion and catalysis.

2. Crystal and electronic structure of pure copper

2.1 Bulk copper

As shown in Figure 4, copper (Cu) possesses a face-centered cubic (FCC) crystal structure, where copper atoms reside at corners and face centers of the crystal unit cell, with the number of nearest neighboring atoms reaching the maximum 12, that is, closely packed. The lattice constant of bulk Cu is approximately 3.61 Å. The electrical and thermal conductivities of Cu at room temperature are 5.96 × 10⁷ S/m and 398 W/(m·K), respectively. The electronic configuration of Cu is [Ar] 3d¹⁰4s¹, indicating a fully occupied 3d shell and semi-filled 4 s orbital, resulting in the typical metal bands, without a forbidden band gap between the valence band (VB) and conduction band (CB). Besides, Cu demonstrates specific light absorption properties. For bulk Cu, it owns good absorption in the visible spectral range and results in the typical metallic color. Besides, plasmonic resonance is a ubiquitous phenomenon in Cu, which is the excited states of collective oscillations of free electrons in a metal, under the irradiation of light. For Cu nanomaterials, the resonance absorption can be tuned from the ultraviolet to the visible-light region.

Figure 4.

Crystal structure of bulk copper.

2.2 Low-index surface of copper

Copper can expose a variety of crystalline facets, among which the low-index facets are particularly active and have garnered significant attention. Following we will elucidate the atomic and electronic structures of several low-index Cu facets.

2.2.1 Cu(100) surface

Figure 5 illustrates the atomic and electronic structures of the Cu(100) surface [14, 15]. Figure 5(a) depicts the crystal structure of Cu, with the Cu(100) surface highlighted in gray on the upper panel, the lower panel shows the top-view image, and unit cell is marked with a square. Figure 5(b) illustrates the three-dimensional (3D) Brillouin zone (BZ) of bulk Cu and its projection onto the (100) plane. Figure 5(c) displays the typical scanning tunneling microscope (STM) image of Cu(100) surface, with each bright dot corresponding to a Cu atom, it illustrates an obvious relationship between the atomic arrangement in the STM image in Figure 5(c) and the crystal structure from Figure 5(a). Figure 5(d) presents the theoretically calculated bands and experimentally measured ones by angle-resolved photoelectron spectroscopy (ARPES) with hν = 27 eV on the Cu(100) surface along Γ-M direction.

Figure 5.

(a) Crystal structure of Cu(100). (b) 3D BZ of bulk Cu and the surface projected BZ of Cu(100). (c) Typical STM image of Cu (100) [14]. (d) Tight-bond calculated bulk band for Cu(100) surface (gray shaded), and the red line represents the experimentally obtained bands by ARPES spectra with hν = 27 eV [15].

2.2.2 Cu(110) surface

Figure 6 illustrates the atomic and electronic structure of the Cu(110) surface [14, 16]. Figure 6(a) depicts the crystal structure of Cu, highlighting the Cu(110) surface in gray at the top, and displaying the corresponding atomic arrangement at the bottom, with the rectangle-shaped unit cell indicated. The corresponding BZ is depicted in Figure 6(b). Furthermore, the STM image in Figure 6(c) shows a regular array of bright dots, and the period is consistent with the atomic arrangement in Figure 6(a). Finally, the high-resolution surface state (SS) of Cu(110) is shown in Figure 6(d), informing a parabolic dispersion.

Figure 6.

(a) Crystal structure, (b) BZ, and (c) typical STM image of Cu (110) surface [14]. (d) ARPES intensity plot of Cu(110) surface state [16].

2.2.3 Cu(111) surface

Figure 7 illustrates the atomic and electronic structures of the Cu(111) surface [14, 17]. Figure 7(a) depicts the crystal structure of Cu, with the Cu(111) surface highlighted in gray in the upper panel, and the top-view shown in the lower panel. Figure 7(b) presents the corresponding BZ. Additionally, Figure 7(c) displays the typical STM image of the Cu(111) face, with the atomic arrangement aligned with that in Figure 7(a). Finally, Figure 7(d) presents the ARPES result of Cu(111) face, with the Shockley-type SS and sp band clearly observed.

Figure 7.

(a) Crystal structure, (b) BZ, and (c) typical STM image of Cu (111) surface [14]. (d) ARPES spectra of pristine Cu(111), the SS state and sp band are indicated with a white arrow and blue line, respectively [17].

2.3 Facet-dependent catalysis of copper nanomaterials

The distinct crystalline surfaces of Cu normally own different physical and chemical properties, including the catalysis activity and the corresponding reaction pathways. Therefore, surface modulation is an important research hotspot in catalysis.

Figure 8(a) depicts the decomposition kinetics of formate (HCO₂) on Cu(111), Cu(110), and Cu(100) surfaces [18] through density functional theory (DFT, solid line) and potential energy surface (PES, dashed line), which shows great coincidence within 10 meV. It can be seen that the activation barrier heights for HCO₂ decomposition on Cu(111) are close to those of Cu(100) and lower than those of Cu(110), while the values are 1.104, 1.155, and 1.483 eV, respectively. For the mean translational energy ⟨E_trans⟩ of the CO₂ product (Figure 8(b)) and mean kinetic energy of adsorbed H atom (Figure 8(c)) during HCO₂ decomposition, the former one obtains more kinetic energy on Cu(111) and Cu(100) surfaces, but the latter one obtains most energy on Cu(110) surface. These results undoubtedly inform the facet-dependent catalysis of Cu material.

Figure 8.

(a) Comparison of stationary point energies of HCO₂ decomposition on relaxed Cu(111), Cu(100), and Cu(110) surfaces. (b) Mean translational energy ⟨E_trans⟩ of CO₂ product from HCO₂ decomposition. (c) Distribution of the maximum kinetic energy of adsorbed H atom (E_{kin_H_max}) during HCO₂ decomposition, the arrows indicate the mean E_{kin_H_ma} [18].

Figure 9 illustrates the electrochemical reduction of CO₂ on different facets of single crystal Cu [19]. Although the onset potentials for C₂H₄ formation are consistently 300–400 mV more negative than those for CO product on Cu(100), Cu(110), and Cu(111) surfaces, the values on each surface are different, that is, Cu(100) (−0.7 V, −0.3 V) < Cu(110) (−0.8 V, −0.5 V) < Cu(111) (−0.9 V, −0.6 V), informing a facet-dependent activity for CO₂ reduction.

Figure 9.

Faradaic efficiencies (FE) of CO₂ reduction products formed on (a) Cu(100), (b) Cu(110), and (c) Cu(111) surfaces. The dashed lines mark the onset potentials of CO and C₂H₄ formation [19].

3. Crystal and electronic structure of typical copper-based compounds

As we know, there are numerous copper-based compounds reported, with distinct structures and properties. This section will introduce the crystal and electronic structures of various classes of copper-based compounds, encompassing semiconductors, alloys, semimetals, and superconductors.

3.1 Copper-based semiconductor

Semiconductor is a kind of material whose electrical conductivity falls between that of a conductor and insulator. The properties of semiconductors are primarily determined by their energy band structure, separated by an <5 eV energy band gap between the valence band (VB) and conduction band (CB). At absolute zero temperature and without excitation of the external field, VB is completely occupied, and CB is entirely unoccupied, thereby preventing the flow of electrons from forming a current. However, as the temperature increases, or under the irradiation of light with enough energy, partial electrons can be excited from VB to CB, allowing the flow of current. There are numerous copper-based semiconductors, typically including CuO, Cu₂O, Cu₃N, and so on.

CuO is a kind of semiconductor with the monoclinic unit cell and space group C2/c, as depicted in Figure 10(a). Each copper atom is surrounded by four oxygen atoms, and each oxygen atom is similarly surrounded by four copper atoms. Figure 10(b) illustrates the calculated band structure of CuO. The gray portion represents the band gap of Cu, and the Fermi level (E_F) located near VB, indicating a p-type semiconductor [20]. Hall effect tests reveal that CuO has a resistivity of 0.26 Ω cm, mobility of 0.12 cm²/V∙s, and a carrier concentration of 7.4 × 10¹⁹ cm⁻³ [21]. Figure 10(c–d) displays the SEM and TEM images of CuO nanoparticles fabricated using a simple sol-gel method. The CuO nanoparticles are uniformly distributed, spherical in shape, with particle sizes ranging from 20 to 50 nm. The optical band gap of CuO varies between 1.2 and 2.1 eV, attributed to differences in the preparation process and sample quality. The band gap of the above CuO nanoparticles is estimated to be 1.2 eV(Figure 10(e)) [22].

Figure 10.

(a) Crystals structure and (b) calculated electronic band structure of CuO [20]. (c) SEM image, (d) TEM image and (e) Tauc plot of CuO nanoparticles [22].

The crystal structure of Cu₂O is depicted in Figure 11(a). The space group of Cu₂O is Pn3m, and it possesses a cubic structure, with oxygen atoms located at eight vertex positions and one body center position in the cell, while four copper atoms are situated at the midpoint of the line connecting the vertices and the body center. Figure 11(b) illustrates the calculated band structure of Cu₂O, where the gray portion represents the band gap, and E_F is positioned near VB [20]. Hall effect tests suggest that Cu₂O is a kind of p-type semiconductor, with a resistivity of 149 Ω∙cm, mobility of 51 cm²/V∙s, and a carrier concentration of 1.5 × 10¹⁵ cm⁻³ [21]. The band gap of the Cu₂O film prepared by electrodeposition is 2.1 eV (Figure 11(c)) [23]. Figure 11(d–f) shows the Cu₂O films prepared via the magnetron sputtering (MS) method at different temperatures. As the temperature increases from 300 K to 600 K and 1070 K, the films transition from fibrous grains to columnar grains, and the grain size increases. The statistical grain sizes are 79 ± 17, 228 ± 57, and 884 ± 373 nm, respectively [24].

Figure 11.

(a) Crystal structure, (b) electronic band structure of Cu₂O [20]. (c) Plot of (αhν)² vs. hν for Cu₂O [23]. (d-f) SEM images of Cu₂O films grown at different temperatures, (d) 300 K, (e) 600 K, and (f) 1070 K [24].

As shown in Figure 12(a), Cu₃N possesses an anti-ReO₃ cubic structure with a space group of Pm$\overline{\mathit{3}}$m and a lattice constant of a = 3.82 Å. The Cu₃N crystal lattice is composed of Cu₆N octahedrons through vertex sharing [25]. While each N atom is surrounded by six Cu atoms, each Cu atom is shared by two N atoms. Figure 12(b) displays the calculated band structure of Cu₃N. The valence band maximum (VBM) and conduction band minimum (CBM) are located at R and M points, respectively, informing an indirect band gap of 1.4 eV [26]. Ultraviolet photoelectron spectroscopy (UPS, Figure 12(c)) reveals that the VBM of Cu₃N located at 0.42 eV below E_F, UV-visible (UV-vis) absorption spectrum measurement (Figure 12(d)) gives the optical band gap to be 1.92 eV, then CBM should be located at 1.50 eV above E_F. Comparatively speaking, pristine Cu₃N’s VBM is closer to E_F than CBM, suggesting Cu₃N is a p-type semiconductor [27]. Figure 12(e–f) shows the SEM and high-resolution TEM images of Cu₃N prepared by MS method, revealing uniform spherical nanoparticles with an approximate size of 40 nm, while the TEM image shows (001) plane with the crystal space of 0.383 nm [28].

Figure 12.

(a) Crystal structure and (b) calculated band structure of Cu₃N [25, 26]. (c) UPS spectra, the inset is enlarged around E_F to estimate the position of the valence band maximum. (d) UV–vis absorption spectrum [27]. (e) SEM and (f) high-resolution TEM images of Cu₃N [28].

3.2 Copper-based alloy

Copper-based alloys are kinds of composites composed of Cu and other elements, such as Zn, Al, N, Mn, Pb, and so on. They are extensively utilized due to their favorable electrical and thermal conductivity, corrosion resistance, and workability. This subsection introduces several copper-based alloys.

Ni_0.7Cu_0.3 alloy is a promising catalyst toward hydrogen evolution [29]. As shown in Figure 13, Ni_0.7Cu_0.3 alloy films can be prepared through a facile potentiostatic electrodeposition method (Figure 13(f)). They reveal a highly porous structure consisting of well-organized 3D nanosheets (SEM, Figure 13(a–b)), with Cu and Ni uniformly distributed (EDS mapping, Figure 13(c–e)), and mainly show metallic states (XPS, Figure 13(g)).

Figure 13.

(a) Low- and (b) high-magnified SEM images of Ni_0.7Cu_0.3 alloy film via electrodeposition. (c-e) EDX mapping of Cu, Ni, and O elements for the as-prepared sample. (f) XRD patterns and (g) XPS results of Cu 2p and Ni 2p [29].

As depicted in Figure 14, Ag_0.8Cu_0.2 alloy, which can be applied in zinc-air batteries, was synthesized on nickel foam via a two-step electrochemical substitution reaction. The morphology of the Ag_0.8Cu_0.2 alloy exhibits a dense and uniform dendritic shape, with Ag and Cu elements uniformly distributed. The clear lattice stripes in HRTEM image and sharp diffraction spots suggest a high crystallinity [30].

Figure 14.

(a) The FE-SEM image, (b) HRTEM image, (c) SAED pattern, and (d) EDS overlay image of the Ag_0.8Cu_0.2 catalyst [30]. Red, purple, and green colors indicate Cu, Ag, and Ni, respectively.

Except for the binary Cu-based alloy, ternary Cu-based alloy is another broadly applied kind. For example, Figure 15 illustrates the structure of ternary CoFeCu alloy on Ni foam (NF), an efficient catalyst for oxygen evolution reaction (OER, for more information refer to Section 4.3). As the copper content increases from 0 mmol to 40 mmol, the morphology changes from nanosheets to nanoparticles, branched structures, and dendritic morphology (Figure 15(a–d)) [31]. The TEM and elemental mappings suggest a uniform distribution of Co, Fe, and Cu elements. Besides, the crystal structure of the CoFeCu alloy is schematically depicted in Figure 15(e).

Figure 15.

SEM images of CoFeCu/NF samples with different content of Cu, (a) 0 mmol, (b) 10 mmol, (c) 20 mmol, and (d) 40 mmol. (e) TEM image and corresponding elemental distribution images and (f) schematic crystal structure of CoFeCu [31].

3.3 Copper-based topological semimetal

Topological semimetal is a kind of novel material whose CB and VB are connected at specific points or lines around E_F in the reciprocal space. In recent years, Cu₃PdN has been proposed as a Dirac semimetal, stabilized by the C₄ rotational crystal symmetry. As illustrated in Figure 16(a), Cu₃PdN adopts an anti-perovskite crystal structure (space group P$\overline{\mathit{m}}$3m), with a nitrogen atom located at the cube’s center, Cu atoms occupy the octahedral vertices, and Pd located at the corner site. Figure 16(b) shows the 3D bulk BZ and (001) projected surface BZ. For the calculated electronic band structure, in the absence of spin-orbital coupling (SOC), the VB and CB are dominated by the Pd 4d (blue) and Pd 5p (red) states, respectively. Accompanied by the energy band inversion around R point, the Dirac points near E_F are observed (Figure 16(c)). When introducing SOC, a tiny 62-meV gap is opened for the Dirac node along R-X direction (Figure 16(d)) [32]. Experimentally, the prepared Cu₃PdN nanocrystals show a cube morphology (Figure 16(e)) and a negligible band gap of 0.2 ± 0.1 eV (Figure 16(f)) [33].

Figure 16.

(a) Crystal structure of anti-perovskite Cu₃PdN. (b) Bulk and projected (001) surface BZ, the orange rings and red points indicate the nodal line and Dirac points, respectively. Calculated electronic band structure of Cu₃PdN without (c) and with (d) SOC [32]. (e) UV–vis absorption spectrum, the corresponding Tauc plot is inserted on the right-up corner. (f) TEM image with statistical size inserted [33].

Monolayer Cu₂Si is also theoretically and experimentally recognized as a topological semimetal. Figure 17(a) illustrates the crystal structure. As shown in Figure 17(b), first-principle calculations reveal two hole-like bands (α, β) and one electron-like band (γ) in the energy band structure, which form closed contours on the Fermi surface (Figure 17(c)) with hexagonal, hexagram, and circular shapes, respectively. Besides, the γ-energy band intersects linearly with the α and β bands, forming two Dirac node loops (NL1, NL2) centered at the Γ point and protected by mirror inversion symmetry, as illustrated in Figure 17(d). Experimentally, depositing Si atoms onto the Cu(111) surface through the molecular beam epitaxy (MBE) method can enable the formation of monolayer Cu₂Si films. High-resolution ARPES tests have also revealed the constant energy counters (CECs) at E_F (Figure 17(e)) and nodal point (Figure 17(f)), consistent with the theoretical prediction [34].

Figure 17.

(a) Top and side views of the atomic structure of monolayer Cu₂Si. (b) Calculated band structure, (c) Fermi surface, and (d) momentum distribution of the nodal loops (NL1, blue; NL2, orange) of Cu₂Si without SOC. The blue, orange, and green lines in (c-d) correspond to α, β, and γ bands in (b), respectively. (e-f) Experimental ARPES CECs at E_F and nodal point for monolayer Cu₂Si/Cu(111) [34].

3.4 Copper-based superconductor

Superconductor is a kind of unique material that exhibits a sudden disappearance of electrical resistance and complete repulsion of magnetic fields, known as the Meissner effect, below the critical temperature (T_c) [35]. Under a superconducting state, the current within the superconductor can flow unimpededly without heat loss. This subsection provides several examples to introduce copper-based superconductors.

As shown in Figure 18(a), polycrystalline samples of Ba₂CuO_4-y were synthesized using solid-state reaction at high pressure (∼18 GPa) and high temperature (∼ 1000°C). This sample exhibits an obvious superconducting transition at about 73 K in both zero-field-cooled (ZFC) and field-cooled (FC) modes (Figure 18(b)). This enhanced T_c than the isostructural La₂CuO₄-based counterparts can be attributed to the compressed octahedron structure, in which the 3d3z²-r² orbitals are lifted above the 3dx²-y² orbitals (Figure 18(c)) [36].

Figure 18.

(a) Typical XRD pattern, (b) magnetization, and (c) Cu-O bond lengths of Ba₂CuO_4-y superconductor [36].

Crystalline [Cu₃(C₆S₆)]_n (Cu-BHT) film is another kind of superconductor, and it can be synthesized through liquid-liquid interfacial reaction. Figure 19(a–c) present the atomic structure and high-resolution TEM images of Cu-BHT. Simply speaking, Cu-BHT is composed of stacked π-d conjugated 2D nanosheets with Cu²⁺ ions and BHT ligands, and forming the famous kagome structure, that is, woven arrangement of six corner-shared triangles and form sixfold symmetry of structure composed of hexagons and triangles. In Figure 19(d), the in-plane resistivity is plotted against temperature, revealing a sharp superconducting transition at around 0.25–0.3 K. Furthermore, the origination of the unconventional superconductivity of Cu-BHT is attributed to the strong electron correlations, due to T_c/T_F of Cu-BHT is 0.025 and belongs to the range of strongly correlated superconductors [37].

Figure 19.

(a–b) Top-view and side-view of the crystal structure of Cu-BHT, the triangular color blocks indicate the kagome structure. (c) HRTEM image. (d) Temperature dependence of the in-plane resistivity. (e) Uemura plot of T_c against the effective superfluid density [37].

4. Application in energy conversion and catalysis

Over the past few decades, copper and its compounds have been applied in many areas of energy conversion and catalysis, including electrocatalysis of hydrogen evolution, oxygen reduction reaction, carbon dioxide reduction, and so on. Moreover, they contribute significantly to photocatalysis, enabling processes such as water decomposition and wastewater treatment. Additionally, in oxidation reaction catalysis, they play a pivotal role in oxidizing and utilizing organic compounds to produce environmentally friendly products. These applications are significantly important for improving energy utilization and mitigating environmental pollution. This section will briefly introduce the typical application of Cu and its compounds in energy conversion and catalysis.

4.1 Plasmonic catalysis

Plasmonic catalysis usually utilizes the plasmonic effect-induced hot electrons of metal and its compounds to enhance the catalytic reactions. The core principle involves the emergence of localized surface plasmon resonance (LSPR) upon light irradiation with a specific wavelength, which induces oscillations of free electrons on material surfaces and generates a strong electric field. This strong electric field produced by LSPR can interact with adjacent reactants or catalysts, leading to alterations in their chemical behavior. Additionally, LSPR can generate lots of charge carriers and induce photothermal effects [38]. Moreover, the plasmonic property can be tuned by the inherent characteristics, dimensions, and morphology of the materials [39].

Au, Ag, and Cu are the most commonly employed plasmonic materials, and Cu owns the advantages of being non-precious, cost-effective, and readily available. Furthermore, Cu nanoparticles (Cu NPs) exhibit superior LSPR effects within the UV-visible and even near-infrared range, making them broadly applied in water splitting, CO₂ reduction, and so on. As shown in Figure 20, the experimentally synthesized Cu NPs via photoreduction show obvious SPR absorption around 600 nm, and the photocatalytic activity can be optimized through adjusting the precursor Cu²⁺ amounts. As a result, the maximum hydrogen (H₂) and oxygen (O₂) evolution rates from water splitting reach 9.02 and 4.48 μmol‧h⁻¹, respectively [40].

Figure 20.

(a) Schematic illustration of the synthesis and photocatalysis of Cu NPs. (b) Absorption spectrum. (c) The photocatalytic gas evolution from water with the Cu NPs prepared with different volumes of precursor [40].

Li et al. reported that the hybridization of plasmonic Cu NPs onto TiO₂ to form x wt.%-Cu/TiO₂ can consistently increase the light absorption between 400 and 750 nm (Figure 21(a)). Besides, the decreased photoluminescence (PL) spectra (Figure 21(b)) inform the enhanced photogenerated carrier separation and reach the best state for 0.13 wt.%-Cu/TiO₂. Furthermore, the introduction of Cu NPs markedly improves the photocatalytic hydrogen production rate from 666.42 μmol⋅g⁻¹‧h⁻¹ for pure TiO₂ to 2853.53 μmol‧g⁻¹⋅h⁻¹ for 0.13 wt.%-Cu/TiO₂ (Figure 21(c)). This heightened efficiency is attributed to the plasmonic nature of Cu and the resultant photothermal effect [41].

Figure 21.

(a) UV–vis DRS absorption spectra. (b) PL emission spectra. (c) H₂ evolution rate over x wt%-Cu/TiO₂ [41]. (d) UV–vis absorption spectra of Cu/ZnO catalyst and ZnO support. (e) Stability test of methanol production rate from CO₂ reduction over the Cu/ZnO catalyst at 220°C with and without visible light irradiation. (f) Arrhenius plots under dark and light conditions [42].

Wang et al. reported that load Cu onto ZnO support to form Cu/ZnO catalysts can effectively improve the visible-light absorption with the peak centered at 584.3 nm (Figure 21(d)). Moreover, the methanol production rate from CO₂ reduction can be improved from 1.38 μmol‧g⁻¹‧min⁻¹ under dark to 2.13 μmol‧g⁻¹‧min⁻¹ under visible-light irradiation (Figure 21(e)). Concurrently, the apparent activation energy is decreased by about 40% from 82.4 to 49.4 kJ‧mol⁻¹ (Figure 21(f)). These results indicate that the SPR-induced hot electrons of Cu NPs can effectively assist the activation of reaction intermediates during methanol synthesis [42].

4.2 Hydrogen evolution reaction (HER)

HER is an important half-reaction for (photo)electrocatalytic water splitting. For the standard electrolytic cell, it is composed of cathode, anode, and electrolyte. HER normally occurs at cathode. Under the external electric field, electrons can move to the cathode surface and participate in the reduction reaction to generate H₂. Besides, the electrolyte’s acid-base property can influence the HER reaction route. For acidic electrolytes, the reaction can be expressed as 2H⁺+2e⁻ → H₂. For alkaline electrolytes, it follows: 2H₂O + 2e⁻ → H₂ + 2OH⁻ [43, 44].

Cu and its compounds are widely employed as effective HER catalysts. For example, incorporating Cu₂S into CdZnS (CZS) to form snowflake-shaped 2% Cu₂S/CZS heterojunction (Figure 22(a)) can not only enhance the light absorption property (Figure 22(b)), but also improve the hydrogen evolution within 5 hours from 92.5 μmol for pristine CZS to 295.2 μmol for 2% Cu₂S/CZS (Figure 22(c)), due to the reduced agglomeration of granular CZS by snowflake-shaped structure of Cu₂S. Additionally, 2% Cu₂S/CZS shows great HER stability (Figure 22(d)) [45]. Similar experiments reveal that the deposition of Cu NPs can further facilitate the separation of photogenerated carriers in TiO₂/CuInS₂ (Figure 22(e)), and improve the HER activity even when TiO₂/CuInS₂ reaches a saturated state (Figure 22(f)) [46].

Figure 22.

(a) SEM image of 2%Cu₂S/CZS (CdZnS). (b) UV–vis DRS and (c) H₂ evolution of CZS, Cu₂S and 2%Cu₂S/CZS. (d) Cycling H₂ generation experiment [45]. (e) PL spectra and (f) H₂ generation of TiO₂, TiO₂/CuInS₂, and TiO₂/CuInS₂/Cu under >420 nm visible light [46].

4.3 Oxygen evolution reaction (OER)

OER is another half-reaction for PEC water splitting, which happens at the anode. Similar to HER, the acid-base property of electrolytes affects the OER electrochemical reactions. Under acidic conditions, it follows 2H₂O → 4H⁺+O₂ + 4e⁻. Under alkaline conditions, OER proceeds as 4OH⁻ → 2H₂O + O₂ + 4e⁻. For OER, efficient catalysts are crucial to lower the activation energy and enhance the efficiency of electrochemical conversion, necessitating dedicated research for efficient OER catalysts to be crucial [47].

Cu and its compounds are widely used catalysts in OER reactions. As shown in Figure 23, CuO is composited with CoMn₂O₄ (CMO), forming CuO-CMO nanostructures, to enhance the OER performance of CMO. By optimizing the CuO concentration, CuO-CMO-16% demonstrates superior performance, with an overpotential of 277 mV at 10 mA‧cm⁻² (Figure 23(a)), the lowest Tafel slope of 43 mV dec⁻¹ (Figure 23(b)), larger electrochemical active surface area (ESCA = C_dl/C_sp, Figure 23(c–e)), and good long-term stability (Figure 23(f)) [48].

Figure 23.

(a) LSV curve for CuO-CMO-x with varying composition of CuO. (b) Tafel plot analysis. (c-e) C_dl plots for CMO, CuO, and CuO-CMO-16%. (f) Long-term stability test for CMO and CuO-CMO-16% [48].

Porous Cu-CoP (Cu₁Co_xP) nanoplates were employed as OER catalysts, and Cu₁Co₁₀P with a molar ratio of Cu to Co to be 1:10 shows the highest current density (Figure 24(a)) and low overpotential of 252 mV at 10 mA cm⁻² (Figure 24(c)). Besides, Cu₁Co₁₀P exhibits a low Tafel slope (89.1 mV dec⁻¹, (Figure 24(b))), largest active surface area (37.21 mF cm⁻², (Figure 24(d))), the lowest impedance (Figure 24(e)). Moreover, Cu₁Co₁₀P shows superior stability under alkaline conditions at 1.48 V vs. RHE (reversible hydrogen electrode) (Figure 24(f)). The improvement in OER performance can be attributed to Cu doping and morphology modulation of porous structure that induce more surface electronic states and rapid electron transfer [49].

Figure 24.

(a) LSV curves, (b) Tafel plots, (c) overpotential at current density of 10 mA‧cm⁻², (d) scan-rate dependent current density, and (e) Nyquist plots of CoP and Cu₁Co_xP samples. (f) Long-term test for Cu₁Co₁₀P [49].

4.4 Oxygen reduction reaction (ORR)

ORR is a crucial chemical reaction in fuel cells and metal-air batteries, and garnering extensive research attention in recent years. ORR involves the reduction of oxygen (O₂) to water (H₂O) or hydroxide (OH⁻). As depicted in Figure 25, ORR can proceed through two-electron and four-electron pathways. The crucial distinction between the two ORR pathways relies on the cleavage of the O▬O bond. If the O▬O bond is broken, the reaction follows the two-electron ORR pathway; otherwise, it proceeds through the four-electron ORR pathway [50]. Based on pH conditions, ORR can be further categorized into four reactions [51]:

Figure 25.

Two-electron and four-electron pathways of ORR [51].

In acidic conditions:

Four-electron reduction: O₂ + 4H⁺ + 4e⁻ → 2H₂O;

Two-electron reduction: O₂ + 2H⁺ + 2e⁻ → H₂O₂.

In alkaline conditions:

Four-electron reduction: O₂ + 2H₂O + 4e⁻ → 4OH⁻;

Two-electron reduction: O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻.

Coupling Cu-CuFe₂O₄ nanohybrid with conductive carbon to form Cu-CuFe₂O₄/C has been utilized as a highly efficient electrocatalyst for the ORR. As shown in Figure 26(a), the well-defined ORR peak with a current density of −1.49 mA‧cm⁻¹ at −0.33 V is observed for Cu-CuFe₂O₄/C, exhibiting sharper and more pronounced characteristics and more positive ORR onset potential than that of other materials. The results of linear-sweep voltammetry (LSV) in Figure 26(b) also indicate that the onset and half-wave potentials for Cu-CuFe₂O₄/C, that is, −0.139 and − 0.39 V vs. Ag/AgCl, respectively, are more positive than others, suggesting a superior ORR efficiency of Cu-CuFe₂O₄/C, which can be attributed to the presence of metallic Cu and the reduced size of NPs on the carbon substrate. Furthermore, Koutecky-Levich (K-L) analysis between −0.4 and −0.7 V indicates that the ORR on Cu-CuFe₂O₄/C proceeds a four-electron process (Figure 26(c)–(d)) [52].

Figure 26.

(a) Cyclic voltammetry (CV) of several Cu-based materials in O₂- (red lines) and N₂-saturated (black lines) 0.1 M KOH electrolyte. (b) LSVs in an O₂-saturated 0.1 M KOH electrolyte at 1600 rpm. (c) K-L plots between −0.4 and − 0.7 V. (d) Plot of electron transfer vs. potential for Cu-CuFe₂O₄/C hybrid [52].

As depicted in Figure 27, the ultrathin flower-like 2D Cu/Cu₂O nanosheets grown through the low-temperature method in an ice/water mixed environment (273–278 K) demonstrate excellent ORR activity. CV curves exhibit a distinct reduction peak with an onset potential of 0.987 V and a half-wave potential of 0.921 V at 273 K (Figure 27(a)). First-principle calculations reveal that the abundant active Cu₂O stepped atoms can result in excellent low-temperature ORR activity. Figure 27(b) shows the calculated adsorption energies of OO^*, OOH^*, O^*, and OH^*. While the reaction steps of Cu₂O(111) and Cu₂O(220) exhibit a downhill trend, Cu(111) includes an uphill tendency and is not favorable for the ORR reaction. Furthermore, the difference in the step trends of Cu₂O(111), Cu₂O(220), and Cu(111) can be attributed to the d-band center theory, that is, the d-band centers of Cu₂O(220) (−0.65 eV) and Cu(111) (−0.78 eV) are more closer to E_F level (0.0 eV) than Cu(111) (−1.26 eV) [53].

Figure 27.

(a) Low-temperature CV curves of 2D Cu/Cu₂O nanosheets in N₂- or O₂-saturated 0.1 Mol‧L⁻¹ KOH solution at a scan rate of 50 mV‧s⁻¹. (b) First-principle calculated free energy for ORR over 2D Cu/Cu₂O nanosheets [53].

4.5 Nitrogen reduction reaction (NRR)

NRR is a kind of reaction that can convert molecular nitrogen (N₂) into ammonia (NH₃) or other nitrogen-containing compounds, which are extensively applied in medicine, chemical engineering, and environmental sciences. It is a complex, multistep process involving several intermediates and electron transfer steps, such as adsorption, electron transfer, intermediate generation, and product release. Initially, the N₂ molecule is adsorbed onto the surface of the catalyst, followed by an electron transfer that facilitates the gradual breaking of the N☰N bond. Subsequently, the N₂ molecules are progressively transformed into various intermediates, such as N₂H^*, NH^*, NH₂^*, and ultimately yield NH₃ [54]. Besides, the six-electron process NRR is competitive with two-electron HER as their similarity in equilibrium potential in both acidic and alkaline electrolytes. Then, it is crucial to design a specific electrocatalyst that prefers to adsorb N atom than H atom, that is, suppress HER and enhance the selectivity of NRR [55].

In recent years, there has been a surge in the application of Cu-based catalysts in NRR. For instance, the in situ modification of Cu₂O with unsaturated Cu-MOF to form Cu-MOF/Cu₂O heterostructures shows an excellently boosted NH₃ production rate (7.16 mmol/m²/h), even superior to the noble-metal Pt (Figure 28(a)). Besides, Cu-MOF/Cu₂O exhibits stable performance after five cycles (Figure 28(b)). The enhanced NRR performance can be attributed to the unsaturated Cu(II) sites and the porous structure of Cu-MOF, which can selectively absorb and activate N₂ molecules [56].

Figure 28.

(a) NH₃ production rate of Pt, Cu₂O, Cu-MOF and Cu-MOF/Cu₂O. (b) Recycling test of Cu-MOF/Cu₂O [56].

As shown in Figure 29, anchoring zero-valent Cu NPs onto reduced graphene oxide (Cu NPs-rGO) can serve as an efficient electrocatalyst for NRR [57]. It achieves an ammonia (NH₃) yield of 24.58 μg‧h⁻¹‧mg⁻¹_cat and a Faraday efficiency (FE) of 15.32% at −0.4 V vs. RHE (Figure 29(b)). Furthermore, the production rate and FE show excellent cycling stability, the current density exhibits good long-term stability.

Figure 29.

(a) TEM image of the Cu NPs-rGO. (b) V_NH3 and FEs at various potentials. (c) Cycling test for NRR at −0.4 V. (d) Time-dependent current density curve for 30 h at −0.4 V [57].

4.6 Carbon dioxide reduction reaction (CO₂RR)

CO₂RR is a process that converts CO₂ into renewable energy. The CO₂RR mechanism encompasses four key stages: adsorption, activation, electron transfer, and product desorption. Initially, CO₂ molecules undergo adsorption onto the electrode surface. Subsequently, the adsorbed CO₂ undergoes activation to enhance its reactivity with electrons. The activated CO₂ molecules then react with the electrons on the electrode, accepting the electrons and forming a reduction intermediate. This intermediate subsequently undergoes further reactions to yield the final product. CO₂RR is a complex multi-electron process, and different reaction routes give rise to distinct intermediates and products, including single-carbon (C₁; HCOOH, CH₃OH, CH₄), two-carbon (C₂; C₂H₄, C₂H₆, C₂H₅OH), and even multi-carbon (C_n, n ≥ 3) products. For example, a two-electron transfer results in the formation of formic acid (HCOOH) through the reaction CO₂ + 2e⁻ + 2H⁺ → HCOOH, a six-electron transfer yield methanol (CH₃OH) via CO₂ + 6e⁻ + 6H⁺ → CH₃OH + H₂O, and a twelve- and fourteen-electron process generate ethyl alcohol (C₂H₅OH) and ethane (C₂H₆) by 2CO₂ + 12e⁻ + 12H⁺ → C₂H₅OH + 3H₂O and 2CO₂ + 14H⁺ + 14e⁻ → C₂H₆ + 4H₂O, respectively. The selectivity and yield of CO₂RR are crucial parameters for assessing the process’s efficiency [58].

A nanodefective Cu nanosheet (n-CuNS) is synthesized and proved to be an efficient catalyst for CO₂RR to ethylene (C₂H₄) [59]. As shown in Figure 30(a), n-CuNS exhibits superior ethylene partial current density at −1.48 V vs. RHE (66.5 mA‧cm⁻²) than nondefective CuNS (5 mA‧cm⁻²) and CuNP (8.5 mA‧cm⁻²). Besides, n-CuNS also shows an enhanced ethylene faradaic efficiency (FE) of 83.2% (Figure 30(b)), informing high selectivity. DFT calculations reveal that the introduction of Cu defects can enhance the adsorption of key reaction intermediates ^*CO, ^*OCCO, and OH⁻ (Figure 30(c)). Besides, the defective Cu(111) surface of n-CuNS decreases the reaction barriers for 110 meV in the CO dimerization reaction (^*CO + ^*CO➔^*OCCO), which is a rate-determining step for ethylene production (Figure 30(d)).

Figure 30.

(a) Ethylene (C₂H₄) partial current density and (b) FE from electrochemical CO₂RR at various potentials for different catalysts. (c) Comparison of adsorption energy of key intermediates on different facets. (d) Energy diagrams and geometries of CO dimerization on OH⁻ adsorbed and defective (red) and nondefective (black) Cu(111) of CuNS [59].

Choi et al. reported that constructing intimate atomic Cu-Ag interfaces on the surface of Cu nanowires through a two-step process (Figure 31(a)) can significantly enhance the methane (CH₄) selectivity of CO₂RR. As shown in Figure 31(b), the CH₄ production from CO₂RR on Cu₉Ag₁NWs begins to increase at −1.12 V vs. RHE with FE_CH4 of 63.29% ± 4.85%, and reaches a maximum FE_CH4 of 72% at −1.17 V vs. RHE. Besides, Cu₉Ag₁NWs show obviously superior CO₂RR performance than CuNWs (Figure 31(c)), due to the synergistic effect of CO-Ag^* and H-Cu^* for enhanced CH₄ selectivity at low overpotential [60].

Figure 31.

(a) Schematic process of preparing CuAgNWs. (b) FEs of Cu₉Ag₁NWs in 0.1 M KHCO₃ at room temperature and atmospheric pressure. (c) Methane (CH₄) FEs of Cu₉Ag₁NWs, CuNWs and Cu_8.2Ag_1.8NWs samples [60].

4.7 Single-atom catalysis (SAC)

SAC is an innovative catalytic technology in which the catalyst consists of individual atoms rather than clusters or multiple atoms. Typically, these catalysts anchor individual atoms to the surface of a solid carrier, ensuring both stability and high catalytic activity. A distinctive feature of SAC is its extremely high atomic utilization and density of active sites, stemming from the ability of each atom to act as an active site in the catalytic reaction. Moreover, the atomically dispersed nature allows SAC’s electron clouds interact more efficiently with the reactant and substrate, further enhancing the catalytic activity. Due to its elevated activity, selectivity, and efficient atom utilization, SAC has become a rising star in catalysis research [61].

As shown in Figure 32, outstanding H₂ evolution performance is achieved through loading highly dispersed Cu single atoms onto TiO₂ and forming CuSA-TiO₂ [62]. The dispersion of atomic Cu is confirmed through the high-resolution high-angle annular dark-field (HAADF) STEM characterization (Figure 32(a, b)). The highest hydrogen yield occurred at a CuSA loading of 1.5 wt%, achieving an H₂ evolution rate of about 100 mmol‧g⁻¹‧h⁻¹, highlighting the pivotal role of CuSA loading (Figure 32(c)). Furthermore, it shows highly stable performance, even after 380-day storage in the lab (Figure 32(d)). Besides, the electrochemical impedance spectra (EIS) suggest that 1.5 wt% CuSA-TiO₂ exhibits lower impedance than pure TiO₂ (Figure 32(e)), more favorable for carrier transfer. The PL spectra in Figure 32(f) demonstrate that the photogenerated carriers of TiO₂ are effectively suppressed after CuSA loading, thereby promoting the separation of photogenerated carriers. These results imply that the anchoring of CuSA on TiO₂ can provide more active sites, enhance the photogenerated carrier separation and transfer, and promote the H₂ evolution.

Figure 32.

(a) High-resolution HAADF STEM image of CuSA-TiO₂. (b) Line profile marked in (a). (c) Photocatalytic H₂ evolution rate of TiO₂ and TiO₂ loaded samples with different ratios of Cu SACs. (d) Cyclic photocatalytic activity of water splitting for 1.5 wt% CuSA-TiO₂. (e) ESI and (f) PL spectra of TiO₂ and CuSA-TiO₂ [62].

Through a semi-transformed strategy, the Cu SAC is embedded into carbon dots to form Cu-CDs with the coordination of two N and two O. The synthesized N,O-coordinated Cu SAC (also CuN₂O₂) shows effective electrochemical CO₂RR activity to produce methane (CH₄), with the CH₄ partial current density of Cu-CDs 25 times higher than that of CuPc at −1.64 V vs. RHE, and the maximum turnover frequency (TOF) of Cu-CDs reaches a value of 2370 h⁻¹, suggesting a great CO₂RR activity (Figure 33(a)). Besides, the Cu-CDs catalyst shows stable performance of current density and high CH₄ Faradaic efficiency (78 ± 2%) (Figure 33(b)), revealing a remarkable selectivity of CO₂RR to CH₄. DFT calculations of the limiting potentials of the products of CO₂RR and HER are depicted in Figure 33(c), informing that the superior CH₄ selectivity of CuN₂O₂ (Cu-CDs) originates from the more negative potentials of CH₄ depart from other products for Cu-CDs, so that CH₄ can be exclusively produced with highest FE and less by-products [63].

Figure 33.

(a) Partial CH₄ current density (right y-axis) and TOFs (left y-axis) of Cu-CDs, CDs + Cu²⁺, and CuPc at different potentials. (b) Stability of current density and FE of Cu-CDs (black) and CuPc (red). (c) The limiting potentials of the products of CO₂RR and HER on the CuN₂O₂(Cu-CDs), CuN₄, and Cu(111) [63].

5. Conclusions

This chapter provides a comprehensive introduction to the applications of copper and its compounds in energy conversion and catalysis. Based on the introduction of the crystal and electronic structure of bulk Cu and the representative low-index surfaces, several kinds of Cu-based materials with different band gaps are further illustrated, including semiconductors, semimetals, alloys, and superconductors. We find that the bonded elements and crystal structure can greatly influence the electronic band structure of the Cu-based compounds, and further induce various physical-chemistry properties, including light absorption property, conductivity, catalytic performance, and so on. At last, the typical application of Cu-based compounds in several important catalytic processes, utilizing solar energy and/or small quantity of electric energy, are introduced with typical examples. In general, Cu and Cu-based materials exhibit broad and effective applications in the critical energy conversion and catalysis fields, including plasmonic catalysis, HER, OER, ORR, NRR, CO₂RR, and SAC.

6. Perspectives

Despite significant progress in the field of energy conversion and catalysis using copper and its compounds have been achieved, there still have some various challenges and issues that demand attention. Firstly, how to design materials and further improve the activity, stability, and selectivity of specific reactions. Secondly, developing convenient theoretical and experimental methods for precise control of the atomic structure and electronic band structure of Cu-based compounds and the related heterojunction, as well as the number, location, and coordination environment of outer surface/inner layer active sites. Thirdly, an effective route for quantity production to meet the requirements of industrialization.

In the future, emphasis should be placed on catalytic activity, stability, selectivity, and quantity production, which are the keys to achieve industrialization. Initially, attention must be directed toward structural material design, involving alterations in specific surface area, pore size, and constructing heterostructure to enhance catalytic activity. Subsequently, stability can be heightened through surface modification utilizing corrosion-resistant and high-temperature-resistant materials. Furthermore, the establishment of stability in reaction intermediates via bond optimization between product and catalyst surface is crucial for controlling product selectivity. Lastly, artificial intelligence (AI)-based model study and data mining from historical database can be employed for structure design, performance prediction, and structure-relationship analysis, this should be quite helpful for shortening the development cycle of high-performance catalysts. Besides, efficient synthesis methods for fast and large-amount preparation must also be developed.

In summary, Cu and its compounds demonstrate substantial potential for applications in the realm of energy conversion and catalysis. Through sustained and in-depth research and exploration in future, Cu and its compounds could be believed to realize more potential and play a vital role in the field of energy conversion and catalysis.

Acknowledgments

Project is funded by the Talent Fund of Hubei University of Technology (grants BSQD2020112, BSQD2020107, BSQD2020104), Science and Technology Research Project of Education Department of Hubei Province (grant Q20201402), and the Outstanding Talent Foundation for Green Industry Leading Plan of HBUT (JCRC2021003).

Conflict of interest

The authors declare no conflict of interest.