A General Strategy for Decoupled Hydrogen Production from Water Splitting by Integrating Oxidative Biomass Valorization

Recent catalytic routes for the preparation and the upgrading of biomass derived furfural and 5-hydroxymethylfurfural

Furans represent one of the most important classes of intermediates in the conversion of non-edible lignocellulosic biomass into bio-based chemicals and fuels. At present, bio-furan derivatives are generally obtained from cellulose and hemicellulose fractions of biomass via the acid-catalyzed dehydration of their relative C6-C5 sugars and then converted into a wide range of products. Furfural (FUR) and 5-hydroxymethylfurfural (HMF) are surely the most used furan-based feedstocks since their chemical structure allows the preparation of various high-value-added chemicals. Among several well-established catalytic approaches, hydrogenation and oxygenation processes have been efficiently adopted for upgrading furans; however, harsh reaction conditions are generally required. In this review, we aim to discuss the conversion of biomass derived FUR and HMF through unconventional (transfer hydrogenation, photocatalytic and electrocatalytic) catalytic processes promoted by heterogeneous catalytic systems. The reaction conditions adopted, the chemical nature and the physico-chemical properties of the most employed heterogeneous systems in enhancing the catalytic activity and in driving the selectivity to desired products are presented and compared. At the same time, the latest results in the production of FUR and HMF through novel environmental friendly processes starting from lignocellulose as well as from wastes and by-products obtained in the processing of biomass are also overviewed.

Decoupling hydrogen production from water oxidation by integrating a triphase interfacial bioelectrochemical cascade reaction

Water electrolysis to produce H₂ is a promising strategy for generating a renewable fuel. However, the sluggish-kinetics and low value-added anodic oxygen evolution reaction (OER) restricts the overall energy conversion efficiency. Herein we report a strategy of boosting H₂ production at low voltages by replacing OER with a bioelectrochemical cascade reaction at a triphase bioanode. In the presence of oxygen, oxidase enzymes can convert biomass into valuable products, and concurrently generate H₂O₂ that can be further electrooxidized at the bioanode. Benefiting from the efficient oxidase kinetics at an oxygen-rich triphase bioanode and the more favorable thermodynamics of H₂O₂ oxidation than that of OER, the cell voltage and energy consumption are reduced by ~0.70 V and ~36%, respectively, relative to regular water electrolysis. This leads to an efficient H₂ production at the cathode and valuable product generation at the bioanode. Integration of a bioelectrochemical cascade into the water splitting process provides an energy-efficient and promising pathway for achieving a renewable fuel.

Hydrogen and Potassium Acetate Co-Production from Electrochemical Reforming of Ethanol at Ultrathin Cobalt Sulfide Nanosheets on Nickel Foam

The sluggish reaction kinetics of the anodic oxygen evolution reaction increases the energy consumption of the overall water electrolysis for high-purity hydrogen generation. In this work, ultrathin cobalt sulfide nanosheets (Co₃S₄-NSs) on nickel foam (Ni-F) nanohybrids (termed as Co₃S₄-NSs/Ni-F) are synthesized using cyanogel hydrolysis and a sulfurization two-step approach. Physical characterizations reveal that Co₃S₄-NSs with a 1.7 nm thickness have abundant holes, implying the big surface area, abundant active edge atoms, and sufficient active sites. Electrochemical measurements show that as-synthesized Co₃S₄-NSs/Ni-F have excellent electrocatalytic activity and selectivity for ethanol oxidation reaction and hydrogen evolution reaction. Due to their bifunctional property of Co₃S₄-NSs/Ni-F nanohybrids, a symmetric Co₃S₄-NSs/Ni-F∥Co₃S₄-NSs/Ni-F ethanol electrolyzer can be effectively constructed, which only requires a 1.48 V electrolysis voltage to reach a current density of 10 mA cm^-2 for high-purity hydrogen generation at the cathode as well as value-added potassium acetate generation at the anode, much lower than the electrolysis voltage of traditional electrochemical water splitting (1.64 V).

Coaxial Ni-S@N-Doped Carbon Nanofibers Derived Hierarchical Electrodes for Efficient H2 Production via Urea Electrolysis

Electrochemical water splitting into hydrogen is a promising strategy for hydrogen production powered by solar energy. However, the cell voltage of an electrolyzer is still too high for practical application, which is mainly limited by the sluggish oxygen evolution reaction process. To this end, hybrid water electrolyzers have drawn tremendous attention. Herein, coaxial Ni/Ni₃S₂@N-doped nanofibers are directly grown on nickel foam (NF), which is highly active for hydrogen evolution reaction. Meanwhile, the Ni₃S₂@N-doped nanofibers on NF prepared in an Ar atmosphere display superior urea oxidation reaction performance to previously reported catalysts. The cell voltage is about 1.50 V in urea electrolysis to deliver a current density of 20 mA cm^-2, lower than that of a traditional water electrolyzer (1.82 V). The current density is around 77% relative to its initial value of 20 mA cm^-2 after 20 h, superior to Pt/C|Ir/C-based urea electrolysis (14%). It is found that the synergistic effect between metallic Ni and Ni₃S₂, as well as the interfacial effect between metal centers and N-doped carbon, favors the initial dissociation of H₂O and the adsorption/desorption of H* with thermal neutral Gibbs free energy. Meanwhile, the in-situ generated NiOOH on the outer surface of Ni₃S₂ possessed lower electrochemical activation energy for urea decomposition. Meanwhile, the abundant oxygen vacancies in electrodes could expose more active sites for the adsorption of intermediates, including H* and OOH*. It is also found that the hierarchical nanostructure of densely packed nanowires provides ideal electronic and ionic transport paths for fast electrocatalytic kinetics. The present work indicated that the modulation of compositions and hierarchical nanostructure is effective to prepare efficient catalysts for H₂ production via urea electrolysis.

Artificial Heterointerfaces Achieve Delicate Reaction Kinetics towards Hydrogen Evolution and Hydrazine Oxidation Catalysis

Electrochemical water splitting for H₂ production is limited by the sluggish anode oxygen evolution reaction (OER), thus using hydrazine oxidation reaction (HzOR) to replace OER has received great attention. Here we report the hierarchical porous nanosheet arrays with abundant Ni₃ N-Co₃ N heterointerfaces on Ni foam with superior hydrogen evolution reaction (HER) and HzOR activity, realizing working potentials of -43 and -88 mV for 10 mA cm^-2 , respectively, and achieving an industry-level 1000 mA cm^-2 at 200 mV for HzOR. The two-electrode overall hydrazine splitting (OHzS) electrolyzer requires the cell voltages of 0.071 and 0.76 V for 10 and 400 mA cm^-2 , respectively. The H₂ production powered by a direct hydrazine fuel cell (DHzFC) and a commercial solar cell are investigated to inspire future practical applications. DFT calculations decipher that heterointerfaces simultaneously optimize the hydrogen adsorption free energy (ΔG_H* ) and promote the hydrazine dehydrogenation kinetics. This work provides a rationale for advanced bifunctional electrocatalysts, and propels the practical energy-saving H₂ generation techniques.

Boosting hydrogen generation by anodic oxidation of iodide over Ni-Co(OH)2 nanosheet arrays

For overall water electrolysis, the hydrogen evolution reaction (HER) is severely limited by the sluggish kinetics of the anodic oxygen evolution reaction (OER). Therefore, replacing the OER with a more favorable anodic oxidation reaction with remarkable kinetics is of paramount significance, especially the one that can produce value-added chemicals. Moreover, time-saving and cost-effective strategies for the fabrication of electrodes are helpful for the wide application of electrolysis. Herein, thermodynamically more favorable iodide electrooxidation over Ni doped Co(OH)2 nanosheet arrays (NSAs) in alkaline solution is presented as the alternative to the OER to boost the HER. And the active species are determined to be the reverse redox of the Co(iv)/Co(iii) couple. Remarkably, a negative shift of voltage of 320 mV is observed at a current density of 10 mA cm-2 after using iodide electrolysis to replace ordinary water splitting. The synthetic strategy and iodide oxidation in this work expand the application of Co-based materials in the field of energy-saving hydrogen production.

Innovative Protocols in the Catalytic Oxidation of 5-Hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) has been identified as one of the most promising biomass-based multi-purpose platform molecules. Innovative protocols, namely electrocatalysis, photocatalysis, and microwave (MW)-assisted chemistry, as well as continuous-flow systems, add a new dimension and another promising toolbox for the oxidation of HMF in recent years. This Minireview deals with recent progress in the catalytic oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) and other intermediates using noble, non-noble, and metal-free systems deploying emerging protocols. Selective HMF downstream oxidation products could be obtained not only via common catalyst modifications, namely nature of the metal, preparative method, and the property of deployed support, but also by using innovative processes.

Electrodeposition of hybrid nanosheet-structured NiCo2O4 on carbon fiber paper as a non-noble electrocatalyst for efficient electrooxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

Hybrid nanosheet-structured NiCo₂O₄ on CFP as a self-supporting electrode for electrochemical oxidation of HMF to FDCA.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Hydrogen production by electrochemical reaction using ethylene glycol with terephthalic acid

Hydrogen production by electrolysis reaction of ethylene glycol and terephthalic acid in alkaline solution. When ethylene glycol and terephthalic acid were added to water together, more hydrogen was produced.

Critical practices in conducting electrochemical conversion of 5-hydroxymethylfurfural

This work systematically studied the effects of various experimental parameters on the catalytic activity and product yield of HMF conversion and affords a rigorous assessment protocol for conducting electrochemical conversion of HMF.

Electro-catalytic oxidation of HMF to FDCA over RuO2/MnO2/CNT catalysts in base-free solution

Up to 72% yield of 2,5-furandicarboxylic acid was achieved in a neutral electrolyte through the electro-oxidation of 5-hydroxymethylfurfural over a ruthenium–manganese mixed oxide catalyst.

Nitrogen modulated NiMoO4 with enhanced activity for the electrochemical oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

NiMoO4 catalyst modified with nitrogen can significantly improve the electrocatalytic oxidation performance of HMF to FDCA.

Integrating hydrogen production with anodic selective oxidation of sulfides over a CoFe layered double hydroxide electrode

Replacing the sluggish oxygen evolution reaction (OER) with oxidation reactions for the synthesis of complex pharmaceutical molecules coupled with enhanced hydrogen evolution reaction (HER) is highly attractive, but it is rarely explored. Here, we report an electrochemical protocol for selective oxidation of sulfides to sulfoxides over a CoFe layered double hydroxide (CoFe-LDH) anode in an aqueous-MeCN electrolyte, coupled with 2-fold promoted cathodic H₂ productivity. This protocol displays high activity (85–96% yields), catalyst stability (10 cycles), and generality (12 examples) in selective sulfide oxidation. We demonstrate its applicability in the synthesis of four important pharmaceutical related sulfoxide compounds with scalability (up to 1.79 g). X-ray spectroscopy investigations reveal that the CoFe-LDH material evolved into amorphous CoFe-oxyhydroxide under catalytic conditions. This work may pave the way towards sustainable organic synthesis of valuable pharmaceuticals coupled with H₂ production.

Replacing anodic OER with selective sulfide oxidation produces sulfoxide-related pharmaceutical compounds over a CoFe-LDH catalyst with enhanced HER, providing a sustainable protocol for valuable pharmaceuticals synthesis without external oxidants.

Sustainable 2,5-furandicarboxylic synthesis by a direct 5-hydroxymethylfurfural fuel cell based on a bifunctional PtNiSx catalyst

A nanocatalyst with an interface between Pt and NiSx on carbon black (PtNiSx/CB) was synthesized, exhibiting superior electrocatalytic activities toward the oxygen reduction reaction (ORR) and 5-hydroxymethylfurfural (HMF) oxidation reaction. Based on bifunctional PtNiSx/CB, a new type of direct HMF fuel cell was assembled, which can continuously convert HMF to value-added 2,5-furandicarboxylic acid with stable discharging performance.

Partially exposed RuP2 surface in hybrid structure endows its bifunctionality for hydrazine oxidation and hydrogen evolution catalysis

Replacing the sluggish anode reaction in water electrolysis with thermodynamically favorable hydrazine oxidation could achieve energy-efficient H2 production, while the shortage of bifunctional catalysts limits its scale development. Here, we presented the scalable one-pot synthesis of partially exposed RuP2 nanoparticle-decorated carbon porous microsheets, which can act as the superior bifunctional catalyst outperforming Pt/C for both hydrazine oxidation reaction and hydrogen evolution reaction, where an ultralow working potential of -70 mV and an ultrasmall overpotential of 24 mV for 10 mA cm-2 can be achieved. The two-electrode electrolyzer can reach 10 mA cm-2 with a record-low cell voltage of 23 mV and an ultrahigh current density of 522 mA cm-2 at 1.0 V. The DFT calculations unravel the notability of partial exposure in the hybrid structure, as the exposed Ru atoms are the active sites for hydrazine dehydrogenation, while the C atoms exhibit a more thermoneutral value for H* adsorption.

Self-derivation-behaviour of substrates realizing enhanced oxygen evolution reaction

Self-derivation-behaviour of substrates is utilized to fabricate monolithic electrodes for oxygen evolution, in which the selected substrate functions as both the precursor of the active catalyst and a conductive support. In particular, NiFe layered double hydroxide (LDH) can be directly derived from the surface metal of commercial NiFe foam (NFF). Moreover, the as-prepared monolithic electrode exhibits enhanced activity and durability, originating from the resultant defective nanosheet structure and autologous catalyst-support features.

Alcohol oxidation as alternative anode reactions paired with (photo)electrochemical fuel production reactions

(Photo)electrochemical cells that produce fuels have often relied on water oxidation to complete the redox cycle. Here, the authors discuss alcohol oxidation as an alternative reaction and consider general mechanistic features of oxidation electrocatalysts.

Photocatalytic Conversion of Lignin into Chemicals and Fuels

Lignin, an underutilized component of lignocellulosic biomass, is regarded as a rich reservoir for the production of aromatic chemicals and fuels. Despite extensive research in recent years, lignin's potential is far from being fully unlocked. Photocatalysis that uses sustainable solar energy to drive lignin conversion under mild conditions has been identified as a promising strategy and received growing research interest. This review aims to present a critical introduction to the photocatalytic conversion of lignin, including a summary of lignin conversion pathways and mechanisms, as well as the latest cutting-edge innovations on photocatalyst design and reactor construction. Moreover, the screening of solvents and regulation of other key factors that are involved in photocatalytic lignin conversion are also elucidated and future perspectives and challenges for photocatalytic conversion of lignin into valuable products are discussed.

A Precious-Metal-Free Hybrid Electrolyzer for Alcohol Oxidation Coupled to CO2 -to-Syngas Conversion

Electrolyzers combining CO2 reduction (CO2 R) with organic substrate oxidation can produce fuel and chemical feedstocks with a relatively low energy requirement when compared to systems that source electrons from water oxidation. Here, we report an anodic hybrid assembly based on a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) electrocatalyst modified with a silatrane-anchor (STEMPO), which is covalently immobilized on a mesoporous indium tin oxide (mesoITO) scaffold for efficient alcohol oxidation (AlcOx). This molecular anode was subsequently combined with a cathode consisting of a polymeric cobalt phthalocyanine on carbon nanotubes to construct a hybrid, precious-metal-free coupled AlcOx-CO2 R electrolyzer. After three-hour electrolysis, glycerol is selectively oxidized to glyceraldehyde with a turnover number (TON) of ≈1000 and Faradaic efficiency (FE) of 83 %. The cathode generated a stoichiometric amount of syngas with a CO:H2 ratio of 1.25±0.25 and an overall cobalt-based TON of 894 with a FE of 82 %. This prototype device inspires the design and implementation of nonconventional strategies for coupling CO2 R to less energy demanding, and value-added, oxidative chemistry.

Photoelectrochemical Decomposition of Lignin Model Compound on a BiVO4 Photoanode

The photoelectrochemical decomposition of lignin model compounds at a BiVO₄ photoanode is demonstrated with simulated sunlight and an applied bias of 2.0 V. These prototypical lignin model compounds are photoelectrochemically converted into the corresponding aryl aldehyde and phenol derivatives in a single step with conversion of up to ≈64 % over 20 h. Control experiments suggest that vanadium sites are electrocatalytically active, which precludes the need for a redox mediator in solution. This feature of the system is corroborated by a layer of V₂ O₅ deposited on BiVO₄ serving to boost the conversion by 10 %. Our methodology capitalizes on the reactive power of sunlight to drive reactions that have only been studied previously by electrochemical or catalytic methods. The use of a BiVO₄ photoanode to drive lignin model decomposition therefore provides a new platform to extract valuable aromatic chemical feedstocks using solar energy, electricity and biomass as the only inputs.

Preparation of CoS2 supported flower-like NiFe layered double hydroxides nanospheres for high-performance supercapacitors

Layered double hydroxides (LDHs) are a kind of classic pseudocapacitive materials with lamellar structure and large specific surface area, which have attracted swinging attention in the electrochemical energy storage area. The CoS₂@Ni is synthesized through a hydrothermal process, followed by surface generation of the flower-like nickel-iron layered double hydroxide (NiFe-LDH) nanospheres through a hydrothermal process, which is directly used to design a binder-free electrode with a splendid capacitance capability. The as-synthesized NiFe-LDH@CoS₂@Ni electrode presents an outstanding specific capacitance of 11.28 F cm^-2 (3880 F g^-1) at 2 mA cm^-2 (1.17 A g^-1) in a three electrodes system. Also, the all-solid-state asymmetric supercapacitor (ASC) is combined utilizing the NiFe-LDH@CoS₂@Ni hybrid as the positive electrodes and active carbon covered Ni foam as negative electrodes, respectively. The as-fabricated ASC exhibits a high energy density of 15.84 Wh kg^-1 at the power density of 375.16 W kg^-1 and can be able to lighten a blue LED indicator for more than 30 min, revealing that the prepared NiFe-LDH@CoS₂@Ni owns great potential in the aspect of practical applications. Therefore, the prepared NiFe-LDH@CoS₂@Ni with outstanding electrochemical properties could be applied for high-performance supercapacitors.

Tailorable Electrocatalytic 5-Hydroxymethylfurfural Oxidation and H2 Production: Architecture-Performance Relationship in Bifunctional Multilayer Electrodes

Water electrocatalytic splitting is considered as an ideal process for generating H₂ without byproducts. However, in the water-splitting reaction, a high overpotential is needed to overcome the high-energy barrier due to the slow kinetics of the oxygen evolution reaction (OER). In this study, we selected the 5-hydroxymethylfurfural (HMF) oxidation reaction, which is thermodynamically favored, to replace the OER in the water-splitting process. We fabricated three-dimensional hybrid electrocatalytic electrodes via layer-by-layer (LbL) assembly for simultaneous HMF conversion and hydrogen evolution reaction (HER) to investigate the effect of the nanoarchitecture of the electrode on the electrocatalytic activity. Nanosized graphene oxide was used as a negatively charged building block for LbL assembly to immobilize the two electroactive components: positively charged Au and Pd nanoparticles (NPs). The internal architecture of the LbL-assembled multilayer electrodes could be precisely controlled and their electrocatalytic performance could be modified by changing the nanoarchitecture of the electrode, including the thickness and position of the metal NPs. Even with a composition of the identical constituent NPs, the electrodes exhibited highly tunable electrocatalytic performance depending on the reaction kinetics as well as a diffusion-controlled process due to the sequential HMF oxidation and the HER. Furthermore, a bifunctional two-electrode electrolyzer for both the anodic HMF oxidation and the cathodic HER, which had an optimized LbL-assembled electrode for each reaction, exhibited the best full-cell electrocatalytic activity.

Prospects of Value-Added Chemicals and Hydrogen via Electrolysis

Cost is a major drawback that limits the industrial-scale hydrogen production through water electrolysis. The overall cost of this technology can be decreased by coupling the electrosynthesis of value-added chemicals at the anode side with electrolytic hydrogen generation at the cathode. This Minireview provides a directory of anodic oxidation reactions that can be combined with cathodic hydrogen generation. The important parameters for selecting the anodic reactions, such as choice of catalyst material and its selectivity towards specific products are elaborated in detail. Furthermore, various novel electrolysis cell architectures for effortless separation of value-added products from hydrogen gas are described.

Electrochemical Reactors for CO2 Conversion

Increasing risks from global warming impose an urgent need to develop technologically and economically feasible means to reduce CO2 content in the atmosphere. Carbon capture and utilization technologies and carbon markets have been established for this purpose. Electrocatalytic CO2 reduction reaction (CO2RR) presents a promising solution, fulfilling carbon-neutral goals and sustainable materials production. This review aims to elaborate on various components in CO2RR reactors and relevant industrial processing. First, major performance metrics are discussed, with requirements obtained from a techno-economic analysis. Detailed discussions then emphasize on (i) technical benefits and challenges regarding different reactor types, (ii) critical features in flow cell systems that enhance CO2 diffusion compared to conventional H-cells, (iii) electrolyte and its effect on liquid phase electrolyzers, (iv) catalysts for feasible products (carbon monoxide, formic acid and multi-carbons) and (v) strategies on flow channel and anode design as next steps. Finally, specific perspectives on CO2 feeds for the reactor and downstream purification techniques are annotated as part of the CO2RR industrial processing. Overall, we focus on the component and system aspects for the design of a CO2RR reactor, while pointing out challenges and opportunities to realize the ultimate goal of viable carbon capture and utilization technology.

Mechanistic Investigation of Biomass Oxidation Using Nickel Oxide Nanoparticles in a CO2-Saturated Electrolyte for Paired Electrolysis

A highly efficient CO₂ electrolysis system could be created by introducing biomass oxidation as an alternative anodic reaction to the sluggish oxygen evolution reaction in a CO₂-saturated and near-neutral electrolyte. Here, we successfully demonstrate anodic biomass oxidation by synthesizing 5 nm nickel oxide nanoparticles (NiO NPs). NiO NPs show a unique electrocatalytic activity for 5-hydroxymethylfurfural (HMF) oxidation under near-neutral conditions, exhibiting an anodic current onset (1 mA cm^-2) at 1.524 V versus the reversible hydrogen electrode and a total Faradaic efficiency of ≤70%. Electrokinetic and in situ ultraviolet-visible spectroscopic analyses suggest that a redox active nickel hydroxide species is formed on the surface of NiO electrocatalysts during HMF oxidation, and this oxidation of Ni(II) hydroxide to Ni(III) oxyhydroxide could be the rate-determining step. This mechanistic study of biomass oxidation in a CO₂-saturated electrolyte provides insight into constructing a highly efficient system for the paired electrolysis of CO₂ reduction and biomass oxidation.