**1. Introduction**

Biocatalysts represent an alternative to conventional catalysts, providing a number of advantages that include availability from renewable resources, biodegradability and high selectivity [1]. Enzymes are proteins and catalyse a wide variety of reactions that have applications in a range of industrial processes [2,3]. They are extremely effective biological catalysts, highly selective and can operate under mild conditions (ambient temperatures, physiological pH and atmospheric pressure). When used in organic synthesis, enzymes can obviate the need to protect and activate functional groups. Enzymes are generally soluble in water and can avoid the use of organic solvents, resulting in the generation of less waste [4–6]. Oxidoreductases are a class of enzymes that catalyse redox reactions [7] such as oxygenation, dehydrogenation, oxidative bond formation and electron transfer reactions [8]. Over the last four decades, oxidoreductases have been coupled successfully with electrochemistry in biofuel cells [9–12] and biosensors [13–16]. The use of oxidoreductases in biocatalysis has gained significant attention in the synthesis of a variety of chemicals, such as chiral compounds [17–19], biofuels [20] and ammonia [21]. While biocatalysts are most commonly used in batch reactors, the development of biocatalytic flow reactors is of increasing interest as it can bring significant advantages that include improved mass and heat transfer, lower costs and higher yields [22,23]. This review describes oxidoreductases and bioreactors from an electrochemical perspective, with a focus on the electrocatalytic activity of oxidoreductases and methods to improve their use.
