With respect to the negative role of carbon dioxide on our climate, it is clear that the time is ripe for the development of processes that convert CO(2) into useful products. The electroreduction of CO(2) is a prime candidate here, as the reaction at near-ambient conditions can yield organics such as formic acid, methanol, and methane. Recent laboratory work on the 100 A scale has shown that reduction of CO(2) to formate (HCO(2)(-)) may be carried out in a trickle-bed continuous electrochemical reactor under industrially viable conditions. Presuming the problems of cathode stability and formate crossover can be overcome, this type of reactor is proposed as the basis for a commercial operation. The viability of corresponding processes for electrosynthesis of formate salts and/or formic acid from CO(2) is examined here through conceptual flowsheets for two process options, each converting CO(2) at the rate of 100 tonnes per day.

This paper reports an investigation into the electro-reduction of CO 2 in a laboratory bench-scale continuous reactor with co-current flow of reactant gas and catholyte liquid through a flow-by 3D cathode of 30 # mesh tinned-copper. Factorial and parametric experiments were carried out in this apparatus with the variables: current (1-8 A), gas phase CO 2 concentration (16-100 vol%) and operating time (10-180 min), using a cathode feed of [CO 2 + N 2 ] gas and 0.45 M KHCO 3 (aq) with an anolyte feed of 1 M KOH(aq), in operation near ambient conditions (ca. 115 kPa(abs), 300 K). The primary and secondary reactions here were respectively the reduction of CO 2 to formate (HCOO ) ) and of water to hydrogen, while up to ca. 5% of the current went to production of CO, CH 4 and C 2 H 4 . The current efficiency for formate depended on the current density and CO 2 pressure, coupled with the hydrogen over-potential plus mass transfer capacity of the cathode, and decreased with operating time, as tin was lost from the cathode surface. For superficial current densities ranging from 0.22 to 1.78 kA m )2 , the measured values of the performance indicators are: current efficiency for HCOO ) = 86-13%, reactor voltage = 3-6 Volt, specific energy for HCOO ) = 300-1300 kWh kmol )1 , space-time yield of HCOO ) = 2 · 10 )4 -6 · 10 )4 kmol m )3 s )1 , conversion of CO 2 = 20-80% and yield of organic products from CO 2 = 6-17%. Nomenclature a specific surface area of cathode, m )1 a k Tafel constant for reaction k, V a 2Cu Tafel constant for reaction 2 on copper, V a 2Sn Tafel constant for reaction 2 on tin, V CD current density, kA m )2 CE k current efficiency for reaction k, -D diffusion coefficient of CO 2 (aq), m 2 s )1 d wire diameter in cathode mesh, m d b bubble diameter, m E electrode potential, V(SHE) E cell full-cell operating voltage (absolute value), V E r,k reversible electrode potential of reaction k, V(SHE) E o k standard electrode potential of reaction k, V(SHE) DE voltage window in operation of 3D flow-by electrode, V F Faraday's number, kC kmol )1 G gas load in 3D flow-by electrode, kg m )2 s )1 h liquid hold-up in 3D flow-by electrode, -I current, kA i superficial current density, kA m )2i iL mass transfer limited superficial current density for reaction 1, kA m )2 j k partial real current density of reaction k, kA m )2 j 1L mass transfer limited real current density for reaction 1, kA m )2 K¢, K 0 , K 1 reaction equilibrium constants, -, M kPa )1 , M k F mass transfer coefficient due to forced convection, m s )1 k G mass transfer coefficient due to gas (H 2 ) generation, m s )1 k M combined mass transfer coefficient, m s )1 L liquid load in 3D flow-by electrode, kg m )2 s )1 P total pressure, kPa(abs) p CO 2 partial pressure of CO 2 , kPa(abs) p H 2 partial pressure of H 2 , kPa(abs) R gas constant, kJ kmol )1 K )1 Re bReynolds' number for bubble generation at electrode, -Re g Reynolds' number for gas flow = v g d q g / l g , -Re l Reynolds' number for liquid flow = v l d q l / l l , -SE specific energy for formate p...

This paper reports experimental and modeling work for the laboratory scale-up of continuous ''trickle-bed'' reactors for the electro-reduction of CO 2 to potassium formate. Two reactors (A and B) were employed, with particulate tin 3D cathodes of superficial areas, respectively, 45 · 10 À4 (2-14 A) and 320 · 10 À4 m 2 (20-100 A). Experiments in Reactor A using granulated tin cathodes (99.9 wt% Sn) and a feed gas of 100% CO 2 showed slightly better performance than that of the tinned-copper mesh cathodes of our previous communications, while giving substantially improved temporal stability (200 vs. 20 min). The seven-fold scaled-up Reactor B used a feed gas of 100% CO 2 with the aqueous catholyte and anolyte, respectively [0.5 M KHCO 3 + 2 M KCl] and 2 M KOH, at inlet pressure from 350 to 600 kPa(abs) and outlet temperature 295 to 325 K. For a superficial current density of 0.6-3.1 kA m À2 Reactor B achieved corresponding formate current efficiencies of 91-63%, with the same range of reactor voltage as that in Reactor A (2.7-4.3 V), which reflects the success of the scale-up in this work. Up to 1 M formate was obtained in the catholyte product from a single pass in Reactor B, but when the catholyte feed was spiked with 2-3 M potassium formate there was a large drop in current efficiency due to formate cross-over through the Nafion 117 membrane. An extended reactor (cathode) model that used four fitted kinetic parameters and assumed zero formate cross-over was able to mirror the reactor performance with reasonable fidelity over a wide range of conditions (maximum error in formate CE = ±20%), including formate product concentrations up to 1 M.Keywords Carbon dioxide Á Continuous reactor Á Electro-reduction Á Formate Á 3D electrode Á Scale-up Á Tin granule cathode Nomenclature a 1 ,a 2 Tafel constant for reaction 1, 2 (V) b 1 ,b 2 Tafel slope for reaction 1, 2 (V decade À1 ) C Concentration of KCl (M) CECurrent efficiency (-) d p, average Average particle diameter (m) E a,1 , E a,2 Activation energies for reactions 1 and 2 (kJ kmol À1 ) E cell Full-cell operating voltage (absolute value) (V) E 1 , E 2 Electrode potential for reaction 1 and 2 (V(SHE)) E r,1 , E r,2 Reversible electrode potential for reaction 1 and 2 (V(SHE)) G Gas flow rate (mL STP min À1 ) H Height of 3D cathode (m)Exchange current densities for reactions 1 and 2 (kA m À2 ) j 1 ,j 2 Partial real current density for reaction 1, 2 (kA m À2 ) j 1L CO 2 mass transfer limited current density for reaction 1 (kA m À2 ) k 1 , k 2 Electrochemical rate constants for reactions 1 and 2 (m s À1 ) L Catholyte liquid flow rate (mL min À1 ) P cathode Cathode side pressure (kPa(abs))

This paper reports an experimental investigation into the effects of five process variables on the performance of a bench-scale continuous electrochemical reactor used in the reduction of CO 2 to potassium formate, and interprets the data in terms of reactor engineering for a (speculative) industrial process for electro-reduction of CO 2 . The process variables: temperature, catholyte species, catholyte conductivity, cathode specific surface area and cathode thickness were studied, along with CO 2 pressure and current density, in a set of factorial and parametric experiments aimed to unravel their main effects and interactions. These variables showed complex interdependent effects on the reactor performance, as measured by the current efficiency and specific energy for generation of formate (HCO 2 ) ). The ''best'' result has a formate current efficiency of 86% at a superficial current density of 1.3 kA m )2 , with a product solution of 0.08 M KHCO 2 and specific electrochemical energy of 260 kWh per kmole formate. The combined results indicate good prospects for process optimization that could lead to development of an industrial scale reactor. NomenclatureC catholyte composition CE current efficiency (dimensionless) E cathode potential (VSHE) E cell full-cell operating voltage (absolute value) (V) E o Standard equilibrium electrode potential (VSHE) GDE gas diffusion electrode i geometric (superficial) current density (kA m )2 ) i max maximum geometric (superficial) current density (kA m )2 )Me cathode material P CO 2 pressure (Bar(abs) or kPa(abs)) P cathode cathode side pressure (kPa (abs)) T temperature (K) t operating time (h) X 1 , X 2 , X 3 factorial variables defined in Tables 6, 9, 10, 13, 14, 16 and 17 y volume fraction (i.e. mole fraction) in gas phase (dimensionless) s thickness of 3D cathode (m)

Topological defects, with an asymmetric local electronic redistribution, are expected to locally tune the intrinsic catalytic activity of carbon materials. However, it is still challenging to deliberately create high‐density homogeneous topological defects in carbon networks due to the high formation energy. Toward this end, an efficient NH3 thermal‐treatment strategy is presented for thoroughly removing pyrrolic‐N and pyridinic‐N dopants from N‐enriched porous carbon particles, to create high‐density topological defects. The resultant topological defects are systematically investigated by near‐edge X‐ray absorption fine structure measurements and local density of states analysis, and the defect formation mechanism is revealed by reactive molecular dynamics simulations. Notably, the as‐prepared porous carbon materials possess an enhanced electrocatalytic CO2 reduction performance, yielding a current density of 2.84 mA cm−2 with Faradaic efficiency of 95.2% for CO generation. Such a result is among the best performances reported for metal‐free CO2 reduction electrocatalysts. Density functional theory calculations suggest that the edge pentagonal sites are the dominating active centers with the lowest free energy (ΔG) for CO2 reduction. This work not only presents deep insights for the defect engineering of carbon‐based materials but also improves the understanding of electrocatalytic CO2 reduction on carbon defects.