Nanoporous 6H-SiC Photoanodes with a Conformal Coating of Ni-FeOOH Nanorods for Zero-Onset-Potential Water Splitting

Abstract

A surface-nanostructured semiconductor photoelectrode is highly desirable for photoelectrochemical (PEC) solar-to-fuel production due to its large active surface area, efficient light absorption, and significantly reduced distance for charge transport. Here, we demonstrate a facile approach to fabricate a nanoporous 6H-silicon carbide (6H-SiC) photoanode with a conformal coating of Ni-FeOOH nanorods as a water oxidation cocatalyst. Such a nanoporous photoanode shows significantly enhanced photocurrent density (j_ph) with a zero-onset potential. A dendritic porous 6H-SiC with densely arranged holes with a size of ∼40 nm on the surface is fabricated by an anodization method, followed by the hydrothermal deposition of FeOOH nanorods and electrodeposition of NiOOH. Under an illumination of AM1.5G 100 mW/cm², the Ni-FeOOH-coated nanoporous 6H-SiC photoanode exhibits an onset potential of 0 V versus the reversible hydrogen electrode (V_RHE) and a high j_ph of 0.684 mA/cm² at 1 V_RHE, which is 342 times higher than that of the Ni-FeOOH-coated planar 6H-SiC photoanode. Moreover, the nanoporous photoanode shows a maximum applied-bias-photon-to-current efficiency (ABPE) of 0.58% at a very low bias of 0.36 V_RHE, distinctly outperforming the planar counterpart. The impedance measurements demonstrate that the nanoporous photoanode possesses a significantly reduced charge-transfer resistance, which explains the dramatically enhanced PEC water-splitting performance. The reported approach here can be widely used to fabricate other nanoporous semiconductors for solar energy conversion.

Keywords: Ni−FeOOH nanorods; nanoporous silicon carbide; photoelectrochemical water splitting; water oxidation cocatalyst; zero-onset potential.

Figure 1

SEM images of the planar 6H-SiC (SC) and nanoporous 6H-SiC samples (PSC6, PSC12, PSC18, PSC24) prepared at 6, 12, 18, and 24 V, respectively. Top-view SEM images of SC (a), PSC6 (b), PSC12 (c), PSC18 (d), PSC24, (e) and cross-sectional SEM images of SC (f), PSC6 (g), PSC12 (h), PSC18 (i), and PSC24 (j). High-magnification images (k–o) measured on the denoted squares of images (f–j).

Figure 2

SEM images of the Ni–FeOOH-coated planar 6H-SiC (Ni–FeOOH/SC) and the Ni–FeOOH-coated nanoporous sample PSC18 (Ni–FeOOH/PSC18). Top-view image (a), cross-sectional image (b), and histogram of the Ni–FeOOH diameter distribution, and (c) of the Ni–FeOOH/SC. Top-view image (d), cross-sectional image (e), and histogram of the Ni–FeOOH diameter distribution, and (f) of Ni–FeOOH/PSC18.

Figure 3

High-resolution TEM image of the Ni–FeOOH nanorods.

Figure 4

XPS spectra of Ni–FeOOH deposited on 6H-SiC. XPS full survey spectrum (a), Fe 2p (b), Ni 2p (c), and O 1s (d) spectra of Ni–FeOOH.

Figure 5

PEC performance of the Ni–FeOOH-coated planar 6H-SiC (Ni–FeOOH/SC) and Ni–FeOOH-coated PSC18 (Ni–FeOOH/PSC18) photoanodes. Chopped current density–voltage (j–V) curves of the Ni–FeOOH/SC (a) and Ni–FeOOH/PSC18 (b) photoanodes. Note the unit of μA/cm² for the photocurrent in (a) and the unit of “mA/cm²” for (b). Applied-bias-photon-to-current efficiency (ABPE) plots of Ni–FeOOH/SC (c) and Ni–FeOOH/PSC18 (d). All of the PEC measurements are done in a 1.0 M NaOH solution under AM1.5G 100 mW/cm² illumination.

Figure 6

PEC water splitting by the Ni–FeOOH/PSC18 photoanode. (a) Current density versus time (j–t) curve of the Ni–FeOOH/PSC18 photoanode at 1 V_RHE under steady-state AM1.5G 100 mW/cm² illumination in a 1.0 M NaOH solution. (b) Measured H₂ and O₂ volumes during (a). The dotted lines show the theoretical volumes of H₂ and O₂ with 100% faradaic efficiencies, respectively.

Figure 7

Nyquist plots for the Ni–FeOOH/SC (a) and Ni–FeOOH/PSC18 (b) photoanodes measured at 0.4 V_RHE under AM1.5G 100 mW/cm² illumination.

Figure 8

Schematic illustrations of the planar Ni–FeOOH/SC and porous Ni–FeOOH/PSC18 photoanodes for PEC water splitting. The planar photoanode exhibits a high light reflection at the surface, while the porous photoanode traps light within the pore structure. The penetration depth (D_λ) of light, the width of the space-charge region (W_dep), and the carrier diffusion length (L_D) are present in planar and porous photoanodes.