The inchoate horizon of electrolyzer designs, membranes and catalysts towards highly efficient electrochemical reduction of CO2 to formic acid

Abstract

The economic viability of CO₂ reactors is contingent on the selectivity of the CO₂ reduction reaction and the rate of product formation. For this, the rational design of electrolyzers also has a substantial impact on the figures of merit (current density, faradaic efficiency, cell durability). Thus, herein we portray a short review on the shortcomings, challenges and the recent developments on different reactor configurations, components and membrane structures for the efficient electrochemical CO₂ reduction (CO₂R) into HCOO^-/HCOOH. Despite their low CO₂ solubility and poor mass transport, H-type electrolyzers are commercialized due to their screening of a vast number of catalysts. In contrast, membrane-based gas and liquid phase flow reactors break the barriers faced by H-types through the incorporation of gas diffusion electrodes (GDEs) and the membrane electrode assembly (MEA). As the GDE forms the gas-liquid-solid interface, it allows the electrolyzers to generate current densities at the industrial level (200 mA cm^-2). Intriguingly, a continuous liquid fed intermittent flow electrolyzer can control the electrolyte flow at a desired frequency and allow sufficient time for CO₂ gas molecules to effectively reduce into HCOOH. Therefore, a high and stable faradaic efficiency (95%) is achieved in 4 h for HCOOH (576.98 mg) using the boron-doped diamond catalyst. Very recently, a novel strategy to enhance the CO₂R to HCOO^-/HCOOH has been adopted via the recirculation of by-products to the liquid phase MEA flow reactors, which substantially improves HCOO^- selectivity, lowers material costs, and promotes CO₂ mass transfer. In the end, the zero-gap electrolyzer has newly emerged and affords reduced ohmic losses, leading to a straight-forward implementation of industrial systems for CO₂R to value-added products in the future. Besides, the efficiency of HCOO^-/HCOOH production is also explored against proton exchange, anion exchange and bipolar membranes, and the pH of the electrolyte plays a dominant role in deciding the stability and characteristics of the membranes. It is also depicted that the product selectivity depends on different electrolyzer configurations. Recently, bimetallic alloys (Bi-Sn, Bi-In) and 2D layered composites (SnO₂/rGO/CNT) have proven to be potential electrocatalysts (faradaic efficiency > 95%, highly selective and durable) assigned to the abundant active sites for CO₂R. Based on the recent findings and future research directions, we draw reader's attention to construct economic, scalable and energy-efficient CO₂R electrolyzers to realize the techno-economic predictions.