Renewable electricity-driven water splitting to generate H2 is a promising technology for water conversion to chemical fuels. However, the slow electrochemical kinetic reactions such as cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) greatly hinder the energy conversion efficiency and practical application of electrocatalytic water splitting. In particular, up to 90% of the electrical energy is consumed by the anode OER, making it difficult for the HER-OER coupling to significantly reduce the energy consumption for hydrogen production. Using electrochemical oxidation of renewable biomass or biomass-derived intermediates instead of anodic OER, coupled with cathodic HER, can not only reduce the high overpotential of the anodic oxidation reaction but also drive higher currents at low input voltages. Density promotes HER at the cathode while enabling the green synthesis of high-value-added chemicals at the anode, thereby further improving the energy conversion efficiency of electrolyzed water. However, in the harsh catalytic reaction environment, the catalyst is prone to problems such as shedding, corrosion, and dissolution, and the electron transfer and mass diffusion are limited during the reaction process, making its catalytic activity and stability face great challenges.

Based on this, Professor Peng Xinwen and Professor Liu Chuanfu of the State Key Laboratory of Pulp and Paper Engineering of South China the University of Technology cleverly used the three-dimensional hierarchical porous structure of natural wood to wrap cobalt nanoparticles in nitrogen-doped carbon grown in carbonized wood in situ. In nanotubes (Co@NCNT/CW), a wood-based monolithic hierarchical porous "armored" electrocatalyst was fabricated.  The open and well-aligned 3D-oriented microchannels of Co@NCNT/CW facilitate electrolyte transport and gas diffusion. The as-prepared biochar catalyst exhibits excellent trifunctional electrocatalytic performance for HER, OER, and monosaccharide oxidation reactions (MOR).  Co@NCNT/CW can reach a high current density of 500 mA cm-2 at a low overpotential (263 mV), and its performance does not degrade significantly after running for 100 h. In a two-electrode hybrid electrolyzer with Co@NCNT/CW as the cathode and anode coupling HER and MOR, a cell voltage of only 1.36 V were required to drive a current density of 100 mA cm-2, higher than that achieved by conventional bulk water splitting to accomplish. The required voltage is reduced by 420 mV. At the same time, H2 and value-added chemicals can be obtained simultaneously at the cathode and anode. The design of this biochar electrode catalyst provides a promising approach to fabricating highly efficient electrocatalysts for the future production of hydrogen and value-added chemicals.

Fig.1 Schematic diagram of the preparation of wood-derived monolithic armor catalysts

The synthetic route of Co@NCNT/CW is shown in Fig. 1, which mainly includes high-temperature carbonization of natural wood and electrochemical deposition of cobalt precursor, followed by a co-pyrolysis process with a nitrogen source. In the above preparation process, natural wood retains a hierarchical porous structure with highly oriented open channels after carbonization, which provides sufficient space and sites for the integration of Co@ in carbonized wood, which can not only further increase the specific surface area CW, but also improve Electrocatalytic active area, and can effectively promote mass diffusion transport.

Fig.2 Morphology and structural characterization of Co@NCNT/CW catalyst

Fig.3 Chemical composition and structure characterization of Co@NCNT/CW catalyst

Each vertical channel of CW is entirely covered by Co@, and interlaced Co@ grows uniformly in the pores of CW to form an overall hierarchical porous structure. Benefiting from the low tortuosity and hierarchical porous structure, the electrolyte can penetrate the pores of Co@NCNT/CW, which facilitates the release of the gas generated on the catalyst surface from the hierarchical pores and ensures the mass transfer pathway. In addition, Co@ is highly uniformly integrated into the pores inside the CW, which can provide a continuous and fast electron transport path. The introduction of N atoms into carbon materials can further adjust the electronic structure of adjacent C atoms, induce interfacial charge redistribution, and thus stimulate the catalytic activity of C sites.

Fig.4 Hydrogen production performance of Co@NCNT/CW catalyst for electrolysis of water

The Co@NCNT/CW catalyst exhibited excellent HER activity at high current densities, with overpotentials of 238 and 263 mV at high current densities of 200 and 500 mA cm-2, respectively, outperforming commercial Pt/C catalysts. No catalyst degradation was observed after 3000 accelerated cycle tests and 100 hours of chronoamperometry. Furthermore, a dual-electrode electrolyzer was constructed using monolithic Co@NCNT/CW as cathode and anode electrocatalysts.  The Co@NCNT/CW || Co@NCNT/CW electrode exhibits excellent water-splitting activity, can work continuously for 100 hours at 100 mA cm-2, has excellent durability, and has a Faradaic efficiency close to 100%.  The excellent stability of Co@NCNT/CW is attributed to its unique overall 3D hierarchical porous structure and armor coating.

Figure 5 Co@NCNT/CW catalyst monosaccharide oxidation hydrogen production performance

A major challenge of electrocatalytic water-splitting devices is the slow OER kinetics that consumes large amounts of electricity. The coupling of biomass oxidation and HER is an effective strategy to replace traditional water electrolysis for hydrogen production, which can reduce the cost of hydrogen production. The monosaccharide oxidation reaction (MOR) was selected as the anodic OER displacement reaction, and the catalytic activity of Co@NCNT/CW for MOR was higher than that for OER, with a current density of 50 mA cm-2 and a significantly lower overpotential (1.13 V vs 1.54 V).  The Co@NCNT/CW catalyst was used as the cathode and anode electrodes to assemble the hybrid water electrolysis system.  Co@NCNT/CW || produces H2 densities at a voltage 420 mV lower than that required by conventional water-splitting devices. In addition, the continuously operating water electrolysis system exhibited excellent catalytic stability, high hydrogen evolution faradaic efficiency, and anodic xylose conversion up to 97.3%, highlighting the high added value of this strategy in terms of energy-saving hydrogen production and environmental protection. synthesis. Concept advantage of chemicals.

The intrinsic mechanisms of Co nanoparticles and N-doped carbon nanotubes in the MOR and HER processes were revealed by density functional theory (DFT) calculations. On Co@, xylose has higher adsorption energy than H2O molecules and is oxidized preferentially over H2O molecules. In addition, the presence of N-doped carbon nanotubes can tune the electronic configuration of Co nanoparticles, optimize the adsorption energy and dissociation energy of HO molecules, and the free energy of hydrogen adsorption, and create a synergistic effect at the interface, thereby enhancing the catalytic activity.

Fig.6 Reaction mechanism calculated by density functional theory (DFT)

In conclusion, this study developed a wood-derived monolithic sheathed electrocatalyst as a trifunctional gas-diffusion electrode for electrocatalytic water splitting for hydrogen production for energy-efficient hydrogen production and green synthesis of high value-added chemicals. Taking advantage of the synergistic effect of its hierarchical porous structure and metal-carbon substrate, the as-prepared biochar catalyst has excellent catalytic activity and stability. This study provides new ideas for the preparation of highly stable and efficient biochar electrodes, electrocatalysts for energy conversion and storage, and the clean conversion of biomass to organic acids.

The work was published in ", titled "Wood-, for-". The first authors of the article are Li Di and Li Zengyong, doctoral students of the South China University of Technology, and the corresponding authors are Professor Liu Chuanfu and Professor Peng Xinwen of the South China University of Technology.

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