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Biochar-assisted water electrolysis (BAWE) facilitates the high-value utilization of biomass and energy-saving hydrogen production. However, the BAWE efficiency is hindered by slow kinetics of biochar oxidation reaction (BOR) at anode. Herein, to improve the overall efficiency of BAWE, three-dimensional porous nanosheets modified on nickel foam (Ni 1 Fe 2 -LDH@NF) was developed for BOR, and its mechanism was identified. Impressively, the BAWE system exhibited remarkable activity, including a low potential of 1.387 V vs. RHE for BOR over as-synthesized electrocatalyst at current density of 10 mA·cm−2, and excellent long-term stability over 20 h at a high current density of 200 mA·cm−2 and after 1000 CV cycles. The required electricity input was 3.29 kWh·Nm−3H 2 at 10 mA·cm−2 for BAWE with Ni 1 Fe 2 -LDH@NF toward BOR. In addition, the reaction mechanism of BOR including biochar direct oxidation on the electrocatalysts surface and indirect oxidation in the electrolyte was deduced, according to the surface chemistry and composition changes of electrocatalysts and biochar, as well as the products. This work provides an effective strategy towards the enhancement of the BOR process by designing the binder-free electrocatalysts, and facilitates the development of BAWE for energy-efficient hydrogen production and biomass utilization. [Display omitted] • 3D porous Ni–Fe nanosheets modified on nickel foam was developed for enhancing BOR. • Ni 1 Fe 2 -LDH@NF favored the adsorption of OH− and accelerated the charge transfer rate. • Ni 1 Fe 2 -LDH@NF toward BOR demonstrated a potential of 1.387 V vs. RHE@10 mA·cm−2 and stability over 20 h@200 mA·cm−2. • The BOR mechanism including biochar direct oxidation on electrocatalysts and indirect oxidation in electrolyte was deduced. [ABSTRACT FROM AUTHOR]

Biochar-assisted water electrolysis (BAWE) offers a novel strategy for clean hydrogen production and biochar utilization. Here we propose the production of activated biochar via limited biochar electrooxidation over Ni foam and its application to remove Cr(VI). The corn straw-based biochar oxidation reaction delivers 100 mA cm−2@1.528 V vs. RHE, and reaches 98 % Faradaic efficiency for H 2 production. The electrical consumption of 4.16 kWh Nm−3 H 2 for BAWE is lower than 5.07 kWh Nm−3 H 2 for water electrolysis. After 10 h electrolysis at 5 mA cm−2, due to the direct oxidation of biochar at electrode and indirect oxidation by reactive oxygen species in solution, the activated biochar is formed with increased pore size and specific surface area, and full of –OH, C–O, C O, and COOH groups. The removal rate of Cr(VI) by activated biochar reaches 72.79 %, much higher than the 10.96 % for original biochar, which is attributed to the well-developed mesoporous structure of activated biochar for Cr(VI) adsorption, and the enriched –OH and COOH for Cr(VI) reduction and Cr(III) complexation. These findings confirm the feasibility of electrochemical activation of biochar in BAWE process, which demonstrate a new avenue for biochar upgrading and energy-saving hydrogen production for a sustainable future. [Display omitted] • Efficient electrochemical activation of biochar and energy-saving hydrogen production is achieved through BAWE. • Activated biochar with developed pore structure and more O-containing functional groups is formed by electrooxidation. • The adsorption and reduction of Cr(VI) by electrochemically activated biochar significantly increases. [ABSTRACT FROM AUTHOR]

[Display omitted] • Correlation between biochar oxidation reaction (BOR) and electrolytes was determined. • Alkaline electrolyte was more favorable for biochar-assisted water electrolysis. • The higher concentration of H 2 SO 4 and S O 4 2 - slowed down the BOR kinetics. • The highest diffusion coefficient of 3.11 × 10−3 cm2∙s−1 for biochar was obtained at pH of 13.98. Biochar-assisted water electrolysis (BAWE) for hydrogen production is employed to reduce energy consumption by replacing the oxygen evolution reaction (OER) with thermodynamically more favorable biochar oxidation reaction (BOR). Electrolyte, as a carrier to dissolve the biochar particles and form conductive solution, is essential for the transfer and transformation of biochar particles and free radical precursors. Herein, the effects of electrolyte type, concentration, and pH on the BOR performance of BAWE system were investigated. Results show that 1 M KOH solution as an electrolyte was more suitable for the BAWE system, and the BOR of the alkaline BAWE system had the higher electrochemical activity (E 10 = 1.649 V vs RHE, 56 mV∙dec−1) compared with the OER in conventional water electrolysis system (E 10 = 1.982 V vs RHE, 189 mV∙dec−1). The anion adsorption phenomenon appeared in acidic media restricted the BOR kinetics. In addition, the diffusion coefficient of anodic reaction components showed a first decrease and then increase trend when the pH increased. And the diffusion coefficient reached the maximum value of 3.11 × 10−3 cm2∙s−1 at pH of 13.98, which was favorable for accelerating the diffusion of reaction components from the bulk solution to the electrode surface. This work can provide guidance for further improving the diffusion of reaction components, the biochar oxidation rate and hydrogen production in BAWE process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrochemical activation of biochar was achieved via limited electrooxidation. • Biochar-assisted water electrolysis reduced electricity consumption for hydrogen production. • Oxygen-containing groups enriched on biochar during its electrooxidation. • Upgraded biochar by electrochemical activation enhanced its removal of Cr(VI). • Cr(VI) removal mechanism by upgraded biochar follows attraction-reduction-complexation. Biochar-assisted water electrolysis (BAWE) has the potential to reduce the electricity consumption of hydrogen production. However, prolonged electrolysis leads to over-oxidation of biochar, reduced reactivity, and decreasing hydrogen production at cathode. Herein, we proposed the electrochemical activation of biochar via limited biochar oxidation reaction (BOR) and hydrogen evolution reaction (HER) in BAWE process, and the upgraded biochar was employed to remove Cr(VI). Distinct biochar was first pyrolyzed from three kinds of representative biomass components, cellulose, lignin, and their mixture at different temperatures. All of them exhibited excellent BOR activity, especially the biochar derived by the pyrolyzed cellulose and lignin at 800 °C (MBC-800) required a potential as low as 1.319 V vs. RHE@1 mA cm−2 using Pt electrode in 1 M KOH and achieved a Faraday efficiency almost 100% for hydrogen production. BAWE presented an energy-saving electrical consumption of 4.98 kWh Nm−3 H 2 compared to the conventional water electrolysis of 5.32 kWh Nm−3 H 2. Electrochemical oxidation enriched the oxygen-containing groups on biochar surface, which significantly improved its reductivity towards Cr(VI) and complexation with Cr(III), leading to the removal rate of Cr(VI) more than doubled. Results showed the achievement of electrochemical activation of biochar by limited electrooxidation. These findings provide a new route for the high-value utilization of biochar and energy-saving electrolytic hydrogen production. [ABSTRACT FROM AUTHOR]