Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing

Abstract

Monolithic nanoporous metals, derived from dealloying, have a unique bicontinuous solid/void structure that provides both large surface area and high electrical conductivity, making them ideal candidates for various energy applications. However, many of these applications would greatly benefit from the integration of an engineered hierarchical macroporous network structure that increases and directs mass transport. We report on 3D (three-dimensional)-printed hierarchical nanoporous gold (3DP-hnp-Au) with engineered nonrandom macroarchitectures by combining 3D printing and dealloying. The material exhibits three distinct structural length scales ranging from the digitally controlled macroporous network structure (10 to 1000 μm) to the nanoscale pore/ligament morphology (30 to 500 nm) controlled by dealloying. Supercapacitance, pressure drop, and catalysis measurements reveal that the 3D hierarchical nature of our printed nanoporous metals markedly improves mass transport and reaction rates for both liquids and gases. Our approach can be applied to a variety of alloy systems and has the potential to revolutionize the design of (electro-)chemical plants by changing the scaling relations between volume and catalyst surface area.

Fig. 1. 3DP-hnp-Au exhibits control over structure that spans over seven orders of magnitude in length scales, from centimeters to nanometers.

(A to C) Schematic illustrations of (A) 3D printing inks composed of mixtures of Au and Ag microparticles, polymer binder, and solvent (binder and solvent are represented as a green color). (B) The annealing step alloys the Au and Ag phases and removes the polymer binder to yield microscale porosity. (C) The dealloying step selectively removes the Ag phase, yielding the nanoscale porosity. Optical images of the 1-mm scale for multilayer woodpile-like architectures for printing (D), annealing and alloying (E), and dealloying steps (F). Scanning electron microscopy (SEM) images are shown depicting the structural evolution after the printing, annealing (and alloying), and dealloying steps for the 100-μm scale (G to I), 10-μm scale (J to L), 1-μm scale (M to O), and 100-nm scale (P to R). (S and T) Coarsening of the nanostructure after reannealing. Scale bars, 1 mm (D to F), 100 μm (G to I), 10 μm (J to L), 1 μm (M to O and S), and 100 nm (P to R and T).

Fig. 2. Fracture surfaces show that the structure is uniform throughout the dealloyed filaments.

(A) SEM image showing fracture surface of a dealloyed filament. The large grains and faceted surfaces were formed during annealing. (B) Higher-resolution SEM image of the fracture surface and (C) highest-resolution SEM image of the fracture surface showing that nanoscale ligament and pore morphology extend throughout the volume of the printed filaments. Scale bars, 10 μm (A), 1 μm (B), and 100 nm (C).

Fig. 3. 3D-printed Ag-Au structures with different macroscale geometries and microscale architectures.

(A) Optical image of a single-layer array of parallel linear filaments. Optical images of multilayer high–aspect ratio (B) spiral, (C) honeycomb, (D) hollow pillar array, (E) linear simple cubic lattice, and (F) circular radial lattice structures. (A), (B), (C), and (E) shown as-printed, and (D) and (F) shown after annealing. Scale bars, 200 μm (A) and 2 mm (B to F).

Fig. 4. Electrochemical measurements for evaluation of the electrochemically accessible surface area and electric field–driven ion transport in np-Au and 3DP-hnp-Au.

(A) Cyclic voltammograms (scan rate of 10 mV/s in 0.5 M H₂SO₄) collected from np-Au (60 mg) and 3DP-hnp-Au (59 mg) samples showing the Au reduction peaks used to calculate the surface area. (B) Capacitance versus frequency behavior from EIS data. (C) Charging kinetics of np-Au and 3DP-hnp-Au in response to potential jumps from E_i = 0 V to E_F = 0.6 V (versus Ag/AgCl). The time (t_1/2) where the current decreases to its half-maximum value, I/I₀ = 0.5, is used to evaluate the charging kinetics. Dashed line indicates I/I₀ = 0.5.

Fig. 5. Transport and catalytic reactions in np-Au and 3DP-hnp-Au.

(A) Schematic illustration of the liquid flow cell. (B) Calculated and experimental plots of pressure drop versus flow speed show that the hierarchical structure of 3DP-hnp-Au reduces the pressure drop by >10⁵ times compared to np-Au. (C) Schematic illustration of the gas flow through the catalytic fixed bed reactor. GC-MS, gas chromatography–mass spectrometry. (D) Plot of reaction rate versus time that compares the gravimetric reaction rate of 3DP-hnp-Au (80 mg) and np-Au (160 mg) catalysts for methanol oxidation to form methyl formate and carbon dioxide at elevated temperature.