Synthesis and (photo-)electrochemical properties of nitrogen- and carbon-based materials

Chen, Zheng; Dronskowski, Richard (Thesis advisor); Slabon, Adam (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Developing long-term sustainable, renewable and clean energy and aiding in a series of environmental problems while sustaining the growth of human requirements are tremendous challenges. To address these, potential solutions include utilizing photoelectrochemical (PEC) energy conversion technologies to produce hydrogen from water upon harnessing solar, as well as using electrochemical technologies to remove nitrite and nitrate contaminants by reducing them to ammonia. The metal carbodiimide and nitrogen-doped carbon (NDC) materials, as nitrogen- and carbon-based (N,C-based) material components, are employed as electrodes and/or cocatalysts. These materials combine strategies of improving the (photo-)electrochemical efficiencies for application in energy conversion and environmental purification. The synthesis and (photo-)electrochemical properties of N,C-based materials have been investigated in this thesis. In chapter 2, the tin oxide-carbodiimide Sn2O(NCN) is demonstrated as a prospective semiconductor material with a favorable band gap of 2.1 eV. The PEC properties of Sn2O(NCN) and the application of Sn2O(NCN) to couple with CuWO4 thin films to form a heterojunction for PEC water oxidation were investigated. Mott-Schottky measurements reveal that Sn2O(NCN) is an n-type semiconductor with a flat-band potential of −0.03 V vs. reversible hydrogen electrode (RHE). The position of its valence band edge is suitable for PEC water oxidation. Sn2O(NCN) increases the photocurrent density of CuWO4 thin films from 32 μA cm−2 to 59 μA cm−2 at 1.23 V vs. RHE in 0.1 M phosphate buffer (pH 7.0) under backlight AM 1.5G illumination. This upsurge in photocurrent density originates in a synergistic effect between the oxide and oxide carbodiimide, because the heterojunction photoanode exhibits a higher photocurrent density than the sum of its individual components. Structural analyses by means of powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS) reveal that Sn2O(NCN) forms a core-shell structure Sn2O(NCN)@SnPOx during the PEC water oxidation in phosphate buffer. This electrochemical activation is similar to the behavior of Mn(NCN) but different from Co(NCN). In chapter 3, bismuth oxide-carbodiimide Bi2O2(NCN) is demonstrated as a novel mixed-anion semiconductor and structurally related to bismuth oxides and bismuth oxysulfides. Given the structural versatility of these layered structures and the favorable band positions of Bi2O2(NCN), its photochemical properties for PEC water oxidation were investigated. Although Bi2O2(NCN) as a single photoabsorber employed as a photoanode does not generate a noticeable photocurrent density, the fabrication of heterojunctions with WO3 thin-film electrode shows an upsurge of photocurrent density from 0.9 mA cm−2 to 1.1 mA cm−2 at 1.23 V vs. RHE under AM 1.5G illumination in phosphate electrolyte (pH 7.0). Mechanistic analysis and structural analyses employing PXRD, scanning electron microscopy (SEM), XPS, and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM EDX) indicate that Bi2O2(NCN) transforms during operating conditions in situ to a core−shell structure Bi2O2(NCN)@BiPOx. In comparison to WO3/BiPO4, the in situ electrochemical electrolyte-activated WO3/Bi2O2(NCN) photoanode shows a higher photocurrent density owing to superior charge separation across the oxide/oxide-carbodiimide interface layer. Changing the electrolyte from phosphate to sulfate results in a lower photocurrent density. This reveals that the electrolyte determines the surface chemistry and mediates the PEC activity of the metal oxide-carbodiimide. A similar trend could be observed for CuWO4 thin-film electrode instead of WO3 thin-film electrode. These results show the potential of metal oxide-carbodiimides as relatively novel representatives of mixed-anion compounds and shed light on the importance of control over the surface chemistry to enable the in situ activation. In chapter 4, metal-free NDC is considered as a promising functional material for Green Chemistry, however, the structural determination of the atomic positions of nitrogen remains challenging. Directly-excited 15N solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful tool for determining such positions in NDC at natural 15N isotope abundance. Herein, a green approach for the synthesis of NDC using cellulose as a precursor was used, and the electrocatalytic properties and atomic structures of the related catalyst were investigated. NDC(NH3) was obtained by the nitrification of cellulose with HNO3 followed by annealing at 800 °C under NH3/H2 atmosphere. It contained 6.5 wt.% of N and had a surface area of 557 m2 g−1, and 15N ssNMR spectroscopy provided evidence for graphitic N besides regular pyrrolic and pyridinic N. This structural determination allowed to probe the role of graphitic N in electrocatalytic reactions, such as the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and nitrite reduction reaction (NO2RR). The NDC(NH3) catalyst exhibited higher electrocatalytic activities in the OER and HER under alkaline conditions and higher activity for NO2RR in comparison with a reference catalyst NDC(N2), which was prepared by the carbonization of HNO3-treated cellulose and annealing at 800 °C under N2. The electrocatalytic selectivity for NO2RR of NDC(NH3) catalyst is directly related to the graphitic N functions. Complementary structural analyses by means of 13C and 1H ssNMR, SEM, TEM, PXRD, XPS, Raman spectroscopy, and low-temperature N2 adsorption provided solid support to the findings. The results reveal that directly-excited 15N ssNMR spectroscopy at natural 15N abundance is generally capable of providing information on NDC materials if relaxation properties are favorable. It is expected that this approach can be applied to a wide range of solids with an intermediate concentration of N atoms. In chapter 5, electrochemical nitrate reduction into recyclable ammonia is one of the most promoting strategies to tackle nitrate degradation. As promising candidates of electrocatalysts for this strategy, tailored metal-free cellulose-derived NDC materials were prepared from renewable and sustainable cellulose with a combination of HNO3 treatment and carbonization under NH3/H2 atmosphere at 500 °C, 600 °C, 700 °C, 800 °C and 900 °C, respectively. Among the five tailored cellulose-derived NDC materials, the NDC material which was carbonized at 800 °C (NDC-800) showed the highest electrochemical performance, exhibiting 73.1% NH4+ selectivity and nearly 100% NO3− reduction efficiency when prolonging to 48 h CA for the nitrate reduction reaction (NO3RR) performed at the optimal potential of −1.5 V vs. Ag/AgCl in a 0.1 M Na2SO4 with 100 ppm NO3− electrolyte. Complementary structural analyses utilizing SEM, TEM, PXRD, XPS, Raman spectroscopy and low-temperature N2 adsorption provided solid supports. These results show the potential of NDC-800 as a metal-free cellulose-derived NDC material, which offers an appealing and supplementary alternative to efficient electrochemical nitrate reduction to ammonia toward the sustainable development of energy and environment. In chapter 6, the previous chapters of this thesis were concluded and a brief outlook on this area of research was given.